Gentlemen,
I have a spindle which is .636mm diameter. What's the ideal size of bushing to fit around it snugly so there's no play but with enough clearance for a few drops of thin oil? It needs to spin pretty freely but with no wobbling etc.
See if this helps:
Thanks for that, Smolley. Errata: in my OP I meant 6.36mm diameter!
I get around 6.614mm
I think a 6.5mm drill is possible. If you want the absolute best fit, fit the spindle to a drill and insert in the bushing and add valve grinding compound
Looking at my charts the indication is around 50um, but you will need a good reamer set in a machine to get the precision.
6.5mm is the best bet, I reckon. I did find a company that supplies 6.4mm drills but that only allows 1.7 thou clearance. Not sure if that's do-able or not. The other problem is - and this is miles away from my experience - when it comes to bushings, one is supposed to use a reamer after a drill to get the size precisely right and remove the scars of the drilling. Life's complicated when you over-think things as I do. There's doubtless a simpler way of looking at it. However, that does imply the drill needs to be about the same size as the spindle so the reamer can introduce the clearance needed for a freely spinning but play-free result. Maybe.
Unless you have uber precision machines, reaming with a tapered reamer or actually lapping the surfaces to ensure a good fit is all you got mate.
Any issues with differential temperature expansion of shaft and bearing?
Nope, not a problem. Thank f*ck I don't have to worry about that!
Smolley, that was a typo. It was 6.36mm diameter not 0.636. I thought this was kind of an odd size and quite close to 1/4" imperial (and the mechanism in question was made in England in 1975). So I got my good old Moore & Write micrometer out and bingo! It *is* exactly 0.25" and I have managed to find a 1/4" reamer. I think I may be making some progress here now....
There are a few "letter" drills around that size
Sounds Good.
This throws up a profoundly metaphysical conundrum, though. If I have a hole exactly 0.25" and a shaft of exactly 0.25", will the shaft go through the hole if they're both at the same temperature? These are the finer points of engineering which have always eluded me. Perhaps there's something Schopenhauer wrote that covers it.
Thanks, Andy. Turns out the spindle is in fact 1/4" imperial which should simplify things hopefully.
Absolutely
Nothing metaphysical about it.
The short answer is no the shaft wont go through the hole.
Okay, so this would be a good way to get an "interference fit" if that were required, I guess. Find a press and force the shaft in.
1.7 thou should be enough clearance, the issue is more in the tolerance of the drill and the finish.
Reamers can cut a few thou. Perhaps try a few test pieces to see if the clearance is sufficient for the spindle to spin as required?
yes. Although even so it's no a massively tight fit. You would heat the bearing...and freeze the shaft
Agreed.
That's what I plan to do in any event. But I'm just wondering why reamers have the same sizes stamped on them as drills. That would seem to imply that they're intended for use with a drill of the same size, in which case they'd need to be a thou or two larger than the stated size. So a 1/4" reamer would be say 0.2515" if mic'd up. I must try that and see, in fact.
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