X and Y Roller very loose on conduit

This is my first post so hello to all other builders.

First some info:
-Metric parts are printed in PLA on a Migbot Prusa I3. 0.2mm layer 80% infill.
-As a conduit I am using 25mm aluminium round tubing.
-X and Y rollers are mounted so that the double side bearings are inside the frame as shown in the instructions.

  • No steppers or Gt2 belt mounted.

What I am wondering about is how loose the X and Y roller is on the conduit. The side bearings doesn’t even press against the conduit. Is this a normal situation ?

In the middle section the conduit fits very tight, to begin with I thought that it didn’t fit but by pressing the conduit quite hard into the bearings I managed to fit the conduit.

As a result of the very loose X and Y roller I can turn the whole middle section several centimeters.

Can you upload some pictures. Sounds like something is wrong wit the assembly.

Hi Vicious1,

Congratulations with winning the contest, you also got my wote. It is a great design.

I have taken two pictures showing how it is possible to twist the roller on the conduit.

One way
and the other way

Maybe my bearings outer diameter is to small they measure 22mm. But I am using the same type on the middle section where they are pressed very firmly against the conduit.

I haven’t assembled the machine completly but I cannot see how I could have assembled the rollers in the wrong way. The top bearings are pressed in between the sides. And the other bearings are securely mounted. Btw. I am using metric screws.

I have used the holes in the 3D part as they were, meaning no need for drilling.

Any suggestions ?

Sorry wrong links to the pictures I will try again

Can you measure that aluminum tube, just to double check, and you are using M8’s?

Outer diameter measurement of the aluminium tube is:
Minimum 24.8mm and maximum is 25.1mm.

And I am using standard 8mm fully threaded screws.

The tubing fits very tight every place else. Like the corner block, the lock on the roller and so on.

I think that it would be possible to enlarge the top bearing holes in the roller and then force the top bearings down. And in that way force the side bearings to press against the tube. Can you see any problems in the rest of the design if I do that ?

That won’t work the bearings would move over time. Something happened with that print. Are you sure you didn’t scale it up at all?

Are these dimensions really close to yours I have a feeling the 39 will be wrong.

You are right.

39 measures around 39.3mm and 75 measures around 75.3mm.

I have tried different things, and it seems that there is some play between the 8mm screw and the bearing (Bearing inner diameter 8mm, screew 7.75-7.8mm) which is the cause. If I push the bearings towards the center of the tube when tightening the screw. Then the roller fits much better, could be a little tighter, but that can be caused by the .3mm oversize of the 3D print.

Again I must say that your design is very good! If every thing runs with so small tollerances it will result in a great CNC.

I will try to find some other 8mm screws that are closer to 8mm as the one I got.

Thank you very much for your help and great work.

FYI my printed part measures pretty much exactly 39mm and 75mm. You might want to print some calibration cubes and check your stepper calibration values.

My 1/4" bolts (imperial version) fit the holes very snugly. Some of the holes needed a very light filing to get the bolts in. I agree that the loose bolts are probably your main problem.

Hello to all, and my first post.

I have completed the structure of my machine, and experienced the same problem of looseness of the runners on the tubes.
The finished size of the prints were 39.5 mm, height spot on. To cure the looseness, I placed the runner in boiling water for about a minute (water level to the top of the splayed legs) and squeezed the legs together. Was 76 mm across and made them 74.5 mm across.

This made the bearings tight on the tube, perhaps too much so , but I will hot air heat them to ease the pressure. Maybe 75 mm wide would be a better fit. PLA plastic has some interesting properties. I have some bearings on the Z axis that do not touch, so will heat adjust the middle Z parts.

I am using 25 mm chromed steel curtain rails - has a wall thickness of 1 mm, but seem stiff enough. Maybe too much pressure of the bearings would delaminate the chrome coating. We will see.

I have basically used my RepRap printer for small parts, patterns for foundry work etc, so am impressed with the design and thinking that has gone into this CNC Machine.

Thanks for sharing it with us.

Hi all,

I had the same problem. The solution of Bluff Chuffer works! I used a cork of a bottle of Port to get the right dimension.
Thanks all for sharing your experiences and of course thanks to visious1 for his great design and sharing it.