Just to be clear, 8mm is 0,31496 inch, thus slightly larger than 5/16 or 0.3125". If you succeed in pressing an 8mm shaft into a 5.6" bearing without bending or breaking , the bearing inner will most likely crack.
Well, I tried to reply to Stick and Nick, but it did not appear here in the thread. Making a wall cabinet with raised panel doors, and using a set of rail & style cutting bits with top bearings. Straight portions are not a problem as the fence on the router table allows for incremental stock removal until the last pass against the bearing for finish depth. The problem comes with using several passes to remove stock in the arch top rail (Cathedral style) using hand feed. Plan was to begin with a 1 1/4" O.D. top bearing and then step down to reduce the stock removal per pass. Next to last pass made with the original supplied bearing, but with several layers of tape on the bearing. Last pass - remove the tape and run rapidly to prevent burn marks. I checked the top shaft on the bit and it measures 0.3105" so the ASME size will fit (should). Went to my local bearing supply house, looking for metric mearings that might work. Quote was $ 90.00 per bearing. Found some on line, (under 10 bucks) but was concerned that switching between metric and English might create a problem. I guess I did not think through the problem quite enough, or I would have realized that the metric actually had the larger I.D. Just for my enlightenment, should I run into a problem with excess clearance, can one use motor bearing shims ( 0.0030, 0.0050, 0.010 and cut strips as fillers on the inside of a mismatched I.D. and still get good results ??
I thought you were talking about a bearing on the router and not a bearing on a bit. Check with the major manufacturers and on Lee Valley's website. They all have bearings listed that show I.D. and O.D. You can alter a lot of profiles by changing a bearing. For example a round over bit with a smaller bearing will give you a rounded edge with two fillets with the height adjusted properly. You cam get laminate trim bits with oversize bearings to trim too much laminate overhang down before trimming to proper size with the correct bearing on. (They call them overhang bits. Laminate tends to chip if it has too much overhang.) So it's common to use different bearing sizes but the ID of the bearing MUST match the the size of the bit where the bearing mounts on.
Was able to run the arched portion of the top rail with an "English" bearing with a 5/16 I.D. and a 1 3/8" O.D. successfully. Did a dry fit of the panel, rails and styles to check dimensions. Picture of the oversize bearing and the assembled door. Sorry, the phone doesn't always stay clear and focused.
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