HEIDENHAIN TNC 320 User Manual page 106

Please note while programming:
Program a positioning block for the starting point (hole
center) in the working plane with radius compensation R0.
The algebraic sign for the cycle parameter "thread depth"
determines the working direction. If you program the
thread DEPTH = 0, the cycle will not be executed.
The nominal thread diameter is approached in a semi-circle
from the center. A pre-positioning movement to the side
is carried out if the pitch of the tool diameter is four times
smaller than the nominal thread diameter.
Note that the TNC makes a compensation movement in
the tool axis before the approach movement. The length
of the compensation movement is at most half of the
thread pitch. Ensure sufficient space in the hole!
If you change the thread depth, the TNC automatically
changes the starting point for the helical movement.
Danger of collision!
Use the machine parameter displayDepthErr to define
whether, if a positive depth is entered, the TNC should
output an error message (on) or not (off).
Keep in mind that the TNC reverses the calculation for pre-
positioning when a positive depth is entered. This
means that the tool moves at rapid traverse in the tool axis
to set-up clearance below the workpiece surface!
106 Fixed Cycles: Tapping / Thread Milling