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Q: U.S. threads & my lathe

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  • Q: U.S. threads & my lathe

    hello all.
    was hoping someone could clear this up for me.. the thread cutting table on my lathe shows inch-threads in fractions.

    for example:


    i needed to cut a 20tpi thread today.. and my thoughts went like this: 20tpi is 20 threads per inch so 1/20 is about 0.05 inches per thread. i dialed in 13/256 because 13 divided by 256 = 0.050

    i ran a skim cut (scratch!) and it *looks* about right. i dont want to take the plunge and screw up the job.

    am i using the table right? why doesnt it just read "20 tpi" ?? are they fractions that i should be dividing?

    i do metric 99% of the time (i'm in europe) and the metric tables tell me the pitch directly. (1, 1.25, 1.5, etc)

    anyone shed any light?

    ps.. the lathe can also do "D.Pitch" threads.. anyone know what that means?

  • #2
    So I presume you are staying at the Villa Philadelphia in Bulgaria?

    I expect that your metric lathe is providing the closest fractional settings of the metric gearing that approximates the english thread of interest. It can't really say 20tpi since 13/256 = .05078125... Close, but not 20 tpi. Over one inch it will be off by .0156". Over three inches it will be off by one full thread.
    Free software for calculating bolt circles and similar: Click Here


    • #3
      thanks, but its got a big honk'n lever
      that allows me to select:
      1) metric
      2) inch
      3) whitworth
      4) d.pitch (whatever that is!)

      to use the techincal jargon: stuff clogs around inside when i throw the lever!

      villa bulgaria?


      • #4
        Your profile says Location: Philadelphia. The only Philadelphia I could find in Europe is the Villa Philadelphia.

        D. Pitch I would guess is Diametrical Pitch. Not sure why on a lathe, maybe someone else knows. If the lever may be set to inches then how did you come up with 13/256? Can you not do 5/100 (both metric pitches)?
        Free software for calculating bolt circles and similar: Click Here


        • #5
          The DP is definitely Diametral Pitch. DP refers to the SEA gear tooth system. The reason for having DP on a lathe is for cutting worm shaft tooth forms for worm gear set-ups. To find the lead of a DP in inches simply devide by pi.In the SEA and Imperial thread systems the outside diameter of the thread is the first number and the second is the number of threads per inch. Its just another way of getting to the same end point. When I used to have to cut some odd ball threads on Hardinges that weren't in the QC or the change gear chart I'd use the formulas in the Machery's Handbook. IIRC you had to know the actual lead of the thread in inches. For SAE threrads simply take the inverse of the threads per inch
          Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.


          • #6
            >If the lever may be set to inches then how >did you come up with 13/256?

            well, the side of the lathe has a huge placard full of tables. inch/metric/dpitch

            the Inches table doesnt have the numbers i'd expect: 16, 20, etc. instead it has all those strange fractions.. like "13/256".

            says it right on the lathe "13/256"
            i follow the instructions and set all the levers accordingly. only problem is, i dont know what pitch 13/256 is going to get me! i'm hoping 20tpi.

            0.05 in metric would correspond to something in between 1.00 mm and 1.25 -- and i dont have an in-between setting. this, in fact, is how i realized i had an inch thread in the first place.

            thanks for the input so far, i do appreciate it.


            • #7
              This is an example of why non-users of inches are confused by what are claimed to be "english" or "US" units (but aren't).

              Nobody but carpenters uses fractional inches for anything serious.

              The legacy drill sizes in fractions etc are really just decimal sizes, i.e. 3/8 = 375 thousandths, 1/4 = 250 thous, etc. Drill cases here are marked with both the fraction and the decimal for each drill.

              But, metric conversion tables created elsewhere always seem to put fractions down as the sub-inch measurements. No wonder there is confusion and wonderment as to those crazy American inch units.

              Inch measurements for "our" purposes use decimal inches, which is essentially exactly like metric, because we ignore feet or yards, or chains or rods, or other goofy special-purpose length measurements. Just the length of the base unit is off by a factor of 25.4 (2.54 cm/inch or 25.4mm/inch).

              The "rational" (ratio-based) numbers don't always cover the need.

              Now, that table you have....

              1/16 would presumably be 16 tpi directly

              19/512 is probably 27 tpi, but isn't exact.

              I dunno what 11/28 should be, its nearly 2.5 tpi

              9/256 figures to 28.44 tpi....did they mean 28 or 29?

              In mm, it makes much more sense, the fractions are then seen as approximations to "standard" mm pitches. Maybe they just marked the fractions that were closest to the "native mode" metric pitches?

              1/16 would be about 1.59mm pitch (1.6?)

              19/512 would be about .942 mm pitch (0.95?)

              11/28 is 9.98mm, almost 10mm pitch

              9/256 is 0.893mm, did they mean 0.9mm?

              Looks like the folks assumed "US" measurements are fractional (which they are not) and marked the closest fractional equivalent to each metric pitch. it ends up messing you up worse than leaving it alone.

              If you KNEW you had to convert, you could just take the inverse of the pitch and get a tpi out of it. Pitch is in units/turn, while tpi is in turns/unit.

              So a 1mm pitch is 1/25.4 inch per turn, or 25.4 tpi

              Then also, 24 tpi is 1/24 inch per turn, or 25.4/24 = 1.06 mm pitch (actually 1.058333333)

              Now, your 13/256 will get you 19.69 tpi. This is about 1.29mm pitch.

              A real 20 tpi would be a pitch of 1/20, or 0.05 inch. This comes out to 1.27mm pitch.

              [This message has been edited by J Tiers (edited 04-21-2004).]

              [This message has been edited by J Tiers (edited 04-23-2004).]

              Keep eye on ball.
              Hashim Khan


              • #8
                I'm guessing that when you set it to inches it throws in a transposing gear set. This alters the ratio of the spindle to the leadscrew. However, you still have a metric leadscrew and a metric gearbox so the required ratios will be strange numbers. If for instance your lathe has a 4mm acme pitch leadscrew the spindle must then turn 3.1496 times for each turn of the lead screw to produce 20tpi.

                Do you have a SAE thread pitch guage? Just do it empirically and see how close it is.

                J is right, I very rarely work in fractional measurements, even with fractional drill bits. I use my dial calipers to verify the size of drill bits in decimal inches.
                Free software for calculating bolt circles and similar: Click Here


                • #9
                  I'm assuming the Withworth selection is equally screwed up?If so it may not be possible with the gearing you have in place,you may have to cut something close and chase it by hand with a tool made from a tap or die with the right pitch.
                  I just need one more tool,just one!


                  • #10
                    An exact conversion between English and Metric threads is accomplished by a 100:127 tooth compound gear. This ratio (1:1.27) is exact. However, 100 and 127 tooth gears are big and costly and there are some combinations that produce a very close approximation of this ratio and can be used for any but the most demanding threads. The 37:47 ration is often used and the error is only 0.02%.

                    Other ratios can and are used. The Chinese import mill-drills usually have a downfeed that is marked in inches but is really metric and assumes that 25mm = 1". That's off by 1.6%. A 4:5 gear ratio would be off by 1.5748%.

                    I suspect that your lathe does change something when you select English threads and that the compound gear used is close to this 4:5 ratio. The fractions you see on the thread table are likely another approximation to the real thread pitches produced and therefore are even more misleading. The person who made up the table is not very familiar with how English threads are usually specified. Metric threads are usually speced in terms of pitch or the distance from one to the next while English threads are usually speced in terms of the number of threads per inch.

                    The threads will be good enough to fit over a short length for fastening purposes but I wouldn't make any lead screws with them.

                    Paul A.
                    Paul A.
                    SE Texas

                    Make it fit.
                    You can't win and there IS a penalty for trying!


                    • #11
                      I have three East European lathe here that are close to what you have.
                      My big TOS does swing over easily between metric and imperial as it obviously has a decent designed gearbox. The other two, one Czech, one Russion cut all metric in 4 bands but only two bands mean anything in imperial. The other bands are gibberish, things like 17.5638 TPI

                      Where the difference come in, is that the TOS has an imperial leadscrew and conversion gears. The other two have metric pitch lead screws and it doesn't work the same way.

                      There is a work around, you set the gearbox to the exact pitch of the leadscrew and then use change wheels on the drive end as if you were doing metric on an english lathe with no gearbox.
                      You will need trasnsposing gears and you won't be able to disconnect the leadscrew once engaged. Moving back and forward under power is the only option.

                      Any chance of a photo of the change wheel plate, and letting us know a list of the input change wheels and the pitch of the leadscrew in mm.

                      Without this everyone is guessing.

                      John S.

                      Just reread the orginal post and need to ask a question.
                      If you set to 19/xx whatever to get you 20 tpi and scratch a thread on can you do another deeper pass without loosing pitch because you disengaged the 1/2 nuts or do you wind back ?

                      [This message has been edited by John Stevenson (edited 04-23-2004).]

                      Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


                      • #12
                        thanks, everyone, for your input. it is much appreciated.

                        i have some pictures, hope they turned out okay and are legible.


                        there will be 4 photos there "lathe1" through "lathe4"

                        i may show my inexperience by saying this, but i dont think i have any change gears to speak of. that is, the backcover of the lathe reveals only the drive belts. to remove (or change) any gears, i'd have to break the entire gearbox down. so i'm guessing (hoping!) thats not one of my options.

                        john: after the 'scratch' pass i wind back.. reverse direction. never disengage the 1/2 nuts.

                        thanks again,


                        • #13
                          Thanks for the pics.
                          Can we have a better one of the righthand side of the chart?
                          It looks to me as if there is a band labled Whitworth.
                          If so these are are the imperial ones and not the weird ones marked inches.
                          It also looks as if you have an imperial leadscrew of 4 threads per inch. If so then you don't have to keep the 1/2 nuts engaged all the while.

                          John S.

                          Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


                          • #14
                            thanks again. yes i do have a Whitworth scale. it does have all the proper "inch" settings, but i've never used it since i have a table that says INCHES right on it.. and those zany fractions.

                            you're telling me the whitworth will cut a 20tpi if i set it to 20? was it that easy?

                            please explain why i can release the 1/2 nuts.. i always thought that doing so meant losing my timing. (i dont have a thread indicator on the lead screw)

                            and how do i get these nice clean metric threads if i have an imperial leadscrew?

                            just when i thought i knew how to run the thing!


                            ps.. so what exactly is the difference between 'imperial', 'inch', and 'whitworth'? imperial and inch sound to be the same.. but why would i have two tables?


                            • #15
                              <font face="Verdana, Arial" size="2">Originally posted by knucklehead:

                              you're telling me the whitworth will cut a 20tpi if i set it to 20? was it that easy?
                              might be, I think I may agree, but I also want clear pic of right side of charts.

                              <font face="Verdana, Arial" size="2">
                              please explain why i can release the 1/2 nuts.. i always thought that doing so meant losing my timing. (i dont have a thread indicator on the lead screw)
                              You need a thread indicator, and it would be different for metric and inch. With imperial thread, a metric one is do-able but kinda silly.

                              <font face="Verdana, Arial" size="2">
                              and how do i get these nice clean metric threads if i have an imperial leadscrew?
                              Probably it simply has the right gear ratio to advance the carriage properly. It can be done with almost any pitch of LS, but can get pretty silly for some.
                              A "native mode" leadscrew simply makes more things come out to even numbers of teeth with smaller gears.

                              <font face="Verdana, Arial" size="2">
                              ps.. so what exactly is the difference between 'imperial', 'inch', and 'whitworth'? imperial and inch sound to be the same.. but why would i have two tables?
                              I dunno that. might not refer to what you think.

                              It looks like you have a feed table and a thread table. One seems to have the icon of threads, and the other has an icon that appears to be a partly turned workpiece.
                              This is in pic number 1 , the lever at bottom right, and the table above.

                              There is a wiggly line that looks like a thread for right-hand position, and one for left hand position that looks more like turning a part.

                              The gearbox seems to have ratio adjustments that cover many things. Not impossible that a metric to imperial is in there also.

                              I am not clear as to what you shift to move across the table that you showed in pic 3, where we see "metric 26-52" and "inch 26-52".

                              BTW, the 26-52 seems to refer to drive gears that connect the spindle to the box. There is an icon of three gears, two smaller and a big idler.


                              Keep eye on ball.
                              Hashim Khan