om 150 to 200 feet per
minute; for hard bronze, from 35 to 80 feet per minute, the speed
depending upon the composition of the alloy. While these speeds
correspond closely to general practice, they can be exceeded for many
machining operations.

The most economical speeds for a given feed and depth of cut, as
determined by the experiments conducted by Mr. F. W. Taylor, are given
in the table, "Cutting Speeds and Feeds for Turning Tools." The speeds
given in this table represent results obtained with tools made of a good
grade of high-speed steel properly heat-treated and correctly ground. It
will be noted that the cutting speed is much slower for cast iron than
for steel. Cast iron is cut with less pressure or resistance than soft
steel, but the slower speed required for cast iron is probably due to
the fact that the pressure of the chip is concentrated closer to the
cutting edge, combined with the fact that cast iron wears the tool
faster than steel. The speeds given are higher than those ordinarily
used, and, in many cases, a slower rate would be necessary to prevent
chattering or because of some other limiting condition.

=Factors which limit the Cutting Speed.=--It is the durability of the
turning tool or the length of time that it will turn effectively
without grinding, that limits the cutting speed; and the hardness of the
metal being turned combined with the quality of the tool are the two
factors which largely govern the time that a tool can be used before
grinding is necessary. The cutting speed for very soft steel or cast
iron can be three or four times faster than the speed for hard steel or
hard castings, but whether the material is hard or soft, the kind and
quality of the tool used must also be considered, as the speed for a
tool made of ordinary carbon steel will have to be much slower than for
a tool made of modern "high-speed" steel.

When the cutting speed is too high, even though high-speed steel is
used, the point of the tool is softened to such an extent by the heat
resulting from the pressure and friction of the chip, that the cutting
edge is ruined in too short a time. On the other hand, when the speed is
too slow, the heat generated is so slight as to have little effect and
the tool point is dulled by being slowly worn or ground away by the
action of the chip. While a tool operating at such a low speed can be
used a comparatively long time without re-sharpening, this advantage is
more than off