Re: Chatter marks on gear teeth

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Posted by Ron V on March 25, 2013 at 17:50:37:

In Reply to: Chatter marks on gear teeth posted by Tim on March 23, 2013 at 01:07:20:

This is a very common problem and is usually as a result of exceeding the machine's capability.
The machine's capability could mean many things; diameter of the blank, diameter of the hob, pitch, cutting speed, helix angle, left or right hand helix, climb hobbing or conventional, distance from the mounting table to the blank, with tail stock or without, condition of the machine etc, etc.
As LBMAKEM points out; once the diameter of the job exceeds the diameter of the worm gear driving the table - you're in unchartered territory.
The best way to reduce chatter is to slow the cutting speed.
We installed variable speed drives on all our big hobbers. This allows us to tweak the spindle speed to suit the cutting conditions.
Originally the machine had a minimum speed of 45rpm. We found this to be way too fast if we were cutting a large diameter gear, in material like 4140, using a large diameter hob (5" - 6"). We can now cut those same gears @ 15 - 20rpm with no chatter marks. It's slow I know, but it allows us to do jobs when there is no time to outsource.
Below is a link to a video showing the set up of one of the jobs that we were never able to do before installing the VFDs.
The job is done in three cuts.
The machine is just a clunky old Russian made beast and is only rated to cut 2.5DP, in the video it's cutting 1.75DP in 4140.
Note how the job support is almost the same diameter as the root diameter of the job and also note the use of individual clamping bolts rather than just an arbor.

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