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29/64" drill for a 1/2-20 tap in aluminum?

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  • 29/64" drill for a 1/2-20 tap in aluminum?

    Had an interesting weekend working on the go karts. The steering knuckles are 1" OD aluminum tube with threaded inserts welded into the end. The student's drilled all of the inserts with a 29/64" drill bit to be tapped with a 1/2"-20 fine thread tap. Heim joints (ball rod ends) are fine thread, otherwise I would use course threads. I went to tap the first couple of inserts and the tap spun in freely without lubrication. Huh?! that's unexpected. I double checked the diameter of the inserts with a pin gauge and verified diameter. The thread grooves were really shallow, and I was able to spin a bolt into the inserts with 20+ threads engaged and strip the threads by hand. I made some new inserts with a 27/64 bit and the tap wouldn't pilot. So I went up to 7/16 and got a thread I was happy with.

    I searched google and reviewed several different tap-die charts, they all say 29/64. Is there something I am missing with larger diameter aluminum threads?
    If the women don't find ya handsome, they should at least find ya handy

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  • #2


    Here is the steering knuckle design. It didn't matter whether I tapped before or after welding the inserts at the top and bottom of the knuckle.
    If the women don't find ya handsome, they should at least find ya handy

    https://www.facebook.com/WDHSTechClub

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    • #3
      At the risk of offending did you make sure you grabbed the right tap?
      Last edited by Bob La Londe; 04-23-2018, 11:37 AM.
      *** I always wanted a welding stinger that looked like the north end of a south bound chicken. Often my welds look like somebody pointed the wrong end of a chicken at the joint and squeezed until something came out. Might as well look the part.

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      • #4
        Did the 29/64ths actually drill a 29/64s hole. My money is on the hole being WAY oversize on the first batch..

        On the second batch, I'm voting for a crummy tap.

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        • #5
          According to the charts, you get a 75% thread when using 29/64" drill bit tapped with a 1/2"-20.

          It sounds like you have a bad tap.

          Dan
          At the end of the project, there is a profound difference between spare parts and extra parts.

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          • #6
            Originally posted by bobw53 View Post
            Did the 29/64ths actually drill a 29/64s hole. My money is on the hole being WAY oversize on the first batch..

            On the second batch, I'm voting for a crummy tap.
            Best I can tell the tail stock on the school lathe is off center. The entrance to the hole is the right size but the run out from the bent drill bit killed all of the material for the threads.
            If the women don't find ya handsome, they should at least find ya handy

            https://www.facebook.com/WDHSTechClub

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            • #7
              The drill size is 0.453, so it does not take much error in location to "bore" the hole out to an oversize that will not take 1/2-20 threads.

              I know you mentioned this, and it does not affect the drill issue, but fine threads and soft metal do not mix well. Is there no way to either use coarse threads, or use an insert of a harder material?
              1601

              Keep eye on ball.
              Hashim Khan

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              • #8
                Originally posted by J Tiers View Post
                Is there no way to either use coarse threads, or use an insert of a harder material?
                Coarse thread rod ends are available, but they are hard to come by if you are in a pinch. I will plan to add helicoil inserts after this weekend.
                If the women don't find ya handsome, they should at least find ya handy

                https://www.facebook.com/WDHSTechClub

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                • #9
                  My thread charts also mention 29/64" (0.453) and also 11.4mm (0.4488") which is 0.0042" smaller. The female thread depth is 0.0271", so there is not a lot of leeway if the 29/64" is oversize. Going to 7/16" is a good move, as aluminium is soft enough to tap even in an undersize hole.
                  Helicoils are a good method, especially if the thread is adjusted frequently, assuming the wall thickness is sufficient.
                  If a taper tap is available, then measure the diameter of the tip for the drill size.
                  Last edited by old mart; 04-24-2018, 09:06 AM.

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                  • #10
                    Originally posted by Ggerg1186 View Post
                    Coarse thread rod ends are available, but they are hard to come by if you are in a pinch. I will plan to add helicoil inserts after this weekend.
                    Helicoils are cool, you get the strength of the slightly larger thread in the aluminum.. Though you are still going to be fine thread..

                    I'd take it one step further and make a steel insert.. Maybe a course thread 3/4" on the outside, 1/2"-20 on the inside.. Then you have some
                    serious bite in the aluminum, and your rod end is far less likely to pull out of a steel insert.


                    Another thought.. Did you accidentally end up annealing the aluminum when welding, and now you are just dealing with a pile of bubble gum??

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                    • #11
                      Originally posted by bobw53 View Post

                      I'd take it one step further and make a steel insert.. Maybe a course thread 3/4" on the outside, 1/2"-20 on the inside.. Then you have some
                      serious bite in the aluminum, and your rod end is far less likely to pull out of a steel insert.


                      Another thought.. Did you accidentally end up annealing the aluminum when welding, and now you are just dealing with a pile of bubble gum??
                      It's possible we annealed the aluminum. We used MIG welding.

                      I found these at Fastenal, pricey, but just the ticket:

                      https://www.fastenal.com/products/de...1/2%22-20$%7C~
                      If the women don't find ya handsome, they should at least find ya handy

                      https://www.facebook.com/WDHSTechClub

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