Mendel X Axis Carriage - too much clearance from bearings to rod

I am just assembling a Mendel X Axis carriage (using using nophead supplied plastics which appear to be well made) and I've hit a snag.

Where the two groups of triple bearings are fitted I get an ok fit such that two out of three bearings contact the 8mm shaft but it's not great and the lower bearings are essentially doing nothing

On the side with the two pairs of bearings I've got a 9.16mm and a 9.34mm gap between bearing races.so they only contact on the upper side of the shaft.

The bearings are at worst 12.98 /12.99mm diameter, the rod is spot on 8mm.

The wiki only refers to adding washers between the two parts of the carriage to give more clearance but no guidance on what to do if there is too much clearance.

My gut feeling is that I should take a small round file to the holes and make them oval.

Can anyone see a problem with this approach?


Martin

Edited 2 time(s). Last edit at 11/24/2010 09:09PM by m100.

I had this sort of issue and I too got my parts from nophead. Nothing wrong with his parts though.

Firstly, make sure your Mendel is on a level surface. Then make sure your X-bars are level as well (ie: flat with respect to that surface). This way you ensure that the X-bars are running parallel to the base of the machine, and at a 90 Deg angle to the rods. Beware of any front/rear twist in the bars though, as that will throw off your measurement somewhat. The X-carriage won't slide easily if they're twisted though.

While this isn't a foolproof way of making sure it's flat, it works well enough till you tweak things later in the build. (when you adjust the bed)

I didn't do this first off, and ended up with enough of a difference between left/right height that eventually one of those bearings in the 3 bearing assembly snapped the plastic piece it was in after a while sitting there under load. Fortunately some glue suited to ABS solved that.

Note: I also found the distance between rods as detailed on the wiki varies slightly with different parts (size differences), so you may need to adjust them appropriately. I also made sure that I put the flat side of all the vertex pieces (the smooth side that was against the build plate) on the inside of the machine. This way, the measurement of J2 (which has to be the same for all the horizontal rods joining the side triangles) was always consistent without any stresses caused by differences in part height. (eg: if the print stopped 1-2 layers early).

The carriage isn't on the machine just yet. I'm test fitting with a single short length of rod and the slop is such that there appears to be close to zero possibility of the carriage being guided properly, for instance I can easily fit a 0.7mm feeler gauge either in the horizontal or vertical in between the bearing the the rod at the front of the carriage.

On the Y axis bearings I can very easily get a fit such that the rod is fully supported on all bearings, the X axis carriage doesn't even come close.

The overall height is visually ok in that the top of the two parts are fully aligned when bolted together, the overall height of the assembly being 34.25 - 34.52mm. How this compares to the design requirements I'm not sure as I can't seem to get any dimensioning on the models..

So I had another think...and snugged the clamping bolts a lot tighter than I had previously and the overall height reduced to 34.07mm and the clearance from the rod to the bearings reduced to near zero, but I've now got a mismatch in height between the vertical boss on the front edge and the upper carriage and a noticeable distortion when looking on the end of the upper carriage.

So I think I maybe need to reduce the height of the face between the two parts to bring the bearings closer to the rod...or file the bearing mounting holes oval.

Edited 1 time(s). Last edit at 11/24/2010 10:49PM by m100.

The instructions in the wiki are

Quote

Tighten the middle two caps, keep the outer two pairs loose. When fully assembled use a piece of ø8 mm bar to set the distances between the bearings. Slide the bar between the bearings and tighten/slacken the outer two pairs of M4x40 caps to achieve a smooth, low resistance, running fit.

It is normal for the carriage to be deformed to allow the bearings to meet the rod. If you think it is too much you could file or sand the posts in the middle of the upper carriage, but I have not needed to do that on my machine.

Spacings like this can be a little off because it depends on the layer height you slice the parts with.

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[www.hydraraptor.blogspot.com]

Thanks nophead, the deformation needed to get a suitable clearance seemed quite high and I was afraid of cracking the plastic either immediately or having a creep problem and failure at a later date.

After a lot of faffing about I managed to get the design size of the posts from the bottom / top face (these seem a bit undersized but I put that down to the import / conversion process) and then measured what I actually have

For the lower carriage the design is 21.982 (presumably a nominal 22mm) I've got an actual measurement of 22.35 - 22.51

For the upper carriage the design is 11.934 (presumably a nominal 12mm) I've got an actual measurement of 12.19 - 12.32

Taken from the nominal sizes I've assumed this gives me a maximum of 0.83mm extra height, not quite the 1.16 - 1.34 (derived from the 9.16mm and 9.34 gaps) I measured previously.

I think I'll get the sandpaper out