Smithy CNC Mills - Manual (Page 118)

SmithyCNC Programmer’s Reference Manual: Mill Canned Cycles 10-15
Neither code will have any affect when incremental moves with a positive R
value are specified because the R value is added to OLD_Z and that result is
used as the initial level for a G98. The same value is the computed R value so
G99 will also return to the same place.
When the value of R is less than OLD_Z and incremental distance mode is
turned on, G98 will return the tool to the value of OLD_Z. Under those condi-
tions G99 will retract the tool to OLD_Z plus the negative R value. The return
will be below OLD_Z.
10.13 Why use a canned cycle?
There are at least two reasons for using canned cycles. The first is the economy
of code. A single bore would take several lines of code to execute.
Example 1 above demonstrated how a canned cycle could be used to produce 8
holes with ten lines of nc code within the canned cycle mode. The program
below will produce the same set of 8 holes using five lines for the canned cycle.
It does not follow exactly the same path nor does it drill in the same order as
the earlier example. But the program writing economy of a good canned cycle
should be obvious.