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  1. #1
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    Default Drilling and tapping (tight)

    Hi folks.

    I have a good range of high quality taps, I don't as yet have any thread testing tools as a set in the range I'd need would be pretty expensive. I've noticed all the bolts I've bought in the past, cold, stainless, etc are all cold rolled to be slightly under nominal. There are obvious reasons for doing this, but what do you do if you need a very tight fit, one that will turn easily but with no slack.

    I've tried a few tests with delrin, drilling undersize then tapping normally and the results aren't all that different, so I'm guessing I 'm just asking the tap to do more work.

    Before I go breaking a load of expensive taps I thought I'd ask the obvious here, how can I tap a tight hole? Or do I need to find a supplier of perfect bolts?

    Thanks.

  2. #2
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    Default

    I suspect that it is better bolts. I remember when working for a defence contractor that we had a particular grade of bolts specified on the drawing, just to ensure they were the right fit.
    You may have to go to a specialist fastener supplier for 'good' bolts. I would not guarantee the goodness of fasteners from a general hardware shop.

    Michael

  3. #3
    Join Date
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    Default

    Quote Originally Posted by Scruntfundle View Post
    ...but what do you do if you need a very tight fit, one that will turn easily but with no slack.
    You may need to make your own bolts. Either by single-pointing them on a lathe (if you have one) or using a split die and opening the die up to increase the thread diameter.
    Chris

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    Default

    Quote Originally Posted by jack620 View Post
    You may need to make your own bolts. Either by single-pointing them on a lathe (if you have one) or using a split die and opening the die up to increase the thread diameter.
    I guess this is why dies are made adjustable and taps are not?

  5. #5
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    Default

    You can also buy a different fit class of tap (effectively undersize taps) in the EU the tap will have a fit number h1, h2 etc. I have some undersized taps from sutton but I dont know if they are easily order able.

    Sent from my SM-G973F using Tapatalk

  6. #6
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    Default

    What’s the application that requires Aerospace tolerance?

  7. #7
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    Default

    Who said anything about aerospace tolerance?
    Chris

  8. #8
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    Default

    Other option is to try a form tap. I find they produce a marginally tighter hole (I only have experience with m10 form taps and only from one brand so ymmv(

    Sent from my SM-G973F using Tapatalk

  9. #9
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    Default

    You can buy different tolerance taps, but the only real way to ensure a neat fit is to make your own bolts (or have them made to your required tolerance)

  10. #10
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    Quote Originally Posted by jack620 View Post
    Who said anything about aerospace tolerance?
    Expression of speech, wasn’t meant to be taken literally..
    Although once you move beyond ISO/SAE type tolerance you pretty much find yourself in NATO/MIL spec.
    My experience is generally when people start asking about tight tolerance thread it normally means they want to locate the parts using the fasteners rather than locating them with dowels and clamping them with fasteners.

  11. #11
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    Threads are for clamping not precision locating.

    You could use or make shoulder bolts. Threads to clamp and precise shoulder to locate

  12. #12
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    Threads can also be used for things like a vernier depth stop or a fine adjuster on a table saw fence. I recently made these and cut my own male threads to give almost backlash-free movement.

    Admittedly I have no idea what the OP wants to do with his snug fitting threads. I’m just spit-balling.
    Chris

  13. #13
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    Welcome to a TOP FORUM. Sounds like what you need are shoulder bolts.
    Shoulder screws are essentially a socket cap screw that is only partially threaded with a shoulder. ... These bolts are often used on machinery to provide a smooth surface for bearings to ride on. Shoulder bolts are measured by their diameter of shoulder x length of shoulder x thread diameter.
    Shoulder bolt.jpg
    HTH
    Kryn
    To grow old is mandatory, growing up is optional.

  14. #14
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    Where did this shoulder bolt stuff come from?

    All OP asked about is how to get tight mating threads from off-the-shelf bolts...

  15. #15
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    Quote Originally Posted by elanjacobs View Post
    Where did this shoulder bolt stuff come from?
    I'm glad I'm not the only one thinking this!
    Chris

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