Hi,

I had been assisting Art in testing this new enhancement over past couple of days. I promised to document this mod. Here are some relevant notes that pertain to stepper motor setups in particular:

If you are using stepper motors, and employ backlash correction, you MUST use the "Exact Stop" mode in order for this correction to be effective!

The problem was originally manifesting itself as lost steps. It was evident on stepper systems with backlash (BL) correction enabled, and was most prevalent for multi axis moves involving at least one axis having a small travel, less than the size of BL, with the other axis having somewhat larger travel. The most likely issue arose associated with velocity blending, as one axis was sometimes traveling in both directions in order to compensate for the BL. This would cause one of the motors to have a mini-lockup, and miss some steps, due to high step rate being erroneously applied.

The long standing problem has been corrected in Mach3 v1.90.004 and the moves are internally is split up in two sections: BL preload (where Mach makes the BL correction only), and the actual move. This works very well, but due to various issues it can not work in CV (Constant Velocity) mode on the stepper systems.

To test that it is working for you, use a G code file with many small moves. You may want to load the roadrunner.tap, and adjust the feed command to 80% of your maximum IPM. After the program had run, your external, independent DROs (or whatever you use) will be in sync with Mach3. If you use CV mode, this will not likely be the case.

NOTES on Backlash in general:
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Any system will suffer from dynamic backlash to some degree. I say dynamic, as some systems employ force preload, which is fine when they are still, or crawling, but not when they are slewing a heavy cartridge at high IPM. BL correction is a wonderful tool to make an accurate system a superb system. It is not a substitute for good mechanics. Thus, if you have more BL than you can tolerate, look into ball screws etc. Once you had fixed the prevalent mechanics, apply the BL correction. You can expect a 10 to 20X improvement in backlash related accuracy, mainly due to leadscrew thread uncertainty.

For example, my system has about 2 to 3 thou of native backlash. With the BL correction, I can accurately position my tool to better than +/-0.1 thou. If you are starting with 30 thou Acme BL, best you can hope for is +/- 1 to3 thou after the correction. Although this may sound OK to some, do remember that as your tool is cutting, it exerts tangential forces on your carriage, pushing and pulling, depending on direction of the cut and the tool. If the cut is heavy, the carriage will be bouncing back and forth through the before mentioned 0.030" lash region, and you will experience grave inaccuracies, and terrible finish, not to mention danger to your tool and work if using climb milling.

Thus, BL correction, although a great feature, is no substitute for good, sturdy mechanics.

I hope this helps.

Regards,
Chris K.

Chris K
Yerry good post.
Steve

Chris I appear to be having the same problem in version 2.0.033 and I can't seem to locate the "exact stop" setting. Can you point out where it is located.

thanks

Look in general config. You can set it to either exact stop or constant velocity.
You can also use G61 at the start of your code to put the machine in Exact Stop mode, or G64 to put it in Constant Velocity mode.

Thanks ger21

I just found the Exact stop setting.. It was right in front of my face and I did not see it... checked it like 10 times.... Must be going BLIND.

I just tried it out machining air. No more stalling, but a lot of stopping for 1/2 second or so at every change of path (exact stop). Is this going to be good for the parts? Going to get dimples where the stops are at?

Your best bet is to get rid of the backlash so you can use CV mode. Backlash comp in Mach3 doesn't work well with steppers at higher speeds.