Tolerancing
Tolerancing is about how precise a part needs to be. In FRC, we are less precise than you think, except in a few specific places. Tolerancing, however, also extends to a broader problem that comes up constantly: designing parts that are difficult to assemble, impossible to work on once installed, or that don't account for the rest of the robot around them.
If you've ever tried to fit a wrench into a gap that's .25" too small, or discovered that two subsystems collide when they're both moving, or spent an hour assembling something that should have taken five minutes, that's a tolerancing and design-for-assembly problem. Don't be like Joseph.
What actually needs to be precise
Bearing holes
Too big and the bearing wobbles or falls out. Too small and it won't go in.
Match the vendor's recommended hole size exactly. For most FRC bearings this means within about 0.001" of spec.
Shaft center-to-center distance
If two shafts need to mesh gears or run a belt between them, the spacing has to be correct or the gears won't mesh and the belt won't tension properly.
Use the value from your gear/belt calculator. Get this right in CAD and verify it before manufacturing.
Holes that align across multiple parts
If you're bolting two plates together and the holes don't line up, you have a problem.
Use the same CAD model to generate both parts.
What does NOT need to be precise
Bolt holes
Make them clearance holes (0.015" to 0.020" larger than the bolt). This gives you wiggle room during assembly and makes your life significantly easier. For a 1/4" bolt, drill a 17/64" hole.
Overall dimensions of non-mating parts
If a plate comes back from the laser cutter at 11.98" instead of 12.00", it almost certainly doesn't matter.
Lightening pockets, access holes, cable routing slots
These are functional but not precise. A pocket that's 0.05" off from the CAD model has zero impact on the robot.
The real tolerancing problems (designing parts people can actually build and work on)
This is where most of the issues come up in practice. A part might be dimensionally correct but still cause problems because nobody thought about how it gets assembled, accessed, or how it interacts with everything around it.
Think about the order things go together. If part A has to be installed before part B, but part B blocks access to the bolts that hold part A, you have a problem. Walk through the assembly sequence in your head (or better, in CAD) before finalizing the design.
Leave tool clearance around every fastener. A bolt with .25" of space between the head and the nearest wall is impossible to tighten with a wrench. Check that a hex key or socket can physically reach every bolt in the assembly. This is one of the most common issues we run into, and it's entirely preventable in CAD.
Design for disassembly too. At competition, you need to take things apart and put them back together quickly. If removing one mechanism requires disassembling two others first, that's going to cost you matches.
Check range of motion against other subsystems. Your mechanism might work perfectly in isolation, but when it's on the robot alongside everything else, things collide. Move every mechanism through its full range of motion in the top-level CAD assembly while other mechanisms are also in various positions.
Account for wiring and tubing. Wires and cables take up space that doesn't show up in most CAD models. Leave room for cables to route from motors and sensors back to the electronics board without getting pinched by moving parts.
Talk to other subteam members. If you're designing the intake, talk to whoever is designing the superstructure it mounts to. If you're designing the elevator, make sure the electrical team knows where you're putting motors and sensors. A lot of integration problems come from people designing in isolation.
Use slotted holes for anything you're not 100% sure about. If a motor mount needs to slide for belt tensioning, if a sensor position might need tweaking, or if you're not confident in the exact placement of a bracket, slotted holes let you adjust after assembly without re-making the part.
Add extra mounting holes. Holes are free in CAD. If you think you might want to mount something in a slightly different position, add holes for both options now rather than drilling them at competition.
Design shims into the plan. If two parts need to be precisely aligned (like two bearing blocks on opposite sides of a shaft), plan for shimming during assembly. A few washers as shims can fix a 0.010" misalignment that would otherwise bind the shaft.
If you're designing a part and you can't clearly explain how it gets assembled, accessed for repair, and removed from the robot, the design isn't done yet. Go back and fix it before manufacturing.
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