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owens
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sgraber
What is meant is you warm the bearing to the point that when you insert it into the bearing hole that it melts the plastic a bit so it slides into the hole.
I haven't tried that trick with bearings, though I've done it in combination with heating the plastic to get the pulleys onto the motor shafts; if the shaft isn't warm it tends to freeze the pulley in place before it's slid on far enough. And the pulleys are sensitive parts, so I don't want to overheat and distort them. I'm not sure it would work the the bearing holes though. What we found was a problem of out-of-round holes, rather than simply too small, and the plastic really needed to move in order to let the bearings fit in. One thing I did was keep an M8 bolt handy so I could run it through both bearings while the arm was still warm, to make sure they were lined up with each other. Also, I strongly recommend only trying to do one bearing at a time, and re-heating in between; you don't want to try to force them into place or the plastic will distort.
On that point, I strongly recommend trying the oven trick with a scrap piece first. You'll want to get an idea of what Nicholas means by the sound changing - it really does sound like a thud as the plastic softens, but it's also very easy to go too far and make the part too flexible. I've had success with the oven but have also done a couple of variations on his original idea. For the gear arms, I wrapped the gear section loosely in aluminum foil to try to keep it a bit cooler, leaving just the end exposed. And for for the most part I've switched to spot heating with a hairdryer rather than the oven. It helps to have a small enclosure around the part; I use a cardboard box that I could stick the end of the arm into and let the hot air swirl around it. I feel like the hairdryer gives better control, but in both cases I was able to get the bearings and bolts in place without warping the arms (as far as I can tell). This was all on the original GUS; we're still printing the new arms, so haven't tried putting any bearings in yet. . .
Bill.
A metals machine shop will Freeze bearings and bushings for there tight fits.
Ralph