Hello all ...

I have been a happy mach3 user for about a year now and I've been trying out some 3D carvings lately for the first time. I've noticed something odd with very subtle elevation changes -- it's almost as if the machine isn't accounting for acceleration or something which has me thinking it may be CV ...

Here's what i'm seeing - sdc12633

And then weirdly, taller changes in elevation are crisp and very acceptable - sdc12634

Both are from the exact same carving - just different locations. I was using a 1/16" ballnose end mill, running at 80ipm (though in the steeper areas, it barely got over 25ipm) and this is what has me thinking it may be CV.

Would you guys also want to see my gcode? how about my XML file? Is this a CV thing?

Hi Beamer - welcome.

Yes - please post your xml and your gcode.

I figured i'd need to - haha - thanks... here they are...


Thanks for any help you guys can offer - I've tried tweaking the CV settings a few times but haven't had any luck so far.

Well from a quick look I think it IS down to CV. Your accel is VERY low (0.01G). Not sure what to suggest really because the CV tuning params have their limits and your accel is not bringing a lot to the party to work with. I think you'll find that any tuning you do will effectively disable CV before you see anything change. The problem here is your gcode is lots of very short moves so you really need CV. You really need to improve that accel.

Stirling, thank you for your reply. I wondered if my accel was low ... To give you some more background, this is my home built machine and I came to that accel value by trying to drive it as fast as possible without losing steps - I think i've done some tweaks to the machine since then and it looks like I need to revisit that. The mach docs say finding accel is best described as what "feels comfortable" and admits it's pretty vague.

Is there a more systematic approach I could take to find a more workable accel?

The machine does great on 2D work and this is really my first bit of 3D stuff so maybe I should use different settings for the type of cutting I'm doing?

Thanks a ton for looking into this for me - it's very helpful getting confirmation of my suspicions

I would't worry about the 'comfort' thing. I suspect that's more about turning accel DOWN to avoid too much jerk on high accel systems.

On your machine it's about getting as much as you can. Try upping accel until you get stalling and then reduce your max velocity. Try to find a workable compromise.

One thing you could then try if you're still having the same  cutting issues is to use CV distance setting. Turn it on and set it down to something similar to your 'subtle' height changes.