MACHINE-SHOP PRACTICE 421 



In calipering with the micrometer, care must be taken to get 

 the proper touch. If the points of the instrument are crowded or 

 forced to overwork, the resulting error may be TT>\ or even ^-J- in. 

 less than the actual size; if too loose contact is made with the 

 measuring points, the measurement will be larger than the real 

 diameter and equally incorrect. The micrometer should never be 

 laid where excessive heat or widely varying temperatures come in 

 contact with it, as expansion and contraction of the frame injure 

 its accuracy. In very fine and exact work, the heat of the hand 

 may be sufficient to change a micrometer reading. 



482. Machine-Shop Tools. After the machine parts 

 have been cleaned, the remaining operations consist of drilling 

 holes, changing rough and uneven surfaces into smooth and 

 plane surfaces, and so on. This work is done by chipping or 

 turning the metal. For this purpose, tools of very hard steel, 

 attached to one of the different metal-working machines, 

 are used. The most common machines, and the ones gener- 

 ally found in all well-equipped machine-shops, are lathes, 

 planers, milling machines, drills, power hammers, and grind- 

 ing machines. In addition, many special devices or modifica- 

 tions of the above named machines are used for making 

 standard parts. 



483. Classification of Machines. Machines may be 

 divided into the two classes: (1) rotating machines, such as 

 lathes, boring mills, drills, presses, milling and grinding 

 machines; and (2) reciprocating machines, such as slotters, 

 planers, and shapers. 



The speed of machines of the first class is constant through- 

 out each operation, and most of their energy is used to over- 

 come the friction of boring, grinding, or cutting metal, the 

 amount of power consumed by the friction of the machine 

 itself being comparatively small. In machines of the second 



