View Full Version : worm gears, 4th axis

senor J.
10-15-2004, 03:12 PM
Hi everybody heres my worm gear question... I have read that a worm gear setup is prone to a certain amount of backlash. could this be corrected (good enough) In mach2 with the backlash compensation feature? <---main question. I'm interested in a continous fourth axis set up for carving wood ( indexing you say? bah, humbug ) and of course backlash would not be your friend with that (although I think a small amount of backlash could be Ok as long as the gear reduction was high enough say 50 to 1 or 100 to 1) what are everyones thoughts on this. would there be a better system for an "A" axis reduction. Im gonna make a 4th axis even if it comes to blows :boxing:

10-15-2004, 03:19 PM
Just take the trouble to mount your worm in an eccentric bearing housing. This lets you either disengage the worm completely from the gear whilst dialing in your workpiece, or, allows you to snug the worm as tightly into the gear as it can possibly fit. You might even run it a while with some lapping compound on the gear teeth to take off any high spots.

Software backlash compensation is not likely to be satisfactory: your milling cutter will cause the work to jitter even while the rotary axis has no motion command to execute. Backlash comp only works for motion reversals.

senor J.
10-15-2004, 03:33 PM
please pardon my noobishness huflungdung but what exactly is an eccentric bearing housing unit/design
thanks in advance

10-15-2004, 05:38 PM
You know what the CN tower looks like? Well, that looks nothing like an eccentric bearing housing. :D

Seriously though, you would understand that the worm shaft needs to be cradled in a bearing. Now, instead of just mounting that bearing solidly in a hole in the main gearbox, you mount it inside of another sleeve. However, the hole in this sleeve is bored off-center relative to the outside of the sleeve. This is where the eccentricity comes from. This is how you can create a variable gear mesh distance.

10-15-2004, 06:26 PM
You might even run it a while with some lapping compound on the gear teeth to take off any high spots.

Don't do this with brass worms, as the lapping grit will become embedded in the brass and cause premature wear of the pinion.

Fred Smith - IMService

10-15-2004, 07:48 PM
I too want to build a 4th axis, I can get a worm gear set up with a 40 to 1 ratio. I was wondering if anyone knew of a free 4th axis drawing? I want it to have one end with a chuck to hold a round stock, with the tail end to be kinda like a tail stock on a lathe.

senor J.
10-15-2004, 09:39 PM
for my 4th axis I think Im just going to use the headstock off a jet mini lathe. hey mocnc where could I get one of those 40 to 1 wormgear boxes?

10-15-2004, 10:40 PM
Sorry senor J I didn't mean to imply that it was a wormgear box.....I am able to get a worm gear with a regular gear. I am looking for plans for a box that I can use the worm gear. I want to be able to turn pool cues, so thats why I need a 4 or 3 jaw head stock and a tail stock. Any ideas??

10-16-2004, 12:41 AM
Hey guys I designed my 4th axis and had a friend with a machine shop build it. I did not use a worm gear nor any typed of gearhead reduction. BIG mistake. While this setup works fine for soft material, (I forgot to mention this is made specifically for wood) it is not good enough for harder type woods or other materials. I designed the gearbox and used a stepper motor with timing gears and belts for my reduction. I would not go this route again.


10-16-2004, 02:10 AM
I've had an idea cooking for a while now of building a 4th axis using the arbor shaft from a spin indexer. The inexpensive spindexers I have in mind have a ready made 5c collet. 5c collets go up to about 1 1/8" or there are lathe chucks that have a 5c backplate.
It would need a couple of large pillow blocks or the equivelent and a worm gear fitted to it.
5-C Spin Index (http://www.grizzly.com/products/item.cfm?itemnumber=G5649)
I would do it but my life is a mess right now as I am moving to another state. I got to pack up the CNC machine for a while. :mad:
I will try to stay in touch with The CNCZone. :)


senor J.
10-16-2004, 12:34 PM
what about a 6 or 7 inch rotary table converted to operate with a stepper? Anyone have any experience with those? I wonder what the backlash would be like?

10-18-2004, 09:27 AM
Mike, what kind of problems are you having with your rotary axis? I was considering building one with a stepper and belt reduction drive myself, and would like to know the downside(s)? Thanks

10-18-2004, 11:01 AM
Gerry most of the problems are related to me not knowing what I was or am doing! :rolleyes: The first mistake I made was in how I designed my cad drawings to minimise the amount of material I need to make a gunstock blank. Some of the woods I use are very expensive and some are rare enough I have only made one stock from that kind of wood. With that being said I set my programs up with the center line a lot higher in the model than I probably should have. By doing that I had to design the 4th axis with a greater vertical clearance. This then allowed me to hang a lot of the stock to one side oc center which brings in the problem. Flexing of the system while cutting on the back of the stock that is off set from center of the 4th axis. As far as backlash goes I can rotate the axis 20 rotations in one direction, reverse it and rotate 40 in the opposite direction, reverse and bring it back to zero and it will be within a minute part of a degree from where it should be. This is checking it with a level in addition to the Mach2 DRO. That part of it is pleanty accurate for me.

Remember I am doing heavy cuts or at least was. If you are planning something for light work I don't think you will have a problem. If you are cutting metal get a gearhead and build everything stout!


10-18-2004, 12:53 PM
Thanks Mike. I think it will work for what I want to do.