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  • how are speeds & speeds derived?

    While I was doing research to make my speeds & feeds charts, all the data I found was supposedly found by trial and error testing.

    Has no one developed a theoretical method?
    -Dan S.
    dans-hobbies.com

  • #2
    Its based on a formula. Both are dependent on material the cutter is made from and the material being cut. Then there is the geometry of the cutter itself which plays a hugh role in the parameters of the cutter. For the non standard cutters like indexable carbide and variable flute or variable helix end mills you really have to start with manufacturers rated feeds and speeds and go from there.

    Also your machine will have a lot to do with it as well. Rigidity and max HP will determine if your machine can push a cutter to anywhere near its theoretical maximums.

    Feeds and speeds are just a recommended starting point. You have to use your own judgment to see how a cutter is cutting on your machine to know how to proceed from there. Sound, vibration, and the chips themselves will help you find a feed and speed that your machine will be happy with.

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    • #3
      I do it the way Macona suggest in his last paragraph. I have refined the method abit. I place a beer bottle on top the mill when I'm cutting. If the bottle starts to rattle I know I"m working the machine too hard. If the bottle falls off the mill I know I'm climb milling by mistake

      I do listen to my equipment for signs of strain, slowing down of the motor and vibration. If I'm cutting aluminum I just go balls to the wall with the mill until I get alot of smoke, stop and clean off the welded on aluminum to the cutter, and then go a little slower. It works for me.
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      • #4
        Macona,

        I know the speeds and feeds are just a starting point; I'm just shocked at how rough the starting points are.

        The equations that I have seen referenced a lot is the one Taylor came up with back in 1907. From what I have read, all Taylor's equation lets you do is take a known tool material, work material, and tool life, and extrapolate to find a different tool life.

        I have seen other models that take a few more parameters , but basically work the same way. That is to say you use known values and extrapolate.

        I just find it odd that the modern professional machinist is still playing a trial and error guessing game (educated guesses, but still guesses).
        -Dan S.
        dans-hobbies.com

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        • #5
          Originally posted by dan s

          I just find it odd that the modern professional machinist is still playing a trial and error guessing game (educated guesses, but still guesses).
          I don't find it odd at all, its one of those deals where there are just too many variables. Its not just cutter material A and workpiece B and a tool life time.

          You have cutter geometries, which can get insanely complex.

          You have heat distribution, into the cutter, the part and the material, which can be effected by the geometry, cutter material, workpiece material and thickness of the part being cut. Add coolant into the mix and you are at the mercy of pressure to remove steam pockets and the ability of the coolant nozzles to get the coolant where it is needed, don't forget the high pressure additives(concentration), and don't forget the heat distribution properties of the coolant, which can be completely screwed up by improper mixing or tramp oils, (which can screw up your cutter to workpiece interface, which can throw everything out of whack).

          Now add in the machine rigidity and natural and or induced frequencies(NOT a small part of the equation).

          Also the many different depths of cut and chip loads and I think the best you can hope for is a ballpark. Nevermind coatings or variations from manufacturer to manufacturer on the base material of the cutters.

          Even if you could create a reliable model, a lot of the data going into it is going to be trial and error, which is then going to be superceded next week by the new latest and greatest.

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          • #6
            So in other words, crank it until it blows up then back off a quarter turn and run it at that setting?

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            • #7
              dan, machining is not an exact science. As stated, there are to many varaibles to have an exact perfect formula to do things with. If you are going to be a machinist you have to get rid of the engineer/scientist attitude and start machining stuff. You have to learn to machine by the seat of your pants and common sense. All the books you can find to read about machine work is just a start point to really learn how to do it.

              You can read and formulate for 50 years but untill you actually put your hands on a machine and start working you really know nothing about machine work.

              My suggestion is put your books and formulas away and start doing some work. In short time you will know very close what you can run cutters at and even then it will vary for every job.

              Your a perfectionist aren't you? Perhaps an engineer or a doctor or professor?

              Speeds and feeds are derived by doing.

              EDIT: Ok, found your web site, so your a perfectionist techy.
              Last edited by Carld; 12-27-2008, 03:23 PM.
              It's only ink and paper

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              • #8
                Originally posted by Carld
                Your a perfectionist aren't you? Perhaps an engineer or a doctor or professor?
                It's not like we can help it Carl -- we're born that way

                Speeds and feeds are derived by doing.
                The physics of metalworking is an exact science, which is why books like Cincinnati's "A Treatise on Milling and Milling Machines", Moltrecht's Machine Shop Practice, Brown and Sharpe's, ... have several chapters on "the calculus of speeds and feeds" (including Taylor's equation), the physics and metallurgy of chip formation, chip thickness, up- and down-milling, resulting surface finish,...

                That said, a pro machinist who has spent his whole life in the shop can dial-in a machine by sound/feel, but 9 times out of 10 he ends up with the speeds and feeds that are damn close to the tables in the various machinery texts (which you've probably noticed all agree with each other).

                So when an amateur or part-time machinist, who hasn't had a lifetime of experience to learn the correct speeds and feeds, steps up to the machine, he can either d!ck around with it and hope to stumble into the correct speeds and feeds, or he can look up the "optimal" speeds and feeds from Machinery's Handbook, or here, and at least have a reasonable starting point.

                If you look at the speeds/feeds tables you'll notice that it has recommendations for HSS, plain carbide, and coated carbide, with the coated carbide speeds and feeds close to 100% higher than HSS. I've found that the HSS speeds and feeds recommendations work extremely well with the power/rigidity of metalworking machines found in the typical home shop.
                Last edited by lazlo; 12-27-2008, 06:22 PM.
                "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                • #9
                  I started out as a semi perfectionist but when I had to make a living with what I had learned I found that I didn't have time for perfection and the employers or customers didn't want to pay for it. They wanted it RIGHT and as fast as posible without mistakes.

                  My perfectionist attitude fell in the dirt and never got up. I had to make a living and please the boss and customers.

                  Machining is NOT a exact science no matter what many may think. It is exact in what you have to remove to make the part but removing the waste is not an exact science. Perhaps the designing of what you will build is and exact science, but making it is not.

                  To take a perfect design and make it into a working product requires a lot of give and take and the final design is seldom like the original drawings. I and others have seen engineers draw a series of parts that would fit together with such close tolerances that the various parts can't be assembled. The parts had to be remachined with WORKING tolerances. Perfection was a mistake.

                  The last shop I worked at did some prototype work for UofL Speed engineering school. Talk about designs that won't work, we got them.

                  Speed and feed charts are just as many have stated, a guide to start with. Yes, I used them too and many times I just asked my favorite mentor where to start. I really don't mind a person being a perfectionist and they can if they are working in their shop on their stuff. I just hate to see someone reinventing the wheel and that is what dan is doing.
                  It's only ink and paper

                  Comment


                  • #10
                    Originally posted by Carld
                    Machining is NOT a exact science no matter what many may think. It is exact in what you have to remove to make the part but removing the waste is not an exact science.
                    Agree completely. But Dan is asking about how to generate a speeds and feeds table, which is exact part. Making your part after you get the speeds and feeds right (or at least, reasonable or close) is the art.

                    Speed and feed charts are just as many have stated, a guide to start with.
                    Speeds and feeds calculators/tables are optimized to remove the maximum amount of material in the minimum amount of time. In the case of Taylor's equations, it's how many cubic inches of material you can remove in unit time (20 minutes is typical).

                    Taylor's basic equations don't account for power or rigidity, but modern speeds/feeds calculators, like the one I posted, do: K is the equivalent horsepower of the machine (horsepower to the spindle after pulley losses), and there's a parameter for rigidity as well.

                    I just hate to see someone reinventing the wheel and that is what dan is doing.
                    He's trying to make a chart like this, which I have done with Excel, and I find is immensely useful:

                    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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                    • #11
                      The main problem with speeds and feeds isn't finding the optimum, it is understanding why that does not apply to you.

                      usually it is quoted for optimum tool life, maximum production of parts, and hitting consistent dimensional performance, so SPC will work decently, etc. All very nice things, but way far removed from the HSM. Not to mention that they tend to interact, and optimum for one characteristic may be bad for another.

                      So, for the HSM, it is often best to take the quoted numbers, and as a standard practice, take 80% or 90% of the number given. Burns up fewer tools, and still gets the job done just fine.

                      Remember, industry regards cutters as disposable, they just have to cut much more than their cost to be a good life. You may not have extra loaded holders to slap in, etc, and prefer to get better tool life instead of absolute maximum production per dollar.
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #12
                        Originally posted by Carld
                        Your a perfectionist aren't you? Perhaps an engineer or a doctor or professor?
                        B.S. in physics, Software developer by trade.

                        I know that their are two many variables to get you to home plate, but it's 2008 (soon 2009) and it would be nice to get to the infield, instead of just in the ballpark.

                        The HSS doc/feed correction factors from the machinist handbook are presented poorly, but are actually a nice mathematical curve. I plotted the discrete points listed in the machinest handbook, and then curve fitted them.

                        The correlation coefficent is .998 (1 is perfect)


                        The carbide correction factors... FUBAR

                        Lazlo,

                        Have you seen any good models for milling tools, or for any type of carbide tools for that matter.?
                        -Dan S.
                        dans-hobbies.com

                        Comment


                        • #13
                          Any chance I can get my hand on that notebook?

                          Originally posted by lazlo
                          He's trying to make a chart like this, which I have done with Excel, and I find is immensely useful:

                          -Dan S.
                          dans-hobbies.com

                          Comment


                          • #14
                            I was just reading a book on dynamic behavior of materials(Same place I came up with the comment about explosive hardening of Manganese steel).

                            It talks about machining processes inducing failure of the material (that is cutting it with a cutter and making chips) by a process called adiabatic shear banding. It said that bringing the material to the conditions where adiabatic shear banding occurred tended to correlate with what we would see as a nice machined finish.

                            ASB is a process where material impinged by a shock wave is compressed so quickly that it locally heats to a large fraction of its melting point (on a time scale that thermal conductivity cannot equalize the temperatures). This heating and pressure reduces the shear strength of the material locally allowing the material to be cut by a much lower force than would otherwise be required .

                            The hard core nerds who make things like armor have done a lot of computer modeling on adiabatic shear banding. This leads me to believe that with enough really painful theory, one could actually simulate the cutting process with Computational Fluid Dynamics and determine optimal values from first principles and materials properties.

                            I suspect that some of the makers of cutting tools do this kind of simulation already but some of it's pretty fierce math and out of my inclination and current capability. It may be possible that we could come up with some kind of simple equation for the situation that would cause adiabatic shear banding in the material and try to match speeds and feeds to it.

                            That being said, I'd bet the topic would make a great PhD thesis for a CFD guy.

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                            • #15
                              I understand what you and dan s are saying lazlo but I see no reason to make a new version of the old charts that work just fine as a starting point. It's not likely you will ever generate a chart with which you can set an exact rpm and feed rate and get a near perfect result when you cut metal. Perhaps with a CNC but only perhaps because of all the other variables that still exist.

                              I agree with J Tiers, in that the user of the cutter for his/her purposes and expectations of said cutter will be relevant to the speed/feed that should be used. With that said, there is no way a new chart could be any better than a chart from perhaps the 1950's. A chart of 100 years ago may not be relevant to modern tool steels and carbide etc.

                              That is why I still believe that anyone who creats a NEW chart is just reinventing the wheel and the new chart is still relevent to the old wheel and no better.
                              It's only ink and paper

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