Circle is undersized | Reason maybe losing steps?

Hello,

I've been trying to calibrate my machine with a round gauge block with 3.5mm diameter.
When I run a program for a hole with exactly 3.5mm, I always end up with approx. 3.4mm.

So far, I've tried everything:

• Added steps under SCALE -> No success, as when I run a program for a larger pocket, the pocket is too large
• Added BACKLASH -> No success, as a circle isn't a circle anymore
• Changed the Acceleration -> No success, as nothing really changed
• Changed the Velocity -> No success, as nothing really changed
• Complete machine is checked for mechanical errors -> Everything is correct
• Checked for errors in the spindle pitch -> Ran the program at different positions

This is my setup:

• NEMA 23 stepper motor with 4.2A/3.0A (1.8 degree, 200 steps)
• Triple Beast driver with 1/10 micro stepping
• Ballcrew spindle with 5mm lead screw pitch
• MAX_VELOCITY = 40.25
• MAX_ACCELELERATION = 750.0
• STEPGEN_MAXACCEL = 937.5
• SCALE = 400.00
• BASE JITTER = 50000 (I know it's way too high but can this be the problem?)

When I use a straight gauge block with 90mm, the spindle travels the exact 90mm. I have also no measurable backlash and Z-axis works like a charm.
Right now, I have no more idea where the error can be. I'm also not the ultimate professional when it comes to calculating the stepper times but according to the data sheet of my driver I should input 2000ns for all values.

If anyone has any idea why my circle is smaller as programmed or had to deal with a similar problem, you can't imagine how appreciated your suggestion is! :woohoo:

Thanks a lot in advance & best regards,
Hancock

You might consider flex. Everything from the mill/bit to the frame itself.

Try cutting your circle and measure. Then cut the same circle in the same location again and measure that.

Try cutting the circle with a climb cut and measure that.
Then cut another circle with a conventional cut and measure that.

To check for missing steps send your machine to a known location and mark the location, best if you have a dial indicator. Then make your cut and send the machine back to that known location. If the location is correct you are not missing steps.

Rick G

Usually if you are missing steps, the "circle" won't be a circle.

What size tool are you using? Is it sharp? A dull tool will cut a smaller pocket than a sharp one. Try using tool compensation to correct for the under size cut. (If it is a light machine flex is a very likely the culprit.)

Hello,

thanks for the suggestions! It turned out that our Base Jitter was wrong and the driver was giving out the wrong steps or much much rather the ratio was generated wrong. I tested a way of 100mm back and forth and it turned out that there was an offset of 0.09mm. I adjusted the Base Jitter, the Max. Velocity and Max. Acceleration and now it's spot on.

Thanks a lot guys & best,
Hancock