How are "proper" bevel gears cut?

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mcenhillk
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How are "proper" bevel gears cut?

Post by mcenhillk »

I was reading Ivan Law's book on Gears and Gear Cutting last night in bed (as one does) and read that he would be describing a way to approximate cutting bevel gears because proper bevel gears require specialized equipment. This naturally kept me up all night thinking about how conical involute gear teeth are cut. I've already search this forum and the web for discussions about how these specialized machines work but I haven't been able to find much on the subject. I'm not looking to make one of these machines, that would be crazy (crazy enough to work? don't know), just to understand how they work. So, does anyone know of a resource for how proper bevel gears are cut?
pete
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Re: How are "proper" bevel gears cut?

Post by pete »

I would guess Ivan meant a gear hobber. The gear blank and the gear hob are directly geared together for the correct number of teeth. Making one in your own shop? Google Jacobs Gear Hobber. Castings and drawings are available from Collage Engineering Supply in the U.K. Afaik it's capable of cutting crown or bevel gears with some possible shop made extras added. A Google search might turn up exactly how it's done as well. Pre CNC, manual gear hobbing is widely believed to be the most accurate way of producing a true profile to the gear teeth. Blade type cutters to the B&S design only produce an approximate tooth profile for the high and lower tooth counts there optimized for.
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Bill Shields
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Re: How are "proper" bevel gears cut?

Post by Bill Shields »

They still are...
Too many things going on to bother listing them.
mcenhillk
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Re: How are "proper" bevel gears cut?

Post by mcenhillk »

Thanks for the name of that machine pete. Now that I'm starting to build up a small shop, that looks like a dangerously fun project!

However, I don't understand how bevel gears could be hobbed. The diametrical pitch of the bevel changes with the diameter of the gear. A hob has a single DP. I get how Law fakes it with a single cutter and an offset (at least I think I get it). Are all bevel gears approximated because the true conical teeth are too hard to actually manufacture?

Or is this a fundamental misunderstanding about how gears are made?
earlgo
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Re: How are "proper" bevel gears cut?

Post by earlgo »

Two textbooks are on my bookshelf that have full chapters/sections on designing and machining bevel gears.
The first is "Machine Tool Operation - Part II" by Henry Burghardt, McGraw-Hill Book Company Inc copy right 1922 and 1937 (The Maple Press Co, York, PA)
The second is "Advanced Machine Work" by Robert Henry Smith (MIT) 9th edition, The Industrial Education Book Co. Boston, U.S.A. Copyright 1910, 1912, 1915, 1917, 1919, 1922, 1925. (Stanhope Press Boston, USA)
((This was one of my Dad's textbooks at Ohio State University in 1930))
Needless to say, it is a complicated procedure from design, which is covered in detail, to machining, also covered in some detail.
I'm sure every public library has a copy of each. :lol:
This is not a really useful answer, but fun to look up.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
Wolfgang
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Re: How are "proper" bevel gears cut?

Post by Wolfgang »

If memory serves me, accurate bevel gears are produced on a gear shaper.

These machines consist of two shaper rams, each of which may be adjusted such that the appropriate angles and tapers are produced.

Further, to produce the correct tooth profile the blank has to roll while the ram assembly moves parallel to one of the gear's features. This motion is separate and different from the ram's cutting stroke.

All in all a very complex piece of machinery, including the set-up for producing a given gear size, tooth count and pitch.

If anyone here is experienced with such an animal perhaps he could explain it in better detail
Russ Hanscom
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Re: How are "proper" bevel gears cut?

Post by Russ Hanscom »

Somewhere, and I am unable to find it at the moment, or remember the name, there are plans for a combined feed and indexing device that allows true bevel gears to be cut on a common shaper. I had thoughts of making the device once. Obviously a very slow process as only one tooth is cut at a time, but if you need to replicate a particular gear, it will get you there eventually.
pete
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Re: How are "proper" bevel gears cut?

Post by pete »

Yep Wolfgang brought up something I'd forgotten about, those gear shapers. Youtube even has some videos showing how they operate. Maybe a bit outside what you might find in a normal home shop though. :-) As far as the actual hobbing of bevel gears, I'm out of information. I've seen it mentioned that the Jacobs machine can be set up or maybe a better word would be forced into doing it, but that was a long time ago. Another search term for you Mcenhillk is gear skiving. But I'm not sure if that process can do bevel gearing or not. Machining of gear teeth is still a few steps down from high precision gearing though. Grinding complicates it even more. How do they grind a slowly changing tooth profile along the length of bevel gear teeth with manual equipment?

As far as building that Jacobs Gear Hobber. I know there were some build articles, machine set up tips while hobbing gears, and I think some well thought out additions to the original design published in the Model Engineers Workshop magazine in the 1990's - early 2000's. And there may have been more since. Even a one year digital subscription to that magazine gets you access to all those back issues. Additionally gear hobbing is a bit cheaper to buy a single diametrical or module hob that can cut the full range of tooth counts than buying a full set of the B & S type to do the same. And with a manual mill and dividing head and cutting each tooth separately verses hobbing a completely finished gear, it's probably at least 50 times quicker. You'd need quite a few gears to justify building it though. https://www.youtube.com/watch?v=L2M6PYa6mZo has started building one, but he's a bit slow and random with his video production.
mcenhillk
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Re: How are "proper" bevel gears cut?

Post by mcenhillk »

Thanks for the responses. That is exactly what I was looking for, leads on books and terms that I can search for. Found this video of the Gleason 104 running. https://www.youtube.com/watch?v=QTlo6bIIieE

Seems simple enough. Should have one scratched built in a month of Leap Days. :-D
Wolfgang
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Re: How are "proper" bevel gears cut?

Post by Wolfgang »

mcenhillk wrote: Sat Sep 18, 2021 8:11 pm Thanks for the responses. That is exactly what I was looking for, leads on books and terms that I can search for. Found this video of the Gleason 104 running. https://www.youtube.com/watch?v=QTlo6bIIieE

Seems simple enough. Should have one scratched built in a month of Leap Days. :-D
Thanks for that Youtube reference! The various motions are fascinating to watch as they are very subtle. Never seen this kind of gear machining before; not sure that it is a hobbing process, though.

Too bad one cannot see the cutter's teeth. I think they are straight sided, but that is only my opinion.
whateg0
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Re: How are "proper" bevel gears cut?

Post by whateg0 »

I can't see how a bevel gear could be hobbed since the "pitch" effectively changes from the perimeter to the middle.
pete
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Re: How are "proper" bevel gears cut?

Post by pete »

I just don't know enough about how bevel gears are made at production levels or on that Jacobs machine to know for sure so I'm at best guessing about them being hobbed. With larger and much more expensive gears then yes I can see those gear shapers being required, smaller one's then it would be too slow so there has to be a way that's fast. Then again it also depends on the application range the bevel gear is meant for. High loads and extra precision might use one method including post heat treatment grinding. For light loads, lower precision a fairly inaccurate hobbed gear might be good enough? Then there's items like rear wheel drive crown and pinion gears, heat treated, ground on a slow radius for the crown gear and tapered on the pinion, yet there made in the thousands per day to some pretty repeatable levels of precision.
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