Siemens SINUMERIK 840Di sl Programming Manual page 168

Motion commands
4.15 Thread cutting with constant lead (G33)
Cylinder thread
A cylinder thread is described by the thread length and thread lead.
The thread length is entered in absolute or incremental dimensions with one of the Cartesian
coordinates X, Y, or Z. The Z direction is preferred on turning machines. Allowance must
also be made for the run-in and run-out paths, across which the feed is accelerated or
decelerated.
The thread lead is entered at addresses I, J, K, on turning machines preferentially with K.
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Programming Manual, 11/2006, 6FC5398-1BP10-2BA0
Fundamentals