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SFM for HSS on 4140?

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  • SFM for HSS on 4140?

    I've got a 6" dia of 4140 in the chuck that won't be leaving until I've either wrecked it or made a 2.25x8tpi chuck mount. Up until now I haven't turned anything larger than a little over 2 inches and have been wingin' it as far as SFM goes. As soon as I touched a bit to that 6" dia I realized it was time to quit that!

    I also haven't had a clue what my actual spindle speeds are either, which has had a lot to do with why I've been wingin' it. Non-standard pulleys so the manual's no help and if I'da known that when I had it all apart I would've measured them then. Don't have a clue on my backgear speeds either because I didn't count teeth but I can get that ratio by marking and doing the actual rotation. I've already sized up the pulleys with a divider and feel I'm within a few %. They seem to reverse BTW. Only 4 sizes but they're all opposite from each other.

    So ... I've got a "How to Run a Lathe" CD that suggests 60-80 SFM for HSS on medium carbon steels which is my best guess for 4140. I make it out to be 40-50 rpm.

    Yea, nay, or any other recommendations are as always welcome.



  • #2
    4140 in what condition? Hot rolled or cold,dead soft or half hard?

    If it's hot rolled and half hard put the HSS away and break out the carbide.

    If it's cold rolled and half hard 50-60 sfpm and coolant.

    If it's dead soft 80-120 sfpm.
    I just need one more tool,just one!


    • #3
      I thought I got all the pertinent info in there but neglected to mention it's listed in the McMaster as being in an annealed state. Hot from cold I don't know.

      Thanx Wierd.



      • #4

        you can calculate your spindle speeds reasonably closely from the pulley diameters, because the pulley circumferences are proportional to the diameters - (circumference of a circle = pi*D). Measuring the diameters with calipers is close enough for the purpose. For flat belt pulleys it will be reasonably accurate, for V pulleys, where the PCD is slightly less than the outside pulley diameter a bit less so, but still fairly close.

        Say your motor speed is 1720 RPM and you have a 2" pulley on the motor and 10" pulley on the countershaft.
        Then countershaft speed will be 1720 x 2/10 = 344 RPM

        If you are driving an 8" headstock pulley with a 6" countershaft pulley, the headstock spindle speed will be 344 x 6/8 = about 260 RPM,
        or in the reverse case where you drive a 6" headstock pulley with an 8" countershaft pulley, the spindle speed will be 344 x 8/6 = about 460 RPM.

        Last edited by franco; 05-26-2007, 10:54 AM.


        • #5
          Advice on lathe and my work-shop and work-space.

          hi pntrbl

          Try this for a start. Doesn't mention HSS turning speeds but does cover milling/drilling etc. - which will put you in the ball-park.

          And do back and check the index out as well as it should help.

          As regards lathe speeds - see if you can borrow a laser digital tachometer (rev/speed counter). They are accurate to within 2% and dead easy to use - just stick a bit of sticky reflective tape on the lathe. They are too expensive to buy for a "one off"

          They should be in "Tandy" or similar if you need to buy one.

          I have provided the following image which shows the laser tacho near my lathe. The tacho shown is actually a photo of the box the tacho came in - its a better easier shot than using the real thing.

          The tacho is aimed at the small reflective strip you can see on the lathe chuck (after the lathe is running at the selected speed/s) and then just press the "go" button on the tacho, wait until it steadies and then release the tacho "Go" button and read off the speed (revs. per min) on the screen at the back of the tacho - as easy and quick as that.

          I did every speed on every machine in the shop - and just as well as some were a bit off the pace as they were for 60 Hz (USA) and not the 50Hz as here in Australia.

          You can work out the speed (revs per min - RPM) by:
          Cutting speed (ft.per min) = work diam (ins) x PI (3.14) x RPM = ins per min
          ins per min/12 = ft per min

          Simplified, Cutting speed (ft per min) = work diam (inches) x RPM/0.26

          Now moving all that around a bit to get the required RPM for a given work diam and cutting speed you want to use:
          is given as: RPM = cutting speed (ft per min) divided by work diam x 3.8

          Using the tables/charts in the shopswarf info I've attached you and the info above you should be pretty right.

          It is important that the shape of the lathe tool and the job cutting speed (ft. per min) as well as the lathe speed (Revs per min) all be got right.

          I have included a couple of images of the lathe in the shed/work-shop.

          The lathe is an excellent example of what the too-often derided "Chicom" can achieve. It has a mill/drill attached to the back of the lathe bed. The cross slide has a ground/flat area for attaching a removable key-slotted table for use a vertical mill and for fixing various attachments.

          The shed/work shop is a steel-clad, steel portal-framed structure with 8 feet head clearance and is 22 feet wide. The attached car-port and work-space concrete apron is 24 x 22 feet and the enclosed work-shop area is 36 x 22 feet.

          I will post more images shortly.

          I hope this helps.


          • #6
            Thanx guys. My 11" Logan was born with a flat belt set up but got converted somewhere along the way with later model V-belt cones. That and I never did find the right countershaft pulley are why the manual's no help. I've done the measuring and math now and feel I have at least a usable approximation.

            A tach would be nice tho because there's a also a VFD in the loop. Googling around one night I came across a solder it yourself tach kit for $50. I'd have to figure out a pick up on the spindle but then I could mount a 4 digit display right above the VFD. It's automotive in intent and needs 12V but one of them plug in the wall supplies would probably do. It's on the someday list .....

            Anyway, shooting for 100 SFM per Wierd's advice I went right to the cellar and she's cutting just fine. .035 DOC at .010 feed is as far as I'm willing to push it but that produces a cut that doesn't pick up a bit of metal on the backpass. It's clean, clear, and accurate every time.



            • #7
              You can purchase a tachometer like one pictured in oldtiffie's lathe photo on Ebay for ~$32 with postage. Just search for "tachometer". I purchased one several months ago and have used it with my lathe and mill. Works great.


              • #8
                I just crank up the rpm until the chips are starting to turn color when they come off.
                Free software for calculating bolt circles and similar: Click Here


                • #9
                  Originally posted by Evan
                  I just crank up the rpm until the chips are starting to turn color when they come off.
                  Now that's about the easest way to do it, just suck it and see, no need to get confused with all these fangled numbers, just do it.


                  • #10
                    Originally posted by pntrbl
                    I thought I got all the pertinent info in there but neglected to mention it's listed in the McMaster as being in an annealed state. Hot from cold I don't know.

                    Thanx Wierd.

                    Your welcome and no sweat.

                    Hot from cold,Hot finished bar stock will have mill scale and not be held to any kind of close tolerance.It can be over sized or undersized as well as eggshaped.Never depend on a HF round to finish out to the nominal dimension.A 6" OD round might be 6"+ or it might be 5-15/16" when it shows up.
                    Cold finished will be free of mill scale and held to fairly tight tolerances(+.000/-.0015") it will be nearly perfectly round or in the case of square or flat bar nearly perfectly square. This applies to nearly all steel alloys.
                    I just need one more tool,just one!


                    • #11
                      I question the need for 4140 in this application. Leaded steel, low carbon steel or cast iron would be normal materials of choice, for their ease of machining.

                      Semi-finished cast backplates are available from most toolhouses for relatively low prices compared to the expense of a piece of alloy steel. They require far less machining, and will serve the purpose just as well.
                      Jim H.


                      • #12
                        I considered cast iron JC, just don't know any better and wanted to error on the safe side. Didn't have a clue I could buy a semi-finished casting but I'll remember that and thanx for the heads up on it.

                        Really I don't expect this 10" 4 jaw to be particularly useful at 55 lbs. If you need one that big nothing else will do I suppose, but for a daily routine basis I'm on the lookout for something in the 6" range. So I'll be using these lessons to repeat the process someday.

                        I brought an 11' Logan home for a complete teardown, (no part of the machine spared), the day after Xmas last year. Prior to that my machine experience consisted of looking over someone's shoulder and thinking, "Wow" That's cool!" The length, breadth, and depth of my lack of knowledge should never be underestimated .......