Calibration gremlins and odd .001mm -Y jog behavior

I’ve spent the better part of today looking for a gremlin in my Shapeoko as several test parts are coming out under size by .1”-.2” . I checked for run out on the spindle and then setup a dial indicator to see what was going on in my xyz. The hdz looked good and was tracking well, but when I started checking the Y axis I was getting y axis measurements that were .1mm or more off from the jog setting. I stepped down in mm increments jogging to see if larger steps or smaller steps magnified the issue when I notice that at .001mm increments the Y axis wasn’t responding to commands then it would try and catch up later. The X wasn’t as bad as Y but no where near as predictable as the z. So far I’ve retightned the belts, checked for wheel contact and checked all the set screws. The axis’s are as parallel as I can get them

Have you calibrated your steps/mm?

You have to remember, the machine can only move in micro-step increments — so 1/40th of a mm w/ the default belt, pulley and 8x micro-stepping — if you calibrate, then that number become some odd number, and the machine can’t get to certain positions which would otherwise be on the grid.

It’s a trade-off — not calibrating for steps gives one nice numbers in the interface which don’t match where the machine is actually at, calibrating has the machine actually where it is (more-or-less, belts don’t stretch exactly evenly) but you can’t get it to some numeric positions.

Ways to improve this:

  • get motors with more steps/rev. (I rather miss my SO1 which had 400 steps/rev NEMA17s)
  • implement a pulley reduction system (may introduce backlash)

Folks who get really serious about this will machine a part, measure it, then adjust the CAM file to adjust for how things are off.

The other possibility is it’s getting pulled into the cut — leave a roughing clearance and take a finishing pass, see: Adding geometry to cut as a pocket with a finishing pass

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I guess what I’m trying to wrap my mind around is what I’m seeing at the step commanded vs the dial indicator. So maybe I assumed incorrectly that if I’m jogging the Y axis in the - direction 1mm, that I should see 1mm of travel indicated on the dial indicator, I’m not looking at what Carbide Motion thinks. Its the same with the commanding Y to move .001mm I would expect it to move, but the lack of response in some cases, then crazy movement was even more bizarre. I can handle if the shapeoko can’t reliable move in the .001mm range within a small degree, but stepping up in the .01 , .1, 1mm range I’d expect more predictable results, otherwise parts are going to always be significantly off before I even factor in run out and deflection.

Remember, movement in-between the grid points will be interpolated based on the machine movement — it can be surprisingly accurate if one doesn’t get bogged down by minutiae.

Ok maybe I need more clarification on how we are referencing the grid. If the the stepper outputs 1/40th mm per rev, then in theory jogging 1mm should be spot on or pretty darn close, but when its not…where are my steps going? I’m not measuring in carbide motion but at the gantry with a dial indicator. I’m not mad about the state of the machine but trying to establish is something actually wrong, or is 1mm the tolerance of accuracy of movement. Why is my Z behaving so well and the Y and X so un predictable…

Have you calibrated for belt stretch?

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Thanks. That will be my next step. Still not really sure why I’m seeing the jogging issue on the Y axis but I’ll skip ahead to calibrating for belt stretch.

Mr. Adams on the link you provided, what is the .pdf link at the end of the article, it will not let me download it to look at???

if Y axis “hickups” you might have a v-wheel too tight…


That’s a good question — it seems to be one of the resources lost when the server had problems — researching now.

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