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External v. Internal "Turning"

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  • External v. Internal "Turning"

    I think, from past threads, I know the answer to this but...

    If the external turning is producing a cylinder that is, relatively speaking, round and flat on the faced ends, can the bore be something other than round?
    As example, if the external was out by .001, could the bore be out by 10x that, .01?

    Recent project, that worked pretty well, but in checking the bore, it was out of round and the wall thickness measurements (as best I could get, I don't have a mic specifically for taking that measure) seem to confirm this.

    So, I think I can take the placement of the center out of the equation since if it was off the finished bore was way larger than the original drilling.
    Where should I go looking next to make corrections?
    The stick out of the boring bar could have been a bit much but not by a lot...the workpiece itself was out about 3x the diameter, the tool at the start was a brand new insert and at the finish it was still cutting as well as at the beginning...everything was tightened down at the start of the project (applicable gibs e.g.) but not 100% sure of that by the time I was finishing the best I can figure there was one large measurement and 90* opposite one small measurement so oval/elliptical...

  • #2
    If you bored it, it should be pretty darn round. You mentioned wall thickness. Are you actually measuring a lack of concentricity? Lots of things can lead to that, chief among them is that universal chucks never clamp in the same place twice, so if you did one operation, took it out and then re-chucked it, that would cause the bore you just turned to be out compared to the diameter you turned before.
    Stuart de Haro


    • #3
      Also, if it's chucked to tight it will make the OD and ID out of round.
      It's only ink and paper


      • #4
        Another aspect to consider is lead angle if your wall thickness is small. The lead angle on a boring tool will affect the direction of cutting forces on the work. This can cause deflection. For example, take a boring bar positioned parallel to the spindle axis and being fed into the bore. If the lead angle is 0 degrees, the cutting force is also parallel with the spindle axis. If the lead angle is, say, 30 degrees, the cutting force is now directed at an angle to the workpiece. This can deflect the boring bar or, in the case of a thin walled workpiece, the work. The comparative result would be out of round and lack concentricity.
        Last edited by Arthur.Marks; 05-12-2012, 01:24 PM.


        • #5
          Are you actually measuring a lack of concentricity?
          I won't say its not a possibility but I think I had eliminated the potential since the wall thickness variation matched the "long" axis and the "short" axis (the wall thickness at the ends of the long being that bit thinner, the short being that same amount thicker)

          hmmm...while I think about that, I'll ask another related question, if one is making something like this from a solid (as opposed to beginning work from a very thick wall cored round), how would what you say effect things if the finished work was partially in the jaws and partially outside of the jaws?
          If you have a starting work piece of 1.5" diameter solid about 5" long, 3" or so of that could be within the jaws, so 2" sticking out and you then bore all the way through with your first drilling, bore to finished size, say 5/8ths 2.5" deep and counter bore to a 1" diameter about 2" deep. The would leave the finished bore partially out of the jaws but partially within direct jaw pressure as well the counter bore would stop just near the top of the chuck jaws in terms of dimensions.

          arthur.marks: hmmm, get what you are saying, is the implication you want to use boring bars that have as small a lead angle as possible? And, if I at least partially understand, could you then make any bar cut closer to parallel to spindle axis by setting the tool post away from 90* to the spindle axis? To sort of "counter balance" the lead angle concern...

          the set up was roughly the dimensions in my reply to carld (the parts are on a machine and the last thing I want to do is pull them off to measure for certainty , 4 jaw, used a dial indicator to center, faced the outer most end, turned to rough external dimension (this was not much as the round was 1.5" nominal), drilled nearly through entire piece, bored to finish about 1/4" deeper than finished length, counter bored to finished size, removed from lathe, sawed off and faced the sawed end (way less nerve rattling than poor attempts at parting); replaced remaining piece back in lathe, again using dial indicator to center and tried to make use of that little bit of finished bore size from first piece etc.
          The idea was to end up with two parts about 2.25" long, so starting at 5" would allow facing to start, the saw cut and facing each of those ends and then facing the last end and doing as much as possible in the initial set up just seemed the way to go.

          What I know I did not do is really compare the two pieces, they were to be "identical" but I know for the final fitting I made them to fit the mating part. It is possible that I only noticed the OP issue on the second part and though using a dial indicator, the combination of what you three have posted (removing part, jaw pressure and lead angle) and my not being as exact with the indicating as I should be, could account for the discrepancy.

          Thanks for the input.
          Last edited by RussZHC; 05-12-2012, 10:31 PM.


          • #6
            If you started with cold rolled and only took a very small cut to clean up, there might be enough residual stress left to cause distortion as you bored it out. If so, though, that would cause the outside to go out of round as well.