How are "proper" bevel gears cut?

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mcenhillk
Posts: 17
Joined: Sun Aug 22, 2021 6:28 am
Location: Nebraska

Re: How are "proper" bevel gears cut?

Post by mcenhillk »

From watching the video I've posted a few times, it looks like the two cutter wheels change the included angle between them as they rotate around. I haven't figured out why the the gear rotates as the cutters move. I'm assuming that is how the cross section of the tooth profile changes but I'm not betting ANY part of my paycheck on that answer.
TomB
Posts: 495
Joined: Mon Jan 14, 2008 7:49 pm
Location: Southern VT

Re: How are "proper" bevel gears cut?

Post by TomB »

When I owned the ski mountain I used to depend upon a machinist that had a small gear cutting business. He was not some old machinist that had been at it for decades, in fact he was much younger than me. He had numerous new and automated gear cutting machines, located in a shop that had been a Fellows Gear Cutting prototype shop before they had moved to N. Springfield VT. Whenever I'd have to make a hurried run to get something on my ancient ski lifts replaced I'd see dozens of his big machines incrementally hogging out complex gears. When they were nearing completion I could recognize them but I could not follow the machining process well enough to understand how they did their thing. I was not going to him for complicated gears just ordinary small parts and he or his helper could make them on the ordinary lathe and mill off on the side while the expensive machines turned out the cash crop for his small business.

I wish that I could have taken the time to learn more about his shapers (I have several engineering degrees so I could probably follow an explanation) but he was doing me a favor and I did not want to impose on his time any more than necessary.
KellyJones
Posts: 189
Joined: Mon Nov 29, 2010 10:10 pm
Location: Snohomish, WA

Re: How are "proper" bevel gears cut?

Post by KellyJones »

Perhaps I can help. Before I retired, I was the Bevel Gear focal for Boeing Commercial Airplanes engineering department. I spent many hours down in Portland working with the gear cutting machinists.

There are two main ways to cut straight tooth bevel gears. The first is a shaping operation on a two tool generator (Gleason No. 14 machine). The second is a milling operation using two interlocking disc cutters. (If everything works, I have attached a picture of how the interlocking discs work.)

When the discs plunge into the work, they cut a rather narrow slot near where the discs intersect. As you move away from this point, the edges of the discs move away from one another and cut a wider slot. Since the teeth on the discs are straight sided, the work piece (and the discs) roll as if the gear is rotating against a mating gear. This motion generates the involute profile. Finally, the work piece also rotates slightly about an axis 90 degrees to the rotation axis ad this motion created the lengthwise crowning found in Coniflex gears. On older machines, you are still cutting one tooth space at a time.

On more modern machines, the cutter disc geometry is different and you can rotate the work under the cutters and effectively cut all the teeth simultaneously, much like hobbing a spur gear. Usually, the cutter teeth are set 90 degrees to the plane of the cutting disc and the cutting tooth cuts progressively deeper (wider tooth space) as both the tool and the work rotate.

Hope this helps
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Kelly Jones, PE
A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
George Bernard Shaw
(1856-1950)
pete
Posts: 2518
Joined: Tue Feb 10, 2009 6:04 am

Re: How are "proper" bevel gears cut?

Post by pete »

Many thanks for clarifying how it's done Kelly. That explains a lot I hadn't known and I'd guess it's how the profile grinding gets done with probably the same method.
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