Plain milling cutters are designed for milling plain surfaces where the width of the work is narrower than the cutter (Figure K-14). Plain milling cutters less than in. wide have straight teeth. On straight-tooth cutters, the cutting edge will cut along its entire length at the same time. Cutting pressure increases until the chip is completed. At that time the sudden change in tooth load causes a shock that is transmitted through the drive and often leaves chatter marks or an unsatisfactory surface finish.
Light-duty milling cutters have many teeth, which limits their use to light or finishing cuts because of insufficient chip space for heavy cutting. Heavy-duty plain mills (Figure K-15) have fewer coarse teeth, which makes for strong teeth with ample chip clearance. The helix angle of heavy-duty mills is about 45 degrees. The helical form enables each tooth to take a cut gradually, which reduces shock and lowers the tendency to chatter. Plain milling cutters are also called slab mills. Plain milling cutters with a helix angle of more than 45 degrees are
known as helical mills (Figure K-16). These milling cutters produce a smooth finish when used for light cuts or on intermittent surfaces.
Plain milling cutters do not have side cutting teeth and should not be used to mill shoulders or steps on workpieces.