RepRap 3D Printer - Prusa i3 - Build Log: Part 1

Y-Axis Assembly

Start by assembling the structural frame which supports the y-axis. The order of assembly isn't very important, and I found myself going back over assembled components and pulling them apart because I had forgotten something. Just ensure you don't tighten anything beyond finger tight until it's all assembled and you're sure it's correct. 


Thread two washers into the centre of each M10x380mm and trap them with M10 nuts. Then secure the printed y-axis corner to each end with M10 washers and nuts. The y-axis corners should be positioned as far as possible towards the ends to maximize the y-axis travel distance. Slide two LM8UU linear bearings onto a M8x350mm smooth rod and one LM8UU onto the other M8x350mm. The smooth rods then press (with a relatively large force) into the channels in the top of the y-axis corners.



Next assemble the y-axis belt tensioner. Insert the 623ZZ Bearing into the printed bearing guide. I had significant trouble getting mine to fit, and ended up reaming out the bearing guide, then encouraging the bearing with a hammer. Take care not to crack the bearing guide plastic. Heat may help here.

Next install the tensioning bolt into the printed y-axis tensioner. I used an M4x30mm, but a longer bolt would not go astray. The bolt fixes to a captive nut which is inserted into the printed part. Ensure you ream out the hole which takes the M4 (I didn't and had problems with the M4 catching on the plastic, I reamed my hole out to 5mm). I also had problems inserting the captive nut and ended up solving the problem by heating the captive nut up with a soldiering iron prior to inserting it, effectively melting it's way in.

Finally fix the bearing guide into the printed y-axis tensioner part with an M3x35mm with washers and a nyloc nut.

623ZZ Bearing inside bearing guide
Fully assembled y-axis tensioner
Now prepare the remaining two sides of the y-axis frame. Slide the y-axis tensioner you just assembled onto a M8x205mm threaded rod and fix in the centre with washers and M8 nuts. Place M8 nuts and washers on the ends of the rod and on another M8x205mm threaded rod.


Slide the printed y-axis motor mount into the centre of two M8x205mm threaded rods and fix in place with washers and M8 nuts. The exact location of the motor mount is not important for the time being.


Now assemble all four sides of the frame together with nuts at the corners.

Now the aluminium plate for the y-axis carriage needs to be prepared. I also prepared the x-axis frame at the same time. Each of the predrilled holes needs to be tapped with a M3-0.5 thread. This takes quite a bit of care and patience as the taps need to be perpendicular to the surface, and the tap itself is delicate and can easily snap off while tapping. It is also vital to use cutting fluid, as aluminium will bind with the tap, and snap it off. I was able to buy a set of metric taps in Canada at Canadian Tire for about $25. If you aren't confident with getting a perpendicular tap, then use a piece of wood with a 4mm hole, drilled with a drill press, to act as a guide.

I tapped all the holes in one session, regardless if I was sure I was going to use them. If you don't want to go to the effort of tapping, then a simpler option is to drill through the holes with a 4mm bit, and then fix the M3 bolts with nuts instead.

Next affix the printed y-axis belt holder to the back of the aluminium y-axis carriage. If required ream out the holes in the printed part, so the M3's fit nicely. Fix the printed part with M3x10mm and washers.


Position the aluminium y-axis carriage into the completed y-axis frame. The bearing should align with the bearing position slots milled into the carriage.

Move the carriage back and forth to make sure the bearing stay settled in the positioning slots. If they move side-to-side slightly as the carriage moves, this means that the smooth rod aren't perfectly parallel. Adjust the positioning nuts at the corners so that the smooth rods are parallel. Once you are sure everything is okay, tie the carriage down to the bearings with zip-ties as shown.

Fix one of the GT2 belt pulleys to a stepper motor and tighten the grub screws. Then using 2 - M3x10mm screws and washers fix the stepper to the printed y-axis motor mount. Which side to fix the stepper is not critical, but I positioned mine on the left of the mount, as this is going to be closer to the electronics, and will cut down on wiring slightly.

Now that all the hardware is essentially together, everything needs to be aligned before the belt is installed. Slide the carriage right up to the stepper motor. Using the positioning nuts, locate the motor so that the belt pulley and the belt holder are aligned.
Align motor so that the belt will track straight.
Repeat a similar procedure with the y-axis idler at the opposite end of the frame.

Finally install the GT2 belt. Prior to doing this, wind out the y-axis idler screw, so that you have the maximum possible adjustability on the belt tension. Then thread the belt through the belt pulley, idler, and fix to the belt holder.

I had some difficulty fixing the belt to the holder. I found that the most simple way (and way I have seen most around the web) was not giving good results (see photo). The belt wasn't tracking parallel to the frame, because the belt wasn't positioning correctly with the intended location. I fixed my belt as shown below, and this gives a straighter belt.

Wrong - belt doesn't track parallel to return belt or frame

Okay - Belt tracks more parallel to return belt and frame

Finally, take up any slack in the belt with the idler screw.