On 2017-03-18, wrote:

A friend of mine has a Bridgeport knee mill making a loud clicking or
knocking sound in the head. I could not find any obvious reason. So I
sorta just said sorry man I can't find anything. Well about 2 weeks
later Karma I guess, I have a large Jet knee mill 3 phase started doing
the exact same thing.So I am telling myself the same thing, (sorry man
can't find anything ). Has anybody got info as to a guess what it is ? I
am not a machinist just a hobbiest. Thanks for any input you might
offer.

Are these pure step-belt pulleys for speed control, or variable
speed pulleys? If the latter, the moving half of each pulley has a
black plastic (Delrin) sleeve inside it and a key on the shaft screwed
to the inside of the pulley with a Delrin cover over it to minimize play
and friction in the keyway. These eventually wear out, and need to be
replaced. If you don't replace them, the key starts wearing the keyway
in the shaft wider and wider, with a loud "CLACK" as the motor is
started or stopped.

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.

Good Luck,
DoN.
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