Stub Acme Thread Callout Tolerance - 1/1000 or 1/10,000 inch

[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 on std thread gage, Almost all of our threeads are in 1/8 increments.

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.

Can anyone help explain?

- CJ

 

[BillPSU]

All threads should reference some standard unless your company has decided to create a new standard. Dimensions such as pitch diameter are a composite of thread angles, size, and symetry of the thread form. In the technical world of part measurement the 'normal' standard is any gauge must have resolution of 10 times greater than the smallest significant digit. Example is a dimension of .001 the gauge will have a resolution of .0001. Now if the dimension is .0001 the gauge will have a resolution of .00001. These decisions cost money in the gauging and procedures to measure these threads such a temperature control and sometimes other factors need to be controlled because of special materials.

The QA purist will now roast me for not using the correct terms and quoting various standards. But when you call out the standard thread you will also measure the thread with a standard gauge, period. Your manager may be one of those QA purist which you will never convince to change his/her mind. If you find out this point then you will need to decide if fighting a battle on specification callout is worth fighting.

 

[johnchrc]

The battle isn't worth fighting. The standard thread callout controls the gage and gagemaker dimensions. It could be 3.25-6, 3.250-6 or 3.2500-6 SA RH 2G decimal place callout.

Is there no functional purpose for using 4 decimal places for other than a tighter allowance ( smaller by .0001" to .0002" allowance?

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 Gagemenker Pro Program.

Is there a better forum than this question?

- CJ

 

[Cockroach]

Yeah, same here, being using three decimal call-out for twenty-five years. Funny how all of a sudden you need tighter tolerancing.

I think the confusion comes from the various class fits for the Acme, Stub Acme threads. If you have a sloppy fit, then the tight tolerance would obviously not be a consideration. But as the classification gets tighter, the max/min diameters begin to converge, so you need the extra decimal space to make a proper definition.

But a class 2G Stub Acme would only require a three decimal place tolerance.

Regards,
Cockroach

 

[johnchrc]

Cockroach -
I must assume the tighter tolerance is meant to be applied to class 3 fits where the allowance would be 0 at max. material condition and the extra .0001-2" tolerance would theoretically matter. So I think you are right on that the tight 3G & 4G fits where this comes into play.

Using four decimal places on a 2G thread is a waste.

It's interesting that in all of my R&D Facilities the "approved" thread calculators for design documentation have been essentially industry vetted for 20+ years but you can't change the freaking decimal places on locked output cells and they are all set at three decimal places.

- CJ

 

[MiketheEngineer]

And who could possibly measure it - resaonabbly?? And at what temperature.

Once had a guy show me a piece being measured to the nearest 50 millionths. Put my hands around the part and within about 10 seconds - it went to hell.

 

[hydtools]

Just taking the tolerance out of Machinery's handbook, Table 5, Diameter Increment + Pitch Increment, .006*(3.25^-2) + .01225 = .0231, would give 3.2000/3.2231. Just going by the numbers.
I'd call it a non-issue with slightly more tolerance, not less or tighter tolerance, than three-decimal callout.

Ted

 

[KENAT]

Can't remember on the ACME threads but at least for UNS threads I believe the relevant standard specifies that you need 4 dp.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[mrainey]

Can't remember on the ACME threads but at least for UNS threads I believe the relevant standard specifies that you need 4 dp.

Except for class 1B and 2B with minor diameters of 0.138 or larger.

Software For Metalworking

http://closetolerancesoftware.com

 

[johnchrc]

As hdytools said, the actual dimensional difference is very small and doesn't significantly increase tolerance. It is usually around 2/10,000".

I was thinking the thread tolerances were decreasing when I started this thread. If you look at the threads original example it shows the increase in diameter tolerances for a 3.250-6 SA. Because I was angry and had my head up my arse, I didn't even realize it. Some times you can't see the forest for the trees

I just love these threads. It's like a long drawn out thought process.


- CJ

 

[hydtools]

Sometimes a brain fart.

Ted

 

[johnchrc]

Exactly.

- CJ