Major Diameter Before Threading

I've created a chart for the Major Diameter sizes for external threading. The information was pulled from the efunda.com site, heavy cropped and edited to be in a similar format to the handy charts from LMS.

http://doug.freeshell.org/files/machinery/Unified_External_Screw_Threads.pdf
I know this is an old thread but I just wanted to thank you DoogieB for posting the simplest chart ever.
I spent a good hour trying to find a chart for rod diameter prior to threading, just a simple chart for the most common sizes but it wasn't easy, I finally found it here on the good old HM. it's already printed and tapeed on the wall in my shop.
Thank you again DoogieB .
 
What is more important is the pitch diameter. It is the point of contact on your threads. It is an imaginary diameter on the thread where the v of your thread has an imaginary line that goes across your thread where the v has an equal distance that matches the sides to an imaginary line on the top of the v. Now draw a line on the top and bottom of the thread where the v s are equal to the sides and top of the v. There is a tolerance for your class of thread. To simplify it take a letter v now draw a line across the top of the v. It should be the equal distance as both sides are. When rolling a thread vs. Cutting a thread you would turn your stock down to the pitch diameter vs. The major diameter.
A simple formula for cutting internal threads is to subtract your pitch from your major diameter to get your drill size. 5/8 -8 thread. .625-.125. Drill is half inch.
 
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When rolling a thread vs. Cutting a thread you would turn your stock down to the pitch diameter vs. The major diameter.
Huh? If you turn your stock down to the pitch diameter it will be way too small and you will only have half a thread when you are finished... You indeed turn your stock down to a point between the minimum and maximum values of the major diameter for the class of thread you are cutting, and then cut the thread. Am I missing something?
 
Bob, what you said made a lot of sense to me , but I'm still not sure which diameter I should machine a rod before external threading. I found this "thread builder" on the net and this is how they describe diameters:
Diameters:
Major diameter: the maximum and minimum sizes of the outside diameter of a screw
Pitch diameter: the Maximum and minimum sizes where the 1/2 Pitch intersects within the "V" shape of the screw thread. (this is the most important dimension of a thread)
Minor diameter: The maximum and minimum sizes of the smallest diameter of the screw threads
Over wires distance: these are the maximum and minimum distances if using 3 wires of the same size to measure the dimension close to the pitch diameter of an external thread.
Wire Diameters: Diameter of the wires used to measure the dimension close to the pitch diameter the external screw thread.

http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx
 
Did you look at pictures to understand the different diameters? If so, you will see that the pitch diameter is a imaginary point half way along the flank of the thread, and it indeed defines the thread size/location. Read the first diameter on your list:
Major diameter: the maximum and minimum sizes of the outside diameter of a screw
That is the outside diameter of the stock, ready to cut the thread.
screw6.gif
 
Huh? If you turn your stock down to the pitch diameter it will be way too small and you will only have half a thread when you are finished... You indeed turn your stock down to a point between the minimum and maximum values of the major diameter for the class of thread you are cutting, and then cut the thread. Am I missing something?
When I refer to turning your stock down to the pitch diameter it is when you are rolling a thread not cutting a thread. It displaces the metal similar to knurling. It is a much stronger thread. Some times this type of thread is requested or for production when you may be doing multiple operations at the same time like on a screw machine where your drilling from the turret and at the same time a cross slide has a thread roller that comes in from the side. Full speed when thread rolling. The roller comes in and applies pressure and displaces the metal to form your thread.
When cutting a thread it is still your pitch diameter that classes the thread. Obvioulsy within reason but a few thou below or ten though won t make much difference as far as strength but it will make it easier to cut as long as you can still cut to your pitch diameter.
Take threaded rod for example. Major diameter is usually a bit below it s named thread major for sake of easing production.
 
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