Stub Acme Thread Callout Dimension Tolerances to 4 decimal places Why?

[johnchrc]

I have called out SA threads to 3 decimal places for 20 years. Now, I have a manager who requires the dimesion be called out to 4 decimal places. We farm all our equipement out so its checked on Gagemaker, plug and ring gauges, 3-wire method or mating part, All of our SA threads are in 1/8 increments except for the special few.

EXAMPLE (New) EXAMPLE (OLD)
3.2500-6 SA 2G Box 3.250-6 SA 2G Box
Minor Dia: 3.1500/3.1583 Minor Dia: 3.1500/3.158
Pitch Dia: 3.2000/3.2231 Pitch Dia: 3.200/3.223
Major Dia: optional Major Dia: optional

Why is the extra 1/10,000 to 3/10,000 on the dimensions required? Especially for a 2G threads.

Could this be a weak attempt at controlling super thin coatings like NEDOX when no allowance included in the thread dimensions? We typically call out all dimensions apply AFTER coating. Your dealing with coating thickness of .0002-.0003 inch so coating is less than the freaking allowance.

WHY THE TIGHT THREAD CALLOUT?
Is it to match Machinery's Handbook tolerances?
I never have seen SA Threads called out to 4 decimal places.

The standard thread callout controls the plug and ring gage dimensions. It could be 3.25-6, 3.250-6 or 3.2500-6 SA RH 2G decimal place call out. What is the functional purpose for using 4 decimal places on pitch and major dimensions( smaller by .0001" to .0002" allowance? This would apply to all threads.

Just don't understand. None of the SA thread programs we use list dimensions to 4 decimal places. Had to make custom spreadsheet with 4 decimal places or use the Gagemaker Pro Program.

Is there a better forum than this question? Tried Machines and machining.

20 years and suddenly I have to change my thread call outs because of a freaking manager.
- CJ

- CJ

 

[israelkk]

You should follow the thread standard spec such as FED-STD-H28/13 SCREW-THREAD STANDARDS FOR FEDERAL SERVICES SECTION13
STUB ACHE THREADS. The standard gives the dimensions so you do not need to specify them on the drawing at all. By the way, pitch diameter and tolerances are specified to 4 digits accuracy. Only minor and major diameter in the standard are specified to 3 digits accuracy.

 

[johnchrc]

Why should we follow FED-STD_H28/13 Screw Threads for federal Services Section 13. I don't want o be any where near that document with my products.

Do they give dimensions for every single possible thread size combination? We don't always use std 1/8" size increments. At least they use 4 digits accuracy. Whay is that? Guess that is what I keep trying to find out even though it really dosn't mattet to me in the long run. Just stupid question that I was hoping for easy quick answer.

- CJ

 

[israelkk]

johnchrc

If you do not like FED-STD-H28 then how about Machinery's Handbook? You can not invent your new standard and still call it just ACME. You can call it any name you want but not ACME. May be modified ACME but then do not ask why the 4 digits. When you deviate from the standard specifications you need to give full documentation of the thread including the shape and dimensions of the thread elements not just Major, Minor, Pitch. If you decide to deviate from the ACME tolerances then this is not ACME.

 

[johnchrc]

Israelkk,

Now I must back up from my original question to respond to you...
We use Machinery's Handbook as the basis along with traditionally accepted industry practice. I can call the thread STUB ACME all day long and hundreds of thousands have been doing so for decades BECAUSE THAT IS WHAT IT IS. I have probably designed 1,000's of Stub Acme and other straight threads, "standard" and oddball sizes. Please don't make assumptions because you are req'd to follow some government spec that I assume aerospace must follow. Sorry to be so harsh but you are dead wrong and probably not appropriate to respond however, I do appreciate the fact you tried to help.

I am not familiar with your standard but I know know it is not used in my industry. ASME/ANSI B1.8-1988 (R1944) is the basis for SA thread dimensions, This is also basis in Machinery's Handbook.

What bothers me is I ask a question but instead answering, "I'm not sure, or "it could be...", I'm told I must follow FED-STD-H28/13 SCREW-THREAD STANDARDS FOR FEDERAL SERVICES SECTION13. Oh really, if you can show me where I'm required to follow that then I'll apologize and send you a large sum of money. I don't work on them there flying machines so Mr. Gov't doesn't tell me what to use... yet. Our controls are industry generated, as should be.

BACK TO MY QUESTION:
I have never seen the additional 4th dimension on Acme or Stub Acme thread call out. I am not claiming that this practice is wrong nor does it change anything except the # of decimal points to define Major, Minor and Pitch diam's. IT IS NOT THE BIG DEAL AS THIS THREAD WOULD SUGGEST.

I was just CURIOUS what the reason was, other than trying to be accurate to an additional degree. Like I said, most industry thread programs don't give you more than 3 decimal places in thread calculations. If coating, dimensions are applied after coating. does anyone know why my manager suddenly wants everything called out to 4 dimensions? I would ask my manager but he is out of the office and he scares the hell out of me (not really).

I have faith that someone on this website knows or has a guess and I would appreciate your input. Israelkk, take care...

- CJ