Gallery, Projects and General > How do I??

Cutting internal gear teeth

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bogstandard:
Bob,


http://www.arceurotrade.co.uk/Catalogue/Cutting-Tools/Gear-Hobs



John,

After seeing that DVD about gear cutting a few months ago, it gave me a good idea to try.

Very similar to using a freewheeling blank on the lathe for cutting a worm wheel. See attached C-o-C.

I am contemplating stacking gear blanks together, or make long blanks and part off to the thickness required.

The gashing doesn't need to be too deep or long, it is only there until the spiral of the hob starts to work like a worm spindle thread and drive the blank while it is being fed along it. Once the intial cut is done, it is a matter of going back to the start, put a little more cut on, re-engage the teeth and take another cut.

I think it has every chance of doing a good job, but if not, I will most probably make a small hobbing machine.

http://www.collegeengineering.co.uk/Castings/582.htm

The cost of small gearing is the only thing holding me up on a lot of projects I already have a desire to do. They are ridiculously expensive. If this works satisfactorily, then the cost of the hobs, and if needs be, the small hobbing machine, will soon be recouped by the savings made. This set also contains the correct module for the change gears, and other gears on my lathe, so then that opens up other areas to be explored.


John

John Stevenson:
Don't see any reason why that won't work although I'd go for full depth at one pass, light cuts have more chance to bounce about wheras a deep cut into a pre gashed blank should hold tit steady, use a sacrificial gear at the start and end as that's were all the errors getting started and ending will occur.

Personally I wouldn't bother with the CES hobber as once the method is established I'm sure a more robust / universal machine could be fabricated up at 1/4 of the cost.

John S.

bogstandard:
I will suck it and see John. It will be a few months before this project gets to the front of the queue, so I have a little more time to search for answers before then.

If this works OK, I won't even bother with a dedicated hobbing machine. I am not going into production, just making gears for everything I want to do. I reckon if I can cut eight pairs of gears, maybe less, then the hobs will have paid for themselves.


Bob,

Unlike what most people think, they aren't great big stonking beasts of a cutter, but very genteel and superbly made pieces of engineering wonderment.

This is the largest (1.25) and smallest (0.3), the largest is shown against one of my change wheels off the lathe, the cutter is in fact shown upside down.




John

ScroungerLee:

--- Quote from: John Stevenson on October 03, 2009, 06:21:31 AM ---About the only way John,
If the gear is big enough inside another way is to get a small gear of the same pitch turn it dished, harden it, hollow grind it and use this as a cutter indexing round one tooth at a time.

Advantages are finding a ready made gear of the right profile may be easier than hand grinding an involute. As you also cut one tooth you also part cut the teeth to the side. As you index round the multiple cuts will generate a nearly perfect involute.

This is the way it's done commercially but the blank and gear rotate in sync each straoke of the ram.

John S.

--- End quote ---


I have a question about this, using one gear to cut another won't cut deep enough to reach the base circle, will it?

Lee

John Stevenson:
Technically no, the cutters used in industry have been modified to allow clearance.
In the home shop use you can fudge the calculations by adding the clearance to the pitch circle, it will add a gnats cock of backlash to the gear but it will still be a serviceable gear.

To be honest on small model engine gears like wot John is after you would probably be hard pressed to measure any inaccuracy.

One point about involute gears is that even though centre distance and or diameters can be out the gears will still mesh correctly and run OK even though they may have excessive clearance.

John S.

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