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What am I doing wrong?

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  • What am I doing wrong?

    Here is a piece that I just turned down. I tried slow feed and several different turn speeds. The cut pictured was at 1200 RPM, the slowest possible feed setting on my lathe and a .010" pass. This one turned out the best of them all, but can't figure out how to get a glass smooth finish. I tried to keep the insert clean of all chips, but for some reason I couldn't get it any better than this. Was using a coated carbide indexable insert. The piece is supposed to be cold rolled steel. Any ideas?


  • #2
    you getting any harmonics? the repeating pattern seems to indicate so.

    it's hard to chase that out of a setup. you need to minimize too flex and take a good, hard look at every possible variable in your machine which may contribute to vibration...
    -paul

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    • #3
      I would try going full speed at the spindal and maybe try a .020 pass if she has the power. What color are the chips coming off and after cooling? Might need to snug up the gibs, might be flexing. What kinda lathe? try turning the carriage back slowly without retracting, does it cut again and does that cut look better? If going back over it with the tool set at the same depth, it's likely flex.
      Last edited by knudsen; 03-12-2010, 03:28 PM.

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      • #4
        Insert?

        Is it for rouging, medium roughing, finishing, fine finishing, positive, neutral, negative?

        Lots of variables.......

        1018 steel can be difficult as it likes to tear rather than cut.

        There was a discussion recently on alternative tool bits for finishing.

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        • #5
          OK, here we go.

          The lathe is a LeBlond Regal 13x18.
          I don't know if it has harmonics or how to check for them.
          I don't know what the insert is meant for finish wise, it is a coated insert TiNi or something like that.
          Most of the passes I made, I could run the carriage back without retracting and no cut was made, but on one of the higher speed passes, it did cut on the way back. I didn't do it slow enough to take a good cut.

          I have another piece of stock that I am going to try on again tonight, I will try a faster spindle speed, and a heavier cut, see what happens.

          Thanks, you guys are the best

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          • #6
            You might get better results with a sharp high speed steel tool with a small tip radius (like the inserts) 10 degrees back clearance, 5 degrees top rake. The inserts don't have a knife sharp cutting edge. Probably 1 or 2 thou. finish pass.
            For softer materials like cold rolled steel, alum., plastic - a really sharp edge usually gives a better finish.
            Kansas City area

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            • #7
              Originally posted by DebosDave
              Here is a piece that I just turned down. The cut pictured was
              at 1200 RPM, the slowest possible feed setting on my lathe and
              a .010" pass. Was using a coated carbide indexable insert. I
              don't know what the insert is meant for finish wise, it is a
              coated insert TiNi or something like that. The piece is supposed
              to be cold rolled steel.

              What diameter is the material? It looks small, 0.500" or less.

              Using the formula RPM = (CS x 4) รท D, where D is diameter, and where
              CS is Cutting Speed in feet/min, I believe that insufficient RPM may be
              a factor.

              A CS value recommended as a starting point for coated carbide on 1012
              steel is 500-900. If the material is 0.500, then this suggests the spindle
              needs to be running at 4000-7200 RPM.

              For comparison, the CS value for HSS on 1012 is 90-100. For 0.500" dia
              material this suggests spindle speeds of 720-800 RPM.

              .

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              • #8
                I have some high speed cutters too, so I will try one of those this next time and take a small cut and see if that works better.

                Thanks again!

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                • #9
                  May be Material

                  I have had some material that just doesnt wish to machine nice. Rather than beat my already beaten head against the wall i just leave it a couple of thou oversize cover my ways up and use strip of emery cloth. It isnt a nice way to do it but it does work well, hi rpm and sand er./

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                  • #10
                    Welcome to the real world DebosDave. It's next to imposible to get a glass smooth finish on most steels. If you want a glass smooth finish stick with using brass and aluminum.

                    You can try using a shear tool that was discussed here to great extent or you can learn how to machine to .003-.004" oversize and filing to fit and finish.

                    Your going to get some tell you they get a glass finish on everything they do but they are lying. Learn to deal with what you get from a turned finish.

                    A CNC is probably the only machine that will get close to a glass smooth finish.

                    The only thing that can get real close to a glass smooth finish is grinding and/or burnishing.
                    It's only ink and paper

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                    • #11
                      is the incert in tight, also how much of the tool is over hang on the holder, is the holder tight on its mount, might check the gibs and also make sure to lock the compound as well that will make a difference, also yes also you wont get a glass finish to easy on cold or hot rolled , regual steel, i have better luck with 303 316 316L for nice fininsh also on a finsh pass i take the last cut setting and leave it alone and then take several passes, then a bit of emory cloth 150 gritt helps a bit to then take it to the polisher and your good to go, lots of sanding involved in cold rolled with sand paper..

                      just some of my experiences

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                      • #12
                        you'll know when you have harmonics going on. the machine will sing
                        -paul

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                        • #13
                          Try sharp HSS or carbide, either one with a reasonable nose radius. Also, brush some cutting oil on it. I'm also assuming it is supported by a center ... otherwise is could be a lot worse Be sure the tool is on center and you are producing clean chips or turnings.

                          With a sharp, radiused nose you should be able to get a decent finish (with some dependance on the CRS) at just about any speed, as slow as you want. Coated inserts generally are not sharp due the coating and or edge treatment to reduce chipping. They need to be run fast enough so the material flows over the cutting point. On alloy steels, you will smile when you hit this point as the finish is bright and shiny.

                          Den

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                          • #14
                            I think something is funny, maybe just the speed. I don't get a fantastic mirror like finish, but I get a much smoother cut than OP, with cold roll or even hot roll. I usually run that at estimated 2400 RPM, or full speed on low (half that?), on a 7 x with TCMT-21.51 or TCMT-21.52. I don't get the polished look at one end and rough on the other, or the ring like areas (where it dug in?).

                            I was thinking about the test on returning the carriage, if the tool is only getting pulled in part of the time, you might get normal depth some times, then deeper when it pulls in, then it would not cut on the return. On my inferior lathe that bows down and worships your lathe, it pulls in the whole cut, though consistently, and cut on the return.

                            Might also verify your tool tip is dead nutz on center of the work piece.

                            My lathe forgot to say, "Yes me Lord." So I will beat it's ass on your lathes behalf, as soon as I get home.

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                            • #15
                              Begging your indulgence, sir!

                              Originally posted by psomero
                              you'll know when you have harmonics going on. the machine will sing
                              IOW (in other words) "It ain't over 'til the fat lathe sings!"

                              -bill

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