To save expense and labor, a casting should be cut in the most economical way and in the shortest possible time. To do this, the cutting tool should attain the highest possible speed without injuring the metal or casting. The amount of metal removed in a given time represents the cutting capacity of the machine. This depends upon three factors: (1) the speed of the cut; (2) the distance covered by the tool in passing from one cutting part to the next; and (3) the depth of the cut, that is, the thickness of the strip removed from the casting.
The cutting feed of a tool is the thickness of the chip or the travel of the tool sideways during one revolution of the work. Thus, if the feed of a lathe is 1/16 in. for each revolution of the work, the tool moves along the lathe bed 1/16 in. and cuts a chip 1/16 in. wide. Sometimes the feed is given as the number of turns which the work makes while the tool advances 1 in., as 50 or 60 turns to the inch. This is, of course, the same as saying 1/50 or 1/60 in. feed per revolution. The feed is occasionally stated as so many inches per minute, but this is not the common practice. The cut or depth of cut is not the feed; it is the distance the tool enters the work when feeding. Twice the depth of the cut subtracted from the original diameter of the work will give the new diameter after turning.
The rate of feed may vary from 1/125 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 1/4 to 1/5 in. per revolution; on soft steel it should be 1/5 to 1/8 in. per revolution; for finishing cuts on soft cast iron, from 1/15 to 1/25 in. per revolution; and on soft steel, 1/20 to 1/40 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 5/16 in. what is the weight of the volume of metal removed in one minute? 1 cu. in. of metal weighs .277 lbs.
12 X 7/100 X 5/16 = .2625 cu. in. .2625 X .277 = .0727 lb.