for measuring angles. We shall therefore in the following chapters deal principally with special measuring tools, and with such methods in the use of tools which are likely to suggest improvements, or otherwise be valuable to the user and maker of measuring tools.

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

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.^[2]

Shoulder Calipers

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.

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.

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