Replicating a bicycle part

My bike stopped shifting correctly. I found that a plastic cable guide on the bottom of the crank housing had broken. I could not find a part like it on line, so naturally I figured I’d make one.

It’s pretty small, so I used digital calipers to take lots of measurements so I could design the whole thing parametrically.












For strength it should be printed horizontally, and the bottom is curved, so obviously it was going to need lots of support.

To keep the friction low and to prevent it from fracturing I figured it would be better to use Nylon than PLA. I have a spool of Taulman Bridge in black.

On the first try some holes were too small so I had to adjust them, which was not a problem because it only took a half hour to print.  The second one was better, but both of them had layering problems and drooping edges. So I bumped up the temperature and provided a lot more support. I also went to a low layer height of 0.2 mm and slower slower print speed to get finer features.

The third print was clean enough on top but had some porous areas underneath. I decided that would not matter, so the part should be good enough for its purpose.

















Here it is installed.









It seems that I mis-measured or mis-drew the large internal curve. It doesn't lay down against the crank tube properly. But there does not seem to be any vertical force on that area, so I think it will be OK. I'll try it for a while and then check it. If necessary I'll tweak that number and reprint it. That's the cool thing about parametric design in Fusion 360: no need to redraw the whole thing if you just need to change one factor.