This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Cutting drifts having teeth on their sides, similar to large file teeth, are shaped by two methods; small ones not more than 1 in. thick being grooved by filing, and large ones that may be 3 or 4 in. thick being grooved with a planing machine or shaping machine.
The steel suitable for drifts is a tough, well-hammered metal that has not been cast, and the smaller the intended tool the greater is the need to select an elastic fibrous metal which will bend after being hardened, and not be liable to crack in hardening through being too solid. Small thin drifts may be made of a hard Swedish iron, and afterwards partly carbonized to steel the exterior. A drift thus made will sustain a severe bending while in a crooked hole, without being so liable to break as if the entire tool were of steel. The short drifts do not bend while being hammered through a piece of work; they may therefore be made of steel; but all long ones that are comparatively thin are more pliable if made of iron. The hammering of any drift, whether long or short, shakes and tends to break it, and it is advisable to make each one as short as its intended work will permit. Those for drifting small holes often require long handles, similar to that shown in Fig. 107; such a handle is thinner than he portion for cutting, that all its teeth may be driven through the work.
Iron drifts are steeled by being packed in charcoal in boxes; the lids are put on, all the crevices are filled with loam, and a thick layer of loam is put on the ledge, which extends all round the mouth for the convenience of supporting the loam. After all the crevices are thus filled, to keep out the air, the affair is put into a large clear fire, that plenty of room may exist around, and gradually heat all sides of the box at one time. A plate furnace fire will afford a convenient heat, a substitute being a large forge fire; if this is used, the blast is very gently administered until the work is red hot, when the blast is stopped, and the work is allowed to remain at the same heat for 2 hours, during which time the drifts have absorbed the carbon from the charcoal, and the surfaces are steeled. This being done, each one is taken carefully from the charcoal without bruising the edges, and allowed o cool separately, if they are required immediately; if not, the box is taken from the fire; he lid is raised, and the work is allowed to slowly cool while among the charcoal. When the drifts are cold, they are put into order for hardening.
This may be done at any future time, and consists in sharpening the teeth and polishing the surfaces, to make them as they appeared previous to being heated, and when they are to be hardened they are again heated and cooled in water. This second heating is seldom necessary for drifts if they are properly finished previous to steeling, and they may be hardened while hot at the time they are first carbonized. Drifts thus steeled may be softened at any future time when the teeth require sharpening, and again hardened by merely heating and dipping into water, because heating the tool does not liberate the carbon.
This method of carbonizing is also adopted for changing the surfaces of iron screw-taps into steel; taps thus treated are useful for several classes of work, if properly managed.