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Insert Fly Cutters

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  • Insert Fly Cutters

    Alright, so I'm tempted to tweak a fly cutter I'm making to be an insert type. I'm a big fan of the convenience, and since my grinder is somewhere in the mail right now, it's a good fit. However, I've got a few questions for anyone that has made one and their experience (Evan, Boomer, Snuce, others).

    I've got two main questions. The fly cutter will be small, roughly a 1.375" swath (Sherline mill), mainly used for very light facing in aluminum. That said:
    • Angles: What rake and clearance angles have you found to work best for an insert cutter (primarily aluminum cutting here)? Not looking a complete discussion on cutter geometry, but any notes regarding differences between a ground HSS cutter and inserts would be helpful.
    • Insert Type: Triangular are cheap and easy to find, but for a better finish has anyone used a round insert in their homemade cutter? I'd love to use A.R. Warner HSS inserts like I do in my lathe tools, but that limits me to triangular, square, and diamond cutters. Any issues with these shapes causing scratching or excessive hammering?

    Thanks guys.


  • #2
    This is a picture of a HSS bit that works very good for what you propose to do. The bit is rotated out for the photograph in use it is held in the normal position in the flycutter.

    This is easy to grind and it produces an excellent finish. You can experiment with inserts and carbide and you probably won't be convinced until you do but this is the best way to go.
    Byron Boucher
    Burnet, TX


    • #3
      I have a 1/2" sq. holder that uses a triangle insert at a negative setting for cutting hard metal that works very nice. I would suspect you could make a holder that would use a triangle insert with 0 rake that would work fine.

      The cutter with a radius in Boucher's photo would produce a smooth surface on aluminum.

      Why do you want to use carbide on aluminum?
      It's only ink and paper


      • #4

        Over the years I've made a few. Insert and lathe bit types. Some have survived, some have not. These are the survivors.

        I used cheap triangle inserts because, well they're cheap and it's what I had access to. I made them single inserts because they are all under 1" diameters. From 1/2" to 13/16". I needed them to mill S5 and S7 hardened to around 54-58 Rc once in a while. I did break the first 1/2" cutter I made. I think the HT was off on that one. The cutter bodies are made from S7 and HT'ed to 52-54Rc. The insert rake is about 5 degrees and the cutting edge protrudes about a 1/32" out from the body. They cut well and smoothly on Bridgeport and larger machines, even in hardened materials. I've never tried them on anything smaller. If I were to make one 1 3/8" diameter, I'd use at least 2 inserts and preferably 3.

        The other cutters are some I made 20+ years ago. And they look it too. I still use them every once in a while. Then these go dull, I can quickly resharpen and go. When you're out of inserts, you're done. I mounted the bits vertically because I wanted more cutting edges. And by gripping the tools short in the body, you can get a much deeper cut. Because it's more rigid. The tools are mounted with no rake. And they do pound some. If I were going to use this style more often than I have, I would put some rake on them to try and smooth them out a bit. Particularly on heavier, (+.100") cuts. They can leave a pretty nice finish with lighter cuts and properly ground tools though.

        If you think you understand what is going on, you haven't been paying attention.


        • #5
          Check out Evan's flycutter from a bolt: