CNC Control Setup for Milling and Turning:. Peter Smid
setting
These three settings provide flexibility at the setup.
Program Testing and Setup Mode
Various toggle switches being ON or OFF provide great amount of flexibility during program testing and setup. The question that remains to be answered is ‘what else has to be set to make the testing accurate ?’
Keep in mind that program or setup integrity test requires all settings to be completed first. That includes:
▪Program (including subprograms and macros) has to be in CNC memory or available in DNC mode
▪Setup fixture has to be in fixed position
▪Part zero has to be set (G54 - G59)
▪Tools have to be mounted, loaded, registered and ready to use
▪Offsets have to be set - that includes work offset, tool length offset and cutter radius offset
▪Part itself may or may not be mounted in the setup fixture, depending on the type of test performed
Unlike toggle switches which typically select one of two (or three) modes possible, rotary switches select from several different modes. Typical rotary switches found on CNC systems are:
▪MODE selection
▪AXIS selection
▪RAPID override
▪FEEDRATE override
▪SPINDLE override
▪JOG feedrate selection
The Mode Select switch on the operation panel typically offers eight setting in two groups. Each mode has four settings, and only one setting selection can be active at any given time.
| ▪Group 1 | = | Manual Mode | ... four settings |
| ▪Group 2 | = | Automatic Mode | ... four settings |
Manual Mode includes selection of the following four modes of operation:
▪HOME
▪HANDLE
▪JOG
▪RAPID
AUTO mode includes selection of the following four modes of operation:
▪MDI
▪DNC / EXT / REMOTE / TAPE
▪MEM (MEMORY)
▪EDIT
HOME Mode Selection
Most CNC operators use the expression ‘Home’ when they refer to the machine reference position also known as the machine zero. For example, homing the machine is a term that means all axes will be located at the machine reference point (machine zero).
This is a very special position of any CNC machine. It is the true machine origin. Every number that represents axis dimension in the program or settings is somehow related to this point.
In this mode setting, you can move any axis to machine zero. Some machines allow multi-axis motion, other only one axis at a time. Home position also serves as the starting location from which to start a new program, at least for the first part. Most CNC machines require that all axes are zeroed before any program processing can be done. Zeroing the machine - sending all axes home - is one of the first activities after the main power had been turned on for the machine. Several newest CNC machines have a special memory that allows avoiding the manual return, but they are still in minority.
The process of sending each axis HOME after Power ON is fairly consistent for all machine types:
▪Turn power to the machine
▪Turn power to the control
▪Switch to HOME mode selection
▪Select axis
▪Press the + button
▪Repeat for other axes
You may need to hold the + RAPID button for a second or two to start and then let go.
Return to primary machine zero (Home) is always a positive direction in all axes
Generally, it is a good idea to set all relative (REL) positions to zero. There are two major display positions - ABS (absolute) and REL (relative):
| ▪ABS | ... is used for monitoring current tool position |
| ▪REL | ... is used for setup |
As the saying goes, anything can go wrong, but in the HOME mode, the most common problem is over travel. Over travel happens when the selected axis tries to move outside of the machine travel limits. Every CNC machine has a precision limit switch that prevents over travel. When this switch is tripped, the control system will issue an O/T (over travel) alarm.
The most common cause is that the axis position was too close to the home position. The solution is simple - reset the alarm and move the axis that overtraveled further away from machine zero - minimum 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.