Unified screw threads

[MechDesign105]

Hi everyone
I'm sorry if my question is very basic, but I've been reading on the Unified Screw Threads and seem to misunderstand the nomenclature.

How were the UNC and UNF standard defined? Why is for example 2 1/2 - 4 UNC but 2 5/8 is 4-UN and then the 2 3/4" is 4UNC?
Is there a common diameter-pitch ratio for the UNC? For the UNF? Is this diameter-pitch ratio different for the UN?

Is it possible to order a 2 1/2 - 4UNF for example?

Again, I have tried to find answers to my questionning online, to find the logic behing the UNC UNF threads but have not managed to find answers to my questions...

Thanks!

 

[Guest102023]

The standard thread forms are defined independent of the diameters, but of course each diameter is attached to several thread forms (colloquially, 'coarse', 'fine', and 'extra-fine'. So frequently two or more successive diameters use the same thread form.

The thread form for a given diameter (which includes the thread pitch), or diameter-pitch ratio as you call it, is not based on an exact formula, but of course as diameter increases the thread pitch naturally decreases. You cannot order 2½-4UNF; it exists but would not be called UNF. The combinations seem somewhat arbitrary and the origins are probably lost in the mists of time.

Not sure I have exactly answered your question. When it comes to fasteners, there's no such thing as a 'basic' question. Machinery Handbook devotes hundreds of pages to the subject. ASME B1.1, Unified Inch Screw Threads is a good starting point. This reference may help:

http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf

.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[Guest102023]

Correction, as diameter increases the thread pitch INcreases. It's the threads/inch or TPI that decreases.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"

 

[3DDave]

See

http://www.ring-plug-thread-gages.com/ti-UN-vs-UNC-vs-UNF-vs-UNEF-vs-UNS.htm

I'm sure the logic has more to do with stress analysis and production cost leading to certain combinations being more popular than a complicated formula. For example, fine threads are typically stronger, but require more care to make and are more prone to damage. Coarser threads are the opposite, so there are competing pressures, but not absolute drivers. Also, to make the threads requires certain gear ratios in lathes and other thread making equipment, so there is a need to standardize on pitch. Eventually some pitch and diameter combinations are chosen because they fit more applications than others.

tl;dr There isn't any logic, just historical precedent.

 

[tbuelna]

Is it possible to order a 2 1/2 - 4UNF for example?

2.500-4 UNC-XX is a standard thread form that you can use on a drawing or spec without any additional definition being required. There is no standard 2.500" UNF thread form, so it would be incorrect to use this description. But there are standard 2.500" UN or UNS threads defined for every even pitch number between 6 and 20 TPI, which would give you the "fine" thread you are looking for.