Allowance and Tolerance
When designing your own wheels, you need to keep in mind allowances and the tolerances of your parts. Allowance, in engineering and machining terms put simply, is the difference between a part not fitting and fitting together. Tolerance is the variation of a part, due to its properties, conditions or the way it is machined.
Most fabrication services, whether it's for a CNC'd part from carbon fiber or aluminium, will share their expected tolerances, typically you're looking at a deviation of ±0.2-0.3mm of your specified dimension, sometimes even more, especially when CNC'ing aluminium. This also applies to having custom PCB's routed, or parts 3D printed in MJF or SLA. Always look for the chosen services tolerance spec so you have an idea of what to expect or plan for.
To make sure parts fit together as planned, you need to consider these factors. You may find that you need to do some manual sanding of parts if they are too tight, to get the fit that you want, alternatively, you can give your part some extra space, AKA some allowance. It really comes down to what the part is and how important a tight fit is.
Simply put, you're very unlikely to fit a 12mm aviation connector into a 12mm hole, whereas if you make that hole 12.2mm, it will. In this case a little loose but as it has a nut to keep it put, that's not an issue. For shifters or something that moves, this is far more important and may require a few prototypes to get right.
Check out these videos for some more information on this topic.
