When I had to replace the old and used crank set of my daughter's bicycle with a new Shimano FC-M591 44/32/22, I realized too late that the big chain ring it came with did not have mounting holes for the chainguard. So I tried to find a commercial chainguard which I could somehow fit via the 4 mounting screws of the chain ring - but could not find anything reasonable. Thus, I finally ended up designing and printing such a chainguards myself...
The chainguard consists of a thinner outer ring, a thicker inner ring and inner arms connecting the chainguard to the chain ring mounting screws, which are usually 4 or 5 hollow screws located on a circle with 104mm diameter. These chain ring mounting screws consist of an outer barrel with an inside thread and a hollow inner screw which has a 5mm Allen hex key cavity on the outer side and a 4mm hole going through all the way. While it would be possible to attach the chainguard directly with these screw assemblies, I don't think that this would be reasonable, since these screws bear a lot of force and need to be fastened very tightly - no good idea to squeeze any lastic in between...
So we need to somehow use the hole inside these mounting screws to attach the chainguard. The easiest way (but not so elegant) is to use countersunk M4x20 screws from the inside and use an M4 nut as counterpart on the outside of the chainguard. A more elegant solution is to cut an inner M5 thread into the hole of the screws, so that we can the just use M5x16 Allen key bolts from the outside to firmly mount the chainguard. For this you need to first slightly enlarge the holes using a 4.2mm drill bit and then use an M5 tap cut the inner threads.
The SCAD file is fully parametrized. The main parameters are
- the number of teeth of the big chain ring (T)
- the number of mounting screws (H)
- the radius of the circle on which the mounting screws are located (R)
Ready-to-print STL files are provided for some of the standard configurations, using 42/44/48 teeth and 4/5 mounting screws. The files ending in nt use M4 with outer nut traps, all the others are for M5 bolts and tapped threads.
Since it was easy to do, I have also included an STL file with a chainguard for a 44 tooth chain ring which has 4 standard mounting holes for the chainguard on a 155.5mm circle.
Note that the 44 teeth chainguard will just fit in the standard 200x200mm printing region, while the 48 teeth version need an extended printing region of at least 205x200mm.
I printed the chainguard on my DIY box-frame Prusa i3 using a 0.40mm nozzle with 0.25mm layer height and 0.56mm extrusion width. I print on a clear mirror without Kapton tape but thoroughly cleaned with acetone and double concentrated lemon juice.
The supports under the bridge at the crank position need to be teared off after printing. Also the screw holes have a bridging layer inside, so you will need to re-drill the screw holes with a 5mm or 4mm drill (the latter for the nt versions) before mounting the chainguard,