cutting errors when hobbing
cutting errors when hobbing
(OP)
We received some worm gears from a new vendor and we're having problems with them. The gear boxes are noisy. I put dye on the teeth and the dye wore off near the top corner of the tooth. I'm guessing the hob and the blank weren't aligned properly when cutting the teeth. I don't know much about the hobbing process. How is a hob positioned on the blank?





RE: cutting errors when hobbing
RE: cutting errors when hobbing
RE: cutting errors when hobbing
You can encounter problems though if the width of the wheels varies too much. As the centring of the hob is usually only done once for a run, any variation in widths of the blanks is going to determine the position of the tooth profile.
When you “blued up” the test pieces was it done in a box or on test bench? If it was done in a box, were shims used to locate the wheel axially relative to the worm or was the set just assembled any old how? I would have thought that each set ought to be assembled by “blueing”. In a box running in one direction only, we set up wheels to slightly run off centre so as to allow oil to gather on the lead in edge. This is critical for a new set.
RE: cutting errors when hobbing
Our gear box has to run in both directions although the load is typically in one direction only. If everything were to print and at the nominal dimensions, the gear would run .006 offset from the worm centerline. I'm not sure why this was done (this was designed 40 years ago and has been working o.k.). Maybe it was to get the oil in as you said.
I talked to the vendor and he doesn't think it's probable that 1 or 2 gears in 10 would be off. He reminded me that we had talked and decided that we had agreed to decrease the backlash of the gear set by a few thousands. Maybe that has something to do with the problem. I asked him what was involved in setting up the hobber and he said they set the machine, run a gear, inspect it, make adjustments and then run the whole lot. He made it sound like there were two adjustments on the hobber - one to get the teeth centered on the width of the blank and the other an angular adjustment. Is that typical of hobbers (that there is an angular degree of freedom)?
RE: cutting errors when hobbing