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  • Odd fly cutter...

    Left is store-bought. Right is shop-made. Can you tell the subtle difference?


















































    Looks like one is for CW rotation and the other is for CCW. But both came with bits for CCW rotation.

  • #2
    The store bought one pushes the cutting tool away from the centerline. It also required two operations even before the holes were drilled. Either tool will work CW or CCW with the correct cutter.

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    • #3
      I thought the cutting part of the bit should always be on the centerline?

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      • #4
        Originally posted by WaveDude
        I thought the cutting part of the bit should always be on the centerline?
        You're correct about the cutter being on center.

        The other difference is that the store bought one is balanced.

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        • #5
          I think by the time the cutter is in the slot neither is particularly balanced. Empty, the store-bought one looks better balanced, but they don't run empty.

          Both have a logical default operating direction when used conventionally with the store-bought version turning CW. The shop made one may have been built to turn CCW as one of those little lapses of concentration . I'd still toss out the store-bought version and get one that has the cutter seated against the centerline.

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          • #6
            Throwing it out

            The "store-bought" item is much better balanced and will reduce "throw" due to eccentricity of the centre of mass/gravity:



            It has two reductions:
            - the step where the screw heads/adjustments are; and
            - the large "scoop-out" (part of a circle ie an arc) at the front.

            To see the effect of "out of balance", put both in a mill collet and compare the vibration/force/"throw" of each by putting your hand on the mill quill at increasing speeds.

            I made this one on the same principle - it works well but I need to be careful not to get caught up in it!! Because of its large "sweep", I only use it at low revs, but because of its rigidity, I can use some fairly good feed-rates and depth of cut.


            Centripetal (sometimes called centrifugal) force varies as the square of the change in angular velocity (ie the speed or RPM).

            The magnitude of the centripetal force on an object of mass m moving at a speed v along a path with radius of curvature r is:[4]

            F = (mv^2)/r

            The direction of the force is toward the center of the circle in which the object is moving, or the osculating circle, the circle that best fits the local path of the object, if the path is not circular.[5] This force is also sometimes written in terms of the angular velocity ω of the object about the center of the circle:

            F= mrω^2
            from:
            http://en.wikipedia.org/wiki/Centripetal_force

            Which means that if the speed/rpm is doubled the centripetal force increases X 4, if the speed is increased X 3 the force is increased by 3^2 = X 9.

            This is not only a problem associated with fly-cutters but is very much the case with "out of balance" boring heads too where it can be worse if the extended boring tool is fairly long and starts to "whip" as the "free" (cutting edge) end.

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            • #7
              Shouldn't the setscrews push the toolbit against the shoulder with the cutting forces, instead of against the cutting force?
              mark costello-Low speed steel

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              • #8
                Such a tool would require a specific cutter width. By forcing the cutter against the center it allows various cutter widths without need for shims to keep the tool centered. These tools shouldn't be used like roughing cutters so tool flex shouldn't be an issue, but I'd bet that in practice they do in fact beat the snot out of the work .

                All that said, these are not particularly precision tools, and just about any configuration will skim a surface.

                Comment


                • #9
                  Fly-cutters

                  That's true Mark - but it will work the other way.

                  Here is how a fly-cutter works as regards tool-shape at the cutting edge:


                  As long as the tool is within the radial line on the saw blade it will be OK. The line shows the cutter to have top rake and front clearance.

                  The position of the slot the tool is in is not really important at all - but that radial line and the tool-shape sure is.

                  Have a close look at the end of an end milling cutter and you should see what I mean.

                  I have drawn two lines on the saw blade. If were to cut the blade along the blue lines, I'd have the basis of a good fly-cutting tool - but it should be tilted like the saw blade is so as to clear the job:

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                  • #10
                    One seems to have a shank longer than the other-
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      OOOpppps......
                      Last edited by A.K. Boomer; 08-15-2010, 07:59 AM.

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                      • #12
                        Originally posted by Mcostello
                        Shouldn't the setscrews push the toolbit against the shoulder with the cutting forces, instead of against the cutting force?


                        Yes you got it, and its something I immediately seen also, the store bought one is the only one that's properly designed...

                        The "home brew" applies its cutting forces against the set screws and this is a big no-no as set screws work by extremely high unit pressures so if the machining forces/chatter/frequencies are applied against them they will either set into the bit material more or deform their ends - either way tension is lost and one will be wearing the tool bit to the hospital by the end of the day.

                        The home brew guy was a hillbilly and should not be designing tooling,
                        the verification is that we do not cut by the trailing edge of the tool bit, its the leading edge - so there is no way you can line the leading edge of the tool bit up in the configuration that he has cw or ccw(and remain on center) unless you load the set screws directly against the cutting forces...

                        Here's a pic of a properly designed FC (mine) The set screws are pre-loading the cutter into the broad flat tool holders base and the cutters edge remains on centerline...

                        Last edited by A.K. Boomer; 08-15-2010, 08:43 AM.

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                        • #13
                          Originally posted by dp
                          The store bought one pushes the cutting tool away from the centerline. It also required two operations even before the holes were drilled. Either tool will work CW or CCW with the correct cutter.



                          DP,,,, stay away from anything more mechanical than a wheel barrow --- the home brew should be pitched into a scrap heap - your thinking is as dangerous as the guy's who built that piece of crap...

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                          • #14
                            Originally posted by dp
                            I'd still toss out the store-bought version and get one that has the cutter seated against the centerline.


                            You really do have it backasswards...

                            Comment


                            • #15
                              In reality neither fly cutter is properly constructed.

                              1) the slot should be made so that the top of the cutting tool is on center.

                              2) the set screws should apply force to hold the cutting tool to the side of the slot that is on center.

                              3) even though the length of the cutting tool and its placement will vary, effort should be made to balance the fly cutter.

                              BTW, I have worked in shops where fly cutters were banned. We only used HSS or carbide insert face mills due to the spindle bearing damage that can result from excessive use of fly cutters. That said, for a HSMer a fly cutter is an inexpensive way to have a substitute for a face mill.

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