Smithy CNC Mills - Manual (Page 106)

descriptions below) is determined by the setting of the retract_mode: either to
the original Z position (if that is above the R position and the retract_mode is
G98, OLD_Z), or otherwise to the R position. (See G98/G99 below)
10.1 Preliminary Motion
Preliminary motion may be confusing on first read. It should come clear as you
work through the examples in G80 and G81 below. Preliminary motion is a set
of motions that is common to all of the milling canned cycles. These motions
are computed at the time the canned cycle block is encountered by the inter-
preter. They move the tool into the proper location for the execution of the
canned cycle itself.
These motions will be different depending on whether the canned cycle is to be
executed using absolute distances or incremental distances. These motions will
also be affected by the initial position of the z axis when the canned cycle block
is encountered in a program.
If the current Z position is below the R position, the Z axis is traversed to the R
position. This happens only once, regardless of the value of L.
In addition, for each repeat as specified by L, one or two moves are made
before the rest of the cycle: 1. a straight traverse parallel to the XY-plane to the
given XY-position 2. a straight traverse of the Z-axis only to the R position, if it
is not already at the R position.
10.2 G80
G80 turns off all motion. You should think of it as the off position on a rotary
switch where the other positions are the different possible motion modes. In
the EMC interpreter, G80 is one of the modal codes so any other code will
replace it. The result of the following lines of code is the same.
N1000 G90 G81 X1 Y1 Z1.5 R2.8 (absolute distance canned cycle)
N1001 G80 (turn off canned cycle motion)
N1002 G0 X0 Y0 Z0 (turn on rapid traverse and move to coordinate home)
produces the same final position and machine state as
N1000 G90 G81 X1 Y1 Z1.5 R2.8 (absolute distance canned cycle)
N1001 G0 X0 Y0 Z0 (turn on rapid traverse and move to coordinate home)
The advantage of the first set is that, the G80 line clearly turns off the G81
SmithyCNC Programmer’s Reference Manual: Mill Canned Cycles 10-3