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  • Gaps in Unified Screw Thread Series

    Does anyone know why 25 and 26 th/in seem to be missing from the Unified Standard Screw series?

    Here's what lead me to ask:

    A friend of mine has an x-y vise that he is going to use on a small, antique manual drill press. The drill press' column is so close to the axis of the drill that the handle on the top-most feed screw will interfere with the column. So, said friend asked me whether or not I could drill and tap two holes on the other side of the vice so that he could swap the feed screw to the other side, thus avoiding the interference problem.

    However, this vise is made in China, and there is no documentation for it. Surprise! So, I would like to match up the drilled and tapped hole to match the existing bolts, so that said friend could just reuse the same bolts in the newly drilled and tapped holes. Of course, the alternative is to just pick some close-enough threads/inch of some common grade of bolt and be done with it, but if I did that I wouldn't learn anything in the process. And he would have the hassle of having some unused bolts lying around, which will get lost.

    So, not knowing if the thread is a unified screw thread, or metric thread, I used the screw pitch gauge I had at present which is only Unified. From my doodling with the screw pitch gauge, the threads/inch on the actual bolt is between 24 threads/inch and 27 threads/inch. OD of the threads mics as 0.228 inches (5.7912mm ... conspicuously close to the 6mm nominal size in the Machinery's Handbook).

    At this point, the basic question hit me: Why is there a gap in the screw pitch gauge between 24 th/in and 27 th/in?

    I pulled out my Machinery's Handbook 26th Edition, turned to page 1715, which has "Table 2: Diameter-Pitch Combinations for Standard Series of Threads (UN/UNR)" and scanned down the page. Sure enough, there is no 25 or 26 threads/inch anywhere in any column (including Coarse UNF and Fine UNF).

    I conjecture that maybe 25 th/in and 26 th/in is too close to 1mm pitch metric threads? By "too close" I mean that perhaps they figured that users will try to fit "close enough" metric bolts into unified holes, and vice versa, which might yield stripping of bolts. If that is true, then would that explain the other gaps I see in the Unified series?

    By "close enough" I did a quick calculation as follows (with way more precision than is warranted):

    24 th/in --> corresponding single-start pitch is (1/24) == 0.0416666666667in --> (0.0416666666667)*(25.4mm/1in) == 1.05833333333mm

    27 th/in --> corresponding single-start pitch is (1/27) == 0.037037037037in -->(0.037037037037)*(25.4mm/1in) == 0.94074074074mm

    And 0.94074074074mm < 1mm < 1.05833333333mm.

    So, perhaps the bolt is indeed has a metric 1mm pitch metric thread. I see there is a 6mm OD x 1mm pitch in the metric standard, so maybe that is the right bolt size (Nom.Size == 6, pitch == 1 from pg 1757 Table 2).

    Perhaps having Unified threads so close to what I suspect is a very common Metric pitch would be problematic?

    Thanks,
    bg
    Last edited by bgoodr; 06-18-2011, 01:56 AM. Reason: Add URL to picture of x-y vise

  • #2
    Perhaps because "elsewhere" there were/are several already?

    Brass Pipe, all sizes, are 26 TPI
    British Standard Fine 1/4" is 26 TPI
    Model Engineers 7/16" and 1/2" are both 26 TPI

    couldn't find any 25 TPI but that I could see being too close to metric (not absolutely but the "25" makes it easy 4x to get to 100 and in my mind that implies "easy" metric)

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    • #3
      26 threads per inch is very common, it is a bicycle thread.

      Comment


      • #4
        My SB doesn't cut 25 tpi but it does do 27, 26 and 24. 25 tpi is very close to 1mm pitch and would definitely pose a problem of confusion. Although there are many different metric thread "standards" they all include 1mm pitch at various diameters. There are some standard metric threads that are so close to SAE that they are interchangeable for normal thread engagement. 32 tpi is one which is almost exactly the same as .75mm thread pitch.
        Free software for calculating bolt circles and similar: Click Here

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        • #5
          I know you aren't asking about the interference problem, but I don't understand what the trouble is. On my drill press, one handle is to the right, the other handles are to the front. Rather than swapping the lead screw, have you looked at just removing the top section from the dovetail and re-installing it backwards?

          Pics, or it didn't happen.

          Comment


          • #6
            Coming from China, chances are the thread is metric.
            I once bought a QCTP from Shar's and was amused to find that the threads were all metric but because it was being sold into the US the hex socket and the hex height locking nut were both inch sizes. I discovered this because I wanted to put some longer screws in and it didn't match any inch size I could find...

            Michael

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            • #7
              Originally posted by RussZHC
              Perhaps because "elsewhere" there were/are several already?)
              Yes. I guess that is the root of my problem in that the "elsewhere" seems to be spread out over the Handbook versus being sorted all by TPI. I looked all over this huge handbook in vane for the search button.

              Comment


              • #8
                Originally posted by The Artful Bodger
                26 threads per inch is very common, it is a bicycle thread.
                Yep. The index in the Handbook did not show anything for "bicycle thread", and I looked under "thread" and "screw". I wonder if there a online (free) reference for all thread types sorted by TPI and OD?

                Comment


                • #9
                  Originally posted by Evan
                  My SB doesn't cut 25 tpi but it does do 27, 26 and 24. 25 tpi is very close to 1mm pitch and would definitely pose a problem of confusion. Although there are many different metric thread "standards" they all include 1mm pitch at various diameters. There are some standard metric threads that are so close to SAE that they are interchangeable for normal thread engagement. 32 tpi is one which is almost exactly the same as .75mm thread pitch.
                  Ok, then that lends weight to the theory that the bolt I have at hand is a 1mm pitch thread. Is there an online, free, searchable reference for which Metric and Unifiied threads and OD sizes are considered to be interchangeable?

                  bg

                  Comment


                  • #10
                    Originally posted by Boostinjdm
                    I know you aren't asking about the interference problem, but I don't understand what the trouble is. On my drill press, one handle is to the right, the other handles are to the front. Rather than swapping the lead screw, have you looked at just removing the top section from the dovetail and re-installing it backwards?

                    Pics, or it didn't happen.
                    The only pic is pretty useless for this stage of the thread, as it is quite microscopic (is in the original post too): x-y vise. I don't have the drill press and its adjunct xy vise nearby so as to snap some digital photos, but I've requested additional pics from my friend, and will post them back here if he is forthcoming with them.

                    bg

                    Comment


                    • #11
                      There is no law that states a certain thread diameter/pitch must be used. Anyone can make anything he choses for any variety of reasons.

                      For convenience sake, most common fasteners are made to metric or imperial specs, but after that anything goes. Your thread is probably an approximation of a metric thread made for that specific application and chances of finding an over the counter replacement are between slim and none.
                      Jim H.

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                      • #12
                        Originally posted by JCHannum
                        ... and chances of finding an over the counter replacement are between slim and none.
                        Understood. My current plan is to buy/borrow/steal a tap for 6mm OD X 1mm TPI, drill the appropriate hole in some scrap, cross my fingers, and test it out with the bolt.

                        bg

                        Comment


                        • #13
                          Slightly off and on topic.

                          Originally posted by JCHannum
                          There is no law that states a certain thread diameter/pitch must be used. Anyone can make anything he choses for any variety of reasons.
                          Well thats the truth. I saw a paint stripper pad at Home Depot that has the statement that it could only be used with their special tool. I thought hell, that looks just like a 5/8" NC standard grinder thread. Wouldn't go on my grinder! Had to run a tap through it to open it up a bit. It was a 5/8 NC but undersized so it would only fit their tool. What tools!

                          Comment


                          • #14
                            To answer the basic question, the threads commonly found on English System screws (as opposed to metric system screws) are/were based on the gears available for change gears on lathes of yesteryear.

                            The first idea you need to understand is that to cut any given number of TPI, you MUST have ALL the prime factors of that number of TPI somewhere in your gear train or on your lead screw. Most English System lead screws have some simple number of TPI like 4, 6, or 8. A 6 TPI lead screw provides the primes 2 and 3 (2 X 3 = 6) and the 4 and 8 TPI lead screws provide only 2s (2 X 2 = 4 or 2 X 2 X 2 = 8).

                            Since 25 = 5 X 5 and 26 = 2 X 13, these two threads would require gears with two 5s or some multiple of 13 respectively.

                            For maximum versatility, change gears were made in sets "by fours" or "by" some other common number. Or in other words, multiples of four. Thus a set of change gears "by fours" might start with a 16 tooth gear (4 X 4) then the next one be 20 teeth (4 X 5). Then 4 X 6 = 24 teeth. Etc. for 28, 32, 36, etc. This would provide a very versatile set of change gears and if you were allowed to compound them, you could make a very large number of threads with such a set of “by …s” change gears. But there would be only one gear for each multiple of 4 or other base number. The sequence continued up to a number like 15 or 20 so the largest gear in the series may be a 60 - 80 tooth.

                            So, in direct answer to the original question, 25 TPI would require two factors of 5 (5 X 5 = 25). Now, the common sets of change gears did not have any gears with 25 teeth or a higher multiple of that number (50, 75, 100, etc.) Therefore, in order to cut 25 TPI, you would have to use two gears with multiples of 5 and one of them would have to be a compound gear (two gears locked together on one shaft). Or you would need a lead screw with the factor of 5 in it, but these are vary rare on English System lathes. Hence, no 25 TPI in common use.

                            But 26 TPI would be doable on most lathes and it is a fairly common thread as others have stated. Why was it excluded from the “standard threads”? Well; 20, 24, 28, and 32 TPI (note their common difference of 4) are fairly common and numbers like 22, 26, and 30 are half way between them. The plain fact is they are just not needed as 28 is a fairly small jump from 24. You can’t have everything.

                            The above assumes the use of manual change gears, but quick change gear boxes also use a limited set of gears and they operate in sequences much like the manual gears do. In fact, they are much more limited in the threads they can cut and if you look at many of them, you will see that multiples of 13 or 25 may not be included. Cost of more gears vs. their utility. Again, you can’t have everything.
                            Paul A.

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

                            Comment


                            • #15
                              An unidentified pitch that falls between 24 and 26 TPI is likely to be 1mm metric (25.4TPI). But as you can see from the explanation below, this didn't come about due to the possibility of metric/imperial confusion.

                              As standards have progressed, we have had fewer pitch/diameter sizes. As it is, retailers often don't stock the full range of fasteners in the preferred sizes. Imagine how bad it would be if we used all sizes. So, other than the US now doing imperial and metric sizes, the trend has been do eliminate unnecessary sizes. Over the past century, we have reduce the number of diameters we use, reduced the number of pitches we use, and we have reduced the number of pitches we use for each diameter. The cost of having too many sizes outweighs the benefits. Think about how many possible combinations there are of diameter, thread pitch, length, material, strength grade, head style, etc. for even a small number of choices for each.

                              The available settings on a typical lathe gearbox, such as the Norton gearbox, also would be impacted by the sizes used and have an impact on the sizes used. From 4 to 14TPI, you have every integer size. You even have some half and possibly 1/4 or 3/4 sizes but these are more artifacts of the real sizes (i.e. they are half of a real size). Above 14TPI, the odd sizes are eliminated. Above 32TPI, the sizes go in steps of 4. And above 64TPI, the step size is 8. And above 128TPI, the step size is 16. This is largely because the larger and smaller sizes are the result of multiplying or dividing the basic ratios by a power of 2 in the first stage of the gearbox.

                              Here is the threading chart from a South Bend 16" lathe.
                              Code:
                                  4   4-1/2  5  5-1/2  5-3/4 6  6-1/2  7
                                         1  9/8   5/4  11/8  23/16   3/2 13/8   7/4    
                                     ===== ===== ===== ===== ===== ===== ===== =====
                              1/2  |     4 4-1/2     5 5-1/2 5-3/4     6 6-1/2     7
                                1  |     8     9    10    11 11-1/2   12    13    14
                                2  |    16    18    20    22    23    24    26    28
                                4  |    32    36    40    44    46    48    52    56
                                8  |    64    72    80    88    92    96   104   112
                               16  |   128   144   160   175   184   192   208   224
                               
                              teeth|    16   18    20     22    23    24   26    28
                              The column with 23TPI seems to serve no real purpose for imperial threading. Even 100 years ago, those sizes didn't seem to be used. Maybe it was useful with a different change gear combination for doing gear hobs or something (3*24/23 is 3.13 within 0.355% of the mathematical constant pi). The ratios given assume an 8TPI lead screw and the last row is how many teeth you would mesh with a 16 tooth gear to get the ratio for that column. Each row is labeled with the power of 2 ratio from the first stage of the gearbox.

                              To cut a 25TPI thread, you would have needed a 25 (or 50) tooth gear in the train. To cut all the missing integer sizes from 1-80 would be a lot of extra gears. 17,19,23,29,31,37,41,43,47,53,59,61,67,71,73, and 79 are all new prime numbers - 16 extra gears. We already have enough primes with sizes which are multiples of 2,3,5,7,11, and 13 and sizes which are multiples of 9 which isn't a prime but isn't a power of 2 from one, either. Even with change gears instead of quick change gearboxes, you would have need two or three times as many gears, even though you can reuse gears in ways you can't with a quick change.

                              Now, many lathes are trying to provide quick change gear boxes that can do imperial and metric threads and failing at both. There are more ratios than they are willing to provide gears and supporting mechanisms.

                              But there is another fundamental reason for this change in spacing. And that is that the spacing between sizes (both diameters and pitches) should be roughly logarithmic in spacing. If you are dealing with a tiny fastener, then a difference of 50mils in size is a huge change in the strength of the fastener and the space it consumes. If you are starting at 100mils, if the next size up is 50mils larger could be 50% bigger diameter and 125% bigger strength. So, the sizes have to be closer together. But if you have a fastener that is 1" across, 50mils larger is only 5% bigger diameter and 10% more strength. It is just not worth it to manufacture and stock sizes so close together. The sizes that are normally used are, very roughly, a progression of about 20-25% larger each step. And, likewise, you only really need so many pitches per octave (factor of 2) or decade (factor of 10).

                              Even among what we have, we mostly use the UNC sizes, not the UNF sizes. And don't expect to find a #3 or #5 machine screw at your average local retailer. Odd numbered machine screw sizes aren't used much below #6 and have been eliminated over #6. All the sizes over #12 have been eliminated in favor of fractional sizes and even #12 isn't used much. A century ago, there were 22 ASME standard number sizes: you had #0 to #9 in steps of 1 and #10 to #30 in steps of 2. And each of them (except #0) had two, sometimes three thread pitches for 48 total sizes (not counting fractional sizes). Today, #4-40, #6-32, #8-32, and #10-32 are common with #2-56 and #0-80 being used in stuff like eyeglasses and watches and harder to find. I have needed a #5-40 once and it wasn't easy to find. #4-48 is used on your dial indicator tips (48 used to be the preferred pitch for #4). Only 4 of the original 48 sizes/pitches would you expect to find in the first random store you looked in.

                              Much of my info on older sizes comes from the 1920 edition of American Machinists' Handbook:
                              http://en.wikipedia.org/wiki/America...ts%27_Handbook
                              Especially the table on pages 326 and 327. That table isn't in the 1914 edition available online but would be roughly equivalent to combining the three tables starting on page 292.

                              History of the American Screw Company, founded in 1858 at a cost of $1million (about $24million today) and the dominant manufacturer when ASME started standardizing size. They employed 2500 people and had bought out two other screw companies (one founded in 1838) when they started.
                              http://www.artinruins.com/arch/?id=h...=americanscrew
                              https://www.facebook.com/group.php?gid=295002968347

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