cutting errors when hobbing

[BobM3]

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?

 

[diamondjim]

Why do you think these were hobbed?

 

[BobM3]

They asked for and we sent them our hob.

 

[gearcutter]

Unfortunately if it’s a jobbing shop then usually the only way a hob can be positioned axially relative to the wheel is by eye. Having said that though, if the first wheel up on the machine has a centre line marked then, by using witness marks left by the rotating hob there should be no reason why a tolerance of 0.05mm can’t be achieved.
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.

 

[BobM3]

I "blued up" the gear in a gear box. I used the same gear box (and shafts, worm, bearings, etc) with an older gear that we had from the previous vendor. The older gear ran quiet and the dye was taken off across more of the tooth.

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)?

 

[gearpro]

Well if it were me I'd first figure out what size pins to check it with ( M.O.W.) usually a three pin check on a worm gear. Then check it with your old worm gears and see what the diffrence is. Take a blade mic and check the root diameter and compare it with your last good working gear you have had and also check tooth thickness. I havent been involved in worm gears for yrs, we only make helical and spurs, splines, sprokets these days.If your size is off or over sized parts, you'll get that gear box to bind up.Back in the mid 80s we didnt use hobs on worm gears. Looked like a large fly cutter more so then a hob.In regards to angular freedom, really theres not much. If your angles off your job is wrong.