Saturday, 3 September 2011

Time for a few Zzzz’s (After assembling and aligning the Z-Axis)


I have already mentioned the fact that my frame went together in such a way that the bottom cross rods (front and rear) are not quite parallel to each other and perpendicular to the longitudinal bottom rods http://julianh72.blogspot.com/2011/08/main-frame-build-and-alignment.html . Nevertheless, I carried on with the installation of the Z-Axis (in accordance with Part 8 of the Visual Guide http://garyhodgson.com/reprap/prusa-mendel-visual-instructions/ ), hoping everything would sort itself out.

First step is to align the bottom cross rod with the top cross rods (the X-Axis), and mount the Z-Axis round bars in the Z-Axis Motor Mounts – no problems here. As recommended in the Visual Guide,  I spent a bit of time making sure the round rods were truly vertical and parallel with each other, and all seemed to go OK.

I then snapped two PLA Bushings onto each Z-Axis round rod. As I found with the Y-Axis, at first they were quite stiff, but a little bit of light oil, and moving them up and down the rods 20 or 30 times soon had them sliding quite freely.

Next step was to glue the pre-assembled X-Axis assembly onto the PLA bushings. The Visual Guide doesn’t specify the type of glue; I went with Super Glue, and it seems to be holding fine so far. After making sure everything seemed to be well and truly aligned, I left the machine overnight to let the glue dry properly. The next night, I removed the temporary support from the X-Axis, and tried moving the assembly up and down the Z-Rods (being careful to keep the X-Axis assembly horizontal, to avoid “binding” of the PLA bushes against the smooth rods) – it slides quite smoothly – great!

I found that I had to ream the Z-Screw Rod Couplers to get them to fit easily on the motor shafts and the M8 threaded rods. I don’t know if this is typical or not, but a quick pass with a 5 mm drill bit on the motor end and an 8 mm drill bit on the threaded rod end got them clean enough to fit snugly onto both shafts.

Next step was to install M8 nuts into the bottom of the X-Axis Idler Mount and Motor Mount, as well as the “optional” spring and second M8 nut in the top of the two mounts. I very soon found that the springs that were supplied with my kit are too small in diameter to fit over M8 threaded rods, so in my case, the “optional” springs had no option but to be omitted.

I installed the upper M8 nuts anyway – not sure yet whether this was a good move or not. This was all a bit fiddly until I managed to get the M8 threaded Z-Rods into place, so that the M8 nuts could all be held in place by the threaded rods.

Next came the installation of the two Z-Axis motors. The Visual Guide says it is optional to install locating screws for these two motors, citing issues with “binding” of the Z-Axis motion if the whole system is not perfectly aligned. Quite frankly, the whole idea of NOT fixing the motors in place appals my engineering instincts, but the motors are a very snug fit in their RP’ed mounts, and I figure I can always install the screws later, so for now, I have left the Z-Axis Motor mounting screws out.

The motor / screw-rod couplers go on next – no problems here because of my earlier job of reaming them out a bit to fit nicely.

I then wind the two threaded rods up until they engage fully in the couplers (leaving a gap of a few mm between the bottom end of the motor shaft and the top end of the threaded rods, to allow the couplers to “flex” to absorb any misalignment), and tighten all of the screws on the couplers.

After checking that everything seems to be straight and true, I put a spirit level on the X-Axis assembly, and wind one of the Z-Axis threaded rods to level the assembly. I then very carefully wind BOTH Z-Axis rods together to see how the whole Z-Axis motion travels. To my delight (and I must admit, some surprise!), the X-Axis assembly moves smoothly (but slowly) from top to bottom of the Z-Axis.

I don’t want to sound over-confident, but this is all beginning to look like it might just work!

So … off to bed before the next stage of the assembly – assembling and installing the X-Carriage.