How Do You Mill Formed Surfaces? Tips and Precautions

Using a Form Cutter – Milling Formed Surfaces

The cutting edge of a form milling cutter is designed and manufactured to match the workpiece geometry. Its profile is the inverse of the required workpiece surface.

Common form cutters include pointed-tooth and relieved-tooth designs. Pointed-tooth form cutters are difficult to manufacture and sharpen, so relieved-tooth cutters with an Archimedean-spiral flank are commonly used. If the rake angle is not zero, the cutting-edge shape must be corrected by calculation.

A form cutter has a complex edge profile, produces substantial chip deformation, and carries high milling forces. Observe the following precautions:

1. Form cutters are harder and more expensive to manufacture than general-purpose cutters. Use a milling speed and feed rate 20% to 30% lower than for an ordinary cutter. If cutting-edge diameters vary greatly, select speed according to the largest diameter to improve tool life.
2. If the formed surface has a large machining allowance, perform separate roughing and finishing operations. For roughing, use a cutter with a larger rake angle and chip-splitting grooves on the flank to reduce cutting force and improve conditions. For finishing, use a cutter with high profile accuracy and a rake angle γ_w = 0°.
3. Pointed-tooth form cutters must be sharpened with special equipment. On a relieved-tooth cutter, sharpen only the rake face and strictly retain the rake angle used during manufacture.
4. Control flank wear rigorously to extend cutter life.

Using a Gang of Form Cutters – Milling Formed Surfaces

When a surface consists of several straight segments or a combination of straight and curved segments, several form cutters can be assembled into a gang. The combined cutter profile is the inverse of the workpiece surface, and the cutter length and height at every profile point must match the workpiece exactly.