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  • yet another deep hole drilling question

    Looking to drill a hole thru a hardened steel rod. Don't know what the material is, but it feels like around a grade 8 bolt hardness on the inside. The outside is case hardened, but I won't drill thru that, so it doesn't matter. It is a piston shaft for a shock absorber, if that helps any. It has a main section that is 14mm diam for about 9 inches, and then a thinner section 8mm for about 2 inches. I need a hole all the way thru, but probably no wider than .1" or .125" in the thin section (for strength), and I don't really care what diameter in the thicker section, as long as the two holes meet up. I want to run wires thru the hole, so they don't have to be perfectly concentric, nor have a good surface finish.

    I want to try doing this on a lathe, and don't have a high pressure coolant pump, so I probably shouldn't use a gun drill.
    I have done some reading, and some suggestions I ran into include starting the hole with a ball end end mill, and then doing lots of little pecks with successively longer drills.

    Some questions I had are:
    Is a long HSS drill good enough, or do I need something harder like cobalt?
    I believe the drill should have a short flute length?
    What diameter drill should I use for the thicker portion of the shaft?
    What should I do if the hole starts getting crooked halfway thru?
    A good way to make sure the ball end mill starts exactly straight and on center? I was going to put it in a drill chuck that fits into the tail stock, but that doesn't seem like a good idea.

    One problem is that the material seems to really work harden. I have a sample of the rod where it was snapped off in the middle (probably due to some horrible accident), and the texture of the material looked like concrete and a file would bounce off of it. Once I ground some away with a grinder, then the file would start to dig into the material.

    I took the sample to a gun driller, and he said that he could "try" doing it. He wasn't sure, due to the suspected hardness of the material. I think he offered to do $150 for all 5 or so shafts I had. Am I better off just handing the job to him?

    Thanks

  • #2
    I have cut through shafts from shocks with a regular bandsaw. They are ground and chromed but as far as I can tell they are not hardened.

    So you should be able to use any standard drill. If you are drilling deep I would suggest a parabolic flute drill from someone like Guhring. A cobalt version couldnt hurt.

    Use a spotting drill to start.

    Comment


    • #3
      No matter if it's hardened or not, start with a center drill, and then the stubbiest drill first. Increasing the length of the drill as the hole gets deeper. make sure the drills are acurately ground. And, yes, peck drill, being carefull not to load the drill. Needless to say use a lot of coolant and as you go deeper retract the drill all the way out.

      Comment


      • #4
        Shock rods come in two flavors chrome rod which has a thin case under the chrome and a realitively soft center.Then there is Nitro bar which is nitrided 1045,it usually starts off at 38-40rc and then it's nitrided.

        Either should drill,just the nitro bar will be tougher.So long as the drill is started on center and the correct speed is used a normal good quality HSS bit should drill it.

        I would use coolant and don't let the drill bit dwell in the cut.If it's allowed to dwell work hardening will happen.
        I just need one more tool,just one!

        Comment


        • #5
          Originally posted by Rustybolt
          No matter if it's hardened or not, start with a center drill, and then the stubbiest drill first. Increasing the length of the drill as the hole gets deeper. make sure the drills are acurately ground. And, yes, peck drill, being carefull not to load the drill. Needless to say use a lot of coolant and as you go deeper retract the drill all the way out.
          OK, but do NOT use a center drill.

          Use a spotting drill instead. It makes a dimple that the following drill can use, not a hole that will throw it off-line.

          IF you make only the DIMPLE with a center drill, a properly ground lathe cutter, a suitable piece of flint, or whatever, that may be OK. Just don't get a cylindrical section going, and definitely do not drill deep enough get into the center cone.
          1601

          Keep eye on ball.
          Hashim Khan

          Comment


          • #6
            Start the hole with an ordinary centre drill or spotting drill if you have one. Then use the largest bit you are going to use and drill until the flutes are nearly buried, clearing the chips often. Then go down a few sizes and drill no more than a half inch or so. Go back up to the larger drill bit and drill out that half inch. Repeat. This gives room for chips when using the smaller bit and reduces the chip load on the larger bit. That is the biggest problem and what also causes the bit to wander. It's tedious but clear the chips very frequently and use plenty of oil. Do not bury the flutes on the smaller bit, that will cause the bit to wander as the chips pack unevenly. The large bit will follow the hole of the smaller bit.

            Drill slightly over half way, reverse and repeat.

            Stainless steel, 12" x 1" hole with a reduction to 3/4 near the far end. This took all day.

            Free software for calculating bolt circles and similar: Click Here

            Comment


            • #7
              Have you considered EDM?
              Paul A.
              SE Texas

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

              Comment


              • #8
                Originally posted by Paul Alciatore
                Have you considered EDM?
                I just called around. It looks like it would work "for sure", but the cost is about $150 EACH.

                Comment


                • #9
                  Originally posted by Evan
                  Start the hole with an ordinary centre drill or spotting drill if you have one. Then use the largest bit you are going to use and drill until the flutes are nearly buried, clearing the chips often. Then go down a few sizes and drill no more than a half inch or so. Go back up to the larger drill bit and drill out that half inch. Repeat. This gives room for chips when using the smaller bit and reduces the chip load on the larger bit. That is the biggest problem and what also causes the bit to wander. It's tedious but clear the chips very frequently and use plenty of oil. Do not bury the flutes on the smaller bit, that will cause the bit to wander as the chips pack unevenly. The large bit will follow the hole of the smaller bit.

                  Drill slightly over half way, reverse and repeat.
                  That's an interesting idea. But doesn't it also mean that the smaller drill is un-supported along most of it's length while drilling? And also that the smaller drill will not be started on a "properly spotted" (or whatever you call it) point? It seems like an opportunity for the smaller drill to wander. But maybe the larger drill uses it's support to straighten it out?

                  Also, when you pull the small drill out, won't it leave bits of chips along the bore?

                  Comment


                  • #10
                    The smaller drill and the larger drill have the same cutting edge angle I presume. The smaller drill doesn't need to be much smaller, your stated .125 and .100 will do nicely. As long as the drill is properly sharpened so that the edges both cut the same chip load AND it isn't pushed too hard it won't wander unless there are varying areas of hardness in the material. That is also why I suggest only drilling 1/2" at a time. Blow the chips out with air each time you clear the hole. Wear your glasses.
                    Free software for calculating bolt circles and similar: Click Here

                    Comment


                    • #11
                      A ball endmill will give you the straightest start for a drill of the same size. because you can enter the material sufficiently ( 1" ?) to give the drill Flutes ( not tip !) total support

                      In order to drill straight, you need to turn the workpiece, not the drill
                      The difference in accuracy is 10 to 1

                      The proper drill for the second operation requires that its "web" be no bigger than the prior hole diameter . This give freer chip flow, and reduces web forces

                      Rich

                      Comment


                      • #12
                        Did I catch that right? A .125" hole for 9"? I think I would think long and hard about using different material, even if it meant I had to buy it.

                        I guess, like you say, the accuracy isn't too important but it would still take quite a bit of time. I've done some deep holes in the >.5" range and, like Evan says, it soaks up a lot of time. At least it does for me...

                        Comment


                        • #13
                          A gun driller can do it in about 9 minutes
                          He will probably be looking at 2 hours due to chip clearance and repetative cycles

                          Rich

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                          • #14
                            It is not a 9" hole, it is two 4-1/2" holes if drilled from either end. With the speed sufficiently high, drilling with the lathe should not present a problem. A well centered, straight starting hole is the secret. I usually only feed in one or two diameters at a time.

                            It should not be necessary to drill two diameters with a hole this small, it might in fact create problems to do so. If going to a larger size, step drilling can help, but the usual cautions for step drilling apply.

                            If anyone wants to drill some small diameter deep holes, I have a pack of #73 (0.0240") aircraft drills, they are 4" long.
                            Jim H.

                            Comment


                            • #15
                              Originally posted by Rich Carlstedt
                              A ball endmill will give you the straightest start for a drill of the same size. because you can enter the material sufficiently ( 1" ?) to give the drill Flutes ( not tip !) total support

                              Rich
                              I don't know of any long 3/32 ball end mills, so then I'd have to use 1/8, which is on the larger side of my preferred hole size. Maybe it is still ok. But it seems that unless the end mill is mounted EXACTLY straight and on center, it will end up making a bigger hole.

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