11. At what angle should the platen of miller be set to
cut 1 J in. dia. and 8 in. lead?

12. What is the angle at which to set the miller platen
for a 3 in. dia. mill to cut a spiral tooth 28 in. lead?

13. Find the angle for 3 in. mill, one turn in 40 inches.
14. If the spiral teeth in a 3 in. dia. roll are at an angle
of 30°, what would be the length of the roll for one complete
tooth?

15. A spiral gear 10 in. dia. has teeth at 45° angle. What
is length for feed of platen in miller for one complete turn
of gear on centers of spiral head?



SHOP MATHEMATICS 127

16. A milling machine platen has a feed of 2 in. per min-
ute and is cutting a spiral flute reamer | in. dia. with 8 in.
lead. Find angle at which to set the platen and the time re-
quired to cut a flute of 1^ turns.
SPEEDS AND FEEDS

The amount of feed per revolution of a milling cutter
depends somewhat on the number of teeth in the cutter.
For backed off mills the following table is considered good
practice:

Brass, 120 F, P, M,, feed 4§ inches per minute, \ inch deep.
Cast Iron, 60 F. P. M., feed 4f inches per minute, i inch
deep.
Wrought Iron, 48 F. P, M., feed 2 inches per minute, \ inch
deep.
Steel, 36 F, P. M., feed 1^ inches per minute, i inch deep.
In the following problems use the approximate formula

AC
i2=~- for R, P. M, unless otherwise stated.
a

PROBLEMS

17. What speed is required in R, P, M, for a 3^ in. dia.
