Thursday, September 22, 2011

Better, Faster, Stronger...

Over time my Y-axis in particular has gotten more and more troublesome. I was using PLA bushing holders and bronze bushings. When aligned well it worked great but the tolerance was very tight and the slightest change in the rods caused the bushings to bind. I made the mistake of trying to lubricate the setup with some white grease I had for my bicycle, and it worked initially, but eventually it seemed to thicken up and made it worse overall. So I cleaned up the rods and used some light machine oil that at least got me back to where I was before the bike grease blunder.

My copy of Digifab's LM8UU holders
I had ordered some LM8UU linear bearings a while back and finally, about three weeks after the order, the boat from China my new bearings. This was definitely the time for an upgrade. There are several options for bearing holders available on thingiverse, but I chose Digifab's bearing holder because of the provision for cable ties, and the ability to access the attachment screws with the bearing in place. I crossed my fingers through the whole print as the Y-axis had started skipping steps recently. The holders printed out fine and I started making up a new bed using a scrap piece of smoked plexiglass.

Old bronze bushing plate and new LM8UU plate
I decided to go with just the three bearings and I have to say it worked out very well. I was initially concerned because there seemed to be quite a bit of play with just one bearing on the rod, but once all three were mounted to the plate and in place on the machine, it was fantastic just to feel how smooth the movement was and with no noticeable slop.

Fully installed bottom plate with LM8UU bearings
I also decided to try using just three leveling screws instead of the usual four partly because the new holders didn't leave much room near two of the corners of the plate, and partly because I found trying to level four points to be annoying as one always seemed to be loose. I still haven't decided if I like it better or not. Need some more experience to really know whether I want to stick with it.

I used the cable tie trick to tension the Y-axis belt and it worked great. Thanks to whoever posted that on thingiverse (nophead I think). Once I got the belt on, the amount of resistance to movement with the plate increased dramatically. Way more than could be attributed to just the stepper. The only potential source of friction I could see now was where the belt rubs the fender washers next to the 608 bearings. I loosened the nuts holding the fender washers and the resistance dropped significantly. I had no idea the side of the belt could create that much friction from slight misalignment with the bearing. So I moved my bearings a bit and tightened it all back down again and the plate movement was much better.

Printed off a quick rocket shot glass and I have to say I'm completely happy with the new Y-carriage. The walls are much smoother and more consistent. Time to do something about the X-carriage.

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