What does af mean on imperial tools?

Answer 1

American Fractional (tools)

It means that the tool you're looking at is measured in fractions of an inch.

(common tool sizes are 1/4'', 5/16'', 3/8'', 1'', 1 1/8'' amongst many others)

The tools could have easily (but not very practically) been in decimals 1/4'' = 0.25'' or 9/64 = 0.14065"

It actually means Across Flats.So a 1/2 inch AF spanner is exactly that,1/2 inch across the flats of and open end spanner and the nut it fits is exactly 1/2 inch acoss each opposing set of flats.

Some metric spanners are called AF for this reason.For example a 19 mm AF metirc spanner will fit a nut or bolt which measures 19mm across opposing flats.The exceptions are BSW (whitworth) and BSF (British standard fine) where the size marked on the wrench refers to the size of the thread of the bolt or nut that it fits and BA (British Associated) where the spanners are numbered 0 to 10 or 11 with the smaller numbers being larger spanners than the higher ones.

There are conversion charts and explanations on the Baconsdozen metric and imperial tools web site.

AF, Whitworth and BA are all now being replaced by metric however there are still a huge number of cars,motorcycles and other machinery made in an age where planned obsolescence wasn't designed in that need imperial tools to remove and replace their imperial fittings.

The A/F system is associated with American industry's SAE standards, and was intoduced to the UK primarily by both car manufacturers when US firms took over many UK ones, and by NATO for interchangeability of military equipment & tools.

The BA range goes beyond 12BA. Its smallest is 22BA - but you're getting into watch-making there. It is a metric series based actually on the Swiss 'Thury' threads, and in its full form the diameters and hexagons follow a geometrical series broken when the British Standards Institute decided, rather oddly, that the odd numbers were no longer needed. The largest BA size - designated by the digit '0' - is very close to ISO M6 althought not an exact match as the thread angles differ.

All metric spanners are described by their A/F sizes (in millimetres) but the ISO Metric thread and corresponding hexagon range is strangely random, with no real progression or continuity in sizes, pitches and diameter / hexagon proportions throughout the range. Just to make it more fun it is not unknown to find metric but non-standard threads and nut sizes in some Asian Cars, and in isolated non-automotive instances, presumably to force you to buy OEM spares & tools at inflated prices!

BSW and BSF A/F nut and bolt-head dimensions sizes are in constant proportions to the screw diameter.

One BS thread that survives world-wide is 1/4" BSW, for camera tripod screws. Another is the Model-Engineer series (Imperial diameters X 32 and 40 tpi Whitworth form) The derived BSP[ipe] thread series (also Whit. form) has also been harmonised into ISO, rather grudgingly.