Inaccurate after bit change

Hi, I did try to search for this. My apologies if this has been covered elsewhere.I have seen bit change issues regarding the z axis, but this is something different.

I have a design where I carve out the majority of the stock with a 1/8" bit, then clean up some interior curves with a 1/16". That second cut is off by 1-1.5mm on the Y Axis, which might as well be a mile in this case.

I create my work in Fusion 360 and use UGS to send the gcode. I have been getting very good accuracy within any single cut.

My process has been to home the X-Carve, then reset zero. I then jog to the coordinates where I want the job to start from, noting the X, Y, and Z position, then reset again. Once the first cut is done, I change the bit, home the unit (so that I have an absolute frame of reference), reset zero, then jog back the coordinates I had noted previously (I then adjust the Z as necessary, but I like I had mentioned this isn’t where the problem is).

As near as I can tell, I am not returning exactly where I was when I jog to those coordinates again.

Is there a common checklist of things I can do to figure out the cause of this? I have calibrated the machine well enough that traveling from one end of the wasteboard to the other, on either axis, is working out as precisely as I can measure. What I haven’t done is confirmed that it travels from home to an arbitrary point with that degree of accuracy. I am not sure why that would be a different case though.

Anyway, thanks for all thoughts you have on this.

Are you sure your Y-axis is still square (or at least maintaining the same “square-ness”) following your bit change?

When you home the machine, since there’s only a limit switch on one side of the gantry and the second Y-axis motor drive signals are just cloned from the first, really only the side of the Y-axis with the switch is properly homed. If you’re accidentally moving the gantry when performing a bit change, you could be changing the distance between the sides of the gantry respective to each other, which wouldn’t be compensated for when re-homing.

Do you have your $1=255 GRBL parameter set? This maintains maximum holding current on the motors, making it very difficult to accidentally move the gantry or carriage.

No, I’m not sure about that at all. What you’re saying makes sense, so I’ll have to do some tests.

I didn’t even know about that parameter (everything CAD/CAM. gcode etc… is new to me). I’ll check, and if not i’ll figure out how to make sure it’s there going forward. Maybe if it’s sufficiently strong, after the first cut I could just return to zero (not home), change the bit and proceed from there?

Thanks a lot!

Since you’re using UGS, you can just send “$$” (no quotes) in the console, and it will return all of your GRBL parameters and set values. To set the max holding current, send “$1=255” (the setting won’t take effect until you send another move command, i.e. a jog). You can always re-disable the holding current by sending “$1=0”.

If the Y discrepancy is uniform both sides are off the equal amount => unlikely one of the Y´s are dragging behind.
If the Y discrepancy is uneven, larger to one side then you have drift on the “long” side.

I am assuming UGS is sending the same command as Easel does when work zero is set, that point in space is stored.
Meaning after a Homing cycle you can jog to the intended work zero position, zero your work space and carve you go.
After changing tools all you need to do is to re-zero Z and you are good to go.

I would test a similar carve using Easel only in order to rule out the possibility of an error occurring from F360/Post Processor issues.

Xcontroller? Check Reduced Idle current switch (Dip switch #4?), if it is enabled the idle motors only receive 30% of their power.
Also make sure $1=255 as cg49me mentioned.

My work flow with multi-tool jobs are, assuming Easel as sender:

  • Perform homing cycle
  • Jog to intended work zero (XYZ0) and set that as home position/work zero
  • Carve first part
  • Jog to a parking spot (Or use a predefined parking spot like G30 which in my case is Z raised and close to me - to perform tool change)
  • Rezero Z only (G92 Z0)
  • Carve 2nd stage using the stored work zero (which is an offset from machine zero)
  • Done :slight_smile:
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