Often articles of tool steel which are to be threaded give trouble when machined, if annealed by either of the methods mentioned. When annealing steel for taps, extreme care must be exercised, for if the steel is hard, the threading tool will not work properly; if overannealed the steel is not strong enough to withstand the cutting action of the tool, and rough threads result. Either condition is fatal to the production of a good tap.
A satisfactory method of annealing for work of this nature consists in taking an iron box and placing a quantity of hot ashes in the bottom. A board is laid on them and the articles to be annealed are placed on the board after having bean heated to a uniform low red heat. They are then covered with another piece of board and the whole is buried. The wood absorbs heat from the steel fast enough to insure good annealing, while the wood in turn is heated and holds the steel just below a red heat long enough to soften it properly. Steel intended for dies, the surface of which is to be engraved, should be annealed by this method.