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  • Can hardware store hex couplings be used as drill guides?

    Discussion in 'The main mechanical design forum' started by thorq, Apr 9, 2016.

    1. thorq

      thorq Active Member

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      Hello, I have a project where I need to do some pretty straight holes in aluminum square tubing and I am designing a "tool" to help me with that. I am 3d-printing the tool so it's plastic but the bushings will be some sort of metal and I was thinking hex couplings.

      Are they appropriate for driving a drill bit through considering they have threads that are kind of perpendicular to the cutting edges of the metal drill bits? Are they hard enough steel to do the job properly? I only need them for one job that is about 50 drills.

      The tool is quite simple:

      [​IMG]

      Here is the 3D view on Sketchfab:
      https://sketchfab.com/models/60a953dc3a9b4d1f9edbfd82df01ee77/embed
       
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    3. robertjeffery

      robertjeffery Active Member

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      Go for it, I do suggest first drilling out the threads to a clean bore, as otherwise if some swarth gets up inside between the thread and the drill bit, it will jam and rip the threaded hex connectors out of the plastic.
       
    4. thorq

      thorq Active Member

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      I hope that doesn't happen. I'll have to see what metric coupling I could use so that after drilling out the threads I end up with an M5 bore. Maybe M4 but have't seen M4 couplings in the hardware stores around. They all seem to start with M5, M6, M8 ...

      Anyway I am still designing the tool as it will have other functions than do the guiding for the drill bit.

      I am wondering if the drill bit itself will eat up the threads...
       
    5. CPPMable

      CPPMable Well-Known Member

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      I wouldn't use a hex coupling. The threads would probably catch unless you drill them out like robert suggested but that seems like a lot of work to unless you have them laying around. I would suggest just using a press in bushing as a guide. Should be able to find with with the exact ID you want and just go with it. They are pretty cheap as well and can find them at many vendors. They even make special ones for plastic.
       
    6. thorq

      thorq Active Member

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      I figured that bearings must be the best bushings ever... they are hardened steel and they rotate with the drill bit, having no wear due to the cutting edge rotating against a static bushing. I am using in the same project a bunch of F694ZZ bearings so I have re-designed the tool to accommodate the said bearings as temporary bushings. They will be re-used later on when the frame is ready.

      What do you think?


      [​IMG]

      I had to shift the bearings to keep the same hole arrangement as before (need this for 2020 square tubing) and also accommodate these flange bearings as they are.
       
    7. CPPMable

      CPPMable Well-Known Member

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      Interesting!

      The problem with bearings is most are designed only for radial loads unless you get a thrust bearing or others that support axial loads. Plus they are expensive. What you are doing is very similar to many other products on the market such as the Kreg Jigs look it up. Just a simple hardened steel bushing pressed into a plastic jig. I have one and it has lasted many years of abuse.
       
    8. thorq

      thorq Active Member

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      It's just that I don't have the steel bushing but I have the bearing and it will only take whatever forces will be there in 50 holes. I don't think that there will be any other forces than radial ones.
       
    9. Dana

      Dana Well-Known Member

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      Use a "real" drill bushing, from somebody like Carr-Lane, or make one from solid steel stock. Not only are hardware store coupling nuts fairly soft, but threaded holes don't tend to be particularly accurate.
       
    10. lgsp90

      lgsp90 New Member

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      Check out clamp on drill guides from wolfcraft or other wood working tool companies. They are designed to drill dowel pin holes at a true 90 deg to the work surface.
       
    11. K.I.S.S.

      K.I.S.S. Well-Known Member

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      To prevent the drill bit from doing any appreciable damage to the thread, you might also want to consider 'jobbing' bits - these have a limited flute length, and are designed to be more rigid than multi purpose bits. Of course, this depends on your material thickness and also the flexibility you have in your jig design as to whether you can stand it off sufficiently prior to the bit contacting the material.
       

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