Axial rake has a significant effect on axial force and the thrust applied to the spindle and the work piece. The more positive the axial rake, the less axial force. Negative axial rake increases axial force. Positive axial rake lifts the chip away from the milled surface while a negative rake forces the chip back toward the surface.
Radial rake has a major effect on tangential and radial forces. Positive axial rake reduces these forces, minimizing burrs and break-out.
Double negative cutters provide economy and the edge strength required for hard materials and interrupted cuts. But strength is often not required on light finishing cuts and economic gains may be offset by the time spent trying to attain a desired finish. Double negative geometry “pushes” rather than cuts. These higher forces consume more horsepower and create more pressure and heat.
Double positive cutters offer freer cutting action and consume less horsepower but have weaker cutting edges. Lower cutting forces direct less force against the work piece and machine, so there are fewer tendencies to chatter or deflect. Remember, however, that too high a positive angle can tend to reverse the force and lift the work piece into the cutter.
Negative/positive cutters provide the best cutting geometry for finish milling. Positive axial rake, negative radial rake, and the proper lead angle cause chips to be lifted up and out from the finished surface to clear the cutter and work piece. This type of cutter combines the best aspects of negative and positive geometries and provide extremely free cutting action. Excellent chip evacuation leaves the finish-milled surface free of scratches and scoring.
Lead angle also affects cutting forces. Increasing the lead angle from 0° lessens radial force slightly and increases axial force significantly. Generally, chip evacuation is easier with a higher lead angle.