Tc 9-524 fundamentals of machine tools

4-17

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TC 9-524
The slower of the two recommended speeds is used for the
following formulas due to the varying conditions that may
exist, such as the rigidity of the setup, the size of the drilling
machine, and the quality of finish.
RPM = CSx4
D
Where RPM 
= drill speed in revolutions per minute.
CS 
= Recommended cutting speed in surface feet per
minute.
4 
= A constant in all calculations for RPM (except
metric).
D 
= The diameter of the drill itself.
For example, if a 1/2-inch (0.500-inch) twist drill is to cut
aluminum, the formula would be setup as follows:
RPM = 200 X 4 = 800 = 1600 RPM
.500
.500
Thus, the drilling machine would be set up to drill as close
to 1,600 RPM as possible. It is best to use the machine speed
that is closest to the recommended RPM.
metric system of measurement, a different
used to find RPM:
RPM = CS (m) x 320
D (mm)
When using the
formula must be
Where RPM 
= Drill speed in revolutions per minute.
CS 
= Recommended cutting speed in surface
meters per minute.
320 
= A constant for all metric RPM calculations.
D 
= Diameter of the twist drill in millimeters.
For example, if a 15-mm twist drill is to cut medium-carbon
steel, with a recommended cutting speed of 21.4 meters per
minute, the formula would be set up as follows:
RPM= 21.4 x320 = 6848
15 15
RPM = 21.4 x320 = 6.848 = 456.533 RPM
5
15
or 457 RPM
Round this RPM up or down to the nearest machine speed.
4-18
The speeds on these tables are just recommendations and can
be adjusted lower if needed, or to higher speeds if conditions
permit.