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Author Topic: Backlash compensation for a rotary axis ?  (Read 16458 times)

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Offline Greolt

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Backlash compensation for a rotary axis ?
« on: March 08, 2008, 06:16:15 PM »

I've always been of the opinion that backlash is something you deal with via mechanical design and not software.

However I have a worm drive gearbox that I would like to use as the basis for a rotary axis.

It has a very small amount of backlash which I can not eliminate by mechanical means.

I am interested in any ones experiences in a similar situation using backlash compensation in Mach.

The main driven shaft which would become the rotary axis spindle is quite stiff and for engraving
on a cylinder and other light jobs I think it would be OK.  It won't slop back and forth.

But I still have to account for the small amount of movement in the driven shaft before the main shaft will change direction.

Anyone with an informed opinion?

Greg
Re: Backlash compensation for a rotary axis ?
« Reply #1 on: March 08, 2008, 07:36:05 PM »
Hello Greg,
How bout' semi informed ?
I had a similar setup once but it was not motorized. My gearbox had dual output shafts. I mounted a chuck to one end, and a 6" dia. pulley to the other and a handwheel to the input. (would have preferred a motor)
Then I attached a 1/16" aircraft cable to the pulley and wrapped it around a couple of times, then through a small winch pulley to a hanging weight sufficient enough to keep the driven gear permanently against the worm regardless of the direction of feed.
It was sort of crude, but it eliminated ANY possible backlash and worked perfectly for light cutting.
I'm sure there is a software solution too. There's been quite a bit of discussion about that here on the forum.
Just thought I'd share a "Backwoods" technique.
RC

Offline Greolt

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Re: Backlash compensation for a rotary axis ?
« Reply #2 on: March 08, 2008, 08:04:31 PM »
G'day RC

I get what your saying with loading the spindle in one direction.

In this case I would not go that far. 

Unless I see a fairly good prospect of this working well I will go off in some other direction.

I suspect that some of the drawbacks with Mach3's backlash compensation would not apply in this limited situation.

However I would really like to hear some feedback from those who have tried it.

Greg

vmax549

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Re: Backlash compensation for a rotary axis ?
« Reply #3 on: March 08, 2008, 10:00:42 PM »
Greg you still may be able to tighten up the worm. Does the main axle have bearing caps? If so you can shim the plates into the worm decreasing the backlash. I have seen it done.

 The problem with backlash in the worm is there is little resistance to movement on the spindle and it can cause the axis to jitter from the loose play when you are machining inline of the axis and cross the centerline.

(;-) TP

Offline Chip

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Re: Backlash compensation for a rotary axis ?
« Reply #4 on: March 08, 2008, 10:16:44 PM »
Hi, Greg

I setup an  A axis rotational with backlash enabled, Using a sherline rot. table that doesn't have much backlash.

Simulated it with 1/2 handle turn of backlash, Motor tuning setup for Deg moves.

Works Fine Hear.

Just as TP stated, Caution, If it's as you stated, "final output is tight" Engraving should work ok.

Hope this Helps, Chip

   
« Last Edit: March 08, 2008, 10:34:46 PM by Chip »

Offline Greolt

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Re: Backlash compensation for a rotary axis ?
« Reply #5 on: March 09, 2008, 01:10:54 AM »
TP

I wish I could. 

This is a small industrial reduction box and was not designed for precision application.  Final drive is fairly stiff.

Chip

Thanks for trying that. I suspected compensation could work OK in this situation.

I might go ahead and turn up the input adapter and a spindle shaft and give it a "Temp setup" try.

A couple of wood turning chucks that I have should work well.

Greg

Offline jimpinder

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Re: Backlash compensation for a rotary axis ?
« Reply #6 on: March 09, 2008, 03:53:55 AM »
I cannot understand your problem with backlash. Mach3 deals with backlash perfectly. I have a cheap lathe which has quite a lot of backlash. This summer I will deal with it, and probably put in ball screws - but until then Mach 3 deals with it and it is accurate to the thousanth of an inch.

You would be able to, I am sure, deal with backlash on a manual machine, I think we have all done that at one time or another - on Mach3 it is just the same, but the computer does it for you.

All backlash is is the non-movement of the "table" whilst the gears, belts etc etc settle down to pulling the other way.

On any system, to check it - use typed gCode commands, switch off backlash compensation -  move one way - stop, set measure to 0 - move same way for some distance ( the distance does not matter,because backlash has nothing to do with distance, but you can say move one inch and check the table is moving accurately) - stop - reverse back to start.

Your measure should now read 0 - but it will not - this is backlash. Enter it in the table and switch on backlash compensation.

It is simple and accurate and all mechanical systems must have it, otherwise they would seize up - it is only when it gets big enough to notice it becomes a problem.

The only problem I can see with a rotary table is measuring the backlash - I have things to measure to a thou, but nothing that will measure to a degreee (accurately). I suppose you could convert to thous and then convert back - it depends on how you set up the axis.



Not me driving the engine - I'm better looking.

Offline Greolt

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Re: Backlash compensation for a rotary axis ?
« Reply #7 on: March 09, 2008, 04:07:01 AM »
Thanks Jimpinder

I know what backlash is.

I would not consider Mach's software compensation a viable option for a linear axis.

But that's just my opinion.  Each to his own hey.  :)

Greg

Offline Hood

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Re: Backlash compensation for a rotary axis ?
« Reply #8 on: March 09, 2008, 04:28:14 AM »
Greg
 I have never used Machs Backlash because I have never needed to but I have heard there were big improvements made to it a  while back. Have you tried it recently or are you just going on what it was like a year or so ago?
 Hood

Offline Greolt

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Re: Backlash compensation for a rotary axis ?
« Reply #9 on: March 09, 2008, 04:51:29 AM »
No mine is just a philosophical viewpoint.

I am not really anti  Mach's compensation, just software compensation in general. (Although I have read a lot of users hassling with it)

In the type of CNC machines which I am interested in I believe mechanical design is the place to tackle backlash.

Just my opinion, not something I want to debate.  ;D ;D

Having said that, on this gearbox, mechanical measures are not practical so I might try it. :P

Maybe I will be so impressed as to change my opinion.  ;D ;D

Greg