Friction on bearings when tightening y bushing

Trying to assemble my prusa i3 kit and have been stuck on step one for a week!

I have assembled the y section, and as far as I can tell alignment isn't the problem. When I tighten the printed y bushings on to the print bed platform (in this case acrylic) the friction increases dramatically, if I loosen the nuts it lessens, and if I remove the platform entirely the bearings slide perfectly.

I suspected the printed bushings may have been inaccurate, and that drilling the holes bigger would allow me to wiggle the platform into the correct position before tightening it down. I opted to drill out the holes in the acrylic platform rather than the bushings as it was not attached to anything and I could manipulate it more easily. The end result was a print platform that could be positioned loosely over the screws and then tightened down. After this step, and a few more rounds of loosening the threaded rods to attempt to realign I still find the same effect as described.

When I remove all the nuts the platform sits on the bushings held vaguely in place by the screws (which are snug in the bushing holes). It has a lot of play as it moves and this allows the whole thing to roll quite smoothly.


I am out of ideas, the smooth rods seem straight, one of the long threaded rods is slightly bent but as far as I can tell this shouldn't overly affect the smooth rod.

I have been following an 11 part built tutorial which uses an aluminum platform and zipties, the guy lined the whole thing up by eye and it just worked first time. I am considering ordering an aluminum frame but I want to rule out bad parts before I start ordering replacements, largely because as it is I would just as soon order an entire frame as upgrade the acrylic parts as I have not gotten anywhere nailing down what the actual problem is.

I am wondering if there is perhaps a way to test the smooth rails and print platform assembly together absent the frame to rule out the frame alignment as a cause.

Any assistance greatly appreciated!

The under carriage isn't flat or the guide rails aren't parallel. Zip ties to hold bearings is a sure sign of poor design.

Check the link in my sig, below.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

Thank you for replying! I have used various measures to check the rails are parallel but for levelling it I am trusting the flatness of my desk, it appears this may have been a mistake and I will go back to square one.

Re the zip ties, I am using printed bushings that came with my kit, the video guide I am following doesn't cover bushing defects as he just zipties the bearings directly to the aluminium platform. (https://nwreprap.com/prusa_i3_assembly/)

In any case I am very open to the idea of improving the design of my kit, thankyou for the link I shall peruse it when I get home!

most likely youre bushings are not straight. when tighten the screws you push the bearings out of line. Zipties dont have that problem....

About the bushings, I enlarged the holes on the acrylic platform to try to work around that possibility. I can jiggle it around until it 'feels right' but the second I clamp them in place the platform won't move. I have been very careful to not move the platform as I am tightening the nuts but it seems like the platform needs to play as it moves to give a smooth motion.

I have ordered digital callipers, and working on a flat platform to build it on. I may take the whole thing apart and start again. The other thing that occurs to me is that the printed corner brackets for the Y frame may have inaccuracies that throw the frame off. Where I am at now, it seems any part of the kit could be off and I wouldn't know which part to replace. The ziptie+alu frame seems attractive as there would be less or no potential for misaligning the bearings, but in this case I feel I have ruled out or at least compensated for bushing inaccuracies by enlarging the screw holes to allow me to wiggle them into different alignments.

I assume there are 2 bushings a Rod?
If the dont aling it doesnt matter if the Rod can move inside the frame.
Does it work when you only tightening one of the bushings?

When I tried threaded rods joining the supports I found the alignment was dependent on the pitch and I could only adjust in one rev increments, mainly because of the supports having threads in, yours are probably through holes so probably easier to align, but 1 turn is enough to be out, my pitch was much finer than a 8mm rod.

Edited 1 time(s). Last edit at 01/28/2016 09:59AM by MechaBits.

When this didn't slide, my process was as follows, each step resulted in no improvement.

Loosen y corner brackets, align with carriage.
Loosen bushings from carriage, wiggle, tighten bushings.
Remove Carriage, realign, reattach carriage.
Dremel out the holes in the acrylic carriage to allow me to account for inaccurate bushings.
I then more or less repeated the previous steps with no luck. The only way the carriage slides is with the nuts connecting the bushings to the carriage loosened.

My calipers arrived and aren't long enough to measure the width between the rails... I have more or less confirmed that they are the same distance apart at the front as at the back with a bit of creativity, the problem is I am not sure 'more or less' is good enough. They are easily within a mm of tolerance. I cannot confirm that the smooth rails are completely parallel on the x plane.

The threaded rod on the left is bent upwards but I think I did this after a few of the above steps.

I have ordered a new carriage frame and all new rods (the carriage frame is the one in this video and uses zipties to secure to the bearings [www.youtube.com])

Right well, after ordering new rods, frame and print carriage (ziptie bearings) it would appear removing the bushings from the equation results in a perfectly smooth ride before I even went near the rods... I had not considered the bushings may have been throwing the vertical alignment of the bearings off resulting in something I could not correct by drilling bigger holes. Still, new rods!

The new frame has completely different distances for all the holes however, I had made the assumption that such things were standardized for reprap. I have had to take a dremel to the y belt holder already and am now looking at ordering all new printed parts. Anyway problem solved! Thanks for the replies I suspect I will continue to have similar problems throughout this build, and at this rate of replacing parts I will probably have been better off buying an assembled printer, but where's the fun in that!