HUGE tip: design your mechanisms with sketches instead of fiddling around with dimensions in parts.
A part of the core ethos of engineering is ‘iteration’, so the way I see it, the earlier you can iterate in your design the more informed you will be when committing your time and energy to CADding the full assembly. If you can create a sketch that will show you critical dimensions, and how your mechanism will interact with other aspects of the robot, it will allow you to quickly check sizing and tweak parameters on the fly. You would then use this sketch to drive dimensions in your corresponding mechanism CAD.
Here’s a sketch we used to drive our elevator and arm geometry - you can see how it’s pretty basic, but everything essential is in there. We have the scoring locations drawn in, and we are able to move the sketch around to see if our arm + elevator combo was able to reach all our desired scoring locations.
Along those same lines, don’t be afraid to use blocks as a placeholder for mechanisms. As long as you have a rough idea of how large a mechanism is going to be, put it in your assembly so that you can again check for sizing. After that, you can start gradually adding in more detail. My team’s gearboxes usually start off as just a blob of plates and gears, but after we get a better idea of how the robot is coming together, we tweak stuff and eventually make the gearbox “production-ready”.
When we do design gearboxes, we try and draw everything important in one sketch as well. Here’s the sketch for a relatively simple flipped-NEO gearbox I’ve been working on recently.
Essentially, with gearboxes like these, you’re choosing a target gear ratio, and then from there, choosing gears whose combinations allow you to get to that gear ratio. With gears, they transmit power along their pitch diameter, which is simply calculated by PD = # Teeth / Diametric Pitch. Diametric pitch (AKA DP) is simply the number of teeth per inch for a gear. In FRC we mostly use 20DP and 32DP gears (you’ll find 32DP more often in 775pro applications). With gears, we model them in sketches as circles equal to their pitch diameter, and we set them tangent to one another to determine their distance between centers, and thus where to put shafts and bearings.
I can elaborate on anything if needed! Feel free to reach out to me either here or on the FRC Discord.