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Boring Bar / Threading Bar: Flat on top of bar? Proper cutting angle?

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  • Boring Bar / Threading Bar: Flat on top of bar? Proper cutting angle?

    I apologize for the dumb question (this is all still new to me - not machining, but making a conscious effort to get better). Is the flat on the top of the boring bar / threading bar (all of my boring and threading bars have this same characteristic) intended to be aligned with the horizontal plane (the red line in the attached photos) or is the insert supposed to be aligned with the horizontal plane (blue line in the attached pictures)?

    Again, this feels like a dumb question. I've been aligning both my boring bars and my threading bars with the insert oriented in the horizontal plane, but then I wonder why the flat that runs along the top of the bar is oriented so that is is not aligned with the horizontal plane.

    Here's the model number of the particular left-hand internal boring bar that I'm using:



    Setup "A" - this is NOT how I orient the internal threading bar:



    Setup "B" - this IS how I set up the internal threading bar (and my boring bars). The question is that if this orientation is correct, why would the manufacturer bother to machine a flat on the bar because it wouldn't serve any purpose.

    Last edited by AlphaBob; 03-08-2021, 01:04 PM.

  • #2
    Your Pics are not coming through ??

    The flat on the top of the boring bar is for clamping purposes generally , not for alignment purposes
    Rich
    Green Bay, WI

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    • #3
      Picture doesn't work but I'm fairly certain what you are after:
      Boring bars are normally aligned with the flats. Insert has to be angled to provide enough clearance in small bores.

      Boring bars for small holes look weird at first thought:
      https://www.model-engineer.co.uk/for....asp?th=118534

      Click image for larger version

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      Location: Helsinki, Finland, Europe

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      • #4
        Boring and internal threading bars normally have flats on their shanks for horizontal clamping in the toolpost. The insert is normally set at a dipped angle of about 5 to 7 degrees for clearance. I have made some internal threading bars which have the insert angled slightly towards the helix angle of the thread, which slopes in opposite angles depending whether the thread is left or righthanded. That angle would not be more than 2 degrees.

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        • #5
          Originally posted by MattiJ View Post
          Picture doesn't work but I'm fairly certain what you are after:
          Boring bars are normally aligned with the flats. Insert has to be angled to provide enough clearance in small bores.

          Boring bars for small holes look weird at first thought:
          https://www.model-engineer.co.uk/for....asp?th=118534

          Click image for larger version  Name:	641729.jpg Views:	36 Size:	71.4 KB ID:	1932709
          I'm inclined to agree here.

          If the bar has a flat milled down the top then the insert pocket should be machined into the bar at the appropriate angle.

          It's not easy or safe to try to rotate the bar having the set screw sitting on the edge of the flat. It probably would cause the bar to rotate anyway.

          I have bars with and with out flats. The round ones without the flats, namely my Aloris bar is held by a clamp type collet sleeve in the block. It can be rotated to any position.

          Not sure what type of bar the OP has. Perhaps it's missing the anvil.

          JL...........
          Last edited by JoeLee; 03-08-2021, 03:45 PM.

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          • #6
            AlphaBob: You are using the wrong insert in your bars. You can use negative rake inserts in your particular bars, thus getting twice the usage. if you look close at your pics, you will notice the pocket is square, ergo, change the insert to neg. rake inserts. That is why the bottom of the insert pocket is at an angle, to provide proper clearance.
            Sarge41

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            • #7
              Originally posted by sarge41 View Post
              AlphaBob: You are using the wrong insert in your bars. You can use negative rake inserts in your particular bars, thus getting twice the usage. if you look close at your pics, you will notice the pocket is square, ergo, change the insert to neg. rake inserts. That is why the bottom of the insert pocket is at an angle, to provide proper clearance.
              Sarge41
              Are those the OP's pics ? I'm not sure.

              JL............

              Comment


              • #8
                Originally posted by JoeLee View Post
                Are those the OP's pics ? I'm not sure. Just went back and looked. It seems that Mattij posted the pics. I don't know where he got them. My Oh oh maybe.

                JL............
                I assumed they were, maybe not.

                Sarge41
                Last edited by sarge41; 03-08-2021, 04:14 PM.

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                • #9
                  Originally posted by sarge41 View Post

                  I assumed they were, maybe not.

                  Sarge41
                  MattiJ posted those pics and mentioned that the OP's pics didn't show.

                  JL..............

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                  • #10
                    Originally posted by Rich Carlstedt View Post
                    Your Pics are not coming through ??

                    The flat on the top of the boring bar is for clamping purposes generally , not for alignment purposes
                    Rich


                    I disagree, the top flat is for insert alignment.

                    Comment


                    • #11
                      Every bar that I use with rhombic inserts the pocket has a clearance angle whether positive or negative rake inserts.
                      The positive rake triangular insert bars are all flat.
                      Internal threading bars can be anything depending on the insert design, laydown, end mount etc.

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                      • #12
                        Originally posted by JoeLee View Post
                        MattiJ posted those pics and mentioned that the OP's pics didn't show.

                        JL..............
                        This has been quite frequently asked so I just took first pic of boring bars I found with google.

                        If Sarge41 was referring to the pic the pic I showed the boring bars are NOT for negative inserts. OK, the one on the left looks like mis-fitted pocket but it could be also illusion.
                        If anything it looks like square insert on Cxxx pocket.
                        And if I really squint my eyes the smallest insert look like it could be CPxxx on a CCxx pocket (15 degree clearance on insert and 7 degrees on insert)
                        Location: Helsinki, Finland, Europe

                        Comment


                        • #13
                          Originally posted by DR View Post



                          I disagree, the top flat is for insert alignment.
                          Can you explain your concern ?

                          My comment is :
                          Positive rake cutters always present themselves with a positive or flat approach to the bore centerline
                          Negative Rake Bar inserts always cock the insert to meet Bore centerline.
                          If the boring bar is made for negative rake cutters ( as seen in a posters photo) The manufacture sets the correct angle for his bar, and the flat is on top of the Bar for clamping .
                          (edit) when I say clamping, I am responding to the posters comment when he wanted to know why ( if ?) the flat was not on a horizontal plan
                          Rich
                          Last edited by Rich Carlstedt; 03-09-2021, 10:23 AM.
                          Green Bay, WI

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                          • #14
                            ??Question??
                            If the bar with the insert is pointing downward
                            I always learned that to be a negative rake.
                            I also have learned you can have a negative
                            rake toolholder and a positive insert, which can
                            result in the total angle presented to the work
                            to be positive as a whole. If these holders are
                            angled downward, is the tip of the insert still
                            on centerline? It should be, because if not,
                            what you dial in is NOT what size gets cut.
                            I never got a good answer on all of this.
                            So are these negative bars with a super positive
                            insert, for net positive rake ? Do you set the
                            tip on center? Need more info, other than,
                            "they are all that way". This helps not at all.

                            -D
                            DZER

                            Comment


                            • #15
                              Originally posted by Doozer View Post
                              ??Question??
                              If the bar with the insert is pointing downward
                              I always learned that to be a negative rake.
                              I also have learned you can have a negative
                              rake toolholder and a positive insert, which can
                              result in the total angle presented to the work
                              to be positive as a whole. If these holders are
                              angled downward, is the tip of the insert still
                              on centerline? It should be, because if not,
                              what you dial in is NOT what size gets cut.
                              I never got a good answer on all of this.
                              So are these negative bars with a super positive
                              insert, for net positive rake ? Do you set the
                              tip on center? Need more info, other than,
                              "they are all that way". This helps not at all.

                              -D
                              Always on center.

                              My old shars boring bar set for a 1/2" boring head at home has flat rake pockets. The new ones I got at work are quite negative. I guess to do smaller holes. They work worse for lathe work.
                              21" Royersford Excelsior CamelBack Drillpress Restoration
                              1943 Sidney 16x54 Lathe Restoration

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