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Balancing Fly Cutter

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  • Balancing Fly Cutter

    Alright guys, here's a question (all pertain to a tool blank/lathe tool single point fly cutter).

    Based on your experience, how important is it to have the tip of the cutting tool on the center line of fly cutter? By my reasoning, if the tool blank is centered in the fly cutter, and there is minimal grinding on one edge to get the cutting edge, the edge will be offset by roughly half the blank width, such than when spun in a mill there will be come normal motion of the to the workpiece. To ensure only tangential motion, the tool blank can be offset, but this leads to an imbalanced tool.

    The reason I bring this up is I need to build a quick flycutter for a Sherline size mill, and my balanced design is not one I've seen before (and I'm curious as to why not). A good chunk of fly cutters are balanced along the axis of the tool (with the balance here affected by how much the tool sticks out), but none that I've seen do it normal to the tool.

    So here's some pictures of what I'm planning to make. Anyone see an argument for or against this?

    EDIT: This was drawn backwards by accident. Cutter is drawn to spin in reverse.

    Last edited by Joel R; 02-27-2011, 06:03 PM.

  • #2
    If I understand you correctly, and I'm not at all sure I do, then I think a minimally ground tool will effectively result in considerable negative back rake, and will require more than normal end clearance before it will cut.

    I'm failing to see what you hope to gain with that configuration.
    Lynn (Huntsville, AL)


    • #3
      So what about when it's cutting? The force required to cut metal going to introduce a force vector out at the tool bit's diameter. Worse, it's a pulsating vector, because it's there when it's cutting and it disappears on the back stroke. Compare that to force on the tool/holder when it's out of balance - isn't that going to be a constant, less jerky force that doesn't matter very much?

      I don't know how to calculate this, but If you're taking a few thou off aluminum, I'd imagine balance of a fly cutter may matter, because the cutting force is so small, and the speed is likely high, exaggerating any imbalance.

      But if you're taking a cut in steel, then it's hard for me to imagine balance mattering at all, because the speed is slower (less matter of balance,) and the required cutting force is much greater. Whack-Whack-Whack I don't think balance matters much, unless maybe it's way off.

      Not that it's easily possible, but wouldn't adding more than 1 tooth be a better way of balancing a fly cutter? I'm not talking mass balance, but cutting force balance.


      P.S. On second thought -- would an out of balance fly cutter give a pre-load effect to the spindle bearings? When the cutting edge slaps the edge of the workpiece, wouldn't the spindle bearing deflect less, because the deflection slop has already been taken up?

      Edit/P.P.S.: Getting back to the main question about the centerline - it's all about rake angles. You can locate the tool bit smack dab in the center, but look at the angle it forms with the diameter, it makes your tool have negative radial rake. You'll have to grind your tool differently, where you'll probably grind tons of positive rake just to get neutral.
      Last edited by Ryobiguy; 02-27-2011, 05:32 PM.


      • #4
        If you have a lathe, Just consider boring inside, Your tool is way above center.
        Meaning while it won't need much clearance, it will effectively have a negative rake (Blunt like) cutting edge. Great for chewing harder steels with carbide... Not really seen much HSS with negative rake, but then, you might be using a carbide insert or brazed carbide tool in there.

        Sometimes it recommended to be a tiny bit above center with a boring bar.. But im not sure if that flycutter is too extreme above center.
        Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


        • #5

          You've got the analysis right on the cutting face of the toolbit. If the toolbit is centered it creates a negative rake. Moved to have the FACE of the toolbit on center means zero rake and moved a little more creates a positive cutter.

          I've seen some flycutters with part of the body cut away but I'm not sure it makes much contribution. Depending on how long your toolbit is, the cutaway might be just right or might not. Then you've also got the toolbit length which could introduce another imbalance. In the final analysis I think the cutting forces are so much greater magnitude than the balance forces they almost disappear in the equation.

          My only other thought is to think through left hand/right hand cutting. Your graphic looks like the toolbit centerline is arranged for a counterclockwise spindle. If that's what you've got, great you're good to go.
          "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill


          • #6
            Alright guys, after reading a couple of the replies, I realized I forgot to mention something. It's been awhile since I drew this up, but when I did, it put this "backwards", i.e., the tool would rotate in reverse (will be fixed when made). The edge of the blank facing the viewer in the second picture is on the centerline (actually 0.010 off to account for grinding). Disregard any angles on the tool blank... it was put there just for weight/balance information.

            With that out of the way, I may still be confused. If the tool is exactly on centerline, am I still seeing excessive rake angles (prior to grinding)?

            EDIT: Looks like TGTool caught my mistake before I finished writing this reply.


            • #7
              Its really easy to 'overthink' things like this - as been said many times what we do aint gonna be to NASA specs.

              I recently needed to get a really nice slick finish that I knew I could get with a fly cutter, but the pieces were 5 & 6" wide! (The base pieces for the little precision drill press) I had the large 3" dia 'sto-bought' fly cutter that took 3/8" bits, but that would only stretch to about 4 1/2" - So I just made one. First milled the 3/8" slot out to accept 1/2", then found a 4" piece of 1/2" key stock, brazed a piece of carbide on it and had the necessary length to span the piece. Its seriously not in balance obviously, but it cleaned up the piece nicely, and I have used it several other times to reach across other over sized pieces.

              If everything seems to be going well, you have obviously overlooked something........


              • #8
                Joel, my question is, What do you mean by the tool is on the center line?

                Do you mean the leading edge of the cutter is on center line or do you mean the center of the cutter is on the center line?

                As already stated, if the leading edge is on center line it will be a near neutral rake. If the center of the cutter is on the center line of the flycutter the cutter will be cutting with a negative rake.

                If you want to balance the flycutter take some metal off the heavy side of the flycutter body like the flycutter in Bill's photo.
                It's only ink and paper


                • #9
                  I don`t think balance will make any difference. I have used every type you can imagine and they all work.If you thank it is hanging out too far just slow the rpm. I have even used the cross hole in a boring head with a bar sticking ouy 12 inches and fly cut a piece . Just run it slow.
                  Every Mans Work Is A Portrait of Him Self


                  • #10
                    The edge of the tool that's doing the cutting should be on the centerline. This puts the groove that the tool sits in off towards one side of the body. At the same time, if you had equal lengths of tool bit sticking out at both ends, the tool is balanced in that way, but it's obviously off balance the other way. It's this second balance that I think you're referring to. It might be insignificant at normal speeds because the tool bit takes the place of the metal machined out for the groove. Only the extended portions of the tool bit will cause an imbalance then, and this can be helped by adding weight to the side of the flycutter body. I said side- it would probably be what you'd call the front, depending on how you're looking at it, the front being where the setscrews are normally put to hold the toolbit.

                    Personally I would prefer to have the cutting edge side of the toolbit slightly behind the center line to gain a slight positive rake without having to grind on that edge. Of course this puts the groove even more off center, but you should also look at this another way- the heavier or more massive the flycutter body is, the more stable it will be generally. The more meat it has, the less you would need to stick the cutting edge out the side, and the less the imbalance would be as well. The larger the diameter of the fly cutter 'disc', the more room there is to create the groove slightly behind center without losing strength in the body.

                    I have more theory on building flycutters, but I've just been called away. Back in a while.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                    • #11
                      For gosh sake. Talk about mountains out of mole hills. :-)