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  • Slitting Saw, newbie??

    I am trying to teach myself to machine and recently bought a Grizzly G0619 mill.

    My question is when using a slitting saw, how do you figure the rpm's and cutting speeds? Is it the same as an endmill? When I do, they are turning so slow that I have vary little torque. I looked at a cold saw (metal cut off saw) with a 14" blade and it was spinning much faster than what I was spinning a 4" blade (both are carbide tipped blades). What am I missing? I can't find this covered in any of the books I have.

  • #2
    Yes, a slitting saw of any significant diameter does need to run fairly slowly.

    A 4" dia. saw has a circumference of approximately a foot -- actually a bit over, but call it a foot. To get a cutting speed of 100 fpm, which might be appropriate to cut mild steel with a HSS sawblade, the saw has to turn 100 rpm.

    Now, you're talking carbide, and you don't say what you're cutting. The carbide ought to let you boost the fpm, and if you're sawing aluminum instead of steel you can crank it up even more.

    But basically yes, you figure it like an end mill. At least I do.
    ----------
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    • #3
      Going slow you SHOULD have tons of torque. but that is if you get slow speed by belts and/or gears.

      That vertical milling machine has a brushless DC motor, and while it has a stated range from 100 to 1750 RPM on spec sheet (0-2000 in catalog), it seems to do that by electronic speed control, and not by belts, other than a 2:1 speed changer.

      Because of the speed and torque issue, it may just not be suitable for a large slitting saw, which is mare a cutter intended for a horizontal mill. Horizontal mills typically can be run below 100 RPM, with full torque.

      Even though it will go at 100 rpm, and despite being set in the low speed range, you are getting about 1/10 HP at 100 RPM. If you turn up (because of carbide) to 200 or 250, you can get 1/5 or 1/4 HP applied to the saw.

      You still will be torque limited to the basic motor torque, (or 2x in slow speed), so that may be an unacceptable (but unavoidable) limitation. With belt or gear drive, at 100 RPM you would have about 17 x the basic motor torque. Instead, you may have no more than 2x.

      By contrast, my small horizontal mill has a 1/4 HP motor, but is all belt drive plus 6:1 back gear. When set to run at a slow rpm for larger cutters, the machine happily chews through steel with a 2" or 4" wide high helix "slab mill" cutter. It has all sorts of torque, far in excess of the motor basic torque, because all the speed reduction is mechanical only, multiplying torque.
      CNC machines only go through the motions.

      Ideas expressed may be mine, or from anyone else in the universe.
      Not responsible for clerical errors. Or those made by lay people either.
      Number formats and units may be chosen at random depending on what day it is.
      I reserve the right to use a number system with any integer base without prior notice.
      Generalizations are understood to be "often" true, but not true in every case.

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      • #4
        Surface feet per Minute X 12 divided by PI X Cutter Diameter (inches) = RPM ???

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        • #5
          Yes, you figure it like an end mill. Or any other cutting tool, for that matter.

          I currently have a mini-mill (such as the G8689), that I have and can work with tools far larger than what the spec sheet says I can. The trick is to push to the upper edge of the suggested speed and take excessively light cuts. Yes, it sucks. But it can be done.

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          • #6
            Originally posted by knedvecki
            Surface feet per Minute X 12 divided by PI X Cutter Diameter (inches) = RPM ???
            I think you mean: divided by cutter diameter, don't you?
            Lynn (Huntsville, AL)

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            • #7
              Originally posted by knedvecki
              Surface feet per Minute X 12 divided by PI X Cutter Diameter (inches) = RPM ???
              I think it's supposed to be SFM / 12 (= in./min.) divided by Pi x Cutter Dia.(circumference (in.)) = RPM.

              edit: Nope! he was right the first time!
              I cut it twice, and it's still too short!
              Scott

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              • #8
                Correct, you calculate it like an endmill
                And yes, your mill just may not be up to the task of running any kind of large diamiter cutter like a flymill or sliting saw, or even a larger boring head.

                This is the big problem in wide speed 'VFD'/variable speed electric drives, And why most good VFD conversions of belt/gear driven mills retain the pullys/gear box, even if its going to be a CNC and the speed all controled automaticly, its still a HUGE benifit to be able to alter the gearing on the motor manualy via actualy pully/gear ratios.
                Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                • #9
                  Originally posted by lynnl
                  I think you mean: divided by cutter diameter, don't you?
                  Not really. As he said, divided by (Pi times cutter dia. in in.).

                  If simplified, the formula will be:

                  RPM=SFM x 4 / Cutter diameter (in inches)
                  Mike
                  WI/IL border, USA

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                  • #10
                    OK, I was figuring it right then and it is just the limitation of my small mill. The thing that confused me was the metal cut off saw at work (I am a mechanic, not a machinist by trade) spins a larger 14" blade at a much higher rpm. I try to cut most things close to size at work but needed to cut a 1.5" thick piece of steel at home. Even taking light passes, it was bogging down to much and I ended up waiting until I could take it to work to cut it.

                    Space is the big premium at my house/shop. I only have a 8'x12' shed for my hobbies. I have a my mill, an Atlas 10" lathe, two reloading presses and my ammo and shooting supplies all in that space. I guess I will have to try and make room for a saw now too.

                    Thanks for all the responses! I am learning a lot here and try to only ask questions after searching as much as I can first.
                    Last edited by 50BMGBOB; 10-01-2011, 09:22 PM.

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                    • #11
                      Some of the guys here have adapted a second tool holder head to their mills for extremely high speed milling. The same idea can be used to reduce the speed to a geared down crawl. The existing spindle drives a cog belt and a clamping arm attaches to the quill (assumes your quill doesn't spin) and has a simple bearing pair and a tool holder such as a good drill chuck or collet chuck.

                      The add-on spindle can be mounted to the power head if needed. This is an example if a simple add-on spindle:

                      http://www.cncathome.com/spindles.html

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                      • #12
                        I think you need a horizontal bandsaw...... Get a HF or similar Asian one if you must, although I have and like an old Atlas saw with hydraulic feed.....

                        Every metal shop needs a horizontal bandsaw for cutting stock. Some of them (like my Atlas) flip up to be a vertical bandsaw as well, although the table is typically tiny.

                        The problem with your mill is also that the saw is on a vertical spindle. That makes cutting a piece off a longer "stick" exceptionally hassle-prone....... the head interferes, you have to hold a stick of metal vertical, but there is no good vise for it, generally the whole process is a mess.... if it is even possible.
                        Last edited by J Tiers; 10-02-2011, 01:35 AM.
                        CNC machines only go through the motions.

                        Ideas expressed may be mine, or from anyone else in the universe.
                        Not responsible for clerical errors. Or those made by lay people either.
                        Number formats and units may be chosen at random depending on what day it is.
                        I reserve the right to use a number system with any integer base without prior notice.
                        Generalizations are understood to be "often" true, but not true in every case.

                        Comment


                        • #13
                          I work on large container cranes at the Port on Oakland. We have saws and welders there for repairs, just no machine tools. I am lucky in that I can do most of my cutting there. I cut this piece there but made it 3/8" to long. That is a lot to mill off with my little mill. Like I said, space is the premium at home. I am debating between a cold cut off saw or a hand held band saw for home. Both have good and bad points. I would love a larger saw, but that will have to wait until I move to a bigger shop, and that isn't even in the picture right now.

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                          • #14
                            Originally posted by MichaelP
                            Not really. As he said, divided by (Pi times cutter dia. in in.).

                            If simplified, the formula will be:

                            RPM=SFM x 4 / Cutter diameter (in inches)
                            Yes, it does become right when you put parentheses around the "pi X cutter dia." I wasn't reading it that way.

                            Oh well, whatever.
                            I just think of it in terms of your reduced approximation: 4XSFM divided by dia. (in inches).
                            Lynn (Huntsville, AL)

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                            • #15
                              Have you tried just cranking upt he speed and taking the light cuts as suggested? I'd go by ear and you should hear a low buzzing sound, not a high pitched squeal. With the carbide you don't have to worry about wearing the teeth out as much.

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