I see it in Machinery's Handbook - defined as :

Designating UNS Threads.—UNS screw threads which have special combinations of
diameter and pitch with tolerance to Unified formulation have the basic form
designation set out first followed always by the limits of size.

The pitch diameter tolerances used in Table 3 for all classes of the UNEF,
12-UN, 16-UN, 20-UN, 28-UN, and 32-UN series and the UNS series, are based on a
length of engagement of 9 pitches and are applicable for lengths of engagement
of from 5 to 15 pitches.

Martin


Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
"Our Republic and the Press will Rise or Fall Together": Joseph Pulitzer
TSRA: Endowed; NRA LOH & Patron Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Originator & Charter Founder
IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com/

On 9/24/2010 11:03 PM, Ned Simmons wrote:
On Fri, 24 Sep 2010 18:46:48 -0700, "Bill Noble"
wrote:



wrote in message
. au...
1/2"X 12 TPI is British Standard Whitworth.
I have no idea where you could find them in the U.S.
If you decide to turn them, note that the thread angle is 55 degrees not
60.


whitworth taps are commonly available - I happen to have some because of
british cars, but they are not at all hard to find - and ebay is a source
for them cheap -

They may not be Whitworth. I measured a tap I have here that's marked
1/2-12 USF and it's definitely a 60 degree thread. I've no idea what
USF stands for -- perhaps it's related to the old USS (United States
Standard) thread.

I repaired an old 36" bandsaw that used 1/2-12 fasteners and had to
single point several replacement bolts. The saw was probably late 19th
century. Old enough that the wheels had steel spokes and steam bent
oak rims.