Monday, 22 August 2011

Main Frame – Build and Alignment

Having sourced all the parts necessary for the main frame, it was time to start the actual assembly. I downloaded and printed a colour copy of Gary Hodgson's excellent “Visual Instructions” http://garyhodgson.com/reprap/prusa-mendel-visual-instructions from the Mendel Prusa Assembly Wiki page http://reprap.org/wiki/Prusa_Mendel_Assembly . While having on-screen pictures and videos is all very well, I find there is nothing quite like a good hard copy alongside you when you are attempting any sort of complex project!

The two triangular end frames went together very easily (now that I had sourced some nuts that actually fitted my threaded rods!) I was very careful with my measuring, and when I was finished, the two frames were identical equilateral triangles, which could be overlaid on top of each other very nicely in any orientation. I was very pleased with the results - so far, so good!

Next came the front, rear and top threaded rods. Again, following the instructions carefully, on my first evening’s work I had completed the basic assembly of the main frame – not bad!



… or so I thought!

On closer inspection, the frame was sitting with only three feet on the ground – the fourth foot (the rear left corner) was sitting about 5 – 8 mm above the ground.

No big deal, I thought – I simply slackened off all the front, rear and top nuts to loosen the joints, stacked up some decent weight on the frame to hold it flat on the ground, and very carefully, I gradually snugged up all of the nuts to an even tightness.

When everything was tight, I confidently removed the weight – but the back left foot sprung up off the floor again!

I went through this process several times, but no matter what sequence I used, every time I finished, the frame was very slightly out of alignment again. I am not sure exactly what is going on, but I suspect that the rod holes in the RP’ed corner vertices are not perfectly square onto the face of the vertices, so when everything is tightened, one or more of the front, rear and / or top rods are not quite at a perfect right-angle to the end frames.

The only way I could resolve the issue of the uneven feet was to also slightly adjust my perfectly-formed end frames. I was very reluctant to do this, because the end frames seemed to be so perfectly aligned, but I could see no other way around it!

Eventually, I got the whole frame rigid and sitting firmly on the ground – but if you look very carefully, you can see that the cross rods are not all perfectly parallel to each other. Whether this matters or not, or whether it is best to “smear” the alignment imperfections across the whole frame (as I have effectively done) or to keep “perfect” end frames and live with imperfect alignment of the feet, I am not quite sure. I guess the proof of the pudding will come when I am ready to fire up the machine and see what happens.

More updates to follow, as my build progresses.