Wednesday, 30 November 2011

Smooth Operator

You know you're an engineer when ...

... it ain't broke, but you still gotta fix it!

Having got my machine working nicely, I could see a number of areas for improvement; in particular:

a) I had quite a bit of "wobble" on the Z-Axis screw rods
b) The PLA slide bushes on the X and Y axes were OK, but were prone to a little bit of "slip-stick" friction, which would lead to "shudder" which could be seen in the filament make-up of printed parts.

So, I printed and installed some new Z-Axis Couplers http://www.thingiverse.com/thing:9622 to control the Z-Axis wobble.

A warning to anyone who replicates this:

In my original build phase, the illustrated guide recommended not installing mounting screws on the Z Motors http://julianh72.blogspot.com/2011/09/time-for-few-zzzzs-after-assembling-and.html , because the wobble is a known issue with the Prusa Mendel, and you have to allow the Z Axis drive system to flex a bit. Thinking that the new couplers were designed to solve this problem, I foolishly installed some screws to hold the motors down. (Somehow. it offends my engineering sensibility to have mechanical components which are not fully mounted!)

DON'T FALL FOR THE SAME MISTAKE!!!!!

Even though the Z Axis is now MUCH smoother, there is still some mis-alignment, and if the motors are held down tightly, you will probably hear all sorts of nasty grinding noises (or worse - and bust a PLA slide bush on the Z-Axis like I did!) Luckily, I had some spare PLA Bushes, so was able to fix this quickly,and immediately removed the Z Axis motor screws again. With the motor again free to flex a bit, the Z axis is now really nice and smooth again.

I also bought some LM8UU linear bearings on eBay http://www.ebay.com.au/itm/330621488095?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649 to install on the X and Y axes.

I made some LM8UU snap-fit holders http://www.thingiverse.com/thing:7755 out of PLA for the Y axis . (I'm not printing in ABS yet, coz I don't have a heated bed yet, although I have ordered the heat bed.) These came out OK, but are a bit brittle, so some of the snap-fit retainers broke when I was inserting the bearings; however, they are still functional, so I installed them anyway. I will make myself another (hopefully stronger) set when I replace the whole print bed to install the heated bed.

 I also printed myself a LM8UU X-Carriage http://www.thingiverse.com/thing:13054 out of PLA. This came out very nicely - nice and solid, and no damage when I installed the linear bearings.



So ... does it make any difference? In a  word: YES!

The Z-Axis wobble is almost entirely eliminated, and the X and Y axis movement is very noticeably improved - super-smooth and whisper quiet, and this all translates into really nice prints. I'll post some pictures of my current improved print results shortly.

 I still have some spare LM8UUs, which I intend to install on the Z axis using something like this http://www.thingiverse.com/thing:9936 maybe. However, I will probably wait until I can print ABS, because I'm not comfortable printing the X Axis brackets out of PLA, because the X Motor is mounted directly onto one of the X Brackets, and I am afraid it will soften and distort from the heat generated by the motor. (The X Carriage is separated from both the Hot End and the Extruder Motor, so I was happy to try that in PLA -  hopefully it doesn't suffer from heat distortion problems. Time will tell - and I didn't throw the original X Carriage away, just in case I damage my new X Carriage.)