Posted by BillS on August 22, 2013 at 12:29:04:
In Reply to: hobbing high helix gears posted by steve green on August 22, 2013 at 11:33:55:
If you can set the table angle to 73.5 - RH hob angle (or to 73.5 + LH hob angle), you can probably find change gears and a setup. A RH cutter reduces table angle slightly, but that may not be the best choice, so we will explore further. As for machines, hobbers designed to cut worms as well as gears are most likely to work in this case.
One thing to consider is:
A large helix angle has a large force component that would act like climb cutting no matter from which direction the table advances. Think of it this way: 95% of the cutting force is tangent to the workpiece circumference for your large angle. For a LH workpiece, the direction tends to loosen a RH nut (if one is used to mount the workpiece on an arbor).
If you use a left hand hob, you would reverse the index gears with an additional idler to reverse rotation of workpiece. This would still tend to loosen a workholding nut, but would exert 95% of cutting force to oppose the index train, which is what you want. The LH hob would increase the table angle rather than decrease it, which may not be a problem.
Note that the size of the angle, not its hand, is the culprit. Perhaps a LH cutter might be a good choice for a large RH helix as well...
The trick is, what can you get away with without resorting to a new cutter.
Arthur Lehman experienced a problem that broke the cutter, and he did an excellant article about destructive vibration. My impression was that cutting forces were exciting the index worm and gear to dance, so that normal frictions were almost eliminated. This is a dynamic situation and cannot be tested with static forces. His was a large diameter workpiece with a large lever arm. (see his article). I suspect that he is climb hobbing when the viration "breaks out" so the cutter is trying to "pull" rather than oppose the index.
Normally you try to set up a hobber to produce cutting forces that oppose workpiece rotation. This dampens vibration, removes backlash, and avoids circumstances that would pull (or climb cut) rather than push against the workpiece.
So you want to pick a machine that is tight (minimal backlash) and cannot be easily back driven through the index worm and change gears. This might imply a single start index worm.
At least with 16 Pitch and small diameter (i.e. small lever arm), the cutting forces and excitation should be low compared to friction and momentum on a reasonably large and/or tight machine. But be aware of the possibility.
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