I am using stainless steel smooth rods (A2/V2A). When I use the standard LM8UU type linear bearings on these rods, the hardened steel balls of the bearings will end up cutting ugly grooves into the rods. Even though I can reduce this problem by rotating the rods by a few degrees around the axis from time to time, I don't like this combination of soft and hard steel.

One solution is to use brass bushings instead of the linear steel bearings. Brass on steel has a very low friction coefficient and causes very little abrasive wear.

But as it turned out, I could not find any ready made brass bushings around here. So I looked in my garage if I could find any brass objects from which I could make some DIY bushings... And what did I find? A handful of M8 brass nuts!

When I bored out the thread with a 8mm drill bit and chamfered the rims, I was amazed how easily the nuts moved on the 8mm smooth rods. So I decided to give it a try and use these DIY bushings on the X- and Y-Axes of the new box-framed Prusa i3 I am building.

As I could not find any reference of someone else already having used the same type of simple brass bushings, I had to adapt myself the X- and Y-carriages. The frame for one bushing is simple, as it essentially consists in a nut trap with two holes for a zip tie which retains the nut.

The Y-axis was relatively simple. However, my first try of using a separate frame from each of the four bushings was not successful. It turns out that it's not so easy to align the nuts precisely enough. Combining the two bushings from each rod into a one frame made the alignment and mounting of the print bed much easier. Two STL files are included, one for 70mm spacing of the two nuts and the other for 100mm spacing. The corresponding SCAD file is also provided.

For the X-axis things are a bit more complicated, as one has to integrate the nut bushing frame into the Prusa i3 source code. The included x-carriage STL files are for MXL, T2.5 and GT2 belts. But since I use MXL belts on my i3, only that STL file has been tested. As my ad-hoc modifications of the original i3 code are rather "quick-and-dirty" and I don't want to copy the whole source code into this project, I have just uploaded a diff-file showing the modifications I made to Prusa's x-carriage.scad, for those who want to know exactly what I did.

As to the performance of my new brass nut bushings: The X- and Y-carriages glide at least as easily on my (now finished) i3 as they do on my Mendel which uses LM8UUs. However, when sliding the carriages by hand, I noted that the nut bushings are rather sensitive to pushing/pulling off-center. But since the belts apply their force at the center, the carriages glide very nicely and precisely when moved by the steppers.

So far I am quite satisfied with the first prints I did on my new i3. Thus, the brass nut bushings seem to work well. But of course, this is only a first go at it. It remains to be seen how the bushings will perform in the long run... So I'd be quite interested to hear from others who have done some work with these or other DIY brass bushings.

Update 2014-02-02: I also uploaded the files for a version of the x-carriage which uses 4 bushings instead of the standard 3. Given the short axial length of a single nut bushings, I think that it makes sense to have two on each rod.

Update 2014-02-10: I just realized that I can also use these bushings for non-standard sized smooth rods (i.e. anything other than the usual 8mm or 10mm rods), such as e.g. the nice 9mm rods that can be salvaged from most outdated Epson inkjet printers.