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Backlash compensation again - and Xbox controller
« on: June 20, 2012, 11:28:22 AM »
I am running the latest M3 lockdown version on my lathe and mill, and have also installed the addon for using the Xbox wireless game controller.  Backlash is (so far) not a problem on the mill which has ballscrews but is on the lathe (converted Myford) so at the weekend I decided to enable backlash compensation.  I measured the BL on the two slides and entered the right values in the DROs and set the speed to 20% as recommended.  I didn't touch the Shuttle Accel setting as the manual doesn't mention it though I see in some of the forum postings that it seems to be relevant - so probably set to the default value which seems to be 1.  The Z backlash is about 0.08 mm, Z about 0.18.

Then trying the lathe using the Xbox controller I found some very peculiar behaviour.  Continuous jogging using a joystick in one direction worked fine, but when I reversed direction, first I think it moved slightly in the same direction as before, then moved much too far in the other direction at a speed that varied wildly, first going at normal jog speed, then slowed right down, then speeded up again.  I haven't tried any actual cutting as this behaviour seems absurb, obviously something wrong.

So, questions if anyone can help please.  First, anyone with experience using BC on a lathe?  Or BC with Xbox controller?  And what is the significance of the shuttle accel parameter, is it relevant at all with the Xbox controller and if so what should I set it to?  

TIA - John.

Offline Hood

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Re: Backlash compensation again - and Xbox controller
« Reply #1 on: June 20, 2012, 04:24:03 PM »
never usedbacklash but Rich has a rule which seems to work, 50-50-50 rule I think he calls it. Cant really remember what all are for but almost certain its
50%  speed and 0.05 for shuttle, cant remember what the other one is for but try these and see.
Hood

Offline RICH

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Re: Backlash compensation again - and Xbox controller
« Reply #2 on: June 20, 2012, 06:01:02 PM »
John,
I have not used backlash in long time because I got rid of it on all my machines. Worth the time and effort!
I strongly recommend you do the same.

The  50-50-50 rule which you may want to consider is as follows:

50% - The max velocity is 50% of where your steppers will start to skip
50% - Set the shuttle Wheel setting in configuration to .0050  to .050 ( .5 too slow )
50% - Backlash speed

The Shuttle Wheel Accel is in seconds. How exactly is the" time" applied to the backlash "cycle"? Well I don't really know.
 It's internal to Mach. By observing the difference in how the motor reacts to different settings you will see that it affects the overall time
 it takes for Mach to take up the backlash and at say a value of 5 you will find that over five seconds the handwheel / shaft will turn
 rather slowly. At 0.005 sec its almost instantaneous. In the real slow setting, and say you jog a distance of .1"  you will find there
seems to be almost 3 distinct motor responses in the cycle,....... a small rotation then deceleration...... then backlash taken up
over a time...and then  accel and move to finish the distance.

So to get of flavor of it you need to start slow, observe, listen to see what is happening over an operating range.
Work your way up in settings.

RICH
Re: Backlash compensation again - and Xbox controller
« Reply #3 on: June 21, 2012, 05:32:29 AM »
Thanks Rich.  I had another go yesterday with a bit more success.  One thing seems to be that you can't make a jog move smaller than the BC compensation distance on a parameter, so if I choose an incremental jog of 0.1 mm it doesn't work but 1 mm does.  The behaviour on an axis is then exactly as you described with the shuttle accel setting at 1.  Unfortunately when I try to enter a different shuttle wheel setting it is not recognised - maybe I'm having DRO finger trouble, but I was wondering how this "shuttle" is related to "shuttle express" which I used to use as a manual input device but now of course is replaced with the Xbox control.  I've also realised that there is a fundamental problem with BC because if it doesn't exactly compensate the backlash I guess it can get progressively inaccurate?

Offline RICH

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Re: Backlash compensation again - and Xbox controller
« Reply #4 on: June 21, 2012, 06:36:31 AM »
John,
BC may be limited as you describe, I just don't remember since it's been a while since i used it.

Quote
if it doesn't exactly compensate the backlash I guess it can get progressively inaccurate
How repeatable BC will provide for addressing the non-movement depends on the mechanics of the axis. ie;
If the backlash is from a non preloaded or adjusted bearing there can be differences in the actual backlash, small maybe,
but it will be present. One would need to do some testing to confirm just how well it does when running a varying code with a lot of back and
forth moves.  

If you want to do accurate lathe work ( threading comes to mind) or 3D milling ( lots of code with small moves) then get rid of the backlash.  

RICH
Re: Backlash compensation again - and Xbox controller
« Reply #5 on: June 22, 2012, 10:30:34 AM »
Another update.  I've now made much more progress having worked out how to change the shuttle accel setting.  Basically I think if you type a new number into this field and press "enter" the general config screen just closes, and the new number typed in is lost.  Technique seems to be to type in the new number, then click the mouse cursor to plave the insertion point somewhere else on the screen, then press OK or enter.  This then saves the new parameter setting.

So I set the parameter to .005 s and BC works much more as expected now and with a bit of experimental tweaking of the backlash parameters it works much more consistently.  I can see though that if the backlash varies along the screw then one could build up errors during a run.

My reason for wanting to get BC working is for taper turning away from the headstock, but I must say I can't see any reason why backlash should be a problem for any CNC operations carried out in a way which is eqivalent to manual turning (such as threading) - am I missing something here?

Ideally I would like to eliminate the backlash, but the problem with the Super 7 is that the cross-slide doesn't have enough room for a ballscrew of the usual diameters (say 12 mm) as the nut is too large.  I have found 8mm ballscrews that would fit but they are much more expensive.  Also converting the leadscrew involves major dismantling of the apron that I'm not ready to face yet!

Another small observation on the Mach documentation, I assume that the term "shuttle accel" in the general config screen has nothing whatever to do with the use of a "shuttle" as an MPG - the documentation is practically silent on its significance, so maybe its name could be changed and a few more words given on what it does?

Anyway, thanks once again for the assistance,

John.

Offline RICH

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Re: Backlash compensation again - and Xbox controller
« Reply #6 on: June 22, 2012, 08:30:38 PM »
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am I missing something here
Yes, you will think you died and went to heaven when you don't have to worry about BC!
Trying to cut  certain class threads is almost impossible. Forget multiple threading. And holding close tolerance is rather difficult.

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found 8mm ballscrews that would fit but they are much more expensive
Yep, soon as you get away from the normaly purchased sizes the price goes up. You can have a look at what I did to put a ball screw in my 6" Atlas. Lot of work but was worth the effort. My lathe X axis ball screw came from a high end 2 axis positioning unit and was lucky to find it.

Have fun,
RICH