The left-hand thread
Thread is not equal to thread
Left-handed threads or also left-turning or left-rising threads are those which can be screwed together counterclockwise. These are marked with the abbreviation LH.
LH here stands for left-handed or “left hand”.
With this type of thread it should be avoided that the thread loosens by itself. This is the case with the left-hand bicycle pedal, for example.
In addition, the left-hand thread is often used as a safety lock for gas bottles to prevent life-threatening mix-ups.
Cutting left-hand threads
Cutting the internal thread for left-hand threads
Cutting left-handed threads is identical in sequence to making right-handed threads. The decisive difference lies in the direction of rotation of the tool.
For left-handed threads, the lead of the tap tip has a shape ground to the right.
Cutting the external thread for left-hand threads
The same applies to the cutting of the external thread for left-hand threads. There are special dies for the production of left-handed threads.
Otherwise, the information on thread size and dimension remain the same.