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% | |
| 1% | = 325 / 100 | = 3.25 |
| 70% | = 3.25 × 70 | = 227.5 mm/min |
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