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 settings is between 0 and 200 percent, or in some cases only between 0 and 150 percent, in 10 percent increments. When set to 0%, there will not be any axis motion, although there are exceptions on some older machines.
When running a new part, start with FEEDRATE OVERRIDE switch set to between 10 and 50 percent for safety
Even experienced CNC programmers often program cutting feedrate a little too low or a little too high. This is not unexpected, as it is always difficult to predict the exact effect of the tooling and setup on speeds and feeds ahead of time. If the CNC operator is qualified and authorized to make changes to the cutting feedrates, it is important to follow a certain procedure for each tool used by the program:
▪Start the first part in low override setting (10-50%)
▪Adjust the override during dry run (usually up)
▪Watch for feedrate deviations of each tool
▪Keep track of the ‘best’ override rate for each tool
▪Recalculate the new feedrate for each tool
▪Modify actual program settings for the F-address
It is not unusual to adjust spindle speed as well, depending on the actual cutting conditions. Other factors, namely width and depth of cut will also have an effect on speeds and feeds, but that usually involves major program changes, not always quickly done at the machine.
Optimizing Programmed Feedrate
If the cutting feedrate in the program work without adjustments, there is nothing to do but run the program at 100% feedrate override. In many cases, some adjustments will be necessary, even if they are small. The following two examples will show two typical changes -one for a feedrate that to be decreased, and the other for a feedrate that has to be increased.
If the best cutting conditions are achieved when the feedrate override switch is set below 100%, it means the programmed feedrate was too high.
In the first program example, using metric units, the programmed feedrate is set to F325.0 or 325 mm per minute. This feedrate proves to be too high, and the operator finds the best cutting conditions at 70% setting of the feedrate override switch. What will the change be in the program, so the tool can cut at 100% (no override)?
| 325.0 | = 100% |
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