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
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
NOTES on Backlash in general:
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
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.