Rockwell Automation 8520 9/Series CNC Lathe - Manual (Page 620)

Thread Cutting
Chapter 25
25-20
4. The cutting tool is returned along the Z axis at a rapid feedrate to the
Z axis position prior to the G21 block.
5. Program execution continues on to the next block.
G21 is modal. Following passes need to contain only a new value for the
infeed (X value). The other parameters programmed in the G21 block
remains in effect.
The G78 multipass threading routine can be programmed to cut parallel,
face, or tapered fixed lead threads (variable lead threads may only be cut
using a G34 block). This routine performs a predetermined series of
threading steps designated by a single program block. The G78 block
contains all of the necessary information to cut the complete thread. When
executed, the routine makes multiple passes over the thread until the
programmed root depth is reached. The control automatically generates all
threading passes necessary to reach the programmed root depth.
In effect, with the exception of the different infeed types, the multipass
threading routine is executed as if many G21 single pass threading cycles
were being executed. The key difference between the two features (aside
from infeed types) is that the multiple pass cycle only requires one block to
do the job of many single pass threading blocks. The G78 multipass
threading routine is a non-modal G-code (unlike G21). This routine is
executed only when a block contains a G78.
A finishing pass is also available with the multipass threading routine.
The size of the finishing pass, and whether a finishing pass is performed at
all, is determined by the system installer in AMP. If a finishing pass is
made, it is typically to improve final thread finish by removing a
significantly smaller amount of material with the last pass executing across
the thread by the G78 routine.
The two different chamfering features (threading retract and threading
chamfer) described in section 25.1 may also be used with this multipass
thread cutting routine. The parameters to enable these and to determine
their angle and length are set in AMP. The thread chamfer feature must
also be enabled through PAL.
The conditions to be satisfied to execute this thread cutting routine are
described in section 25.1, thread cutting considerations.
25.6
O.D. & I.D. Multipass
Threading Routine (G78)