Smithy CNC Mills - Manual (Page 27)

the leading edge of the tool to cut more heavily. Typical values are 29,
29.5 or 30.
The number of "spring passes" is given by the H- value.
Spring passes are additional passes at full thread depth. If no additional passes are
desired, program H0.
Each pass begins at a position on the drive line. It consists of
1. An X traverse to the depth for this pass
2. A pause for the spindle to reach index position
3. A spindle-synchronized Z feed along the thread
4. A traverse to the original X
5. On all passes but the last, a traverse Z move to the
beginning point for the next pass
The tool will pause briefly before each threading pass, so a relief groove will be
required at the entry unless the beginning of the thread is past the end of the
material.
The exit move (traverse to original X) is not synchronized to the spindle speed. With
a slow spindle, the exit move might take only a small fraction of a revolution. If the
spindle speed is increased after several passes are complete, subsequent exit moves
will require a larger portion of a revolution, resulting in a very heavy cut during the
exit move. This can be avoided by providing a relief groove at the exit, or by
not changing the spindle speed while threading.
The sample program g76.ngc shows the use of the G76 canned cycle, and can be
previewed and executed on any machine using the sim/lathe.ini configuration.
2.18 G81 to G89: Canned Cycles
The canned cycles G81 through G89 have been implemented as described in this
section. Two examples are given with the description of G81 below.
All canned cycles are performed with respect to the currently selected plane. Any of
the three planes (XY, YZ, ZX) may be selected. Throughout this section, most
of the descriptions assume the XY-plane has been selected. The behavior is always
analogous if the YZ or XZ-plane is selected.
SmithyCNC Programmer’s Reference Manual:G Codes
SmithyCNC Programmer’s Reference Manual: G-Codes 2-12