Surface finish is the result of tool marks or irregularities left by the cutting edges of a milling cutter. These irregularities are:
Roughness: the measurement of tool marks in terms of RMS (Root Mean Square); measured with a relatively short sampling length and suppresses waviness.
Waviness: widely spaced irregularities that underlie the roughness.
Flatness: the overall condition of the entire milled plane; measured by large straightedges or feelers.
Laps: blending of successive passes; normally a function of how well the milling head is squared to the table and how rigidly it is held in position.
Bearing: the supporting quality of a milled surface; a combination of all these irregularities.
How the machine affects finish.
The entire machine setup must be rigid since any type of looseness or lack of rigidity will affect a milled surface finish. The “heel” or trailing edge of the cutter should clear the work piece. Therefore, the spindle should be tilted very slightly in the direction of feed.
If the cutter is flat to the work piece, (1) the finished surface is recut by the back side of the cutter, (2) the cutting edges can carry small chips that scratch the surface, (3) more friction creates heat build-up in the work piece and cutting edges, and (4) increased cutter contact can induce chatter.
Too much spindle tilt creates excessive “dish” or scallops. The effect is magnified as cutter diameter increases.