CALIPERS, DIVIDERS, AND SURFACE GAGES
In the present chapter we shall deal with the simpler forms of tools used
for measuring, such as ordinary calipers, and their use; surface gages;
special attachments for scales and squares, facilitating accurate
measuring; and vernier and beam calipers. The descriptions of the tools and
methods referred to have appeared in MACHINERY from time to time. The names
of the persons who originally contributed these descriptions have been
stated in notes at the foot of the pages, together with the month and year
when their contribution appeared.
Setting Inside Calipers
[Illustration: Figs. 2 and 3 - Fig. 4 - Setting Inside Calipers]
It is customary with most machinists, when setting inside calipers to a
scale, to place one end of the scale squarely against the face of some true
surface, and then, placing one leg of the caliper against the same surface,
to set the other leg to the required measurement on the scale. For this
purpose the faceplate of the lathe is frequently used on account of its
being close at hand for the latheman. The sides of the jaws of a vise or
almost anything located where the light is sufficient to read the markings
on the scale are frequently used.
The disadvantages of this method are, first, that a rough or untrue object
is often chosen, particularly if it happens to be in a better light than a
smooth and true one, and, second, that it is very hard to hold the scale
squarely against an object. It is easy enough to hold it squarely
crosswise, but it is not so easy a matter to keep it square edgewise. As
can be readily seen, this makes quite a difference with the reading of the
calipers, particularly if the scale is a thick one.
Figs. 2 and 3 show this effect exaggerated. _B_ is the block against which
the scale abuts. The dotted line indicates where the caliper leg should
rest, but cannot do so, unless the scale is held perfectly square with the
block. Fig. 4 shows a method of setting the calipers by using a small
square to abut the scale and to afford a surface against which to place the
leg of the caliper. The scale, lying flat on the blade of the square, is
always sure to be square edgewise, and is easily held squarely against the
stock of the square as shown. This method has also the advantage of being
portable, and can be taken to the window or to any place where the light is
satisfactory. When using a long scale, the free end may be held against the
body to assist in holding it in place.[2]
Shoulder Calipers
[Illustration: Fig. 5. Shoulder Calipers]
In Fig. 5 are shown a pair of calipers which are very handy in measuring
work from shoulder to shoulder or from a shoulder to the end of the piece
of work. For this purpose they are much handier, and more accurate, than
the ordinary "hermaphrodites." The legs are bent at _AA_ so as to lie flat
and thus bring the point of the long leg directly behind the short one
which "nests" into it, as at _B_, so that the calipers may be used for
short measurements as well as for long ones.
Double-jointed Calipers to Fold in Tool Box
In Fig. 6 are illustrated a pair of large calipers that can be folded up
and put in a machinist's ordinary size tool chest. The usual large caliper
supplied by the average machine shop is so cumbersome and heavy that this
one was designed to fill its place. It can be carried in the chest when the
usual style of large caliper cannot. It is a very light and compact tool.
It is a 26-inch caliper, and will caliper up to 34 inches diameter. The top
sections are made in four pieces, and the point ends fit between the top
half like the blade of a knife, as shown in the engraving. Each side of the
upper or top section is made of saw steel 1/16 inch thick, and the lower
part or point of steel 1/8 inch thick. The double section makes the tool
very stiff and light.
The point section has a tongue _A_, extending between the double section,
which is engaged by a sliding stud and thumb nut. The stud is a nice
sliding fit in the slot, and the thumb nut clamps it firmly in place when
in use. _B_, in the figure, shows the construction of the thumb nut. _C_ is
a sheet copper liner put between the washers at _A_. The dotted lines in
the engraving show the points folded back to close up. The large joint
washers are 1¾ inch diameter, and a 5/8-inch pin with a 3/8-inch hexagon
head screw tightens it up. The forward joints are the same style, but
smaller. The main joint has two 1¾-inch brass distance pieces or washers
between the two main washers. The top section is 12½ inches between
centers, and the point sections 15 inches from center to point. Closed up,
the calipers measure 16 inches over-all.
[Illustration: Fig. 6. Large Double-jointed Calipers]
Kinks in Inside Calipering
Close measurements may be made by filing two notches in each leg of an
inside caliper so as to leave a rounded projection between, as shown at
_E_, Fig. 7. Then, with an outside caliper, _D_, the setting of the inside
caliper, _B_, is taken from the rounded points. The inside caliper can be
reset very accurately after removal by this method. A still better way is
to have two short pins, _CC'_ set in the sides of the inside caliper legs,
but this is not readily done as a makeshift. To measure the inside diameter
of a bore having a shoulder like the piece _H_, the inside caliper _F_ may
also be set as usual and then a line marked with a sharp scriber on one
leg, by drawing it along the side _G_. Then the legs are closed to remove
the caliper, and are reset to the scribed line. Of course, this method is
not as accurate as the previous one, and can be used only for approximate
measurements.
[Illustration: Fig. 7. Methods of Inside Calipering]
To get the thickness of a wall beyond a shoulder, as at _K_, Fig. 7, set
the caliper so that the legs will pass over the shoulder freely, and with a
scale measure the distance between the outside leg and the outside of the
piece. Then remove the caliper and measure the distance between the caliper
points. The difference between these two distances will be the thickness
_M_.
Inside Calipers for Close Spaces
In Fig. 8 are shown a pair of inside calipers which are bent so as to be
well adapted for calipering distances difficult of access, such as the
keyway in a shaft and hub which does not extend beyond the hub, as
indicated. With the ordinary inside calipers, having straight legs, and
which are commonly used for inside work, it is generally impossible to get
the exact size, as the end which is held in the hand comes in contact with
the shaft before both points come into the same vertical plane. The
engraving plainly shows how calipers for this purpose are made, and how
used. Any mechanic can easily bend a common pair to about the shape shown
to accommodate this class of work.[3]
[Illustration: Fig. 8. Inside Calipers for Close Spaces]
Surface Gage with Two Pointers
Figs. 9 and 10 show a special surface gage, and illustrate an original idea
which has been found to be a great saver of time and of milling cutters. It
can also be used on the planer or shaper. By its use the operator can raise
the milling machine table to the right height without testing the cut two
or three times, and eliminate the danger of taking a cut that is liable to
break the cutter. This tool is especially valuable on castings, as raising
the table and allowing the cutter to revolve in the gritty surface while
finding the lowest spot is very disastrous to the cutting edges.
[Illustration: 10]
To use this surface gage, the pointer marked _C_ in Fig. 9 is set to the
lowest spot in the casting, and then the pointer _B_ is set from it with
perhaps 1/32 inch between the points for a cut sufficient to clean up the
surface. Pointer _C_ is then folded up as shown at _C'_ in Fig. 10, and the
table is raised until the pointer _B_ will just touch the under side of the
cutter as shown at _B'_ in Fig. 10. In this way the table is quickly
adjusted to a cut that will clean the casting or other piece being
machined, and with no cutting or trying whatever.[4]
To Adjust the Needle of a Surface Gage
[Illustration: Fig. 11. Method of Adjusting the Needle of a Surface Gage]
[Illustration: Fig. 12. Scale Attachment for the Square]
Fig. 11 illustrates a method of adjusting the needle of a surface gage. To
set the gage 3¾ inches from the table, get somewhere within ¼ inch of the
mark on the square. With the thumb and forefinger on hook _A_, turn the
needle till it reaches the point desired. By turning the needle, it will
travel in a circular path, on account of the bend near the point, and thus
reach the desired setting.
Scale Attachment for the Square
Fig. 12 shows a device for attaching a scale to a square. This combination
makes a very convenient tool to use when setting up work for keyseating, as
is illustrated in the engraving, in which _S_ is the shaft to be splined
and _C_ the milling cutter. It is also a very handy tool for truing up work
on the boring mill or lathe. At the upper left-hand corner, is shown the
construction of the parts, which are made of dimensions to suit the size of
the scale and the square. For the combination to be successful, it is
essential that the blade of the square is the same thickness as the
scale.[5]
Attachment for Machinist's Scale
[Illustration: Fig. 13. Convenient Attachment for Machinist's Scale]
Fig. 13 shows a very convenient appliance. It will be found very useful in
the machine shop for setting inside calipers to any desired size. The gage
is clamped over the rule wherever desired, and one leg of the calipers set
against the gage, the other leg being brought flush with the end of the
scale.[6]
Setting Dividers Accurately
To set dividers accurately, take a 1-inch micrometer and cut a line
entirely around the thimble as at _A_, Fig. 14, and then, with the
instrument set at zero, make a punch mark _B_ exactly one inch from the
line on the thimble. If less than one inch is wanted, open out the
micrometer and set the dividers to the dot and line so as to give one inch
more than the distance wanted. Now with the dividers make two marks across
a line, as at _a_ and _b_, Fig. 14, and then set the dividers to one inch
and mark another line as at c. The distance from _c_ to _b_ is the amount
desired, and the dividers can be set to it. Great care must, of course, be
exercised, if accurate results are required.
[Illustration: Fig. 14. Method of Setting Dividers Accurately]
Combination Caliper and Divider
The combination caliper and divider shown in Fig. 15 is one that is not
manufactured by any of the various tool companies. It is, however, one of
the handiest tools that can be in a machinist's kit, as it lends itself to
so many varied uses, and often is capable of being used where only a
special tool can be employed. The illustration suggests its usefulness. The
tool can be used as an outside caliper, as an inside caliper, and as a
divider. The common form of this tool has generally only one toe on the
caliper legs, but the double toes save the reversal of the points when
changing from outside to inside work. The divider points may be set at an
angle, which permits of stepping off readily around the outside of a shaft
at angular distances, where the ordinary dividers are useless. A number of
other uses could be mentioned, but any intelligent mechanic can readily
suggest them for himself.
[Illustration: Fig. 15. Combination Caliper and Divider]
Attachment for Vernier Calipers
While vernier and slide calipers are very handy shop tools, their
usefulness is much more limited than it ought to be for such expensive
instruments. In order to increase the usefulness of these tools, the
attachments shown in Fig. 16 may be made. In the upper left-hand part of
the engraving the details of a useful addition to the caliper are shown.
_A_ is made of machine steel, while the tongue _B_ is of tool steel,
hardened and ground and lapped to a thickness of 0.150 inch, the top and
bottom being absolutely parallel. This tongue is secured to _A_ by the two
rivets _CC_. The thumb-screw _D_ is used for fastening the attachment to
the sliding jaw of the vernier or slide caliper. In the upper part of the
engraving is shown the base, which is of machine steel, with the slot _F_
milled for the reception of the fixed jaw of the caliper. The set-screws
_GGG_ are put in at a slight angle so that the caliper will be held firmly
and squarely in this base. In the figure to the left these pieces are shown
in the position for forming a height gage, for which purpose the attachment
is most commonly used. As a test of the accuracy of its construction when
the attachment is placed in this position, the tongue _B_ should make a
perfect joint with the fixed jaw of the caliper, and the vernier should
give a reading of exactly 0.150. When it is desirable that the tongue _B_
should overhang, the base _E_ is pushed back even with the stationary jaw,
as shown in the engraving to the right. In this position it is used for
laying out and testing bushings in jigs, etc. The illustration shows the
tool in use for this purpose, _K_ being the jig to be tested. All
measurements are from the center line upon which the bushing No. 1 is
placed. Taking this as a starting point we find the caliper to read 1 inch.
Bushing No. 2, which is undergoing the test, should be 5/8 inch from this
center line. It has a ¼-inch hole, and we therefore insert a plug of this
diameter. Now adjust the tongue of the caliper to the bottom of this plug
(as shown in the engraving) and the vernier should read 1.625 minus
one-half the diameter of the plug, or 1.500, and any variation from this
will show the error of the jig. In this case the top surface of _B_ was
used and no allowance had to be made for its thickness. In case the bottom
surface is used, 0.150 must be deducted from the reading of the caliper.
[Illustration: Fig. 16. Attachment for Vernier Calipers]
It is very easy to make a mistake in setting a bushing, and such a mistake
is equally hard to detect unless some such means of measuring as this is at
hand. It often happens that jigs and fixtures are put into use containing
such errors, and the trouble is not discovered until many dollars' worth of
work has been finished and found worthless. The illustration shows but one
of the many uses to which this attachment may be applied. The figures given
on the details are correct for making an attachment to be used upon the
Brown & Sharpe vernier caliper, but for other calipers they would, of
course, have to be altered to suit.[7]
Improved Micrometer Beam Caliper
[Illustration: Fig. 17. Improved Micrometer Beam Caliper]
In a beam caliper having a sliding micrometer jaw with or without a
separate clamping slide, it is necessary to have the beam divided into unit
spaces, at which the jaw or slide may be accurately fixed, the micrometer
screw then being used to cover the distance between the divisions; but it
is difficult to construct a beam caliper of this type with holes for a
taper setting pin, at exactly equal distances apart; consequently a plan
that is generally followed in making such tools is to provide as many holes
through the slide and beam as there are inch divisions, each hole being
drilled and reamed through both the slide and beam at once. If it were
attempted to drill the holes through the beam at exactly one inch apart,
having only one hole in the clamping head and using it as a jig for the
purpose, it would be found very difficult, if not impossible, to get the
holes all of one size and exactly one inch apart. The design of the
micrometer beam caliper shown in Fig. 17, which has been patented by Mr.
Frank Spalding, Providence, Rhode Island, is such, however, that it is not
necessary to drill more than one hole through the clamping slide. The beam
_F_ is grooved longitudinally, and in the groove are fitted hardened steel
adjusting blocks in which a taper hole _D_ is accurately finished. Between
the blocks are filling pieces _G_, which are brazed or otherwise fastened
in the groove. Holes are drilled, tapped, and countersunk between the
blocks and the filling pieces _G_, in which are fitted taper head screws
_EE_1_. The construction is thus obviously such that the blocks may be
shifted longitudinally by loosening one screw and tightening the other. In
constructing the caliper, the holes through the beam are drilled as
accurately as possible, one inch apart, and centered in the longitudinal
groove, but are made larger than the holes in the blocks, so as to provide
for slight adjustment.
Large Beam Caliper
[Illustration: Fig. 18. Large Beam Caliper]
Fig. 18 shows a large beam caliper designed for machinists and
patternmakers. It consists of a beam _MN_ and the legs _R_ and _S_, made of
cherry wood to the dimensions indicated. The legs are secured in position
on the beam by means of the thumb screws _A_, which jam against the gibs
_C_ at the points of the screws. The gibs have holes countersunk for the
screws to enter, to hold them approximately in place, and the nuts _B_ are
of brass, fitted into the filling pieces _P_ that keep them from turning.
The filling pieces are riveted to the legs by means of cherry dowels _D_.
One leg _S_ is provided with a fine adjustment consisting of flexible steel
spring _H_, ending in a point which is adjusted by the thumb screw _E_.
This screw is locked in adjustment by the check nut _G_ bearing against the
brass nut _F_, which is inserted in the leg as shown.[8]
* * * * *