AF spanners

Ginger DJ

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Here's one for ya, Just been asked a question to which i really don't know the answer (rare for me!). What does AF stand for on AF spanners?
 
Across flats. That is distance from one face to another in a parallel type of lines way, IYSWIM.
Ah, you've got me doubting myself now and I was so smug getting the first answer in. Pride cometh before a fall and all that. :o Is it Whitworth then where the distance is across the face? :confused:
 
American fine


Nope.....

http://www.gomog.com/allmorgan/whitworth.html

Meanwhile, our impudent rebel colony in the America ;-) introduced their own standards in 1918 with the National Fine (NF) and National Coarse (NC) threads approved by the Society of Automotive Engineers (SAE). These were still imperial sizes, and had pitch sizes measured in teeth per inch (TPI), but the thread form had an included angle of 60 degrees, and the spanner size was determined by the 'across flats' measurement of the hex bar from which the bolt was formed. Be careful not to confuse the letters AF when used to mean 'across flats', with AF used (erroneously) to indicate 'American Fine' thread. This is actually 'National Fine' and should be called NF or UNF.
 
BSW -where the 5/8" thread originally required a 5/8" spanner until the hex size was resduced 1/16" in WW2 to save steel, hence the expression:
"Please pass me the 9/16" spanner so I can loosen this 5/8" nut".
Obvious, innit?
Nutty?
 


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