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Thread: 4-40 thread problem

  1. #1

    4-40 thread problem

    I'm trying to find info about the major thread dia tolerances of a 4-40 set screw. Here's our problem.
    We've been providing a product with a 4-40 helicoil installed in ABS plastic for which our customer has been installing a 4-40 allen driven cap screw. All works fine.... 1000's of parts.
    Now they want to use that hole for a 1" long, 4-40 allen driven set screw and it's binding up.
    Parts were rejected stating "threads are too tight".
    We can run a "go" guage in there. It's a little snug but can go. The old cap screw works fine.
    Then the new set screw is still tight.
    We measured the major diameter of the set screw and find that it's way at the top of tolerance
    .1112" But also it's oblong. Measuring from .1095-.1112". ( could this be part of the trouble?)
    Regardless, is there any info about set screws being made to the high end of thread diameters or better yet... any info refering to using a set screw with a helicoil?
    Last edited by art; 03-30-2011 at 09:33 AM.

  2. #2
    Lead Engineer RWOLFEJR's Avatar
    Join Date
    Mar 2011
    Rochester Pennsylvania
    Sure there's going to be some variation or tolerance to every nut and bolt made... and maybe your bolts were made 2A and the set screws are 3A (Although your measurement suggests they're 2A...) But regardless the set screws are to spec so you need to look at the entire situation. I'm not a big fan of the Helicoils but yes... they do have their place. Your problem is either with the coil or the tapped hole prior to coil insertion... or possibly just "the nature of the beast." Helicoils have an allowance for a snug fit on the initial insertion of a bolt. Do a search and they'll list all sorts of specs that they meet and one of those specs as an allowance for the first time a bolt is run in it could be snug and could further seat the coil. So these tiny bolts had a tiny allen wrench to run them in when it was a cap screw. Now it's a much smaller wrench to run the set screws in and it'll feel tighter with the even thinner wrench giving under the torque.

    Or... Could be your tap is worn some. Take a gage to your tapped holes prior to inserting the helicoils. (Helicoil sells them for their funky taps) Another thing to check to help determine if there's an actual problem or a "by design" thing would be to run a set screw into one of the rejected parts... then remove it... Then run it back in and see what sort of torque it takes the next go at it. It would also be a good idea to get torque readings on both good and rejected parts using both the original cap screws and the new set screws. This will give you information to better understand what's happening and explain what's going on to your customer.

    Good luck,

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