Saturday, 10 January 2009

Cutting worm gears on a Lathe


Here is the finished worm gear holder with a worm gear blank in the holder and a finished worm gear on top for comparison. The cutter is a standard M3 Tap.

Note the 1.5mm PTFE disk under the worm gear blank (3mm thick) this is to centre the blank to the cutter and to support the gear during cutting whilst helping the gear to turn on the pin.

The pin is just a piece of axle with a coupling sleeve on to stop it falling through the gear holder.

I marked a line on each blank to show when it had done a full turn and with the RPM of the lathe set to about 200, advanced the gear holder into the cutter 0.05 mm per turn of the gear.

It could probably have been done quicker but I was'nt in a rush, the job worked perfectly and the cutter never broke.

11 comments:

nophead said...

Hi Andy,
Very nice. Can't wait to see how well it works.

Larry_P said...

Very clever!
I'd looked at worm gears [bostongear.com] but the cost seemed daunting. This is an interesting "spin" on the problem. Should be doable using a mill instead, I think. Correct?

-- Larry

BodgeIt said...

Like it really interested in your approach to this problem.
As I havent got a Lathe yet.
I'm going for the L bracket and bearing method from Demented.
your method really appeals to me though as you will have almost 360 degrees of grip using your worm gears
Great stuff it just looks like it will work so well.

aka47 said...

Thanks guys, much appreciated.

Larry it should be doable on a mill with a bit of finesse.

BodgeIt, Demented's work is excellent and not every one has a lathe. So Demented's work is still very important.

I am currently toying with using threaded rod as a rack and worm gears cut this way as pinions. I guess in a much larger size than M3 though maybe M8+.

Likewise the cost of gears (spur in particular) or any reasonably precise off the shelf mechanical components seemed to have a heavy price tag attached.

aka47 said...

PS I can't claim any credit for the approach to cutting worm gears, I found it on the internet on a mini lathe site from a guy that was using this method to make telescope positioner components

Forrest Higgs said...

So what kind of rotational rate were you using when you cut the teeth on that gear with the M3 die? I'm assuming from looking at the rig that the gear rotation was controlled by the die cutting action. Is that correct?

Larry_P said...

Andy,

First off, if you have the link to the site where you learned this technique, would you please post it?
Meanwhile I have a question regarding pitch diameter (or number of teeth): How do you prevent "fractional gear teeth" during the machining process? As you cut the teeth, the outer diameter of the gear is slightly reduced, no? Thus, the circumference (and number of teeth that will fit around it) is also decreasing. Do you start with a diameter that is slightly over an integer number of teeth, such that by the end of the cutting, you're down to an even number? (And do the partially-cut teeth continue to line up as you cut increments deeper?)

-- Larryember

aka47 said...

Forest, Yes indeed the thread pattern on the tap drives the gear blank round as it cuts. Note the Blank has a starter notch turned into it around the middle to help get the tap deeper into the gear quicker.

The articles suggest making your own cutter if you don't have a tap to hand by running a slit down some studding. Providing the material of the studding is harder than your gear material and the cut makes the edges sharp enough it will function as a cutter.

I went for the use a tap option for ease.

My lathe was running at 200RPM.

Larry, Links as requested, there are more that a search of goggle will throw up but these were the two primary ones that set me along this route.

http://mini-lathe.org.uk/worm.shtml

http://www.atmsite.org/contrib/JSAPP/wormgear/wormgear.html

Yes you are correct the wheels are best set up to have a cut(ie inner not the outer diameter) an integral divisor of the pitch you are cutting to.

Funily enough I just set out to mess with 10mm brass blanks and it all worked so well I kept on going.

Unlike spur gears I Think because of the angle of cut etc and the fact that that the gear teeth are expected to follow curves etc I think the outer part of the cut circumference has a wider pitch (Or it would'nt mesh) this does seem to allow for some difference from the ideal diameter and may remesh a poorly calculated fit.

I reality I haven't done enough of these to actually really know I am currently just guessing.

please do have a go and let us know how you get on.

Rinkle said...

I always like to know about worm gears, you posted very nice blog about cutting worm gears on a lathe. I like it and much appreciated.
Worm Gears

Mike said...

This has been seen on a few mini-lathe sites as well:
http://www.bedair.org/Worm/Worm.html
http://webpages.charter.net/mbonfire/wormgear.htm
Great work everyone1

AKA47 said...

Nophead has done some work with these too and found that they just seem to magically work out.

In at least one of the articles you posted they have setup indents around the circumference using a mill to drill them.

I think this is a way of forcing alignment.

The Meccano machine gear cutter uses another gear as a template for cutting spur gears.

http://edwards.web.users.btopenworld.com/meccano/modelgear.htm