DoN. Nichols wrote:


If the step belt pulleys version -- I don't know, but perhaps
the spline which couples the upper pulley to the spindle is dry and
worn.
OK, two more things to think of. If the spindle spline is badly worn, it
will clatter when taking short (as in along the length of the cutter) cuts.
What I mean here is that the cutting flute is not continuously cutting
workpiece material, like when cutting 1/8" sheet with a 1" end mill, for
instance. Each cutter flute digs in and the comes free. This will rattle
the spline. Easy to check, just twiddle the spindle nose and see how much
it can be turned CW/CCW before it hits the other side of the driving spline.
This will generally only rattle when cutting, and run totally quiet when
running without cutting.

One other thing is a shot V belt. If the V belt has started to break down
in one spot, it will ride lower in the pulley V groove. As the thin spot
alternately rounds the driven, then the driving pulley, it will cause large
speed-up/slow-down of the spindle. It can actually start the whole machine
shaking! I saw this on a lathe once, it was QUITE dramatic.

Jon