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Understanding Backlash Compensation
« on: December 15, 2018, 06:34:42 AM »

I'm using Mach3 mainly because there is this thing called "Backlash Compensation". So I figured out what my backlash is for each axis using a dial indicator, when I stumbled upon this fundamental problem of this compensation. Maybe its just how it works, maybe I'm using it wrong.

Some numbers first. I set my X-axis to 0.07mm backlash. Without backlash enabled, if I were just to reverse the direction when milling and mill a 100mm pocket, the pocket would be 99.93mm long. That's why I enabled this 0.07mm compensation. With this, when I just reversed (!) direction, Mach3 will add another 0.07mm to the lenght of my desired travel and the pocket will become truly 100mm.

Now image I would just go ahead in that direction and extend this 100mm pocket for anoter 100mm. Now the pocket will NOT be 200mm but it will be 200.07mm. Because Mach3 will add another 0.07mm to my feed lenght, however I'm still going in the same direction as before hence in this case theres NO compensation needed. I thought BC is there to compensate for play in the axis travel.

Backlash Compensation should only be calculated when there was actually a change of the direction you're milling. (but here's the next problem: how should Mach3 know what my last direction I moved was? Should be doable programming wise for sure, but now things get complicated)

Right now, when I do g0x100 five times in in one sequence, I will end up with a 500.35mm track, because the program will always add this 0.07mm to every move I do... When there is no compensation needed at all!

Please tell me I am using BC not correct. Because If this is how this "works", I can't see whats the purpose of this is at all. Thanks.
« Last Edit: December 15, 2018, 06:38:25 AM by Bobthebuilder »

Offline RICH

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Re: Understanding Backlash Compensation
« Reply #1 on: December 15, 2018, 08:23:10 AM »
Backlash comp is only applied when there is a change in direction and the intent is to compensate for non-movement of the axis.
I have never seen anything like what you are posting over my many years  on this forum.

BC works, it is just a "band aid approach" to fixing a mechanical problem that should be fixed / minimized in the first place.

1. An axis should be calibrated and tuned first.
2. Then determine the amount of non-movement.
3. Determine what is causing the backlash. Try to fix  / reduce the non-movement.  
4. Once you have enabled BC for whatever amount , save settings and exit Mach3 and reopen Mach3.
5. Now adjust the other settings that affect how backlash is done. You will find this info  in Members Docs>
6. After changes done in 5. above, save settings, exit Mach3 and reopen Mach3.

The above is for Mach3 using just the PP. If you are using an external motion controller, plug in, whatever, then consult the info for that devive specific to BC.

Re: Understanding Backlash Compensation
« Reply #2 on: December 16, 2018, 04:38:41 AM »
I can confirm Rich's words, BC does work at least for small backlash.  I haven't got it to work on a lathe without ballnut on the leadscrew where the backlash is much greater, but one gets used to only cutting in one direction on the lathe anyway.
Re: Understanding Backlash Compensation
« Reply #3 on: December 16, 2018, 01:43:46 PM »
Hi All
Backlash works on my machine, .003 thou on the x axis, It makes for accurate movements but does not stop the table moving .003 thou when the cutter forces are in the wrong direction!!
Re: Understanding Backlash Compensation
« Reply #4 on: December 16, 2018, 01:54:13 PM »
If you have backlash you should not climb mill even with compensation on, as the backlash will allow the cutter to drag itself forward possibly breaking the tool.