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  • Fly Cutter Position For Pass

    Hey guys, I have a fairly quick question about fly cutting.

    I've got a 1" square bar of hot roll steel that I'm squaring and cleaning up, and the best tool I have for it is a fly cutter. The cutter diameter minimum is about 1.5", up to about 3". Should I make it about as short as I can and drive it down the centre of the part, or should I make it more like 2"? Should I offset it so the edge of the tool diameter is near one of the part edges? Is a ~90 degree entry into the part better than an angled entry?

    Of course, I should run it so the part force is aimed at the fixed jaw.

    I'm also thinking it may not actually matter.

  • #2
    Hi Matt!

    I'ts not the sort of thing I'd worry too much about. If you're trying to achieve extreme accuracy by considering tram error of the mill, then it's really your choice of feed axis that will matter more than anything. Unless you're taking one heck of a lot more material off than I imagine you would be with a fly cutter, the cutting forces should be small enough that I can't see that affecting much of anything.

    Since the angle of any tram error is fixed, and so is the width (short face) of the surface of your work, your surface error across the short face of the work due to tramming error will not change no matter what the tool stick-out is. The tool stick-out will exaggerate the effect (not the angle) of any tram error, though, so the thickness of the part after your pass might be off from what you expected if you don't take that into account.

    I always set the tool in as far as I can to cover the work surface, and feed right up the middle. For most work I do that's going to mean maximum 0.005" depth of cut when fly cutting on my tiny mill. If you're taking off a lot of meat then you might want to make some of these considerations, but otherwise I wouldn't sweat it at all.
    Last edited by mars-red; 12-12-2016, 12:28 PM.
    Max
    http://joyofprecision.com/

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    • #3
      That really is a good Question,

      lots of variables there for sure, hitting it "dead on" creates more of the "interrupted cut" effect as there's less dwell time on the work piece and the entry is abrupt, can be a little harder on the tooling and spindle and all that, can also produce a "ripple effect" in the work piece ---

      Gradually entering into one side reduces this effect drastically and also creates way more time on the work piece itself and reduces rippling in both ways and is easier on the machine and tooling...

      My vote is if your confident with your tram then offset the cutters entry to one side, maybe do a clean up pass on the other side to see if your off in fore and aft tram...

      again good question though, and swath of cutter plays into everything too, expanding on that real quick - "Generally" if you don't need the swath don't use it - use the cutter in it's most compact and stable form (short) fly cutters along with boring heads are a few of the milling tools that you can actually increase cutter speeds drastically without changing spindle speeds,
      the speed increase is generally associated with less rigidity, not a great combo esp. for guys like me with step pulley machines that are already on the lowest of settings...

      last but not least - depends how much your removing and what kind of material - even with mild steel "skin cuts" can be short duration interrupted and if your cutter is good you should have no problemo's.
      Last edited by A.K. Boomer; 12-12-2016, 12:54 PM.

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      • #4
        My tram is alright, IIRC somewhere near 2/10ths per inch in X and Y, perfectly acceptable for a goober like me. I previously fly cut the faces of a 3x4" block for my indicator stand project and with the mic found it to be 1.2 thou thickness variance between the big faces.

        This particular task is just scale removal and cleanup. Working on a precision level and this will be the base. Going to get scraped on the bottom.

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        • #5
          Depends on how slow your mill can go. Larger radius cutter needs lower RPM spindle speed.

          allan

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          • #6
            Originally posted by mattthegamer463 View Post

            This particular task is just scale removal and cleanup. Working on a precision level and this will be the base. Going to get scraped on the bottom.
            You should be fine as far as not causing any abruptness then , but If it's something precision I would still offset, take you initial cut, then step to the opposing side without changing depth of cut, take another go at it and carefully watch the pattern, there are details that will show you if you have a good tram or not - Ideally you should lay down another pattern on top of the old one, but still be able to fully see the old one on both sides... should look like a crosshatch...

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            • #7
              Just in case, there are proper methods used for squaring up rough stock, some on here and some on You-tube that you can look at - procedures and sequences used to ensure you don't end up with a part that is not totally squared.

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              • #8
                SOP for me is 1) use a the fly cutter just a little larger than the work. 2) Have the edge of the work, the side where the cutter enters. close to a tanget to the cutter. That greatly reduces the impact of the cutter, ie at whatever feed the chip is thinner there than at the halfway point of the cutter. Goes against the advice of some, but I think accuracy comes from the tram, the tool will flex where ever it is, and putting extreme accuracy and milling together has to be some sort of oxymoron Also, entry angle matters more when trying to get material offer, a lot less so on a super skinny finish cut. I can't ever recall thinking much more deeply on it than that.....I suppose by using the right sized cutter, you'll be doing both - going down the middle and entry at close to a tangent
                Last edited by Mcgyver; 12-12-2016, 01:38 PM.
                in Toronto Ontario - where are you?

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                • #9
                  Originally posted by A.K. Boomer View Post

                  Gradually entering into one side reduces this effect drastically and also creates way more time on the work piece itself and reduces rippling in both ways and is easier on the machine and tooling...

                  My vote is if your confident with your tram then offset the cutters entry to one side, maybe do a clean up pass on the other side to see if your off in fore and aft tram....
                  I vote this way and as small a diameter as possible
                  ...lew...

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                  • #10
                    Depends on where you want the red hot chips to fall ;-)

                    I like to keep the fly cutter small in dia. Make a pass of 1/2 the width. then the other 1/2 on a second pass.
                    If needed clean up with a third pass down the center.

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                    • #11
                      An entry and exit from the cut that is longer and more tangential will result in a lighter chip load and less slamming at the beginning and end of the cutter swing through the metal. So a smaller diameter of cut and right down the middle is how I've done it. Mind you I've not done a whole lot of fly cutting. Mostly I just use a larger size end mill.

                      You making one of those bent glass tube precision levels that showed up in a recent past thread?
                      Chilliwack BC, Canada

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                      • #12
                        Originally posted by BCRider View Post
                        An entry and exit from the cut that is longer and more tangential will result in a lighter chip load and less slamming at the beginning and end of the cutter swing through the metal. So a smaller diameter of cut and right down the middle is how I've done it. Mind you I've not done a whole lot of fly cutting. Mostly I just use a larger size end mill.

                        You making one of those bent glass tube precision levels that showed up in a recent past thread?
                        Yep. Although, minus the bending part. I couldn't find a decent looking tube online for a reasonable price, and couldn't find local glass suppliers, but I did find a .375 ID .500 OD piece of lab glass someone had whipped up into a flower stem holder at the junk store, $0.50. Got 5" of straight tube out of it that should be useful.

                        Not being able to bend, I hope, won't be the death of me. It'll be a learning experience if it's overly sensitive, I guess. Might be able to retrofit the bending feature in later, if the glass will allow. Also going to use a rule below the glass, which may not be great. If it bugs me too much I'll see if I can mark the glass with a dremel. I ordered a small pipette over a month ago from China and it hasn't shown up, I've stopped holding my breath. I can work on the base portion in the meantime anyway and decide later.

                        Quick drawing in PaintCAD '98 of the movement, I think for a single pass would give the most tool time in the part, and after the initial part of the cut should be pretty low impact on entry? I'm pretty confident.

                        In the case of this task, squareness/parallelism isn't as critical as a basically-flat surface to start scraping from. The level will be calibrate-able so any parallelism errors should be cancelled out there.

                        Part is black, vise is green, fixed jaw on the right side, fly cutter is red, direction of rotation and part movement marked with arrows.
                        Last edited by mattthegamer463; 12-12-2016, 05:35 PM.

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                        • #13
                          Do not even THINK about marking the tube with anything that scratches it. That's what is called a "glass cutter" after all. Especially if you wish to try wedging it upwards to give the tube a slight arching so it indicates level.

                          The other thread used the same sort of tubing you're considering and just puts a touch of pressure under the middle of the tube to flex it slightly. Glass is a lot more flexible than some might think and for the sort of sensitive level you're after it only needs to flex a very few thou. It'll easily tolerate that.... provided you don't carve it with a Dremel......

                          The ruler under the tube will also be highly prone to parallax errors in reading it. Far better to make up a flat bar with the index marks on it then run a slot part way through it along the middle with a ball end mill that is the same size or slightly larger than the tube so the marking ruler sits down onto the tube and the surface is at the same level as the bubble. It's easy to see and won't risk damage to the tube.

                          With the arrows shown in your sketch the cutter is nearing the end of its pass. And with the offset you show it is hitting the side of the piece with the thickest part of the cut and exiting with the trailing cut. On top of this the geometry you show is set up to provide the most self feeding force. But reverse the direction of travel of the black arrow to indicate the cutter moving upwards from where you show it and all that changes. Now you're "pushing" the fly cutter into the cut so it won't self feed. And the cutter is going to enter the cut with a smaller chip load which should provide an easier transition and hopefully a nicer surface finish. The exit will leave a big burr but that can be filed off.
                          Chilliwack BC, Canada

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                          • #14
                            BC I think he may be cutting how your describing - the arrow is the parts movement "towards" the cutter... I think. ?

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                            • #15
                              DOH!... I'll bet you're right.

                              I don't know why but I usually think about the part and everything else is relative to that. Which doesn't make much sense when you think about it since the cutter is stationary on a mill and the part moves. I guess it's a "me" thing....
                              Chilliwack BC, Canada

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