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  • Threadcutting RPM?

    Alright, I know it varies, but I need a general number.

    My 10x22 lathe has a six preset speeds, the minimum is 150 RPM, and I'm wanting to avoid tinkering with the electric bits. I may decide to go that route, but I think if I instead gear it down (using the tensioner as a place for new gears) through two short belts instead of one long one, I could cut it down to, say, 50.

    I can never remember--is it diameter or circumference of a pulley that affects the ratio?

  • #2
    Usually whatever speed you are comfortable using as you will not be able to run the machine at the optimum speed anyway as you have to disengage the half nuts at the correct time...
    Precision takes time.

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    • #3
      Originally posted by Deus Machina
      I can never remember--is it diameter or circumference of a pulley that affects the ratio?
      I suppose circumference is more correct. One pulley revolution results in one circumference worth of belt going by.
      However, since there is a linear relationship between the two, you can use either, just be consistent.
      Mike

      My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

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      • #4
        I find, for many things, especially coarse threads, like 8 or 9 TPI, that between 30 and 50 RPM is best.

        For me 150 would be unacceptable, but I also tend to work on larger parts, up to the full available swing on the machine. Trying to take cuts on a steel workpiece 10 inch diameter, at 150 rpm is a problem. It is almost 400 FPM, where 100 is a typical recommended speed. leads to burnt cutters

        And for threading, at 150 RPM, an 8 tpi thread x 8 threads long goes past in 3.2 seconds.

        You have 0.4 seconds per revolution, rather short time to decide to pull out the threading tool, and accomplish that.

        Unless you are already the "human CNC", it may be easier to work with slower rpm..... you are likely not doing production, time is not critical.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #5
          Depends on the TPI and if theres a shoulder in the way

          Basicly, Take your TPI, Multiply by 60. Lets say 10tpi, so 600 rpm = 1 inch per second. Rather fast. So about 1/3rd that would be nice. 200rpm at 10TPI = 0.33" per second. A little easyer to disengage the half nut in a reasonabley sized run out groove. (say, 0.25" Groove, about 0.66 seconds to disengage the half nut at that speed)

          Your free to thread faster if the groove is bigger, or the TPI is much higher, Or theres no shoulder.
          Of course, Doing very fast threading towards the headstock can be.. unnerveing to say the least. And doing really coarse threads.

          And Tiers makes a good point, SFM must also be calculated and not exceeded for your cutter type.
          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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          • #6
            If you thread at 150 rpm you better have very fast reflexes and very good hand eye coordination.
            It's only ink and paper

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            • #7
              Threading

              Very very slow is best!
              Bill
              I cut it off twice and it's still too short!

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              • #8
                I realize money doesn't grow on trees, but when it does, seriously consider upgrading to a VFD for your lathe...you'll wonder why it didn't come that way in the first place.

                In the mean time here's a handy little pulley ratio calculator I picked at random.

                http://www.culvermotor.com/Engineeri...alculator.html
                Home, down in the valley behind the Red Angus
                Bad Decisions Make Good Stories​

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                • #9
                  Hi,

                  An honest question, because I actually don't understand. Why do you guys always thread into shoulders? I stopped doing that after the first time I did that.

                  dalee
                  If you think you understand what is going on, you haven't been paying attention.

                  Comment


                  • #10
                    Originally posted by dalee100
                    Hi,

                    An honest question, because I actually don't understand. Why do you guys always thread into shoulders? I stopped doing that after the first time I did that.

                    dalee
                    Dale,

                    Think of a shaft like from a mower spindle.

                    It will have a pulley/sheave with a nut holding it on.

                    Most times the threaded area is smaller than the pulley/sheave area.

                    That is why sometimes threading up to a shoulder is needed.

                    Hope this helps you understand a little better.

                    Brian
                    OPEN EYES, OPEN EARS, OPEN MIND

                    THINK HARDER

                    BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

                    MY NAME IS BRIAN AND I AM A TOOLOHOLIC

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                    • #11
                      It sounds like you and I have the same lathe more ore less, 150 RPM has been a problem for threading certain things, I find myself stopping the lathe and turning it by hand, especially threading to a shoulder. I built a hand crank that pinches on the ID of the spindle for just that.

                      Soon I'll have mine changed over to 3 phase with a VFD, If I spec'd everything out correctly I should be able to go from 60 - 2000 RPM before I need to change a belt. I'll be able to spin it quite a bit faster than the stock 2400 also

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                      • #12
                        Too bad they didn't give at proper slow speed ....but if its not tight to a shoulder, a couple of hundred rpm's is ok for threading...one hand on the feed, one the lever, you hit your mark and pull back backthe tool a fast half a turn while dissengaging the lever....doesn't take that much practice to get good at it; split second timing like a race car driver lol. Of course thats for run of the mill stuff...i'd not be going that fast for very coarse threads.....for that I don't have an answere other than what you're doing; devise a way to slow it down....but for me anyway, very coarse threads aren't that common, at least to a shoulder.

                        when it is tight to a should, reverse the lathe spindle direction and put the tool post at the back of the work - you'll then thread from head stock to tailstock.
                        .

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                        • #13
                          Originally posted by Iraiam
                          I find myself stopping the lathe and turning it by hand, especially threading to a shoulder. I built a hand crank that pinches on the ID of the spindle for just that.
                          +1 on that tip. A hand crank makes a huge difference in threading, chamfering large diameters, cutting pulley grooves and cutting with a form tool when you don't have a "granny" back-gear.
                          Milton

                          "Accuracy is the sum total of your compensating mistakes."

                          "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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                          • #14
                            I do a lot of standard fine pitches 16 to 20 at about 750 to 1000. You can actually get a pretty good rhythm going.

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                            • #15
                              Originally posted by bborr01
                              Dale,

                              Think of a shaft like from a mower spindle.

                              It will have a pulley/sheave with a nut holding it on.

                              Most times the threaded area is smaller than the pulley/sheave area.

                              That is why sometimes threading up to a shoulder is needed.

                              Hope this helps you understand a little better.

                              Brian

                              Hi,

                              I've had to single point up a shoulder many times. First, I relive to minor diameter at the shoulder with a cut-off tool. Just like a stripper bolt is done. I like about 1/8" wide "landing zone". Then I turn the tool over, reverse the spindle and thread out away from the spindle. Doesn't even require messing with the lead screw because that will run properly in reverse.

                              Any 3-phase machine can do this. Heck, even my cheap crappy HF 8x14 is fully able to do it.

                              It just puzzles me I guess. Not that there isn't more than one way to skin the kittycat.

                              dalee
                              If you think you understand what is going on, you haven't been paying attention.

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