Mysterious Z axis errors

[dachopper]

Hi everyone,

Started routing a piece of plywood using UCCNC, with a cut that is uniform and parallel from end to end, to find that my Z axis is loosing position as it moves along the piece.

The first time I was running constant velocity mode, with max acceptable error of 30 microns. I ran 95cm in length, the Z error was actually to push the Z axis into the piece further ( create a deeper cut ) , and over 95cm, I had a change in Z position of about 1 to 2mm deeper cut.

The second time, I changed the constant velocity mode max error to 50 microns, decreased the Z axis max acceleration by 60%, and max speed to 30% to discount the motor missing steps due to load at higher speed/accelerations. This time the Z errors appeared to occur much faster, and after about 20cm, the Z had plunged down an extra 4.5mm.

The G-code is correct, and I confirmed that without running the program, jumping from start to finish showed that the work was 4 to 5 mm deeper at one end vs the other.

My thinking is that it somehow has something to do with Constant velocity mode ( although this is not supposed to create compound errors, just errors between point a and point c ) - or my controller not hacking it, or not providing enough holding power sometimes ( G540 )

The wierd thing is, when I slowed down the z axis and accelerations, it got worse - which you would think would make better if it was a load related issue?

Still wainting the leadshines to arrive, so need to persist with the G540 for now


Any ideas?

Nick

[Neale]

Was the cut supposed to be uniform in Z (i.e. no Z movement along the cut) or was there some Z movement?

[Clive S]

Was the cut supposed to be uniform in Z (i.e. no Z movement along the cut) or was there some Z movement?

Are you sure that the cutter is not being pulled out of the collet?

[dachopper]

So, the cutter is firmly in the collet. The part is uniform z from end to end, but across the part ( side to side ) there is z movement up in the middle, and down at each side. The CNN pass is parallel going across the part, move along by .5 mm, then across ... hundreds of times.

[Neale]

If there is a lot of Z movement, and you are sure that the mechanical side of things is spot on, have you seen this thread?

[dachopper]

Thanks for pointing out that thread Neale -

I found something funny happening to the Y axis in my UCCNC software, where my slaved axis was re-copying settings from another axis, including the wrong step and Dir information when I changed a specific value on one of the pages ( I'm not talking about the value of boxing slave, it's something else like if I make a change to the acceleration value, then the slave dumps all info and re-imports all info including the wrong pin and direction assignment ) . I still haven't figured out which value it is because I don't change the settings enough, but the randomly see my Y axis trying to go opposite directions as a result sometimes of me changing a few parameters.

When I went back in to fix the issue ( not knowing why it had changed, or how ) in the confusion of trying to get my Y axis motors to spin in the right direction ( together ) - I Clicked on active low, so I had some dir / pin steps on active low, and some on active High for my X and for my Z axis.

Guess where the errors were in my cut - in my z axis was the biggest, but X also had a left / right 1-2mm error by the end. So hopefully in a couple hours I can restart the beast up, and get my first workpiece cut out !