the part program, any rapid tool motion is called by the G00 command. G00 has no feedrate associated with it; how fast the tool moves depends on the machine specifications - actual rapid rate cannot be programmed. Modern CNC machining centers can rapid at a fairly common rate of 60 000 mm/min (2360 ipm) or even higher. As much as this speed is welcome for production, it is generally too fast for setup and first part run. Using the override switch to slow down the rapid motion is a welcome relief, when needed.
Rapid override switch indicates the percentage of the maximum amount of rapid motion that the machine is capable of
Generally, CNC machining centers that do not exceed 30 000 to 38 000 mm/min (1180-1500 ipm) have a selection range different from machines with ultra high rapid motion. Compare the two rotary switches available from the CNC operation panel:
Machine manufacturer may choose a different range. What is important to understand is that machines with very high rapid rate need finer adjustments. A comparative example will follow, but let’s look at the F0 selection first.
F0 Selection
On some machines, there is a selection that does not specify the actual percentage of the rapid rate reduction and is identified as F0. If available, this will be the lowest percentage, but a percentage that can be changed by system parameter. Other percentages are fixed. For example, if 25% is the lowest fixed setting, you can make the F0 setting to be 20%, 15%, 10%, and so on.
Rapid Override Comparison
Consider the difference between two machines - as an example, one with a maximum rapid rate of 30 000 mm/min (1180 ipm) and another with a fast rapid rate of 60 000 mm/min (2360 ipm):
Depending on the setting of F0 (10% used) the table shows how fast the CNC machine will rapid with different override settings.
Rapid Override On Lathes
Most CNC lathes have a rapid override setting in the range of 0-25-50-100%. The main difference on CNC lathes is that the X-axis, because of its shorter travel distance) will have a rapid rate about of only one half of the rapid rate for the Z-axis. For example:
| X-rapid rate | = | 12 000 mm/min [ 472 ipm ] |
| Z-rapid rate | = | 24 000 mm/min [ 944 ipm ] |
| X-rapid rate | = | 20 000 mm/min [ 787 ipm ] |
| Z-rapid rate | = | 40 000 mm/min [ 1575 ipm ] |
For multi-purpose lathes (mill-turn or turn-mill type), each additional axis will have its own maximum rapid rate assigned by the manufacturer.
Also keep in mind, that some machines use push-button switches rather than rotary switches. In all cases, the functionality of the Rapid Override Switch is always the same.
A few suggestions that should come handy for CNC operator at any level of experience:
Use RAPID OVERRIDE switch for setup and verifying the first part of the run
For running a new part:
When running a new part, start with RAPID override switch set to between 10 and 50 percent for safety
For production, the fastest cycle time is required. Once the setup has been confirmed as correct and the part program has been optimized, run at 100%:
After verifying both setup and program, run production at 100% rapid rate for the best cycle time
Modern CNC machines are well built and can withstand high rapid rates repeatedly.
Another very useful override switch is the Feedrate Override switch. In the part program, a cutting feedrate is programmed using the F-address. Feedrate is associated with cutting motions, using commands G01, G02, G03 as well as various cycles and several other motion commands. There are two methods of programming a feedrate, mainly depending on the machine type:
| ▪Feedrate per minute | ... typically used for milling |
| ▪Feedrate per revolution | ... typically used for turning |
Feedrate per minute is normally used to program cutting motion on CNC milling machines and machining centers, while feedrate per revolution is programmed for cutting motion on CNC lathes. Depending on the current units (G20 for imperial units or G21 for metric units), cutting feedrate will use the following format:
Programming examples - Metric:
| F250.0 | ... 250 millimeters per minute | 250 mm/min |
| F0.3 | ... 0.3 millimeters per revolution | 0.3 mm/rev |
Programming examples - Imperial:
| F10.0 | ... 10 inches per minute | 10 ipm |
| F0.012 | ... 0.012 inches per revolution | 0.012 ipr |
The purpose of the Feedrate Override switch is to optimize feedrates provided by the part program
Feedrate Optimization
The programmed feedrate is always 100% and can be decreased or increased at the CNC machine by using the Feedrate Override switch.
Typical range of a feedrate override switch
