Training the Veteran Rookie Coach: Press Fitting Bearings

CNC routers may not have an absolute accuracy of +/-0.001", but you can repeatably make holes that are +/-0.0005" if you adjust your tool diameter compensation in your CAM program. We typically undersize our endmills 0.005"-0.007" and do a 0.005" finishing pass to get good snap-fit bearing holes, but it typically takes a few tries to get it dialed in on a new router.

I agree that most parts, on most FRC bots, don’t need to be held to tenths tolerances. In fact, I’d argue that most things made in most “conventional” machine shop don’t need to be held to tenths. I’d even question the ability of most FRC teams to measure accurately to that tolerance and have confidence in said measurement.

As a former R&D machinist, with >30 years of experience making parts to a few tenths (.0002") tolerance, one of my personal goals in FRC has been to teach the padawan engineers of tomorrow how small that value really is and if it’s really needed.
Nothing is benign. All decisions have consequences. Much of my energy is spent trying to teach critical thinking.

It’s too easy to add decimal points in CAD, or call out tolerances in the title block, without really understanding that in the “real world”, somebody will have to pay extra money for the part because of overly tight tolerances or dimensions. In addition to parts costing more, there will be delays cuz it takes more time/patience to set up and make high tolerance parts. And, the part inspector will take more time to inspect, or reject a lot more parts (needlessly).

Ack! I’m getting flashbacks of crazy, over toleranaced parts just writing this!

To the OP.
FRC Bearing installation can be a slip fit, press (light or heavy) fit, or “drops” in the hole. IMO, ideally they should be a light press. That way, they’re easy to install/remove but don’t fall out accidentally. BTW, the hole also needs to be round in addition to the proper size.
If you buy cheap bearings, be sure to measure the outside diameters in the lot order to determine what might be a nominal size for you to aim the hole to be.


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