Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: MCONCEPTS on October 01, 2009, 10:03:24 PM
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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.
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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
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If you cut a square, is it perfectly square?
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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?
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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
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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.
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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.
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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
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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.
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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.
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mconcepts
try these settings for a starting base.play with a program that had notacable bad move.
set backlash speed to 45% -- set shuttle speed to 0.1 , this is a real good setting --- set look ahead to 5 lines.
play with this setup a little bit and see if you notice something.
bill
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Rule of thumb i have recommended to start with is 50-50-50.
-BACKLASH SPEED = 50%
- SHUTTLE SPEED = .050 ( i use .005, but try the slower speed first )
- MOTOR VELOCITY SETTING = 50% of max velocity ( if motors skip at 100 ipm reduce to 50 ipm)
Now increase your velocity up to 70% of max ( you are leaving 30% margin for cutting ) and try,
then start decreasing the shuttle speed down in increments to .005, trying each downward setting.
I would leave the backlash at 50% but if your velocity setting is at 70% of max, then 100 % may be just fine.
You need to play some as each machine is different.
RICH
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OK. I'm playing around with the setting but it's really frustrating. Each time I think I have it set properly it screws up again. I can get it set to complete a Job correctly (after having to cut test pieces), then I move to a new Job and the X axis is messing up, test again, works fine with compensation turned off, change the settings and now it's working again, then 30 minutes cutting later the Z axis screws up, test again now I've fixed that problem... Now I have to rerun the entire job to make sure it isn't screwing up somewhere else. I'm testing 90% and cutting 10%. I can't seem to find a setting that will make it work correctly all the time. Each time I can change the settings to fix the current problem but then it screws up somewhere else. It's messing up on really easy code that I don't understand. Below is the latest example.
N59767 G02 X15.5063 Y9.8769 I-.5063 J-.8769
N59768 G01 X15.5688 Y9.9851
N59769 G02 X15.5688 Y9.9851 I-.5688 J-.9851
N59770 G00 Z.1
N59771 M05
N59772 G53 Z0.
N59773 G53 X0. Y0.
N59774 M01
All it has to do is finish the pocket, move to Z +.1 turn off the spindle and move to X0 and Y0. Instead it moves to Z +.1 then moves right back down to -.1875 where it finished pocketing and moves across the part (cutting where it shouldn't) and stops at some random 8.??? 9.???? postion and stops. The funny thing is that the code is fine and the probem is repeatable. Only thing that makes it work correctly is turing off compensation. I Still don't understand what these settings actually do. What does Shuttle Accell have to do with Backlash compensation and what does the speed % have to do with why it would work correctly or not. It didn't work at all when set at 100%, worked but messed up occasionally at the default setting of 20% and now it is messing up at 50%. Do I just try 75% and see what happens? ??? I know it can't be anything else like noise or grounding issues since when it does mess up It's always repeatable in exactly the same place in the G code and it always goes away when Backlash compensation is turned off. Very Strange. I'm also only running at 20-30ipm with a 1/4" 2 flute cutter so I'm not pushing alot of speed for wood routing.
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First off.....you need to have the appropriate backlash set for each axis. Your feedrates are not that high.
What works for one axis may not be appropriate for another. So three axis need to be played with to get the combo right for you.
Shuttle Accel - It's how fast the backlash will be implemented during non-motion. Try this so you can see what i just wrote. Set the shuttle accel to 5, jog at 10 ipm or so and then jog the other way. Backlash will comp will come in on a change of directiion, and if you watch a handwheel on the motor or the pulley, you will see it takes forever. Now change it to say .5 and see the difference. Try .005.
Let me remark here, should you have a lot of backlash and a fast shuttle acceleration ( i use .005 ), and use a small end mill, don't be suprised if you break them. The cutter can't take the forces. I use 1/64" ball ends and it will break in a heart beat and they are not cheap!
It sounds to me like you are missing steps. When you miss a step, the axis has a mind all of it's owne, and it's off to where ever.
There are folks in here that run routers and work with wood and provide some first hand experience.
RICH
RICH
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I have different Backlash settings for each axis. I'm not using any for Z but I have tested and set X and Y correctly to get a perfect Circle. I tried the setting for the Shuttle Accel but got wierd results. With the setting at 5 and moving in one direction and then changing directions it changes directions quickly, slows down over about a 5 inch distance and eventually comes to a complete stop then slowly accelerates over the next 5 inches of travel until it's back up to full speed. I found this strange becasue I would have thougth it would have stopped at the point of switching directions then accelerated back to speed not change a full speed, slow down to a stop then accelerate back to speed. Does this seem right?
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It's been some time since i fooled with it a lot to see just the affect of the different settings.
I just tried it in version .028 and it does behave differently than what i remember.
Dosen't seem right as you said.
RICH
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Had the issue again on another Job last night..... I haven't gotten a chance to test this yet but I think I might have figured out the issue. For the last two times I have had the compensation issue it has been right before a tool change, after finishing the final cut. I think even the time before that it was the same situation but I didn't notice it util it started cutting after the tool change. I started having the backlash problem reappear after I upgraded to BobCAD 23 although the code looks fine and I hadn't cut enough after thinking the issue was originally fixed to assume it had anything to do with the BobCAD upgrade from V21 but BobCAD 23's CAM Tree and Mill/Router post automatically turns on and off the router for me in between tool changes (I really like V23 CAM alot, so no complaints here). I think I'm getting the bad movements in the table when backlash compensation is turned on and the router is being turned off after the last cut. The Mach Mill/Router post was setup to turn the router off immediatly after finishing the last cut on the tool change and then move to home X0,Y0. This is where it misses steps, it never properly makes it to X0,Y0. I've now modified the Mach Post file to have it turn the Router off after it reaches X0,Y0 and not before so I don't get any electrical surges/interference when the router gets shut off and backlash compensation is occuring after the final cut on it's movement to X0,Y0. I think this will solve my issues even though turning off the router at that point without backlash compensation turned on has never caused me an issue once it just seems to be a combination of the two that is causing the issues but I'll test and let you know. I have several posts that I can recreate the issues with.