Face milling cutters of less than 6 in. diameter are sometimes called shell end mills. The cutter body, made from heattreated alloy steel,can be used almost indefinitely, and only the cutting inserts need to be changed. Face milling cutters are usually mounted directly on the spindle nose and held in position by four cap screws. The back of the cutter is counterbored to fit closely over the outer diameter of the spindle
nose. The driving keys of the spindle nose fit the keyseats in the back of the face mills (Figure K-81). Another method of mounting face mills is the flat back mount (Figure K-82). It uses a centering plug located in the machine spindle taper to center the cutter. When a face mill is mounted on the machine spindle, it is important that all mating surfaces between the cutter and the spindle be clean and free of any
nicks or burrs. Even the smallest particle between the mating surfaces will make the required alignment of the cutter impossible.
After the cutter has been mounted on the machine, a check with a dial indicator should show the cutter to run true or to be out of alignment less than .001 in. Face mills are made as light-duty cutters with numerous teeth for finishing cuts or as heavy-duty cutters with fewer teeth and heavier bodies for roughing operations. Positive-rake angles produce a good surface finish, increase cutter life, and use less power in cutting (Figure K-83). Positive-rake angles are effective in machining tough and work-hardened materials.
Cutting pressures, which may deflect thin-walled workpieces, are smaller with positive-rake cutters than with negative-rake cutters under the same conditions. Zero or negative-rake cutters are strong and will give good service under heavy impact or interrupted cutting conditions (Figure K-84).
Negative-rake inserts create high cutting pressures, which tend to force the workpiece away from the cutter. Negativerake inserts should not be used on work-hardened materials or ductile materials such as aluminum or copper.