Re: Breaking of hob when cutting high helix angle gears


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Posted by Bill S on July 29, 2003 at 13:18:17:

In Reply to: Breaking of hob when cutting high helix angle gears posted by Russell on July 29, 2003 at 12:06:04:

One thing I would look for is the effect of climb cutting. If the cutting action is climb, then like a milling machine, the cutter tends to pull itself into the workpiece. If machine allows this to happen (due to backlash, spindle flexure, etc.)this can put enormous chip load on the cutter.

Usually, you want to cut such that the cutting force opposes the feed direction (and opposes the workpiece direction of rotation). Otherwise, you depend completely on your machine having no backlash in table or work spindle.

I think, since the cutter is LH and the workpiece is LH, the workpiece rotation is such that the cutter is trying to pull the workpiece ahead of its normal rotation. At low helical angles, this may not be a problem, but at 53 Deg, a large component of cutting force is pulling the workpiece in the direction of rotation. It takes very little motion to cause excessive load on the cutter tooth, and perhaps overcome the radial feed pressure as well.

You can look for signs on the other pieces cut before. If the tooth is roughly cut, suspect spindle flexure and backlash. If not then suspect a sudden "jump" of the radial infeed caused by cutting force, or slack in the table ways, or even loosening of the workpiece on the spindle.

A right hand cutter would avoid this scenario, since the setup for a RH cutter would reverse the workpiece rotation and make the cutting forces oppose workpiece rotation, and machine backlashes.



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