The machine trade is divided into machine construction, tool-making, and die-sinking. Under the first type of work are grouped all those operations, discussed in previous sections, which have to do with making and repairing machinery.

Tool-making requires a higher degree of skill than the average machinist usually possesses. The work ranges from making shop tools, such as jigs, boring bars, templates, etc., to the making of fine hand-tools and instruments of precision. The principles involved in tool-making are the same as in other machine-work. Greater care, however, must be exercised because of the greater accuracy demanded, as sometimes work is required to be exact to the ten-thousandth part of an inch. When such accuracy is expected the work is ground to size on a universal grinder, which grinds either cylindrical or plane surfaces.

Fig. 203.   Screw Pitch Gauge.

Fig. 203. - Screw Pitch Gauge.

Drop forgings, in which a form is made to conform to the desired shape, are made by means of dies or blocks of steel.

The work of making the dies is called die-sinking (Fig. 204). As it is necessary to work on the metal by various means, making allowance for shrinkage and the flow of metal, a considerable amount of skill is required to work out these forms and finish them so that the product will be properly made.