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must I use a 2-flute end mill to cut a keyseat?

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  • must I use a 2-flute end mill to cut a keyseat?

    I believe the prevailing wisdom when cutting a slot is that one should use a 2-flute end mill. I need to cut a keyseat in a 1.25" shaft to match the one in an H1 bushing, so it will ultimately accept a 1/4x1/8" key. I just looked - I don't have a 1/4" 2-flute end mill. I do have clean sharp 3-flute and 4-flute end mills of that size. So I want to know if it's absolutely imperative that I use the 2-flute. What bad thing happens if I don't?

    metalmagpie

  • #2
    It can work , drill a hole first, helps... if you dont drill you have to ramp in.

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    • #3
      Do you have anything smaller than a 1/4"? Use a smaller size and then open it up to the size needed. I was taught that using an on size cutter will make an oversized keyway.

      lg
      no neat sig line
      near Salem OR

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      • #4
        I never heard of such a rule. The main thing to determine is how close your endmill cuts to size. Here's what I would do: Use the cutter you have on a piece of scrap stock with appropriate speeds and feeds. Measure the slot to see if it is a good fit. Should be within a 0.001" of target width. If not then adjust cutter or process to suit.

        Just a thought: A woodruff key cutter will work without introducing tool tip runout into the critical width dimension. No need to worry about this if the endmill is running true. Hope this helps.

        Best Regards,
        Bob

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        • #5
          The best way to do a keyway is to hog out most of the material with a smaller endmill, then finish to size. If you just go in there with a full size one, it will cut off to one side. Then the keyway will be too wide with a rough finish on one side. You can try it on a piece of scrap to see what happens. A full size 2 flute won't be as bad as a full size 4 flute, but still not the best way to go. I would use a 3/16 down the middle, then down each side to leave .005 to .010 per side to full depth, or just shy, then you can use the 1/4" 4 flute to finish.Usually, it's safer to just finish with the smaller cutter, because then you have finer control over the final width. You may want to mike the key to get a slight press fit, .0005 to .001.
          Kansas City area

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          • #6
            Or if it's a slot that runs off the end of the shaft like some/many do such as motor shafts you can simply come in from the open end.

            I believe that most of the worry is based the lack of end cutting on most four flute cutters. And perhaps a secondary worry about the four flutes not clearing chips fast enough and tending to cut over size due to the issue. Perhaps try a scrap piece first to check for size? And if it cuts a little over size by too much try with a constant light air blast?
            Chilliwack BC, Canada

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            • #7
              Originally posted by Toolguy View Post
              The best way to do a keyway is to hog out most of the material with a smaller endmill, then finish to size. If you just go in there with a full size one, it will cut off to one side. Then the keyway will be too wide with a rough finish on one side. You can try it on a piece of scrap to see what happens. A full size 2 flute won't be as bad as a full size 4 flute, but still not the best way to go. I would use a 3/16 down the middle, then down each side to leave .005 to .010 per side to full depth, or just shy, then you can use the 1/4" 4 flute to finish.Usually, it's safer to just finish with the smaller cutter, because then you have finer control over the final width. You may want to mike the key to get a slight press fit, .0005 to .001.
              I don't think you'd want any "press fit" for a key in a shaft. It needs to be able to be removed often to access bearings etc.
              ...lew...

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              • #8
                I use an undersize end mill and sneak up on the size.
                In general, it is the convention to have a key a press fit
                in the keyseat of the shaft and a slip fit in the keyway
                in the hub.
                If you want to get a tight fit with a one-shot operation,
                carbide end mills tend to run .0002" or a bit more
                undersize, to get the tight fit that might be required.
                If you also try cut a keyseat in one shot, remember
                end mills deflect sideways to the direction of travel.
                So to combat this, plunge to depth, power feed the
                length of the keyseat that you want to cut, and get
                out. No zig zagging around, else you get a loose fit.,


                -Doozer

                -Doozer
                DZER

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                • #9
                  Originally posted by Lew Hartswick View Post
                  I don't think you'd want any "press fit" for a key in a shaft. It needs to be able to be removed often to access bearings etc.
                  ...lew...
                  If it is not a press fit in the shaft, you will get fretting corrosion from movement.

                  -D
                  DZER

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                  • #10
                    Use a keyway end mill, they have a shallow depth of cut normally 2 or less diameters. They are also +.0000" -.xxxx" depending on diameter.
                    The best way, as mentioned, is to use a smaller tool and walk it off allowing you to climb both sides.
                    https://www.mscdirect.com/product/details/87397030
                    Last edited by Bented; 10-01-2020, 04:40 PM.

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                    • #11
                      Rules of thumb are meant to be broken. No reason you can't use a 4 four flute for slotting, just gotta be careful about chip evacuation. Long as you can get the chips out, doesn't matter how many flutes you're using

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                      • #12
                        What I learned in school, think I got it right..
                        key way comes in from an end or a groove feature,,
                        key seat is captive, no open ends ..often involves fitting a piece of key stock , but not always.

                        now any concern removing tight key... simply drill and tap the key, then insert screw to jack it out...easy peasy..
                        and if you can stand the tapered end, horizontal wheel cutters, are fastest, more accurate for cutting keyways.

                        the last 2 I figured out myself., or by observation.

                        Comment


                        • #13
                          As I understand it, it is not a chip issue that makes a cutter cut oversized. If a three or four flute cutter is trying to cut the entire keyway at one pass, then one or two flutes will be cutting at one or both sides, but a third flute can/will be cutting at the material in the center of the slot. It is that flute, which is cutting near the center that tends to push the entire tool sideways and that causes it to cut a bit wide on one side. The other side will probably be cut close to where it should be so overall, the slot tends to be too wide.

                          A two flute cutter will not have any flutes cutting near the center while it's two flutes are cutting at the sides. So it will not experience any sideways forces while it is actually cutting. Also the cutting forces will be radially symmetrical. That means that the cutting forces of one will tend to cancel those of the other. This is balanced about the center axis of the cutter and there should be no net deflection while these two flutes are cutting the side wall of the slot. There may be some unbalance while one flute is cutting near the center of the slot and the other flute is cutting air at the rear, but since there are no flutes cutting at the sides while this is happening, the slot is not enlarged. This is probably why it is suggested that a two flute cutter be used for cutting a slot or keyway.

                          All of that being said, running a smaller diameter cutter down the center first will mostly or completely eliminate this problem and will therefore produce an accurate slot or keyway. I suspect the two flute "rule" is mostly from commercial shops where time is money and they do not want to use extra time for the extra passes that a smaller diameter cutter would require. The cost of stocking extra tool(s) is less than the cost for the machinist's time. If you are a hobbyist working in your own shop and on your own time, then you can safely ignore it and use other means as suggested to get an accurate slot.

                          Nothing here is carved in stone.



                          Originally posted by BCRider View Post
                          Or if it's a slot that runs off the end of the shaft like some/many do such as motor shafts you can simply come in from the open end.

                          I believe that most of the worry is based the lack of end cutting on most four flute cutters. And perhaps a secondary worry about the four flutes not clearing chips fast enough and tending to cut over size due to the issue. Perhaps try a scrap piece first to check for size? And if it cuts a little over size by too much try with a constant light air blast?
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

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                          • #14
                            When I worked in a machine shop we used 3 flute, Weldon,end mill designed for key slots. These were short end mills and were used cutting full depth.
                            John b. SW Chicago burbs.

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                            • #15
                              2 flute is called a slot drill for a reason. It can plunge. A lot of 4 flute cant.

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