Posted by Tom Ramsey on November 29, 2011 at 10:47:58:
In Reply to: Re: lead angle correction - worm gear hobbing posted by BillS on November 29, 2011 at 08:43:45:
The further you move away from center the more the lead changes not to be confused with helix angle. The lead for the actual worm is calculated using the actual P.D. of the worm. Worm hobs are ground oversize so that the worm will mesh in the center of the throat of the worm gear and not straddle it.
When sharpened the O.D. of the cutter will be reduced to the point of no longer usable life but will still be ovesize. The helix angle does not change. This is why you set the machine to 0' unless the tool itself is marked with a "set to" angle.
You cannot use a regular hob unless the P.D of the hob is the same as the worm.
The ultimate proof is in the gear tester set up. Even the testers with vernier scales are not accurate enough. We set ours with a CMM. If you don't have that luxury you can do a check with a height gage and compare the axis'. Once you establish that you can check center distance in a similar fashion or use your gear tester.
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