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Offline Jeff_Birt

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Re: Backlash compensation
« Reply #10 on: July 28, 2012, 06:33:10 PM »
With software backlash comp the controller is guessing about how to compensate for the backlash. Let's same you home your machine, it runs toward the home swtich, hits it and then backs off. At this point Mach knows which side of the backlash will be on (toward the home switch.) Anytime the axis reverses Mach will throw in some extra steps on that axis to take up the backlash. There are two big problems with this approach:

1) The amount of backlash you program into Mach must be exact. If you machine has more or less backlash than you program in Mach will be adding the wrong number of steps at each axis reversal.
2) The forces of the cutting process tend to push the table around so there is no way to tell where an axis really is so the backlash compensation will likely make things worse. If you have every been climb milling on an old manual mill you have likely seen the table jump like this.

I had a chance to meet the guys from Tormach at the CNC Workshop. Nice guys and nice machines. With a larger, more rigid machine that that you can take much larger cuts than you can on a Taig. Unless your using only really small bits I wouldn't guess that the 10K spindle speed would be limiting.
Happy machining , Jeff Birt
 
Re: Backlash compensation
« Reply #11 on: July 28, 2012, 07:06:57 PM »
It may be "guessing", but its guessing by the amount you tell it.  As you can see by the example posted it wasn't just "guessing".  It was losing position by a little bit on each pass.  It was either exceeding the limits programmed (most likely acceleration) and losing steps or it was adding consecutively more steps in one diraction than in the other.  That just doesn't make sense.  Its ok to say you don't know too.  

My curiosity is peaked though.  I intended to try the same thing in axact stop although its much slower, and again with backlash speed set at a lower percentage of machine speed. 


I suspect either backlash compensation doesn't play well with CV mode or that somehow it doesn't use the standard accleration set when adjusting the screw for backlash. 

I would note the Y is the axis with the worst backlash, and always has been the one to go bad first and fastest under use.

« Last Edit: July 28, 2012, 07:10:43 PM by Bob La Londe »

Offline RICH

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Re: Backlash compensation
« Reply #12 on: July 30, 2012, 12:12:31 AM »
Bob,
All I can say is that if one wants to do accurate 3d work then minimise / eliminate backlash and have the right components for the machine.
I found doing 3D with backlash is like playing pool with a noodle for a cue stick........... ???
Same goes for doing accurate lathe  / threading work.

Carry on folks...... ;)
RICH
Re: Backlash compensation
« Reply #13 on: July 30, 2012, 09:20:53 PM »
Rich, I know there is no substitute for a tight accurate machine.  It still doesn't explain the radical failure and progressive loss of position illustrated. Especially when it cut the previous pass perfectly.  No changes to machine settings between passes, and yes cranking down the machine as best I can without using backlash compensation IS the way I have been doing 3D work.  Given the nature of the feedback I have received though I am going to have to conclude that BC as it is now is a waste of time.  A sloppy or rough cut would be expected, but what was shown is a progressive loss of position.  Backlash was measured with a dial indicator before experimenting with the compensation feature.  

Jeff, I don't know how you are getting sub .001" backlash with stock bearings and plastic pin couplers.  I finally broke down and ordered some angular contact bearings to replace the skate bearings.  I already converted the Z to helical coupler and will convert the X&Y when I convert the machine to ball screw.  (I have screws on the way.)

Tormach... push comes to shove I use a lot of .125 and smaller cutters for fine detail work.  There is a huge difference in material removal rates between 10K and 30K.  Even a 1/4" cutter likes speeds upto 24K.  


« Last Edit: July 30, 2012, 09:24:14 PM by Bob La Londe »

Offline Jeff_Birt

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Re: Backlash compensation
« Reply #14 on: July 31, 2012, 12:20:04 PM »
Bob, I have no idea why you would not believe me about what backlash I'm seeing. I'm telling you what I see on multiple machines which is also what Taig says it will so. It sounds like your just pushing things beyond their mechanical design.
Happy machining , Jeff Birt
 
Re: Backlash compensation
« Reply #15 on: July 31, 2012, 01:24:20 PM »
Bob, I have no idea why you would not believe me about what backlash I'm seeing. I'm telling you what I see on multiple machines which is also what Taig says it will so. It sounds like your just pushing things beyond their mechanical design.

Obviously because I am a total idiot and incapable of doing simple experiments like cranking down everything until it stalls and then backing it off enough to run even to do a simple no cutting load test of backlash under ideal circumstances.  

I did exactly that yesterday.  I started by turning backlash comp off obviously.  No load.  Cranked down nuts until it stalled, then backed them off.  Did the same thing with gibbs.   Mobil way oil on the saddles and spindle oil on the screws to give it the best chance of good results.  Found there was a some backlash in the bearings so I replaced them with exactly the same part number bearing (I had spares on hand) and found there was still some back lash in the bearings.  Sorry, cheap skate bearings are going to vary, but I knew that.  Then I did some rapid change tests and found with everything tightened just so and well lubricated there was still about .0005 to .0007 just in the plastic pin couplers.  Less if I loosened up the table, but then what's the point.  LOL.  

I also did some cycle tests to make sure of repeatability.  The machine was not losing steps.  With a hundred cycles on each axis repeatability was sub .001" while I was testing.  Tests were run travel limit to travel limit on X & Y.  

I took your word that is "possible", but I still doubted the hundreds of hours claim.  With relatively new (less than a couple hundred hours) screws and nuts  and new bearings it checked at .0015 on X and .0025 on Y apx with everything as tight as I can make it without losing steps.  Checked with two different dial indicators.  One was a Starret.  One was a Shars import.  

Now maybe I have a lemon, or maybe I am really am an idiot, but after spending all afternoon experimenting (not the first time) I do have some difficulty with the the sub .001 for hundreds of hours without adjustment claim even with light cutting loads.  I'ld be hard pressed to believe it is possible on this machine atleast even with virtually no cutting loads.  I can probably get it under .001 with ACBs and helical flex couplers, but I would still be doubtful about being able to keep it there for hundreds of hours without adjusting it.  I could be wrong though.  I hope so.  The worst of the backlash does appear to be in the bearings.  Since I couldn't find ACBs that small with grease seals I'll have no choice but to set up some kind of oil mechanism on them.  

I would note that the Z axis way plate on the machine was never straight from the factory, and I had to shim it in order to get it close.  Maybe I did get a Monday morning Taig, but the the design just doesn't impress me that its capable of hundreds of hours of actual cutting with sub .001 backlash with stock components without having to readjust it.  

When I got your "it must be you" comment I decided to test it to the best of my ability.  You have setup a bunch of these afterall, and I have setup one.  

BUT NOBODY WHO HAS RESPONDED TO THIS HAS YET TO GIVE A GOOD ANSWER ABOUT WHY THE BACKLASH COMPENSATION CAUSED THE RESULTS I GOT.  Which was the original point of the thread.  I do have an idea about what could cause it if it were machining in mixed milling mode with lots of short (high resolution) reversals on an axis, but this was not the case.  The cut shown was done in conventional mode with each cut pass only having short move reversals on the Z which amazingly only dialed in at .002 backlash before the cut.  I have not checked it after because I have been playing with the X & Y for this round of testing.  Sure a sloppy machine may jerk and move depending on varying cutting loads, but a consistent loss of position per cut pass in one direction doesn't make sense.  

Added:
I might add in order to isolate causes of backlash I turned the screws by hand for some of my tests. 



« Last Edit: July 31, 2012, 01:32:49 PM by Bob La Londe »
Re: Backlash compensation
« Reply #16 on: August 10, 2012, 03:52:13 PM »
Well, Jeff, I might owe you an apology.  I dialed it down to mostly the bearings and the pin couplers.  When I installed my ACBs (with my own spacer so they worked like paired ACBs) I got it down pretty close to .001.  I was on the phone with Taig this morning over an issue with another part and I discovered something.  There is supposed to be a similar spacer between the skate bearings that Taig uses that was never installed on my machine that makes the skate bearings perform similarly to paired ACBs.  It was never there on any of my 3 axis. 

Offline Jeff_Birt

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Re: Backlash compensation
« Reply #17 on: August 14, 2012, 12:56:44 PM »
Glad to hear you are getting it sorted Bob. I figured something was amiss as what you were seeing was different that the norm on Taig machines. I would not have guessed as the spacer plates being missing though. On the coupling straws I have found that keeping the gap less than the recommended 1/16" really helps. Also make sure that the end of the leadscrew (or nylock nut) is not actually touching the stepper shaft. Sometimes it takes me a few tries to get the coupler half on the stepper motor in just the right place so as to ensure a small gap between them. If they touch you can actually put enough pressure on the stepper shaft to cause problems. I have also tried a few alternative couplers be have never been happy with the results.
Happy machining , Jeff Birt