Have you ever tried to make a left hand thread with a tap? Using G84, even if you enter left rotation at the beginning, causes the rotation to change to right after reading the cycle. But there is a way for everything, as the reader of this page informed us.

G84 – Face tapping cycle for lathes – left hand thread version + example.

Cycle diagram:

G84 X(U)_ C(H)_ Z(W)_ R_ P_ F_ K_ M_ ;

X_ C_ : Hole location data
Z_: Distance between point R and the bottom of the hole
R_ : The distance from the starting level to the level of point R
P_ : Dwell time at the bottom of the hole
F_ : Feed rate
K_ : Number of repetitions (if needed)
M_ : M code to clamp the C axis (if needed)

OK. It looks the same, so where is the magic? We need to add the M176 code before running the cycle, and just after canceling the cycle M177 and that was all we needed to be happy.

M176 : Turning on the conversion of spindle revolutions (change of revolutions to the left)

M177 : Turning off spindle revolutions recalculation (restores right revolutions)

Part of code on M12x1L example (machine tested):

(LEFT THREAD M12x1)
M176 (TURN ON SPINDLE RPM CONVERSION)
M29S80 (RIGID THREADING INCLUDED)
G18
G84Z-12.8R0.P0F1.0 (CALL THREADING CYCLE)
G80 (CYCLE CANCELLATION)
M177 (DISABLING THE RECALCULATION OF SPINDLE RPM)
G0Z4
Z40

Depending on the machine and type of control, this may vary. It’s best to test it on “dry run”. I don’t see much information about it in my DTR. I’m posting it to help others. On behalf of the CNC community, I thank the reader for the valuable information.