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Press fit hex into round hole?

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  • Press fit hex into round hole?

    What size round hole should I make in 6061 AL so I can press fit a 5/16-24 locknut into the hole. I want to be able to use the enclosing aluminum to spin on/off the nut and tighten it down.

    Probably similar to a plastic knob with a metal insert. I don't want the nut slipping in the hole.

    Are there different ways to achieve the same goal?

    Thanks.

    - T

  • #2
    How thick is the aluminum? Should be as thick as the nut.I think any torque or vibration and the aluminum will fail. Harbor Freight sells a nut riveter that installs nut into sheet metal just like a pop rivet.Someone may make them for heavier gauge material
    What is the application? Maybe someone could throw in a different method if we knew what you are building.

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    • #3
      The part is going to be made out of aluminum barstock so the hole will be as deep, and most likely, deeper than the locknut itself. The edge of the locknut will be flush to the edge of the hole.

      I want to use a locknut because I want the locking capabilities of the locknut (as opposed to just tapping a hole). This isn't the type of application where locktite could be used.

      My tunnel vision is pointing to using a locknut pressed into a hole, but the functionality needed is a 5/16-24 thread with locking capability. The part will be screwed on and off several times but it isn't a high torque application and the vibration requires it to be locked on.

      Thanks.

      - T

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      • #4
        Unless you cut a hex recess in the aluminum shaft the nut will start slipping in the bore when you try to tighten the nut.

        If the wall is thick enough you can use opposing set screws to tighten against the flats on the nut to retain the nut.

        To determine the size hole to press the nut in measure across the high points on the nut and reduce the dimension by .050" and that leaves .025" on each side to press in. Then with the set screws locked against the nut it should work for a while until stress works it loose.

        How thick is the wall going to be?
        It's only ink and paper

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        • #5
          My Modern Machine shop Handbook says a 5/16 locknut should be..

          0.500 ins across the flats

          0.552 ins across the corners.

          I dont think you'll find a chart showing the exact dimensions for forcing a hex fit into ally ..so a few test runs may be needed..

          You could try drills in the range from..

          0.5 or 1/2 ins
          0.5156 or 33/64
          0.5313 or 17/32
          0.5469 or 35/64

          Rob

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          • #6
            It's best to measure the nut your going to use because the maker may not use the standards in the book.
            It's only ink and paper

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            • #7
              out side the box, iam thinking a counter sunk set screw to secure it and prevent it from ever spinning,

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              • #8
                A wrap of momofilament line around the male threads will provide the same effect as a locknut. If you are concerned about the threads in the aluminum, use a Helicoil for the threads.
                Jim H.

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                • #9
                  I remember you when we were in kindergarten! You're that kid that kept pounding square blocks into round holes.

                  Why not just get a piece of 1/2" CRS Hex bar stock, bore a hole down the middle of it, and sharpen the corners....Presto! A hex broach!
                  Last edited by saltmine; 05-13-2011, 08:06 PM.
                  No good deed goes unpunished.

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                  • #10
                    For aluminum make the hole diameter the same as the dimension across the flats. For locking, use a Stover nut and grind the "corners" so the vertices of the flats are sharp and pound it in. A Stover nut is all metal and has a built in washer on one side so it won't go in too far.

                    One problem with press fits of steel into aluminum is that if it gets much warmer than the temp at which it was assembled the steel part will fall out.
                    Free software for calculating bolt circles and similar: Click Here

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                    • #11
                      What about this? Drill and tap a hole from the side of the knob for a 10-24 nylon screw. Then drill and tap your 5/16 hole.

                      Install a nylon screw long enough to extend into the 5/16" hole. This should lock the knob in place. As the nylon screw wears out, tighten or replace.
                      When I get Time... I'll...

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                      • #12
                        If this is a one off, your press fit nut concept should work fine. This may be obvious, but you should machine the hole with stepped diameters. The nut should be pressed in to a shoulder, preventing the nut from pulling through the part in use.

                        For a production part, you should take a look at locking helicoils. They have a deformed thread in the middle of the coil which grabs the screw. I work on components for Formula 1 cars, and they use these extensively as they are light weight, re-usable, and very effective under high vibration conditions. They use standard Helicoil taps, and installation tools. Get them from McMaster carr.

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                        • #13
                          Originally posted by Evan
                          For aluminum make the hole diameter the same as the dimension across the flats. For locking, use a Stover nut and grind the "corners" so the vertices of the flats are sharp and pound it in. A Stover nut is all metal and has a built in washer on one side so it won't go in too far.

                          One problem with press fits of steel into aluminum is that if it gets much warmer than the temp at which it was assembled the steel part will fall out.
                          a stover nut is a one time use nut...

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                          • #14
                            Combining a couple of the suggestions you have a short length of the hole bored large enough to center the nut, then a smaller ID in which to press it. I'd probably go a tad larger than the dimension across the flats for the smaller bore. Part of what might become a problem is if the nut cuts the aluminum as the corners are pressed in. If they do, then the nut will want to fall out when the temperature of the part rises significantly. If the corners can skid into the bore under pressure, the aluminum will become stretched around the nut and a lot more of a temperature rise will be required to make the nut come loose. It is entirely possible that by using a lube you can avoid the first circumstance.

                            The bolt being inserted will only require as much torquing as the nyloc insert decides. It's not that high, and the inserted nut should be stable and not rip out. If there's going to be side forces against the bolt or threaded rod piece being used, you might bore the hole deeper for the inside end of the bolt or rod to nest into a short way.
                            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                            • #15
                              Can you mount the locknut on a piece of threaded rod, turn the hex to a cylinder and thread the outside? Then, thread the hole, loctite & screw the nut in. This will be more secure than a simple press fit, and will resist the pulling forces when in use.

                              Ian
                              All of the gear, no idea...

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