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New Machine Configuration & Backlash Comp Issues
« on: February 22, 2012, 08:49:05 PM »
Before I start, I would like to say that I have spent the last week or so reading forum after forum about the various woes associated with backlash compensation. It was only about halfway through my search that I realized the odd relationship between mach 3 builds and the G540 controller... I guess its been cleared up since 09'-10'??

For the last couple of weeks I have been struggling to put the finishing touches on my 'JoesCNC Hybrid" rack and pinion CNC router. The main issue is that when enabled, the backlash compensation causes very rough movement when the compensation is actually utilized. Here is a short video where the machine moves around a 10" x 10" square, ending with a diagonal move to the center:


Thus far, I have tried the following:
1.Tore down and rebuilt each of the mechanical components twice in an effort to remove as much mechanical backlash as possible. In general, I am very pleased with the results: backlash of 0.0043" in the y-axis and 0.0070" in the x-axis (zero-backlash nut in the z-axis does a great job).
2. Re-read the mach 3 manual and performed a careful step-by-step re-installation of the software.
3. Verified BIOS settings for the parallel port.
4. Sent my controller back to Gecko to have them take a look at it. They couldn't find anything specifically wrong with the drivers, but they replaced the motherboard as a precaution.
5. Tuned the g540 trim.
6. Changed shuttle accel setting. I read several accounts of people changing this value from 0.005 (my current setting) to 0.05, and 0.5; the vibration that you hear in the video gets worse and worse as I go toward 0.5 setting.

Naturally, I can turn off backlash compensation and everything sounds / moves fine. I wanted to see if anyone else has run into this combination of symptoms before I request phone support, but if it gets to that point, I will post the results. My XML file is attached.

« Last Edit: February 22, 2012, 08:53:38 PM by NiOBoBeO »

Offline ger21

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Re: New Machine Configuration & Backlash Comp Issues
« Reply #1 on: February 22, 2012, 11:02:51 PM »
This video may help you with setting up backlash comp.

But, to be honest, I don't think many router users use backlash comp. The best thing is always to get rid of the backlash. Are you using spring loaded pinions?

Also, I doubt that anyone can fix the issue over the phone. Getting backlash comp to work "correctly" may require a long and tedious trial and error process, of trying multiple combinations of settings until you find what works for you. There's no magic settings that will work for all machines.

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Re: New Machine Configuration & Backlash Comp Issues
« Reply #2 on: February 23, 2012, 06:00:02 AM »

Thanks for the reply. I have seen the video before and my settings are the same (0.005s shuttle accel / 100% backlash velocity). The R&P setup is spring loaded- I'll attach a picture. As I mentioned before, I have achieved as little backlash as I can without major design alterations.

Why would CNC routers typically not employ backlash compensation??

I understand that there is no universal fix to this type of issue, but from what I can see, the shuttle accel and % velocity that the backlash travels at are the main influential factors. Are there other settings that compensation is a function of?

I cannot say for sure, but I feel like this issue used to be less pronounced; I would like to try the build that I initially started with and see what happens. I found the change log, but is there anywhere to download the previous Mach 3 builds?