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drills and round or 3 lobe holes (deflecting the hijack)

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  • drills and round or 3 lobe holes (deflecting the hijack)

    Trying not to hijack the other thread so egregiously.......

    Originally posted by Evan
    Explain why a 2 flute milling cutter does NOT make a 3 lobed hole. While explaining that also explain why a 2 flute drill should.
    Why should I explain ANYTHING? Do you make the rules?

    I'll do it if I want to, and right now I have no objection to doing that, so why not?

    A drill does, because the flexibility of the shank and long fluted section allows it....

    Drills are rarely perfectly ground. Even if perfectly ground, the material drilled is rarely perfectly uniform.

    if, or I should probably say "when" one side of the drill digs a little deeper, or hits a more resistant spot in the material, (or if that side is just ground a little "out") then it wants to slow down.

    The flexibility of the long fluted section allows the whole end of the drill to "swing around" the part that has 'caught" on the hard spot, which makes the "wankel-like" curve.

    if this occurs in the same spot a couple times, it may actually establish the pattern, and the drill will tend to catch on the non-round spot at one end of the "wankel curve", which makes ANOTHER one "advanced" from the first. On it goes, ending up with a three-lobed hole, if the 'clearance lands" of the drill don't end up cutting away the hole back to round. In thin material that may not happen, and the hole will stay lobed.

    A three-flute drill will do the exact same thing, in this case making a SQUARE hole, which is one way of avoiding the Watts drill internal mechanism.....use a long flexible shank to exaggerate the effect, and shape the bit to improve the square form.

    Any two fluted rotary cutter has the tendency to cut 3 sides, ...... even an end mill.

    The rigidity of the cutter, the holder, the spindle, and drive head relative to the work determines the degree to which the effect is 'expressed".

    A mill is usually good on the rigidity factor, and the cutter is typically short and large diameter. So the lobing is unlikely to be expressed on a scale that is important to you.
    Last edited by J Tiers; 06-19-2011, 01:37 AM.
    1601

    Keep eye on ball.
    Hashim Khan

  • #2
    Sorry Jerry, it wasn't a hijack and the answer is back on the other thread where it belongs.
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      of course it is a hijack aka an OT diversion.

      The man asked about end mill holders, and now you are discussing the shape of the hole a drill makes, or an end mill makes.

      So long as it does not reliably make a round and exactly to-size hole, it is not relevant. And it does NOT make a reliably to-size and round hole.

      Your apology to me is noted, and accepted.


      Originally posted by Evan
      No, it is different. The reason that it doesn't produce a 3 lobed hole is because it doesn't have a point. If you follow the path of the centroid of a rotating Reuleaux Polygon it decribes a curve that is generated by a short line which orbits around the geometric centre. This is precisely equvalent to the tip of a standard drill bit that is ground so that the cutting edges are unequal. If the tip is ground precisely and better yet if the tip is split point as well then that tendency goes away completely.

      http://curvebank.calstatela.edu/reu/reuleaux.htm

      This is not true, it is simply WRONG

      the square hole drills have no point to them. the end is square.

      This is a Watts drill. Find the point on it..... It makes a square hole. The same "N+1" effect occurs with 2 flutes. Guiding makes it more reliably to-size and on-location, but the basic effect is always present.

      Last edited by J Tiers; 06-19-2011, 11:48 AM.
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        I have always been under the impression that you get tri-lobe holes when the point of the drill is off center, or there is some other flaw in the point geometry.
        James Kilroy

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        • #5
          Originally posted by jkilroy
          I have always been under the impression that you get tri-lobe holes when the point of the drill is off center, or there is some other flaw in the point geometry.
          Off-center drill point makes an oversized hole, for sure, and it may or may not make a tri-lobe. The tendency to make a tri-lobe with ANY slight difference in the forces on one flute exists regardless.

          That may be a drill fault, or it may be a material inconsistency.

          The reason drills work pretty well is that they have a point that tends to stabilize them. This is exactly opposite to what Evan said, which appears to be that a 2 flute end mill has no point and so won't do the tri-lobe..... In actual fact, it will do it WORSE because of having no point..
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            Polygon-shaped holes are caused by using any drill or mill bit that has to do center-cutting.

            Drilling a pilot hole that is 50% of the finished hole greatly reduces the problem, with both drills and end mills. End mills basically don't wobble at all if you make a pilot hole first, they don't even need to be perfectly centered on the drilled pilot hole--but then end mills aren't convenient to buy in every size that drills can be found in, either.



            It'd be nice if someplace made "finishing drills". Like, a regular 115-pc drill set, but with flutes that are only 20% or so as deep as usual... they would cut a straighter smoother hole than a regular drill, but they would also lift the material up and out, unlike a straight-flute chucking reamer.

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            • #7
              Originally posted by DougC_582
              Polygon-shaped holes are caused by using any drill or mill bit that has to do center-cutting.

              What is your reasoning on this?

              I ask because I am quite sure that it is not correct, one bit of evidence being the Watts drill, for which you HAVE TO drill a pilot hole or it cannot work. The Watts drill has no center cutting ability.

              I am pretty certain that if you DO have to do center-cutting, particularly with a protruding point, that that will STABILIZE the bit somewhat, and REDUCE the tendency to make an N+1 polygon.

              If you think about it, to make a polygon, the drill must rotate in such a way as to actually move the point sideways. Obviously if it is restrained by needing to move the point sideways through metal, that is less likely to happen.

              if you make the drill point angle flatter, I am claiming that makes the drill less stable and more likely to generate a polygon.
              1601

              Keep eye on ball.
              Hashim Khan

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