Not satisfied with tolerances

I bought my X-carve 9 months ago and haven’t been able to get it to cut with acceptable tolerances. Most work I do doesn’t need to be exact so I have managed. Sometimes I make a more precise piece with outer distance at 56 mm but it cut 55,5-55,7 mm. Some months ago it turned out to be acceptable 56,95 mm (and I don’t now why it “succeded” that time).
When I route a 6 mm slot it’s turns out dead 6 mm.

I have:

  • Adjusted the V-wheels (even changed them since there was a play between the bearing and the washers)
  • Adjusted belt tension
  • Stiffened the maker slide
  • Checked the size on bits
  • Calibrated stepper motor current
  • Calibrated stepping

Before I calibrated it was +1 mm on 700 mm (both X and Y) but I have adjusted it exactly.

I had some deflection before I did the stiffening mod and changed the V-wheels but it didn’t improve.

I searched the forum and discovered others had the same problem - but there was no real solution. So hit me with new theories and suggestions!

Please share more information… a photo and/or video of what you’re trying to accomplish. Also a description of the material. I’m sure we can help you get dialed in.

In order to cut accurately you need to:

  • know the effective cutting diameter of the endmill when cutting along both axes, and along a diagonal
  • calibrate the belts for belt stretch
  • plan the cuts so that the machine isn’t pulled off the toolpath (this latter doesn’t apply as much on a more rigid / upgraded machine) — look up climb vs. conventional milling and roughing vs. finishing passes

The usual diagnosis/calibration aid for this is a traditional machinist test piece, a stacked diamond, circle, square: https://www.shapeoko.com/wiki/index.php/Calibration_and_Squaring_the_Machine#Diamond_Circle_Square_Test

XY have a stepper with 200 steps, 8 microsteps to travel 40mm so your system resolution is 40/200*8 = 0,025mm. There are two sides on a square so if you are lucky they are on the same side of the positioning tolerance, if not you get 0,05mm difference.

Pololu states that setting the amps too high will prevent the microsteps to work properly.

My spindle has 0,05mm runout so in addition to the bit size which may differ it cuts 0,1mm bigger than expected.

I think aiming for 0,15mm on a belt system is more realistic.

I measured once again. The slot is 9 mm deep and it’s 6 wide at the bottom measured with digital caliper and but approximatelly 6.1 mm at the top. The bit is also 6. Can’t really get other readings.

So you suggest I try to turn down the potentiometers?

Would say it’s needless to put up pics. It’s just a 56 x 56 mm square, 3 mm deep in birch. But the result is pretty much the same on other materials or dimensions.

Here is my diamond, round and square test I use to evaluate the precision of my machine.

Every shape is cut in both inside and outside version, then measured and test fitted as an inlay. Visually they seem to fit exactly.

To be honest, the Y axis is spot on and the X axis shows a little backlash (circle is an diagonal positioned oval and the Y side of the square drifts away) of 0,2mm. Probably a loose belt.

For me nothing to worry about, hand cut parts for my model airplane never made it consistently below a 0,5mm tolerance so a consistent 0,2 is near perfect.

From the Pololu website: background on microsteps, must read
For the microstep modes to function correctly, the current limit must be set low enough (see below) so that current limiting gets engaged. Otherwise, the intermediate current levels will not be correctly maintained, and the motor will skip microsteps.

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