438 APPLIED SCIENCE 



of many levers, varying in thickness by thousandths of an 

 inch, and each designated by a number. By applying these 

 levers successively, it is possible to determine the thickness 

 of a piece of metal. A similar arrangement of plugs, called 

 thickness plugs, are used for determining the diameter of a 

 hole. 



507. Screw Pitch Gauge. It is very desirable at times 

 to be able to tell quickly the pitch of a screw thread. This 



may be done by means of a screw 

 pitch gauge, which consists of 

 many thin pieces or leaves of 

 steel fastened to a holder (Fig. 

 203). On the e'dge cf each piece 

 of steel there are teeth correspond- 

 ing to a standard thread section.. 

 Fie;. 203. Screw Pitch By placing leaves successively over 

 Gauge. ne thread until a leaf coincides, 



it is possible to determine the screw pitch. 



508. Division of Machine-Shop Trades. 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 bo exer- 

 cised because of the greater accuracy demanded, as some- 



