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Establishing Good Centre Holes for Long Part Turned Between Centres

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  • Establishing Good Centre Holes for Long Part Turned Between Centres

    I've got a head scratcher that I haven't had to deal with before and I'm at an impasse for how to solve it at the moment. 10x18 benchtop lathe.

    I've got a steel rod, 18" long and 1-1/4" diameter, which can't pass into my spindle bore. I can turn it between centres to do the features I need, which is just to turn down the diameter on the ends of the rod, but I need to keep the new features concentric with the rest of the rod surface. So how can I put the centre drill holes into the ends of the rod and maintain concentricity with the existing surface?

    My only idea is to use a steady rest, which will be a big hassle but should work. That will let me drive the work and get the "wobble" to a minimum at the end where I'm drilling. Then flip, do the other end, put it between centres, do the cut on the far end, flip, do the cut on the 2nd end, done.

    Need a bigger lathe. This keeps happening.

  • #2
    Yes, that's a job for your steady rest.
    Facing, center drilling long work.
    A quick and easy way to set your steady rest, is to first chuck the bar, then slide your steady up close to your chuck, and use a piece of paper to 'feel' your steady fingers dragging on the bar. (good enough for center drilling).
    Now slide your steady back to where you need it, and snug it down.

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    • #3
      To machine a shaft that is perfectly concentric start with a piece of material that's larger than your finish size
      and an inch or so longer than you need. Set it up and put a centre in one end and machine a slight shoulder
      on the end that goes in the chuck so you can give it a good push with the centre in the tailstock. Turn the
      whole shaft to the required diameter, cut off the excess on the chuck end and put in centre on that end. Done.
      Keith
      __________________________
      Just one project too many--that's what finally got him...

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      • #4
        Steady rest is the way to go. And its not hassle at all, just bolt it to your bed and eyeball the fingers to center. Set it to very end of the bar and it doesn't need to be anywhere perfectly on center with long parts.
        And I prefer to finish the center hole with tiny boring bar if I need best results. Center drilling can be off from rotation center by slight amounts depending on this and that but small boring bar takes care of that.

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        • #5
          Originally posted by KiddZimaHater View Post
          Yes, that's a job for your steady rest.
          Facing, center drilling long work.
          A quick and easy way to set your steady rest, is to first chuck the bar, then slide your steady up close to your chuck, and use a piece of paper to 'feel' your steady fingers dragging on the bar. (good enough for center drilling).
          Now slide your steady back to where you need it, and snug it down.
          True, however the OP did not specify the the material other than "steel rod" hot rolled, CF round etc.
          Either of the two I mentioned are not perfectly round so your could be chasing your tail trying to set it up to run perfectly concentric. Ground stock on the other hand would be a better choice if you looking for that kind of precision.

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

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          • #6
            Matt's suggestion works, if you have enough bed at the end for the carriage et al, or bring it by and I'll put them in holding the work in a collet
            .

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            • #7
              Another approach:

              Take a piece of 2" dia aluminum (or whatever you have) and counterbore it 1 1/4" dia X about an inch deep. In the same lathe set up drill a hole equal to the O.D. of the center drill you want to use thru the back of the counterbored surface. Then, use this as a drill jig on each end of the bar. If you want to go a step further put some set screws on the side of the jig which intersect the 1 1/4" dia to hold the jig in place as you center drill it with either a drill press or even a hand drill.

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              • #8
                One way round this---

                Originally posted by Mcgyver View Post
                Matt's suggestion works, if you have enough bed at the end for the carriage et al, or bring it by and I'll put them in holding the work in a collet
                Providing you need accuracy only to thous and not tenths is to take a piece of material larger than your shaft, maybe an inch or so thick. Bore it out to about 3/4" deep to be a smooth sliding fit on the piece you wanr to work on and, at the same setting drill ream or bore a hole through to be a smooth sliding fit for your chosen center drill
                IF really pushed for space careful drilling with a hand drill or in a drill press will give tolerable centre holes.
                However, if you have the length between tailstock and chuck you can get a reasonable job Without even using a steady. With a steady, you will soon see if adjustment of fingers is needed.
                This trickery can even be done on hot rolled material to give reasonably central centres and can avoid having to turn temporary surfaces to run in steadies.
                Be very wary of work moving out of chucks if the steady is not properly aligned.
                Hope these hints help, David Powell.

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                • #9
                  I should have mentioned the rod is cold finished, so its smooth but roundness is unknown.

                  MattiJ's suggestion should be okay, I think there's enough room there if I hang the tailstock off the end of the bed a little bit, and maybe scrounge up my smallest chuck.

                  There's not enough room for my carriage beyond the steady so I can't bore the centre hole after drilling to take care of angular error. It'll have to do without.

                  This is a little job for my neighbour so I asked him if he actually needs the rest of the diameter turned down as well, which would make my life easier.

                  DATo's idea is pretty darn excellent and definitely a back pocket tip for the future, as well as plan B for this.

                  Thanks for the tips guys, I'll try to take a swing at it tonight and see what happens.

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                  • #10
                    I would go with DATo's drilling jig, but would probably use steel.

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                    • #11
                      Down at the junk yard there is a machine with a Milwaukee drill motor holding a center drill behind a 3 jaw open center chuck. It is for drilling the centers of long rods before they go in a lathe.

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                      • #12
                        I have a 'vee' block holder for my QCTP. When I need to center-drill a long shaft I mount the shaft in the vee block holder, mount a center drill in the chuck, and drill it using the carriage for feed.

                        The vee stays on center vertically, so I only need to adjust the shaft for in-out centering. Only takes 30 seconds. There isn't much in the way of cutting forces drilling a center, and I've done it with up to 2" 4140 shafts several feet long just by mounting the shaft in the vee at its center - the axial stickout isn't really a huge problem.

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                        • #13
                          Originally posted by Sun God View Post
                          I have a 'vee' block holder for my QCTP. When I need to center-drill a long shaft I mount the shaft in the vee block holder, mount a center drill in the chuck, and drill it using the carriage for feed.

                          The vee stays on center vertically, so I only need to adjust the shaft for in-out centering. Only takes 30 seconds. There isn't much in the way of cutting forces drilling a center, and I've done it with up to 2" 4140 shafts several feet long just by mounting the shaft in the vee at its center - the axial stickout isn't really a huge problem.
                          V ”block” in qctp sounds good and handy. faster than mounting steady rest.
                          I have used also the ball bearing ”nudge tool” for things that are not super critical. Works suprisingly well even if it lacks any up-down guidance.

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                          • #14
                            A bigger lathe

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                            • #15
                              I ended up using my steady to do the centre drilling and it worked well. Got the part done with a thou or 2 runout and the neighbor is happy.

                              Sent from my SM-G930W8 using Tapatalk

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