Need Help!- X axis over travelling during run.

[JayCop]

When cutting a 5"x5" square my machine consistanly cuts two sides on the Y axis perfectly at 5" and 5 1/16" on the X axis sides. What could be causing the x-axis to repeatedly go over by 1/16"? When each axis is tested individually with a 1" dial indicator both are within a + or - thousandth or two. The repeatability seems to be very good.

I am using CNCRouterParts Rack and Pinion on both x and Y axis. The X axis is dual driven by 420oz steppers. My power supply is 36V 8.8amp. I am also using a gecko 510 board.

Any help will be greatly appreciated.

[PaulRowntree]

What happens with larger and smaller shape dimensions along X?
Does the gcode look right?

[ger21]

What if you cut it in different places on the machine?

[JayCop]

I will test some more sizes tomorrow, and on different places on the machine. I did try some smaller sizes but did not measure them and they are at my shop.

These were cut toward the middle of the Y axis towards the - direction of the X axis but not close to the softlimits.

I will attach the gcode tomorrow when I pull it off my machine computer. The dxf file was made with sketchup and the gcode with vectric cut2D.

[flojor10]

You should try backlash compensation, using this my new machine gets +/_ .0005in everytime.

[JayCop]

What backlash settettings did you use?

[JayCop]

I tested a 10" square and got a different result with the x-axis being short about 1/16". I have attached the g-code to both of the files I cut, as well as a picture of the 10" square with measurements.

[PaulRowntree]

I see that the gcode is using arcs to round the corners of the 10" 'square'. Could this be interfering with your physical measurement of the 'square's ' dimensions?

1/16" seems too large for backlash, but having a constant error on one axis that does not scale with piece size sure looks like backlash.

[JayCop]

The interesting thing is that on the 10" square it is short 1/16" and on the 5" square it was over sized 1/16". I cut the 10" considerable slower than the 5" one and turned my acceleration on the x-axis down to 10.

How can I stop cut2d from rounding the corner? The entry point corner is the only one that looked rounded when I cut it.

[PaulRowntree]

Originally Posted by JayCop The interesting thing is that on the 10" square it is short 1/16" and on the 5" square it was over sized 1/16". I cut the 10" considerable slower than the 5" one and turned my acceleration on the x-axis down to 10. How can I stop cut2d from rounding the corner? The entry point corner is the only one that looked rounded when I cut it.

Sorry, I missed that the 1/16" was short on one, long on another. And I also missed that you are cutting a profile outside a square, so the arcs pivot at the corner of the 10x10, leaving a squarer corner than the toolpath suggests without considering the tool as well. Actually, turning down the acceleration can cause corner problems if in CV mode, because the toolpath has to start turning sooner in order to keep the velocity constant, thus rounding it off.

[JayCop]

Should I turn off CV, or turn up accel?

[PaulRowntree]

I always have CV on, and look to the corners to see if it is rounding. To avoid problems, have higher acc (I have seen suggestions to look for the max acc that you machine can support without loosing steps, then reduce it by 10-20% for safety). To see if CV is a problem, just turn it off and look for differences in the quality of the cut (the machine will jerk around more)
CV problems are always at the corners. Large shapes should have the flats at the right positions even if there are CV problems.

Are the X and Y edges of the cuts at truly 90 degrees, or is it skewed at all?