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damping a test bar

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  • damping a test bar

    i decided to check how straight my lathe cuts. i had 400 mm of this thick walled aluminum pipe, chucked it up and had a go at it. while the first half want well i got terrible chatter at the end:

    Kostenloses Bilderhosting

    so i filled the pipe with sand using an allthred and two washers and tried again. it was a little better but not much difference, you can see that section above. so next i poured some way oil in there and let it soak in over night. well, what a difference, this time it worked. i was able to turn a smooth end on the pipe:

    bilder upload

    so now i can check the lathe over 300 mm or so.

  • #2
    If you are going to have to fill it with sand/oil use solid round. Both are going to cause some sag at the outer end. Maybe a super light cut with razor-sharp tooling is better?

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    • #3
      for the test i will be using a 2 mm radius. i wanted to rought it in first and kept the insert so a comparison could be made. its a 40/20 mm od/id bar, so it can be regarded as solid in terms of stifness. i think this is a simple way to extend the lenght of the bar over the usual 150 mm. might even be an idea for the next boring bar i make.
      Last edited by dian; 12-01-2016, 01:37 AM.

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      • #4
        For a cut like this if you're using an insert that produces some spring in the work then it'll reflect in the cuts on the test bar. For this case I suggest a keenly sharp HSS tool for making the skim cuts for final measuring.

        When I made the test bar for my own lathe I set it up with a live center and hogged off about 1 to 1.5 mm through much of the middle length leaving just a band at the end that was full diameter and about 6 to 8mm wide. Same at the chuck. that way I only had to skim cut the two bands and measure. The nice sharp HSS cutter skimmed these off neatly with no spring as shown by the lack of any grooving if I stopped the cut and let it sit. If you can't do that with the insert you're using then it's springing the test bar and that's no good

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        • #5
          Definitely cut the "collars". The test you are doing is, over here, often called the "two collars" test for just that reason. Both are cut in one pass, using a fine power feed.

          A 400mm tube is pretty long... about 16" US. If it is indeed 40mm diameter, that is a lot of unsupported length vs the diameter. Even so, that is a lot of chatter, and it is over a lot of length. Are you sure you have no other problems with the machine?

          Also, insert tooling is never truly sharp unless you sharpen it, Carbide often does not hold up if sharpened to a fine edge, and is commonly somewhat blunt, so that it will not cut well with a depth of cut under perhaps 0.1 mm. So the tool pressure is probably significant. Use very sharp HSS for this test. It may also be best to use steel tube instead of aluminum, depending on what type aluminum it is, because a softer aluminum will not cut cleanly, may be more susceptible to chatter, and it may take much fiddling to get a good cut.
          1601

          Keep eye on ball.
          Hashim Khan

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          • #6
            thanks for the suggestions, but this works. i just wanted to share it.

            btw, i dont bother with the collars any more. what good does that do? its probably meant for machines without powerfeed. i just let the lathe do its job.

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            • #7
              I just found it faster to machine the one collar then manually move to the next in a few seconds instead of waiting for 7 or 8 minutes while it cut the part between. I could make two more measurements and adjustment trials in that much time.

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              • #8
                16 inches overhang is crazy for high accuracy unless you are shearing off a few or sub 10ths... sag will make it cut higher and try just pushing the bar end with your finger - it will deflect.

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                • #9
                  Another option is to part off a couple of sections from the aluminium tube and press them onto a length of round steel bar. The "sections" become the collars.

                  Norman

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                  • #10
                    Yeah, a two collar test. Long stick-outs like that do tend to chatter regardless of feeds, speeds, or tool geometry. The best practice is to dampen the vibration without deflecting the work.

                    I place a wood block between the center and the end of the work holding it there with just enough force to hold the block. The friction against the block is enough to dampen leaving a smoother cut.

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                    • #11
                      Originally posted by J Tiers View Post
                      Definitely cut the "collars". The test you are doing is, over here, often called the "two collars" test for just that reason. Both are cut in one pass, using a fine power feed.

                      A 400mm tube is pretty long... about 16" US. If it is indeed 40mm diameter, that is a lot of unsupported length vs the diameter. Even so, that is a lot of chatter, and it is over a lot of length. Are you sure you have no other problems with the machine?

                      Also, insert tooling is never truly sharp unless you sharpen it, Carbide often does not hold up if sharpened to a fine edge, and is commonly somewhat blunt, so that it will not cut well with a depth of cut under perhaps 0.1 mm. So the tool pressure is probably significant. Use very sharp HSS for this test. It may also be best to use steel tube instead of aluminum, depending on what type aluminum it is, because a softer aluminum will not cut cleanly, may be more susceptible to chatter, and it may take much fiddling to get a good cut.
                      I don't know what you consider to be "truly" sharp but I have Walter inserts that will cut you finger if you just barely touch them. I use them for sneaking up on a final dimension. They are quite durable also. I have often done interrupted cuts on steel spline shaft and they will still cut your finger when I finished. I got bloody testing how sharp they stayed.

                      They are a ground insert and last a long time.
                      How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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