=Effect of Lubricant on Cutting Speed.=-_Turning and Boring A specialized treatise for machinists, students in the industrial and engineering schools, and apprentices, on turning and boring methods, including modern practice with engine lathes, turret lathes, vertical and horizontal boring machines

higher cutting speed can be used, if a stream of soda water or other

cooling lubricant falls upon the chip at the point where it is being

removed by the tool. In fact, experiments have shown that the cutting

speed, when using a large stream of cooling water and a high-speed steel

tool, can be about 40 percent higher than when turning dry or without a

cooling lubricant. For ordinary carbon steel tools, the gain was about

25 per cent. The most satisfactory results were obtained from a stream

falling at a rather slow velocity but in large volume. The gain in

cutting speed, by the use of soda water or other suitable fluids, was

found to be practically the same for all qualities of steel from the

softest to the hardest.

Cast iron is usually turned dry or without a cutting lubricant.

Experiments, however, made to determine the effect of applying a heavy

stream of cooling water to a tool turning cast iron, showed the

following results: Cutting speed without water, 47 feet per minute;

cutting speed with a heavy stream of water, nearly 54 feet per minute;

increase in speed, 15 per cent. The dirt caused by mixing the fine

cast-iron turnings with a cutting lubricant is an objectionable feature

which, in the opinion of many, more than offsets the increase in cutting

speed that might be obtained.

Turret lathes and automatic turning machines are equipped with a pump

and piping for supplying cooling lubricant to the tools in a continuous

stream. Engine lathes used for general work, however, are rarely

provided with such equipment and a lubricant, when used, is often

supplied by a can mounted at the rear of the carriage, having a spout

which extends above the tool. Owing to the inconvenience in using a

lubricant on an engine lathe, steel, as well as cast iron, is often

turned dry especially when the work is small and the cuts light and

comparatively short.

=Lubricants Used for Turning.=--A good grade of lard oil is an excellent

lubricant for use when turning steel or wrought iron and it is

extensively used on automatic screw machines, especially those which

operate on comparatively small work. For some classes of work,

especially when high-cutting speeds are used, lard oil is not as

satisfactory as soda water or some of the commercial lubricants, because

the oil is more sluggish and does not penetrate to the cutting point

with sufficient rapidity. Many lubricants which are cheaper than oil are

extensively used on "automatics" for general machining operations. These

usually consist of a mixture of sal-soda (carbonate of soda) and water,

to which is added some ingredient such as lard oil or soft soap to

thicken or give body to the lubricant.

A cheap lubricant for turning, milling, etc., and one that has been

extensively used, is made in the following proportions: 1 pound of

sal-soda, 1 quart of lard oil, 1 quart of soft soap, and enough water to

make 10 or 12 gallons. This mixture is boiled for one-half hour,

preferably by passing a steam coil through it. If the solution should

have an objectionable odor, this can be eliminated by adding 2 pounds of

unslaked lime. The soap and soda in this solution improve the

lubricating quality and also prevent the surfaces from rusting. For

turning and threading operations, plain milling, deep-hole drilling,

etc., a mixture of equal parts of lard oil and paraffin oil will be

found very satisfactory, the paraffin being added to lessen the expense.

Brass or bronze is usually machined dry, although lard oil is sometimes

used for automatic screw machine work. Babbitt metal is also worked dry,

ordinarily, although kerosene or turpentine is sometimes used when

boring or reaming. If babbitt is bored dry, balls of metal tend to form

on the tool point and score the work. Milk is generally considered the

best lubricant for machining copper. A mixture of lard oil and

turpentine is also used for copper. For aluminum, the following

lubricants can be used: Kerosene, a mixture of kerosene and gasoline,

soap-water, or "aqualine" one part, water 20 parts.

=Lard Oil as a Cutting Lubricant.=--After being used for a considerable

time, lard oil seems to lose some of its good qualities as a cooling

compound. There are several reasons for this: Some manufacturers use the

same oil over and over again on different materials, such as brass,

steel, etc. This is objectionable, for when lard oil has been used on

brass it is practically impossible to get the fine dust separated from

it in a centrifugal separator. When this impure oil is used on steel,

especially where high-speed steels are employed, it does not give

satisfactory results, owing to the fact that when the cutting tool

becomes dull, the small brass particles "freeze" to the cutting tool and

thus produce rough work. The best results are obtained from lard oil by

keeping it thin, and by using it on the same materials--that is, not

transferring the oil from a machine in which brass is being cut to one

where it would be employed on steel. If the oil is always used on the

same class of material, it will not lose any of its good qualities.

Prime lard oil is nearly colorless, having a pale yellow or greenish

tinge. The solidifying point and other characteristics of the oil depend

upon the temperature at which it was expressed, winter-pressed lard oil

containing less solid constituents of the lard than that expressed in

warm weather. The specific gravity should not exceed 0.916; it is

sometimes increased by adulterants, such as cotton-seed and maize oils.

CHAPTER III

TAPER TURNING--SPECIAL OPERATIONS--FITTING

It is often necessary, in connection with lathe work, to turn parts

tapering instead of straight or cylindrical. If the work is mounted

between the centers, one method of turning a taper is to set the

tailstock center out of alignment with the headstock center. When both

of these centers are in line, the movement of the tool is parallel to

the axis of the work and, consequently, a cylindrical surface is

produced; but if the tailstock _h_{1}_ is set out of alignment, as shown

in Fig. 1, the work will then be turned tapering as the tool is

traversed from _a_ to _b_, because the axis _x--x_ is at an angle with

the movement of the tool. Furthermore the amount of taper or the

difference between the diameters at the ends for a given length, will

depend on how much center _h_{1}_ is set over from the central position.

[Illustration: Fig. 1. Taper Turning by the Offset-center Method]

[Illustration: Fig. 2. Examples of Taper Work]

The amount of taper is usually given on drawings in inches per foot, or

the difference in the diameter at points twelve inches apart. For

example, the taper of the piece shown at _A_, Fig. 2, is 1 inch per

foot, as the length of the tapering surface is just twelve inches and

the difference between the diameters at the ends is 1 inch. The conical

roller shown at _B_ has a total length of 9 inches and a tapering

surface 6 inches long, and in this case the taper per foot is also 1

inch, there being a difference of 1/2 inch in a length of 6 inches or 1

inch in twice that length. When the taper per foot is known, the amount

that the tailstock center should be set over for turning that taper can

easily be estimated, but it should be remembered that the setting

obtained in this way is not absolutely correct, and is only intended to

locate the center approximately. When a taper needs to be at all

accurate, it is tested with a gage, or by other means, after taking a

trial cut, as will be explained later, and the tailstock center is

readjusted accordingly. There are also more accurate methods of setting

the center, than by figuring the amount of offset, but as the latter is

often convenient this will be referred to first.

=Setting Tailstock Center for Taper Turning.=--Suppose the tailstock

center is to be set for turning part _C_, Fig. 2, to a taper of

approximately 1 inch per foot. In this case the center would simply be

moved toward the front of the machine 1/2 inch, or one-half the required

taper per foot, because the total length of the work happens to be just

12 inches. This setting, however, would not be correct for all work

requiring a taper of 1 inch per foot, as the adjustment depends not only

on the _amount_ of the taper but on the _total length_ of the piece.

[Illustration: Fig. 3. Detail View of Lathe Tailstock]

For example, the taper roller _B_ has a taper of 1 inch per foot, but

the center, in this case, would be offset less than one-half the taper

per foot, because the total length is only 9 inches. For lengths longer

or shorter than twelve inches, the taper per inch should be found first;

this is then multiplied by the _total_ length of the work (not the

length of the taper) which gives the taper for that length, and one-half

this taper is the amount to set over the center. For example, the taper

per inch of part _B_ equals 1 inch divided by 12 = 1/12 inch. The total

length of 9 inches multiplied by 1/12 inch = 3/4 inch, and 1/2 of 3/4 =

3/8, which is the distance that the tailstock center should be offset.

In this example if the taper per foot were not known, and only the

diameters of the large and small ends of the tapered part were given,

the difference between these diameters should first be found (2-1/2-2 =

1/2); this difference should then be divided by the length of the taper

(1/2 / 6 = 1/12 inch) to obtain the taper per inch. The taper per inch

times the _total_ length represents what the taper would be if it

extended throughout the entire length, and one-half of this equals the

offset, which is 3/8 inch.

=Example of Taper Turning.=--As a practical example of taper turning let

us assume that the piece A, Fig. 4, which has been centered and

rough-turned as shown, is to be made into a taper plug, as indicated at

_B_, to fit a ring gage as at _C_. If the required taper is 1-1/2 inch

per foot and the total length is 8 inches, the tailstock center would be

offset 1/2 inch.

[Illustration: Fig. 4. Taper Plug and Gage]

To adjust the tailstock, the nuts _N_ (Fig. 3) are first loosened and

then the upper part _A_ is shifted sidewise by turning screw _S_. Scales

are provided on some tailstocks for measuring the amount of this

adjustment; if there is no scale, draw a line across the movable and

stationary parts _A_ and _B_, when the tailstock is set for straight

turning. The movement of the upper line in relation to the lower will

then show the offset, which can be measured with a scale.

When the adjustment has been made, nuts _N_ are tightened and the part

to be turned, with a dog attached, is placed between the centers the

same as for straight turning. The taper end is then reduced by turning,

but before it is near the finished size, the work is removed and the

taper tested by inserting it in the gage. If it is much out, this can be

felt, as the end that is too small can be shaken in the hole. Suppose

the plug did not taper enough and only the small end came into contact

with the gage, as shown somewhat exaggerated at _D_; in that case the

center would be shifted a little more towards the front, whereas if the

taper were too steep, the adjustment would, of course, be in the

opposite direction. A light cut would then be taken, to be followed by

another test. If the plug should fit the gage so well that there was no

perceptible shake, it could be tested more closely as follows: Draw

three or four chalk lines along the tapering surface, place the work in

the gage and turn it a few times. The chalk marks will then show whether

the taper of the plug corresponds to that of the gage; for example, if

the taper is too great, the marks will be rubbed out on the large end,

but if the taper is correct, the lines throughout their length will be

partially erased.

[Illustration: Fig. 5. Setting Work for Taper Turning by use of Caliper

Gage]

Another and more accurate method of testing tapers is to apply a thin

coat of Prussian-blue to one-half of the tapering surface, in a

lengthwise direction. The work is then inserted in the hole or gage and

turned to mark the bearing. If the taper is correct, the bearing marks

will be evenly distributed, whereas if the taper is incorrect, they will

appear at one end. Tapering pieces that have to be driven tightly into a

hole, such as a piston-rod, can be tested by the location of the bearing

marks produced by actual contact.

After the taper is found to be correct, the plug is reduced in size

until it just enters the gage as at _C_. The final cut should leave it

slightly above the required size, so that a smooth surface can be

obtained by filing. It should be mentioned that on work of this kind,

especially if great accuracy is required, the final finish is often

obtained by grinding in a regular grinding machine, instead of by

filing. When this method is employed, a lathe is used merely to

rough-turn the part close to size.

[Illustration: Fig. 6. Side View showing Relative Positions of Gage and

Work]

When the amount that the tailstock center should be offset is determined

by calculating, as in the foregoing example, it is usually necessary to

make slight changes afterward, and the work should be tested before it

is too near the finished size so that in case one or more trial cuts are

necessary, there will be material enough to permit this. When there are

a number of tapered pieces to be turned to the same taper, the

adjustment of the tailstock center will have to be changed unless the

total length of each piece and the depth of the center holes are the

same in each case.

=Setting the Tailstock Center with a Caliper Tool.=--Another method of

setting the tailstock center for taper turning is illustrated in Fig. 5.

The end of an engine piston-rod is to be made tapering as at A and to

dimensions _a_, _b_, _c_ and _d_. It is first turned with the centers in

line as at _B_. The end _d_ is reduced to diameter _b_ up to the

beginning of the taper and it is then turned to diameter _a_ as far as

the taper part _c_ extends. The tailstock center is next set over by

guess and a caliper tool is clamped in the toolpost. This tool, a side

view of which is shown in Fig. 6, has a pointer _p_ that is free to

swing about pivot _r_, which should be set to about the same height as

the center of the work. The tailstock center is adjusted until this

pointer just touches the work when in the positions shown by the full

and dotted lines at _C_, Fig. 5; that is, until the pointer makes

contact at the beginning and end of the taper part. The travel of the

carriage will then be parallel to a line _x--x_, representing the taper;

consequently, if a tool is started at the small end, as shown by the

dotted lines at _D_, with the nose just grazing the work, it will also

just graze it when fed to the extreme left as shown. Of course, if the

taper were at all steep, more than one cut would be taken.

[Illustration: Fig. 7. Obtaining Tailstock Center Adjustment by use of

Square]

If these various operations are carefully performed, a fairly accurate

taper can be produced. The straight end _d_ is reduced to size after the

tail-center is set back to the central position. Some mechanics turn

notches or grooves at the beginning and end of the tapering part, having

diameters equal to the largest and smallest part of the taper; the work

is then set by these grooves with a caliper tool. The advantage of the

first method is that most of the metal is removed while the centers are

in alignment.

[Illustration: Fig. 8. Second Step in Adjusting Tailstock Center by use

of Square]

=Setting the Tailstock Center with a Square.=--Still another method of

adjusting the tailstock for taper turning, which is very simple and

eliminates all figuring, is as follows: The part to be made tapering is

first turned cylindrical or straight for 3 or 4 inches of its length,

after the ends have been properly centered and faced square. The work is

then removed and the tailstock is shifted along the bed until the

distance _a--b_ between the extreme points of the centers is exactly 1

foot. The center is next offset a distance _b--c_ equal to one-half the

required taper per foot, after which a parallel strip _D_, having true

sides, is clamped in the toolpost. Part _D_ is then set at right angles

to a line passing from one center point to the other. This can be done

conveniently by holding a 1-foot square (preferably with a sliding head)

against one side of _D_ and adjusting the latter in the toolpost until

edge _E_ of the square blade is exactly in line with both center points.

After part _D_ is set, it should be clamped carefully to prevent

changing the position. The angle between the side of _D_ and an

imaginary line which is perpendicular to axis _a--b_ is now equal to

one-half the angle of the required taper.

The axis of the part to be turned should be set parallel with line _E_,

which can be done by setting the cylindrical surface which was

previously finished, at right angles to the side of _D_. In order to do

this the work is first placed between centers, the tailstock being

shifted along the bed if necessary; the tail-center is then adjusted

laterally until the finished cylindrical surface is square with the side

of _D_. A small try-square can be used for testing the position of the

work, as indicated in Fig. 8. If the length of the work is less than 1

foot, it will be necessary to move the center toward the rear of the

machine, and if the length is greater than 1 foot, the adjustment is, of

course, in the opposite direction.

[Illustration: Fig. 9. A Lathe Taper Attachment]

=The Taper Attachment.=--Turning tapers by setting over the tailstock

center has some objectionable features. When the lathe centers are not

in alignment, as when set for taper turning, they bear unevenly in the

work centers because the axis of the work is at an angle with them; this

causes the work centers to wear unevenly and results in inaccuracy.

Furthermore, the adjustment of the tailstock center must be changed when

turning duplicate tapers, unless the length of each piece and the depth

of the center holes are the same. To overcome these objections, many

modern lathes are equipped with a special device for turning tapers,

known as a taper attachment, which permits the lathe centers to be kept

in alignment, as for cylindrical turning, and enables more accurate work

to be done.

[Illustration: Fig. 10. Sectional View of Taper Attachment]

Taper attachments, like lathes, vary some in their construction, but all

operate on the same principle. An improved form of taper attachment is

illustrated in Figs. 9 and 10. Fig. 9 shows a plan view of a lathe

carriage with an attachment fitted to it, and Fig. 10 a sectional view.

This attachment has an arm _A_ on which is mounted a slide _S_ that can

be turned about a central pivot by adjusting screw _D_. The arm _A_ is

supported by, and is free to slide on, a bracket _B_ (see also sectional

view) that is fastened to the carriage, and on one end of the arm there

is a clamp _C_ that is attached to the lathe bed when turning tapers. On

the slide _S_ there is a shoe _F_ that is connected to bar _E_ which

passes beneath the toolslide. The rear end of the cross-feed screw is

connected to this bar, and the latter is clamped to the toolslide when

the attachment is in use.

When a taper is to be turned, the carriage is moved opposite the taper

part and clamp _C_ is fastened to the bed; this holds arm _A_ and slide

_S_ stationary so that the carriage, with bracket _B_ and shoe _F_, can

be moved with relation to the slide. If this slide _S_ is set at an

angle, as shown, the shoe as it moves along causes the toolslide and

tool to move in or out, but if the slide is set parallel to the carriage

travel, the toolslide remains stationary. Now if the tool, as it feeds

lengthwise of the work, is also gradually moved crosswise, it will turn

a taper, and as this crosswise movement is caused by the angularity of

slide _S_, different tapers are obtained by setting the slide to

different positions.

By means of a graduated scale _G_ at the end of slide _S_, the taper

that will be obtained for any angular position of the slide is shown. On

some attachments there are two sets of graduations, one giving the taper

in inches per foot and the other in degrees. While tapers are ordinarily

given in inches per foot on drawings, sometimes the taper is given in

degrees instead. The attachment is set for turning tapers by adjusting

slide _S_ until pointer _p_ is opposite the division or fractional part

of a division representing the taper. The whole divisions on the scale

represent taper in inches per foot, and by means of the sub-divisions,

the slide can be set for turning fractional parts of an inch per foot.

When slide _S_ is properly set, it is clamped to arm _A_ by the nuts

_N_. Bar _E_ is also clamped to the toolslide by bolt _H_, as previously

stated. The attachment is disconnected for straight turning by simply

loosening clamp _C_ and the bolt _H_.

=Application of Taper Attachment.=--Practical examples of lathe work,

which illustrate the use of the taper attachment, are shown in Figs, 11

and 12. Fig. 11 shows how a taper hole is bored in an engine

piston-head, preparatory to reaming. The casting must be held either in

a chuck _C_ or on a faceplate if too large for the chuck. The side of

the casting (after it has been "chucked") should run true, and also the

circumference, unless the cored hole for the rod is considerably out of

center, in which case the work should be shifted to divide the error.

The side of the casting for a short space around the hole is faced true

with a round nose turning tool, after which the rough-cored hole is

bored with an ordinary boring tool _t_, and then it is finished with a

reamer to exactly the right size and taper.

This particular taper attachment is set to whatever taper is given on

the drawing, by loosening nuts _N_ and turning slide _S_ until pointer

_P_ is opposite that division on the scale which represents the taper.

The attachment is then ready, after bolt _H_ and nuts _N_ are tightened,

and clamp _C_ is fastened to the lathe bed. The hole is bored just as

though it were straight, and as the carriage advances, the tool is

gradually moved inward by the attachment. If the lathe did not have a

taper attachment, the taper hole could be bored by using the compound

rest.

[Illustration: Fig. 11. Lathe with Taper Attachment arranged for Boring

Taper Hole in Engine Piston]

The hole should be bored slightly less than the finish size to allow for

reaming. When a reamer is used in the lathe, the outer end is supported

by the tailstock center and should have a deep center-hole. The lathe is

run very slowly for reaming and the reamer is fed into the work by

feeding out the tailstock spindle. The reamer can be kept from

revolving, either by attaching a heavy dog to the end or, if the end is

squared, by the use of a wrench long enough to rest against the lathe

carriage. A common method is to clamp a dog to the reamer shank, and

then place the tool-rest beneath it to prevent rotation. If the shank

of a tool is clamped to the toolpost so that the dog rests against it,

the reamer will be prevented from slipping off the center as it tends to

do; with this arrangement, the carriage is gradually moved along as the

tailstock spindle is fed outward. Some reamers are provided with

stop-collars which come against the finished side of the casting when

the hole has been reamed to size.

After the reaming operation, the casting is removed from the chuck and a

taper mandrel is driven into the hole for turning the outside of the

piston. This mandrel should run true on its centers, as otherwise the

outside surface of the piston will not be true with the bored hole. The

driving dog, especially for large work of this kind, should be heavy and

stiff, because light flexible clamps or dogs vibrate and frequently

cause chattering. For such heavy work it is also preferable to drive at

two points on opposite sides of the faceplate, but the driving pins

should be carefully adjusted to secure a uniform bearing on both sides.

The foregoing method of machining a piston is one that would ordinarily

be followed when using a standard engine lathe, and it would, perhaps,

be as economical as any if only one piston were being made; but where

such work is done in large quantities, time could be saved by proceeding

in a different way. For example, the boring and reaming operation could

be performed much faster in a turret lathe, which is a type designed for

just such work, but a turret lathe cannot be used for as great a variety

of turning operations as a lathe of the regular type. There are also

many other classes of work that can be turned more quickly in special

types of machines, but as more or less time is required for arranging

these special machines and often special tools have to be made, the

ordinary lathe is frequently indispensable when only a few parts are

needed; in addition, it is better adapted to some turning operations

than any other machine.

Fig. 12 illustrates how a taper attachment would be used for turning the

taper fitting for the crosshead end of an engine piston-rod. Even though

this taper corresponds to the taper of the hole in the piston, slide _S_

would have to be reset to the corresponding division on the opposite

side of the central zero mark, because the taper of the hole decreased

in size during the boring operation, whereas the rod is smallest at the

beginning of the cut, so that the tool must move outward rather than

inward as it advances. The taper part is turned practically the same as

a cylindrical part; that is, the power feed is used and, as the carriage

moves along the bed, the tool is gradually moved outward by the taper

attachment.

[Illustration: Fig. 12. Taper Attachment Set for Turning Taper End of

Piston-rod]

If the rod is being fitted directly to the crosshead (as is usually the

case), the approximate size of the small end of the taper could be

determined by calipering, the calipers being set to the size of the hole

at a distance from the shoulder or face side of the crosshead, equal to

the length of the taper fitting on the rod. If the crosshead were bored

originally to fit a standard plug gage, the taper on the rod could be

turned with reference to this gage, but, whatever the method, the taper

should be tested before turning too close to the finished size. The test

is made by removing the rod from the lathe and driving it tightly into

the crosshead. This shows how near the taper is to size, and when the

rod is driven out, the bearing marks show whether the taper is exactly

right or not. If the rod could be driven in until the shoulder is, say,

1/8 inch from the crosshead face, it would then be near enough to finish

to size by filing. When filing, the lathe is run much faster than for

turning, and most of the filing should be done where the bearing marks

are the heaviest, to distribute the bearing throughout the length of the

taper. Care should be taken when driving the rod in or out, to protect

the center-holes in the ends by using a "soft" hammer or holding a piece

of soft metal against the driving end.

[Illustration: Fig. 13. Tool Point should be in same Horizontal Plane as

Axis of Work for Taper Turning]

After the crosshead end is finished, the rod is reversed in the lathe

for turning the piston end. The dog is clamped to the finished end,

preferably over a piece of sheet copper to prevent the surface from

being marred. When turning this end, either the piston reamer or the

finished hole in the piston can be calipered. The size and angle of the

taper are tested by driving the rod into the piston, and the end should

be fitted so that by driving tightly, the shoulder will just come up

against the finished face of the piston. When the taper is finished, the

attachment is disengaged and a finishing cut is taken over the body of

the rod, unless it is to be finished by grinding, which is the modern

and most economical method.

=Height of Tool when Turning Tapers.=--The cutting edge of the tool,

when turning tapers, should be at the same height as the center or axis

of the work, whether an attachment is used or not. The importance of

this will be apparent by referring to Fig. 13. To turn the taper shown,

the tool _T_ would be moved back a distance _x_ (assuming that an

attachment is used) while traversing the length _l_. As an illustration,

if the tool could be placed as high as point _a_, the setting of the

attachment remaining as before, the tool would again move back a

distance _x_, while traversing a distance _l_, but the large end would

be under-sized (as shown by the dotted line) if the diameters of the

small ends were the same in each case. Of course, if the tool point were

only slightly above or below the center, the resulting error would also

be small. The tool can easily be set central by comparing the height of

the cutting edge at the point of the tool with one of the lathe centers

before placing the work in the lathe.

[Illustration: Fig. 14. Plan View showing Method of Turning a Taper with

the Compound Rest]

=Taper Turning with the Compound Rest.=--The amount of taper that can be

turned by setting over the tailstock center and by the taper attachment

is limited, as the centers can only be offset a certain distance, and

the slide _S_ (Fig. 9) of the attachment cannot be swiveled beyond a

certain position. For steep tapers, the compound rest _E_ is swiveled to

the required angle and used as indicated in Fig. 14, which shows a plan

view of a rest set for turning the valve _V_. This compound rest is an

upper slide mounted on the lower or main cross-slide _D_, and it can be

turned to any angular position so that the tool, which ordinarily is

moved either lengthwise or crosswise of the bed, can be fed at an angle.

The base of the compound rest is graduated in degrees and the position

of these graduations shows to what angle the upper slide is set. Suppose

the seat of valve _V_ is to be turned to an angle of 45 degrees with the

axis or center, as shown on the drawing at _A_, Fig. 15. To set the

compound rest, nuts _n_ on either side, which hold it rigidly to the

lower slide, are first loosened and the slide is then turned until the

45-degree graduation is exactly opposite the zero line; the slide is

then tightened in this position. A cut is next taken across the valve by

operating handle _w_ and feeding the tool in the direction of the arrow.

[Illustration: Fig. 15. Example of Taper Work Turned by using Compound

Rest]

In this particular instance the compound rest is set to the same angle

given on the drawing, but this is not always the case. If the draftsman

had given the included angle of 90 degrees, as shown at _B_, which would

be another way of expressing it, the setting of the compound rest would,

of course, be the same as before, or to 45 degrees, but the number of

degrees marked on the drawing does not correspond with the angle to

which the rest must be set. As another illustration, suppose the valve

were to be turned to an angle of 30 degrees with the axis as shown at

_C_. In this case the compound rest would not be set to 30 degrees but

to 60 degrees, because in order to turn the work to an angle of 30

degrees, the rest must be 60 degrees from its zero position, as shown.

From this it will be seen that the number of degrees marked on the

drawing does not necessarily correspond to the angle to which the rest

must be set, as the graduations on the rest show the number of degrees

that it is moved from its zero position, which corresponds to the line

_a--b_. The angle to which the compound rest should be set can be found,

when the drawing is marked as at _A_ or _C_, by subtracting the angle

given from 90 degrees. When the included angle is given, as at _B_,

subtract one-half the included angle from 90 degrees to obtain the

required setting. Of course, when using a compound rest, the lathe

centers are set in line as for straight turning, as otherwise the angle

will be incorrect.

Rules for Figuring Tapers

+---------------------+---------------------+--------------------------+

| Given | To Find | Rule |

+---------------------+---------------------+--------------------------+

|The taper per foot. |The taper per inch. |Divide the taper per foot |

| | | by 12. |

| | | |

|The taper per inch. |The taper per foot. |Multiply the taper per |

| | | inch by 12. |

| | | |

|End diameters and |The taper per foot. |Subtract small diameter |

| length of taper in | | from large; divide by |

| inches. | | length of taper, and |

| | | multiply quotient by 12.|

| | | |

|Large diameter and |Diameter at small |Divide taper per foot by |

| length of taper in | end in inches. | 12; multiply by length |

| inches and taper | | of length of taper, and |

| per foot. | | subtract result from |

| | | large diameter. |

| | | |

|Small diameter and |Diameter at large |Divide taper per foot by |

| length of taper in | end in inches. | 12; multiply by length |

| inches, and taper | | of taper, and add result|

| per foot. | | to small diameter. |

| | | |

|The taper per foot |Distance between | Subtract small diameter |

| and two diameters | two given diameters| from large; divide re- |

| in inches. | in inches. | mainder by taper per |

| | | foot, and multiply |

| | | quotient by 12. |

| | | |

|The taper per foot. |Amount of taper in | Divide taper per foot by |

| | a certain length | 12; multiply by given |

| | given in inches. | length of tapered part.|

+---------------------+---------------------+--------------------------+

=Accurate Measurement of Angles and Tapers.=--When great accuracy is

required in the measurement of angles, or when originating tapers,

disks are commonly used. The principle of the disk method of taper

measurement is that if two disks of unequal diameters are placed either

in contact or a certain distance apart, lines tangent to their

peripheries will represent an angle or taper, the degree of which

depends upon the diameters of the two disks and the distance between

them. The gage shown in Fig. 16, which is a form commonly used for

originating tapers or measuring angles accurately, is set by means of

disks. This gage consists of two adjustable straight-edges _A_ and

_A_{1}_, which are in contact with disks _B_ and _B_{1}_. The angle

[alpha] or the taper between the straight-edges depends, of course, upon

the diameters of the disks and the center distance _C_, and as these

three dimensions can be measured accurately, it is possible to set the

gage to a given angle within very close limits. Moreover, if a record of

the three dimensions is kept, the exact setting of the gage can be

reproduced quickly at any time. The following rules may be used for

adjusting a gage of this type.

[Illustration: Fig. 16. Disk Gage for Accurate Measurement of Angles and

Tapers]

=To Find Center Distance for a Given Taper.=--When the taper, in inches

per foot, is given, to determine center distance _C_. _Rule:_ Divide the

taper by 24 and find the angle corresponding to the quotient in a table

of tangents; then find the sine corresponding to this angle and divide

the difference between the disk diameters by twice the sine.

_Example:_ Gage is to be set to 3/4 inch per foot, and disk diameters

are 1.25 and 1.5 inch, respectively. Find the required center distance

for the disks.

0.75

---- = 0.03125.

The angle whose tangent is 0.03125 equals 1 degree 47.4 minutes; sin 1 deg.

47.4' = 0.03123; 1.50 - 1.25 = 0.25 inch;

0.25

----------- = 4.002 inches = center distance C.

2 x 0.03123

=To Find Center Distance for a Given Angle.=--When straight-edges must

be set to a given angle [alpha], to determine center distance _C_

between disks of known diameter. _Rule:_ Find the sine of half the angle

[alpha] in a table of sines; divide the difference between the disk

diameters by double this sine.

_Example:_ If an angle [alpha] of 20 degrees is required, and the disks

are 1 and 3 inches in diameter, respectively, find the required center

distance _C_.

---- = 10 degrees; sin 10 deg. = 0.17365;

3 - 1

----------- = 5.759 inches = center distance _C_.

2 x 0.17365

=To Find Angle for Given Taper per Foot.=--When the taper in inches per

foot is known, and the corresponding angle [alpha] is required. _Rule:_

Divide the taper in inches per foot by 24; find the angle corresponding

to the quotient, in a table of tangents, and double this angle.

_Example:_ What angle [alpha] is equivalent to a taper of 1-1/2 inch per

foot?

1.5

--- = 0.0625.

The angle whose tangent is 0.0625 equals 3 degrees 35 minutes, nearly;

then, 3 deg. 35 min. x 2 = 7 deg. 10 min.

=To Find Angle for Given Disk Dimensions.=--When the diameters of the

large and small disks and the center distance are given, to determine

the angle [alpha]. _Rule:_ Divide the difference between the disk

diameters by twice the center distance; find the angle corresponding to

the quotient, in a table of sines, and double the angle.

_Example:_ If the disk diameters are 1 and 1.5 inch, respectively, and

the center distance is 5 inches, find the included angle [alpha].

1.5 - 1

------- = 0.05.

2 x 5

The angle whose sine is 0.05 equals 2 degrees 52 minutes; then, 2 deg.

52 min. x 2 = 5 deg. 44 min. = angle [alpha].

[Illustration: Fig. 17. Setting Center Mark in Line with Axis of Lathe

Spindle by use of Test Indicator]

[Illustration: Fig. 18. Jig-plate with Buttons attached, ready for

Boring]

=Use of the Center Indicator.--=The center test indicator is used for

setting a center-punch mark, the position of which corresponds with the

center or axis of the hole to be bored, in alignment with the axis of

the lathe spindle. To illustrate, if two holes are to be bored, say 5

inches apart, small punch marks having that center-to-center distance

would be laid out as accurately as possible. One of these marks would

then be set central with the lathe spindle by using a center test

indicator as shown in Fig. 17. This indicator has a pointer _A_ the end

of which is conical and enters the punch mark. The pointer is held by

shank _B_ which is fastened in the toolpost. The joint _C_ by means of

which the pointer is held to the shank is universal; that is, it allows

the pointer to move in any direction. Now when the part being tested is

rotated by running the lathe, if the center-punch mark is not in line

with the axes of the lathe spindle, obviously the outer end of pointer

_A_ will vibrate, and as joint _C_ is quite close to the inner end, a

very slight error in the location of the center-punch mark will cause a

perceptible movement of the outer end, as indicated by the dotted lines.

When the work has been adjusted until the pointer remains practically

stationary, the punch mark is central, and the hole is bored. The other

center-punch mark is then set in the same way for boring the second

hole. The accuracy of this method depends, of course, upon the location

of the center-punch marks. A still more accurate way of setting parts

for boring holes to a given center-to-center distance is described in

the following:

=Locating Work by the Button Method.=--Among the different methods

employed by machinists and toolmakers for accurately locating work such

as jigs, etc., on the faceplate of a lathe, the one most commonly used

is known as the button method. This scheme is so named because

cylindrical bushings or buttons are attached to the work in positions

corresponding to the holes to be bored, after which they are used in

locating the work. These buttons, which are ordinarily about 1/2 inch in

diameter, are ground and lapped to the same size and the ends squared.

The diameter should, preferably, be such that the radius can be

determined easily, and the hole through the center should be about 1/8

inch larger than the retaining screw, so that the button can be shifted.

As an illustration of the practical application of the button method, we

shall consider, briefly, the way the holes would be accurately machined

in the jig-plate in Fig. 18. First the centers of the seven holes should

be laid off approximately correct by the usual methods, after which

small holes should be drilled and tapped for the clamping screws _S_.

After the buttons _B_ are clamped lightly in place, they are all set in

correct relation with each other and with the jig-plate. The proper

location of the buttons is very important as their positions largely

determine the accuracy of the work. A definite method of procedure that

would be applicable in all cases cannot, of course, be given, as the

nature of the work as well as the tools available make it necessary to

employ different methods.

[Illustration: Fig. 19. Setting a Button True Preparatory to Boring, by

use of Test Indicator]

In this particular case, the three buttons _a_, _b_ and _c_ should be

set first, beginning with the one in the center. As this central hole

must be 2.30 and 2.65 inches from the finished sides _A_ and _A_{1}_,

respectively, the work is first placed on an accurate surface-plate as

shown; by resting it first on one of these sides and then on the other,

and measuring with a vernier height gage, the central button can be

accurately set. The buttons _a_ and _c_ are also set to the correct

height from side _A_{1}_ by using the height gage, and in proper

relation to the central button by using a micrometer or a vernier

caliper and measuring the over-all dimension _x_. When measuring in this

way, the diameter of one button would be deducted to obtain the correct

center-to-center distance. After buttons _a_, _b_ and _c_ are set

equidistant from side A_{1} and in proper relation to each other, the

remaining buttons should be set radially from the central button _b_ and

the right distance apart. By having two micrometers or gages, one set

for the radial dimension _x_ and the other for the chordal distance _y_,

the work may be done in a comparatively short time.

[Illustration: Fig. 20. Testing Concentricity of Button with Dial Gage]

After the buttons have been tightened, all measurements should be

carefully checked; the work is then mounted on the faceplate of the

lathe, and one of the buttons, say _b_, is set true by the use of a test

indicator as shown in Fig. 19. When the end of this indicator (which is

one of a number of types on the market) is brought into contact with the

revolving button, the vibration of the pointer _I_ shows how much the

button runs out of true. When the pointer remains practically

stationary, thus showing that the button runs true, the latter should be

removed. The hole is then drilled nearly to the required size, after

which it is bored to the finish diameter. In a similar manner the other

buttons are indicated and the holes bored, one at a time. It is evident

that if each button is correctly located and set perfectly true in the

lathe, the various holes will be located at the required

center-to-center dimensions within very close limits.

[Illustration: Fig. 21. Drilling a Bushing Hole]

Fig. 20 shows how one of the buttons attached to a plate in which three

holes are to be bored is set true or concentric. The particular

indicator illustrated is of the dial type, any error in the location of

the button being shown by a hand over a dial having graduations

representing thousandths of an inch. Fig. 21 shows how the hole is

drilled after the button is removed. It will be noted that the drill is

held in a chuck, the taper shank of which fits into the tailstock

spindle, this being the method of holding small drills. After drilling,

the hole is bored as shown in Fig. 22. The boring tool should have a

keen edge to avoid springing, and if the work when clamped in position,

throws the faceplate out of balance, it is advisable to restore the

balance, before boring, by the use of a counter-weight, because the

lathe can be rotated quite rapidly when boring such a small hole.

[Illustration: Fig. 22. Boring a Bushing Hole]

When doing precision work of this kind, the degree of accuracy will

depend upon the instruments used, the judgment and skill of the workman

and the care exercised. A good general rule to follow when locating

bushings or buttons is to use the method which is the most direct and

which requires the least number of measurements. As an illustration of

how errors may accumulate, let us assume that seven holes are to be

bored in the jig-plate shown in Fig. 23, so that they are the same

distance from each other and in a straight line. The buttons may be

brought into alignment by the use of a straight-edge, and to simplify

matters, it will be taken for granted that they have been ground and

lapped to the same size. If the diameter of the buttons is first

determined by measuring with a micrometer, and then this diameter is

deducted from the center distance _x_, the difference will be the

distance _y_ between adjacent buttons. Now if a temporary gage is made

to length _y_, all the buttons can be set practically the same distance

apart, the error between any two adjacent ones being very slight. If,

however, the total length _z_ over the end buttons is measured by some

accurate means, the chances are that this distance will not equal six

times dimension _x_ plus the diameter of one button, as it should,

because even a very slight error in the gage for distance _y_ would

gradually accumulate as each button was set. If a micrometer were

available that would span two of the buttons, the measurements could be

taken direct and greater accuracy would doubtless be obtained. On work

of this kind where there are a number of holes that need to have

accurate over-all dimensions, the long measurements should first be

taken when setting the buttons, providing, of course, there are proper

facilities for so doing, and then the short ones. For example, the end

buttons in this case should first be set, then the central one and

finally those for the sub-divisions.

[Illustration: Fig. 23. Example of Work illustrating Accumulation of

Errors]

=Eccentric Turning.=--When one cylindrical surface must be turned

eccentric to another, as when turning the eccentric of a steam engine,

an arbor having two sets of centers is commonly used, as shown in Fig.

24. The distance _x_ between the centers must equal one-half the total

"throw" or stroke of the eccentric. The hub of the eccentric is turned

upon the centers _a--a_, and the tongued eccentric surface, upon the

offset centers, as indicated by the illustration. Sometimes eccentrics

are turned while held upon special fixtures attached to the faceplate.

[Illustration: Fig. 24. Special Arbor for Turning Eccentrics]

When making an eccentric arbor, the offset center in each end should be

laid out upon radial lines which can be drawn across the arbor ends by

means of a surface gage. Each center is then drilled and reamed to the

same radius _x_ as near as possible. The uniformity of the distance _x_

at each end is then tested by placing the mandrel upon the offset

centers and rotating it, by hand, with a dial indicator in contact at

first one end and then the other. The amount of offset can also be

tested either by measuring from the point of a tool held in the

toolpost, or by setting the tool to just graze the mandrel at extreme

inner and outer positions, and noting the movement of the cross-slide by

referring to the dial gage of the cross-feed screw.

[Illustration: Fig. 25. Turning an Engine Crank-pin in an Ordinary

Lathe]

=Turning a Crankshaft in a Lathe.=--Another example of eccentric turning

is shown in Fig. 25. The operation is that of turning the crank-pin of

an engine crankshaft, in an ordinary lathe. The main shaft is first

rough-turned while the forging revolves upon its centers _C_ and _C_{1}_

and the ends are turned to fit closely the center-arms _A_ and _A_{1}_.

After the sides _B_ and _B_{1}_ of the crank webs have been rough-faced,

the center-arms are attached to the ends of the shaft as shown in the

illustration. These arms have centers at _D_ and _D_{1}_ (located at the

required crank radius) which should be aligned with the rough pin, when

attaching the arms, and it is advisable to insert braces _E_ between the

arms and crank to take the thrust of the lathe centers. With the forging

supported in this way, the crank-pin and inner sides of the webs are

turned and faced, the work revolving about the axis of the pin. The

turning tools must extend beyond the tool-holder far enough to allow the

crank to clear as it swings around. Owing to this overhang, the tool

should be as heavy as possible to make it rigid and it is necessary to

take comparatively light cuts and proceed rather cautiously. After

finishing the crank-pin and inside of the crank, the center-arms are

removed and the main body of the shaft and the sides _B_ and _B_{1}_ are

finished. This method of turning crankshafts is often used in general

repair shops, etc., especially where new shafts do not have to be turned

very often. It is slow and inefficient, however, and where crankshafts

are frequently turned, special machines or attachments are used.

[Illustration: Fig. 26. LeBlond Lathe with Special Equipment for

Crankshaft Turning]

=Special Crankshaft Lathe.=--A lathe having special equipment for

rough-turning gas engine crankshaft pins is shown in Fig. 26. This

lathe is a heavy-duty type built by the R. K. LeBlond Machine Tool Co.

It is equipped with special adjustable headstock and tailstock fixtures

designed to take crankshafts having strokes up to about 6 inches. The

tools are held in a three-tool turret type of toolpost and there are

individual cross-stops for each tool. This lathe also has a roller

steadyrest for supporting the crankshaft; automatic stops for the

longitudinal feed, and a pump for supplying cutting lubricant. The

headstock fixture is carried on a faceplate mounted on the spindle and

so arranged as to be adjustable for cranks of different throw. When the

proper adjustment for a given throw has been made, the slide is secured

by four T-bolts. A graduated scale and adjusting screw permit of

accurate adjustments.

The revolving fixture is accurately indexed for locating different

crank-pins in line with the lathe centers, by a hardened steel plunger

in the slide which engages with hardened bushings in the fixture. The

index is so divided that the fixture may be rotated 120 or 180 degrees,

making it adjustable for 2-, 4- and 6-throw cranks. After indexing, the

fixture is clamped by two T-bolts which engage a circular T-slot. The

revolving fixture is equipped with removable split bushings which can be

replaced to fit the line bearings of different sized crankshafts. The

work is driven by a V-shaped dovetail piece having a hand-nut

adjustment, which also centers the pin by the cheek or web. The crank is

held in position by a hinged clamp on the fixture. The tailstock fixture

is also adjustable and it is mounted on a spindle which revolves in a

bushing in the tailstock barrel. The adjustment is obtained in the same

manner as on the headstock fixture, and removable split bushings as well

as a hinged clamp are also employed.

The method of chucking a four-throw crank is as follows: The two

fixtures are brought into alignment by two locking pins. One of these is

located in the head and enters a bushing in the large faceplate and the

other is in the tailstock and engages the tailstock fixture. The

crankshaft is delivered to the machine with the line bearings

rough-turned and it is clamped by the hinged clamp previously referred

to and centered by the V-shaped driver. The locking pins for both

fixtures are then withdrawn and the machine is ready to turn two of the

pins. After these have been machined, the fixtures are again aligned by

the locking pins, the two T-bolts of the headstock fixture and the

hinged clamp at the tailstock are released, the indexing plunger is

withdrawn and the headstock fixture and crank are turned 180 degrees or

until the index plunger drops into place. The crank is then clamped at

the tailstock end and the revolving fixture is secured by the two

T-bolts previously referred to. After the locking pins are withdrawn,

the lathe is ready to turn the two opposite pins.

[Illustration: Fig. 27. Diagrams showing Arrangements of Tools on

LeBlond Lathe]

=Operation of Special Crankshaft Lathe.=--The total equipment of this

machine (see Fig. 27) is carried on a three-tool turret tool-block. The

method of turning a crankshaft is as follows: A round-nosed turning tool

is first fed into a cross stop as illustrated in the plan view at _A_,

which gives the proper diameter. The feed is then engaged and the tool

feeds across the pin until the automatic stop lever engages the first

stop, which throws out the feed automatically. The carriage is then

moved against a positive stop by means of the handwheel. The roller

back-rest is next adjusted against the work by the cross-feed handwheel

operating through a telescopic screw, and the filleting tools are

brought into position as at _B_. These are run in against a stop,

removing the part left by the turning tool and giving the pin the proper

width and fillets of the correct radius. If the crankshaft has straight

webs which must be finished, two tools seen at _b_ are used for facing

the webs to the correct width. During these last two operations, the

crank is supported by the roller back-rest, thus eliminating any

tendency of the work to spring.

[Illustration: Fig. 28. (A) Spherical Turning with Compound Rest. (B)

Concave Turning]

After one pin is finished in the manner described, the back-rest is

moved out of the way, the automatic stop lever raised, the carriage

shifted to the next pin, and the operation repeated. The tools are held

in position on the turret by studs, and they can be moved and other

tools quickly substituted for pins of different widths. This machine is

used for rough-turning the pins close to the required size, the

finishing operation being done in a grinder. It should be mentioned, in

passing, that many crankshafts, especially the lighter designs used in

agricultural machinery, etc., are not turned at all but are ground from

the rough.

=Spherical Turning.=--Occasionally it may be necessary to turn a

spherical surface in the lathe. Sketch _A_, Fig. 28, shows how a small

ball-shaped end can be turned on a piece held in a chuck. The lathe

carriage is adjusted so that the pin around which the compound rest

swivels is directly under the center a. The bolts which hold the swivel

are slightly loosened to allow the top slide to be turned, as indicated

by the dotted lines; this causes the tool point to move in an arc about

center _a_, and a spherical surface is turned. Light cuts must be taken

as otherwise it would be difficult to turn the slide around by hand.

[Illustration: Fig. 29. Spherical Turning Attachment for Engine Lathe]

Sketch _B_ illustrates how a concave surface can be turned. The

cross-slide is adjusted until swivel pin is in line with the lathe

centers, and the carriage is moved along the bed until the horizontal

distance between center _b_ of the swivel, and the face of the work,

equals the desired radius of the concave surface. The turning is then

done by swinging the compound rest as indicated by the dotted lines. The

slide can be turned more evenly by using the tailstock center to force

it around. A projecting bar is clamped across the end of the slide at

_d_, to act as a lever, and a centered bar is placed between this lever

and the tailstock center; then by screwing out the tailstock spindle,

the slide is turned about pivot _b_. The alignment between the swivel

pin and the lathe centers can be tested by taking a trial cut; if the

swivel pin is too far forward, the tool will not touch the turned

surface if moved past center _c_, and if the pin is too far back, the

tool will cut in on the rear side.

=Spherical Turning Attachments.=--When spherical turning must be done

repeatedly, special attachments are sometimes used. Fig. 29 shows an

attachment applied to a lathe for turning the spherical ends of

ball-and-socket joints. The height or radius of the cutting tool and,

consequently, the diameter of the turned ball, is regulated by adjusting

screw _A_. The tool is swung around in an arc, by turning handle _B_

which revolves a worm meshing with an enclosed worm-wheel. As will be

seen, the work is held in a special chuck, owing to its irregular shape.

[Illustration: Fig. 30. Attachment for Turning Spherical End of Gasoline

Engine Piston]

Another spherical turning attachment is shown in Fig. 30. This is used

for machining the ends of gasoline engine pistons. The cross-slide has

bolted to it a bar _A_ carrying a roller which is pressed against a

forming plate _B_ by a heavy spring _C_. The forming plate _B_, which is

attached to a cross-piece fastened to the ways of the lathe bed, is

curved to correspond with the radius required on the piston end, and

when the tool is fed laterally by moving the cross-slide, it follows the

curve of plate _B_. The piston is held in a special hollow chuck which

locates it in a central position and holds it rigidly.

In connection with lathe work, special attachments and tools are often

used, especially when considerable work of one class must be turned;

however, if a certain part is required in large quantities, it is

usually more economical to use some semi-automatic or automatic turning

machine, especially designed for repetition work.

=Turning with Front and Rear Tools.=--In ordinary engine lathe practice,

one tool is used at a time, but some lathes are equipped with

tool-holders at the front and rear of the carriage so that two tools can

be used simultaneously. Fig. 31 shows a detail view of a lathe in which

front and rear tools are being used. These tools are of the inserted

cutter type and the one at the rear is inverted, as the rotary movement

of the work is, of course, upward on the rear side. This particular

lathe was designed for taking heavy roughing cuts and has considerable

driving power.

[Illustration: Fig. 31. Front and Rear Tools used for Roughing]

The part shown in this illustration is a chrome-nickel steel bar which

is being roughed out to form a milling machine spindle. It is necessary

to reduce the diameter of the bar from 5-7/16 inches to 3-3/4 inches for

a length of 27 inches, because of a collar on one end. This reduction is

made in one passage of the two tools, with a feed of 1/32 inch per

revolution and a speed of 60 revolutions per minute. The use of two

tools for such heavy roughing cuts is desirable, especially when the

parts are required in large quantities, because the thrust of the cut on

one side, which tends to deflect the work, is counteracted by the thrust

on the opposite side.

[Illustration: Fig. 32. Lo-swing Lathe for Multiple Turning]

Sometimes special tool-holders are made for the lathe, so that more than

one tool can be used for turning different surfaces or diameters at the

same time, the tools being set in the proper relation to each other. The

advantage of this method has resulted in the design of a special lathe

for multiple-tool turning.

=A Multiple-tool Lathe.=--The lathe shown in Fig. 32 (which is built by

the Fitchburg Machine Works and is known as the Lo-swing) is designed

especially for turning shafts, pins and forgings not exceeding 3-1/2

inches in diameter. It has two carriages _A_ and _B_ which, in

conjunction with special tool-holders, make it possible to turn several

different diameters simultaneously. At the front of this lathe there is

an automatic stop-rod _C_ for disengaging the feed when the tools have

turned a surface to the required length. This stop-rod carries

adjustable stops _D_ which are set to correspond with shoulders, etc.,

on the work. The rod itself is also adjustable axially, so that the

tools, which are usually arranged in groups of two or more (depending

upon the nature of the work), can be disengaged at a point nearer or

farther from the headstock as may be required, owing to a variation in

the depth of center holes. For example, if it were necessary to feed a

group of tools farther toward the headstock after they had been

automatically disengaged, the entire rod with its stops would be

adjusted the required amount in that direction.

[Illustration: Fig. 33. Lo-swing Lathe arranged for Turning a Steering

Knuckle]

The gage _G_, which is attached to a swinging arm, is used to set the

stop bar with reference to a shoulder near the end of the work, when it

is necessary to finish other parts to a given distance from such a

shoulder or other surface. The use of this gage will be explained more

fully later. Cooling lubricant for the tools is supplied through the

tubes _E_. The lathe shown in the illustration is arranged for turning

Krupp steel bars. A rough bar and also one that has been turned may be

seen to the right. The plain cylindrical bar is turned to five different

diameters, by groups of tools held on both carriages.

[Illustration: Fig. 34. Plan View showing Method of driving Steering

Knuckle and Arrangement of Tools]

=Examples of Multiple Turning.=--Figs. 33 and 34 show how a Lo-swing

lathe is used for turning the steering knuckle of an automobile. Four

tools are used in this case, three cylindrical surfaces and one tapering

surface being turned at the same time. For this job, the four tools are

mounted on one carriage. The taper part is turned by the second tool

from the headstock, which is caused to feed outward as the carriage

advances by a taper attachment. This tool is held in a special holder

and bears against a templet at the rear, which is tapered to correspond

with the taper to be turned. This templet is attached to a bar which, in

turn, is fastened to a stationary bracket seen to the extreme left in

Fig. 33. This part is finished in two operations, the tool setting being

identical for each operation, except for diameter adjustments. As the

illustrations show, three of the four tools employed are used for

straight turning on different diameters, while the fourth finishes the

taper.

These pieces, which are rough drop forgings, are first reduced to the

approximate size. When it becomes necessary to grind the tools, they are

reset and those parts which have been roughed out are turned to the

finished size. The average time for the first operation, which includes

starting, stopping, turning and replacing the piece, is one minute,

while for the second operation with the finer feed, an average time of

two minutes is required. The work is driven by sleeve _S_, which fits

over the spindle and is held in position by the regular driver, as

shown. This sleeve is notched to fit the knuckle, so that the latter can

easily and quickly be replaced when finished.

One of the interesting features of this job lies in the method of

locating the shoulders on each knuckle, at the same distance from the

hole _H_ which is drilled previously, and which receives the bolt on

which the knuckle swivels when assembled in a car. As soon as the

knuckle has been placed between the centers, a close-fitting plug _P_

(Fig. 33) is inserted in this hole and the indicator arm with its

attached gage or caliper _G_ is swung up to the position shown. The

stop-rod on which the stops have been previously set for the correct

distance between the shoulders is next adjusted axially until the gage

_G_ just touches the plug _P_. The indicator is then swung out of the

way, and the piece turned. If the next knuckle were centered, say,

deeper than the previous one which would, of course, cause it to be

located nearer the headstock, obviously all the shoulders would be

located farther from the finished hole, provided the position of the

stops remained the same as before. In such a case their position would,

however, be changed by shifting the stop-rod until the gage _G_ again

touched the plug thus locating all the stops with reference to the hole.

As the adjustment of the stop-rod changes the position of the taper

templet as well as the stops, it is evident that both the shoulders and

the taper are finished the same distance from the hole in each case. The

connection of the bracket (to which the templet arm is attached) with

the stop-rod is clearly shown in Fig. 33. This bracket can either be

locked to the ways or adjusted to slide when the stop-rod is moved.

[Illustration: Fig. 35. First and Second Operations on Automobile

Transmission Shaft--Lo-swing Lathe]

The part illustrated in Fig. 35 is an automobile transmission shaft. In

this particular case, cylindrical, tapering and spherical surfaces are

turned. The upper view shows, diagrammatically, the arrangement of the

tools and work for the first operation. After the shaft is "spotted" at

_A_ for the steadyrest, the straight part _C_ and the collar _B_ are

sized with tools _S_ and _R_ which are mounted on the left-hand

carriage. A concave groove is then cut in collar _B_ by tool _R_, after

which spherical end _D_ is formed by a special attachment mounted on the

right-hand carriage. This attachment is the same, in principle, as the

regular taper-turning attachment, the substitution of a circular templet

_T_ for the straight kind used on taper work being the only practical

difference.

[Illustration: Fig. 36. Axle End turned in One Traverse of the Five

Tools shown]

After the surfaces mentioned have been finished on a number of pieces,

the work is reversed and the tools changed as shown by the lower view.

The first step in the second operation is to turn the body _E_ of the

shaft with the tool _T_ on the left-hand carriage. The taper _F_ and the

straight part _G_ are then finished, which completes the turning. It

will be noted that in setting up the machine for this second operation,

it is arranged for taper turning by simply replacing the circular

templet with the straight one shown. When this taper attachment is not

in use, the swiveling arm _M_, which is attached to a bracket, is swung

out of the way.

The method of driving this shaft is worthy of note. A dog having two

driving arms each of which bears against a pin _N_ that passes through a

hole in the spindle is used. As the ends of this pin, against which the

dog bears, are beveled in opposite directions, the pin turns in its hole

when the dog makes contact with it and automatically adjusts itself

against the two driving members of the dog. The advantage of driving by

a two-tailed dog, as most mechanics know, is in equalizing the tendency

to spring slender parts while they are being turned.

[Illustration: Fig. 37. Lathe Knurling Tool having Three Pairs of

Knurls--Coarse, Medium and Fine]

In Fig. 36 another turning operation on a lathe of this type is shown,

the work in this case being a rear axle for a motor truck. The turning

of this part is a good example of that class of work where the rapid

removal of metal is the important feature. As the engraving shows, the

stock, prior to turning, is 3-1/2 inches in diameter and it is reduced

to a minimum diameter of 1-1/16 inch. This metal is turned off with one

traverse of the carriage or by one passage of the five tools, and the

weight of the chips removed from each end of the axle is approximately

12 pounds. The time required for the actual turning is about 9 minutes,

while the total time for the operation, which includes placing the heavy

piece in the machine, turning, and removing the work from the lathe, is

12 minutes. The axle revolves, while being turned, at 110 revolutions

per minute and a feed equivalent to 1 inch of tool travel to 60

revolutions of the work is used. It will be noticed that the taper

attachment is also employed on this part, the taper being turned by the

second tool from the left. As the axle is equipped with roller bearings,

it was found desirable to finish the bearing part by a separate

operation; therefore, in the operation shown the axle is simply roughed

down rather close to the finished dimensions, leaving enough material

for a light finishing cut.

=Knurling in the Lathe.=--Knurling is done either to provide a rough

surface which can be firmly gripped by the hand or for producing an

ornamental effect. The handles of gages and other tools are often

knurled, and the thumb-screws used on instruments, etc., usually have

knurled edges. A knurled surface consists of a series of small ridges or

diamond-shaped projections, and is produced in the lathe by the use of a

tool similar to the one shown in Fig. 37, this being one of several

different designs in common use. The knurling is done by two knurls _A_

and _B_ having teeth or ridges which incline to the right on one knurl

and to the left on the opposite knurl, as shown by the end view. When

these two knurls are pressed against the work as the latter revolves,

one knurl forms a series of left-hand ridges and the other knurl

right-hand ridges, which cross and form the diamond-shaped knurling

which is generally used.

If the surface to be knurled is wider than the knurls, the power feed of

the lathe should be engaged and the knurling tool be traversed back and

forth until the diamond-shaped projections are well formed. To prevent

forming a double set of projections, feed the knurl in with considerable

pressure at the start, then partially relieve the pressure before

engaging the power feed. Use oil when knurling.

The knurls commonly used for lathe work have spiral teeth and ordinarily

there are three classes, known as coarse, medium and fine. The medium

pitch is generally used. The teeth of coarse knurls have a spiral angle

of 36 degrees and the pitch of the knurled cut (measured parallel to the

axis of the work) should be about 8 per inch. For medium knurls, the

spiral angle is 29-1/2 degrees and the pitch, measured as before, is 12

per inch. For fine knurls, the spiral angle is 25-3/4 degrees and the

pitch 20 per inch. The knurls should be about 3/4 inch in diameter and

3/8 inch wide. When made to these dimensions, coarse knurls have 34

teeth; medium, 50 teeth; and fine knurls, 80 teeth.

[Illustration: Fig. 38. Hendey Relieving Attachment applied to a Lathe]

The particular tool illustrated in Fig. 37 has three pairs of knurls of

coarse, medium and fine pitch. These are mounted in a revolving holder

which not only serves to locate the required set of knurls in the

working position, but enables each knurl to bear against the surface

with equal pressure. Concave knurls are sometimes used for knurling

rounded edges on screw heads, etc.

=Relieving Attachment.=--Some lathes, particularly those used in

toolrooms, are provided with relieving attachments which are used for

"backing off" the teeth of milling cutters, taps, hobs, etc. If a

milling cutter of special shape is to be made, the cutter blank is first

turned to the required form with a special tool having a cutting edge

that corresponds with the shape or profile of the cutter to be made. The

blank is then fluted or gashed to form the teeth, after which the tops

of the teeth are relieved or backed off to provide clearance for the

cutting edges. The forming tool used for turning the blank is set to

match the turned surface, and the teeth are backed off as the result of

a reciprocating action imparted to the toolslide by the relieving

attachment. The motion of the toolslide is so adjusted that the tool

will meet the front of each tooth and the return movement begin promptly

after the tool leaves the back end of the tooth.

[Illustration: Fig. 39. Relieving a Formed Cutter]

These attachments differ somewhat in their construction and arrangement

but the principle of their operation is similar. Fig. 38 shows a Hendey

relieving attachment applied to a lathe. A bracket carrying the gearing

_A_ through which the attachment is driven is mounted upon the main gear

box of the lathe, and the special slide _B_, which is used when

relieving, is placed on the cross-slide after removing the regular

compound rest. The gears at _A_ are changed to suit the number of flutes

or gashes in the cutter, tap or whatever is to be relieved. If we assume

that the work is a formed milling cutter having nine teeth, then with

this particular attachment, a gear having 90 teeth would be placed on

the "stud" and a 40-tooth gear on the cam-shaft, the two gears being

connected by a 60-tooth intermediate gear. With this combination of

gearing, the toolslide would move in and out nine times for each

revolution of the work, so that the tool could back off the top of each

tooth. (The gearing to use for various numbers of flutes is shown by an

index plate on the attachment.) The amount of relief is varied to suit

the work being done, by means of a toothed coupling which makes it

possible to change the relative position between the eccentric which

actuates the toolslide and the cam lever, thereby lengthening or

shortening the reciprocating travel of the tool.

[Illustration: Fig. 40. Relieving Side of Angular Milling Cutter]

=Application of Relieving Attachment.=--Some typical examples of the

kind of work for which the relieving attachment is used are shown in

Figs. 39 to 42, inclusive. Fig. 39 shows how a formed milling cutter is

relieved. The toolslide is set at right angles to the axis of the work,

and the tool moves in as each tooth passes, and out while crossing the

spaces or flutes between the teeth. As the result of this movement, the

tops of the teeth are backed off eccentrically but the form or shape is

the same from the front to the back of the tooth; hence, a cutter that

has been relieved in this way can be ground repeatedly without changing

the profile of the teeth, provided the faces are ground so as to lie in

a radial plane.

When relieving, the cutting speed should be much less than when turning

in order to give the toolslide time to operate properly. A maximum of

180 teeth per minute is recommended, and, if wide forming tools are

used, it might be advisable to reduce the speed so low that only 8 teeth

per minute would be relieved. It is also essential to use a tool having

a keen edge, and the toolslide should work freely but be closely

adjusted to the dovetail of the lower slide. Before beginning to back

off the teeth, it is a good plan to color the work either by heating it

or dipping into a strong solution of copper sulphate. This will enable

one to see plainly the cutting action of the tool in order to stop

relieving at the proper time.

[Illustration: Fig. 41. Relieving a Right-hand Tap]

Fig. 40 shows a method of relieving the teeth of an angular cutter. For

an operation of this kind the toolslide is swiveled around at right

angles to the side that is to be relieved. By the use of an additional

universal joint and bearing to permit the toolslide to be swung to a

90-degree angle, the teeth of counterbores, etc., can be relieved on the

ends. When the attachment is used for relieving inside work, such as

hollow mills and threading dies, the eccentric which controls the travel

of the toolslide is set so that the relieving movement is away from the

axis of the cutter instead of toward it. This change is made by the

toothed coupling previously referred to, which connects the cam lever

and oscillating shaft, the latter being turned beyond the zero mark in a

clockwise direction as far as is necessary to obtain the desired amount

of travel. For internal work it is also necessary to change the position

of the opposing spring of the toolslide, so that it will press against

the end of the slide and prevent the tool from jumping into the work.

[Illustration: Fig. 42. Relieving a Hob having Spiral Flutes]

Fig. 41 shows how a right-hand tap is relieved. The ordinary practice is

to first set the tool the same as for cutting a thread. The motion of

the toolslide is then adjusted so that the tool on the forward stroke

will meet the front of each tooth, and start back as soon as the tool

leaves the end of the land or top of the tooth. Taps having a left-hand

thread can be relieved by two different methods. With the first method

the cut starts at the cutting edge of each tooth, and ends at the

"heel," the tool moving in toward the center of the work. With the

second method, the cut begins at the heel and discontinues at the

cutting edge, the tool being drawn away from the work during the cut.

When using the first method the tap must be placed with the point toward

the headstock, the shank end being supported by the tailstock center.

This is done by providing an extension or blank end at the point of the

tap long enough to hold the driving dog. With the second method, the tap

is held between centers the same as one having a right-hand thread, but

the travel of the toolslide is set the same as for inside relief.

=Relieving Hobs or Taps Having Spiral Flutes.=--With this attachment,

taps or hobs having "spiral" or helical flutes can also be relieved. (A

spiral flute is preferable to one that is parallel to the axis, because

with the former the tool has cutting edges which are square with the

teeth; this is of especial importance when the lead of the hob or tap

thread is considerable.) When relieving work having spiral flutes (as

illustrated in Fig. 42), the lead of the spiral and the gears necessary

to drive the attachment are first determined. After the attachment is

geared for the number of flutes and to compensate for the spiral, the

lead-screw is engaged and the backing-off operation is performed the

same as though the flutes were straight. The carriage should not be

disengaged from the lead-screw after starting the cut, the tool being

returned by reversing the lathe.

When gearing the attachment for relieving a tap or hob having spiral

flutes, the gears are not selected for the actual number of flutes

around the circumference but for a somewhat larger number which depends

upon the lead of the hob thread and the lead of the spiral flutes. Let

us assume that a hob has 6 spiral flutes and that the attachment is

geared for that number. The result would be that as the tool advanced

along the thread, it would not keep "in step" with the teeth because the

faces of the teeth lie along a spiral (or helix which is the correct

name for this curve); in other words, the tool would soon be moving in

too late to begin cutting at the proper time, and to compensate for

this, the attachment is geared so that the tool will make a greater

number of strokes per revolution of the work than the actual number of

flutes around the circumference.

With this attachment, the two gears listed on the index plate for the

actual number of flutes are selected, and then two compensating gears

are added, thus forming a compound train of gearing. The ratio _R_ of

these compensating gears is determined as follows:

_r_ + 1

R = -------

_r_

in which

_r_ = _L_ / _l_;

_L_ = lead of spiral;

_l_ = lead of hob thread.

For example, if a hob has a pitch circumference of 3.25, a single thread

of 0.75 inch lead, and 6 spiral flutes, what compensating gears would be

required?

The lead _L_ of the spiral flutes is first determined by dividing the

square of the circumference _C_ of the hob at the pitch line by the lead

_l_ of the hob thread. Thus lead _L_ = _C^2_/_l_, or, in this case, _L_

= 3.25^2/0.75 = 14 inches, approximately. Then _r_ = 14 / 0.75 = 18-2/3.

Inserting these values in the formula for ratio R,

18-2/3 + 1 19-2/3 19-2/3 x 3 59

_R_ = ---------- = ------ = ---------- = --

18-2/3 18-2/3 18-2/3 x 3 56

Hence, the compensating gears will have 56 and 59 teeth, respectively,

the latter being the driver. As the gears for 6 flutes listed on the

regular index plate are, stud-gear 60 teeth, cam-shaft gear 40 teeth,

the entire train of gears would be as follows: Gear on stud, 60;

_driven_ intermediate gear, 56; _driving_ intermediate gear, 59;

cam-shaft gear, 40. It will be understood that the position of the

driving gears or the driven gears can be transposed without affecting

the ratio.

=Classes of Fits Used in Machine Construction.=--In assembling machine

parts it is necessary to have some members fit together tightly, whereas

other parts such as shafts, etc., must be free to move or revolve with

relation to each other. The accuracy required for a fitting varies for

different classes of work. A shaft that revolves in its bearing must be

slightly smaller than the bearing so that there will be room for a film

of lubricant. A crank-pin that must be forced into the crank-disk is

made a little larger in diameter than the hole, to secure a tight fit.

When a very accurate fitting between two cylindrical parts that must be

assembled without pressure is required, the diameter of the inner member

is made as close to the diameter of the outer member as is possible. In

ordinary machine construction, five classes of fits are used, _viz_;

running fit, push fit, driving fit, forced fit and shrinkage fit. The

running fit, as the name implies, is employed when parts must rotate;

the push fit is not sufficiently free to rotate; the other classes

referred to are used for assembling parts that must be held in fixed

positions.

=Forced Fits.=--This is the term used when a pin, shaft or other

cylindrical part is forced into a hole of slightly smaller diameter, by

the use of a hydraulic press or other means. As a rule, forced fits are

restricted to parts of small and medium size, while shrinkage fits have

no such limitations and are especially applicable when a maximum "grip"

is desired, or when (as in the construction of ordnance) accurate

results as to the intensity of stresses produced in the parts united are

required. The proper allowance for a forced fit depends upon the mass of

metal surrounding the hole, the size of the work, the kind and quality

of the material of which the parts are composed and the smoothness and

accuracy of the pin and bore. When a pin or other part is pressed into a

hole a second time, the allowance for a given tonnage should be

diminished somewhat because the surface of the bore is smoother and the

metal more compact. The pressure required in assembling a forced fit

will also vary for cast hubs of the same size, if they are not uniform

in hardness. Then there is the personal factor which is much in evidence

in work of this kind; hence, data and formulas for forced fit allowances

must be general in their application.

=Allowance for Forced Fits.=--The allowance per inch of diameter usually

ranges from 0.001 inch to 0.0025 inch, 0.0015 being a fair average.

Ordinarily, the allowance per inch decreases as the diameter increases;

thus the total allowance for a diameter of 2 inches might be 0.004 inch,

whereas for a diameter of 8 inches the total allowance might not be over

0.009 or 0.010 inch. In some shops the allowance is made practically the

same for all diameters, the increased surface area of the larger sizes

giving sufficient increase in pressure. The parts to be assembled by

forced fits are usually made cylindrical, although sometimes they are

slightly tapered. The advantages of the taper form are that the

possibility of abrasion of the fitted surfaces is reduced; that less

pressure is required in assembling; and that the parts are more readily

separated when renewal is required. On the other hand, the taper fit is

less reliable, because if it loosens, the entire fit is free with but

little axial movement. Some lubricant, such as white lead and lard oil

mixed to the consistency of paint, should be applied to the pin and bore

before assembling, to reduce the tendency of abrasion.

Allowances for Different Classes of Fits

(Newall Engineering Co.)

+-----+--------------------------------------------------------------+

| | Tolerances in Standard Holes[1] |

|Class+------------+---------+---------+---------+---------+---------+

| | Nominal | Up to | 9/16"-1"| 1-1/16"-| 2-1/16"-| 3-1/16"-|

| | Diameters | 1/2" | | 2" | 3" | 4" |

+-----+------------+---------+---------+---------+---------+---------+

| | High Limit | +0.0002 | +0.0005 | +0.0007 | +0.0010 | +0.0010 |

| A | Low Limit | -0.0002 | -0.0002 | -0.0002 | -0.0005 | -0.0005 |

| | Tolerance | 0.0004 | 0.0007 | 0.0009 | 0.0015 | 0.0015 |

+-----+------------+---------+---------+---------+---------+---------+

| | High Limit | +0.0005 | +0.0007 | +0.0010 | +0.0012 | +0.0015 |

| B | Low Limit | -0.0005 | -0.0005 | -0.0005 | -0.0007 | -0.0007 |

| | Tolerance | 0.0010 | 0.0012 | 0.0015 | 0.0019 | 0.0022 |

+-----+------------+---------+---------+---------+---------+---------+

| Allowances for Forced Fits |

+-----+------------+---------+---------+---------+---------+---------+

| | High Limit | +0.0010 | +0.0020 | +0.0040 | +0.0060 | +0.0080 |

| F | Low Limit | +0.0005 | +0.0015 | +0.0030 | +0.0045 | +0.0060 |

| | Tolerance | 0.0005 | 0.0005 | 0.0010 | 0.0015 | 0.0020 |

+-----+------------+---------+---------+---------+---------+---------+

| Allowances for Driving Fits |

+-----+------------+---------+---------+---------+---------+---------+

| | High Limit | +0.0005 | +0.0010 | +0.0015 | +0.0025 | +0.0030 |

| D | Low Limit | +0.0002 | +0.0007 | +0.0010 | +0.0015 | +0.0020 |

| | Tolerance | 0.0003 | 0.0003 | 0.0005 | 0.0010 | 0.0010 |

+-----+------------+---------+---------+---------+---------+---------+

| Allowances for Push Fits |

+-----+------------+---------+---------+---------+---------+---------+

| | High Limit | -0.0002 | -0.0002 | -0.0002 | -0.0005 | -0.0005 |

| P | Low Limit | -0.0007 | -0.0007 | -0.0007 | -0.0010 | -0.0010 |

| | Tolerance | 0.0005 | 0.0005 | 0.0005 | 0.0005 | 0.0005 |

+-----+------------+---------+---------+---------+---------+---------+

| Allowances for Running Fits[2] |

+-----+------------+---------+---------+---------+---------+---------+

| | High Limit | -0.0010 | -0.0012 | -0.0017 | -0.0020 | -0.0025 |

| X | Low Limit | -0.0020 | -0.0027 | -0.0035 | -0.0042 | -0.0050 |

| | Tolerance | 0.0010 | 0.0015 | 0.0018 | 0.0022 | 0.0025 |

| | High Limit | -0.0007 | -0.0010 | -0.0012 | -0.0015 | -0.0020 |

| Y | Low Limit | -0.0012 | -0.0020 | -0.0025 | -0.0030 | -0.0035 |

| | Tolerance | 0.0005 | 0.0010 | 0.0013 | 0.0015 | 0.0015 |

| | High Limit | -0.0005 | -0.0007 | -0.0007 | -0.0010 | -0.0010 |

| Z | Low Limit | -0.0007 | -0.0012 | -0.0015 | -0.0020 | -0.0022 |

| | Tolerance | 0.0002 | 0.0005 | 0.0008 | 0.0010 | 0.0012 |

+-----+------------+---------+---------+---------+---------+---------+

[1] Tolerance is provided for holes, which ordinary standard reamers can

produce, in two grades, Classes A and B, the selection of which is a

question for the user's decision and dependent upon the quality of the

work required; some prefer to use Class A as working limits and Class B

as inspection limits.

[2] Running fits, which are the most commonly required, are divided into

three grades: Class X for engine and other work where easy fits are

wanted; Class Y for high speeds and good average machine work; Class Z

for fine tool work.

=Pressure for Forced Fits.=--The pressure required for assembling

cylindrical parts depends not only upon the allowance for the fit, but

also upon the area of the fitted surfaces, the pressure increasing in

proportion to the distance that the inner member is forced in. The

approximate ultimate pressure in pounds can be determined by the use of

the following formula in conjunction with the accompanying table of

"Pressure Factors."

=Pressure Factors=

+-----+-----++-----+-----++-----+-----++------+------++------+------+

|Diam-|Pres-||Diam-|Pres-||Diam-|Pres-||Diam- |Pres- ||Diam- |Pres- |

|eter,|sure ||eter,|sure ||eter,|sure ||eter, |sure ||eter, |sure |

|In- |Fac- ||In- |Fac- ||In- |Fac- ||In- |Fac- ||In- |Fac- |

|ches | tor ||ches | tor ||ches | tor ||ches |tor ||ches |tor |

+-----+-----++-----+-----++-----+-----++------+------++------+------+

|1 | 500 ||3-1/2| 132 ||6 | 75 || 9 | 48.7 ||14 | 30.5 |

|1-1/4| 395 ||3-3/4| 123 ||6-1/4| 72 || 9-1/2| 46.0 ||14-1/2| 29.4 |

|1-1/2| 325 ||4 | 115 ||6-1/2| 69 ||10 | 43.5 ||15 | 28.3 |

|1-3/4| 276 ||4-1/4| 108 ||6-3/4| 66 ||10-1/2| 41.3 ||15-1/2| 27.4 |

|2 | 240 ||4-1/2| 101 ||7 | 64 ||11 | 39.3 ||16 | 26.5 |

|2-1/4| 212 ||4-3/4| 96 ||7-1/4| 61 ||11-1/2| 37.5 ||16-1/2| 25.6 |

|2-1/2| 189 ||5 | 91 ||7-1/2| 59 ||12 | 35.9 ||17 | 24.8 |

|2-3/4| 171 ||5-1/4| 86 ||7-3/4| 57 ||12-1/2| 34.4 ||17-1/2| 24.1 |

|3 | 156 ||5-1/2| 82 ||8 | 55 ||13 | 33.0 ||18 | 23.4 |

|3-1/4| 143 ||5-3/4| 78 ||8-1/2| 52 ||13-1/2| 31.7 ||.... | .... |

+-----+-----++-----+-----++-----+-----++------+------++------+------+

Assuming that _A_ = area of fitted surface; _a_ = total allowance in

inches; _P_ = ultimate pressure required, in tons; _F_ = pressure factor

based upon assumption that the diameter of the hub is twice the

diameter of the bore, that the shaft is of machine steel, and the hub of

cast iron, then,

_A_ x _a_ x _F_

_P_ = ---------------

_Example:_--What will be the approximate pressure required for forcing a

4-inch machine steel shaft having an allowance of 0.0085 inch into a

cast-iron hub 6 inches long?

_A_ = 4 x 3.1416 x 6 = 75.39 square inches;

_F_, for a diameter of 4 inches, = 115 (see table of "Pressure

Factors"). Then,

_P_ = (75.39 x 0.0085 x 115)/2 = 37 tons, approximately.

=Allowance for Given Pressure.=--By transposing the preceding formula,

the approximate allowance for a required ultimate tonnage can be

determined. Thus, _a_ = 2_P_ / _AF_. The average ultimate pressure in

tons commonly used ranges from 7 to 10 times the diameter in inches.

Assuming that the diameter of a machine steel shaft is 4 inches and an

ultimate pressure of about 30 tons is desired for forcing it into a

cast-iron hub having a length of 5-1/2 inches, what should be the

allowance?

_A_ = 4 x 3.1416 x 5-1/2 = 69 square inches,

_F_, for a diameter of 4 inches, = 115. Then,

2 x 30

_a_ = -------- = 0.0075 inch.

69 x 115

=Shrinkage Fits.=--When heat is applied to a piece of metal, such as

iron or steel, as is commonly known, a certain amount of expansion takes

place which increases as the temperature is increased, and also varies

somewhat with different kinds of metal, copper and brass expanding more

for a given increase in temperature than iron and steel. When any part

which has been expanded by the application of heat is cooled, it

contracts and resumes its original size. This expansive property of

metals has been taken advantage of by mechanics in assembling various

machine details. A cylindrical part which is to be held in position by a

shrinkage fit is first turned a few thousandths of an inch larger than

the hole; the diameter of the latter is then increased by heating, and

after the part is inserted, the heated outer member is cooled, causing

it to grip the pin or shaft with tremendous pressure.

General practice seems to favor a smaller allowance for shrinkage fits

than for forced fits, although in many shops the allowances are

practically the same in each case, and for some classes of work,

shrinkage allowances exceed those for forced fits. In any case, the

shrinkage allowance varies to a great extent with the form and

construction of the part which has to be shrunk into place. The

thickness or amount of metal around the hole is the most important

factor. The way in which the metal is distributed also has an influence

on the results. Shrinkage allowances for locomotive driving wheel tires

adopted by the American Railway Master Mechanics Association are as

follows:

Center diameter, inches 38 44 50 56 62 66

Allowance, inches 0.040 0.047 0.053 0.060 0.066 0.070

Whether parts are to be assembled by forced or shrinkage fits depends

upon conditions. For example, to press a driving wheel tire over its

wheel center, without heating, would ordinarily be a rather awkward and

difficult job. On the other hand, pins, etc., are easily and quickly

forced into place with a hydraulic press and there is the additional

advantage of knowing the exact pressure required in assembling, whereas

there is more or less uncertainty connected with a shrinkage fit, unless

the stresses are calculated. Tests to determine the difference in the

quality of shrinkage and forced fits showed that the resistance of a

shrinkage fit to slippage was, for an axial pull, 3.66 times greater

than that of a forced fit, and in rotation or torsion, 3.2 times

greater. In each comparative test, the dimensions and allowances were

the same.

The most important point to consider when calculating shrinkage fits is

the stress in the hub at the bore, which depends chiefly upon the

shrinkage allowance. If the allowance is excessive, the elastic limit of

the material will be exceeded and permanent set will occur, or, in

extreme cases, the ultimate strength of the metal will be exceeded and

the hub will burst.

CHAPTER IV

THREAD CUTTING IN THE LATHE

When threads are cut in the lathe a tool _t_ is used (see Fig. 2),

having a point corresponding to the shape of the thread, and the

carriage is moved along the bed a certain distance for each revolution

of the work (the distance depending on the number of threads to the inch

being cut) by the lead-screw _S_ which is rotated by gears _a_, _b_ and

_c_, which receive their motion from the spindle. As the amount that the

carriage travels per revolution of the work, and, consequently, the

number of threads per inch that is cut, depends on the size of the gears

_a_ and _c_ (called change gears) the latter have to be changed for

cutting different threads. The proper change gears to use for cutting a

given number of threads to the inch is ordinarily determined by

referring to a table or "index plate" _I_ which shows what the size of

gears _a_ and _c_ should be, or the number of teeth each should have,

for cutting any given number of threads per inch.

[Illustration: Fig. 1. Measuring Number of Threads per Inch--Setting

Thread Tool]

[Illustration: Fig. 2. Plan and Elevations of Engine Lathe]

=Selecting the Change Gears for Thread Cutting.=--Suppose a V-thread is

to be cut on the end of the bolt _B_, Fig. 2, having a diameter of 1-1/4

inch and seven threads per inch of length, as shown at _A_ in Fig. 1,

which is the standard number of threads per inch for that diameter.

First the change gears to use are found on plate _I_ which is shown

enlarged in Fig. 3. This plate has three columns: The first contains

different numbers of threads to the inch, the second the size gear to

place on the "spindle" or "stud" at _a_ (Fig. 2) for different threads,

and the third the size of gear _c_ for the lead-screw. As the thread

selected as an example has 7 threads per inch, gear _a_ should have 48

teeth, this being the number given in the second column opposite figure

7 in the first. By referring to the last column, we find that the

lead-screw gear should have 84 teeth. These gears are selected from an

assortment provided with the lathe and they are placed on the spindle

and lead-screw, respectively.

[Illustration: Fig. 3. Index Plate showing Gear Changes for Threading]

Intermediate gear _b_ does not need to be changed as it is simply an

"idler" for connecting gears _a_ and _c_. Gear _b_ is mounted on a

swinging yoke _Y_ so that it can be adjusted to mesh properly with

different gear combinations; after this adjustment is made, the lathe is

geared for cutting 7 threads to the inch. (The change gears of many

modern lathes are so arranged that different combinations are obtained

by simply shifting a lever. A lathe having this quick-change gear

mechanism is described in the latter part of this chapter.) The work _B_

is placed between the centers just as it would be for turning, with the

end to be threaded turned to a diameter of 1-1/4 inch, which is the

outside diameter of the thread.

=The Thread Tool.=--The form of tool used for cutting a V-thread is

shown at _A_, Fig. 4. The end is ground V-shaped and to an angle of 60

degrees, which corresponds to the angle of a standard V-thread. The

front or flank, _f_ of the tool is ground back at an angle to provide

clearance, but the top is left flat or without slope. As it is very

important to grind the end to exactly 60 degrees, a gage _G_ is used,

having 60-degree notches to which the tool-point is fitted. The tool is

clamped in the toolpost as shown in the plan view, Fig. 2, square with

the work, so that both sides of the thread will be cut to the same angle

with the axis of the work. A very convenient way to set a thread tool

square is illustrated at _B_, Fig. 1. The thread gage is placed against

the part to be threaded, as shown, and the tool is adjusted until the

angular sides of the point bear evenly in the 60-degree notch of the

gage. The top of the tool point should be at the same height as the

lathe centers, as otherwise the angle of the thread will not be correct.

[Illustration: Fig. 4. Thread Tools and Gage for testing Angle of End]

=Cutting the Thread.=--The lathe is now ready for cutting the thread.

This is done by taking several cuts, as indicated at _A_, _B_, _C_ and

_D_ in Fig. 5, the tool being fed in a little farther for each

successive cut until the thread is finished. When these cuts are being

taken, the carriage is moved along the bed, as previously explained, by

the lead-screw _S_, Fig. 2. The carriage is engaged with the lead-screw

by turning lever _u_ which causes the halves of a split nut to close

around the screw. The way a lathe is handled when cutting a thread is as

follows: After the lathe is started, the carriage is moved until the

tool-point is slightly beyond the right end of the work, and the tool is

fed in far enough to take the first cut which, ordinarily, would be

about 1/16 inch deep. The carriage is then engaged with the lead-screw,

by operating lever _u_, and the tool moves to the left (in this case 1/7

inch for each revolution of the work) and cuts a winding groove as at

_A_, Fig. 5. When the tool has traveled as far as the thread is wanted,

it is withdrawn by a quick turn of cross-slide handle _e_, and the

carriage is returned to the starting point for another cut. The tool is

then fed in a little farther and a second cut is taken as at _B_, Fig.

5, and this operation is repeated as at _C_ and _D_ until a "full"

thread is cut or until the top of the thread is sharp. The thread is

then tested for size but before referring to this part of the work, the

way the carriage is returned to the starting point after each cut should

be explained.

[Illustration: Fig. 5. Thread is formed by taking a Number of Successive

Cuts]

When the tool is withdrawn at the end of the first cut, if the carriage

is disengaged from the lead-screw and returned by hand, the tool may or

may not follow the first cut when the carriage is again engaged with the

lead-screw. If the number of threads to the inch being cut is a multiple

of the number on the lead-screw _S_, then the carriage can be returned

by hand and engaged with the lead-screw at random and the tool will

follow the first cut. For example, if the lead-screw has six threads per

inch, and 6, 12, 18 or any number of threads is being cut that is a

multiple of six, the carriage can be engaged at any time and the tool

will always follow the original cut. This is not the case, however, when

the number of threads being cut is not a multiple of the number on the

lead-screw.

One method of bringing the carriage back to the starting point, when

cutting threads which are not multiples, is to reverse the lathe (by

shifting the overhead driving belts) in order to bring the tool back to

the starting point without disengaging the carriage; in this way the

tool is kept in the same relation to the work, and the carriage is not

disengaged from the lead-screw until the thread is finished. This is a

good method when cutting short threads having a length of say two or

three inches; but when they are longer, and especially when the diameter

is comparatively large (which means a slower speed), it is rather slow

as considerable time is wasted while the tool is moving back to its

starting point. This is due to the fact that the carriage is moved

slowly by the lead-screw, but when disengaged, it can be traversed

quickly by turning handle _d_, Fig. 2.

A method of returning the carriage by hand when the number of threads

being cut is not a multiple of the number on the lead-screw is as

follows: The tool is moved a little beyond the right end of the work and

the carriage or split nut is engaged with the lead-screw. The lathe is

then turned forward by hand to take up any lost motion, and a line is

made on the lathe bed showing the position of the carriage. The

positions of the spindle and lead-screw are also marked by chalking a

tooth on both the spindle and lead-screw gears, which happens to be

opposite a corner or other point on the bed. After a cut is taken, the

carriage is returned by hand to the original starting point as shown by

the line on the bed, and is again engaged when the chalk marks show that

the spindle and lead-screw are in their original position; the tool will

then follow the first cut. If the body of the tailstock is moved against

the bridge of the carriage before starting the first cut, the carriage

can be located for each following cut by moving it back against the

tailstock, and it will not be necessary to have a line on the bed.

[Illustration: Fig. 6. Indicator used when Cutting Threads]

=Indicator or Chasing Dial for Catching Threads.=--On some lathes there

is an indicator for "catching threads," as this is called in shop

language. This is a simple device attached to the carriage and consists

of a graduated dial _D_ and a worm-wheel _W_ (see Figs. 2 and 6) which

meshes with the lead-screw, so that the dial is revolved by the

lead-screw when the carriage is stationary, and when the carriage is

moved by the screw, the dial remains stationary. The indicator is used

by engaging the carriage when one of the graduation lines is opposite

the arrow mark; after a cut is taken the carriage is returned by hand

and when one of the graduation lines again moves opposite the arrow, the

half-nuts are thrown into mesh, as before, and this is repeated for each

successive cut, thus causing the tool to always come right with the

thread. If the number of threads per inch is even, engagement can be

made when any line is opposite the arrow, but for odd numbers such as 3,

7, 9, 11, etc., one of the four long or numbered lines must be used. Of

course, if the thread being cut is a multiple of the number on the

lead-screw, engagement can be made at any time, as previously

mentioned.

=Principle of the Thread Indicator.=--The principle upon which the

thread indicator operates is as follows: The number of teeth in

worm-wheel _W_ is some multiple of the number of threads per inch of the

lead-screw, and the number of teeth in the worm-wheel, divided by the

pitch of the screw, equals the number of graduations on the dial. For

example, if the lead-screw has six threads per inch, the worm-wheel

could have twenty-four teeth, in which case the dial would have four

divisions, each representing an inch of carriage travel, and by

sub-dividing the dial into eighths (as shown) each line would correspond

to 1/2 inch of travel. The dial, therefore, would enable the carriage to

be engaged with the lead-screw at points equal to a travel of one-half

inch. To illustrate the advantage of this suppose ten threads per inch

are being cut and (with the lathe stationary) the carriage is disengaged

and moved 1/6 inch or one thread on the lead-screw; the tool point will

also have moved 1/6 inch, but it will not be opposite the next thread

groove in the work as the pitch is 1/10 inch. If the carriage is moved

another thread on the lead-screw, or 2/6 inch, the tool will still be

out of line with the thread on the work, but when it has moved three

threads, or 1/2 inch, the tool will then coincide with the original cut

because it has passed over exactly five threads. This would be true for

any number of threads per inch that is divisible by 2. If the thread

being cut had nine threads per inch or any other odd number, the tool

would only coincide with the thread at points 1 inch apart. Therefore,

the carriage can only be engaged when one of the four graduations

representing an inch of travel is opposite the arrow, when cutting odd

threads; whereas even numbers can be "caught" by using any one of the

eight lines.

This indicator can also be used for "catching" fractional threads. As an

illustration, suppose 11-1/2 threads per inch are to be cut, and the

carriage is engaged for the first cut when graduation line 1 is opposite

the arrow; engagement would then be made for each successive cut, when

either line 1 or 3 were opposite the arrow, or in other words at spaces

equal to a carriage movement of 2 inches. As the use of the indicator

when cutting fractional threads is liable to result in error, it is

better to keep the half-nuts in engagement and return the carriage by

reversing the lathe.

=Replacing Sharpened Thread Tool.=--If it is necessary to sharpen the

thread tool before the thread is finished, it should be reset square

with the work by testing with the thread gage as at _B_, Fig. 1. The

carriage is then engaged with the lead-screw and the lathe is turned

forward to bring the tool opposite the partly finished thread and also

to take up any backlash or lost motion in the gears or half-nut. If the

tool-point is not in line with the thread groove previously cut, it can

be shifted sidewise by feeding the compound rest _E_ in or out, provided

the latter is set in an angular position as shown in the plan view, Fig.

If the thread tool is ground flat on the top as at _A_, Fig. 4, it is

not a good tool for removing metal rapidly as neither of its two cutting

edges has any slope. In order to give each cutting edge a backward

slope, it would be necessary to grind the top surface hollow or concave,

which would be impracticable. When a course thread is to be cut, a tool

shaped as at _B_ can be used to advantage for rough turning the thread

groove, which is afterward finished to the correct depth and angle by

tool _A_. This roughing tool is ground with a backward slope from the

point and the latter is rounded to make it stronger.

=Use of Compound Rest for Thread Cutting.=--Another form of thread tool

is shown at _A_, Fig. 7, which is very good for cutting V-threads

especially of coarse pitch. When this tool is used, the compound rest

_E_ is set to an angle of 30 degrees, as shown, and it is fed in for the

successive cuts by handle _w_ in the direction indicated by the arrow.

It will be seen that the point a of the tool moves at an angle of 60

degrees with the axis of the work, thus forming one side of the thread,

and the cutting edge _a--b_, which can be set as shown at _B_, forms the

opposite side and does all the cutting. As this edge is given a backward

slope, as shown, it cuts easily and enables threading operations to be

performed quickly. Threads cut in this way are often finished by taking

a light cut with a regular thread tool. The cutting edge _a--b_ is

ground to an angle of 60 degrees (or slightly less, if anything) with

the side, as shown by sketch _A_.

When cutting threads in steel or wrought iron, some sort of lubricant is

usually applied to the tool to preserve the cutting end and give a

smooth finish to the thread. Lard oil or a mixture of equal parts of

lard oil and paraffin oil are often used for this purpose. If the thread

is small, the lubricant may be applied from an ordinary oil can, but

when cutting comparatively large threads, it is better to have a stream

of oil constantly playing upon the tool-point. This constant flow may be

obtained by mounting a can having a spout leading to the tool, on a

bracket at the rear of the carriage.

[Illustration: Fig. 7. Cutting Thread by using Compound Rest]

[Illustration: Fig. 8. (A) V-thread. (B) U. S. Standard Thread. (C)

Square Thread. (D) Left-hand Thread. (E) Double Square Thread. (F)

Triple Square Thread]

=Threads Commonly Used.=--Three forms of threads or screws which are in

common use are shown in Fig. 8; these are the V-thread (_A_), the U. S.

standard (_B_), and the square thread (_C_). The shapes of these threads

are shown by the sectioned parts. The V-thread has straight sides which

incline at an angle of 60 degrees with each other and at the same angle

with the axis of the screw. The U. S. standard thread is similar to the

V-thread except that the top of the thread and bottom of the groove is

left flat, as shown, and the width of these flats is made equal to 1/8

of the pitch. The square thread is square in section, the width _a_,

depth _b_ and space _c_ being all equal. All of these threads are

right-hand, which means that the grooves wind around to the right so

that a nut will have to be turned toward the right to enter it on the

thread. A left-hand thread winds in the other direction, as shown at

_D_, and a nut is screwed on by turning it to the left.

=Multiple Threads.=--Threads, in addition to being right-and

left-handed, are single, as at _A_, _B_, _C_ and _D_, double, as at _E_,

and triple, as at _F_, and for certain purposes quadruple threads or

those of a higher multiple are employed. A double thread is different

from a single thread in that it has two grooves, starting diametrically

opposite, whereas a triple thread has three grooves cut as shown at _F_.

The object of these multiple threads is to obtain an increase in lead

without weakening the screw. For example, the threads shown at _C_ and

_E_ have the same pitch _p_ but the lead _l_ of the double-threaded

screw is twice that of the one with a single thread so that a nut would

advance twice as far in one revolution, which is often a very desirable

feature. To obtain the same lead with a single thread, the pitch would

have to be double, thus giving a much coarser thread, which would weaken

the screw, unless its diameter were increased. (The lead is the distance

_l_ that one thread advances in a single turn, or the distance that a

nut would advance in one turn, and it should not be confused with the

pitch _p_, which is the distance between the centers of adjacent

threads. Obviously the lead and pitch of a single thread are the same.)

=Cutting a U. S. Standard Thread.=--The method of cutting a U. S.

standard thread is the same as described for a V-thread, so far as

handling the lathe is concerned. The thread tool must correspond, of

course, to the shape of a U. S. standard thread. This tool is first

ground to an angle of 60 degrees, as it would be for cutting a V-thread,

and then the point is made flat as shown in Fig. 9. As will be recalled,

the width of this flat should be equal to 1/8 of the pitch. By using a

gage like the one shown at _G_, the tool can easily be ground for any

pitch, as the notches around the periphery of the gage are marked for

different pitches and the tool-point is fitted into the notch

corresponding to the pitch wanted. If such a gage is not available, the

width of the flat at the point can be tested by using, as a gage, a U.

S. standard tap of the same pitch as the thread to be cut.

When cutting the thread, the tool is set square with the blank, and a

number of successive cuts are taken, the tool being fed in until the

width w of the flat at the top of the thread is equal to the width at

the bottom. The thread will then be the right size provided the outside

diameter _D_ is correct and the tool is of the correct form. As it would

be difficult to measure the width of this flat accurately, the thread

can be tested by screwing a standard nut over it if a standard thread is

being cut. If it is being fitted to a tapped hole, the tap itself is a

very convenient gage to use, the method being to caliper the tap and

then compare its size with the work.

[Illustration: Fig. 9. U. S. Standard Thread, Thread Tool, and Gage]

A good method of cutting a U. S. standard thread to a given size is as

follows: First turn the outside of the blank accurately to diameter _D_,

and then turn a small part of the end to diameter _r_ of the thread at

the root. The finishing cut for the thread is then taken with the tool

point set to just graze diameter _r_. If ordinary calipers were set to

diameter _r_ and measurements taken in the thread groove, the size would

be incorrect owing to the angularity of the groove, which makes it

necessary to hold the calipers at an angle when measuring. To determine

the root diameter divide 1.299 by the number of threads per inch and

subtract the quotient from the outside diameter. Expressing this rule as

a formula,

/1.299\

_r_ = _D_ - ( ----- )

\ _N_ /

in which _D_ equals outside diameter; _N_, the number of threads per

inch; and _r_, the root diameter. The number 1.299 is a constant that is

always used.

[Illustration: Fig. 10. End View of Lathe Headstock]

=Cutting a Left-hand Thread.=--The only difference between cutting

left-hand and right-hand threads in the lathe is in the movement of the

tool with relation to the work. When cutting a right-hand thread, the

tool moves from right to left, but this movement is reversed for

left-hand threads because the thread winds around in the opposite

direction. To make the carriage travel from left to right, the

lead-screw is rotated backwards by means of reversing gears _a_ and _b_

(Fig. 10) located in the headstock. Either of these gears can be

engaged with the spindle gear by changing the position of lever _R_.

When gear _a_ is in engagement, as shown, the drive from the spindle to

gear _c_ is through gears _a_ and _b_, but when lever _R_ is raised thus

shifting _b_ into mesh, the drive is direct and the direction of

rotation is reversed. The thread is cut by starting the tool at _a_,

Fig. 8, instead of at the end.

[Illustration: Fig. 11. End of Square Thread Tool, and Graphic Method of

Determining Helix Angle of Thread]

=Cutting a Square Thread.=--The form of tool used for cutting a square

thread is shown in Fig. 11. The width _w_ is made equal to one-half the

pitch of the thread to be cut and the end _E_ is at an angle with the

shank, which corresponds to the inclination _x--y_ of the threads. This

angle _A_ depends upon the diameter of the screw and the lead of the

thread; it can be determined graphically by drawing a line _a--b_ equal

in length to the circumference of the screw to be cut, and a line

_b--c_, at right angles, equal in length to the lead of the thread. The

angle [alpha] between lines _a--b_ and _a--c_ will be the required angle

_A_. (See end view of thread tool). It is not necessary to have this

angle accurate, ordinarily, as it is simply to prevent the tool from

binding against the sides of the thread. The end of a square thread tool

is shown in section to the right, to illustrate its position with

relation to the threads. The sides _e_ and _e_{1}_ are ground to slope

inward, as shown, to provide additional clearance.

When cutting multiple threads, which, owing to their increased lead,

incline considerably with the axis of the screw, the angles for each

side of the tool can be determined independently as follows: Draw line

_a--b_ equal in length to the circumference of the thread, as before, to

obtain the required angle _f_ of the rear or following side _e_{1}_; the

angle _l_ of the opposite or leading side is found by making _a--b_

equal to the circumference at the root of the thread. The tool

illustrated is for cutting right-hand threads; if it were intended for a

left-hand thread, the end, of course, would incline in the opposite

direction. The square thread is cut so that the depth _d_ is equal to

the width. When threading a nut for a square thread screw, it is the

usual practice to use a tool having a width slightly greater than

one-half the pitch, to provide clearance for the screw, and the width of

a tool for threading square-thread taps to be used for tapping nuts is

made slightly less than one-half the pitch.

=Cutting Multiple Threads.=--When a multiple thread is to be cut, such

as a double or triple thread, the lathe is geared with reference to the

number of single threads to the inch. For example, the lead of the

double thread, shown at _B_, Fig. 12, is one-half inch, or twice the

pitch, and the number of single threads to the inch equals 1 / 1/2 = 2.

Therefore, the lathe is geared for cutting two threads per inch. The

first cut is taken just as though a single thread were being cut,

leaving the work as shown at _A_. When this cut is finished the work is

turned one-half a revolution (for a double thread) without disturbing

the position of the lead-screw or carriage, which brings the tool midway

between the grooves of the single thread as indicated by dotted lines.

The second groove is then cut, producing a double thread as shown at

_B_. In the case of a triple thread, the work would be indexed one-third

of a revolution after turning the first groove, and then another third

revolution to locate the tool for cutting the last groove. Similarly,

for a quadruple thread, it would be turned one-quarter revolution after

cutting each successive groove or thread.

There are different methods of indexing the work when cutting multiple

threads, in order to locate the tool in the proper position for cutting

another thread groove. Some machinists, when cutting a double thread,

simply remove the work from the lathe and turn it one-half a revolution

by placing the tail of the driving dog in the opposite slot of the

faceplate. This is a very simple method, but if the slots are not

directly opposite or 180 degrees apart, the last thread will not be

central with the first. Another and better method is to disengage the

idler gear from the gear on the stud, turn the spindle and work

one-half, or one-third, of a revolution, as the case might be, and then

connect the gears. For example, if the stud gear had 96 teeth, the tooth

meshing with the idler gear would be marked with chalk, the gears

disengaged, and the spindle turned until the chalked tooth had made the

required part of a revolution, which could be determined by counting the

teeth. When this method is used, the number of teeth in the stud gear

must be evenly divisible by two if a double thread is being cut, or by

three for a triple thread, etc. If the stud is not geared to the spindle

so that each makes the same number of revolutions, the ratio of the

gearing must be considered.

[Illustration: Fig. 12. Views illustrating how a Double Square Thread is

Cut]

=Setting Tool When Cutting Multiple Threads.=--Another method, which can

sometimes be used for setting the tool after cutting the first groove of

a multiple thread, is to disengage the lock-nuts from the lead-screw

(while the spindle is stationary) and move the carriage back whatever

distance is required to locate the tool in the proper position for

taking the second cut. Evidently this distance must not only locate the

tool in the right place, but be such that the lock-nuts can be

re-engaged with the lead-screw. Beginning with a simple illustration,

suppose a double thread is being cut having a lead of 1 inch. After the

first thread groove is cut, the tool can be set in a central position

for taking the second cut, by simply moving the carriage back 1/2 inch

(one-half the lead), or 1/2 inch plus the lead or any multiple of the

lead. If the length of the threaded part were 5 inches, the tool would

be moved back far enough to clear the end of the work, or say 1/2 + 5 =

5-1/2 inches. In order to disengage the lock-nuts and re-engage them

after moving the carriage 5-1/2 inches (or any distance equal, in this

case, to one-half plus a whole number), the lead-screw must have an even

number of threads per inch.

Turning and Boring A specialized treatise for machinists, students in the industrial and engineering schools, and apprentices, on turning and boring methods, including modern practice with engine lathes, turret lathes, vertical and horizontal boring machines · 第 8 / 10 章

=Effect of Lubricant on Cutting Speed.=--When turning iron or steel a