Elastic Stress Concentration Factor(s) for UNC Threads

[mcclain]

It is my understanding that UNC external threads do not have a specific root radius. Without knowing the radius how would one estimate the elastic stress concentration factor?

Any thoughts?

Thanks

 

[MintJulep]

While it is true that the "UN" thread form - be it course or fine - does not require a radius, it would be a difficult thing to find an actual "UN" thread form fastener in production today - at least for fasteners an inch or smaller.

Below an inch, you are almost certain to get a "UNR" thread form, which has defined limits on the radius between the flank and the bottom of the thread. The limits are 0.108P to 0.144P.

 

[Tunalover]

MintJulep-
Are you saying that a UNRC can be a UNC? I deal with fasteners all the time below an inch major diameter: 2-56 UNC (.086-56 UNC), 4-40 UNC (.112-40 UNC), 6-32 UNC (.138-32 UNC), 8-32 UNC (.164-32 UNC), 1/4-20 UNC (.250-20 UNC) and so on and was never aware that I was getting UNRC fasteners instead of UNC fasteners. Can you explain some more?


Tunalover

 

[MintJulep]

Pretty simple really.

Below an inch, externally threaded fastener threads are almost universally produced by rolling.

It's easier and more economical (fewer rejections) to roll the UNR form than the UN form. So almost all rolling dies are made to produce the UNR form.