Misalignment, Part Movement, or Deflection
Because a hollow mill has a balanced cutting action, it will try to center
the part, thus moving the part. When the tool is stationary, as you reach
the end of the cut, near the spindle, the part will no longer move to let
the stock get into the center of the hollow mill. the tool will cut small
at the end of the cut and leave drag marks when retracted. Misalignment can
be proven to a customer if the tool is stationary by removing two opposite
blades. depending upon whether the misalignment is horizontal or vertical
moving two opposite blades will correct the misalignment. By turning the
tool to the perfect spot you should be able to cut a perfectly straight
part with two opposite blades.
If the tool is rotating but misaligned you should get a correct sized part
eccentrically off center. You can still have drag mark problems and a slightly
tapered cut, even when the tool is rotating, if there is part deflection or
movement. In that case, drag marks should appear only on one side of the part.
If all blades are not ground alike, this can produce the same effect as
misalignment if the tool is stationary. If the tool is rotating with blades
that are not identical, the tool probably cuts slightly oversize with one
blade doing all of the cutting.