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threading ?

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  • threading ?

    Hey it has been a while but i don't have easy access to the net anymore. I still drop bye and read the posts. Glad you are feeling better Thrud.

    To my point. When making threads on a shaft i seem to have a problem as to what size to make the shaft and how deep to cut the threads. For example. If i was making a 1/2" by 13 thread shaft. Should i machine the shaft down to .490 and then cut the threads.

    Thanks in advance Spkrman15

  • #2
    I'm sure others can offer more authoritative advice, but in my classes we were taught to turn the blank screw about .005" undersize. That always seemed to work well for me.


    • #3
      recently i began to investigate the same type of problem when asked to make a custom bolt. While I had had my lathe for some time my threading skills were nil. After some reading and about 5 bolts here's what i have found. To cut the thread that you speak of turn the shaft to 1/2" then with the top slide set to 29 1/2" degrees feed a 60 degree threading tool bit perpendicular to the work piece for a distance of .742/pitch, or .742/13 which is .057. This is what I have read, however I found that when using "store bought" nuts (grade 5 from NAPA) the above calculation produces a fit that is a bit on the loose side. I have found that a feed of .71/pitch produces a very smooth fit with no objectionable play. Having said all that, it is only based on my reading and experience of cutting maybe 10 threads on a lathe. Perhaps one of these folks with years of experience can educate us?

      randy t


      • #4
        Depends on what type of cutter is being used.

        Using a simple 60 degree cutter:

        Look up in Machinery's Handbook or similiar thread reference chart for the major diameter of the thread size in question. For a 1/2-13 the major diameter for a 2A class is listed as .4985 to .4876". Turn to that diameter, stay on the small side for easier threading.

        Using a full profile carbide insert:

        With this tool the cutter creates the thread-vee and also establishs the major diameter simultaneously. In this case use any convenient starting diameter, 1/2" is fine.

        Whatever threading tool/method is used the final depth of cut needs to be measured. Thread wires are least expensive method, $30 set(?). Other methods are thread micrometers, expensive. And thread ring gages, also expensive.

        A general rule of thumb for threading:

        For external threads use the smallest allowable starting diameter as shown in the thread chart under major diameter.

        For internal threads use the largest tap drill size shown in the threading chart under minor diameter.


        • #5
          (deleted. ...dup post)

          [This message has been edited by lynnl (edited 03-03-2003).]


          • #6
            When i manual thread I turn down to the size (i.e. 1/2-13..turn down to 1/2")

            But when I thread on the CNC, I turn about .005 or so under so I don't have to file or sand the sharp crest...

            Get yourself a "Center Gage" looks like a little has all the thread depths on the back of the gage...Any catalog would carry one for about $4.00 for import or up to $13.00 for Starrett...get the cheap one...

            Very worthwhile investment...



            • #7
              a agree with brent i do it just like him. save time and don't make the major diameter slow you down. mic a bolt. i worked in a srcew machine shop and ill tell you they dont spend much time doing treads,die heads are fast but thread rolls kick butt big time.
              single point is what makes a real machinst who can make mutiple leads,(2 start,3 start,4 start,left hand metric or whatever).

              one time i had a fast job making a 1" 8 left hand for a pump drive shaft they olny needed 3 " of thread but the shaft was 15' long, i put it in the lathe and cut it in 5 minuts(in frount of the costomers) and the boss chewed my a$$ for going way to fast.
              threading will get you more money in a shop if it is different.
              i have done #4 40 to 8" pipe and left and right of all sizes and acme,metric,and british standards up to 72 mm.
              by the way if it is landing gear for a plane olny turn it .001 under and file till the nut or Gage goes on.
              good luck .


              • #8

                you wrote: "by the way if it is landing gear for a plane olny turn it .001 under and file till the nut or Gage goes on."

                Just so no one takes you seriously, this was a joke, right?

                Files have no place in life-critical threading operations. Actually, files have no place in any properly done threading operation.