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  • Rear toolposts

    Just curious who (here) uses them and for what, in particular.
    Dedicated cut-off?

    Just wondering pros/cons.

    Tony

    Edit: are they meant to be used with the spindle running in the "normal" direction? or "backwards".
    In normal direction, tool in RTP would be upside and cutting forces would want to lift carriage.
    Lift carriage = bad.
    Last edited by Tony; 01-03-2012, 04:16 PM.

  • #2
    Hi Tony,

    A rear post is on the tuit list, as I've used one before and got on very well with it - My plan is a 2 or more way turret or QC rear post with upside-down parting blade and a chamfer tool (as a minimum).

    Agreed, an upside-down rear parting tool might tend to lift, but more importantly it won't cause the work to climb the tool if the cut's too heavy, it'll force it down and this and any carriage/tool lift will reduce the depth of cut so it reduces the tendency to dig in, which is what often breaks parting tools On a "sturdy" lathe the tendency to lift isn't so much of an issue (I'd guess the carriage on mine probably weighs as much as the average hobby mini lathe - I'm spoiled ) and on a lighter lathe with well-adjusted gibs it shouldn't be a problem.

    Bear in mind that cutting force due to infeed on a front tool will also tend to lift the rear of the carriage from the way (hence most lathes have a keeper running under the rear way), tilting it around the front V-way, and this should be allowed for in the lathe's design...

    An upside-down tool has the advantage that chips clear more easily too (thanks, Gravity!), and sometimes it's very useful, e.g. if using an internal threading tool in a blind bore, when it's easiest to run the lathe in reverse and cut away from the chuck, avoiding knackering the tool in the bottom of the bore (well, it works for me!)

    Just my ha'pennorth,
    Dave H.
    Rules are for the obedience of fools, and the guidance of wise men.

    Holbrook Model C Number 13 lathe, Testa 2U universal mill, bikes and tools

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    • #3
      I have used them on occasion, but my current lathe's taper attachment doesnt allow for one and they arent really convenient (quite often in the way) unless youre running production.

      The other reason why I wouldnt run one, was bc I once had a rear mounted parting tool launched at me while trying to cut a really deep and skinny slot with too much tool overhang. The piece grabbed the tool, and when running upside down/forward it whistled past my ear before I knew it was gone. Admittedly my own fault, but made me reconsider the direction of possible bullets.
      "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

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      • #4
        As Dave, post 2
        John

        I used to be indecisive. Now I'm not so sure , but I'm not a complete idiot - some bits are still missing

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        • #5
          My lathe has t-slots that run the length (front to back) of the carriage. The compound can be slid from extreme front to the back at will. A factory option was dual compounds.

          I've used it a few times on the back but it really doesn't buy me much. It's certainly awkward to get to and to read the compound dials unless I move to the back of the lathe.

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          • #6
            Originally posted by lakeside53
            My lathe has t-slots that run the length (front to back) of the carriage. The compound can be slid from extreme front to the back at will. A factory option was dual compounds.

            I've used it a few times on the back but it really doesn't buy me much. It's certainly awkward to get to and to read the compound dials unless I move to the back of the lathe.
            You don't want to be using a compound for parting (it's like clamping onto rubber ) use a dedicated big solid rear toolpost, part under power & get on with the next piece
            John

            I used to be indecisive. Now I'm not so sure , but I'm not a complete idiot - some bits are still missing

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            • #7
              ??? I wasn't referring to parting operations from the back. I was typically threading or getting some tool into a weird position.

              I part from the front using the compound (for tool holding) all day long. Heck, today I was using a big fat carbide insert to do that in 2 inch 304 stainless, and under power (carriage). Perfect. I don't know anyone that uses the compound alone for parting, but I suppose there is always someone.

              It all depends on the lathe. My current lathe and compound isn't made of rubber at all. On a smaller lathe (10 inch) I owned I replaced the compound with a big block of steel. That helped all operations, not just parting.
              Last edited by lakeside53; 01-04-2012, 12:49 AM.

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              • #8
                Rear mounted tool post

                I first replaced the compound with this aluminum block for increased rigidity.


                That was a significant improvement andworked so well that I then constructed this rear tool post also from aluminum.



                It is shown with an upside down parting tool mounted in a Dorian wedge type QCTP. I have been somewhat concerned about the tool lifting on the QCTP. I have not used it for parting. I have used it for modifying parts mounted on a threaded arbor. By turning in reverse and cutting on the backside the part is tightened on the arbor and the visibility is much better. I used aluminum because it was in stock, easier to machine, and lighter to handle. For parting on the backside I will probably made a dedicated parting holder. The visibility for rear parting also concerns me. I think it will work better but I would sure like to see what is going on.
                Byron Boucher
                Burnet, TX

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                • #9
                  Wouldn't be without one on a lathe personally - put one on the Myford when I first got it around 1980 - it's in it's third entity now, but in constant use, parting off, boring, second op tools , there's a zil;lion uses for 'em. When I bought the 9x20 as a second lathe, late last year, a rear tool post was the first thing I added, followed closely by a tumbler reverse.

                  Here it is 'under construction'
                  Last edited by Tel; 01-04-2012, 12:50 AM.
                  Tel

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                  • #10
                    Aluminium for lathe additions

                    Originally posted by Boucher
                    I first replaced the compound with this aluminum block for increased rigidity.


                    That was a significant improvement andworked so well that I then constructed this rear tool post also from aluminum.



                    It is shown with an upside down parting tool mounted in a Dorian wedge type QCTP. I have been somewhat concerned about the tool lifting on the QCTP. I have not used it for parting. I have used it for modifying parts mounted on a threaded arbor. By turning in reverse and cutting on the backside the part is tightened on the arbor and the visibility is much better. I used aluminum because it was in stock, easier to machine, and lighter to handle. For parting on the backside I will probably made a dedicated parting holder. The visibility for rear parting also concerns me. I think it will work better but I would sure like to see what is going on.
                    Aluminium additions

                    Yes, it's easy to machine and get a nice finish, but there is always a quid-pro-quo. Aluminum is a third the stiffness of ferrous materials!

                    NzOldun

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                    • #11
                      Rear tool base material choices.

                      Well Aluminum wouldn’t be the best candidate for construction of a boring bar nor would it have been my first pick for this project. It has actually worked quite satisfactorily. In this case the journey up the stress/strain curve is a short one and the difference in the slope of the path is insignificant. In the HSM environment it will out last me.
                      Byron Boucher
                      Burnet, TX

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                      • #12
                        rear tool post

                        No one has said anything about how you can time tools with a back post. If you turn and face a shoulder with the front post then use the back post to face end of part you can hold dead lengths from the end to the shoulder. Great, if you have to hold .001 length.
                        Jim

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