BSW: the Whitworth coarse thread
Whitworth is regarded as the first standardised screw thread and is still found wherever British engineering was built in: on classic cars, machine tools, agricultural machinery, in shipbuilding. VÖLKEL makes 276 hand taps for it to DIN 352 in HSS-G, as three-piece sets and as individual taper, second and bottoming taps, from BSW 1/16 x 60 to BSW 3" x 3.5.
55° and rounded crests
Two features set Whitworth apart from the later systems: a flank angle of 55° instead of 60°, and rounded crests and roots instead of flat ones. The rounding makes the thread less sensitive to notch effects. Recutting a Whitworth thread with a UN tool destroys exactly that contour.
The trap at 1/4 inch
BSW 1/4 has 20 threads per inch, and so does UNC 1/4. Diameter and pitch match, the flank angle does not. A Whitworth screw will therefore go into a UNC thread, but it bears on only part of the flanks and can strip under load. At 1/2 inch the thread counts differ, 12 against 13, and it jams straight away.
The thread counts get fine quickly
At the small end the series becomes surprisingly fine: BSW 1/16 has 60 threads, 3/32 has 48, 1/8 has 40. Only from about 3/16 inch does it behave like a coarse thread. For the same diameters there is a finer series in BSF.
Three-piece set
Taper tap with a six to eight thread lead, second tap with four to five, bottoming tap with two to three, marked by one ring, two rings or no ring on the shank. Tap drill sizes are in the article calculate tap drill size.
Left-hand versions under BSW left hand, complete imperial sets under BSW and BSF, dies for the external threads among the round dies.
