Measuring Tools/Chapter 2

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Measuring Tools  (1910) 
Various Authors
Chapter 2

CHAPTER II

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

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Figs. 2 and 3Fig. 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.[1]

Shoulder Calipers

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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 ⅛ 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 ⅝-inch pin with a ⅜-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.

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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.

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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.[2]

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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.

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Figs. 9 and 10. Surface Gage with Two Pointers

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.[3]

To Adjust the Needle of a Surface Gage

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Fig. 11. Method of Adjusting the Needle of a Surface Gage

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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.[4]

Attachment for Machinist's Scale

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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.[5]

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.

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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.

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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 ⅝ 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.

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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.[6]

Improved Micrometer Beam Caliper

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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 EE1. 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

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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.[7]


  1. M. H. Ball, April, 1902.
  2. M. H. Ball, February, 1901.
  3. Harry Ash, April, 1900.
  4. M. H. Ball, March, 1903.
  5. Ezra F. Landis, May, 1902.
  6. L. S. Brown, March, 1903.
  7. C. W. Putnam, October, 1901.