Gear hobbing process question, what causes difference on one flank vs the other

[DDoug]

How is he going to measure it ? Without very specialized equipment, you can't. CMM doesn't cut it.

It's like heat treating, only worse. Most people don't have Rockwell testers and have to go by what the heat treater says. If the heat treater messes up, how will you know ?

Gear measuring equipment is way more complex than that. I've only had one or two customers in my entire life who could measure the gears I've made.

Another problem is, all those numbers that software puts on a data sheet ? Pretty much meaningless. There's only a couple that actually have any effect. So that's also another layer of b.s. in the process.

So the vendor quotes a job, but can't inspect/verify ?
Maybe the OP should send some of the gears out to a place that can verify those numbers "independent Inspection".

 

[EmGo]

So the vendor quotes a job, but can't inspect/verify ?
Maybe the OP should send some of the gears out to a place that can verify those numbers "independent Inspection".

It's not that simple. Many, or even most smaller places that cut teeth can't do analytical measurements. And the ones that can, charge for the extra labor.

And then you have to be able to interpret the charts, which is a skill in itself.

All you'd see from an expensive inspection on these is, there's a big chunk out of the teeth on one side. Well whoop-dee-doo, I can see that with my encalibrated eyes

Gear machines are not like lathes and mills. If they are set up correctly, and the measure over wires or span measurement is correct, then the teeth will be correct.

The problem here is that (from looking at the photos) the part moved on the arbor. It's too big of a gouge to be just a dull cutter and appears to get deeper with successive teeth. This is what a shaper can do if the cutter or part moves. But we don't have enough info to determine that specifically, just a guess on my part from seeing this before. And it really doesn't matter, that particular gear is scrap. Time to check the rest. If ALL of them are like that, then more investigation is needed.

p.s. It's not even necessarily that the vendor was irresponsible. If this is just one part, stuff can happen. You make fifty, check the first one and maybe every five in between for size, or even every one, but if one blank had a problem -- was .010" thinner than your fixture, or the guy thought it was tight but wasn't, well, a bad part does slip through sometimes. Life happens, to all of us.

I still think the design is bad from a quietness standpoint tho.

 

[DDoug]

Learned something new today, disappointed but did learn something.
so all those numbers and tolerances my engineer sweated over don't get checked ?
I recall sending cut sections of teeth to a lab for micro hardness testing (to check case depth & induction depth) all kinds of testing.

 

[EmGo]

so all those numbers and tolerances my engineer sweated over don't get checked ?

Most of them, no. And to be honest, there was no reason for him to sweat over them. There's a few things that do count. If you get these right, the rest of it don't make no nevermind (unless maybe the parts are going into space and have to work for fifty years at minus 300 F.)

I recall sending cut sections of teeth to a lab for micro hardness testing (to check case depth & induction depth) all kinds of testing.

Hardness is one of the things that matters. Tooth-to-tooth spacing matters. The lead matters. Involute matters. But those pages of numbers that a computer program spits out ? Most of it is irrelevant. And a lot of the stuff you get off a tester ? It takes some experience to interpret those charts. Even there, some things matter while others do not.

For instance, on highly stressed parts you may be more concerned with grain flow than you are with the lowest point of single-tooth contact (a number that computers may care about but in real life, not so important.) So it could be more worthwhile to cut the part apart and do nital etch or x-rays and so on to see the grain structure, rather than care about some of those non-essential numbers that software likes.

It's kind of like CAD programs that are so proud of themselves for drawing teeth. But no gear shop needs that crap. We don't draw teeth, it's a waste of time. Looks cool for kids but makes no difference to the product. All you need is the blank, a circle. Circle is pretty quick to draw and does the job. In practice, with the proper setup and good cutters, size measurement will do the job. Maybe add lead in there, and if it's a better-than-9 gear, run an involute check on the first part just to feel happy. For almost all commercial gears, that'll do it.

edit: If you buy a lot of gears and want to qualify them better ? I'd say a little rolling tester would be valuable. If the parts roll together smoothly without more than a few thou in center variation, that's a worthwhile incoming check to do. If they don't, then go to a more thorough bunch of tests. Otherwise, you're good to go.

 

[SBAER]

I have actually had the cost of inspecting a set of gears that make up a reducer by a third party using one of those specially equipped CMMs . It was in the range of $3000 for 5 different gears.

 

[EmGo]

I have actually had the cost of inspecting a set of gears that make up a reducer by a third party using one of those specially equipped CMMs . It was in the range of $3000 for 5 different gears.

Ouch. This is why I shake my head when people talk about "bringing back manufacturing." I'm sure this is not totally unreasonable for a US company to charge. Take the price of the machine, training, land and building costs, admin overhead, insurance, utilities, all the stuff that having a shop requires, and this is probably what they have to charge.

For one-third that cost me and the Assist can hop on a train, go up to Nanjing Two, throw the gears on their late-model Zeiss gear tester, spend the night, have dinner for the qc guys in the shop, pay the factory, and go back home the next day. Sometimes the shop guys insist on buying our dinner instead, on the factory. China wants manufacturing. They make it easy. The US does not.

Fight it all you like but that's like emptying the ocean with a teaspoon. Most of the people in the US do not want manufacturing. All of the ones with power. We're a tiny minority.

 

[SBAER]

Valid point on the difficulty of bringing back manufacturing. The whole manufacturing ecosystem is degraded in NA at this point.

 

[EPAIII]

Say all you want about why there is no manufacturing in the US, but there is one reason and one reason only and that is COST. Plain and simple, products, at least most products, just cost less if made in other countries. And that includes the cost of shipping them half way around the world. The US priced itself out of business and I do not see that changing any time soon.

I am not bringing politics into this, it is just a plain, simple fact.

Ouch. This is why I shake my head when people talk about "bringing back manufacturing." I'm sure this is not totally unreasonable for a US company to charge. Take the price of the machine, training, land and building costs, admin overhead, insurance, utilities, all the stuff that having a shop requires, and this is probably what they have to charge.

For one-third that cost me and the Assist can hop on a train, go up to Nanjing Two, throw the gears on their late-model Zeiss gear tester, spend the night, have dinner for the qc guys in the shop, pay the factory, and go back home the next day. Sometimes the shop guys insist on buying our dinner instead, on the factory. China wants manufacturing. They make it easy. The US does not.

Fight it all you like but that's like emptying the ocean with a teaspoon. Most of the people in the US do not want manufacturing. All of the ones with power. We're a tiny minority.