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Making very small, oddball thread, flathead screw

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  • Making very small, oddball thread, flathead screw

    How best could one make a small, #4 and smaller, SAE or Metric replacement flathead screw, going into an existing tapered hole. The head would be the most difficult, especially something that small. Could you take a longer & larger screw with the correct head taper that has a thread minor dia. larger than the major dia. of the screw you want; reduce the head top dia. to correct, remove enough of the old threads & cut the oddball thread & cut to length. Is it likely the head would fit or is there a better way to cut the head and thread from stock.

  • #2
    I'd make the screw from scratch... if you can get away with a flat-blade screwdriver drive, as opposed to cross-point or hex socket. If it's an inch screw, 82-degree included head angle. Metric, 90.

    After turning the thread and head angle, you could part it off. Screw it into a block, and run a small end mill or slitting saw into the head to make your slot for the screwdriver. One could also use a bandsaw, but... results would be questionable.

    Especially since an existing screw would probably be harder than plain stock.


    • #3
      You mention #4 or smaller and oddball thread. What oddball thread? I have dies for 4-40, 4-36, 4-48, and perhaps more #4 threads. Those are "standard" #4 threads. Or do you need something really odd, like 4-45?

      If you try to cut down an existing screw you will run into several problems. First, how to hold it. The head will not be large enough to get a good grip for cutting new threads. And if you use a screw that is big enough to remove all the old thread, then you will need to completely reform the head anyway. So, you are better off with fresh stock that can be easily gripped and cut the new screw off after it is mostly complete.

      Anyway, I would just start with a piece of round stock and turn the thread end. Thread it. then turn the head starting with the underside. Finally the top of the head. Some heads would not be complete before parting it off so you would need to make a matching female hole in a piece of scrap and screwing it in for finishing.

      As for the drive recess, that could be done several ways. First, there is the simple straight slot. Shop made screws are probably the origin of the straight slot.

      Or you could drill a small hole and use a rotary broach to cut an Allen or spline hole.

      Or you could use a hardened screwdriver tip and a lot of pressure to create a Phillips recess if the metal is soft enough. You would do the final finish on the head after this operation.

      Or do an external hex for a socket wrench or hex driver. Or a square head, which is simpler. I can see it now, a #4 screw on my 10" rotary table and a magnifier so I can see the work. Talk about overkill. I guess a spin index would be more in keeping with the size of the work.

      If it is a really odd thread, you will need whatever change gears that are needed to cut it.

      Frankly I would do whatever I can to allow the use of a standard screw. $0.05 vs. $0.50 for the stock and hours and hours of work.
      Last edited by Paul Alciatore; 10-25-2015, 07:49 PM.
      Paul A.

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


      • #4
        Could you use those little dremel disks to slot it?


        • #5
          flat head screw - can we assume its a slotted head? just turn and thread in the lathe , use a slitting say for the slot.