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lathe cutting tool; above or below axis, which is better?

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  • lathe cutting tool; above or below axis, which is better?

    Tonight, I didn't have my QTCP ready, so I was forced to using an HSS chinese boring bar to turn OD's on some some arbors I needed (to finish the QCTP with). After adjusting the tool heigh a bunch of times (and angles), I managed to produce a configuration that cut surprisingly smooth. I pushed my luck and turned the compound so that the boring bar was nearly parallel, with the bar I was cutting. RPM was around 1000 and the part was 0.800" OD. I kept pushing the compound angle closer and closer to 0 - I think I must have gotten within less than a degree. The finish was absolutely great, about as good as ground, nice and consistent throughout. Due to my curiosity and negligence, I pushed it in shoulder and broke the bit. The bit is was about 3/16ths between the cutting tip and the shank, 1/4" at the shank. The kind of swarf that was being produced was similar to dust; mixed with WD40 it produced a slurry that dripped off the cutter.

    ...

    as hard as I tried with another boring bar, I could never get another one of these boring bars to produce the same kind of finish. I've tried various angles in all three axes, and various tool heights.

    ...

    It got me thinking about something I read here once; either above or below the cutting axes, one is better than the other. Which was it? I think it was, you're better off being slightly above axis than slightly below axis. My second setting was making horrible noises ...

  • #2
    I have always gone for slightly below but I am a rookie. If you are slightly above and the bit grabs as it gets pulled down it also gets pushed out digging it in deeper. Slightly below center it just gets pulled down a bit gets unhooked and moves on.

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    • #3
      I don't understand why your using a boring bar to turn the OD of something. You should be using a lathe tool which would be anything from 1/4" square to maybe 1/2" square depending on the size of your lathe.

      You should have the tip of the cutter at the axis of the lathe spindle. That is where you will get the best cut. DO NOT set the tip above the axis of the spindle, the tool will rub on the front. Sometimes you can set the tool a few thousandths low.

      If a boring bar inside of a bore it needs to be on the center line (axis) to cut correctly.
      It's only ink and paper

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      • #4
        I was taught to place a lathe tool ON CENTER, not above or below. That has worked for the last 45 years, for me.

        DJ

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        • #5
          Originally posted by mechanicalmagic
          I was taught to place a lathe tool ON CENTER, not above or below. That has worked for the last 45 years, for me.

          DJ
          That's an ideal scenario. Let me rephrase the question; would you approach from above or from below?

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          • #6
            Ok, If you always keep the tip of the cutter on the center line of the work you will not have any problems.

            Is that clear enough? Yes, that is an ideal situation and is what should be done.
            It's only ink and paper

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            • #7
              id have to say that well over 90% of the time i set my cutter just a slight hair below center when turning an OD when boring an ID its above and the cutter is just a hair twisted down ward makes a nice finish, and when parting i now have adopted going just a hair above center ..

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              • #8
                Originally posted by Elninio
                That's an ideal scenario. Let me rephrase the question; would you approach from above or from below?
                I don't approach. I have a single purpose height gauge, it's fixed ON CENTER. My cross slide is flat, the height gauge is used to set the tool once, then I cut. (OK, to get picky, it may be +-.002".) But I never intentionally set high or low.

                DJ

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                • #9
                  On the OD set your tool on center or very VERY slightly below NEVER above. On the ID it is opposite. This is because when on the OD, being above would mean that if your tool dug in, the DOC (depth of cut) would get deeper, not what you want. If you set it on center or below, the DOC would reduce, which is a good thing. You can probably figure out why being above is ok when cutting on the ID.
                  James Kilroy

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                  • #10
                    Below - your rake angle is slightly negative, but you'll have more than adequate relief (clearance).

                    Above - more positive rake angle, but likely inadequate relief, causing the cutter to rub and chatter.

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                    • #11
                      Originally posted by jkilroy
                      On the OD set your tool on center or very VERY slightly below NEVER above. On the ID it is opposite. This is because when on the OD, being above would mean that if your tool dug in, the DOC (depth of cut) would get deeper, not what you want. If you set it on center or below, the DOC would reduce, which is a good thing. You can probably figure out why being above is ok when cutting on the ID.
                      It's good to hear that some of you know this from experience. I would have reasoned that having it only slightly above on the OD would put greater load on the cutter, reducing vibrations. I'd rather reap the advantage of undercutting a part over improved finish, than machining a part twice because I cut too much ...

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                      • #12
                        According to the south Bend lathe works publication; How to Run A lathe, "the cutting edge of the cutter bit should be about 5 deg. above center". Form tools, like threading tools should be placed with the cutting edge on center. This also applies to taper turning.
                        I have used this rule for about 45 years, it has served me well.

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                        • #13
                          Leo St. Clair in "Design and Use of Cutting Tools" suggests the tool tip be 1% of the cutting diameter above center; he assumes front relief of 8 degrees. The idea is that if you attempt too great an infeed the tool will rub just under the cutting edge, making excessive infeed difficult due to the increased pressure needed. Also, if the tool nods downward the tool will rub, helping to support the tool which reduces the tendency to chatter. When the tool tip is below center, chatter can be a problem because this effect is lost.

                          I've been using this approach since reading the book over a year ago and it has worked well for me. I set the tool about on center and adjust the tool up if it chatters and down if it takes more infeed pressure than expected to cut.

                          Parting sometimes requires pausing to adjust tool height as the diameter decreases so this technique works best with a QCTP to make changing tool height easy.

                          John
                          Location: Newtown, CT USA

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                          • #14
                            5 degrees is 0.5 mm above center on a 12mm workpiece!!!!!

                            Phil

                            Originally posted by JCD
                            According to the south Bend lathe works publication; How to Run A lathe, "the cutting edge of the cutter bit should be about 5 deg. above center".

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                            • #15
                              Here's the piece;

                              The taper was turned with carbide, on center aligned with a magnifying glass. The two oval patched on the shiny part are the jaw marks, I didn't tighten it very hard. The piece is about 3/4" diameter at the largest part. The white 'tube' on the side in the background is the wire from my earbuds (headphones), to give you an idea of the size. The finish isn't exagerated, I guess I should have compared it along side an HSS bit ...
                              Last edited by Elninio; 03-19-2011, 10:55 PM.

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