Missing Steps Poss TB6600 Fault

[Neale]

I've just been through a similar exercise on my own machine which has only just reached the point where I can start using it. First cuts doing some simple profiling were fine, but a bit of vigorous 3D V-carving left the cutter a couple of mm down on its starting point. I fixed mine, but your problem is very slightly different.

First - you end up high rather than low in Z. Typically, the error is the other way (as it was with mine) as the big load on Z is when you are raising the spindle as you don't just have inertia but also the deadweight of the spindle plus Z platform. Missing steps, slipping coupler, will generally have effect when the spindle is on the way up. In my case, I had stupidly forgotten to put a flat on the end of the Z ballscrew and the pulley was slipping. Easy fix.

Secondly, you have an error light flashing. Don't know what that means on your drivers, but it doesn't sound right! Do you have any kind of manual that says what it might mean?

Thirdly, you are using a laptop. Turning off power management is a small step in the right direction but I wouldn't trust a laptop to deliver reliable real-time pulses. Processors for mobile (basically, battery) systems can do some odd things where the processor just goes off and does some internal housekeeping. Doesn't affect normal use but it's a bit of a killer to realtime operation. That's why Mach3 is not officially supported on laptops. You might expect this to give random axis movement, though, and you are reporting a consistent change in one direction, so maybe that's not too big a problem in your case.

Lastly, and my best guess at the problem, is that you have the step signal inverted. It's not always obvious how to wire the BOB to stepper driver, and it is possible to get the signal wires the wrong way round (red face at my end - I wired mine very carefully but obviously not carefully enough). Mach3 wants the driver to operate on a rising edge of a pulse but if the signal is inverted, it's triggering on the falling edge instead. The quick fix is to go into "ports and pins" in Mach3 and swap the step signal for the Z axis to active low. What happens in practice is that on every direction change, you lose a step. On my machine I could check this by using my Z height-setting touchplate; I just repeated the height setting operation many times and (looking at machine coordinates, not work coordinates) saw that each time, the spindle had gone down about 4-6 microsteps (based on Mach3 DRO readings). Swapped signal polarity and now get repeatability within a couple of microsteps time after time. You are either going to need a height touchplate to do this, though, or use a dial gauge to measure actual height after multiple up and down movements.

Or it could be electrical noise...

Good luck!