This form of die, Fig. 135, is adjusted by means of a clamp collar as shown in Fig. 136. In some shops it is the only form of screw-threading die used for screw-machine work. When so used, it should be fitted to one of the holders on hand, provided there is one of the proper size.

Average dimensions of spring dies are given in Table V. These sizes are used by a manufacturing concern employing a great many screw-threading dies of this description. It is not necessary to follow the proportions given, as they are intended only as a guide, and may be changed to suit circumstances.

For uniform and well-finished threads, two dies should be used, one for roughing, and one for finishing.

Table V. Dimensions Of Spring Screw-Threading Dies

Size of screw

(in)

Outside diameter

(in)

Length

(in)

1/2

to

1/1

1/2

1 1/4

1/4

to

1

1

1 2/4

2/8

to

I

1

2

1/2

to

2/4

1 1/4

2 1/4

3/4

to

1

1 3/4

2 1/2

1

to

1 1/4

2

3

1 1/4

to

1 1/4

2 1/2

3 1/2

11/2

to

2

3 1/4

4

Where many dies of a size are made, it is best to have a holder with a shank fitting the center hole of some lathe. The stock can be machined to size and cut to length. The clearance hole in the back of the die should be first drilled somewhat larger than the diameter of the screw to be cut. For dies up to and including | inch, this excess in size should be 1/32 inch; for dies 1/8 to J inch, it should be 3/14 inch; for dies 1/2 inch and over, it should be from 1/16 to 1/8 inch. After drilling the clearance hole, the die should be reversed in the holder, and drilled and tapped the same as a round die, using a hob to finish the threads to size.

For general work, the die should have four cutting edges, making the lands about one-sixteenth the circumference of the screw to be cut. Chamfer about three threads. The length of the threaded portion of the die should not exceed one and one-quarter times' the diameter of the screw to be cut. To produce the cutting edges, use a 45-degree double-angle milling cutter. Fig. 137, which should be of sufficiently large diameter to produce a cut, as shown in Fig. 138.

The chamfered edges should be relieved, and the cutting edges finished with a fine file. Stamp the size and number of threads on the back end of the die, as shown in Fig. 138, and then harden.

Hardening

The die should be heated in a tube and hardened in a jet of water coming up from the bottom of a tank, in order that the water may enter the threaded portion. The die should be hardened a little farther up than the length of the thread, and should be moved up and down in the bath to prevent a water line; the temper should be drawn to a full straw color.

Malleable Iron Collars

Where many clamp collars are used, castings of malleable iron or gun metal may be made from a pattern; the hole should be cored to within 1/16 inch of finish size, drilled, and reamed. When the screw hole has been drilled and tapped and the collar split, it is ready to use. If the surfaces are finished, the cost is materially increased.

Illustration Of Spring Die

The form of spring die shown in Fig. 139 is especially adapted for heavy work; the jaws, being heavy and well supported by the cap, do not spring when taking heavy cuts. One end of the cap has an internal thread which screws on to the end of the shank, thus drawing the cutting end of the tool securely against the shank. This also provides a means of adjusting the size of the cutting end, as the cap is tapered on the inside at the outer end to fit the taper on the outside of the jaws. A locking nut fastens the cap securely when it has been set to the right size. The cutting end of the die has grooves, as shown at a. These grooves engage with tongues on the shank to prevent turning.

Special Form of Spring Die.

Fig. 139. Special Form of Spring Die.