Left-hand UNC threads
Left-hand imperial threads are rarely in a catalogue and mostly needed just when a repair depends on one: on American machinery, on turnbuckles, on shaft ends with changing direction of rotation. VÖLKEL makes 96 hand taps for left-hand UNC to DIN 352 in HSS-G, as three-piece sets and as individual taps, from UNC No. 6 x 32 LH to UNC 1" x 8 LH.
Same thread, other direction
Profile, 60° flank angle and thread count are identical to right-hand UNC. So the same tap drill table applies, set out in the article calculate tap drill size. Only the helix is reversed: the tool turns anticlockwise, and the short reverse turn to break the chip goes clockwise.
The set stays three-piece
Because UNC is a coarse thread, left-hand work also uses three tools: taper tap with a six to eight thread lead, second tap with four to five, bottoming tap with two to three. For the finer UNF left hand series two are enough.
How to recognise it later
At first glance a left-hand thread looks like any other. Hold the screw upright and the threads rise to the left instead of to the right. If the part is not marked, somebody may later start clockwise and destroy the first threads. The article identify and cut left-hand threads correctly shows the signs.
Further left-hand series: metric left hand, BSW left hand, UNF left hand. Complete left-hand sets are under assortments.
