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Boring hole tapered

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  • Boring hole tapered

    Was boring 4 holes in 3/4 aluminum today on a Bridgeport with their boring head and the holes were tapered. Quite a bit smaller at the bottom. (no exact dimension but on the order of ten or twenty thou) The cutter was one of a set that has been around for many years
    of those brazed carbide tips on a slightly tapered shank, see picture "I hope will show up". :-)
    So what can cause this?? I thought "spring" in the bar but then thinks I, the radial force should be no different at the bottom than at the
    top. So why??
    Here goes an attempt to post a link for a couple pix.

    Well the links look OK :-)
    SO any Ideas?? I ran a few more passes without changing the setting and they managed to open up but would still like to know WHY.

  • #2
    Lack of clearance on the cutting tip in the z-axis?? Just a guess. IDK.


    • #3
      I'd be interested in the answer. That's a long shaft for being so slender.

      Wonder if it has something to do with BUE combined with that long shaft?

      At the end of the project, there is a profound difference between spare parts and left over parts.

      Location: SF East Bay.


      • #4
        First if there is no finish probalem I would wonder if the quill might be loose. 2) A long slender boring bar is always a recipe for problems. Plus brazen carbide tools are not the best thing for aluminum. Sharpening carbide really needs diamond wheels. Green wheels do not do that good of a job. The wheel will chip the carbide leaving edge ragged. Shop made bars that take positive take inserts can do wonders. One type is that sold by Mesa Tools. Or make your own
        Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.


        • #5
          Originally posted by MyrtleLake View Post
          Lack of clearance on the cutting tip in the z-axis?? Just a guess. IDK.

          +1 on lack of clearance.


          • #6
            Clarance is a good possibility, especially using the bar on a minimum size hole. But that does not seem to be an issue here, the hole is of good size.

            Also take a look at the corner of the boring bar cutting edge. Odds are it has some radius.

            What happens in resistant material, is that you take a cut... all looks good. But the radius on the "corner" adds a radial force to the cutter, tending to push it inwards.

            So the second pass the bar is pushed in a tiny bit more, then it finds the place where the forces balance out. So the entrance diameter is a hair bigger than the final. And the transition is just inside the hole, not at the actual mouth, because the rounded corner does not develop full radial force until it is cutting fully into the hole.

            So, the next pass, the transition is cut a bit, but the cutter is again pushed in a bit, and the transition moves down into the hole a bit more, because the smallest diameter is in a little bit, and the full radial force is not developed until the radius is fully into the smaller area.

            Each pass moves the transition inward by maybe 1/3 or 1/2 the radius of the corner. Result is that the mouth of the hole and the bottom are of different diameters.

            A boring bar with a sharp corner tends to cut only on the end, especially if it has good rake, like Criterion or Bokum. Even their radiused bars have a good deal of rake. The flat "plow" type carbide has no rake, and is far more easily forced a tiny bit off. After a few passes you can be into real taper.

            Probably spring cuts will help. Same with HSS cutters that are dead sharpp, and have some rake.
            CNC machines only go through the motions


            • #7
              Originally posted by Illinoyance View Post
              +1 on lack of clearance.
              Yes that, as well as lack of clearance on the back side too; maybe it's just the picture but it looks like the cutter is rotated in an attempt to give it a positive rake. Don't do that, those boring bars need to be set for a neutral rake.

              That first image shows lots of dust-like aluminum particles instead of chips; good indication that it's rubbing instead of cutting. The surface finish of the hole also looks like the result of rubbing. Set the cutting tip to a neutral rake and sharpen it, this'll go away. Those boring bars can leave a very nice finish when set properly. BTW a little WD40 in the cut is helpful too.

              I sharpen mine with a diamond lap, and don't radius or chamfer the tip unless it's chipped. If it is chipped, a sharp chamfer still cuts fine without any hole taper.

              If all else fails, make a spring pass or two until the hole diameter is even. I'm betting the main problem is not having the tool perpendicular to the cut though, and maybe a bit dull.
              Last edited by Yondering; 10-19-2017, 12:49 PM.


              • #8
                The setup is too long for it's rigidity, when it contacts the surface it cuts the inscribed radius, as it penetrates the material the inward forces deflect the tool so the hole closes as it progresses.
                If you benefit from the Dunning-Kruger Effect you may not even know it ;-)


                • #9
                  Do you lock the slide after you turn the dial?, I also agree more clearance on the cutting tip.


                  • #10
                    As others have said.. It's deflecting more the deeper you cut.. I remember having the same problem at some point in the past.


                    • #11
                      Originally posted by Magicniner View Post
                      The setup is too long for it's rigidity, when it contacts the surface it cuts the inscribed radius, as it penetrates the material the inward forces deflect the tool so the hole closes as it progresses.
                      If it's correctly sharpened and with the proper clearance it won't do that though. I use these same type of bars for the same operation regularly and don't get tapered holes.


                      • #12
                        The first picture sure does look like it's rotated to get a positive top rake. And that does not bode well for the edge clearance on an unmodified cutter of that sort. So I vote for lack of side clearance too. And certainly from having a similar set I can say for sure that when not modified if I were to rotate the cutter like that to give a positive top rake then there would most surely be side clearance issue and rubbing. And rubbing would be an obvious reason for a taper of that much.
                        Chilliwack BC, Canada


                        • #13
                          Thanks folks. I did rotate it a bit for top rake and didn't check the grind to see if it had enough (or any) clearance ground on the edge. I'll do that tomorrow
                          if I can remember . :-)


                          • #14
                            Make sure it's sharp too. Those boring bars generally are not sharp enough when new; sometimes they don't have enough (or any!) clearance angle either.


                            • #15
                              Do you use that boring head a lot? Is it an inexpensive one? One cause could be the boring head's slide might be working its way towards the middle.