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Chipload and how can I tell I got it right?

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  • Chipload and how can I tell I got it right?

    How can you define a good chip load? I thought it was more art then science but then I read this:

    20. Chipload from IPM, RPM and Number of Teeth in Cutter

    from this ad in ebay:

    http://cgi.ebay.com/Machinist-Machin...1%7C240%3A1318

    So, is proper chipload determination more of an art or a science? I note sometimes my chips are thinner then at other times and I'd really like to know when I've got it right as I expect it will effect cutter wear. Is it a relationship of thickness of chip to it's width and if so, what would be a reasonable relationship?
    Last edited by Your Old Dog; 12-31-2008, 08:16 AM.
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    Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

    It is true, there is nothing free about freedom, don't be so quick to give it away.

  • #2
    That looks like a handy unit.

    The chipload calculation using IPM RPM and tooth count really gives average chip thickness. I say average because chips taper. Thin at the start of the cut, thick at the end when milling conventionally, and opposite for climb milling.
    To get "total chipload" you have to factor in DOC, and length through the cut. That is when art and experience start to take over for one offs. You have to juggle your machine, endmill, surface finish, feed, stepover, and depth per pass.

    I visualize Feed, WOC, and DOC as a three dimensional graph. I try to set feed by the book, and then vary WOC and DOC inversely to suit the situation.

    Does that make a lick of sense?

    Comment


    • #3
      Every material has an ideal feed rate, measured in inches per minute. This is translates into a chip load per tooth -- .002" to .005" is typical for a HSS endmill, and the ideal feed rate for aluminum is twice the ideal feed rate for mild steel, since aluminum shears a lot easier than steel.

      Since feed rate is a function of tooth load, you also have to take into account the number of teeth -- a 4-flute endmill run at the same feed rate will have half the chip load of a 2-flute endmill, so you have to run the 4-flute at twice the linear speed to get the same chipload.

      That may sound overly complicated, but it boils down to the fact that if you run the feed too fast (i.e., crank the table too fast), each flute on your endmill is taking too big of a bite, and you get lousy surface finish (tearing), and you'll kill the cutter in short time. If you run the feed rate too slow (i.e., crank the table too slow), each flute on your endmill is taking too little of a cut, and you're re-cutting the workpiece over and over, cutting swarf, and basically dulling the cutter without doing a lot of work.

      I keep a little chart taped to my milling machine, and I just look up the ideal feed by diameter of the endmill and number of teeth, but if you have a computer in the shop, this is a great calculator:

      http://www.custompartnet.com/calcula...speed-and-feed
      "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

      Comment


      • #4
        By the way, the concept of ideal speeds and feeds apply to any machine tool, and the ideal feed rate for a material is the same whether the tool is a mill, lathe, drill press, or shaper.

        Speeds and feeds calculations are explained in every machinist text, but here's a short but sweet online tutorial from the University of Wisconsin:

        Calculating Speeds and Feeds for Twist Drills
        "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

        Comment


        • #5
          Originally posted by lazlo

          That may sound overly complicated, but it boils down to the fact that if you run the feed too fast (i.e., crank the table too fast), each flute on your endmill is taking too big of a bite, and you get lousy surface finish (tearing), and you'll kill the cutter in short time. If you run the feed rate too slow (i.e., crank the table too slow), each flute on your endmill is taking too little of a cut, and you're re-cutting the workpiece over and over, cutting swarf, and basically dulling the cutter without doing a lot of work.
          Seems like too little feed is less about "re-cutting", and more about "rubbing" and not cutting. At a certain chipload, the "chip" is too small for the edge to cut. So several edges rub, then one takes a cut, and it repeats over and over. The edges that "rub" get heated, since no chip exists to take away heat. Edges get dulled, dubbed-over, and shiny, especially if the low chipload is from excess surface speed.

          The "re-cutting" happens at any feed, it has only to do with direction of cut, since conventional milling pushes out chips ahead of the cut, ready to fall in and be cut again. But you need to cut, and not rub, and the bigger the chip, the more damaging it would be. With light feed, you rub, get thin chips, and not very many of them.

          I find that most cutters "like to cut", meaning that an honest cut on the work rarely dulls them. Mostly it is a light cut rubbing off the edge, or a heavy cut breaking teeth, that causes damage, in my experience.

          I have no doubt that re-cutting has an effect, it can double or triple the effective depth of cut from the perspective of the edge....
          1601

          Keep eye on ball.
          Hashim Khan

          Comment


          • #6
            Originally posted by J Tiers
            Seems like too little feed is less about "re-cutting", and more about "rubbing" and not cutting. At a certain chipload, the "chip" is too small for the edge to cut. So several edges rub, then one takes a cut, and it repeats over and over. The edges that "rub" get heated, since no chip exists to take away heat. Edges get dulled, dubbed-over, and shiny, especially if the low chipload is from excess surface speed.
            Sure, that sounds right Jerry -- better analogy than re-cutting.

            And like most of us have found empirically, it's better to err on the side of too little feed than too much feed.
            "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

            Comment


            • #7
              Actually, I usually slow the cutter, and crank up the feed, at least with a horizontal mill.

              I have burned the edge off more cutters than I have broken, so obviously I have not had the feed high enough.
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #8
                Originally posted by J Tiers
                I have burned the edge off more cutters than I have broken, so obviously I have not had the feed high enough.
                You need to use a speeds and feeds table

                (Donning my asbestos suit and ducking all the tomatoes )...
                "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                Comment


                • #9
                  Originally posted by lazlo
                  You need to use a speeds and feeds table

                  (Donning my asbestos suit and ducking all the tomatoes )...
                  Actually, it has been due to mystery metal, which work hardened....... not what I thought it was! And some time ago, at that......

                  Besides, I have found that the tables are pretty useless, because I am NOT trying to get max chips per cutter, and I don't have the type machines the tables are for....

                  So the feeds don't work out like that... and horizontal mills have many factors that affect how the feed works out..... like getting the cutter started into the cut..... would sound like a jackhammer if done per the chart..... you gotta ease in, and then go faster when 2 or 3 teeth are cutting.
                  1601

                  Keep eye on ball.
                  Hashim Khan

                  Comment


                  • #10
                    Don't want you to think I posted and then ran off. I made a reply after the 3d or 4th post but it doesn't show so.....


                    Thanks guys, the explanation is very easy to understand. I'll continue to listen to the sound of the mill and my auto X feed groaning to tell when I've got it right. I haven't burnt any cutters but have snapped a 2 and for a full blown, balls to the wall machinist rookie of only 2 years I guess I got something right
                    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                    Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                    It is true, there is nothing free about freedom, don't be so quick to give it away.

                    Comment


                    • #11
                      The definitive answer

                      Below is the definitive answer, even worth a second read for those that have seen it before. :

                      ...................................
                      Newsgroups: rec.crafts.metalworking
                      Subject: Re: Speeds and feeds horizontal milling
                      From: Robert Bastow <[email protected]>
                      Date: Tue, 02 Feb 1999 23:33:41 GMT

                      Brian Evans wrote:

                      > Read with interest the discussion on feeds and speeds
                      >
                      > Any advice from anyone who's had some horizontal milling experience?
                      >
                      > Thanks, Brian


                      Milling speeds and feeds are a real can of worms..not because there aren't
                      readily available GUIDELINES..but simply because different people have different
                      (honest) opinions based upon a whole range of different experiences.

                      THE ONE CONSTANT FACTOR IS CUTTING SPEED IN FEET PER MINUTE..It doesn't matter
                      whether I am using a 1" diameter slitting saw on my lathe cross slide, or you
                      are running a 10" diameter x 12" long slab mill on a 60 HP Cincinnati mill..if
                      we are both using HSS cutters on hot rolled steel, we are both limited to the 80
                      to 100 surface feet per minute.

                      You may find it hard to believe that, during a six year apprenticeship, during
                      which I ran SCORES of different mills..from the teensiest Instrument Mill to 48
                      foot Planer Mills...I never was taught, nor did I find it necessary to apply any
                      "magic formulae"

                      But the reason for that is simple..THERE AREN'T ANY!!

                      The objective is to remove metal as quickly (therefore economically) as
                      possible. In the early days of (particularly horizontal) Mills it was common
                      practice for manufacturers to rate and compare them in terms of "CUBIC INCHES OF
                      METAL REMOVED PER MINUTE" And, believe me, some of those old slabmillers could
                      shift IMPRESSIVE amounts of metal.

                      But there are so many other variables..some you have control over..width and
                      depth of cut, feed per tooth, coolant, tool geometry, SHARPNESS!! etc.

                      And there are a whole HOST more, that you , generally, do NOT have control
                      over..Age and CONDITION of the machine. Size of the machine, rigidity of its
                      design, its dynamic behaviour under load. the part itself, its rigidity and
                      clamping etc etc.

                      Heck a Kray Mainfrain couldn't calculate all the "BEST" parameters for all the
                      jobs and all the machines in a large shop.

                      So how DID we do it? As do it we did..most Jobs were "on ticket" ...piecework!
                      Commensurate with meeting specs. on fit and finish..we were paid to shift metal
                      as fast as possible.

                      In reality it was nowhere as complicated as one might imagine!

                      Get a job ticket, go to machine..never seen it before!

                      Clamp down job, install cutters. Quick reference to Starrett Chart pinned in
                      lid of tool box (No-body figured it out in their head..the chart was quicker,
                      especially on a Monday morning!!) X" dia at 90 ft/min = Y rev/min. Crank the
                      speed change dial (on most CINCI's, Kearney & Trecker, Herbert's etc the speed
                      and feed changes were through crank handles on large dials.

                      Now set the depth and width of cut. HMMmm! In MOST cases the fastest way to
                      shift metal was to engage as much of the cutter as possible get as many teeth
                      cutting as wide and deep as possible at the same time.."Bury the Bugger" the
                      saying went. That way you removed more metal per tooth, per rev and were less
                      likely to wear the cutter out before the job was done

                      Limiting factors..HP..got MORE than enough. Machine rigidity..slide conditions
                      etc...NO IDEA..never seen the bugger before..only one way to find out though!!
                      What's next..the work piece..this one is sturdy enough to take some
                      "elbow"...Set Up..NO PROBLEM..we soon learned to fasten things down so Dynamite
                      wouldn't shift 'em..before tickling them with fifty plus Horsepower.

                      Whats left?..the FEED rate..you know, how many thou per tooth per rev..I have
                      absolutely NO IDEA until all the other factors start inter-acting and the whole
                      stage play gets into the dress rehersal.

                      Curtain up time, light a fresh fag and take a last swig of cold tea.

                      Lights, curtain.. One last check around...spindle clear, feed disengaged, SAFETY
                      CHECK..these machines are NOT fitted with an "OUCH" switch. No "oily rags" about
                      (apprentices rubbernecking) No laborer shoveling chips out of the back of the
                      machine. Bootlaces tied, floor clean and dry..two or three clear escape
                      routes..nothing to trip or fall over. Did I mention safety glasses? Safety
                      WHAT??

                      Deep breath. Concentrate. Start spindle. Coolant, GENTLY feed cutter into job BY
                      HAND. Feel the cut, feel the whole set up shuddering and settling into
                      equilibrium as the cutter digs deeper and the motor starts to push some serious
                      horspower into its job slowly build up the hand feed rate until the cutter is
                      "Buried"..In full depth and width.

                      Continue to gradually increase the feed rate, as every sense and instict strains
                      for any sign of trouble. So far so good..you figured the right starting
                      points..now you and the machine begin to understand each other and trust starts
                      to grow..We are NOT going to hurt each other or let each other down are
                      we!!!..Still gradually increasing the hand feed pressure..the machine, now all
                      the slack is taken up, all the castings have bent and bedded into unity..is
                      READY!! Quick check of the chips, cutter seems happy coolant flow good...You're
                      READY.

                      NOW!! Lets show them what we REALLY CAN DO!! start to crank the feed faster
                      and faster until you feel that first shuuder of discomfort..back off a
                      bit...engage power feed and crank the selector handle fast until you start to
                      feel the power feed catch up with and overtake your hand feed. Ease off on the
                      crank handle..let the machine take over..But don't let go yet..Every sense organ
                      is tuned in as the machine settles down to a steady pace after its quick
                      acceleration..everything feels, sounds, smells, good....turn up the feed another
                      notch..settles down..happy..turn it another...settles happy...turn it
                      another,,machine grunts..unhappy..turn it back...happy. You just arrived at the
                      CORRECT FEED PER TOOTH !!!!

                      Slowly you relax, letting your hands creep away from the knobs and handles..the
                      machine munches on..in equilibrium..chips and coolant sound like frying
                      bacon..machine is bunched into and happy with its job. You turn to find the
                      cigarette..after that first puff..has burned away. Light another. wipe your
                      hands..gradually your senses retreat from the machine. as it does what it does
                      best..shifting metal.

                      You have a bond of trust now. You and that Machine. It will let you know in
                      good time..in your secret language..if something start to go amiss. It trusts
                      you, to hear and respond, before any harm befalls it. You are a team now..both
                      doing the job you do best.

                      Now you realise your throat is dry!! no cold tea left, check the clock check the
                      job...ten minutes left "in cut", before you need to stop and replace the
                      workpiece.

                      You turn, and without a backward glance, you stride confidently toward the
                      canteen for a welcome "cuppa" On the way we happen to meet.."Hey Robert" you
                      ask "what feed rate are you using on that job?" "Haven't a clue" say I "go
                      check the dials..I'm off for a cuppa!!"

                      It may not be the answer you want Brian..But I'm afraid it's the only one I can
                      give you!

                      Happy milling.

                      Robert Bastow
                      .................................................. ..

                      Comment


                      • #12
                        Thanks a million Pfilbur for taking the time to run that down and post it here. It's confirmation for me of the way I've been doing it but wondered if I was right.
                        - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
                        Thank you to our families of soldiers, many of whom have given so much more then the rest of us for the Freedom we enjoy.

                        It is true, there is nothing free about freedom, don't be so quick to give it away.

                        Comment


                        • #13
                          Originally posted by philbur
                          Below is the definitive answer, even worth a second read for those that have seen it before. :
                          That's exactly my point Philbur -- that's Teenut (Robert Bastow)'s famous post to rec.crafts.metalworking. It was a lengthy (and polite) discussion about calculating speeds and feeds, and most of the pro machinists were advocating calculating (or looking up) the correct speeds and feeds, when Teenut went into his poetic missive.

                          Unless you've got 50 years in the business like Teenut or Lane, you're not going to be able to "feel" the correct feeds and speeds like a master Tool and Die maker can...

                          But more to the point, with all that experience and art, a Master Tool and Die Maker is empirically finding the same values that have been calculated for you in the Machinery's Handbook. Yes, you need to tweak those values according to the power and rigidity of your machine, but mild steel will always cut best at 100 SFPM, regardless of what machine, and what cutter, and aluminum will always cut best at 200 SFPM...

                          So you can spend a lifetime trying to learn the correct speeds and feeds by feel, or you can just look it up in a table, and get most of the benefits of their extensive experience in 3 minutes

                          That's not saying that after looking up the correct cutter speed you're going to start making parts like Lane, but at least you have taken care of one source of errors...
                          Last edited by lazlo; 01-01-2009, 11:23 AM.
                          "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

                          Comment


                          • #14
                            This might be oversimplifying but the way I was shown was

                            On steel with HSS tools as an example, watch the chips, if they look right "nice sixes and nines (depending on the material of course) and they aren't smoking blue then thats a good indicator that you have it right.
                            The shortest distance between two points is a circle of infinite diameter.

                            Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                            Southwestern Ontario. Canada

                            Comment


                            • #15
                              For those of us lacking the years of experience and tool & die maker training (that's me!!), this was something that I found helpful. It is from a Popular Science magazine article that I clipped and saved years ago. I have no idea how accurate the results this gives are, I would guess they are pretty conservative. I've used this little formula for years to get me in the ballpark, and get decent results. It is from the October 1972 issue of Popular Science magazine, page 106 if the link does not work for some reason:

                              LINK!!

                              I wrote the formula and chart up on a laminated 3x5 and keep it by my machines. I don't refer to it as much as I used to, but find it handy from time to time.

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