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i hate to ask but. . od threading formula

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  • i hate to ask but. . od threading formula

    how do you search for the info. in a past post someone described how to find the actual od for turning threads based on number of threads and mean diameter. or maybe someone real nice will just post that formula again and i promise i will not ask ever again. thanks very much.
    davidh (the old guy)

  • #2
    The OD is the diameter.Not shure what you are asking. root diameter , pitch diameter are what size to drill are bore hole for intermal threads. ?
    Every Mans Work Is A Portrait of Him Self
    http://sites.google.com/site/machinistsite/TWO-BUDDIES
    http://s178.photobucket.com/user/lan...?sort=3&page=1

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    • #3
      Originally posted by davidh
      how do you search for the info. in a past post someone described how to find the actual od for turning threads based on number of threads and mean diameter. or maybe someone real nice will just post that formula again and i promise i will not ask ever again. thanks very much.
      davidh (the old guy)
      it is all in the pitch (tpi).
      this is not exact but typically just works:

      .65 / tpi ~-> depth/height of thread to be added/subtracted from nominal diameter you happen to be using.

      .75 / tpi ~-> total amount to move the compound set at 29.5 deg (start at zero)
      --
      Tom C
      ... nice weather eh?

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      • #4
        oh crap i didn't make myself clear. i want to thread 1" fine thread on a shaft. what diameter should the shaft actually be turned to before i start threading ?
        aah, that make more sense that last nights question.

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        • #5
          A 1" shaft should be 1". It can be slightly undersize, and standard 1" cold rolled steel or free turning steel can be used without further machining.

          Threading can upset the material slightly, and cause the finished thread to be oversized, light application of a file will correct that.
          Jim H.

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          • #6
            Tables etc.

            Try these - but you will get a better image if you down-load them and view them in your preferred viewer (I prefer "Irfanview" - but anything that can ear a *.jpg file will do:
            http://i200.photobucket.com/albums/a...w_threads1.jpg

            http://i200.photobucket.com/albums/a...w_threads2.jpg

            http://i200.photobucket.com/albums/a...w_threads3.jpg

            http://i200.photobucket.com/albums/a...w_threads4.jpg

            http://i200.photobucket.com/albums/a...w_threads5.jpg

            http://i200.photobucket.com/albums/a...w_threads6.jpg

            http://i200.photobucket.com/albums/a...w_threads7.jpg

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            • #7
              "or maybe someone real nice will just post that formula again"

              Here it is, just subtract 1% from the nominal diameter.
              For example, 1/2 inch diameter rod would be turned down
              to .495" before threading ( .5" - .005" = .495" ). If you
              forget again it's in "Machine Shop Trade Secrets" by James
              Harvey or you can just find the correct diameter in
              "Machinery Handbook".

              Ed P

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              • #8
                ten thousand thanks. . . . . got it and copied it. . .
                davidh

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                • #9
                  The Unified thread form is standardized. People new to machining should look into it and make a few practice pieces that assemble together before moving on to making working parts. All screw thread manufacturing data is either reduced to formulas based on nominal data or available in tables of preferred sizes. At the very minimum the home shop owner should have in his library some common reference book of machine shop practice. "Machinery's Handbook" is but one of them.

                  Most everything in the machinist's trade is governed by one standard or another. You may rebel against it but if you wish to make parts that interchange with the outside world you have to at some point reconcile your work with established standards. Only home model makers and builders of one of a kind apparatus have the luxury of flauting standards in their work. Even then standard items like ball bearings, common machine screws, snap rings etc have to fit somehow in their non-standard whole.

                  I know of no standard formula deducting 1% of the nominal diameter for the actual diameter. It works very well but it's not a standard way of doing things. The OD tolerance is spelled out in a table for the 1 -12 UNF class 2A (even the designation is standardised) thread you pose for your example but it varies slightly for thread class. I don't mean to be snobbish about this but standards do rule the machine shop. They are an agreed way of doing things that promotes interchangability and common practice. Thus a shaft made in Brazil and a bearing made in Indonesia can be assembled in China with castings from Poland, machined in Italy that can be sold in Scandanavia, transported to Anartica or Africa and installed without fitting or alteration into French or Russian built equipment there and serve reliably for years.

                  Since a large part of a machinist's effort involves compliance to one standard or another and his success in the shop dependends on his skill at doing so. It only follows that a book of such standards would be an important asset to his work.

                  So:

                  1) Start a library.

                  2) Let the first purchase be "Machinery's Handbook" (12th edition or later) or some other commonly available machine shop reference.

                  Purchased new shop references can be expensive. You can find them on eBay for $10 and less.
                  Last edited by Forrest Addy; 08-26-2008, 01:44 PM.

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                  • #10
                    I totally agree with Forrest on this. Buy a Machinery Handbook and other refference books and use them.

                    You must use the accepted standardized measurements for interchangeability.
                    It's only ink and paper

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                    • #11
                      Originally posted by Carld
                      I totally agree with Forrest on this. Buy a Machinery Handbook and other refference books and use them.

                      You must use the accepted standardized measurements for interchangeability.
                      And I'd be happy to sell you a 24th ed. I have a spare. :-)
                      ...lew...

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                      • #12
                        On the major diameter being the theoretical diameter, in this a case a 1" thread. The only external threads that actually are that way are class 3 threads, and that is the absolute max.

                        I like the theoretical minus 1%, its not "correct" but from the threads I just looked at real quick, it will land you in the correct diameter for pretty much any standard thread, coarse to extra fine, any class.

                        As for the Machinery Handbook, I have quite a few, and from the 40's until now, there really isn't much of a difference. I don't understand how they keep coming out with new versions and people keep buying them. On the 26th edition I have they haven't even gotten around to putting R-8 tapers in there. If you want 40 pages of data on some obscure taper common in the roaring '20's its in there, but if you want some info on something that has been in use for only 30 or 40 years, you are SOL.

                        The 24th edition had one sentence on J-threads, which a lot of people deal with a lot(especially me), the 26th and 27th editions don't even have that one sentence. Want to know about a spindle nose that has been in common use for 30-40 years, its not in there.

                        The only truly useful thing in there, is basic thread data, thats all I ever use it for, some of the cutting data is OK, but thats outdated, the fastener data is good, but when was the last time that changed and how often do you need that.

                        There has to be a better book out there.

                        Quick summary, grab an old one off of e-bay, because there is some neat stuff in there, just not a lot of useful stuff for anything remotely modern, no point in wasting $$ on something new that hasn't changed in 50+ years.

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                        • #13
                          Forrest Addy wrote:
                          "I know of no standard formula deducting 1% of the nominal diameter for the actual diameter. It works very well but it's not a standard way of doing things."

                          You are, of course, absolutely right. I should have been more specific
                          and said that it was not the standard but "a rule of thumb".

                          Ed P

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                          • #14
                            Originally posted by bobw53
                            There has to be a better book out there.
                            There is.
                            Modern Machine Shop's Handbook for the Metalworking Industries:

                            http://www.amazon.com/gp/product/156990345X

                            My MH now sits idle.

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                            • #15
                              thanks for the input. i do have a 19th machinerys handbook i bought new when i started my first job (about 100 years ago) and found it tonite, buried in a bunch of other stuff.

                              so i attempted to cut my threads tonite in a turned down shaft. . . i turned it to .990 for a 1"-8 thread . how good am i ? ? ? well, all went quite well, tool was resharpened and properly shaped according to the fish tail i have. great, now i just need to concentrate on this and take small cuts. . .

                              i started out with .005 on the compound for the first three or four cuts. ( i have a decent lathe from the tool room at diamond tool and horseshoe) i thnk its a sheldon 13. anyway, got to the final cut and i didn;t back out of the damn thread fast enuf and the material turned in the chuck. . . just a tad, but enuf to completely mess up the next cut. . .

                              so if i did this every day or even every week i would be very disappointed in what i did, but making threads about three times a year and i will forgive myself for being carless. not checking and double checking everything.. i should have tightened the chuck that extra ummph when i put the stock in it. and i should have backed out the cross feed a tad sooner.

                              the good part, i scanned a bit of the chapter on threads in the machinerys handbook before i started. all the info i needed was right there as i was told. but sometimes its more fun to ask as so many in forum are so very eager to share their knowledge. for that i am grateful.

                              i still don;t know quite enuf to be dangerous but i getting close.

                              thanks for the replys. i will attempt another threading episode tomorrow night.

                              davidh (the old guy)

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