MACHINE-SHOP PRACTICE 423 



The rate of feed may vary from 1^-5 in. per revolution to 1 in. 

 per revolution. A well-known authority on lathe practice gives the 

 following figures and directions for roughing cuts. On soft cast 

 iron the feed should be f to i in. per revolution; on soft steel it 

 should be i to i in. per revolution; for finishing cuts on soft cast 

 iron, from iV to ^V in. per revolution; and on soft steel, Vo to 

 ^V in. per revolution. 



The volume of metal removed from a casting in the unit time of 

 one minute is equal to the cutting speed multiplied by the feed 

 multiplied by the depth of cut. To illustrate: If the speed of 

 cut is 1 ft. per minute, the feed .07 in., and the depth of cut yV in. 

 what is the weight of the volume of metal removed in one minute? 

 1 cu. in. of metal weighs .277 Ibs. 



12 X iiU X TG - .2625 cu. in. 

 .2625 X .277 - .0727 Ib. 



485. Theory of Cutting Metal. A harder material will 

 cut or scratch a softer material. Therefore the cutting tool 

 must always be harder than the stock. Different metals, 

 such as iron, wrought iron, steel, copper, brass, etc., possess 

 different grades of hardness. The harder the metal, the 

 greater the friction, the quicker the tool is worn away, and 

 the greater is the strain on the machine. For these reasons 

 it is necessary to have different cutting speeds for different 

 metals. If this speed is exceeded or if too great a cut is re- 

 moved, the heat will be correspondingly greater. This heat 

 may be sufficient to take the temper out of the steel or warp 

 the stock. Therefore it is very important to know the best 

 cutting speed and feed in all cases, so that the maximum 

 output of the machine can be obtained. 



486. Kinds of Lathes. Probably the most essential 

 machine in the general machine-shop is the lathe. It is 



