Mendelmax y-axis 180 degree bearing retrofit
by dreameredeemer, published
In case you haven't heard, the mendelmax 1.5 y-axis suffers severely from being overconstrained; for an excellent explanation of what this means and how to fix it in general, see bilsef's post here: http://goo.gl/U2XJN
The gist of it is that one of the rods needs to be constrained only vertically, by a setup alternatively called an idler, follower, or 180 degree bearing. Using cartridge roller bearings for this is slightly less than ideal since they're not designed to take a point load (effectively), especially on the outer shell... but an abuse of spec with the risk of rapid wear is better than an entire machine being completely useless if you ask me. At any rate, this makes the laser cut carriage glide freely as it should.
You'll need some approximation of the following hardware:
2 m6x30 hex cap bolts
4 m6 nuts
4+ m6 washers
2 606 bearings
2 m5x20 allen cap bolts
2 m5 lock nuts
4 m5 washers
Print the part. The scad is mostly parametric, so you can adjust things as necessary (further fiddling may be necessary). 606 bearings are the biggest usable OD--they just barely rub the insulation.
Install the bearings on the printed part. Pull the idler rod and remove the bushings, then mock up the fit and mark/outline where it will go; it will end up about center in both dimensions. The part can rotate 10 degrees to close a gap in case the bearings are too far apart. Pull the crossmember and drill holes as per your marks. Mount the part to it (you'll want to install them opposite of what's in the picture so that you can get a wrench on the nut) and reassemble, reinstall the carriage. Push it back and forth over and over, basking in the glory of a carriage that glides so nicely.
Next steps are to design a new belt clamp positioned as close as possible to the bushinged rod aka master rod. Then make sure your x-axis isn't overconstrained in the same manner the y was (many x carriage designs are overconstrained). The z-axis is a little different; having bushings on the idler side x rods somewhat decouples the z rods in that dimension, so even though it looks like the y-axis, z isn't overconstrained.
Despite all this, I'm probably going to end up buying (grudgingly) the new y-axis kit, mostly because the 8mm rods aren't thick enough to handle the weight of the aluminum plate.