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  • Back side tool holder question

    There was a recent article in one of the Magazines recommending upside down threading on the back side of the work piece. I can't get my head around this one. A picture would be worth a 1000 words. My Nardini lathe has slots that could be usefull. How do you adjust something like this?
    Byron Boucher
    Burnet, TX

  • #2
    You're not supposed to get your head around it, you're suppose to stand on your head

    Whether turning, threading or parting off, you want to set the tool upside down and keep all other normal tool to work relations the same except that your compound is now the ceiling of your shop and vice versa.

    If this helps you to get a better grasp on the process, remember that your cutting tool is mounted where it is for your convenience. If you could mount the tool anywhere in a path 360 degrees around the part, it would work the same way as it normally does.

    If your work deflects a bit from its own weight and length, the rear mounted tool will effectively be a little BELOW center if it was on center close to the chuck.

    I've used rear mounted tools on a small Maximat since the lathe had inadequate compound to center height for the AXA toolpost I was using. Mounting the tool upside down basically gave me a 3/8" boost in height by rear mounting (3/8" tools).

    Hope this helps a bit and doesn't obfuscate things further Den

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    • #3
      Originally posted by nheng
      If this helps you to get a better grasp on the process, remember that your cutting tool is mounted where it is for your convenience. If you could mount the tool anywhere in a path 360 degrees around the part, it would work the same way as it normally does.

      Den
      Sorta, but mounting in the front with normal rotation pushes the tool (and compound/cross/saddle) down onto the bed. Reverse turning or rear tool turning will try to lift the saddle/cross/compound and requires tighter adjustments (or HEAVY assembly and light cuts) to avoid chatter as discussed on the recent ID thread topic.

      Main theory on rear turning is based on fulcrum and force orientation. Mounted in the traditional front orientation, forces push down on end of tool and any flex resulting from cutting pressure tends to push the tool deeper into the material, increasing the force, increasing the flex, taking a bigger bight, ... SNAP! This is sometimes called a "runaway system". But mounted on the rear with standard rotation of material, the force is now pushing up on the tip of the tool, and any flex is going to tilt it BACK and away from material, reducing force, and seeking equilibrium. Again, the big down side is that the compound/cross/slide/toolpost is built assuming downward force on the front. Applying a significant lifting force from the back is not ideal, but many people find the trade-offs favorable on certain machines...
      Russ
      Master Floor Sweeper

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      • #4
        I guess one of the points of confusion regards a comment in another thread and again in one reply here concerns using a quick change tool holder on the back side. Would it hold in reverse or would the tool holder pop off? I have both a spare Dorian and a 4 sided.
        Byron Boucher
        Burnet, TX

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        • #5
          BadDog did a good job of pointing out some of the practical limits to consider with rear mount tools. Some machines have plastic hold-downs on the rear of the saddle which will have more flex that steel or bronze. If there is wear under the edge of the saddle it can also lift if not adjusted for. You could put a DTI on the saddle to ways toward the rear and lift up on the saddle to check for looseness.

          Once a toolholder is mounted and the wedge is pulled down, it should act almost as if solid. If its a piston type toolpost, it's symmetrical anyway.

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          • #6
            My rear tool post has never popped off. It would be simple to secure it more if you were worried about it.

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            • #7
              foidemore, it's the only way to bore.
              IE, boring bar upside down at the back.
              It has many advantages, not least of which is, you can see better what you are doing
              Just got my head together
              now my body's falling apart

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              • #8
                Originally posted by Boucher
                There was a recent article in one of the Magazines recommending upside down threading on the back side of the work piece. I can't get my head around this one. A picture would be worth a 1000 words. My Nardini lathe has slots that could be useful. How do you adjust something like this?
                Are refering to "Threading Upside Down and Backwards" by R. A. Zellman Page 39 of the 2007 HSM bonus issue. ?

                I do not see in the article where he refers to a rear position tool holder. He does suggest an alternative technique to traditional single point threading by :
                1) reversing spindle
                2)installing tool upside down
                3) reversing carriage feed to feed left to right
                4) advancing tool in at 90 degrees to the work instead of the traditional 30 degrees


                I have not tried this but I think it would work well on a small mini lathe And would come in handy on a small cnc.
                if you mount the tool in the back position while doing the above, the tool would rub rather than cut and attempt to cut a left hand thread.
                I agree a photo would have clarified the setup.
                Tin
                Ad maiorem dei gloriam - Ad vitam paramus

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