Chainguard for bicycles - fully parametrized

thingiverse

When I swapped out my daughter's old and worn-out crank set for a brand-new Shimano FC-M591 44/32/22, I made a crucial mistake - the big chain ring that came with it didn't have mounting holes for a chainguard. That's when I realized too late that finding a commercial chainguard to fit via the four mounting screws of the chain ring wouldn't be easy. Thus, I was forced to design and print one myself. The chainguard consists of two main parts - a thinner outer ring and a thicker inner ring - connected by inner arms that attach to the chain ring's mounting screws. These screws are usually 4 or 5 hollow ones located on a circle with a diameter of 104mm. Each screw features an outer barrel with an inside thread and a hollow inner part with a 5mm Allen hex key cavity on one side and a 4mm hole through it. While I could have attached the chainguard directly to these screws, that wouldn't be practical since they bear a lot of force and need to be tightened firmly - no room for any kind of flexibility in between. So, we need to find another way to attach the chainguard using the holes inside these mounting screws. One simple yet not-so-elegant solution is to use countersunk M4x20 screws from the inside and an M4 nut as a counterpart on the outside of the chainguard. A more elegant option would be to cut an inner M5 thread into the hole of each screw, allowing us to attach the chainguard using M5x16 Allen key bolts from the outside. To do this, we first need to enlarge the holes slightly using a 4.2mm drill bit and then use an M5 tap to cut the inner threads. The SCAD file is fully parametrized with three main parameters: * The number of teeth on the big chain ring (T) * The number of mounting screws (H) * The radius of the circle where the mounting screws are located (R) Ready-to-print STL files for standard configurations, using 42/44/48 teeth and 4/5 mounting screws, are also provided. The files ending in _nt_ use M4 with outer nut traps, while all others use M5 bolts and tapped threads. I've also included an STL file for a chainguard designed specifically for a 44-tooth chain ring with four standard mounting holes on a circle of 155.5mm diameter. Note that the 44-teeth chainguard will fit within a standard 200x200mm printing region, while the 48-teeth version requires an extended printing area of at least 205x200mm.