# How to measure special threads



## Brento (Apr 8, 2022)

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?


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## mmcmdl (Apr 9, 2022)

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 .


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## Brento (Apr 9, 2022)

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.


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## mmcmdl (Apr 9, 2022)

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 !


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## mmcmdl (Apr 9, 2022)

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 !


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## Brento (Apr 9, 2022)

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.


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## mmcmdl (Apr 9, 2022)

You're ahead of the game then .   You ever wonder why cad plated bolts never thread correctly ?


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## Brento (Apr 9, 2022)

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.


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## Brento (Apr 9, 2022)

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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## Brento (Apr 9, 2022)

No i dont Dave


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## mmcmdl (Apr 9, 2022)

26 TPI . Single is .0251 so double height would be .0502 to make it easy on you .


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## Brento (Apr 9, 2022)

?


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## KyleG (Apr 9, 2022)

Since my copy of Machinery’s Handbook lives at work, I use this website at home:





__





						UN imperial screw thread calculator
					






					theoreticalmachinist.com
				




It will do standard as well as nonstandard diameters and pitches, and it gives tolerances for the various thread classes. It also gives the “over wires” measurements if that’s how you measure them.


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## SLK001 (Apr 9, 2022)

This site gives you all the data you probably need.  If your thread isn't in the pull down menu, then you can enter it in the "Basic Diameter" and "Threads per Inch" boxes.


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## Chewy (Apr 9, 2022)

Try this:
3/8-26
OD .375
Core .3340
Radius .006414
Depth .0205
Pitch .0385
Effective Dia.  .3545
TPI 26  
Tapping Drill 8.5mm


1/4-26
OD .250
Core .2090
Radius .006414
Depth .0205
Pitch .0385
Effective Dia. .2295
TPI 26 
Tapping Drill #4

60 Degree Cycle Engineers Standard Threads


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## Chewy (Apr 9, 2022)

Here is a link to files:
anyflip.com/yafd/anxo
My copy was found on oldengine.org which is now defunct.  Found a post that the new site will be:  
mikeandjensoldengines.com

I printed out all the pages and keep them in a thread notebook, along with Machinery pages and any other site information. Saves a lot of time figuring out what I have.  And a WHOLE LOT LESS pages then Machiery pages each time!!!!

Always looking for obscure and ancient knowledge.  If any of the readers have bookmarks, please post some.  I learned a long time ago that if I have a question, most likely somebody in the past already found the answer.


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## Brento (Apr 9, 2022)

@Chewy i did find the threads from you link. I will be saving that for sure. I did not find tolerances however like class fits for it in that link.

@SLK001 @KyleG i tried there at first and it wouldn’t work but bc bozo me didn’t think i could put in different information.


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## Chewy (Apr 9, 2022)

They may not have class fits.  I'm thinking that was stuff developed later on when standards became necessary.  If you have the original, simply make the new ones to the same size.


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## Brento (Apr 9, 2022)

I believe the customer is sending me nuts and some dies


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## Chewy (Apr 9, 2022)

Word of caution.  Don't be surprised if the dies are carbon steel and not HSS.


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## Brento (Apr 9, 2022)

My plan is to single point most of the stuff if i can. Depends on how many he needs. He is sending me stuff to quote prices.


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