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I can't seem to get Backlash Compensation to work...
« on: October 01, 2009, 10:03:24 PM »
I have a 4'x8' DynaCNC Router and I have oval shaped circles. I've adjusted the gear racks for the X and Y and have minimized the machine backlash but I still get circles that are visibly not circular. I tried to configure Backlash Compensation and used the youtube video posted by Hoss Machine. I set all the settings, checked the enable backlash checkbox and restarted Mach3. The circles look exactly the same with Backlash turned on as they do with it turned off. I adjusted the settings to .07 on X and Y (alot of Backlash) still no change. Any idea what I could be doing wrong? The machine cuts fine otherwize. Just want to tune the backlash out as much as possible. I'm using version 1.90. This is a Stepper motor table using CandCNC controller.

Offline RICH

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Re: I can't seem to get Backlash Compensation to work...
« Reply #1 on: October 02, 2009, 12:13:55 AM »
The backlash will take out the non-movement of the axis. But if the steps per unit are not correct for the circle you are doing you can get an oval. Also if the axis system varies over distance.
I may be wrong, but the backalsh comes into play at change in the axis movement and it is not implemented over the  quadrant of the circle ( there are some programs that do that ). What is the measured backlash of each axis?
RICH

Offline ger21

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Re: I can't seem to get Backlash Compensation to work...
« Reply #2 on: October 02, 2009, 07:35:16 AM »
If you cut a square, is it perfectly square?
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: I can't seem to get Backlash Compensation to work...
« Reply #3 on: October 02, 2009, 09:59:28 AM »
I'll test cutting a square tonight and measure it just to make sure my issue are 100% with backlash and not the motor settings, altought from what I have cut so far, everthing turns out the size and position it should. I originally drilled my table using the router with 128 evenly spaced holes on the 4X8 table. It worked perfect but I could tell I had small backlash issues when changing directions.
The circle issue appears to be mostly backlash in the X axis and I will see if I can reduce it further with Machine adjustment. I tested cutting a 2" circle positioned at +1",+1", clockwise and with the machine starting at 0,0 and starting cutting at X2",Y1". As it rotates around the circle clockwise starting at the right side of the circle it cuts straight down for a small distance as the X axis changes direction from where it started, then starts the turn (creating an egg shape at the lower right side of the circle, does the circle well from their until it gets to the upper left side of the circle where it does the same as the lower right when the x axis changes direction, again, egg shaped upper left of circle. What supprized me is that the backlash settings in Mach didn't affect the shape of the circle at all, leading me to believe it isn't working. Even putting in .07 as the X and Y backlash, no change at all. It should have changed the shape of the circle a little. I'll keep testing. It has backlash settings for XYZ and ABC.... what are the ABC settings for? Is that for machines with more than 3 Axis?

Offline RICH

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Re: I can't seem to get Backlash Compensation to work...
« Reply #4 on: October 02, 2009, 12:08:24 PM »
Just something very quick as i got to go.
Since the backlash is taken you still can have axis issues gain or decrease. Now if i wanted to create the perfect circle i would do some testing at that particular diameter and actualy chang my steps per unit to accomplish it.
That's how i get a close tolerance bearing fit using the mill. But normaly this would not be something you would  do as it's really not practical and you need to test.
RICH
Re: I can't seem to get Backlash Compensation to work...
« Reply #5 on: October 02, 2009, 07:24:19 PM »
OK. Tested a 2" square... The square looks square but when measured, the width is 2.005 measured top and bottom and the heigth is 2.025 on the right and left sides. The corners appear to be perfectly square, It's just slightly taller than wide but that could also be a Backlash issue.
Re: I can't seem to get Backlash Compensation to work...
« Reply #6 on: October 02, 2009, 08:36:01 PM »
Ok. Figured it out. If I set "Backlash Speed % of Max" at 100 as suggested by HossMachine on Youtube, backlash doesn't work for me at all, it does nothing as if it was turned off. If I set it back to the default of 20 then backlash compensation works great! I have about .015 backlash in the X and Y axis and I'll play around with it further but I've got a very nice circle now.
Re: I can't seem to get Backlash Compensation to work...
« Reply #7 on: October 04, 2009, 04:27:11 PM »
Ok. I have it working but I found a big issue... I tested the Backlash settings and got it working perfectly. Then I went today to cut out parts from a 4'x8' piece of 3/4 ply and the first part cut perfect. I have to do multiple passes at 1/4 depth to cut the parts and on the second part, it cut 1/4 depth all the way around, then went around the last 90 deg corner where the G code tells the Z axis to move to +.2" then back down to .5" depth to cut the next pass. The Z axis starts to move up and then the stepper motor makes some strange noise (similar to binding), then moves down .5. At this point it's actually cutting about .70 depth and on the final pass makes a nice deep cut in my table. I spent hours looking at the CAM and the G Code and checking for binding in my table. Nothing is wrong. I  tested the same code maybe 12 times, each time it has the same issue in the same spot of the G code, the Z axis gets off when it try's to move up .2" after rounding the final corner. Mach thinks it moved up but the stepper never made it. Finally I turned off Backlash compensation and it works fine. For some reason the backlash compensation is conflicting with my Z axis movement. It appears that Mach is trying to do the X axis compensation at the same time my Z axis command is telling it to change height and it doesn't like it. Anyone know if this is a known but? I didn't catch it until the bit had already cut thru my spoilboard and into my table! :o

Offline ger21

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Re: I can't seem to get Backlash Compensation to work...
« Reply #8 on: October 04, 2009, 04:43:00 PM »
For some reason the backlash compensation is conflicting with my Z axis movement.

What's happening is the backlash comp is trying to move too fast, causing lost steps. What is your backlash speed % set to? try lowering it. Also try adjusting the shuttle accel setting on the general config page.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: I can't seem to get Backlash Compensation to work...
« Reply #9 on: October 04, 2009, 05:23:51 PM »
Gerry, Thanks for the reply. My Speed % is set to the default of 20 and the Shuttle is set to .005..... I did what I should have done in the first place and upgraded to the latest stable release of Mach3 after my last post and I ran the same G code and it worked fine, so something changed between the releases. Anyway, it's fixed now. Do you think I need to mess with the Speed % or Shuttle if I have it working now? I'm not exactly sure what they do, but it cuts very nice with it set to the way I have it now.