Screws for holding metal parts together have countersunk, round, or cheese-shaped heads. For sizes below &-in in diameter
SCREWS screws cut with the B.S.A. Thread are recommended; and for larger sizes those with British Standard Whitworth threads.
Holes through which metal screws have to pass imist be bored to the full size of the screw in the held part; and be of the smaller tapping diameter in the holding part, to allow for threads being-cut in the surrounding metal. A tapping plate is needed for the selection of the proper size of drill for the smaller hole. It contains pairs of holes, one of a pair just allowing the screw to pass, while the other fits the drill that should be used for the hole that has to be tapped.
Taps for cutting threads are sold usually in sets of three for each size: a taper, for starting the thread; a second, for opening it out; and a plug, for cleaning to full size. In thin metal the first only may be needed. It should be worked in just far enough to allow the screw to enter rather stiffly.
A tap should be well oiled before use, and be rotated to right and left with a tap wrench. Taps, being of highly tempered steel, are brittle and liable to snap if unduly forced. It is better to enlarge the hole slightly with a broach than to risk breaking the tap.
If a number of screws are needed to hold two parts together, it is of the greatest importance that the holes in both parts should register exactly. If they do not, some of the screws will refuse to enter until the holes in the held part have been enlarged. The two parts should be clamped firmly together, and bored with the drill of tapping size. The holes in the held part are then enlarged to full size with a larger drill, a broach or a reamer.