How to measure special threads

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Brento

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So i have a 3/8-26 stud and a 1/4-26 stud threaded on both sides. Obviously not normal sized screw sizes. I have the formula to get the pitch diameter

Pitch diameter of external thread = Major diameter of external thread-(0.650*Pitch)
dp = d-(0.650*P)

Only thing id like to know is how to figure out allowances and tolerances. Obviously us machinists try to be spot on but how does one come up with the allowance for a tolerance or even a class fit?
 
Brent , here's what I was trying to explain , but it is somewhat complicated until you get used to it . There are constants for all size threads , regardless of major diameters . Constants work off thread pitch as well as class limits . Here is one of my books from my apprenticeship days . This has ALL the info one could ever need . I will say that it is pretty simple after seeing how everything works together . :encourage:
 

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I will have to run through the machinery’s handbook but i was have trouble finding 26tpi in the book. I swear i looked at every table to find a 26tpi.
 
There could be some trig work involved , but it's easy like I said . You do it a couple times and you're a master at it ! ;)
 
I'll add this .............................this really gets fun when your cutting threads for the Gubmint . Class 3 threads that get plated , anodized , hard coated etc . The plater / outside vendor ALSO has tolerances on the coatings . Some are in , some are out of the metal etc . In this case , all nominal dimensions have to be taken into account . As an average , the pitch diameter has to measure 3 times smaller than the nominal plating thickness . When finish plated/coated , this will bring the PD back into spec . ( Hopefully the vendor is good ) .

Is your head spinning yet ? I realize it's early ! :big grin:
 
I actually know about that surprisingly! My last job we had lots of parts that got coated. My problem is i didnt have to do any calculations. But i do remember there was lots of tolerances bc we had to leave room for plating.
 
You're ahead of the game then . :encourage: You ever wonder why cad plated bolts never thread correctly ? :grin:
 
I have never had to worry about plating or heat treat before but id like to know how to find limits for say thread classes and then from there it would be nice to learn how to account for plating. I am a big index card guy so even if it is in the book of hard to find and understand knowledge (aka machinery’s handbook) i would write the formula the formula definitions and step by step process on going through the formula using an example. That is where it is honestly nice to have someone who knows the formulas and how to do it teach us guys who cant.
 
I am very seldom a book reader and that is half the time my downfall. Looking in the machinery’s handbook is usually just going over my head when i read it or can never find the right info.
 
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