By Orville R. Hagans and D. L. Thompson
Basic Mechanics - Lesson 4
31. MAKING SMALL STEEL SCREWS
Small steel screws can be bought in assortments for any make of American watch, and for most of the Swiss models, however, it is often the case that the screw needed is not on hand, or not easily secured, therefore, it is a convenience to be able to make one or to refinish a marred or rusted one.
As in making taps, it will be necessary to have an assortment of screwplates or dies, if the correct thread is to be made. If a screw has been lost, the thread diameter and pitch can usually be determined from another screw from the movement, of which there are generally two or more of the same diameter and pitch of thread, which is used to select the correct hole in the screw-plate or die. Otherwise, a tap of the same diameter and pitch of the thread of the screw hole . will have to be used to select the proper die.
To make a small screw, select a piece of staff-wire, or drill-rod, which is a little larger in diameter than the head of the screw which is to be made, or of the counter-sink made for it, and heat it to pale blue. Turn the tap and, as .shown in Fig. 21, at a, in the same manner as in making a tap, but with a square shoulder and then turn on the threads, as shown at b. The length of the head is now to be marked and the diamete of the head turned to correct size, allowing a little for grinding and polishing, after which the screw is to be severed from the wire with a cutting-off graver, as shown in Fig. 21, b, or with a square graver as shown by the dotted lines. 'l'he screw is then reversed in the lathe and held by the tap, closing the chuck on it lightly, and the flat of the head turned true, after which the thumbnail is placed on the center of the flat to guide the file and the slot filed as shown in Fig. 22, being careful to file the slot to the same depth 011 each side of the head.
The slot can also be cut with a small thin circular-saw, as shown in Fig. 23, the screw being held in a pin-vise which is to be rested on the T-rest.
The screw is to be hardened in the usual manner and then tempered to dark blue on a bluing slip. This temper is sufficient to give the threads the proper strength; to prevent the slot from being easily burred by a screwdriver; and to take a good polish. If a screw is to be given a high polish, it should be tempered to a bright blue, but screws having very small taps will break quite easily when so tempered.