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  • #16
    The formula is too much like work.....

    with a 4" diameter part the RPM equals the SFM, so if you need 100 SFM, use 100RPM.

    For 2" diameter, use twice the speed. 1" use 4x. 1/2" use 8x speed. f you have an 8" diameter part, use half the speed, so 50 RPM to get 100SFM.

    DONE.

    It's the same information, but done by simple proportions.
    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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    • #17
      Everyone here has been really helpful with formulae to calculate things. But if you look around on the web, or in the Machinery Handbook, you'll find charts with diameter vs RPM that provide you with the SFPM. HERE IS ONE.

      Similarly there's lots of different versions that give you SFPM for different materials from doing a web search for "cutting speeds for different metals".

      Now one thing that the speeds for metals never says is that these are the MAXIMUM suggested speeds. Consider that these charts are intended for use in industry where time is money and they want the most material removed in the least time. And they might well be using some great honking heavy machine. So don't be shocked if you find your home machine can and should be used at some lower SFPM cutting rate.

      Similarly each material has it's own traits which you'll learn quickly. I've got some steel here from the scrap yard that I'm pretty sure is 4130 or StressProof or some other really tough alloy. I know that if I try to turn it too fast with my HSS cutters that it heats up and work hardens and then strips the end off my cutting tool. But if I slow it way down and keep the edge engaged so it doesn't rub then it cuts fine with the HSS. And by slow down I mean REALLY slow down to a fair bit less than the "hard steel" SFPM chart entry suggests.

      Your selection of cutting speed will also matter to the tool life. Ever watch Keith Fenner on You Tube? I'm always a bit surprised that he does a lot of his machining operations at relatively low RPMs for drilling and milling even in relation to the tools he's using. But then I bet he doesn't need to sharpen then all that often. Since I've come to somewhat mimic that I've noticed that I'm not dulling my drills anywhere near as soon as in the past.

      So do the math with the formulae above or find and print out the charts that make the most sense to you and have the materials you mostly use and stick them on the wall. And from there go with what works for you within the limits of those charts.

      Stainless can also be another material that defies the suggestions. Or it may be that the chart suggestions can only be met with flood cooling. Some stainless I've turned needed to be run at WAY lower a SFPM than the charts indicated and on top of that I had to ensure that the tool didn't rub at all. It needed to dig in and stay engaged or it would work harden the stainless and then it was like grinding the edge off the HSS. So you'd think switching to carbide would be the way to go, right? Wrong..... I ruined two or three of the carbide tips for the clearly wrong sort of insert and brazed carbide tools I had at the time. The solution in the end was to run the darn stuff at about 1/4 the SFPM listed in the charts for stainless and STILL keep it well lubricated. But that's what happens when we pick out "Mystery Metals" from the scrap dealer.

      If I had to guess at it I'd say that I mostly work to around 50 to 70% of the SFPM of the charts. And I think my tooling stays sharp for longer for that reason.
      Chilliwack BC, Canada

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      • #18
        Bc your problem with stainless and carbide is likely the wrong grade of carbide.

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        • #19
          Originally posted by MrWhoopee View Post
          I believe they are suckered in by the term "high speed steel". I had to convince a newbie that 400 rpm was too fast for a HSS tool in a 3 in. diameter fly cutter in steel. Once he slowed down to about 100, the whole thing worked much better. CSx4/dia. is a very easy formula to remember and will get you close enough to start making chips (instead of resharpening your tool).
          High Speed Steel got its name in the 1930s when, relative to carbon steel tools, it did allow much higher speeds to be used. I agree its a misleading name in the present day and age, but I still like it, and use HSS tooling a lot. It probably helps that I built up a good stock of 'Eclipse' brand HSS tool stock (A good UK brand from back in the day) so don't use any of the modern Chinese HSS which can be a bit variable.
          'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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          • #20
            https://littlemachineshop.com/mobile/speeds_feeds.php

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            • #21
              Way back when I started getting interested in this stuff and there was very little of what we now call the Web, I stumbled on a short article in an old Popular Science magazine that I had that addressed just this discussion. It ended up at the same formula that had been brought up here already. While it leaves out lots and lots of the nuances involved, it so simplified things that even a machinist know-nothing like me could use it to good effect. I still have a copy pinned to the wall in my shop, it's been there since I had a shop. I suspect that I have posted this at some point in the past.

              Now that we have the WWW, you can find it here, starting on page 106, it's called:

              "How you can cut metal like a pro"


              Easily digested and works ok for the vast majority of stuff to get you in the ballpark if you otherwise have no idea where to start.

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              • #22
                Originally posted by enginuity View Post
                This is exceptionally wise advice. It really depends on the equipment. Monarch 10EE? I'd start with the posted industrial figures. Chinese or Southbend bench lathe? Much much slower.

                I work on both ends of the spectrum - program and setup highly rigid CNC turning centres and also run small bench lathes.

                If you have small hobby type equipment you should find yourself running on the slower end of your lathe for most things. If you have a backgear you'll be surprised by how often you will use it.

                The good news is most cutting tools, including most of the fine grain carbide (which is pretty much all carbide these days) doesn't mind running slow, provided you have adequate feed (chip load). In manual machining feed is where the art and experience comes into the equation - you need to look at the chips and feel the forces in the machine to determine how things are progressing.
                I agree most of the time I run 600 to 800 rpm and .008 per rev on my Grizzly. .060 depth of cut. I use modified tool holders because I have a lot of surplus inserts that I have collected over the years.

                Most home jobs are not needing to be at max metal removal. It isn’t like I’m turning a profit off my work


                Sent from my iPhone using Tapatalk

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                • #23
                  Stainless ..... I ruined two or three of the carbide tips for the clearly wrong sort of insert and brazed carbide tools I had at the time.
                  Originally posted by 754 View Post
                  Bc your problem with stainless and carbide is likely the wrong grade of carbide.
                  Oh, it most certainly was. Still is for that matter. I've yet to really effectively wrap my head around the whole carbide tooling thing. I won't get into it in this thread as I've posted about my trials and tribulations in other threads.
                  Chilliwack BC, Canada

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                  • #24
                    (Metric) Cutting speed table I made and printed to A4 size to mount to shop wall:
                    https://www.dropbox.com/s/6ujdafhnn3...able.xlsx?dl=0

                    Everyone feel free to re-use and adapt to your own use.
                    Location: Helsinki, Finland, Europe

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