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  • SFM calculating

    Fellow machinists: my students and I are doing an experiment we are going to find as many different formulas for calculating spindle speed as we can then graph the results...we want to test to see which is the closest and to see how many there are

    Currently I use RPM = SFM x 4 รท diameter...

    What do you use?

    Thanks!!!

  • #2
    what?

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    • #3
      I use that too, have for 65 years. Peter
      The difficult done right away. the impossible takes a little time.

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      • #4
        Well, the actual formula for translating between RPM and SFM with the diameter (d) being measured in inches is:

        RPM = (12 x SFM) / (3.141 x d)

        This simplifies to:

        RPM = 3.820 x SFM / d

        The 4 in the often quoted equation as you have stated is actually an approximation of 12 / 3.141 which actually equals 3.820. The error is only 4.72% so it is good enough for most shop purposes and a lot easier to remember and use. I doubt that most motors used on our machines are that accurate.
        Paul A.
        SE Texas

        Make it fit.
        You can't win and there IS a penalty for trying!

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        • #5
          You either get one formula (the exact right one) or multiple approximations, which usually are close enough for anything.

          ..so, what is the point of this?
          Amount of experience is in direct proportion to the value of broken equipment.

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          • #6
            That is what I was taught to use CS x 4/ D where CS is the cutting speed in Feet Per Minute and D is the Diameter of the Workpiece or Cutter, whichever applies.
            Will

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            • #7
              Some of you grumpy old farts have forgotten what it was all about when you were a student. The point for a student is what you learn on the journey, not what you see when you get there.

              It seems like a practical exercise in understanding the relationship between rpm and surface speed, or maybe even more fundamental, the relationship between diameter and circumference.

              Phil

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              • #8
                RPM = sfm( M/s ) x 320 / cutter dia in mm


                note this is for metric cutters

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                • #9
                  Marty,

                  Did you explain to your students the meaning (or geometric meaning with some arithmetics, if you will) of the formula you gave them? May be then the absurdity of the project will become more apparent.

                  I'm sure you realize that "4" in your formula is really 12 (as in inches per foot) divided by pi, don't you?
                  Mike
                  WI/IL border, USA

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                  • #10
                    Originally posted by philbur
                    Some of you grumpy old farts have forgotten what it was all about when you were a student. The point for a student is what you learn on the journey, not what you see when you get there.

                    It seems like a practical exercise in understanding the relationship between rpm and surface speed, or maybe even more fundamental, the relationship between diameter and circumference.

                    Phil

                    You nailed Phil! Students get frustrated when there is more than one way to skin a cat!

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                    • #11
                      I use my sight and sound to judge sfm.
                      Andy

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                      • #12
                        Originally posted by vpt
                        I use my sight and sound to judge sfm.
                        I used to do that too until I discovered my tool wear rate was way out of line. Having no talent for that, I bought a laser tachometer

                        Marty's formula is the one I use, too, but pretty much was self-discovered after a few sessions with the calculator.

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                        • #13
                          blackadder, its 318, right? (i mean, 300 is easier.)

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                          • #14
                            http://www.monstertool.com/monster_t...lculators.html
                            jack

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                            • #15
                              Originally posted by dian
                              blackadder, its 318, right? (i mean, 300 is easier.)
                              its 320 as in my post I know I am dyslectic but you made me check my post


                              eg 320 x 24 /12
                              give you 640 rpm for a 12 mm cutter in steel ( en1a )


                              but in practice I use a machinist pro calculator ;-) expensive but they do so much more

                              Stuart

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