of 25 mm or one inch should be sufficient.
The best precaution to overtravel when homing the axes is not at the beginning of the day but at the end of the day. When the machine is ready to be shut off, make sure that each axis is at least 25 mm or one inch away from machine zero. There should be no overtravel when you use this method.
If you find moving each axis separately to the HOME position time consuming, there is an alternative. Use the MDI (Manual Data Input) mode and the proper program command. Keep in mind that there is a big difference between sending machine home in absolute and incremental mode. For setup at the machine, the incremental mode is the one to use.
Program command G28 for the primary machine zero (or G30 for the secondary machine zero) is a command that is not always easy to understand, particularly by CNC operators.
G28 or G30 is simply defined as Machine Zero Return
The real definition is longer and more complex:
G28 is the machine zero return command that will move the specified axis to primary machine zero through an intermediate position
The same applies to G30, except the motion is to the secondary machine zero position.
The intermediate position is often used in programming to avoid an area that can cause collision - this is not the case when zeroing machine axes, and the intermediate position is not necessary. Here are some examples how to zero machine (send all axes home):
On a CNC machining center:
| G91 G28 X0 | (MACHINE ZERO IN X ONLY) |
| G91 G28 Y0 | (MACHINE ZERO IN Y ONLY) |
| G91 G28 Z0 | (MACHINE ZERO IN Z ONLY) |
| G91 G28 B0 | (MACHINE ZERO IN B ONLY) |
| G91 G28 X0 Y0 Z0 | (MACHINE ZERO IN XYZ) |
On a CNC lathe:
Incremental mode uses U and W for X and Z axes:
| G28 U0 W0 | (MACHINE ZERO IN XZ) |
U0 or W0 can also be used as a single axis motion.
HANDLE Mode Selection
Selecting the HANDLE mode, you are transferring all manual motions from the control panel to the device that is an integral part of all CNC machines - the handle.
Fanuc calls this manual device Manual Pulse Generator (MPG), but nobody else does - it is a handle, plain and simple. Most handles are manual devices, but many new models are digital, at least in the sense that they include a small screen display that can show much more then position only.
Regardless of the design, the handle is a device used for manual axis motion, commonly used in setup. Its design usually includes a coiled cable that can be extended into the working area. During setup, you select the axis you want to move and the amount of increment, then use the wheel to move by a very precise distance. The handle can be set in such a way that the minimum increment (that is the smallest amount of motion) is:
▪0.001 mm
or
▪0.0001 inches
These amounts are standard on all CNC machining centers and CNC lathes. Because of the many features the handle offers, a separate chapter has been developed for that purpose - see chapter named Setup Handle.
JOG Mode Selection
The word jog means nothing more than continuous feed. This mode (and the following Rapid mode) have no relation to any program features. Both are strictly used in manual mode, particularly during part setup.
RAPID Mode Selection
Rapid mode is logically related to the Jog mode and located next to it. While JOG mode is used for motions that require manual feedrate, the RAPID mode is used in manual operations to provide rapid motion of the selected axis.
In conjunction with the axis selection and direction of motion, any single axis can be moved in two directions. Velocity of the rapid motion will also depend on the setting of the Rapid Override Switch (described later).
For safety reasons, the rapid motion will only take place, while the ± switch is being pressed.
During setup, it is safer to start with medium or slow rapid motion and increase it as necessary
MDI Mode Selection
MDI stands for Manual Data Input. This feature enables the CNC operator to enter one or more program blocks as a group and process them together. MDI mode is for temporary use only, for example, during setup or testing. It is not intended to run production programs.
MDI mode can be called programming on the fly - if you want to use this mode, you have to have at least rudimentary knowledge of programming. MDI mode uses actual program codes to achieve certain results.
For example, the following MDI command will move the X-axis by negative 271.450 mm, in rapid mode:
G91 G00 X-271.450
The motion will start at the current tool position and end 271.450 mm away in the negative X direction.
The process is simple:
▪Select the MDI mode
