Thread Standard Question

[USMechE6]

Hi All,

I'm selecting a thread standard for a nominal 1-9/16" diameter engagement, but am confused by what I see in Machinery's Handbook. The internal threads/inch of 6, 8, 12 and 16 are all listed as UN, then 18 is listed as UNEF and then 20 is listed back at UN. Can anyone explain why the 20 threads per inch would not even be considered fine-pitch while the 18 is listed as extra-fine?

I like the slightly greater clearance the minor diameter of the 20 will provide for a solid piece that has to fit thru before the externally-threaded feature comes in, but I am aware of the pros/cons associated with coarse/fine/extra-fine, so I want to make sure I understand the basis for this designation. Some of the other nominal diameters have this, too - 1-1/2", 5/16" to name a couple.

- Eric

 

[BrianPetersen]

You gave us the length, but not the diameter. What is it?

 

[btrueblood]

UNC, UNF and UNEF are standard thread pitches in the sense that you can walk into a well-stocked(*) hardware store and ask for coarse, fine, or extra-fine fasteners of whatever size, and get a certain pitch that mates with corresponding nuts.

The other UN sizes are not commonly available, i.e. you will need to have mating hardware specially made to that pitch; but the standard covers all of the tolerances.

(*)This is MY definition of a well-stocked hardware store, YMMV on finding EF threaded fasteners in any hardware store near you...

 

[USMechE6]

Sorry meant that was the engaged diameter (looking at internal and external). No lengths in my previous post, edited it for clarity.

 

[3DDave]

It's a happenstance that probably stems from the transition from only having the UN thread designation to adding clarifying letter(s) to the base "UN". I think this was in order to label the threads that are preferred.

The linked chart shows how arbitrary the conversion was.

http://www.amesweb.info/Screws/Tables/Thread-Pitch-Chart-UNC-UNF-UNEF-UN.aspx