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Eccentric turning - not intentional

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  • Eccentric turning - not intentional

    Hello,
    I have a PM 1030V metal lathe that I mostly use for making bamboo fly rod ferrules from nickel silver solid rod and a few other similar metals. The ferrules are basically just a cylinder with a dam of material left in the middle to act as a moisture barrier.
    I drill from both ends of the rod and start the holes with a center drill, drill/ream to the appropriate depth and size then turn the outside diameter to the desired size. The piece is held with an ER-32 collet and a live center while turning.

    The issue is that after I drill/ream both sides and turn the outside diameter down, the hole is no longer centered in the piece near the collet. It is off center like I was trying eccentric turning. I attached an image - a pic is worth a thousand words.

    The tail stock and head stock are aligned and the lathe holds less than .001 over about 3 inches when turning. I checked the spindle run out using a dial indicator and it is negligible - around .0004.

    I am kind of at a loss at this point on what else to trouble shoot. I suspect this is an easy fix but I have exhausted all of my limited machining knowledge.

    Anyone have any ideas?

    Patrick

  • #2
    Is your raw material round?

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    • #3
      Yes - 3/8 and 5/16 round rod.

      Patrick

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      • #4
        No. I mean is it actually round. Lot's of raw material has lobes on it. Especially hot rolled steel. If you don't chuck up the part in exactly the same spot you will have problems.

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        • #5
          Probably not exactly round. FWIW I used the exact same procedure with an Atlas 618 lathe and didn't have this issue.

          Patrick

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          • #6
            Change the order of machining. Drill and ream one end then turn the OD. Part off, turn the piece around and hold it in a collet to drill and ream the other end. Now both ends should be concentric to the OD.

            Or, if you have all the parts cut to length to start with, drill and ream one side, rough-turn the OD, flip the part, do the same in the collet for the other end then finish the OD as you have been.
            Peter - novice home machinist, modern motorcycle enthusiast.

            Denford Viceroy 280 Synchro (11 x 24)
            Herbert 0V adapted to R8 by 'Sir John'.
            Monarch 10EE 1942

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            • #7
              I think you should check the runout of the ER collet chuck.

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              • #8
                how eccentric (in this craft everything is always off...there is no 'dead on' . And what material?

                Please try and get the picture up, it will help. but basically what Peter and Illinoyance said.

                my bet is in crappy collets. Put a similar size piece you believe to a straight cylinder (ie a dowel pin) in the collet and measure what TIR there is?
                in Toronto Ontario - where are you?

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                • #9
                  Originally posted by Illinoyance View Post
                  I think you should check the runout of the ER collet chuck.
                  I'll double check this tonight. I checked but it is worth redoing. Other than that I think my best option is to follow Peters instructions from above.

                  Patrick

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                  • #10
                    If the collets are not running true even Peter's method will leave some eccentricity.

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                    • #11
                      Image is not working but to me it looks like more runout than what collet chuck would cause.
                      Here's the image if I had better luck:
                      Location: Helsinki, Finland, Europe

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                      • #12
                        How far out of the chuck does the material protrude?
                        Len

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                        • #13
                          Just to straighten out the issue.

                          Put a piece of your stock in the ER collet and turn a short section round and true. Then bring up the spot drill and poke in your drill pilot. At this point, you should be able to observe if the centering effort has resulted in a location concentric with the turned OD.

                          If this looks good, Drill the hole and observe the results. An improperly sharpened twist drill will cut a wild hole.



                          Try boring the holes instead of drilling. Sometimes all it takes is boring the first several millimeters depth.

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                          • #14
                            Be sure to use a very short rigid spotting drill or centerdrill. The point shape of the spotting drill is important. A conventional grind on the drill leaves a small flat at the end of the web. That can cause the drill to walk when it contacts the work. If that happens you can see the drill wobbling when you start the hole.
                            You might also want to check the tailstock alignment. If the spotting drill is not on center when it contacts the work there will be a greater tendency for drill to walk.

                            You didn't say how far the part projects from the chuck when you start drilling. If it projects a long way that will exacerbate the problems related to runout. Also, with ER collets the workpiece needs to held for the full length of the collet. If the work is chucked only part way in the collet the back end of the collet will collapse more than the front, resulting in a tapered bore. That will allow the work to wobble in the chuck resulting in an eccentric hole.

                            I suggest the following steps to find the cause of the problem:
                            Verify that the collet is running true.
                            Make sure the part is held full length of the collet.
                            Check tailstock alignment.
                            Use a rigid spotting drill

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                            • #15
                              ps

                              Regardless of what the tailstock alignment might be, Striking the center and boring the hole will always run concentric with the turned OD .

                              Again, it need not be the entire hole depth. How many of these do you need at one time... Is "efficiency" an issue?

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