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Need Help with Boring Bar

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  • Need Help with Boring Bar

    I just received a new boring bar mad by APT.

    It has a squared off end that fits into the tool holder of my quick change.

    However, when I inserted the bar into the holder I found that the carbide bit was not aligned to the true horizontal. The front is rotated downward by 5 to 10 degrees. Since the back end is squared off I cant rotate it so that it is level.

    From the operator side of the lathe, the tip that is closest to the operator is lower than the side that is farthest away.

    I would have expected the that the edge would be perfectly horizontal. Is this the way it should be?

    If so, how should it be aligned?



  • #2
    Sounds like you have a boring bar for negative rake inserts. Yep, they're supposed to be that way. Just set the cutting edge on center and you're set.

    However, if you're a home shop machinist, I think you'll like a positive rake bar much better. They cut easier with less machine power. The neg. bars are mainly for heavy cuts on more powerful lathes. I've used both but I like positive rake bars better overall.

    Maybe you can exchange it for another bar? Be sure to get the appropriate insert for your bar also. Positive inserts for postive bars etc. I have found the Travers catalog helpful in understanding insert nomenclature.


    • #3
      I bought it from Enco and as I look at their end-view picture of it I see the downward rake.

      What part of the tool should be at what height?

      That is, should the tip be at the center line of the part I am boring?


      • #4
        I have a full set of APT boring bars - TPG 222 size and 322 size. I love them.

        The reason for the tool not being straight horizontal is to provide positive rake in boring. The flats line up the bar right.

        It is important to remember that on manual machines, positive rake is generally the rule for the light duty type, or with the TPG (positive rake) inserts. On the OD of the part, the tool tip is UP from the rest of the tool. In boring, a negative rake situation is actually a positive rake situation (you are now reverse of OD - physics, tip down is actually tip up or positive for the ID). Bore bars as a rule are set "tip down" compared to "tip up" as done on OD. These tools have a bit more positive rake on the ID to allow for tool clearance as well.

        Trust the tool, they cut great. If they did not, I would not have invested in three of each size for my school shop. They are also extremely versatile.
        CCBW, MAH


        • #5
          By the way, center the tool tip, even to the point of slightly above center (which is like slight below center on the OD)

          Everything is bacwards in boring.

          the tool is right, do not exchange it.
          CCBW, MAH


          • #6
            Very cool! So the very tip (which is closest to me) should be aligned with the center of he work?

            I'm glad the APT is a keeper. Shame the drill set I just bought isn't. ;-)


            • #7
              I have several boring bar sets and they are all the same - all of them use positive inserts.

              The reason they do this is because in smaller holes the insert would drag and prevent cutting to proper depth. This also gives better chip clearance. Scott is correct on setting the boring bar slightly above center height. The bars do deflect enough that it does not matter. Do not exceed an overhang of 6:1 (overhang Vs. Boring bar diameter) unless it is a solid carbide (they can go to 11:1 or more) bar.