Tc 9-524 fundamentals of machine tools

LATHE OPERATIONS
faster for workplaces of small diameter and slower for
workplaces of large diameter. The proper cutting speed for a
given job depends upon the hardness of the material being
machined, the material of the tool bit, and how much feed and
depth of cut is required. Cutting speeds for metal are usually
expressed in surface feet per minute, measured on the
circumference of the work. Spindle revolutions per minute
(RPM) are determined by using the formula:
12 X SFM = RPM
3.1416 X D
Which is simplified to:
4 X SFM = RPM
D
Where SFM is the rated surface feet per minute, also
expressed as cutting speed.
RPM 
is the spindle speed in revolutions per minute
D 
is the diameter of the work in inches.
in order to use the formula simply insert the cutting speed of
the metal and the diameter of the workpiece into the formula
and you will have the RPM.
7-26

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TC 9-524
Turning a one-half inch piece of aluminum. cutting speed
of 200 SFM. would result in the following:
4 x 200
= 1600 RPM
1/2
Table 7-2 in Appendix A lists specific ranges of cutting
speeds for turning and threading various materials under
normal lathe conditions, using normal feeds and depth of
cuts. Note that in Table 7-2 the measurement calculations are
in inch and metric measures. The diameter measurements
used in these calculations are the actual working diameters
that are being machined. and not necessarily the largest
diameter of the material. The cutting speeds have a wide
range so that the lower end of the cutting speed range can be
used for rough cutting and the higher end for finish cutting. If
no cutting speed tables are available, remember that,
generally. hard materials require a slower cutting speed than
soft or ductile materials. Materials that are machined dry.
without coolant. require a slower cutting speed than
operations using coolant. Lathes that are worn and in poor
condition will require slower speeds than machines that are in
good shape. If carbide-tipped tool bits are being used, speeds
can be increased two to three times the speed used for high-
speed tool bits.
Feed
Feed is the term applied to the distance the tool bit
advances along the work for each revolution of the lathe
spindle. Feed is measured in inches or millimeters per
revolution, depending on the lathe used and the operator’s
system of measurement. Table 7-3 in Appendix A is a guide
that can be used to select feed for general roughing and
finishing operations. A light feed must be used on slender and
small workplaces to avoid damage. If an irregular finish or
chatter marks develop while turning. reduce the feed and
check the tool bit for alignment and sharpness. Regardless of
how the work is held in the lathe, the tool should feed toward
the headstock. This results in most of the pressure of the cut
being put on the work holding device. If the cut must be fed
toward the tailstock. use light feeds and light cuts to avoid
pulling the workpiece loose.