Helical Gear Cutting Lathe Attachment

[cfellows]

For years I've been infatuated with the idea of cutting helical gears. While I've always found a way to avoid the use of helical gears, they can add a certain elegance to engine valve trains and they have a kind of fascination about them. I wanted to be able to cut and match gears of different angles, mostly for cross shaft applications but have been put off by the seeming complexity of the mathematics involved. However, I think I finally have it figured out and have begun work on an attachment for my 11" logan metal lathe that will let me cut small helical gears of pretty much any angle and up to about 1" in diameter or so.

This is total experimental and may take a number of iterations to get it to work right, and may come to a dead end at some point if I can't figure it out. It will work by using sliding, rotating spindle. The axial movement of the spindle will be controlled by a triangular piece of aluminum sheet, bent around the hub of the indexing collar. As the spindle is turned, a follower will run along the edge of the angular aluminum piece and cause the spindle to advance at a rate determined by the angle. The indexing collar which has 24 holes, will allow me to index the aluminum angle to cut different numbers of teeth in the gear. By the way, the helical gear on the end of the spindle is just for illustration to show where the helical gear blank will be afixed.

Here's some pictures of what I have so far:

Chuck

 

[John S]

Very interesting, I'll keep an eye out on this.

Worried about cutting forces, what about a strongish spring pushing the indexing washers against the template and a lever on the washers to control feed and give some feel to the cutting ?

Another way would be to have a bowden cable from the cross slide over a fixed roller going to the OD of non index wheel.
The OD would have to be worked out to give you the correct helix angle.
Large OD, small helix and small OD to give large helix.

The maths shouldn't be too hard.

John S.

 

[Captain Jerry]

Chuck, that is a terrific challenge. I wish you the best of luck. I hope it doesn't take you too long to work it out. You have put my mind in a terrible state of boggle. I think I see where you are heading but then I don't. You have shown a terrific tease with the first post. I'm hooked.

Jerry

 

[steamer]

Hey Chuck,

How about gearing it to your leadscrew for the helix. Then superimposed the index on that for each gear tooth......Like a real helical set up on a universal mill......Cool! ;D

Dave

 

[larry1]

Chuck, Making gears is one thing I would like to work with, Ithink that your work is really great. Larry1

 

[cfellows]

Very interesting, I'll keep an eye out on this.

Worried about cutting forces, what about a strongish spring pushing the indexing washers against the template and a lever on the washers to control feed and give some feel to the cutting ?

Another way would be to have a bowden cable from the cross slide over a fixed roller going to the OD of non index wheel.
The OD would have to be worked out to give you the correct helix angle.
Large OD, small helix and small OD to give large helix.

John S.

John,

I've considered the cutting forces although I don't claim to have any certainty about the outcome. The spring idea sounds like a good possibility. My plan is to run the work against the cutting edge (as opposed to climb cutting). The aluminum template will pull the blank into the cutter as the spindle is turned. I also plan to only use brass or aluminum for gear blanks. I've discarded the idea of using a cable or other means to control kickback since I would have to change several parameters for every different gear I cut.

Chuck

 

[cfellows]

Hey Chuck,

How about gearing it to your leadscrew for the helix. Then superimposed the index on that for each gear tooth......Like a real helical set up on a universal mill......Cool! ;D

Dave

I thought about this, but it would require mounting the gear blank in the lath spindle then building a separate motor to drive the cutter mounted on the cross slide. Also, the gear combinations required for the different leads on helical gears would require some pretty wild gear ratios to drive the lead screw. I know some people have done this, but I've never seen a picture or a video of what it looks like.

I did come up with a design for geared compound that I could mount on the cross slide to hold the blank. It would have its own lead screw which would be geared to the spindle holdind the gear blank. Since I have a 48 pitch gear set which includes just about every size from 20 to 100 tetth, I could probably make this work and will keep considering it.

Chuck

 

[Dan Rowe]

Hi Chuck,
This is a very interesting thread. I like your spindle indexer I might borrow that idea.

Have you considered milling the helix on the spindle and using a pin in the bearing block guide the motion as the spindle moves forward? It would mean a new spindle for every helix angle which might be a drawback.

Dan

 

[cfellows]

A little more progress. I made a custom fixture for bending the aluminum template that guides the axial movement of the spindle as it rotates. The first picture shows the template after it has been bent into a circle and fitted to the hub of the indexing wheel on the spindle:

The template starts as a 1/8" thick piece of aluminum. It is a truncated right triangle and the difference in width between the two ends exactly equals the lead of the gear tooth for one full revolution. The length is (nearly) long enough to go completely around the hub of the indexing wheel. It should be noted that the angle of the cut is not equal to the finished angle of the gear tooth.

Here you can see the template blank clamped to the bending fixture using a pair of vice grips.

The initial bend is done with my fingers:

The bend is then finished using a crescent wrench:

Got a bit of tuning to do, but I think this part will work well.

Chuck

 

[John S]

Nice,
I'm still on the same page.

 

[Deanofid]

I thought about this for about an hour the day you first posted it. Then my little head started smoking.
This sure clears up a lot. Thanks Chuck.
It won't make a difference how many teeth there are on each blank, right? As long as a given set of
gears are the same angle, this works. Is that correct?
Width of the blank will be a factor, though? In order to make a full cut across the face of the gear, I mean.

Dean

 

[cfellows]

I thought about this for about an hour the day you first posted it. Then my little head started smoking.
This sure clears up a lot. Thanks Chuck.
It won't make a difference how many teeth there are on each blank, right? As long as a given set of
gears are the same angle, this works. Is that correct?
Width of the blank will be a factor, though? In order to make a full cut across the face of the gear, I mean.

Dean

Unfortunately just about every gear requires a different template. It depends on the the angle, the pitch, and the number of teeth. The template in the picture is for a drive gear which has 6 teeth, 80 degree angle (from the axis), and 40 pitch. The gear blank will be about 7/8" diameter and the finished gear will look more like a worm gear with six teeth. The driven gear will be 10 degrees, 12 teeth, and will be about 5/16" in diameter. I chose these angles and ratios because the drive gear will be on a crankshaft which is typically about 3/8" diameter or bigger. The driven gear will be on a 1/8" shaft to drive a valve shaft. I know it's hard to visualize, but the drive gear will be almost 3 times bigger in diameter than the driven gear, but will have half as many teeth.

Chuck

 

[rklopp]

Check out this article on skew gears on Ron Chernich's model engines site, and the links that lead from it.
http://www.modelenginenews.org/techniques/skews.html Ron'll probably scold me for jumping out of the frames on his site.

 

[Deanofid]

Thanks Chuck. I'll just let my head smoke a while longer.


Dean

 

[tel]

You're not the only one with a smokin' 'ead Dean. I'm right out in the wilderness here - keep the details coming Chuck and perhaps the penny will drop!

 

[cfellows]

Check out this article on skew gears on Ron Chernich's model engines site, and the links that lead from it.
http://www.modelenginenews.org/techniques/skews.html Ron'll probably scold me for jumping out of the frames on his site.

Yeah, I had already found and studied this information in some detail. It's really what made things click for me!

Chuck

 

[cfellows]

Nice,
I'm still on the same page.

John, I had seen this video some time back but just went back to look at it this morning and discovered it to be yours (I think?).

[ame=http://www.youtube.com/user/John5tevenson#p/u/10/fps0OR1eF_s]http://www.youtube.com/user/John5tevenson#p/u/10/fps0OR1eF_s[/ame]

Do you have any information on how you made that single point cutter? I've got a bunch of commercial circular gear cutters, but would like to use a single point (correct term?) like yours for my helical gear cutting.

Thx...
Chuck

 

[cfellows]

I think I'm ready to try cutting a gear. The spring I have installed is not quite as strong as I'd like, but it's better than nothing. I've also installed a small ball bearing that presses against the aluminum template to force axial movement of the spindle as it rotates. I made the bearing bracket double ended so could reverse the body if need be for working on the other side of my milling vise. I'm also thinking about knurling the indexing wheel for a better grip. Or, I may make a handle that will give me more leverage.

 

[cfellows]

After my first use of the gear cutting attachment, I think it's definitely a success. I, however, have to take a little more time to use it correctly. Two problems with this first gear which is 6 teeth cut on an 80 degree angle. The first problem occured on the first tooth. I was cutting too aggressively and the screw which holds the blank in plase spun loose letting the gear blank spin. Kind of boogered up the perimeter of the blank. The second issue is that I miscounted the number of holes on one of the teeth so the spacing on that tooth is too small. So, the combination of those two problems makes the gear look a little rough, but with a little practice, I think it's going to work great!

The first picture shows the gear still in the cutter attachment positioned at about the middle of one of the teeth.

This second picture shows a closeup of the gear.

Chuck

 

[larry1]

Chuck, That isreally a neat tool. Looks like it did great job of cutting this gear, and yes I am learning a lot and not even in the shop, for a while.