Wandering Y Axis.....

[Lester]

Hi Guys,

Looking for some help with my home converted Amat30lv. Have made a few parts with success however I am struggling to to keep the Y axis from drifting whilst using adaptive tool paths. The picture show my issue, the pocket was started in the centre using a helix before moving to the thin right side and then back to the left. You can see the jump in the part where the mill returns to the centre to machine the fatter left side, but the y axis has drifted (this is obvous as well if i ask the machine to return to x0,y0)



Am running:
DM860N drivers
4Nm Nema stepper motors (wired in bipolar parallel)
ST-V2 breakout board
UC300ETH
UCCNC
Have tried running with a 48v and 60v power supply.
16mm ballscrews on X and Y and a 20mm on Z. All 5mm pitch.



Was worried I was loosing steps because I was trying to run too quickly approx 2500mm/min rapids (with a 1000mm/min cuttuing speed and a small DOC), so lowered the rapids to 1300mm/min (at 400mm/min with a bigger DOC). The drift seemed to get smaller, but I'm guessing this could be due to the larger DOC, less direction changes?

Am thinking it could possibly be due to my gib adjustment. After watching John Sauders' video on his Tormach and having a play I managed to get the Y axis 'lost motion' to be minimal and think there is about 0.001" of rocking when measuring the saddle from the chip guard. I did however struggle to get the X axis twist (measured from the table to the head to be less than 0.004" without tightening the gib so much that the motion is less consistent and jumpy.

Is this gib adjustment something I should be looking into more or is there anything else I should be looking into?

Also interested what kind of speeds and accelerations people are managing to achieve with similar setups.

Thanks,
Lester

[Neale]

Seen something like this on my own machine. In my case, it was on Z and the more direction changes, the worse it became. It was because the Z stepper driver wiring between the motion controller (integrated with breakout board in my case) and stepper driver had its polarity switched. The stepper driver interprets a level change to generate a step to the motor. If it steps on the wrong pulse edge, you effectively lose one step (microstep, in practice) on every direction change. With only a few direction changes, this might not show up but adaptive clearing generates a lot of direction changes.

Exactly how you fix this will depend on how the breakout board is wired to the stepper driver (in mine, this was a differential connection which has a lot of advantages but is a bit unpredictable from the polarity point of view) but the fix is likely to be as easy as s swapping a couple of wires, or changing the pulse polarity setting on the step port/pin for the Y axis. Can't help directly with that as I do not use UCCNC.

[Lester]

Seen something like this on my own machine. In my case, it was on Z and the more direction changes, the worse it became. It was because the Z stepper driver wiring between the motion controller (integrated with breakout board in my case) and stepper driver had its polarity switched. The stepper driver interprets a level change to generate a step to the motor. If it steps on the wrong pulse edge, you effectively lose one step (microstep, in practice) on every direction change. With only a few direction changes, this might not show up but adaptive clearing generates a lot of direction changes.

Exactly how you fix this will depend on how the breakout board is wired to the stepper driver (in mine, this was a differential connection which has a lot of advantages but is a bit unpredictable from the polarity point of view) but the fix is likely to be as easy as s swapping a couple of wires, or changing the pulse polarity setting on the step port/pin for the Y axis. Can't help directly with that as I do not use UCCNC.

Cheers for the help Neale, that sounds very similar to my issue.

Think my wiring would be in a 'single ended'/common anode config at the moment. Whats the benefit to the differential connection?

I know where the pulse polarity setting is so can try to change that later, not sure if i can see why one axis would be different to the other two (if my wiring is correct.......).

[Neale]

Differential wiring, if the hardware allows, is much more noise-resistant, which can help avoid false stepping and other interference issues. In practice, standard single-ended connections usually work OK, but I would always go differential if it's possible. Why the step pulse polarity is different on one axis (if it is...) is a question only you can answer! But more seriously, it's so quick to check that it's worth having a look.

I found this a frustrating fault. I had been using my CNC router very happily with no problem until I did a bit of detailed 3D engraving with it, and found that the cutter slowly seemed to sink further into the work. All the time, Mach3 was recording the correct depth. Obviously, the fault had always been there but with simple profiling cuts where Z is over-depth anyway, a tiny drift was never going to show up but with lots of direction changes, it was a different story. What you have shown looks very familiar but in Y.

[m_c]

If it steps on the wrong pulse edge, you effectively lose one step (microstep, in practice) on every direction change.

Just a minor correction. You actually lose 2 steps on the direction change, as the drive first moves one step in the wrong direction, so it takes another second step to get back to where it was before the direction change.

It's also worth noting that some drives (Leadshine in particular) require a relatively long direction setup time, which some controllers might not be able to do.

[JAZZCNC]

I know where the pulse polarity setting is so can try to change that later, not sure if i can see why one axis would be different to the other two (if my wiring is correct.......).

This is a common issue and happens with lots of drives/controllers and never gets noticed by most as they don't run toolpaths with a high number of direction changes like adaptive or 3D work.
The reason it can be different on one drive to another is often that it's set that way in software at the factor. The polarity switch just flips the state.

There is an easy test just knock up a test file that runs 200 lines of short back n forth G0 moves with last move back to Zero, 10mm is plenty. Then Zero axis and Mark the spot. If all is correct it will return to mark. Do this on all Axis.

[JAZZCNC]

Was worried I was loosing steps because I was trying to run too quickly approx 2500mm/s rapids (with a 1000mm/s cuttuing speed and a small DOC),

Pretty sure you must mean 2500(mm/min), not (mm/s) if not then that's your problem right there...Lol

[Lester]

Pretty sure you must mean 2500(mm/min), not (mm/s) if not then that's your problem right there...Lol

Whoops, of course.




Ps. To all of you, you absolute legends. Tried jazz's test after adjusting the step polarity. Seemed OK... Attempted the same pocket, absolutely perfect!


Few mins later and we have half a chain adjuster!

Over the moon, cheers guys!


Sent from my Mi MIX 2 using Tapatalk

[Neale]

Good stuff - nice to know that that was the problem! One of those things that can really worry you but with a trivial fix - once you know what it is... I lost a lot of sleep over that one when it happened to me, until eventually I discovered that the error was exactly equal to the size of one microstep times number of direction changes (I think, with apologies to m_c if my memory is playing tricks - I think I only lost one step per direction change).