I will echo the advice above, that there are currently so many COTS gearbox options from AndyMark, Vex, West Coast Products, and others which fit together without machining that you will likely improve both your competetiveness and practical learning more by exploring the COTS options prior to making your own gearboxes.
When/if you decide to take the plunge and make your own gearboxes, here are some bits of advice I've mentally compiled reading other threads (3946 has no machining resources, so I can't back most of these up with personal experience):
- Do not count on water jet for cutting with critical dimensions, such as press-fit bearing holes. Either mill these holes (best), or water jet them slightly under-size and ream to the proper size.
- Use relatively large radii on any pockets (lightening holes) in the gearbox plates. This will slightly simplify the work if done on a mill, but more importantly will distribute the stress better than a sharp inside corner.
- The most reliable solution to support an axle is to provide one support bearing at each end, with no clip rings between these bearings. (Clip ring grooves are stress risers much like sharp corners in pockets.) If you decide to cantilever a shaft (especially for a drive wheel), the connection between the two bearing plates should be significantly more than a few bolts. Most teams solve this by pressing the bearings into opposite faces of a piece of rectangular tubing. Some then put the gears inside the tube, others outside (usually inboard of the tube).
- [Also true for many COTS gearboxes] Consider well where to use steel vs aluminum. For a typical well-designed two-stage reducing gearbox, the two most stressed components are usually the output shaft and the small gear on the cluster shaft.