Tram changing based on position?

Has anyone seen a problem where tram is changing depending on where the spindle is positioned?

I’m using a 1" surfacing bit, it does a pretty good job of show how off the spindle is. I had it dialed in nicely, so I ran a surfacing pass on the wasteboard. I had a 1/4" stepover, that was enough before to show if I had issues.

Now I’m seeing patterns on the wasteboard. Everything is fine left-to-right, but front to back the board goes from ‘ripples’ (slightly out of tram) to dead flat and back to ripples, about every 4". The effect is perfectly consistent left-to-right.

I doubt the extrusions are doing it, so the only thing I can think of is the V wheels. Could they be out-of-round enough to show this effect? That would at least explain why the effect is periodic.

v-wheels perimeter is about 2" though, but who knows, maybe the effects of several mechanical things add up to a 4" period. Out of curiosity, could you upload have a picture ? How many stripes are there along the Y dimension? Can you feel any slop in the Z carriage when you rock it front and back manually ?

I think it might be closer to 3" for the circumference? If they are 23.5mm v-wheels, the circumference is 73.8mm which is about 2.9".

I can’t check right now but I seem to remember that the inner diameter inside of the V groove (on which the wheel actually rides) was around 19mm, hence the ~2" (inner) circumference. But I may well be wrong and I’ll grab my caliper tonight to double check that (unless someone else does first)

Yeah… I had an odd feeling about that too that made me cross my eyes trying to figure it out, wondering if they truly sit on the apex of the V or on the sloped walls of the V.

I get about 70mm rolling distance on the old belt driven Z which uses, I think, the same V wheels and similar rail profile, which is consistent with a 22mm rolling diameter.


Thanks for confirming @LiamN, so 4" period is…something else.

OK, I put a ruler on it, and it’s actually about a 3" pitch.



That looks like it could well be V wheels, it could well be that they’re just over-tensioned and developing flat spots, alternatively they could be damaged. I think damage is less likely as this is symmetric across the length of X which would require the same damage left and right.

If you were to disengage both Y drive belts and roll the X axis back and forth, I suspect you would feel a lumpy bumpy movement as the V Wheels roll over the flat spots.

My suggestions in order of effort are

  1. Release the pre-load on all the Y plate V wheels so the lower wheels spin freely, then carefully re-tighten just up to the point where you feel some friction on the lower wheels but they still turn reasonably easily. (You should move the machine to one of the positions where the wasteboard is flat for this bit). Run the machine back and forth (N to S) for a few minutes and try again, the flat spots roll out fairly quickly in my experience.

  2. Remove all of the Y plate V Wheels one at a time and visually inspect them, they should be smooth and round and rotate cleanly on their bearings. Make sure that the shim washers etc. all go back in the right places when you put them back.

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OK, I’ll try re tensioning the V wheels and see if that’s the problem. I suspect I did have it too tight, as I was not able to easily rotate the lower wheels.

I’ve unloaded them and now can rotate the lower wheels, I’ll run it back and forth a few times then try another surfacing pass.

You shouldn’t be able to rotate the lower wheels w/o the carriage wanting to move — see:

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Thanks everyone, it looks like too-tight wheels was the problem. I re-adjusted them, and things are much better - the tram is now consistent across the whole board.


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