Wednesday, 24 August 2011

Assembling the Y-is-there-always-something-which-doesn’t-quite-fit?-Axis

[Note to any readers who are only now happening upon this blog, and are contemplating undertaking a similar adventure of their own: The postings are in reverse chronological order (newest at the top), so if you want to read this as a guide to your own build project (and to benefit from the lessons of my own learnings and mistakes!), you might want to start at my first posting at the bottom http://julianh72.blogspot.com/2011/08/first-post-inspirations.html  (dated 20 August 2011) and work your way up.]

Having assembled and aligned the main frame to the best of my modest abilities, it was time to commence the assembly of the Y-Axis, comprising the two rails which run from front to back carrying the print-bed, the print-bed carriage itself, and the Y-Axis belt-drive system. I was a little bit nervous about the overall alignment of the frame (see my previous posting  http://julianh72.blogspot.com/2011/08/main-frame-build-and-alignment.html ), and in particular the fact that the Y-Motor mount was a bit warped (a consequence of the practical limitations of precision of home-baked RP parts, I guess) – hopefully this would all resolve itself in the following steps.


Again following Gary Hodgson's excellent “Visual Instructions” http://garyhodgson.com/reprap/prusa-mendel-visual-instructions I first installed the two round-bar rails, and then aligned them against the long sides of the acrylic print bed bottom plate which was supplied with my kit. I then snapped 2 PLA bushings onto each rail. At first, they were quite tight, but they all freed up nicely with a light smear of electric shaver oil and running them along the rails 20 or 30 times.

The instructions say to “put a dab of glue on the top side of the bushings”, but don’t say what sort of glue. I used “Super Glue”, hoping for the best. (So far, fingers crossed, it seems to be working well for all of the glued joints – PLA-to-Acrylic, PLA-to-ABS, and for some ABS repairs which I will talk about in following posts.) I then aligned and placed the acrylic print-bed bottom plate onto the bushings, put some weight on to keep the glued joints closed, and went to bed. (I didn’t want to take any chances with applying forces to the glued joints until they were REALLY ready.)

What I DIDN’T do was to measure the dimensions of the supplied acrylic print-bed bottom plate before I glued it on. In hindsight, I should have!

The print-bed bottom plate is slightly wider than the clear space between the main frame bottom corner vertices, which means that the print-bed cannot travel quite to the ends of the rails, limiting my Y-Axis movement to about 170 mm, rather than the 200 mm that the Prusa is supposed to be capable of. The clear space between the corner vertices is 234 mm; the print-bed bottom plate is supposed to be 140 mm x 225 mm (which would fit between the corner vertices with about 4 mm to spare on each side), but the supplied plate is 140 mm x 240 mm, so it clashes with the bottom corner vertices.



I have decided to proceed with the build accepting the slightly limited Y-Axis travel for now – hopefully this won’t cause me any grief later on, but for anyone who has the same kit as me:  you may want to check the dimensions BEFORE you install the Y-Axis print-bed bottom plate, and consider modifying it (and the top plate as well!) first.

Installing the motor and belt all went without a hitch – apart from the fact that the Y-Motor Bracket is slightly distorted, so the motor shaft is slightly skew to the toothed belt. This would probably be a big deal if it was a geared drive, but the motor and print-bed both seem to move smoothly, so hopefully this will be fine. I will only know for sure when I get power to the motors later in the build.


The only other slight issue I have is that the bottom run of the toothed belt is making slight contact with the threaded rod which will anchor the bottom ends of the Z-Axis rods. It is only the slightest contact on the smooth side of the belt, and it doesn’t seem to be catching, but I will probably tape over the threaded rod to avoid premature wear and tear on the belt.

[EDIT (07 September 2011): The Visual Guide has since been amended to show the cross threaded rod now goes UNDER the bottom bars of the end frames, which would solve my problem:  http://issuu.com/garyhodgson/docs/prusa_mendel_visual_instructions/35?viewMode=fullscreen  - I will probably move it later, when I get into the commissioning and fine tuning stage.]

Anyway – another milestone achieved – and now on to the assembly of the X-Axis.



Edit 08 September 2011:

I have been contacted by another Brisbane-based RepRapper who also purchased the same kit at about the same time as me (his was delivered a bit later than mine), and he also had no troubles getting the M8 nuts to fit on his M8 threaded rods. He confirmed that the packaging for the nuts was the same as mine, and he has measured his threads and confirms they are standard M8 1.25 mm pitch. (I don't have the tools to confirm the thread pitch of my M8 nuts, but I have measured my threaded rods and confirm they have 1.25 mm pitch.) When we catch up with each other, I'll make sure I have samples of my threaded rod and nuts, and we'll see if we can figure out what the difference between his kit and my kit are.

So for now, the root cause of my problem remains unresolved (although I have a work-around of buying replacement nuts) - as Alice would say: "Mysteriouser and mysteriouser ..."