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how to drill correctly with a center drill?

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  • how to drill correctly with a center drill?

    I'm not a real turner, I use a lathe intuitively, and have long been interested in how to use center drills correctly?
    They often broke, but then I stopped clamping the tailstock quill so that the drill in the quill had some play - and the drills stopped breaking!
    But the question arises whether the centering accuracy has deteriorated or not when the drill is loosely mounted, what do you think? Click image for larger version

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  • #2
    First, make sure your tailstock is correctly aligned with the headstock. As centre drills are small diameter, the higher speeds are better for them. Try to get the tool that is used for facing off, dead on centre so it leaves no pip.

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    • #3
      If you NEED to keep the quill unlocked it may be that your tail stock quill isn't centered on the head stock axis. Or your chuck is a bit off center. You can check your tailstock and chuck for centering with a couple of pieces of scrap metal and a magnetic base with dial indicator..

      To check the actual quill first set up the magnetic base to check for an droop or rise. Extend the quill by at least half way. Set the mag base and dial down on the cross slide and adjust the cross slide to pick up the top crest of the quill. Run the carriage back and forth and see if the reading changes. If it does then you have some droop or rise. Take note of the values and direction for later. And check this both for the quill floating and clamped. If the tail stock body is worn a little this will change things. At some point you might want to line bore and sleeve the tail stock body if the play is excessive.

      With that done we can how check for side to side and vertical centering. Retract most of the quill so it extends a couple of cm only. Chuck up a slug of scrap which has a diameter a little bigger than the quill. Machine it down until it's quite close in size to the quill. Matching the diameters isn't needed but does add extra points Take note of the diameters for both so you can split any difference for the next check. Now you can use the dial gauge set up to read the crests. Then alter the setup so the dial gauge tip is at center height and do the same from slug to quill. With those numbers and noting any radius difference you will quickly know if your quill is centered with the head stock spindle axis for both height and horizontal.

      You can do the same thing with the tail stock's drill chuck. But with the added task of setting it into the taper at 4 or more positions to check for being off center.

      A buddy's older South Bend turned out to be rather worn on the dovetails of the tail stock's base. It was even wear so it still sat solidly. But the tail stock quill was drooping and it was lower than the axis. We fixed it by drilling two good size but shallow holes in the base which we used to hold two brass shim plugs. The plugs being adjusted for height carefully until the quill was level and centered on the headstock axis as measured using this same procedure. We only needed TWO plugs for this as for whatever reason the wear was all at the "front" of the base. But we were ready for two at the rear if it turned out to be needed. The shallow holes is so the plugs don't wander around

      And of course setting the side to side runout is just a case of using the two screws that are used to set center or used to set up for taper turning.

      I found that when I did this for my own lathe and again with my buddy's lathe that the center drills didn't "pull" in any direction when entering the work.
      Chilliwack BC, Canada

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      • #4
        Drilling is drilling. Yes you line it up and carefully drill. Drill needs to be sharp. If you are to use the center for grinding or precision turning, you bore the 60° included angle with a small boring bar. Most center drills DONT make a perfect round hole.
        For general use, just drill it. We are talking precision... .0001 TIR . Most dont work this close and only drill the centers. I rarely use center drills, but use Curcuit Board drills instead for a pre-drill pilot hole. On the Engine lathe, I will turn off the spindle and continue applying lite pressure on the drill while the chuck is slowing down. This get rid of any chatter and makes a rounder hole. Wont take you long to become center drill expert !!

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        • #5
          Thank you, I have fulfilled the requirements for centering the tailstock, when grinding a long cylinder using a rotating center, the difference in diameters at different ends is not worse than 0.02mm.
          The ends of small drills with a thinner diameter less than a millimeter break.
          I understand that inaccuracies can occur in the taper and drill chuck, this is possible.
          How do I hope the drill is centered automatically when the quill clamp is relaxed, and this does not affect the centering accuracy of the part, is it?

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          • #6
            Small center drills can break from extremely small amounts off-center. Less than the centering accuracy of the tailstock. They are hard to use unless you are certain of perfect centering accuracy before you do any drilling.
            2730

            Keep eye on ball.
            Hashim Khan


            It's just a box of rain, I don't know who put it there.

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            • #7
              The quill in its bore in the tailstock should not be so loose a fit for the lock to affect it.

              There are four factors relating to tailstock quill accuracy, the position sideways, the height and the need for the quill to be in line with the headstock in both axes. If this is correct, the quill will be true whether it is withdrawn or extended.

              Some alloys of steel are not easy to drill or turn, so always use a little lubricant when centre drilling. I give my centre drills a quick check for their sharpness before deciding which end to use.
              Last edited by old mart; 05-07-2021, 02:43 PM.

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              • #8
                Originally posted by old mart View Post
                The quill in its bore in the tailstock should not be so loose a fit for the lock to affect it.

                There are four factors relating to tailstock quill accuracy, the position sideways, the height and the need for the quill to be in line with the headstock in both axes. If this is correct, the quill will be true whether it is withdrawn or extended.
                But, generally, it ain't..... because nothing is perfect, there is always some clearance, and wear makes it larger.

                The art of making perfect things with imperfect tools........
                2730

                Keep eye on ball.
                Hashim Khan


                It's just a box of rain, I don't know who put it there.

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                • #9
                  Thank you all for your advice, I will analyze everything carefully.
                  But does the drill itself find the center of rotation in the rotating part or does it make a mistake?

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                  • #10
                    I don’t really know if it makes a difference or not but a lot of times when I get to the depth I want, I will give it some feed pressure and then lock the quill down and let it sit there for a few seconds or so.

                    Theory is if something is a little off locking down the quill will essentially cause the center drill to be a “single point tool”.

                    I don’t know if it actually works but it makes me feel better.

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                    • #11
                      Originally posted by oxford View Post
                      I don’t really know if it makes a difference or not but a lot of times when I get to the depth I want, I will give it some feed pressure and then lock the quill down and let it sit there for a few seconds or so.
                      Thank you, I think this is valuable advice and I will try to do so.

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                      • #12
                        The main thing that causes off center drilling in my experience is leaving a very small nub after facing. If I'm real careful about avoiding that then a short stiff center drill with the minimum amount of extension (drill or quill) gets me close enough that I'm not worried. If I'm going to be boring after drilling I don't bother with a center drill and use a small stub drill about the size of the web/ chisel point on the final drill.

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                        • #13
                          Originally posted by mattthemuppet View Post
                          The main thing that causes off center drilling in my experience is leaving a very small nub after facing. If I'm real careful about avoiding that then a short stiff center drill with the minimum amount of extension (drill or quill) gets me close enough that I'm not worried. If I'm going to be boring after drilling I don't bother with a center drill and use a small stub drill about the size of the web/ chisel point on the final drill.
                          I also always work with minimal tool lengthening to maintain the rigidity of the system, but for centering I have to relax the tailstock quill so as not to break the centering drill.
                          It's already night with us, I'll go to bed, tomorrow I'll check one idea ..

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                          • #14
                            I'm not sure why you're breaking center drills. I've only ever broken one and that was drilling into hard nasty scrapbin steel.

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                            • #15
                              Originally posted by oxford View Post
                              I don’t really know if it makes a difference or not but a lot of times when I get to the depth I want, I will give it some feed pressure and then lock the quill down and let it sit there for a few seconds or so.

                              Theory is if something is a little off locking down the quill will essentially cause the center drill to be a “single point tool”.

                              I don’t know if it actually works but it makes me feel better.
                              That's a clever trick ! ! ! Turns the center drill into a bit of a mini boring bar. Not perfectly but it should reduce any runout by at least half.

                              I suppose the other thought would be to lightly pinch the quill lock so it can still move with only a slight drag which would remove any drooping.
                              Chilliwack BC, Canada

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