Gantry Rail Height - Y-Axis Rail slightly above rollers

Mostly Printed CNC - MPCNC Troubleshooting
#1

Hi All
I am putting my J type (25.4mm) machine together and have noticed that my X-Axis gantry rail sits 1-2mm above its corresponding rollers, not flat down on them (Y-Axis rail sits down flat on its rollers). This means it can rock with the middle assembly and Y-Axis rail as the pivot point. It’s not a lot but it is there. I checked the rail and it is straight.

Is there a height adjustment I’ve missed? All I can think of is that the rail spacers are too thick which lifts the Y-Axis rail slightly (Y rail is on top). The Y rail runs through the top of the Middle Assembly which lifts the X-Axis rail slightly above its rollers.

Has anyone else noticed this? Am I getting too fussy?!

Thanks in advance.

Jono

#2

That’s a new one. Not sure what would cause that, are all your rails relatively straight? A bowed rail might cause that.

#3

I checked the Y-Axis Rail but not the others. I’ll do that tonight and report back.

Like I said, maybe the Rail Spacers are too thick? What are the dimensions of the Spacers?

Also, how much of a gap should there be between the Bottom Corner and Top Corner parts when they are assembled with rails and spacers but not tightened down? The Top part doesn’t sit down on top of the Bottom part, intentionally I believe so that they can clamp the rails, but how much of a gap should there be?

#4

If your spacers printed poorly they could cause that. In that case either reprint them or sand them down a bit.

There is no way to measure the spacers, two opposite circular cuts.

Gap is a few mm, like you said so you can clamp it.

#5

Thanks viscious1. The Spacers printed pretty well unless they are over-extruded but I don’t think so. I was thinking I would sand the Spacers to try to fix the problem.

#6

I spent some time on this last night, even sanded down a couple of Spacers but that didn’t work. I think I have found the cause of the problem however. When I released all the tension bolts on the Middle Assembly the rocking disappeared. Tightening Tension Bolt B (the long 5" one)_ seems to be the culprit but Tension Bolts C also to some extent. I didn’t have these extremely tight to begin with, no cracking plastic sounds!

I need to re-tension the Middle Assembly but can’t seem to get it right. I can get all the bearings engaged except for one in the lower XYZ (see attached pic).

I am trying to tighten the bolts just enough so that none of the XY or XYZ parts can move and get the XYZ bearings engaged. XY bearings are fine, easy to tension but that last XYZ bearing is a bugger.

MidAssem.jpg
MidAssem.jpg610×529 55.2 KB

#7

I spent yet more time on this last night and have finally got it resolved. After more tightening, rocking, loosening, repeat but add in lots of swearing, I started looking at if a shim of some sort would help.

I ended up putting a washer on the 5" long Tension bolt B, between the 2 XY parts. With this in place I found I could then get even rail tension by adjusting both TBA’s, without causing the rail rock. This also kept the Z rails straighter and in line with the Z lead screw (previously I could feel tension on the Z lead screw which I knew wasn’t right).

The above is what worked for me, it may not be right, but it is working at the moment to resolve the issue I had. Time will tell if it is correct. Hopefully this weekend I will get all my motors wired up to my controller, frame screwed down squarely to my table, belts on and then some test drawings done. That would be awesome because I am itching to get this going and use it in anger!

#8

Nice, Whatever gets you working!

You might want to get some calipers out and check your printers zaxis. Move it 100mm and see if it is spot on.