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Thread: Machinists, get ready to shoot me

  1. #1
    Insider PBSteve's Avatar
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    Machinists, get ready to shoot me

    I've got a part I want to press-fit/force fit. I am going by ANSI B4.1-1967, with a fit class FN2. My shaft is 304 SS, and my hole is in 6061-T6.

    The dimension I'm coming up with on my shaft is 0.1942" (+0.0003 -0.0000), and the dimension on my hole is .1935" (+0.0005 -0.0000). They're both lathed parts, and the interference fit is 0.25" long.

    This seems... expensive. Someone tell me it isn't too bad or I'm doing something wrong/stupid?
    Last edited by PBSteve; 09-22-2017 at 08:59 PM. Reason: Went to a standard sized (#10) reamer
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    Insider PBSteve's Avatar
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    Not to mention it's within the realm of Type II anodizing variations.
    "He died on that hill even though no one was attacking"
    I work for the company building the Paragon...once we figure out a name

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    Can you share the specific application? Even if it's not paintball, I'd maybe look to use epoxy but have machined channels to hold it. Obviously threads could be an option. Then you could stake it (common practice in firearms).

  4. #4
    Typically you wouldn't anodized something before you press fit it in my limited experience dealing with it. Look at the old wgp body press fit feednecks for an example they don't anodize them before they press them in.

    Did you make sure the part still has the crush you need during thermal expansion because they will expand and contract at different rates.

    A tenth tolerance isn't to hard to hold on a decent lathe, assuming it's a short piece, stainless isn't the easiest to do it with though.
    Last edited by Florypb505; 09-23-2017 at 10:41 AM.

  5. #5
    Insider PBSteve's Avatar
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    Quote Originally Posted by Florypb505 View Post
    Did you make sure the part still has the crush you need during thermal expansion because they will expand and contract at different rates.
    Ooo good point, forgot to consider that. Nevermind! I think a slip fit and epoxy is the call. Thanks everyone.

  6. #6
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    Also look at a temperature fit. Freeze the inner piece, heat the outer piece. Put them together, and then they seat.

    Not sure the tolerance you might need, but they make it fit all the time.

  7. #7
    Insider PBSteve's Avatar
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    Yeah, that's called a shrink fit and uses the same dims as the friction fit according to the ANSI standard.

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    Just a question for the group - what's the advantage of using interference fits over a thread? Simplifies machining? Shorter engagement lengths?

    I tend to trust interference/temperature fit, since the crushing forces are massive (not always intuitive, but the expansion*the material modulus gives you the crushing force). A slip fit with epoxy scares the bejeesus out of me since it implies the load is on the binder in shear.

    The BMW M30 uses a temperature fit on the timing chain sprocket where it slips onto the crank. I thought that was really cool - installation takes a 450F oven and some quick work. Came off with a gear puller/no heat, though.
    "So you've done this before?"
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  9. #9
    The advantages I see are that threads allow a little bit of slop that the press fit doesn't. Also it is more costly to make nice threads then just reaming a hole and turning down one piece to the size you need.

    The other thought is if you rip out a thread you are sol more often then not. You rip out a press fit you have more options to try and fix it.
    Last edited by Florypb505; 09-24-2017 at 10:12 AM.

  10. #10
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    Threads are dirt cheap as long as they're a standard profile and pitch so they can be cut with common tools. Personally, I'd trust threads with a permanent thread locker over a slip fit. Depending on how your threads are done, you can stake it and they won't come loose under a reasonable use case. Assuming this is for the paragon plunger, I think you'll be fine with threads and some locker (though uninformed users will inevitably try to take it apart).

    The m2 bolt is multiple pieces, BTW. We haven't has a single report of one separating. It's threads and locker.
    Last edited by ironyusa; 09-24-2017 at 10:17 AM.

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