ine or coarse. Ordinarily, fine feeds are used for finishing steel,
especially if the work is at all flexible, whereas finishing cuts in
cast iron are often accompanied by a coarse feed. Fig. 15 illustrates
the feeds that are often used when turning cast iron. The view to the
left shows a deep roughing cut and the one to the right, a finishing
cut. By using a broad flat cutting edge set parallel to the tool's
travel, and a coarse feed for finishing, a smooth cut can be taken in a
comparatively short time. Castings which are close to the finished size
in the rough can often be finished to advantage by taking a single cut
with a broad tool, provided the work is sufficiently rigid. It is not
always practicable to use these broad tools and coarse feeds, as they
sometimes cause chattering, and when used on steel, a broad tool tends
to gouge or "dig in" unless the part being turned is rigid. Heavy steel
parts, however, are sometimes finished in this way. The modern method of
finishing many steel parts is to simply rough them out in a lathe to
within, say, 1/32 inch of the required diameter and take the finishing
cut in a cylindrical grinding machine.

[Illustration: Fig. 15. Roughing Cut--Light Finishing Cut and Coarse
Feed]

=Effect of Lubricant on Cutting Speed.=--When turning iron or steel a
higher cutting speed can be used, if a stream of soda water or other
cooling lubricant falls upon the chip at the point where it is being
removed by the tool. In fact, experiments have shown that the cutting
speed, when using a large stream of cooling water and a high-speed steel
tool, can be about 40 percent higher than when turning dry or without a
cooling lubricant. For ordinary carbon steel tools, the gain was about
25 per cent. The most satisfactory results were obtained from a stream
falling at a rather slow velocity but in large volume. The gain in
cutting speed, by the use of soda water or other suitable fluids, was
found to be practically the same for all qualities of steel from the
softest to the hardest.

Cast iron is usually turned dry or without a cutting lubricant.
Experiments, however, made to determine the effect of applying a heavy
stream of cooling water to a tool turning cast iron, showed the
following results: Cutting speed without water, 47 feet per minute;
cutting speed with a heavy stream of water, nearly 54 feet per minute;
increase in speed, 15 per cent. The dirt caused by mixing the fine
cast-iron turnings