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Helical grooves

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  • Helical grooves

    Suppose you take two strips of sheet metal, perhaps a half inch wide, and wrap them around a rod such that there is no space left around the rod. You have formed a spiral assemblage. Fasten one strip to the rod, and the other is free to move along the groove that it now resides in.

    Place a tube over that, and fasten the tube to the movable strip. If you rotate the tube with respect to the rod, you get an axial displacement, like a nut running on a threaded rod.

    This is obviously a very coarse thread- you might be lucky with this example to get 1 turn per inch. Now consider the post about the two directional thread that some Russian guy made. There was discussion about errors in the angle of the flanks or something because of the coarse thread- am I going to see some similar effects with my sheet metal strips?

    I'm assuming the strips have straight sides, a proper rectangular cross section, before being wound onto the rod. There will be an effect on the edges of the strip as they are wound onto the rod- the strip has to stretch on the outside and shrink on the inside as it's being wound, so that is naturally going to change the shape of the edges. But putting this effect aside- will the flanks of these mating strips change their orientation with respect to each other when wound onto the rod? The assumption is that if the edges of the strips had full contact when the strips are laid side by side on a flat surface, then when wound onto the rod they will still have that full contact.

    I'm thinking that the only change I'll see is edge deformation as the strips are bent to form around the rod. The square thread form should remain perfect. Does that make sense?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    What you describe makes total sense. And in theory it would act just like you say. The thread pitch being reliant on the total width of the the two strips.

    However doing this in real life would likely require the aid of some form of machine or tensioning device that has some serious mojo associated with it. The bigger the core and thinner the strips the easier of course. But if you're trying to tightly wind some 16Ga around a 1" diameter core it could make for some interesting language if the setup isn't pretty skookum.

    Oh, and I'd include a small spacer of some sort between your two strips, The width to be included in the "pitch" addition so your thread has a touch of play in the end.

    Depending on what diameter you have to winde the s
    Chilliwack BC, Canada


    • #3
      If your strips had no thickness, the edges would touch perfectly. For strips with some thickness, winding the strips around the former rod will cause the strips to become wider at the bottom (where they touch the former rod), narrower at the top / outside. This will result in a sharp vee groove between the strips. Angle of the vee groove will be determined by the former rod diameter and the strip thickness.

      To get perfect edge face to face contact, you'll need to start with a strip of trapezoidal form (wider face outwards). On winding the strips, a parallel gap will then appear, giving your thread some "backlash" or working clearance.

      All of the gear, no idea...


      • #4
        The flank error issue from the other thread is due to the compound angles presented by the width of the top of the threading tool as it attacks the part at varying lead angles. So the higher the lead angle, the more the included angle (only when measured *inline* with the axis of lead angle) changes. The example was given in the thread of holding a playing card and viewing it square from above. As the card is rotated while still viewing from above, the effective width of the card on the axis presented to your eye will change. So in that case you'd see a changing width rather than a changing angle. Because a threading tool has width at one end and an effective point at the other, this width basically only changes at the big end, which makes the included angle change as the tool (or rather, in the case of threading on the lathe, the helix angle) is rotated. This problem is generally specific to a 2 dimensional (it should only touch in two dimensions as a line contact with sufficient clearance) profile being rotated as it's passed through a part.

        Your sheet metal example would not work exactly the same, but as already noted, there would absolutely be some distortion there that would cause you problems with a "zero gap" fit of the sliding member. I'm not sure if you're asking this as a theoretical exercise or if you are asking because you have some sort of plan to actually make something... That would probably make a difference in the answers you'll get. It suffices to say that forming such an object would not be trivial.
        Last edited by eKretz; 12-22-2020, 09:56 AM.


        • #5
          What if instead of wrapping both strips around the male former, one was twisted into the bore of the tube. I think that the sloping sides would now match better.


          • #6
            Somewhat off the original topic, but if I were going to try to actually make a very light duty linear actuator thingy like this, I'd try not with sheet metal strips but with suitably thick wire. I haven't thought through the geometry entirely, but it seems intuitive that it would 'just work' - or at least some of the distortion issues mentioned would go away.
            "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979


            • #7
              That helical wire trick has been used on Verdict lever indicators. The lever inner end fits in the open wound wire and turns it to work the pointer. The helix must be superbly wound to be accurate.


              • #8
                It is a hypothetical question- though I have made a mechanism this way to act as a jig to machine something on the mill. It is now clear to me that forming a thread by winding the strips doesn't introduce the error, while machining the thread as in the video does allow for an error to occur.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-