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  • CNC machined part, screw holes

    Discussion in 'The main mechanical design forum' started by king-panther, Apr 3, 2014.

    1. king-panther

      king-panther Member

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      I am designing something that will be made by a CNC machine, how would I design it that it allows a screw to be screwed in? Because if the surface is smooth, there is nothing for the screw to grip, also, can i design it universally to fit all screws of the same length and radius?
       
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    3. nicbirt

      nicbirt New Member

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      Two basic options:
      1. Use self-tapping screws and provide a tapping hole (less than the screw diameter so the screw can cut its thread through the material). You may want to consider thickening the material locally if more thread engagement is required.
      2. Use a 'rivnut' and a machine screw. The 'rivnut' is is an insert that contains a threaded hole (a nut) and is squeezed in place like a pop rivet.
       
    4. ElZylcho

      ElZylcho Member

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      I'd say use the tapping holes, but use a thread tap instead of self-tapping screws. Especially if this part is intended to be easily removed and reinstalled.
       
    5. KevinC

      KevinC Well-Known Member

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      The theory with screw is that it uses the friction created by the normal force (how tight you screw the screw in). So increasing normal force or increase friction will work.

      Increase normal force can be achieved by screwing the screw in with more torque, or put a lock washer in there to provide more normal force so the screw won't work it's way loose.

      Increasing friction can be achieved by just keeping the part free of oil. Put some lock tide in there to make sure it doesn't work it's way loose is a good idea as well depends on your application.

      Tap your hole good is essential. Have fun!

      Hope it helps
       

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