are usually limited to sizes below 1⁄2 inch. Sometimes the solid die is cylindrical in shape, with a slot through one side which enables one to obtain a slight degree of adjustment by squeezing the slotted portion together. Large dies, or the sizes over 1⁄2 inch, are usually

Fig. 66.—Showing Holder Designs for One and Two Piece Thread Cutting

Dies.

made in two pieces in order that the halves may be closed u or brought nearer together. The advantage of this form of d is that either of the two pieces may be easily sharpened, and a it may be adjusted very easily the thread may be cut by eas

stages. For example, the die may be adjusted to cut large, which will produce a shallow thread that will act as an accurate guide when the die is closed up and a deeper thread cut.

A common form of die holder for an adjustable die is shown at Fig. 66, . As will be apparent, it consists of a central body portion having guide members to keep the die pieces from falling out and levers at each end in order to permit the operator to exert sufficient force to remove the metal. The method of adjusting the depth of thread with a clamp screw when a two-piece die is employed is also clearly outlined. The diestock shown at B

is used for the smaller dies of the one-piece pattern, having a slot in order that they may be closed up slightly by the clamp screw. The reverse side of the diestock shown at B is outlined below it, and the guide pieces, which may be easily moved in or out, according to the size of the piece to be threaded by means of ec5 centrically disposed semi-circular slots in the adjustment plate, are shown. These movable guide members have small pins let into their surface which engage the slots, and they may be moved in or out, as desired, according to the position of the adjusting plate. The use of the guide pieces makes for accurate positioning or centering of the rod to be threaded. Dies are usually sold in sets, and are commonly furnished as a portion of a complete outfit

such as outlined at Fig. 67. That showr has two sizes of diestock, a tap wrench, eight assorted dies, eight assorted taps, and a small screw driver for adjusting the die. An automobile repair shop should be provided with three different sets of taps and dies, as three different standards for the bolts and nuts are used in fastening automobile components. These are the American, metric (used on foreign cars), and the S. A. E. standard threads. A set of pipe dies and taps will also be found useful.

1811

Fig. 68.-Common Forms of Inside and Outside Calipers.

Measuring Tools.-The tool outfit of the machinist or the mechanic who aspires to do machine work must include a number of measuring tools which are not needed by the floor man or one who merely assembles and takes apart the finished pieces. The machinist who must convert raw material into finished products requires a number of measuring tools, some of which are used for taking only approximate measurements, such as calipers and scales, while others are intended to take very accurate meas

urements, such as the Vernier and the micrometer. A number of common forms of calipers are shown at Fig. 68. These are known as inside or outside calipers, depending upon the measurements they are intended to take. That at A is an inside caliper, consisting of two legs, A and D, and a gauging piece, B, which can be locked

Fig. 69.-View Showing Utility of Combination Inside and Outside Friction Joint Calipers.

to leg, A, or released from that member by the screw, C. The object of this construction is to permit of measurements being taken at the bottom of a two diameter hole, where the point to be measured is of larger diameter than the portion of the hole through which the calipers entered. It will be apparent that the legs

A and D must be brought close together to pass through the smaller holes. This may be done without losing the setting, as the guide bar B will remain in one position as determined by the size of the hole to be measured, while the leg A may be swung in to clear the obstruction as the calipers are lifted out. When it is desired to ascertain the measurements the leg A is pushed back into place into the slotted portion of the guide B, and locked by the clamp screw C. A tool of this form is known as an internal transfer caliper.

The form of caliper shown at B is an outside caliper. Those at C and D are special forms for inside and outside work, the former being used, if desired, as a divider, while the latter may be employed for measuring the walls of tubing. The calipers at E are simple forms, having a friction joint to distinguish them from the spring calipers shown at B, C and D. In order to permit of ready adjustment of a spring caliper, a split nut as shown at G is sometimes used. A solid nut caliper can only be adjusted by screwing the nut in or out on the screw, which may be a tedious. process if the caliper is to be set from one extreme to the other several times in succession. With a slip nut as shown at G it is possible to slip it from one end of the thread to the other without turning it, and of locking it in place at any desired point by simply allowing the caliper leg to come in contact with it. The method of adjusting a spring caliper is shown at Fig. 68, H. The caliper shown at Fig. 69, A, is known as a "hermaphrodite," and is so called because it can be used for measuring both diameters of pieces or bores of holes. It is provided with a removable and adjustable point, two being provided with each device. One of these is curved, the other is straight. The point is firmly held in position by a knurled nut and washer and draw bolt, and is given additional support by the loop at the end of the caliper leg. The auxiliary caliper point furnished with these tools makes it possible to convert the tools readily into an inside caliper. Two uses of this tool are shown at Fig. 69, B and C. These are self-explanatory.

An important tool that should be included in the equipment of every shop where machine tools are used is shown at Fig. 70, A.