Importance of Speed Element. The speed at which cutting is done is an important matter. This varies with the shape of the tool, the quality of the metal being worked, and the strength of the lathe. The amount of metal removed in a given time is, therefore, equally variable. It is impossible to make a correct estimate of the time that a given piece of work will require, unless all of the above elements are known. For approximate estimates, the cutting speed for carbon tool steel cutting tools may be taken to range about as follows: \

In cast iron

from

30

to

40 feet per minute

In wrought iron

from from

25 15

to to

30 feet per minute 40 feet per minute

In steel

In brass

from

60

to

100 feet per minute

Suppose a wpought-iron shaft 6 feet long and 4 inches in diameter is to be turned. Let the lathe be capable of carrying a feed of 1/32 inch per revolution. The shaft has a circumference of 4X3.1416 = 12.5664 inches. To give the tool a cutting speed of 25

feet per minute the shaft must make 25X12/12.5664 = 24 revolutions per minute (approximately), giving a feed of 1/32 X 24 = 3/4 inch in that interval of time. With a travel of 3/4 inch per minute, it will take the cutting tool on the lathe carriage (6 X12) ÷ 3/4 = 96 minutes to take a cut the whole length of the shaft.

The amount of feed is really the governing element. This may be as much as 3/16 inch per revolution, and, for finishing cuts, may not be more than 1/100 inch. The depth of the tool cut also influences the time required to finish a given piece of work, and this may vary from 1/100 to 1/2 inch, depending entirely upon the shape of the tool and the strength of the lathe.