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Letter drills: what was the system developed for?

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  • Letter drills: what was the system developed for?

    Y'know... out of all the sets of drills (mm, fractional, number, letter) the letter set seems the least used and the most ambiguous. Is there a logic behind these sizes?

  • #2
    I heard somewhere that Letter Drills originated in the music industry.
    Piano wire sizes?
    Just to be a smarta$$, I always ask for a Letter E drill instead of 1/4"


    • #3
      I don't think they are so little used, you need some of them for tap
      drills and the F makes a very nice hole for a 1/4 rod when used as a
      "tommy bar". I just used one today for that purpose. They started
      with number 1 at an inconvient place so when it was necessary to
      go bigger I guess letters was a better choice than negative numbers.


      • #4
        Probably just evolved with no logic, like wire gauges

        No real answers here -

        but some useful tables (no chairs)



        I used to be indecisive. Now I'm not so sure , but I'm not a complete idiot - some bits are still missing


        • #5
          Letter drill sizes are simply a continuation of the numbered drill size progression. It beats calling them 0 00 000 0000 0000 00000 000000 0000000 ... as some gage sizes do and negative numbers would be confusing

          The history is pretty sketchy but dates back to the chaotic early days of wire gages where there was a lack of formal standards and about 19 different wire gage systems in use.

          Numbered drill bits are basically the same size as a stubs steel wire gage but about 1 mil oversize (half a mil on really small ones). So basically, they are a clearance hole for the same numbered/letter wire. The stub gage sizes also switched to letter for sizes 0.234" and above.

          Stubs steel wire guages are not the same as Stubs iron wire gages (aka English Standard Wire and Birmingham gages).

          My 1920 edition of American Machinists Handbook shows letter and number sized drills. The stubs IRON wire gage was adopted as a formal standard in Great Briton in 1884 but dates back to the early 19th century. It is still used for medical needles, catheters, and sutures. Stubs Steel Wire Gage lives on in drill bits.

          Stubs made wire, including tool steel wire, and drill bits.

          In 1887 there were 30 gage systems in use, including 19 for wire.

          As for the infrequent use, in large rod sizes small differences in size are less important and fractional sizes give an adequate rage of sizes for most uses and the standardization of bolts quite some time ago dictated the use of fractional sizes for 1/4" and above.


          • #6
            Whitis, that was very informative. Thank you.


            • #7
              i had wondered myself,briefly. after years of putting up with "almost" fits and difficult tapping i broke down and bought a set of 115. wow! almost everything fits now and tapping is much easier .


              • #8
                Originally posted by wooleybooger
                i had wondered myself,briefly. after years of putting up with "almost" fits and difficult tapping i broke down and bought a set of 115. wow! almost everything fits now and tapping is much easier .

                I got one of the lower-priced TIN coated 115 drill sets. If I need a special size, I drill a close pilot hole with my regular fractional drills and then use the special ones to open the holes up to exact size. So far, I've still got all 115 drills.

                They are very useful.


                • #9
                  Cool post Whitis. I've often wondered where the letter/number system came from...
                  "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."


                  • #10
                    A very old machinist once told me-----

                    that he was told the number drill system was developed in a drill making plant where they were at a total loss of what to call the 80 very odd sizes they made for special order customers. They simply gave the job to the youngest apprentice, and he began by testing all the drills. He found that the old drilling machine he used broke 80 of the tiniest drills they made before he got a good hole, but he also found that he only broke 1 of the biggest they made before getting a good hole.. and of those in between the smaller they were the more he broke. So they simply numbered the drills from one to eighty. Its a nice story, patently untrue, but reassuring when you break a few tiny drills in one day. I like it. regards David Powell.


                    • #11
                      Great story, but it overlooks the effort to create bodies of standards
                      to help facilitate the Industrial Revolution.

                      Reputations and fortunes hung in the balance back in the mid-late 1800's.
                      As individual firms innovated new processes, they developed their own
                      internal standards, some better than others. Afterward, bodies were
                      convened to help distill competing proprietary standards into one or a
                      few common standards.

                      Samual Morse, inventor of the Twist Drill is said to have adopted
                      the Lancashire wire gauge standard. So it helps to look back at how wire
                      gauge standards evolved.
                      It is believed that they originally were based on the series of
                      drawn wires, No. 1 being the original rod, and succeeding
                      numbers corresponding with each draw, so that No. 10,
                      for example, would have passed ten times through the
                      draw plate.

                      Last edited by EddyCurr; 08-27-2010, 11:44 PM.


                      • #12
                        One question comes to mind:"Why are we still using this crap"?


                        • #13
                          EddyCurr, thank you as well. That is the first time I've heard a logical explanation for why the numbers progress inversely to the diameter size.


                          • #14
                            Originally posted by juergenwt
                            One question comes to mind:"Why are we still using this crap"?
                            Good question, Can I get a 1/128" drill set sir?
                            or a 1/256.
                            Or just 1/250 set so I can actualy figure out what the size of the drill is in decimal without a calculator. -_-;
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.


                            • #15
                              One question comes to mind:"Why are we still using this crap"?
                              It makes the imperial system look more feature rich, with much more selections. Compared to the metric system.

                              It also supports the printing companies. I mean, just saying "I want a 3.2 mm drill" and getting one with ... TATAAA ... 3.2 mm without looking up any tables and wasting time browsing through books ...