What could be causing our hob machine vibrations?

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Posted by Ron on June 06, 2005 at 01:14:35:

Hello all,
I need your help. Our 900mm capacity vertical hobber is gradually
developing, what I can only describe as, a kind of harmonic vibration when
cutting 6 module or larger pitches on blanks larger than half the machine's
capacity. The vibration results in chatter marks being left on the tooth
form. When it begins to develop the vibration causes the whole machine to
develop a resonance that can be felt right through to the concrete slab
you're standing on! To make an attempt to rectify the problem we began by
looking at all the obvious things that might cause this eg; work holding,
tool holding, tool cutting edge condition, type of material the blank was
made from, adjusting gibbs on the head and saddle slide ways, saddle
clamping, re tensioning of nuts and bolts located throughout the machine,
tool spindle bearings/end & radial float, machine levelling, etc, etc. The
only thing we feel that is left to do is determining the condition of the
table area. We measured the circular backlash of the table's worm & gear to
be around 0.05mm or .002". We were unable to get any radial movement of the
table although we were able to get around 0.07mm of axial movement. We're
leaving the table's worm shaft bearings until last, although feel that if
they were damaged this would reflect in the table's circular backlash. The
only way to reduce the vibration is to lower the cutting speed to the point
we're its becoming uneconomical to do the job. If anyone has any useful
suggestions please post them.
Many thanks, Ron.

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