Left-hand UNF threads
The combination of imperial thread, fine pitch and left-hand helix is the rarest on the shelf, and exactly for that reason hard to source when you need it. VÖLKEL makes 73 hand taps for left-hand UNF to DIN 2181 in HSS-G, as two-piece sets and as individual taps, from UNF No. 10 x 32 LH to UNF 1" x 12 LH.
Two tools, because it is a fine thread
As with right-hand UNF, the set consists of taper tap and bottoming tap. There is no second tap, because a fine pitch removes less material per thread. The coarse UNC left hand series keeps three tools.
Where these threads are found
Left-hand fine threads do two jobs: they hold where the direction of rotation in service would loosen a right-hand thread, and they prevent mix-ups on connections that must not be swapped. Both occur on clamping fixtures, on shaft ends and on fittings.
While working
Cut anticlockwise, reverse clockwise. A fine pitch also means more turns to depth, more friction and a greater need for lubrication. Suitable products are under lubricants. Tap drill sizes are in the article calculate tap drill size, the signs of left-hand threads in the article identify and cut left-hand threads correctly.
