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 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 , 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!)


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 to install on the X and Y axes.

I made some LM8UU snap-fit holders 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 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 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.)