For good finish milling operation the entire machine set up must be rigid and light because any looseness or lack of rigidity in machine affects the finish adversely. For best results, the spindle should be tilted very slightly in the direction of feed so that cutter does not lie flat to the workpiece.
If the cutter lies flat to the workpiece, then the finished surface is recut by the back side of the cutter, the surface is scratched by the small chips carried by the cutting edges; increased friction creates heat build-up in the workpiece and cutting edges and affect finish and tool life ; chatter is induced due to increased cutter contact.
The spindle tilt should not be much which will result into creation of dish or scallop on the workpiece surface. If the milling machine head is loose on its ways or not in proper alignment to the work mounting surface, the spindle tilt will vary with the constantly varying cutting forces.
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Other tips for good finish in milling are:
i. Use climb or down milling. In conventional or up milling, the cutting edge starts with zero chip thickness and digs into the material; creating lot of rubbing action before the edge can actually get a bite. This rubbing action causes pressure and heat build-up at the finished surface. The pressure and heat developed may also cause the thin chip section cut first, to weld to the cutting edge and be carried around to scratch the finished surface in the next pass.
ii. Cut in the same direction when multiple finish milling passes are required. The direction of cut should preferably be in the direction that forces the milling head into its supporting ways. The depth of cut setting should never be changed between passes. Similarly avoid changing the indexing inserts or changing cutters between passes.
iii. Speed should be high and feed per tooth less, and depth of cut as light as possible (just enough to clean up the surface completely).
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iv. Keep heat out of the part to avoid distortion. Correct coolant in copious amounts should be used.
v. Workpiece surface should be clean so that sharp edges of the finish cutter will not be nicked by passing through rust, scale, or hard spots of un-machined surfaces.
vi. Number of cutting edges in a given diameter cutter should be less dense.
vii. Spindle bearings should be kept tight and spindle run-out maintained to a minimum.
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viii. Cutting edges should be sharp so that they will shave the metal and not rub it. They must also be free of nicks that can scratch the finished surface.
ix. As far as possible, use positive-negative (shear angle) cutters for finish milling. The positive axial rake, combined with negative radial rake plus a bevel angle, cause the chips to be lifted upwards and out from the finished surface and thus constantly thrown clear of the cutter.
Shear angle cutters combine the best of both double positive and double negative types and provide extremely free cutting action. The cutter edge is sharp (when shaving the surface), strong (to take large cutting tangential forces) and has excellent chip evacuation.