Author Topic: Bevel Gear Mill  (Read 10389 times)

Offline mattinker

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Re: Bevel Gear Mill
« Reply #25 on: August 23, 2020, 01:38:11 PM »

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #26 on: August 23, 2020, 02:01:34 PM »
Nope Matt, I haven't. Very interesting, and that must be the Model Engineer/metal tape reference made earlier. I wonder if the same idea could be applied to a fly cutter on a horizontal mill (or on a lathe with boring table)?
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mattinker

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Re: Bevel Gear Mill
« Reply #27 on: August 23, 2020, 02:15:46 PM »
 The shaper is easier than a mill because of the cut direction in relation to the table travel! Have a look at this video!


Offline vtsteam

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Re: Bevel Gear Mill
« Reply #28 on: August 23, 2020, 02:20:54 PM »
Yes, but we'd have to make the shaper, and do that with a vertical slide.

A horizontal mill without vertical feed is easier to make.

And a boring bar between centers on a lathe (with a boring table) is simpler even than that.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline djc

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Re: Bevel Gear Mill
« Reply #29 on: August 23, 2020, 03:03:48 PM »
...I wonder if the same idea could be applied to a fly cutter on a horizontal mill (or on a lathe with boring table)?

Yes it can. It is just a means of getting co-ordinated linear and rotational motion. It is no different in concept to using gears from the mill leadscrew to drive a dividing head for helical milling. Does the same thing without the gears.

As to the machine, have a good sniff around the internet archive for gear-cutting books. There are many old ones now online and being in US, you have better access to Google books than we do.

The classic bevel gear people are Gleason and Bilgram. You can find all their patents online and you can probably find operation manuals for some actual machines at vintagemachinery.org .

You can get some good leads if you put 'bevel gear planer' and 'bevel gear shaper' into Google and look at images. E.g.

https://www.homemodelenginemachinist.com/threads/bilgram-bevel-gear-generator.9042/
https://www.practicalmachinist.com/vb/antique-machinery-and-history/bilgram-bevel-gear-attachment-342628/

See also:

http://vintagemachinery.org/mfgindex/detail.aspx?id=2969&tab=7 (mentions Woodbury's  book - well worth reading)

http://blacksmithandmachineshop.com/a-My-Foot-Hand-treadle-powered-Machines-in-My-Shop-Collection.html (Brown & Sharpe #13H extract from American Machinist)

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #30 on: August 23, 2020, 03:55:59 PM »
Thanks djc, I have lots of gear cutting reference books, but appreciate the suggestions. Thanks definitely for confirming that a horizontal mill can be rigged the same way.  :beer:

The main disadvantage I see to that method is it takes probably a minimum of five passes to cut a decent shaped tooth, and probably seven would be better. If 7, that's 210 passes for a pair of 15 tooth bevel gears.

In addition, on a lathe, each pass would take an adjustment of the carriage, so, 210 adjustments as well. (A shaper with racheting automatic feed would be less tedious.)

With the profiled cutter method on the lathe you would take 90 passes for the same pair. and you would only make 6 carriage adjustments and 6 index adjustments.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #31 on: August 23, 2020, 04:01:08 PM »
hmmm, since I would normally want to make pairs, anyway, would there be a way of cutting two gears at one pass? Might be tricky with two indexing wheels, and getting the gear blanks both exactly positioned in height. But maybe one indexing wheel geared 1:1 to a second arbor.....  :loco:
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mattinker

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Re: Bevel Gear Mill
« Reply #32 on: August 23, 2020, 05:21:10 PM »
Steve,

I think that it is interesting to see the action of the shaper gear set up as it reproduces a rack cutting tool. I understand that your looking for a "simple" way around this, replacing the shaper ram with a flycutter and a reciprocating carriage which traverses to allow the the blank to rotate is possible. It's the involute gear that results from a "rack" profile cutter is the "magic" bit!

Cheers, Matthew

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #33 on: August 23, 2020, 06:36:47 PM »
Yes I agree Matt, I really like that part a lot.  :med:
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vintageandclassicrepairs

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Re: Bevel Gear Mill
« Reply #34 on: August 24, 2020, 03:18:31 PM »
Hi All,
I saw this book on ebay and thought someone here could be interested in Bevel gear theory ?
https://www.ebay.co.uk/itm/GLEASON-12-INCH-STRAIGHT-BEVEL-GEAR-GENERATOR-MANUAL-1941/264813653248

John

Offline awemawson

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Re: Bevel Gear Mill
« Reply #35 on: August 24, 2020, 03:46:18 PM »
I always fancied a 'Sunderland Gear Planer' but I'd have to reinforce the foundations and widen the farm gate  :clap:

Andrew Mawson
East Sussex

Offline engjas

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Re: Bevel Gear Mill
« Reply #36 on: August 26, 2020, 02:31:55 PM »
Hello
Have a look for articles by J. S. Eley on "Constructing a Gear cutting Machine" in Volume 100 of Model Engineer 1949. Long time ago yes but the machine he described was simple, bench top and had a bevel gear cutting facilities. The articles might stir your thoughts. I don't have access to my copies at present but you may find some closer to you.

Good luck on your journey.
John

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #37 on: August 26, 2020, 02:58:23 PM »
For me Andrew it was always building a Tom Jacobs gear hobbing machine which appeared in ME, and in modified form is still available as a set of castings from College Engineering. But that is a BIG project, and I still haven't even finished my lathe project (this year, for sure!)

Wow engias, I missed that one in ME. That's great, I will have to read the whole series -- as mentioned the Jacobs hobber was always a project I had in mind.

I keep waking up at about 4:30 AM by my mind already trying to solve bevel gear cutting problems and geometry. I'm thinking along the lines of Matt's suggestion of an acme/rack shaped cutter, but this time a mill type pivoting around a blank, and cutting a very close-to-true bevel gear involute. And then complicating that by cutting a pair of gears at once. I think it can be done, and I'm close to visualizing what that would look like. But I need a rest between noodling sessions to get it.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #38 on: August 27, 2020, 01:50:44 PM »
My understanding, now:

A rack or single tooth acme shaped cutter on a shaper can cut an ordinary spur gear to an involute profile by rotating the work on its shaft axis and faceting the tooth. It will always be a faceted tooth -- the more strokes per tooth, the more facets, and the finer it can approximate a smooth involute tooth form. This is illustrated by the rig that Matt linked.

A horizontal mill cutter of the rack form can do the same thing faster for the same number of facets, assuming it has more than one cutting tooth. Or cut more facets in the same amount of time, for a finer approximation of an involute tooth form.

In general, the rack shape is easier to form if making a cutter from scratch than a involute profiled cutter. And one rack style cutter serves for all gear tooth numbers, while an involute profiled cutter is suitable for only a range of gear teeth numbers and the actual tooth form cut is approximate.

A hob suitably geared to the work arbor and placed oblique to the work can be arranged to produce an un-faceted (smooth) involute profile because it acts like a continuously variable cutter. J Radford once did a lathe attachment that performed this way. As I visualize it, the tooth won't be completely uniform across the gear, but the profile will be involute at any particular cross section.

Moving on to bevel gears, a hob is not possible because the gear teeth converge.

A single tooth cutter can not be designed to cut a good involute bevel tooth on both edges, no matter how it and/or the work is rotated and translated. And no matter whether the tooth is involute profiled or rack profiled.

It can be designed to be narrower than a bevel gear's tooth, so that it cuts on only one edge, until rotated and translated to cut on the other edge after the prior pass. This is the principle of specialized involute profiled bevel gear cutters. As illustrated by the video Joules linked to earlier.

These work by first clearing the center space of a tooth cut, then they are rotated and translated to widen and shape that same space on first one side, then the other side, yielding 3 passes per tooth cut. Being involute profied cutters, they also only work with a small range of gear teeth numbers so must be available in sets.

The final possibility, and an interesting one for a home workshop bevel gear cutter, is using a rack form tool either in a shaper or mill. A multi toothed mill has the advantage, as usual, of speed, as this method will also create faceted teeth, and the more facets the better the profile, but the longer it takes to complete a tooth.

If I simplify the machine concept, just for visualization's sake, to a fly cutter of rack shape, I can imagine having a horizontal mill spindle which can be positioned around the vertically oriented gear tooth being cut, with the cutter arranged to spin tangential to the bevel gear's tooth cone. And to cut along that axis. It would have to be narrower than the tooth space. As before it would cut only one facet on one tooth face at a time. Multiple passes per tooth side would be required to generate a faceted involute form.

How to move that mill spindle mechanically, vs via cnc and an equation is an interesting puzzle I'm still thinking about.

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline Lew_Merrick_PE

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Re: Bevel Gear Mill
« Reply #39 on: August 27, 2020, 11:30:53 PM »
I guess what has been bothering me about this thread is that, in the terminology I learned as an apprentice, a Bevel Geat is a "cone shaped gear" designed to transmit torque around a "corner."  Thus, the "focal point of the cone is the pivot about which the involute tapers to allow the "corner" to be "turned."  If my gear set "tapers" by (say) .75 from the "base" of the cone to the "point" of the cone, then the unit must "pivot" .375 to either side of the cone's point to successfully generate such a "gear."  I was taught to locate said cone's point concentric with a "pivot bushing" about which the "mount" could pivot and use "precision tapers" to control said "pivot."  Does this make sense?  --  Lew

Offline timby

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Re: Bevel Gear Mill
« Reply #40 on: August 28, 2020, 03:33:04 AM »

A single tooth cutter can not be designed to cut a good involute bevel tooth on both edges, no matter how it and/or the work is rotated and translated. And no matter whether the tooth is involute profiled or rack profiled.

I have seen a bevel gear cutter   that had 2 single tooth cutters on 2 shaper type heads  set on opposing angles and each cutting on 1 side.

I did not note how the indexing was achieved.

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #41 on: August 28, 2020, 10:07:44 AM »
I guess what has been bothering me about this thread is that, in the terminology I learned as an apprentice, a Bevel Geat is a "cone shaped gear" designed to transmit torque around a "corner."  Thus, the "focal point of the cone is the pivot about which the involute tapers to allow the "corner" to be "turned."  If my gear set "tapers" by (say) .75 from the "base" of the cone to the "point" of the cone, then the unit must "pivot" .375 to either side of the cone's point to successfully generate such a "gear."  I was taught to locate said cone's point concentric with a "pivot bushing" about which the "mount" could pivot and use "precision tapers" to control said "pivot."  Does this make sense?  --  Lew

Yes, that's what I was thinking about. Most of the methods mentioned so far only approximate the shape you describe, because they cut as a series of passes, and therefore produce facets rather than smooth shapes. And those that use profiled involute cutters also only approximate the true cone tapered involute shape in form. The rack style if given an infinite number of passes would give a true cone tapered involute profile. This is analogous to how the two profiling methods work on a spur gear.

Since I want to make something that is simple in design and works relatively quickly, but only cuts one gear size and type -- that adds an oddball  wrinkle to the question of which method to use and how to make a device that does it. I'm not shooting for how to make the perfect bevel gear, necessarily, though I'll take it if simple and speedy enough!

Way back in the beginning of this (to me) enjoyable bunch of imaginings about bevel gear shaping, Andrew suggested doing a 3D printed plastic CNC bevel gear, then lost wax (or PLA) investment casting it. Besides the fact that I don't do 3D printing, the time factor for producing bevel gears by that method would be very long. I also don't imagine the finish quality would be very good -- though things have probably advanced, I'm sure in the home shop level 3D world, so I can't say for sure. Anyway, I don't want to do that.

If casting was considered, I suppose I could however take existing well-cut bevel gears as patterns, and use a high temperature rubber mold to duplicate them in zinc alloy -- which I believe has a low enough melting temp, and good enough toughness for my purposes. After all my Craftsman lathe has zinc alloy change wheels and it's over 50 years old. I'm not sure how casting in a rubber compound with zinc would go, or what the finish quality would be, I've never tried it, but it's intriguing.

However this would also belie the title of this thread, which specifically says "mill" meaning cut. And I also boldly mentioned "steel" not zinc. What do you guys think? Should I hem and haw and try to wiggle my way out of what I said I wanted to do, and send for some hitherto untried rubber gunk, or should I continue to try to do what I said and make something weird and of little use to anyone else, with sharp teeth?
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline awemawson

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Re: Bevel Gear Mill
« Reply #42 on: August 28, 2020, 10:18:54 AM »
I suppose for best accuracy and finish you want a process that generates the true involute form like a hob does, and although I'm clear in my mind how that works for a spur gear, for the life of me I can't see how you would do it for a bevel gear.

All sorts of odd shapes can be hobbed  but I'm not aware of a hobbed item the isn't parallel sided  :scratch:
Andrew Mawson
East Sussex

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #43 on: August 28, 2020, 10:31:54 AM »
That's pretty cool timby.  :beer:
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #44 on: August 28, 2020, 10:40:41 AM »
I suppose for best accuracy and finish you want a process that generates the true involute form like a hob does, and although I'm clear in my mind how that works for a spur gear, for the life of me I can't see how you would do it for a bevel gear.

All sorts of odd shapes can be hobbed  but I'm not aware of a hobbed item the isn't parallel sided  :scratch:
Andrew, I rashly stated earlier that a hob couldn't work because the teeth of a bevel gear converge.

But that's maybe not true if the hob only cut on one side of a tooth. Have to think about that for a bit.

(btw. all this stuff I'm thinking about has no doubt already been worked through long before, and resolved. I just like visualizing my way through things myself. This is what madmodding means to me -- well at least the mad part!  :hammer: )
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline RussellT

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Re: Bevel Gear Mill
« Reply #45 on: August 28, 2020, 11:06:48 AM »
I think Lew has got the key point here.

The link that Matt posted in reply number 25 shows how to make an involute form by rotating the blank and the rack profile cutter generates the tooth form.

If the shaper tool moves along a line that intersects the point of the cone of your gears then it will generate an involute form - and because the cone is larger at one end than the other the teeth (and gaps) will be larger at one end than the other.

From what you have said about your use for the gears I am wondering whether you are seeking too much accuracy.  I have only made one bevel gear (a replacement for a hand drill) and after lots of measuring and thinking I realised that the gears were not an ideal form and had just been cut with a single cutter and cut deeper at the wide end.

On the other hand I generally aim to make things as accurately as I can - it's good practice for when I need to.

Russell
Common sense is unfortunately not as common as its name suggests.

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #46 on: August 28, 2020, 11:21:40 AM »
Hi Russell, not sure what your point is here. There have been no disagreements with any points discussed so far, and I've never said I wanted to produce a perfect bevel gear.

The shaper/rack form method produces faceted teeth, and the fineness of the facets depends on your patience and the degree of carriage movement you chose between strokes. Also, if you want a true tapered involute shape on a bevel gear, it can properly only cut on one edge at a time. The same result can be had with a milling cutter, more quickly with the same provisos.

This is just a general discussion about bevel gear cutting so far with no conclusions for a specific device.

With regard to cutting along the cone, that was in post #1: "The main topic of interest would be wedge shape of the teeth. These converge on some imaginary conical center. If the cutter was thinner than the inner space between the teeth it could cut each tooth with two passes along a radial of that cone."

I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #47 on: August 28, 2020, 12:43:53 PM »
I did a quick search for high temp silicone rubber compounds usable for zinc casting, and only found mention of their use in spin casting. More commonly available high temp rubber compounds are used at lower temps with bismuth, lead, and pewter.

The spin casting types for zinc seem to be supplied mainly as prepared disks for proprietary spin casting machines, and require vulcanizing before use. I'm still looking into it, but may not be easily possible to do simple rubber mold casting of zinc in a home shop setup. Zamak 2 seems a preferred alloy because of slower solidification.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline vtsteam

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Re: Bevel Gear Mill
« Reply #48 on: August 28, 2020, 03:54:29 PM »
If commercial bevel gears (like the ones I ordered) really are accurately conical without undercuts, it should actually be possible to mold them in a conventional plaster mold. Though getting them to release might be a little tricky. I already have some bevel gears of the right size on a coiple of steam engines. I might give that a try.  :dremel:
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline mattinker

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Re: Bevel Gear Mill
« Reply #49 on: August 28, 2020, 05:31:28 PM »
Two piece plaster moulds would be quit easy to make! Grease as a release agent.

Cheers, Matthew