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R8 Tight Collets

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  • R8 Tight Collets

    I'm running a Grizzly MD, that I bought used with quite alot of tooling. Since I got it, I noticeed that the .750 collet was quite a snug fit going into the spindle. There is no apperant damage. Just recently, the .500 collet has the same symptoms. It should be noted that all other collets seem to fit fine. I have inspected the internal bore on the spindle, and all appears clean and smooth. It isn't that I can't use these collets, it is just a pain trying to get them out of the spindle when changing. They are import collets as well. Any ideas on what may be causing this? Maybe it is time to upgrade to a better brand of collets. How about some suggestions? Thanks.
    Arbo & Thor (The Junkyard Dog)

  • #2
    Check the collet dimensions against the dimensions in the Machinists Handbook. The bearing dia. should be .950" Check the dimensions against trouble free collets you have. You state there is no apparent damage. Are the troublesome collets new? Any sign of the pin (if your machine has one)in the spindle that prevents rotation of the collet while tightening dragging in/on the slot in the collet? I have had that problem with some 5C collets.
    Paying Attention Is Not That Expensive.

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    • #3
      When I first got my new Enco MD and a set of collets, about half of the collets were either really tight or would not insert in the spindle at all. I immediately assumed that it was the collets that were bad since there was so much variation. I had Enco replace the collets. Surprise! Same results with the new set. So this time I did what I should have and measured the upper shoulder of the all the collets. Sure enough, although there was a LOT of variation (couple of thou !!!) all were at or under the max allowed 0.9495 diameter. Those that wouldnt fit were at or above above approximately 0.948 (my caliper could only measure to .001 at the time). The solution was a new spindle for the MD. Enco promply sent me a replacement, but I had to install it. Kind of a hassle to install, but since doing so, no problems. BTW, dimensions for the R8 taper can be seen at http://www.loganact.com/tips/r8.htm
      Jay

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      • #4
        The weird thing is, the upper shoulder fits fine. It doesn't get tight until I reach the slotted area of the collett. It seems as though they have "sprung" to a larger diameter. I will do some measurements this evening, and post the results.
        Arbo & Thor (The Junkyard Dog)

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        • #5
          Look for a crack in the keyway of the collet. If there is one then the collet is expanded. I've seen it before.

          Also the slightest burr on the edge of the keyway will do it, can be almost invisible.



          [This message has been edited by suprdvn (edited 01-09-2005).]
          Super Dave
          RapidtoCNC.com

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          • #6
            The "upper shoulder" doesn't reach a mating surface in the spindle until the collet is inserted way up into the spindle (at least on my MD). In my Chinese MD, the only "key" in the spindle is a couple of allen head set-screws in the spindle that project into the barrel of the spindle. On my original spindle they had not even been loctited and one had worked its way partially out and was simply sheared off. As mentioned before, this ultimately was not the problem though.

            If your MD is anything like mine (Rong-Fu style), you can remove the cover on the quill stop scale and see the quill. Move the spindle up and down and you should be able to see these set-screws through the slot in the quill. Maybe yours are loose too and one has just screwed itself in a little too far. Be sure to loctite when you get it set right!

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            • #7
              If the key is interfering, it will bind the collet for the whole length.

              Best advice on the key is to get rid of it if it is creating problems. It serves two functions. One is to hold the collet while tightening,and the taper will hold sufficiently for this. The other function is to partially shear and jam the collet in place.
              Jim H.

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              • #8
                I measured some collets this evening with the mic's, and the results are as follows. The 1/2" and the 3/4" are tight, and the 3/8" fits nicely.

                1/2" .9495, and .9281
                3/4" .9507, and .9290
                3/8" .9491, and .9289
                Arbo & Thor (The Junkyard Dog)

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