machining most materials and give good cutting ability to the cutter without sacrificing strength of the
cutter. In milling softer materials, a greater rake angle can be provided to improve cutting ability.
Negative rake angles are provided on carbide-tipped cutters for high-speed milling operations. Since
the angles are both radial and axial, tool life can be increased by increasing the lip angle. For softer
steels, a negative rake angle of 5° to 10° is provided on plain milling cutters with teeth on the periphery.
This angle is increased when medium-carbon and alloy steels are being machined.
Clearance angles are kept on the small side to avoid weakening the cutting edge of the tooth. With a
minimum amount of material behind the tooth, the strength of the tooth is diminished. Clearance angles
of 3° to 5° are used on cutters over ~" in diameter. This is increased on smaller diameter cutters to
prevent the teeth from a rubbing instead of a cutting action.
The type of material being machined affects clearance angles. If cast iron is being machined, 4° to 7°
might be used; nonferrous materials require clearance angles of 10° to 12°. The land on a cutter can be
from 1/32" to 1/ 16" in width, with a secondary clearance back of the land.
ARBOR-MOUNTING MILLING CUTTERS
A milling machine arbor has a shank with a locating taper for locating it so that it will rotate concentrically
with the spindle. The arbor shank is driven by a key on the spindle nose. It is held to the spindle nose by
a draw bar that extends through the hollow spindle. After screwing the draw bar into the end of the arbor
shank for at least, four full threads. The nut is then tightened to hold the arbor firmly in the taper of the
spindle nose.
An arbor-mounting milling cutter has a central hole that closely fits an arbor diameter. A nut at the outer
end of the arbor is turned for tightening all collars and cutters on the arbor. Running the length of the
accurate cylindrical portion upon that the milling cutters are located, milling machine arbors have a
keyway, and thus a key may be used to drive a milling cutter.