New build (my first) can't draw circles.

[mcyoung]

Hello. Sorry to start off on this forum with a help request - am currently at the bottom of the learning curve.

I've recently bought a cheap(ish) CNC machine off EBay. These things come with a controller board that requires a PC with a hardware parallel port to do the real-time stuff so obviously I've ripped that out and replaced with an Arduino+GAUPS shield (which is gShield compatible) running the latest version of GRBL fetched from their source repo.

Despite my ham-fisted efforts it seems to work for straight lines in all axes for linear motion. However I tried the circle example from the GRBL website, adapted to mm, and I get what's in the image below. The preview in Grbl controller 3.6.1 thinks I should get a circle, and so do I. To state the bleeding obvious, I've taped a pencil to the tool housing to do this.

http://www.mycncuk.com/attachment.php?attachmentid=15620&stc=1


It starts in the bottom left of the image, draws the first arc (and a bit) correctly then goes wrong and doesn't make it back to the origin. I don't think it's bad connections or overheating as it's quite repeatable.

Does anybody recognise this kind of problem? Googling isn't turning anything up. Looks like some kind of arithmetic error to me but it really should work as far as I can see. I'm not necessarily looking for a detailed solution (although that would be nice) but right now I'm not even sure where to start looking.

Here's my gcode:

G17 G21 G90 G94 G54 G91.1
G0 Z10
F 500
X-100 Y0.
Z2
G01 Z0. F500
G02 X0. Y100 I100. J0. F500
G02 X100 Y0. I0. J-100. F500
G02 X0. Y-100 I-100. J0. F500
G02 X-100 Y0. I0. J100. F500
G01 Z2 F500
G00 X0. Y0. Z10

[m_c]

Looks like the stepper on the long axis (Y?) is stalling.
What stepper drivers have you fitted, and what voltage power supply are you running?
.
Realistically could be anything from loss of step pulses, a failing driver as it heats up, or something mechanical.

[Robjampar]

I think both x&y are skipping steps;

Sounds like the power supply is hitting its limit, what psu are you using?

Check the voltages when one stepper is running, and then when two are!

[mcyoung]

Looks like the stepper on the long axis (Y?) is stalling.
What stepper drivers have you fitted, and what voltage power supply are you running?
.
Realistically could be anything from loss of step pulses, a failing driver as it heats up, or something mechanical.


I think both x&y are skipping steps;

Sounds like the power supply is hitting its limit, what psu are you using?

Check the voltages when one stepper is running, and then when two are!

Thanks for the suggestions. Having not used this kind of machine before I failed to see that both X and Y were asynchronously missing steps, it just looked like a weird pattern.

I'm using Pololu DRV8825 drivers with an aluminium heatsink on top. This was the problem. I've now replaced the heatsinks with copper ones and put them on the top AND the bottom (should have read the driver documentation better - the bottom is where they're meant to go). It now draws circles correctly. I'm guessing that the drivers were overheating cutting out, then cooling, then restarting, resulting in the bits and pieces of curves I saw.

The rest of the kit is just what came with the machine from China. Undoubtedly many other bits will turn out to be problematical but for now you've solved my problem. Thanks!