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  • Question - Boring on a lathe

    I have a question regarding boring on a lathe, tool flex, and surface finish.

    I am using a 3/4" boring bar with carbide insert (triangular w/60 degree angles). Depth or length of the hole is 3.6". The material is SS. When boring a hole (~1.250 ID), the entrance hole is finalized to the desired size but the exit hole is easily several thou smaller. I find it necessary to make several final passes before the exit hole *comes close* to being the size of the entrance. I assume this is the direct result of tool flex?? If this is the case, and since the boring bar's length is *fixed* and length doesn't change, why would it not flex at the entrance as it does at the exit?

    Regarding surface finish, when a cut is made toward the head stock, the cut is bright and very smooth. When boring through to the opposite end and the carriage travel is reversed, more material is removed but the finish is quite poor in the opposite direction. Why? What should I do to get the same finish coming back as I do going forward? The insert is *new*.

    Harold
    For those having fought for it, Freedom has a flavor the protected will never know.
    Freedom is only one generation away from extinction.

  • #2
    How's the insert mounted verse the work? What insert? You insert may well be wearing before it gets to the far end, or, the SS is work hardening. What type of stainless? What happens if you increase the depth of cut?

    Assuming your triangular insert is not oriented to a 60 degree point right at the work, you'd need to change the lead angle on your insert to bore both ways with the same finish. Hauling it back without increasing the doc on stainless might contribuate to work hardening of the material and lousy surface finish.
    Last edited by lakeside53; 05-15-2009, 12:44 PM.

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    • #3
      The bottom of the cutting tip may be rubbing on the bore if the insert is straight sided. Cutter clearance is very important when boring. Also the height and angle of the cutter in the bore is important. The flex on the bar should be constant from one end of the bore to the other end. Perhaps your spindle bearings may be loose or the gibs on the compound, cross slide or carriage.
      It's only ink and paper

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      • #4
        Hi Guys,

        I think I'm OK with the position of the point against the work piece. If standing at the tail-stock and looking at the tool relative to the bore, the tip is slightly below center and the point is turned to about the 8 o'clock position as compared to perpendicular.

        Carld, I never thought about checking the gibs on the carriage although I did tighten the gibs on the compound.

        Harold
        For those having fought for it, Freedom has a flavor the protected will never know.
        Freedom is only one generation away from extinction.

        Comment


        • #5
          "Spring" cuts can be necessary on less than ideal set-ups & machines. With less tool pressure they can cut oversize though, so I like to rough out, take a spring cut, measure, divide the stock left into a couple of uniform DOC finish passes. I measure in between the finish passes to make sure there aren't any surprises from having less tool pressure.

          On finish passes, I would bring the tool off the work before retracting out. A indicator and mag base set to show cross-slide travel will get you back to where you were. I typically use indicators rather than dials just to avoid issues with backlash, operator error (forgetting to count turns ), etc.

          When I'm having trouble, I find it helpful to run back through and check the basics of the setup: insert/tool sharp, on-center, not rubbing, etc. It's so easy to get to working and forget about those things.
          Jon Bohlander
          My PM Blog

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          • #6
            Having a DRO on my lathe helps me get back to zero (I hope ). I gather that flex is considered normal under these conditions and repeat passes are in fact necessary.

            Harold
            For those having fought for it, Freedom has a flavor the protected will never know.
            Freedom is only one generation away from extinction.

            Comment


            • #7
              Originally posted by hwingo
              Hi Guys,
              I think I'm OK with the position of the point against the work piece. If standing at the tail-stock and looking at the tool relative to the bore, the tip is slightly below center and the point is turned to about the 8 o'clock position as compared to perpendicular.
              Harold
              If I understand you correctly, and depending on the borong bar configuration, you may have created a negative rake condition at the interface of the cutting tip and the part.

              I believe you may have better results if the cutting tip is on center and horizontal. Another issue with the cutting tip being rotated down as you describe is that the dial graduations are no longer accurate in relation to bore size. Fortunately, the bore size would increase less than the dial indicates
              Last edited by Glenn Wegman; 05-15-2009, 04:50 PM.

              Comment


              • #8
                I agree, and it may help to raise the bar above center and keep it level. The bottom of the cutting tip rubbing against the side of the bore is a real issue not to be over looked.

                Actually, if the tool is level and below center it's not cutting negative, it's cutting positive and may be contributing to flex and chatter.
                It's only ink and paper

                Comment


                • #9
                  Hi Guys,

                  If I'm not badly mistaken, when the cutter was above center and at 90 degrees, the heel of the cutter was dragging. That's the main reason I dropped it below center and slightly turned the point down so the point was engaging the work and not the heel. When I get home this evening I will raise the cutter but I am nearly certain that I will still need to rotate the bar so the heel doesn't drag.

                  Harold
                  For those having fought for it, Freedom has a flavor the protected will never know.
                  Freedom is only one generation away from extinction.

                  Comment


                  • #10
                    Grind the heel for proper relief!

                    Comment


                    • #11
                      One thing to remember. Some boring bars do not work. I does not matter how much you paid for it , they are Not all equal. Some work every time some some of the time and some not at all. Been their done that.
                      Every Mans Work Is A Portrait of Him Self
                      http://sites.google.com/site/machinistsite/TWO-BUDDIES
                      http://s178.photobucket.com/user/lan...?sort=3&page=1

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                      • #12
                        Another possibility!

                        Check to be sure your lathe is level - in all directions. This problem sounds like you have a slight twist in the bed.

                        (a carpenter's level is not nearly accurate enough! You'll need to use a very good machinist's level!)

                        Level that thing out, then see if it still turns a taper!

                        Doc-Zeus

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                        • #13
                          I have read, been told and confiirmed on a number of ocassions that a static boring tool (as in a lathe) has a natural tendency to produce a tapered hole, whereas a rotating boring tool (as in a mill) does not. I have yet to identify why.

                          Phil

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                          • #14
                            I have found that a tool will wear after several feet of boring. : )





                            Really you get tool deflection and loose your size quickly.The farther you bore, it builds up.This is a small amount but it does accumulate.Thus the need for a spring or preferably a light finish pass.

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                            • #15
                              While I agree and have experienced the same effect that a bored hole tends to get smaller toward the farthest end, the question is still why?

                              The boring bar support never changes, so the amount od deflection should be the same at the start as in the end. So where/why does it get smaller?

                              I threaded a barrel for a tuner attachment not long ago. The tuner was made on CNC machines, the jam nut and main body all made that way. I know my threads on the OD of the barrel were the same start to finish. But, the jam nut went on fine all the way, then trying the main body (about 1-1/4" long), it went on about 3/4 the way and tightened up. Same scenario as being discussed here.

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