Boring holes in metal requires the use of special tools, the possession of which may on occasion save a great deal of time.
By far the most efficient type of drill is the fluted twist drill, made in diameters ranging from xJ ½ inch up to 3 or more inches. Small drills for making holes up to -fa inch should have parallel shanks of the same diameter as the holes. A geared drill stock to hold them, with self-centreing 3-jaw chuck, can be bought for a few shillings.
A heavier form of stock with a breast pad in place of a handle, so that the driller may press against it with his body, is efficient for drills up to jj-inch diameter. A cheaper alternative is, however, to uso drills with square shanks, fitting an ordinary hand brace of the kind used for boring wood.
Though it may be difficult to bore a hole directly with a brace and, say, a £$-inch drill, the latter will penetrate easily enough if the way is prepared for it by a J-ineh pilot hole made with the hand drill. But a drill should not be used to enlarge a hole only slightly smaller than itself, since it may gain and break.
Before drilling, make a centre-mark with a centre-punch, deep enough to give the drill a hold. In the absence of such a mark the drill will roam about and finally may begin boring in the wrong place. A circle larger than the drill scribed round the centre-mark will show whether the drill is straying or not. If it strays before the bevelled part has entered, a nick should be made on the side to which it should go.
If a hole has to be bored right through, support the metal on a piece of hard wood or softer metal and go gently when the tip begins to come out. Otherwise the drill may dig in and jam. Should this happen, remove the drill and finish from the other side with a broach.
A broach, it should be explained, is a very gently tapering five-sided bar of very hard steel with a tang to sot in a handle. A set of broaches of graduated sizes just overlapping each other is a very useful addition to a drilling equipment, especially valuable when holes in thin metal require enlargement.
A drill should never be run backwards, as doing so wears away the cutting edges from the back, and the drill will not then be able to cut freely.
It is a waste of time drilling with blunt drills. Small drills can be replaced for a few pence, and larger ones are worth the small expense of getting them reground – an operation which needs a special machine.
Except when drilling cast iron, a drill should be kept well lubricated.
The part to be drilled should be secured firmly, so that it may not revolve nor tilt.
When a drill has entered deeper than twice its own diameter, care must be taken not to bend it, as the metal of which, it is made is highly tempered and somewhat brittle.