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  • face milling in the lathe problem

    I was trying to square up some stock in the lathe , it is clamped to the compound, here is the setup

    it is cutting ok and the finish is good the problem is that it acts like a mill that is out of tram.the first part of the cut is down when it gets to rear part of the cut cutting up it is cutting deeper about .002 ,

    what is causing this. what am i doing wrong or what wrong with my lathe?
    thanks for any help
    scariest thing to hear " I am from the government and i am here to help"

  • #2
    Its a well known problem but i cant remember the cause. Most lathes are designed to cut from the centre out or from the edge in, you would not normally allow the tool past the centre point, in which case the fault would never show up.

    The only suggestion i could offer is to increse the swept diameter of the tool so it does not cut on the back stroke. It is probably caused by some minor inaccuracy in the saddle alignment or may be designed-in on a lathe. How about a milling cutter in the chuck and a vertical slide?

    Sorry i cant help more.

    DAve
    If it does'nt fit, hit it.
    https://ddmetalproducts.co.uk
    http://www.davekearley.co.uk

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    • #3
      I have just read in chap 2 of Tubal Cains "milling in the lathe" that a lathe cross slide is not exactly at right angle to the mandrel axis perhaps .002 003 per foot, maybe this is it,,?
      scariest thing to hear " I am from the government and i am here to help"

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      • #4
        Are you sure the work is parallel to the chuck?

        If you think your cross slide is not square to the bed, that is parallel to the chuck then chuck up a short piece of round stock about 3" dia. Take a cut down the side of the round stock and then face it. The side should be square to the face cut. If not you have a problem and now know why you are getting bad results.

        A lathe should hold very close to square for face cuts.
        It's only ink and paper

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        • #5
          It doesn't matter whether the work's parallel to the chuck, as the cutter will make it parallel to the table travel.

          The only two issues I can think of would be the work slipping in the hold-downs, or the cross-slide having some slop in it, so that the upstroke moves the slide slightly, in a way the downstroke doesn't.

          On that same note, I suppose I could see a loose headstock bearing doing the same thing- allowing a slightly different positioning as the cutter goes from the upstroke to the downstroke.

          Either way, the fix would be, as Dave noted, to increase the tool swept area, and only cut your workpiece with the bit going one direction, not both.

          Doc.
          Doc's Machine. (Probably not what you expect.)

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          • #6
            The problem IS as described in the Tubal Cain book, he states that a lathe when correctly set up takes slightly more off at the center than the edge,
            Regards,
            Nick

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            • #7
              Originally posted by NickH
              The problem IS as described in the Tubal Cain book, he states that a lathe when correctly set up takes slightly more off at the center than the edge,
              Regards,
              Nick
              Oh boy! Here we go.

              I don't buy that. I can see that scenario if the tail stock were higher or lower then the quill but that's the only way.
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              • #8
                As I know alittle about this, Mr. Cain is correct. It is accepted practice of scraping in the lathe carriage in mfg. to "throw" the crosslide. A facing cut should be about .0002 to .0003 concave. The idea is that the facing cut stock should be able to sit flat on a smooth surface. I think the .002 error is alittle much but a good indicator and a parallel in a four jaw should illuminate the problem.
                Last edited by daryl bane; 08-24-2007, 03:15 PM.

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                • #9
                  Absolutely right.
                  Lathes are set up to face ever so marginally concave so that the the faced end doesn't wobble like a weeble.
                  On Myfords for example, the carriage is set to face concave by 0.001" in 12".

                  Peter

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                  • #10
                    I must have been brunk and inpoxipated from my arish kofie dish mourning when I sad the wark pees had to be paraell witf da charles, ah, chuck. The rest of my disertation is true or so I thought, I mean think.

                    I'm not so sure if I agree with Mr. Cain but I see no reason to argue, unless, naw, hell, it ain't worth the effort.
                    Last edited by Carld; 08-24-2007, 03:42 PM.
                    It's only ink and paper

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                    • #11
                      No need to guess, it's easy to determine whether your cross slide has the slight angle suggested in Cain's book.

                      Make a small circle near one edge of the face plate, mount an indicator on the cross slide and indicate the circle when it is at about center height on the near side and lock the carriage. Then rotate the faceplate 180 so the circle is at center height on the far side and crank the cross slide out to indicate the center of the circle again. The difference in readings divided by the distance between measuring points is the "error" per inch.

                      Unclear whether it is an "error" or not, depends on your point of view

                      John
                      Location: Newtown, CT USA

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                      • #12
                        Yall are correct. Tighten up the set screws on the gibs on that S outh Bend That will help . been their done that . Your cross slide is tee toddering as it moves across Had same problem once.
                        Every Mans Work Is A Portrait of Him Self
                        http://sites.google.com/site/machinistsite/TWO-BUDDIES
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                        • #13
                          It appears there may be some validity to Mr Cain's way.

                          I got out the accuracy test record for my lathe and looked up the facing cut part. It starts out by stating, "To face hollow or concave only on 12" diameter" and went on to give the dimention achived. While I don't always believe the dimentions written in factory accuracy test sheets, the fact that they test each thing gives insight to what to look for.

                          Sooo, apparantly the lathes are set up to face concave as Mr. Cain states.
                          It's only ink and paper

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                          • #14
                            Pick up a copy of "Milling Operations In the Lathe" by Tubal Cain and he explains it quite clearly. It is the way lathes are normally designed and made that makes it do thusly.
                            Last edited by Al Messer; 08-24-2007, 11:10 PM.

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                            • #15
                              I did as suggested mark the faceplate and rotate 180 while measurin with an indicator mounted on crossslide it was about the same maybe a tenth or 2
                              (0001)difference,, also while the indicator on faceplate about 3 inches from
                              centerline i pushed and pulled the edge of faceplate towards the tailstock about 30 lb push and pull and could measure about .003 deflection is this too much?
                              thanks for all the answers allen
                              scariest thing to hear " I am from the government and i am here to help"

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