Cutting metric left-hand threads by hand
A left-hand thread is used wherever the direction of rotation in service would loosen a right-hand thread: on left wheel studs, on turnbuckles, on bicycle and machine shafts, on gas fittings. VÖLKEL makes 148 hand taps for metric left-hand threads to DIN 352 in HSS-G, as three-piece sets and as individual taps from M 3 x 0.5 LH to M 52 x 5.0 LH.
What changes and what does not
Nothing about the thread itself changes except the direction: profile, flank angle and pitch are identical to the right-hand thread. So the same tap drill table applies, set out in the article calculate tap drill size. What does change is the way you work: the tool turns anticlockwise, and the short reverse turn to break the chip goes clockwise.
The set has three pieces
Just as with the right-hand thread: 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. Buy the set, or reorder a single tap.
Mark it
The most common damage to left-hand threads happens later, not while cutting: somebody starts clockwise and destroys the first threads. Marking the workpiece and the tool clearly costs nothing and prevents it. How to recognise a left-hand thread is shown in the article identify and cut left-hand threads correctly.
Complete sets for left-hand threads are under assortments, machine versions among the machine taps, dies for left-hand external threads among the round dies.
