How Jerky/Uneven does your X Travel Feel If You Hold the Z Carriage?

I noticed a problem this morning when Rapid Jogging in CM where the entire X gantry would visibly shake when traveling X+ or X- (rapid jog speed only).

I loosened my V-wheels a bit and that seemed to help the visible shake, but if I hold my hand on the Z gantry (HDZ in my case) I can feel a bit of shake/jerk/vibration/ general unevenness to the X travel motion. The issue is worse if I apply resistance as it is rapid jogging?

Belts are good, X pulley is tight.

Bent Motor Shaft? Motor itself? or just par for the course?

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That doesn’t sound normal to me.

Does it do the same when you ask it to rapid between compass points?

I’d probably release the X belt at both ends and manually roll the HDZ up and down the X axis to see if that moves smoothly without the belt & motor. I’d also stick a dial gauge on it and check that the HDZ wasn’t deflecting any more than expected.

That’s the rapid moves I’m referencing. Compass point clicks from SW to SE let’s say.

I did this before messing with the V wheels and when the motor had no tension on it felt like it was rotating normally. Keep in mind that I was just turning the shaft with my fingers.

Where am I sticking it and what am I referencing?

Did the HDZ roll up and down the X axis smoothly as well? I’d look for lumps and bumps in the V Wheels or their bearings first.

What sort of frequency of vibration are you seeing / hearing?

Is is a lump lump lump that goes with V wheel rotation or a faster bzzzzz like what you get when you crash the machine into an endstop and the stepper sits buzzing?

The movements you’re looking for are the Z nod and Z rock movements in the diagrams

To do a quick check on these I normally

  • send the machine to the “C” rapid point
  • drop the Z until the spindle is close to the spoilboard, leave the machine on with the motors holding position

Measuring Z nod deflection

  • clamp the dial gauge base down to the spoilboard
  • set the dial plunger on the front of the spindle, close to the collet, you can indicate on the collet or a cutter but these rotate and it’s easy to get bad readings
  • push the collet forward and backward, I use a push with my little finger to ensure I’m only applying 2-3kg of force.
  • with an HDZ and V wheels I’d expect to see < 0.3mm forward & backward, any more than this suggests wobbly V wheels

Measuring Z rock deflection

  • clamp dial gauge base to spoilboard
  • set the dial plunger on the side of the spindle, close to the collet again
  • push the collet left and right with a little finger
  • on the HDZ and V wheels I’d expect to see < 0.4mm left / right (you have V wheel wobble plus belt deflection in this movement axis)

It’s a lot quicker and easier than it sounds once you’ve done it once or twice. You can also generally feel the knock if something is really loose.

It should be easy to check for bent motor shaft, drop the belt, power the machine up, tell it to initialise and watch the pulley to see if it wobbles up and down as it spins.


Yeah I’m feeling a lump, lump, lump; nothing really audible.

Yes, it rolled smoothly.

Deflection testing tomorrow.

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If it’s lump lump then it’s probably V Wheels, maybe one of them has fractured or grown a dent from being over-tensioned, could also be the bearings, maybe one ingested some crud?

When I had my V Wheels over-tight to try to get rid of spindle vibrations the machine would have a really noticeable lump lump in X for the first few minutes of operation thanks the the dents that had formed in the V Wheels whilst the machine was stationary, there’s some evidence of those here

Generally after a few minutes of moving the lumps had flattened out, it was a sign of me abusing the V Wheels though and remember that backlash goes up with V Wheel tension as the extra rolling friction has to be countered by belt extension.



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