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Tapped holes on AL, long term

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  • Tapped holes on AL, long term

    How durable is your typical aluminum alloy (let's say 6061) for a tapped hole that will receive occasional tightening and re-tightening? Tap size would be small, about #10 by 32, screw is steel. My concern is wear and/or cross threading on the soft AL. I am considering pressed-in brass inserts to accept the threads.

    Gary
    Gary


    Appearance is Everything...

  • #2
    why not Heli Coil ???

    Joe B

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    • #3
      7075 is better than 6061 for what you want to do. Then there is always heli-coils. For AL - I would use coarse threads.

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      • #4
        Depends how many threads of engagement and how tight you need to screw it. You can use 75% thread depth in aluminum because its much easyer to cut.

        Why not try some destructive testing? thread some stock the same thickness as you plan to use, and tighten a screw till it strips. If you need to tighten it anywhere near that level, you need a stronger thread (or insert).
        if your worryed about galling, I recall theres thread lubrication jels

        If you are worryed about it someday striping and ruining your life, Use a nut and bolt insted and don't bother taping the hole. Or some kind of replaceable insert. Or realise that even if it strips and attempts to ruin your life, you can then tap it out bigger and use a repair insert/helicoil/etc.

        If its critical that it never fails or you might be injured physicaly/financially, and its going to be removed/inserted a lot by random people, Then go for something like a helicoil right off the bat, Maybe even a insert with a flange or just a bolt/nut.
        For super critical things like aircraft theres many places where the bolts/nuts are not even to be reused once. Sure prevents wear over multiple insertions
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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        • #5
          With coarse thread and a lubricant such as lithium grease threads in 6061-T6 are good for at least a couple of dozen insertions as long as torque limits are respected. In 7075 you can expect similar performance as with steel.

          Why not try some destructive testing? thread some stock the same thickness as you plan to use, and tighten a screw till it strips.
          The screw will break before the threads give up unless there are only a few threads engaged.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Originally posted by Evan
            In 7075 you can expect similar performance as with steel.


            Ok, I've got 7075 alloy as well, so I'll do some testing. The helicoil didn't occur to me, I didn't think they came that small. I'll check that out as well for sure. The part, and hence screw(s), is small, and also meant for the end user to adjust, (loosen-tighten-loosen-tighten sequence) so wear is the primary concern.

            Thanks !

            Gary
            Gary


            Appearance is Everything...

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            • #7
              The number of threads, and the clamping force, have a big effect on how long threads will last in aluminum. So if you can make the thread length reasonable long and you're not cranking the threads too hard, you should be ok.

              But if you decide to go insert, keep in mind that a lot machinists prefer alternatives to heli-coils such as the KeenSerts, if you want more info on this search "heli coil" and "keen sert" on the practicalmachinist.com forum.

              Good luck-

              Paul T.

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              • #8
                You might consider a steel stud loc-tited into the aluminum...

                - Bart
                Bart Smaalders
                http://smaalders.net/barts

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                • #9
                  I try not to use 10-32 threads in aluminum (or other soft material), opting for 10-24 instead. I have experienced lots of instances of 10-32 threads in aluminum galling and stripping with multiple insertions of steel screws.

                  I had to fix my son's little league baseball team's pitching machine because a 1/2-13 thread in the cast aluminum base wore out after many screw tightenings. I used a Re-coil version of a Heli-coil. The threads were for a giant "thumbscrew" used to attach the tripod legs. Even though the screw was only ever hand tightened, it still galled and stripped. Now, with the Re-coil, the repair should outlive the rest of the machine.

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                  • #10
                    Damn near every paintball gun on the planet uses 10-32 screws, typically to hold the grip frame to the body, and the air adapter to the grip frame.

                    Now, while I've had to repair my share of stripped threads, broken screws and damaged holes, I also own markers I've had for ten or even fifteen years, that have seen literally hundreds of dismantlings and rebuildings.

                    I'm a little more experienced than most with that sort of application, but still, those threaded holes hold up very well indeed.

                    The worst offenders are the bodies where the screw only sees perhaps 2-1/2 turns of thread. This was a common problem, which I'd fix by installing a Heli-Coil, and then carefully grinding off the remaining insert 'til it was flush.

                    As the others have noted, if frequent use might be an issue, drop a Heli-Coil, ReKoil, or any one of the other steel/stainless inserts or thread repairs into the hole. Problem solved.

                    Doc.
                    Doc's Machine. (Probably not what you expect.)

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                    • #11
                      I have aluminum items (6061 or similar alloy) that I made over 40 years ago with tapped threads that have held up just fine. One that I can definitely say has had some real use is a photo tripod adaptor to allow a camera to be turned either horizontal or vertical. It has 1/4-20 threads directly in the aluminum and they are practically as good as the day I made them despite being mounted many times on many tripods with steel or SS screws. As long as you don't cross thread them or over torque, there is no reason why the threads can't outlast you.

                      I have seen aluminum threads stripped but, not as many as I have seen in mild steel. Your mileage may vary.

                      10-32 is a fine thread. If you are worried, perhaps you could change it to 10-24 or 1/4-20.

                      If you expect it to really be torqued down or there will be a really high number of assembly/disassembly cycles, then you can go for a better alloy, like the 6061-T6, or 7075, or my personal favorite, Fortal. You can get Fortal from a guy on E-bay in various sized "drops". He is quite generous if you select the "Buy It Now" price and pay promptly as I once got three chunks for the price of one.
                      Paul A.

                      Make it fit.
                      You can't win and there is a penalty for trying!

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                      • #12
                        You might want to try a roll forming tap, they form the threads by displacement. I have only used them a few times but was very impressed with them, the last job I was using 1/4 20 alloy bolt in 6061 I was having trouble pulling the pieces together because I was a little shallow on my tapping they were only going in a 1/2 inch or so, suddenly the bolt spun free I thought I had striped the thread but I had sheared off the bolt I was surprised at how tough those rolled threads were.
                        Darrell

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                        • #13
                          Paul,

                          FORTAL is 7075-T6 aluminum. The only difference is the name.
                          Free software for calculating bolt circles and similar: Click Here

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                          • #14
                            I thought fine threads were stronger than coarse ones in aluminium alloy? Certainly, the head bolts on my bike directly into the alloy case are metric fine, as are most bike spark plugs.
                            Peter - novice home machinist, modern motorcycle enthusiast.

                            Denford Viceroy 280 Synchro (11 x 24)
                            Herbert 0V adapted to R8 by 'Sir John'.
                            Monarch 10EE 1942

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                            • #15
                              Fine threads are stronger than coarse threads but coarse threads have more room for wear. It is wear that is the issue in this application.
                              Free software for calculating bolt circles and similar: Click Here

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