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Form cutting using a Skiving knife

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  • Form cutting using a Skiving knife

    In this thread, the subject of using form tools came up, and Bented brought up a version of a form tool called a skiving knife.

    Coincidentally, I'd been planning to set up some form tools for a few of my more common products, and had been leaning toward trying skiving since I wasn't sure that even my rigid W&S could form some of the pieces without chatter or deformation. Skiving is mentioned- albeit somewhat briefly- in several of my turret lathe books, but not in enough detail to give me confidence. (Of course, I also hadn't really done a lot of online looking.)

    Bent linked a nice little write up, and since that meshed well with what the books said, and since I was needing to get that particular project rolling anyway, I gave it a go. I'd ordered a bar of 01 several months back specifically to make a form tool, so going generally on the new data, I made myself a skiving knife.



    -The heat-treating is terrible, but for the moment this is kind of proof-of-concept.

    I milled down an old shopmade 4-way toolpost that came with the machine, into a rough and ugly- but stiff and functional- holder for said knife.



    And that, with a little trial and adjustment, produces this, in a single pass.



    That's the as-produced surface finish. The light scoring on the raised boss was, I think, due to the cut getting away from the cutting oil. But, the part specs out perfectly- spot on- to the intended dimensions, and does a cut that previously might have taken five minutes, in about thirty seconds.

    I whipped up a quick video for those interested in seeing how the knife works:

    https://youtu.be/BdSA88OjHSo

    Once I get to actually producing the parts, I plan to video a couple more cuts, to better show how it works. It's a lot of fun to watch the cut progress.

    Doc.


    Doc's Machine. (Probably not what you expect.)

  • #2
    Interesting......but isn't it just a shear tool set in a different orientation? Both suffer from not being able to turn to a shoulder. Nice finish, but then it is aluminium. Genuine question, not a criticism or put-down.

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    • #3
      Pretty cool Doc, I'd never heard of that type of operation before. Seems like it might be just the ticket for some of your parts. Well done and nice video.

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      • #4
        Originally posted by IanPendle View Post
        Interesting......but isn't it just a shear tool set in a different orientation?
        -Precisely, though it's worth noting that skiving has been around a very long time, since the flat-belt days at least. (Possibly why skiving was developed- to reduce the HP requirements for form tools.) So arguably, a shear tool is just a smaller and more generalized skiving tool.

        Whatever the etymology, though, it works.

        Both suffer from not being able to turn to a shoulder.
        -What? You'll note a sharp shoulder generated on my part. It's a case of not needing to turn TO a shoulder, but generating a shoulder during the cut. (It's worth noting I'm going to give a little back clearance to the shoulder, to try and get that particular face a little cleaner.

        Nice finish, but then it is aluminium.
        -One of the tricks noted in the link Bented posted, was that the knife is often mounted at a very slight down angle towards the rear, to produce a slight bit of clearance after the cutting edge. The problem with that is, with a flat knife like this, that produces a taper- which, point in fact, I got with the first cut after shimming the knife up. (My only height adjustment.)

        My knife is mounted dead flat, and I surface-ground the top to be smooth. So I likely get a slight "burnishing" action from it.

        Doc.

        Doc's Machine. (Probably not what you expect.)

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        • #5
          That is seriously cool Doc. Thanks for sharing an old technique.
          21" Royersford Excelsior CamelBack Drillpress Restoration
          1943 Sidney 16x54 Lathe Restoration

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          • #6
            that's very neat. Is that the technique when the tool is dramatically below center? So it cuts progressively along the cutting edge as the tool is fed into the work? Cool way of producing features on the part in one pass too. I imagine you could do all sorts of beads and bevels as long as they can be cut into the skiving tool.

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            • #7
              Originally posted by mattthemuppet View Post
              that's very neat. Is that the technique when the tool is dramatically below center?
              -Well, it's done on the underside (generally) of the work, simply because it has to be pulled past it. But the cutting actually takes place across the center- that's why the in-progress cut is curved. The straight edge meets a curved work and produces a curved cut.

              I imagine you could do all sorts of beads and bevels as long as they can be cut into the skiving tool.
              -I'm given to understand that prior to the advent of numerically-controlled machines, almost all "three ball" crank handles, as well as the teardrop-shaped handle itself, were produced by skiving.

              Other than some sort of undercut, I'm having a hard time picturing a profile you couldn't produce with a skiving tool.

              Doc.

              Doc's Machine. (Probably not what you expect.)

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              • #8
                ah yes, that's what I meant. Takes a little mental gymnastics to visualised how the work and the cutting edge interact. Super neat though.

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                • #9
                  Glad that it is working out for you, the tool making may take hours but the speed of production is worth it if making hundreds or thousands of parts..
                  An automatic screw machine with a bar feeder could make hundreds of such parts per hour unattended, set it up, turn it on and let it eat.

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                  • #10
                    Time to put that W&S to work and start cranking out parts!
                    Cayuga, Ontario, Canada

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                    • #11
                      Darn,Im 79 years old and thought I had seen everything.. Very nice and thanks for the good video. Edwin Dirnbeck

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                      • #12
                        If anyone's interested, I shot another quick vid, showing two more cuts- each one I took with two separate cameras, to hopefully give you a better look at how the cut forms.

                        https://www.youtube.com/watch?v=NHutvj77uyI

                        Time to put that W&S to work and start cranking out parts!
                        -That's the idea!

                        Doc.
                        Doc's Machine. (Probably not what you expect.)

                        Comment


                        • #13
                          Originally posted by Bented View Post
                          An automatic screw machine with a bar feeder could make hundreds of such parts per hour unattended, set it up, turn it on and let it eat.
                          -Oh, sure. And a barfed CNC turning center could do the same.

                          There's only three problems with that: One, being self-employed making parts for a tiny niche market, I can't afford one. Two, this is Alaska, not only are there none- as in zero- on the used market, but I also can't afford to have something that size shipped up from the states. And three, I don't know how to run one.

                          But hey, other than that....

                          Doc.
                          Doc's Machine. (Probably not what you expect.)

                          Comment


                          • #14
                            Originally posted by Doc Nickel View Post

                            -What? You'll note a sharp shoulder generated on my part. It's a case of not needing to turn TO a shoulder, but generating a shoulder during the cut. (It's worth noting I'm going to give a little back clearance to the shoulder, to try and get that particular face a little cleaner.
                            Doc,

                            I just watched your second video and now understand how you got the raised diameter and the square shoulder. Thanks for the follow up.

                            Ian.

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                            • #15
                              Originally posted by Doc Nickel View Post

                              -Oh, sure. And a barfed CNC turning center could do the same.

                              A turning center would be a bit of overkill for such parts (-:
                              A simple single spindle mechanical screw machine would spit them out all day long if the material is there, turn it on and go and do something else.

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