Transcriber’s Notes:
Underscores “_” before and after a word or phrase indicate _italics_
in the original text.
Equal signs “=” before and after a word or phrase indicate =bold=
in the original text.
Small capitals have been converted to SOLID capitals.
Illustrations have been moved so they do not break up paragraphs.
Typographical and punctuation errors have been silently corrected.
=MACHINERY’S REFERENCE SERIES=
EACH NUMBER IS ONE UNIT IN A COMPLETE LIBRARY OF
MACHINE DESIGN AND SHOP PRACTICE REVISED AND
REPUBLISHED FROM MACHINERY
NUMBER 135
PRECISION LOCATING AND
DIVIDING METHODS
CONTENTS
Precision Locating Methods 3
Accurate Dividing and Spacing Methods 21
Locating Work for Boring on Milling Machine 32
Copyright, 1914, The Industrial Press,
Publishers of MACHINERY,
140-148 Lafayette Street, New York City
Other books in this series dealing with
the subjects of Toolmaking and kindred
topics are as follows:
No. 31—THREAD TOOLS AND GAGES
No. 64—GAGE MAKING AND LAPPING
No. 107—DROP FORGING DIE SINKING
No. 130—GAGING TOOLS AND METHODS
[Illustration: MACHINERY]
MACHINERY
The Leading
Mechanical Journal
MACHINE DESIGN
CONSTRUCTION
SHOP PRACTICE
THE INDUSTRIAL PRESS
140-148 Lafayette St.
New York City
51-52 Chancery Lane, London
CHAPTER I
PRECISION LOCATING METHODS
The degree of accuracy that is necessary in the construction of
certain classes of machinery and tools, has made it necessary for
toolmakers and machinists to employ various methods and appliances
for locating holes or finished surfaces to given dimensions and
within the prescribed limits of accuracy. In this treatise, various
approved methods of locating work, such as are used more particularly
in tool-rooms, are described and illustrated. These are not given, in
every case, as being the best possible method under all conditions,
because, as every mechanical man knows, the best way may be dependent
upon the element of accuracy with little regard for the time required
to do the work, or this order may be reversed; therefore, one method is
seldom, if ever, the best under all circumstances, and it is necessary
for the workman to consider the conditions in each case and then be
guided by his judgment and experience in determining just how the work
should be done.
Button Method of Accurately Locating Work
Among the different methods employed by toolmakers for accurately
locating work such as jigs, etc., on the faceplate of a lathe, one of
the most commonly used is known as the “button method.” This method 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 ½ or ⅝ inch in diameter, are ground and lapped to the same size,
and the ends are finished perfectly square. The outside diameter
should preferably be such that the radius can easily be determined,
and the hole through the center should be about ⅛ inch larger than the
retaining screw so that the button can be adjusted laterally.
As a simple example of the practical application of the button method,
suppose three holes are to be bored in a jig-plate according to the
dimensions given in Fig. 1. A common method of procedure would be as
follows: First lay out the centers of all holes to be bored, by the
usual method. Mark these centers with a prick-punch and then drill
holes for the machine screws which are used to clamp the buttons. After
the buttons are clamped lightly in place, set them 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. The best method of locating a number of buttons
depends, to some extent, upon their relative positions, the instruments
available, and the accuracy required. When buttons must be located at
given distances from the finished sides of a jig, a surface plate and
vernier height gage are often used. The method is to place that side
from which the button is to be set, upon an accurate surface plate and
then set the button by means of the height gage, allowance being made,
of course, for the radius of the button. The center-to-center distance
between the different buttons can afterwards be verified by taking
direct measurements with a micrometer.
[Illustration: Fig. 1. Simple Example of Work Illustrating Application
of Button Method]
Figs. 2 and 3 illustrate a method which requires only a micrometer. Two
of the buttons are set at the correct distance from one edge of the
plate by measuring from a parallel strip. Obviously, the micrometer
reading will exceed the distance from the center of a button to the
edge of the plate, by the amount equal to the thickness of the parallel
strip plus the radius of the button. The center-to-center distance
between each pair of buttons is also tested as indicated in Fig. 3, by
measuring the overall distance and deducting the diameter of one button.
After the buttons have been set and the screws are tightened, all
measurements should be carefully checked. The work is then mounted
on the faceplate of the lathe and one of the buttons is set true by
the use of a test indicator as shown in Fig. 4. When the dial of the
indicator ceases to vibrate, thus showing that the button runs true,
the latter should be removed so that the hole can be drilled and bored
to the required size. In a similar manner 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 the required distance apart within very close
limits.
[Illustration: Fig. 2. Determining Distance from Button to Edge of
Plate]
Another example of work illustrating the application of the button
method is shown in Fig. 5. The disk-shaped part illustrated is a flange
templet which formed a part of a fixture for drilling holes in flanged
plates, the holes being located on a circle 6 inches in diameter.
It was necessary to space the six holes equi-distantly so that the
holes in the flanges would match in any position, thus making them
interchangeable. First a plug was turned so that it fitted snugly in
the 1¼-inch central hole of the plate and projected above the top
surface about ¾ inch. A center was located in this plug and from it
a circle of three inches radius was drawn. This circle was divided
into six equal parts and then small circles ⅝ inch in diameter were
drawn to indicate the outside circumference of the bushings to be
placed in the holes. These circles served as a guide when setting the
button and enabled the work to be done much more quickly. The centers
of the holes were next carefully prick-punched and small holes were
drilled and tapped for No. 10 machine screws. After this the six
buttons were attached in approximately the correct positions and the
screws tightened enough to hold the buttons firmly, but allow them to
be moved by tapping lightly. As the radius of the circle is 3 inches,
the radius of the central plug, ⅝ inch, and that of each button, ⁵/₁₆
inch, the distance from the outside of the central plug to the outside
of any button, when correctly set, must be 3 ¹⁵/₁₆ inches. Since there
are six buttons around the circle, the center-to-center distance is
equal to the radius, and the distance between the outside or any two
buttons should be 3⅝ inches. Having determined these dimensions, each
button is set equi-distant from the central plug and the required
distance apart, by using a micrometer. As each button is brought into
its correct position, it should be tightened down a little so that it
will be located firmly when finally set. The work is then strapped to
the faceplate of a lathe and each button is indicated for boring the
different holes by means of an indicator, as previously described. When
the buttons are removed it will be found that in nearly all cases the
small screw holes will not run exactly true; therefore, it is advisable
to form a true starting point for the drill by using a lathe tool.
[Illustration: Fig. 3. Testing Location of Buttons]
Fig. 7 shows a method of locating buttons from the finished sides of a
plate, and this same plate with the five buttons attached is shown in
Fig. 6. As the dimensions in Fig. 7 indicate, the holes must not only
be accurate with relation to each other, but also with reference to
the edges of the templet; therefore, it is necessary to work from the
sides as well as the center. The width of the plate was first measured
carefully and found to be 5 inches. As the center-to-center distance
between buttons _B_ and _C_ and also buttons _D_ and _E_, is 2½ inches,
the distance from the center of each outside button to the edge of the
plate is 1¼ inch. A ¼-inch parallel was clamped against the side, as
shown in the illustration, and then the distance from the outside of
each button to the outside of the parallel (1 ¹³/₁₆ inch) was measured
in conjunction with the distance _L_ from the central button. The
distance _L_ was obtained by first determining the center-to-center
distance _M_ which represents the hypotenuse of a right-angled triangle.
_M_² = 1.25² + 1.625²
______________ _____
or _M_ = √1.25² + 1.625² = √4.024 = 2.050 inches.
Therefore, _L_ = 2.050 + 0.625 = 2.675 inches.
In this case, the center button was first located correctly from the
sides and end and then the other buttons were set. When doing precision
work of this kind, the degree of accuracy obtained 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, in order to prevent an
accumulation of errors.
Locating Work by the Disk Method
Comparatively small precision work is sometimes located by the disk
method, which is the same in principle as the button method, the chief
difference being that disks are used instead of buttons. These disks
are made to such diameters that when their peripheries are in contact,
each disk center will coincide with the position of the hole to be
bored; the centers are then used for locating the work. To illustrate
this method, suppose that the master-plate shown at the left in Fig. 8
is to have three holes _a_, _b_, and _c_ bored into it, to the center
distances given.
[Illustration: Fig. 4. Testing Concentricity of Button Preparatory to
Boring Hole in Lathe]
It is first necessary to determine the diameters of the disks. If
the center distances between all the holes were equal, the diameters
would, of course, equal this dimension. When, however, the distances
between the centers are unequal, the diameters may be found as follows:
Subtract, say, dimension _y_ from _x_, thus obtaining the difference
between the radii of disks _C_ and _A_ (see right-hand sketch); add
this difference to dimension _z_, and the result will be the diameter
of disk _A_. Dividing this diameter by 2 gives the radius, which,
subtracted from center distance _x_ equals the radius of _B_; similarly
the radius of _B_ subtracted from dimension _y_ equals the radius of
_C_.
For example, 0.930-0.720 = 0.210 or the difference between the radii
of disks _C_ and _A_. Then the diameter of _A_ = 0.210 + 0.860 = 1.070
inch, and the radius equals 1.070 ÷ 2 = 0.535 inch. The radius of _B_ =
0.930-0.535 = 0.395 inch and 0.395 × 2 = 0.790, or the diameter of _B_.
The center distance 0.720-0.395 = 0.325, which is the radius of _C_;
0.325 × 2 = 0.650 or the diameter of _C_.
[Illustration: Fig. 5. Flange Templet with Buttons Attached]
[Illustration: Fig. 6. Hinge Jig Templet with Buttons Attached]
After determining the diameters, the disks should be turned nearly to
size and finished, preferably in a bench lathe. First insert a solder
chuck in the spindle, face it perfectly true, and attach the disk by a
few drops of solder, being careful to hold the work firmly against the
chuck while soldering. Face the outer side and cut a sharp V-center
in it; then grind the periphery to the required diameter. Next fasten
the finished disks onto the work in their correct locations with their
peripheries in contact, and then set one of the disks exactly central
with the lathe spindle by applying a test indicator to the center in
the disk. After removing the disk and boring the hole, the work is
located for boring the other holes in the same manner.
[Illustration: Fig. 7. Hinge Jig Templet Illustrated in Fig. 6]
Small disks may be secured to the work by means of jeweler’s wax.
This is composed of common rosin and plaster of paris and is made as
follows: Heat the rosin in a vessel until it flows freely, and then add
plaster of paris and keep stirring the mixture. Care should be taken
not to make the mixture too stiff. When it appears to have the proper
consistency, pour some of it onto a slate or marble slab and allow it
to cool; then insert the point of a knife under the flattened cake thus
formed and try to pry it off. If it springs off with a slight metallic
ring, the proportions are right, but if it is gummy and ductile,
there is too much rosin. On the other hand, if it is too brittle and
crumbles, this indicates that there is too much plaster of paris. The
wax should be warmed before using. A mixture of beeswax and shellac, or
beeswax and rosin in about equal proportions, is also used for holding
disks in place. When the latter are fairly large, it may be advisable
to secure them with small screws, provided the screw holes are not
objectionable.
Disk-and-Button Method of Locating Holes
The accuracy of work done by the button method previously described
is limited only by the skill and painstaking care of the workman,
but setting the buttons requires a great deal of time. By a little
modification, using what is sometimes called the “disk-and-button
method,” a large part of this time can be saved without any sacrifice
of accuracy. The disk-and-button method is extensively used in many
shops. Buttons are used, but they are located in the centers of disks
of whatever diameters are necessary to give the required locations.
As three disks are used in each step of the process, it is sometimes
called the “three-disk method.”
To illustrate the practical application of this method, suppose six
equally-spaced holes are to be located in the circumference of a
circle six inches in diameter. To locate these, one needs, besides the
buttons, three disks three inches in diameter, each having a central
hole exactly fitting the buttons. It is best to have, also, a bushing
of the same diameter as the buttons, which has a center-punch fitted
to slide in it. First the center button is screwed to the templet, and
one of the disks _A_, Fig. 9, is slipped over it; then a second disk
_B_ carrying a bushing and center-punch is placed in contact with disk
_A_ and a light blow on the punch marks the place to drill and tap for
No. 2 button, which is kept in its proper place while tightening the
screw by holding the two disks _A_ and _B_ in contact. Next the third
disk _C_ is placed in contact with disks _A_ and _B_ and locates No. 3
button, and so on until the seven buttons are secured in position. The
templet is then ready to be strapped to the lathe faceplate for boring.
[Illustration: Fig. 8. An Example of Precision Work, and Method of
Locating Holes by Use of Disks in Contact]
Of course, it is not possible to use disks of “standard” sizes for
many operations, but making a special disk is easy, and its cost
is insignificant as compared with the time saved by its use. One
who employs this method, especially if he also uses disks to lay
out angles, soon accumulates a stock of various sizes. While it is
desirable to have disks of tool steel, hardened and ground, or, in the
larger sizes, of machine steel, case-hardened and ground, a disk for
occasional use will be entirely satisfactory if left soft.
Another example of work is shown in Fig. 10. This is a jig templet
similar to the one illustrated in Figs. 6 and 7. Sketch _A_ gives its
dimensions and sketch _B_ shows the disk-and-button way of locating the
holes. A steel square is clamped with its stock against the right-hand
edge of the templet and its blade extending across the top. The lower
edge of the blade should be located 0.250 inch from the upper edge of
the templet by the use of size blocks. A 2½-inch disk, touching both
blade and stock, locates hole _C_. Another 2½-inch disk, touching the
first disk and the square blade, locates hole _B_. Next a disk 1.600
inch diameter is placed in contact with the two upper disks and
locates the center hole _A_; and, finally, the disks for holes _B_ and
_C_ are used to locate holes _D_ and _E_.
[Illustration: Fig. 9. Locating Holes on a Circle and Equi-distant by
using Disks and Buttons in Combination]
Two other jobs that illustrate this method may be of interest.
The first one, shown in Fig. 11, required the locating of nine
equally-spaced holes on a circumference of 7⅜ inches diameter. In any
such case, the size of the smaller disks is found by multiplying the
diameter of the circle upon which the centers of the disks are located
by the sine of half the angle between two adjacent disks. The angle
between the centers of adjacent disks equals 360 ÷ number of disks. 360
÷ 9 = 40; hence, in this case, the diameter of the smaller disks equals
7⅜ multiplied by the sine of 20 degrees, or 7⅜ × 0.34202 = 2.5224
inches. 7⅜-2.5224 = 4.8526 inches, which is the diameter of the central
disk.
The templet shown in Fig. 12 required two holes on a circumference 6½
inches diameter, with their centers 37 degrees 20 minutes apart. To
find the diameter of the smaller disks, multiply the diameter of the
large circle by the sine of one-half the required angle, as in the
preceding example; thus 6½ × sin 18 degrees 40 minutes = 2.0804 inches,
which is the diameter of the two smaller disks. The diameter of the
larger disk equals 6½-2.0804 = 4.4196 inches.
Very accurate results can be obtained by the disk-and-button method.
Of course, absolute exactness is equally unattainable with buttons and
a micrometer, or any other method; the micrometer does not show the
slight inaccuracy in any one chordal measurement, while in using the
disks the error is accumulative and the insertion of the last disk in
the series shows the sum of the errors in all the disks. It is only in
cases like the one illustrated in Fig. 9 that we note this, and then,
though in correcting the error, we may change the diameter of the
circle a very slight amount, an exceedingly accurate division of the
circumference is secured.
Use of Two- and Three-Diameter Disks
Fig. 13 illustrates, on an enlarged scale, a piece of work requiring
great accuracy, which was successfully handled by an extension of the
three-disk method. Fourteen holes were required in a space hardly
larger than a silver half-dollar, and, although the drawing gave
dimensions from the center of the circle, the actual center could not
be used in doing the work, as there was to be no hole there; moreover,
a boss slightly off center prevented the use of a central disk, unless
the bottom of the disk were bored out to receive this boss, which was
not thought expedient. Hence, the method adopted was to make the plate
thicker than the dimension given on the drawing, and then bore it out
to leave a rim of definite diameter, this rim to be removed after it
had served its purpose as a locating limit for the disks.
[Illustration: Fig. 10. (A) Layout of Jig-Plate. (B) Disk-and-Button
Method of Locating Holes]
As the holes _A_ and _B_, which were finished first, were 0.600 inch
apart and 0.625 inch from the center, the rim was bored to 1.850 inch
and two 0.600-inch disks, in contact with the rim and with each other,
located these holes. As hole _C_ was to be equi-distant from holes
_A_ and _B_, and its distance from the center was given, the size of
the disk for this hole was readily determined. The disks for holes
_A_, _B_ and _C_ have two diameters; the upper diameters are made to
whatever size is required for locating the disks of adjacent holes, and
they also form a hub which can be used when setting the disks with an
indicator. Hole _D_ was 0.4219 inch from _B_, and calculations based
on this dimension and its distance from the center showed that it was
0.4375 inch from hole _C_.
A “three-story” disk or button was made for hole _D_. The diameter of
the large part was 0.46875 inch and it overlapped disks _C_ and _B_
(the upper sections of which were made 0.375 inch and 0.4062 inch,
respectively), thus locating _D_. Then hole _F_ and all the remaining
holes were located in a similar manner. The upper diameters of disks
_E_ and _D_ were used in locating disks for other adjacent holes,
as well as a hub for the indicator; for instance, to locate a hole
with reference to holes _C_ and _D_, the diameter of the new disk and
the diameter of the upper part of disk _D_, were varied to give the
required location. The relation between the disks _B_, _D_ and _F_ is
shown by the side view.
[Illustration: Fig. 11. Example of Circular Spacing requiring a Large
Central Disk]
It had been decided that no screws should be used in attaching the
buttons or disks to the work, as it was feared that the tapped holes
would introduce inaccuracy by deflecting the boring-tools; therefore
the following method was employed. After all the disks were fastened
in place by clamps, a soft solder of low melting point was flowed
about them, filling the work to the top of the rim. When the solder
had cooled, the clamps were removed, the work transferred to the lathe
faceplate, indicated in the usual way, and the holes bored by a “_D_”
or “hog-nose” drill, guided by an axial hole in each disk, which had
been provided for that purpose when the disks were made. It was thought
that the unequal contraction of the solder and the plate in cooling
might throw the holes “out of square;” however, careful measurements
failed to show any appreciable lack of parallelism in test-bars
inserted in the holes.
[Illustration: Fig. 12. Locating Holes at an Angle by use of Disks and
Buttons]
[Illustration: Fig. 13. Locating Holes by Means of Two- and
Three-Diameter Disks in Contact]
Accurate Angular Measurements with Disks
For setting up a piece of work on which a surface is to be planed or
milled at an exact angle to a surface already finished, disks provide
an accurate means of adjustment. One method of using disks for angular
work is illustrated at _A_ in Fig. 14. Let us assume that the lower
edge of plate shown is finished and that the upper edge is to be
milled at an angle _a_ of 32 degrees with the lower edge. If the two
disks _x_ and _y_ are to be used for locating the work, how far apart
must they be set in order to locate it at the required angle? The
center-to-center distance can be determined as follows: Subtract the
radius of the larger disk from the radius of the smaller disk, and
divide the difference by the sine of one-half the required angle.
[Illustration: Fig. 14. Obtaining Accurate Angular Measurements with
Disks]
_Example_: If the required angle _a_ is 32 degrees,
the radius of the large disk, 2 inches, and the radius of the
small disk, 1 inch, what is the center-to-center distance?
The sine of one-half the required angle, or 16 degrees, is
0.27564. The difference between the radii of the disks equals
2 - 1 = 1, and 1 ÷ 0.27564 = 3.624 inches. Therefore, for an
angle of 32 degrees, disks of the sizes given should be set so
that the distance between their centers is 3.624 inches.
Another method of accurately locating angular work is illustrated at
_B_ in Fig. 14. In this case, two disks are also used, but they are
placed in contact with each other and changes for different angles
are obtained by varying the diameter of the larger disk. The smaller
disk is a standard 1-inch size, such as is used for setting a 2-inch
micrometer. By this method any angle up to about 40 degrees can be
obtained within a very close limit of accuracy. The following rule may
be used for determining the diameter of the larger disk, when both
disks are in contact and the diameter of the small disk is known:
Multiply twice the diameter of the small disk by the sine of one-half
the required angle; divide this product by 1 minus the sine of one-half
the required angle; add the quotient to the diameter of the small disk
to obtain the diameter of the large disk.
_Example_: The required angle a is 15 degrees.
Find the diameter of the large disk to be in contact
with the standard 1-inch reference disk.
The sine of 7 degrees 30 minutes is 0.13053.
Multiplying twice the diameter of the small disk
by the sine of 7 degrees 30 minutes, we have
2 × 1 × 0.13053 = 0.26106. This product divided by
1 minus the sine of 7 degrees 30 minutes
0.26106
= ——————————— = 3.002.
1 - 0.13053
This quotient added to the diameter of the small
disk equals 1 + 0.3002 = 1.3002 inch, which is the
diameter of the large disk.
[Illustration: Fig. 15. Disk-and-Square Method of Accurately Setting
Angular Work]
The accompanying table gives the sizes of the larger disks to the
nearest 0.0001 inch for whole degrees ranging from 5 to 40 degrees
inclusive. Incidentally, the usefulness of these disks can be increased
by stamping on each one its diameter and also the angle which it
subtends when placed in contact with the standard 1-inch disk.
DISK DIAMETERS FOR ANGULAR MEASUREMENT
+------+---------++------+---------++------+---------++
| Deg. | Inch || Deg. | Inch || Deg. | Inch ||
+------+---------++------+---------++------+---------++
| 5 | 1.0912 || 17 | 1.3468 || 29 | 1.6680 ||
| 6 | 1.1104 || 18 | 1.3708 || 30 | 1.6983 ||
| 7 | 1.1300 || 19 | 1.3953 || 31 | 1.7294 ||
| 8 | 1.1499 || 20 | 1.4203 || 32 | 1.7610 ||
| 9 | 1.1702 || 21 | 1.4457 || 33 | 1.7934 ||
| 10 | 1.1909 || 22 | 1.4716 || 34 | 1.8262 ||
| 11 | 1.2120 || 23 | 1.4980 || 35 | 1.8600 ||
| 12 | 1.2334 || 24 | 1.5249 || 36 | 1.8944 ||
| 13 | 1.2553 || 25 | 1.5524 || 37 | 1.9295 ||
| 14 | 1.2775 || 26 | 1.5805 || 38 | 1.9654 ||
| 15 | 1.3002 || 27 | 1.6090 || 39 | 2.0021 ||
| 16 | 1.3234 || 28 | 1.6382 || 40 | 2.0396 ||
+------+---------++------+---------++------+---------++
| _Machinery_ ||
+----------------------------------------------------++
Disk-and-Square Method of Determining Angles
The method shown in Fig. 15 for determining angles for setting up
work on a milling machine or planer, possesses several advantages. No
expensive tools are required, the method can be applied quickly, and
the results obtained are quite accurate enough for any but the most
exacting requirements. As will be seen, an ordinary combination square
is used in connection with a disk, the head of the square being set at
different points on the blade according to the angle that is desired.
Theoretically, a one-inch disk could be used for all angles from about
6 degrees up to a right angle, but in practice it is more convenient
and accurate to employ larger disks for the larger angles.
The only inaccuracy resulting from this method is due to setting the
square at the nearest “scale fraction” instead of at the exact point
determined by calculation. This error is very small, however, and is
negligible in practically all cases. The dimension _x_ required for any
desired angle _a_ can be found by multiplying the radius of the disk,
by the cotangent of one-half the desired angle, and adding to this
product the radius of the disk.
_Example_: The square blade is to be set to an
angle of 15 degrees 10 minutes, using a 2-inch disk.
At what distance _x_ (see Fig. 15) should the
head of the square be set?
Cot 7 degrees 35 minutes = 7.5113,
and 7.5113 × 1 + 1 = 8.5113 inches.
By setting the square to 8½ inches “full,” the
blade would be set very close to the required angle
of 15 degrees 10 minutes.
Locating Work by means of Size Blocks
The size-block method of locating a jig-plate or other part, in
different positions on a lathe faceplate, for boring holes accurately
at given center-to-center distances, is illustrated in Fig. 16. The way
the size blocks are used in this particular instance is as follows: A
pair of accurate parallels are attached to a faceplate at right angles
to each other and they are so located that the center of one of the
holes to be bored will coincide with the lathe spindle. The hole which
is aligned in this way should be that one on the work which is nearest
the outer corner, so that the remaining holes can be set in a central
position by adjusting the work away from the parallels. After the
first hole is bored, the work is located for boring each additional
hole by placing size blocks of the required width between the edges of
the work and the parallels. For instance, to set the plate for boring
hole _D_, size blocks (or a combination of blocks or gages) equal in
width to dimension _A_₁ would be inserted at _A_, and other blocks
equal in width to dimension _B_₁ beneath the work as at _B_. As will be
seen, the dimensions of these blocks equal the horizontal and vertical
distances between holes _C_ and _D_. With the use of other combinations
of gage blocks, any additional holes that might be required are located
in the central position. While only two holes are shown in this case,
it will be understood that the plate could be located accurately for
boring almost any number of holes by this method.
[Illustration: Fig. 16. Method of setting Work on Faceplate with Size
Blocks or Gages]
Incidentally, such gages as the Johansson combination gages are
particularly suited for work of this kind, as any dimension within
the minimum and maximum limits of a set can be obtained by simply
placing the required sizes together. Sometimes when such gages are not
available, disks which have been ground to the required diameter are
interposed between the parallels and the work for securing accurate
locations. Another method of securing a positive adjustment of the
work is to use parallels composed of two tapering sections, which
can be adjusted to vary the width and be locked together by means of
screws. Each half has the same taper so that outer edges are parallel
for any position, and the width is measured by using a micrometer. The
size-block method is usually applied to work having accurately machined
edges, although a part having edges which are of a rough or irregular
shape can be located by this method, if it is mounted on an auxiliary
plate having accurately finished square edges. For instance, if holes
were to be bored in the casting for a jig templet which had simply been
planed on the top and bottom, the casting could be bolted to a finished
plate having square edges and the latter be set in the different
positions required, by means of size blocks. Comparatively large jig
plates are sometimes located for boring in this way and the milling
machine is often used instead of a lathe.
The Master-plate Method
When it is necessary to machine two or more plates so that they are
duplicates as to the location of holes, circular recesses, etc., what
is known as a master-plate is often used for locating the work on the
lathe faceplate. This master-plate _M_ (see Fig. 17) contains holes
which correspond to those wanted in the work, and which accurately fit
a central plug _P_ in the lathe spindle, so that by engaging first one
hole and then another with the plug, the work is accurately positioned
for the various operations.
When making the master-plate, great care should be taken to have the
sides parallel and the holes at right angles to the sides, as well as
accurately located with reference to one another. The various holes
may be located with considerable precision by the use of buttons as
previously described. Of course, it is necessary to have a hole in the
master-plate for each different position in which the work will have to
be placed on the faceplate; for example, if a circular recess _r_ were
required, a hole _r_₁ exactly concentric with it would be needed in
the master-plate. The method of holding the work and locating it with
reference to the holes in the master-plate will depend largely on its
shape. The cylindrical blank _B_ illustrated, is positioned by a recess
in the master-plate in which it fits. The work is commonly held to the
master-plate by means of clamps and tap bolts or by screws which pass
through the work and into the master-plate. Solder is sometimes used
when it is not convenient to hold the work by clamps or screws.
[Illustration: Fig. 17. Master-plate applied to a Bench Lathe Faceplate]
The plug _P_ which locates the master-plate, is first turned to fit
the spindle or collet of the lathe and the outer or projecting end
is roughturned for the holes in the master-plate, which should all
be finished to exactly the same diameter. The plug is then inserted
in the spindle and ground and lapped to a close fit for the holes in
the master-plate. The latter, with the work attached to it, is next
clamped to the faceplate by the straps shown, which engage a groove
around the edge of the master-plate. The first hole is finished by
drilling to within, say, 0.005 or 0.006 inch of the size, and then
boring practically to size, a very small amount being left for reaming
or grinding. The remaining holes can then be finished in the same
way, the work being positively located in each case by loosening the
master-plate and engaging the proper hole in it with the central plug.
It is apparent that by the use of this same master-plate, a number of
pieces _B_ could be made which would be practically duplicates.
The master-plate method of locating work can be applied in many
different ways. It is used for making duplicate dies, for accurately
locating the various holes in watch movements, and for many other
operations requiring great precision. Master-plates are quite
frequently used by toolmakers when it is necessary to produce a number
of drill jigs which are to be used for drilling holes in different
parts having the same relative locations, thus requiring jigs that are
duplicates within very close limits.
When a master-plate is required, that is to be used in making
duplicates of an existing model, the holes are bored in the
master-plate by reversing the process illustrated in Fig. 17. That is,
the central plug _P_ is turned to fit the largest hole in the model and
the latter with the attached master-plate blank is clamped to lathe
faceplate. The first hole is then bored to within say 0.002 inch of the
finish diameter, to allow for grinding, provided the master-plate is
to be hardened. The central plug is then turned down to fit the next
largest hole and the second hole is bored in the master-plate. This
method is continued until all the holes are bored. In order to prevent
any change in the position of the master-plate relative to the model,
it may be secured by inserting dowel-pins through both parts, the work
being held to the lathe faceplate by ordinary screw clamps. If the
holes in the model do not extend clear through, a flat plate having
parallel sides may be interposed between the model and master-plate to
provide clearance between the two and prevent cutting into the model
when boring the master-plate.