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  • Is this safe?

    I want to make an adapter for a 4-1/2 inch electric grinder (10-12 Krpm). I propose using a 5/8 inch joiner nut (for all thread0 as the basis and make a threaded spindle/mandrel that would accept 2 inch or so polishing discs.

    Should retention in the joiner nut be reliant only on the thread or should it be high-strength Loctite'd?
    I could consider cross drilling & placing some sort of pin (roll or spring pin or...) but I wonder if this might be thrown out due to the centrifugal force generated by the grinder shaft's high rpm.

    More importantly, if this whole idea is totally nuts, how should I proceed? Thanks

  • #2
    A tight fitting pin will stay in place if it goes all the way through because centrifugal force will be even on both ends. I think your biggest problem with using ordinary joiner nuts will be poor concentricity and lots of vibration.

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    • #3
      There are some kits on fleabay that screw on a roller part of a belt sander attachment
      https://images.app.goo.gl/xVnkVEypTNCQQfFL8
      Helder Ferreira
      SetŮ’bal, Portugal

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      • #4
        At those speeds, balance is crucial to smooth operation. I would be more inclined to machine an adapter piece, and I'd spend the extra time to make it a close fit.

        Vibration tends to loosen things, and that can happen very quickly. I would not use a threaded coupler in this situation.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Yes, joiner nut idea was nutty. Will machine it properly from one solid piece.

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          • #6
            You mean like this? http://www.flapperadapter.com

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            • #7
              You may want to take a close look at your grinder spindle accuracy first. I say this because a long time ago I made my own piloted valve seat grinder stone holder to fit my angle grinder. Vibrated like crazy until I figured out the wobble was in my grinder spindle. I took the grinder apart so I could use the spindle to hold my stone holder in the lathe to tweak the alignment. Works great now with little vibration and I have never taken it off. I like it because it has plenty of power for roughing.

              Yours is probably fine, just trying to save you some aggravation.

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              • #8
                Do the polishing discs have a maximum recommended rpm on them? How fast doe's the machine designed to utilise the discs run?

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                • #9
                  Originally posted by Dunc View Post
                  Yes, joiner nut idea was nutty. Will machine it properly from one solid piece.
                  Can I go against the grain here? The joiner nut is a perfectly suitable starting place for your idea.

                  The spindle rotation has a tightening effect on everything so no pins, loctite, etc. required.

                  No matter how badly made the nut is, its outer diameter is only circa. one inch, way less than a 4 1/2" dia. 1/4" thick grinding wheel so out of balance vibration is unlikely to be an issue.

                  It would be prudent to look closely at how a standard grinding wheel is located on the spindle and replicate that. There is a straight portion of the spindle onto which a flange fits. The centre of the wheel locates on a boss on the flange. Hence the threads of the spindle do not contribute to location (same as a threaded lathe spindle).

                  If you bore out the end of the coupling nut so it locates snugly on the straight portion of the spindle, you are copying the OEM principle. At the other end, the polishing disk needs to be located nice and concentric with the bore you make.

                  I've always had an idea to make a flapper adapter out of a cheap ER16 straight shank collet chuck. Main issue is annealing the shaft.

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                  • #10
                    Thanksall for the insights. Tungsten dipper has shown that it is difficult to re-invent the wheel. For the price, it is the easiest route.

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                    • #11
                      The simple solution is to use the collet nose off an HF die grinder.



                      The female thread is 3/8-24. Randyjaco posted about using one to make an internal grinding arbor for a tool post grinder over on Shop Floor Talk.

                      I've used one to modify a Matco mini grinder and a second one to make a flapper adapter.

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