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how to get a thread right to bottom of a blind hole?

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  • how to get a thread right to bottom of a blind hole?

    I'm trying to duplicate some fluid valve mount here at work, for a different prototype using the same valve in a different way. The mount is a threaded flat bottom hole with two ports on the flat bottom portion.

    Straight-forward enough, but the factory mounting hole has threads that seem to go right down to the flat bottom surface. The threads are 5/8-24, and I have a bottoming tap but that doesn't put the threads down far enough for the valve to seat properly.

    I bought another 5/8-24 bottoming tap to grind the last couple tapered threads off, but wondering what's the best way to grind the tap to get that thread down as far as possible?

    If I grind it flat, there will be some 'partial' flutes left. I'm wondering if this is a good idea or not.
    Or grind flat, find the first full flute and relieve the ones following that one so that the full flute can dive to the bottom...
    Last edited by T.Hoffman; 11-11-2011, 05:46 PM.

  • #2
    I don't know how close to the bottom you should be and if placement of a relief is possible, but
    you can single point it going the opposite direction (from left to right).

    Or try cutting the tap end after you cut the thread with it normally, first.
    Last edited by MichaelP; 11-11-2011, 05:46 PM.

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    • #3
      They were probably thread milled.

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      • #4
        ...or I guess I could file flat the first thread of the valve ring too, it's plastic.
        It captures 7 or 8 threads deep in the normal mount, one less on the inside end wouldn't hurt anything.

        Didn't think of that before. duh.

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        • #5
          It could have a false bottom. That way they could tap all the way to the bottom of the real hole and then press in a disc with the ports in it to match the holes in the block and the threads would seemingly go all the way to the bottom.
          It's only ink and paper

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          • #6
            Thread milling will get full thread right to the bottom.
            If not possible, another trick is to tap it with a blind-hole tap, then use a second blind-hole tap that has been ground down to remove the lead-in threads, and bottom it out.

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            • #7
              The last thread isn't of much value anyway, since it won't have much support on the end of the male piece. If there was a full thread there it would likely just fold outwards and cause problems. If your tap can cut to full depth till about half of the thread pitch from the bottom, I'd call that good.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                Use an indicator stop. Much depends on your ability to open the half nut or stop the spindle predictably but Ive cut multiple start fast lead threads to a square shoulder and held 0.010" between end of thread and noisy disaster cut after cut - but you absoluteli have to know where the end of the bar is. Only an indicator stop will tell you tha.

                A DRO or a digital gage whon't help you much. A needle approaching a zero is something your perception can react to. Numbers on a display have to be interpreted and only then can one react and open the half nut etc.
                Last edited by Forrest Addy; 11-12-2011, 03:45 AM.

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                • #9
                  sounds like whatever is the "bottom" of the hole might be screwed into the hole leaving the apearance of a blind tap bottom

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                  • #10
                    Actually the original is made in clear acrylic, so no plugs from the bottom or two piece assembly.

                    Molded?

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                    • #11
                      I got flushed in the face and all excited when I saw your thread title and then I saw it.......the question mark
                      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                      Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                      It is true, there is nothing free about freedom, don't be so quick to give it away.

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                      • #12
                        Would it be possible to turn the first couple threads off the valve that screws into the hole???? Then it would seat all the way.

                        JL.......................

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                        • #13
                          Originally posted by T.Hoffman
                          Actually the original is made in clear acrylic, so no plugs from the bottom or two piece assembly.

                          Molded?
                          Why didn't you say that in the first place? Yes, it was most likely molded.

                          Your statement that it "...has threads that seem to go right down to the flat bottom surface" is still a bit ambiguous. Do you mean that it has a thread that stops when the first part of it hits the flat bottom or does it have a thread that continues around from that point as if it were an actual tapped hole that was filled with a threaded plug that has a flat top. The first of these could be cut with a threading tool running in the reverse direction if it was just one thread pitch wide and it was started when in contact with the bottom. The second case could not be cut with any single tool, including a thread milling cutter because the final turn of the thread would be a constantly decreasing fraction of the width of a full thread as it made that last turn at the bottom. Even a four, five, or six flute tap that was cut square would not form this last turn completely accurately. All but one of the teeth at the end would be a fractional part of the thread width and each would cut a different fraction of that Vee. So you would wind up with a final turn that had four, five, or six different shapes, each for a fraction of a turn.

                          Frankly, either of these is very poor design practice. If you are designing a new device, I would strongly recommend that you consider eliminating the last thread or two from the mating part and have a design that is much easier to produce.
                          Paul A.

                          Make it fit.
                          You can't win and there is a penalty for trying!

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                          • #14
                            This same hole is used in several different materials for this valve, some of which I can see machining marks on flat surfaces.

                            Some acrylic both clear and black, and some ABS. That's why I was unsure about it being molded. Could be secondary machining too I suppose.

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                            • #15
                              Originally posted by JoeLee
                              Would it be possible to turn the first couple threads off the valve that screws into the hole???? Then it would seat all the way.

                              JL.......................
                              I was going to say the same thing.

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