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Thread: Press-Fit Sanity Check

  1. #1
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    Question Press-Fit Sanity Check

    The main question:

    1) What are the odds of having a manufactured shaft and bearings 0.025mm too close for a press-fit? Two different sources, could be a metric/imperial mix for all I know. But... that close? I mean, google says 0.025mm interference is right for a press fit and I've measured 0.05mm or thereabouts.

    Intuition says if they're that close, they're probably designed to press together and I've just miss-measured. I am using one of those plug-type transfer gages... and a cheap one at that. But, I'd rather not find out the hard way. Could just be chance. What are the odds?


    Backstory...

    So... my "proof of concept" 3D printed, speed-doubling, spindle for my "Ronter" packed it in. Plastic housing, and using bearings salvaged from a fidget-spinner... not exactly unexpected. What was unexpected was how much better in made that Ronter work.

    I was cutting out bottle rocket fins in coroplast (corrugated plastic sign board) and discovered that a 0.9mm drill bit worked awesome for cutting (because I happened to have an excess supply of said bits). Anyway, I went to cut some more last night, after re-configuring to pre-spindle mode, and it was a melted mess. Even slowed the feed down to match the slower bit speed and still snapped the bit. Even a larger 2mm endmill left a mess (which didn't on my first with-spindle cuts). When cutting this stuff, speed rules, apparently.

    So... plan B is to redo the existing spindle with, hopefully, slightly better bearings and then, eventually, make a real spindle on the lathe with real bearings.

    Thus, 2 questions:

    1) I've got an ER11 collet holder on an 8mm shaft, being the core of my spindle. It mics out at around 7.98mm (or less). The 3 bearings on hand (more junk) I want to press on mic out at 7.93mm (or more). So, I googled and came up with 2 pages that basically say the same thing... good luck. One says I need to get the bearing up to about 600C, the other says 0.025mm is about right for a press-fit. Is this right?

    2) When I go to do this right... guessing at around 3000rpm, what grade of bearing should I be looking to buy? Budget... but I might splurge more than $0.99 for a fidget-spinner this time. I mean, 3000rpm doesn't seem that fast and I'm not looking for any kind of super-accuracy (the machine isn't close to that anyway). But, lasting more than 12 hours might be nice.

    David...

  2. #2
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    2) When I go to do this right... guessing at around 3000rpm, what grade of bearing should I be looking to buy? Budget... but I might splurge more than $0.99 for a fidget-spinner this time. I mean, 3000rpm doesn't seem that fast and I'm not looking for any kind of super-accuracy (the machine isn't close to that anyway). But, lasting more than 12 hours might be nice.
    It's not that fast. 3600 RPM is a very common speed for 60 Hz motors.

  3. #3
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    Bearing up to 600C? Huh? no... for a non sealed bearing 400F is plenty, sealed... maybe 160F. Don't "mic" your bearings... look up the data and go with that. Most bearings are way more accurate then the typical HSM measurements.

    If your shaft is really 7.98mm, is actually "round" and has a good surface finish, just press them on cold.

    3000rpm is not an issue for such small bearings. look up the data... probably 25k+ for non-sealed.
    Last edited by lakeside53; 01-28-2019 at 02:42 AM.

  4. #4
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    Your bearing bore measurement probably has some glitch with tenfold error to expected. Class 1 cheapest, lowest tolerance bearing should be within 7.992..8.000mm, most likely even better than that.
    Either your bearing is some super crappy one or your measurement is screwed. Even the cheapo chinese ones are within the lowest grade 7.992..8.000mm limits. For bog standard brand name like SKF bearing I'd expect to see 7.996 to 7.998 bore.

    https://www.skf.com/pages/jsp/catalo...=tcm:21-123905

    Typically those ER collet holder shafts are slightly undersize. Did you tried if you can slip the bearings on the shaft?
    Last edited by MattiJ; 01-28-2019 at 05:22 AM.

  5. #5
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    Quote Originally Posted by lakeside53 View Post
    Bearing up to 600C? Huh? no... for a non sealed bearing 400F is plenty, sealed... maybe 160F. Don't "mic" your bearings... look up the data and go with that. Most bearings are way more accurate then the typical HSM measurements.

    If your shaft is really 7.98mm, is actually "round" and has a good surface finish, just press them on cold.

    3000rpm is not an issue for such small bearings. look up the data... probably 25k+ for non-sealed.
    Yeah, the temp was from an online calculator... garbage in - garbage out kind of thing. Basically, it's saying it isn't going to happen.

    As for looking up the bearing... no numbers on them.

    The fidget-spinner outside bearings... no problem:
    https://www.vxb.com/608-2rs-Fidget-H...rs-spinner.htm
    (funny that they would sell specific bearings for fidget spinners... I didn't know they did that)

    The center bearings from the same spinners... no number. Might be that they removed the seal. But why?

    But... in looking up the outside ones... I got to thinking. 5/16" converts to 7.9375mm...

    So, now I'm even more confused:

    * A published 8mm shaft is a somewhat loose fit in a published 8mm bearing. It's what I ran with, bit of thick CA on the shaft to act as locktite and it worked well enough until the bearing packed it in.

    * The same fidget spinners come with a different center bearing. No numbers, and they "almost" fit on that 8mm shaft. No seals on those bearings, no numbers either. But, if they really are 5/16"... why did they mix metric and imperial in the same spinner?

    * Making a silly leap and deciding that bearings are built to absolute measurements and the shafts are then made loose or press depending on requirements, why did the Chinese manufacturer of the "ER11 collet on shaft" thing make their shaft a loose fit for 8mm? You think they'd want it better, like even a light press.

    * And... why did I waste 2 hours of my life looking all this stuff up, asking questions, and wondering about it so that I could reuse stupid-cheap bearings? I could have just looked up what I wanted and done it right.


    Well... I am learning, maybe. And, if that bearing really is 5/16" then I did do a pretty good job with the mic.

    So... press fit. 0.05mm interference. I suppose I could dress the shaft down a bit before pressing it together. 0.025mm isn't much though. Might waste $8 for that ER collet holder. But, I already have a spare on the bench, for when I do this right. Learning's good.

    Thanks for the replies,

    David...

    edit: I should explain... they were dumping fidget-spinners at the local store, $0.99 each. I bought one, decided the bearings would be decent for the typical stuff I do, and bought another 10... threw them in my kid's toybox so he can store them for me. I didn't buy them for stuff that runs at speed, like this spindle. They just happened to fit... sort of. I promise I'll buy better bearings for the non-prototype version
    Last edited by fixerdave; 01-28-2019 at 06:18 AM.

  6. #6
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    OK, fidget spinner bearings. That explains a lot.
    Can be 5/16" bore even if the other dimensions are standard metric bearing sizes, bearing manufacturers are known to make bastard childs like that.
    I have also seen "fidget spinner specials" where bearing races are sheared from mild steel tube and balls are non-round judged even by eyeball-o-meter.

    Some time ago I was hunting for 608 bearings with 5/16" bore from everywhere. Some stupid 'Murican electric motor... ended up replacing the entire shaft.

  7. #7
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    Dave, if you are having trouble measuring the bearing bore (I know I definitely would!) maybe turn a scrap piece down until the bearing is a nice snug fit on it and then measure the shaft. At least that way you can get an idea if the bearings are a specific size.

  8. #8
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    Quote Originally Posted by fixerdave View Post
    ... As for looking up the bearing... no numbers on them.

    The fidget-spinner outside bearings... no problem:
    https://www.vxb.com/608-2rs-Fidget-H...rs-spinner.htm
    (funny that they would sell specific bearings for fidget spinners... I didn't know they did that)

    The center bearings from the same spinners... no number. Might be that they removed the seal. But why? ...
    To reduce friction so that they spin longer.

  9. #9
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    Quote Originally Posted by RichR View Post
    To reduce friction so that they spin longer.
    I get the reduced friction part... what I don't get is that A) why they didn't just buy a bearing without a seal and B) if they did pull the seal... why they used a different bearing than the sealed outside ones... I mean, it's almost the same size.

    I suppose I'm trying to second-guess a Chinese engineer (or accountant)... maybe the same guy, I don't know. I'm still rather surprised they actually make "fidget spinner bearings".

    Quote Originally Posted by MattiJ View Post
    OK, fidget spinner bearings. That explains a lot. ... I have also seen "fidget spinner specials" where bearing races are sheared from mild steel tube and balls are non-round judged even by eyeball-o-meter.
    Yeah, I bought one different one... just to see and the bearings in that were a joke. I could probably make better ones from stock. But, these ones, then ones I bulk-bought, seem okay.

    That said... it's entirely possible these factory made fidget-spinner bearings are just seconds or something. It's also possible they're not lubricated enough to handle continuous spinning.

    Like I said... when I do it right, I'll get better bearings. This was only supposed to be a proof of concept. It just worked better than expected, until it didn't. Now it's a shortcut to get going, so I can make those rocket fins for my kid, and the "do it right" gets put off for saner times.

    Also... I'm learning about bearings and press-fits, so that's good.

    David...

  10. #10
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    My son went through a phase where he was making his own 3D printed fidget spinners and selling them at school. I was buying bulk bearings for him. The outward bearings were just generic with colored plastic seals for looks. The middle/center bearings were un-shielded ceramic and they spin like crazy. It's amazing how fast fidget spinners became popular and how fast they seem to fade.
    Work hard play hard

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