Single Pass Bevel Gears

[Captain Jerry]

Yesterday I downloaded a 3D cad (STEP format) of a mitre/bevel gear from Rush Gears and imported it into Alibre to help visualize and analyze.  It seemed to me that there should be a easier and more accurate way to machine these gears than the commonly used 3 pass / constant depth method.  It seems so simple that I was reluctant to think that it could be done.  Today I went to the shop to give it a try.  I think it works!

I had some difficulties based on my equipment and material.  I was just barely able to get my RT/Div head set over to the correct angle and get the cutter to traverse the face at the correct depth without hitting the chuck jaws.  It was only possible to cut in the Y axis direction and the cut had to be made cutting away from the chuck.  In order to get maximum travel, I had to remove the "X" scale from the back of the table. These problems are solely the fault of my poor choice of RT.  Other RT or indexers may not be so limited. 

I do not have a gear cutter so I had to grind a single tooth flycutter style.  This is really not that difficult.  I already had the head which I had used to cut spur gears previously.

The only material that I have that is near the right size  (1/2" diameter) that is not already committed to another project is steel of unknown specifications.

My poor lighting and cheap camera don't do justice to the results but the gears mesh and turn very smoothly.

I will barter (against some future service to be rendered) for my wife's better camera and see if I can get something that you can see.

How? Easy. 

The gear blank is faced at a 50? angle instead of 45?.  The RT axis is set over at a 40? angle instead of 45?.  The cutter profile is a compromise  that only approximates the correct profile but then so is every other cutter except for curves generated by elaborate gear production machinery.  The depth of cut, for which I have no mathematical basis, was set to .040" (for a 1/2" gear).  The result is a tapered tooth of variable depth (height) that is greater at the outside than at the inside and has an effective contact angle of approximately 45?.

The attached gear cross section shows the relative angles.   

HA! You may laugh!  Humph! You may scoff!    I'm going to post this and run for cover.



EDIT:

Sorry. I forgot to add the attachment. Here it is.

[Bezalel]

My poor lighting and cheap camera don't do justice to the results but the gears mesh and turn very smoothly.

Too right Skipper, I can't even see where they mesh   
 
Bez
PS looking forward to the macro photos    hope the negociation goes well
 

[Bogstandard]

Jerry,

You have proved that in this little game we play, lots of times we don't require the exact precision of correctly made gears, but something close will usually achieve what we want.

I'm not saying that imprecise gears should be used all the time, but in those non too critical functions that we come across, they work just fine, so allowing us to make our own 'special' gears.

Nice one


John

[Jo]

I made similair tapered bevels for my Grasshopper, they work adequatley and allow the "balls to fly" but they don't have to actually operate the governor.

Jo

[swilliams]

I like it

Steve

[Ian S C]

Captain Jerry, just what I'm looking for, thats great.  For your 1/2" gear how many teeth?
                                                  Ian S C

[Captain Jerry]

You have proved that in this little game we play, lots of times we don't require the exact precision of correctly made gears, but something close will usually achieve what we want.
Nice one

John

It's a good thing that absolute precision is not required.  We can build little engines that run quite well without rings. We can run flywheels that have not been dynamically balanced.  We can be satisfied with fits that would be rejected by any careful inspector and we can do it with cheap equipment and fading vision.  Hobbed gears with faceted faces are another good example of a good compromise as are bevel gears cut with constant depth/height teeth.  So are Browne & Sharpe precision gear cutters that cover a range of gear teeth.  The single pass method could be improved by the use of precision gear cutters but that doesn't compare with the pleasure of a shop made solution.

I made similair tapered bevels for my Grasshopper, they work adequatley and allow the "balls to fly" but they don't have to actually operate the governor.

Jo

What is your concern?  Do you think the gears are not smooth enough for precision speed control or is it the strength that worries you? A governor gear set should not need to transmit much torque.

Captain Jerry, just what I'm looking for, thats great.  For your 1/2" gear how many teeth?
                                                  Ian S C

The gears that I cut have 12 teeth.  It works out nicely with 6 turns on my 72:1 RT.

[Jo]

I made similair tapered bevels for my Grasshopper, they work adequatley and allow the "balls to fly" but they don't have to actually operate the governor.

Jo

What is your concern?  Do you think the gears are not smooth enough for precision speed control or is it the strength that worries you? A governor gear set should not need to transmit much torque.

No concern, just stating the fact mine don't have to do any work.

Jo

[Captain Jerry]

Oh.  I thought you were saying that they couldn't do any work.

[Ken I]

Jerry,
         The engineer part of my brain that knows exactly how and why gears should be (I worked briefly for ZF) recoils.

The hobbyist in me says what the heck - if it works ....

In my other hobby - racing slotcars - we run plain gears and pinnions with the shafts out of parallel (corrcect orientation) by 20? - otherwise the motor fouls the axle - something the engineer in me says simply just shouldn't work - but it does and its been done like that for a long time.

I have often though of cutting a gear just like you suggested but have never tried it - will definitely give it a whirl next time the occasion arrises.

Ken

[Don1966]

Jerry thanks for simplifying things, this makes it a lot easier for me. I am not an engineer and have to struggle to understand a lot of the concepts in doing all of this. I read and reread to understand things. I am a self taught machinest and only been doing this for three years and part time at that. So thanks for any effort you present that helps get things done.

Don

[Ian S C]

Thanks for that Jerry, I need a pair for a right angle drive for a little ALPHA type hot air engine, for a model boat, I have a similar GAMMA motor in a boat, then found an artical by Koichi Hirata in Japan.  Just out of interest, is it best to use different metals for each of the gears, ie., brass and steel, the first motor has two brass gears, and runs ok.  If the motor does much more than a few dozen hours of running in its life time I don't supose it matters as far as wear is concerned, it's the friction I'm looking at.  Actually I suppose that brass and plastic would be the best mix.  Just thinking out loud.   Ian S C

[Captain Jerry]

I probably shouldn't have started this just before taking a vacation.  Having proven to myself that the concept is valid, I keep thinking about the details and the dimensions.  It is easy to make minor changes and view the results in 3D with Alibre' but it would be nice to be able to confirm in the shop.  When I get back to it, I have to do something about mounting the RT to get better access to the blank.  The 12 tooth gear looks good to me but when I check catalogs for gears, I can't find any mitre gears with less that 18 teeth.  I'm not sure why but I think I will try that too.  It will mean grinding another cutter.  No big deal.

Ian,

In general similar metals promote galling in a sliding contact.   Really good gears are supposed to produce a rolling contact but that is probably not realistic doing it this way.  Still, I see matched brass/brass pairs all the time and nylon or fibre would seem to be a good choice as well.

Jerry

[Captain Jerry]

Jerry,
         The engineer part of my brain that knows exactly how and why gears should be (I worked briefly for ZF) recoils.

The hobbyist in me says what the heck - if it works ....

In my other hobby - racing slotcars - we run plain gears and pinnions with the shafts out of parallel (corrcect orientation) by 20? - otherwise the motor fouls the axle - something the engineer in me says simply just shouldn't work - but it does and its been done like that for a long time.

I have often though of cutting a gear just like you suggested but have never tried it - will definitely give it a whirl next time the occasion arrises.

Ken

So... as an engineer you would have been stymied but a less qualified person just did what was needed and it became SOP.  I wonder how many other innovations have been created by a lack of knowledge.  The opportunities are endless.

[gbritnell]

Hi Jerry,
If one were to grind a tool to accomplish what you have laid out how would one determine what the profile would look like or better yet at what point (outer or inner profile of tooth) would one use?
gbritnell