Pie X diameter of the cutter, in.
Up milling is more commonly used because it is safer. With down milling damage may be caused to the
milling cutter, work piece, and milling machine Down milling the resultant force of the cutter upon the
work piece is directed toward and under the cutter.
This pulls the work piece under the cutter. It is better to have this resultant force directed in the opposite
direction as with up milling. If the work piece is not securely held, it will be drawn into the cutter so fast
that the cutter teeth are unable to make the cut and something will be damaged. If the work piece is held
properly, damage can still occur.
The entire milling machine table will be pulled ahead if any there is any play that is known as backlash,
exists. The next cutter tooth will probably have t~ much material to remove and the cutter will be chipped
or broken. Since down milling is better since it can be done safely, some milling machines are designed
for it. All backlashes must be eliminated.
SPEED, FEED, AND DEPTH OF CUT
Cutting speed as applied to milling can be defined as circumferential speed of the milling cutter
expressed in surface feet per minute (sfpm). It is the distance that the periphery of a milling cutter tooth
travels in one minute.
The revolutions (rpm) refer to the number of revolutions that the cutter makes in one minute. A small
milling cutter must rotate at a higher rpm to cut at the given cutting speed of a larger cutter. A small
cutter is more efficient because it travels a shorter distance.
Feed is the rate at that a work piece is moved toward a rotating milling cutter, that removes material
from its surface. Feed is limited by the depth of the material that can be removed by each tooth of the
milling cutter per revolution. This depth is called the feed per tooth, and its units are inches per tooth.
Feed for milling machines is given in inches per minute, because various milling cutters with different
number of teeth may be employed on a milling machine. The desired feed in inches per minute is set by
quick-change gears, and the power feeds are engaged by control levers at the front of the machine.
Power feeds are usually available for moving the table, saddle, and knee.
Trip dogs are set to disengage power feeds at the correct positions. They are especially useful when
more than one similar work piece is to be machined. Ordinarily it is best to use the largest feed per
tooth that can be employed safely. By doing this it will reduce, the time required for a milling operation
and increases the life of a milling cutter between resharpenings.
Milling cutter life is increased, because the number of cutter-tooth contacts with the work piece surfaces
is reduced. Alight feed may have to be used for a fragile work piece or when it is difficult to hold a work
piece securely. Depth of cut is the normal distance between the work piece surfaces before and after