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Separating cutting and non-cutting time in a CNC cycle

Managers often want to know the percentage of time a machine is actually cutting something in a CNC cycle, but trying to calculate cutting versus non-cutting time can be difficult. But by running two CNC cycles (one without a workpiece) there is a relatively quick and definitely easy way to separate cutting time from non-cutting time.

First of all, let’s define cutting time and non-cutting time:

  • Cutting time is time when the machine is in a cutting mode (G01, G02, G03, etc.).

  • Non-cutting time is everything else (rapid [G00] motions, tool changes, indexes, etc.).

Admittedly, some of the time we attribute to cutting time is related to times when the machine is feeding but not cutting (feeding into and out of a cut). The amount of rapid approach distance, of course, affects how much time is taken during these motions.

First of all, run and time the cycle in the normal fashion – with the feedrate override switch at 100%. You can actually run a part during this time. For the formula that we’ll show, we’ll call this normal feedrate run time.

Second, run and time the cycle with the feedrate override switch set to 200%. You cannot, of course, run a workpiece during this cycle. We’ll call this the double feedrate run time.

With the two times available, apply these two simple formulae to determine cutting time and non-cutting time:

  • Cutting time = (normal feedrate run time minus double feedrate run time) times two

  • Non-cutting time = normal feedrate run time minus cutting time (that you just calculated)

Here is an example. Say your normal feedrate run time is 17 minutes. The double feedrate run time is 9.5 minutes. (17 minus 9.5) times 2 is 15, meaning fifteen minutes of the cycle is cutting time and two minutes of the cycle (17-15) is non-cutting time.

This works because only the cutting motion feedrate is doubled when you turn the feedrate override switch to 200%. For this reason, the difference between the normal feedrate run time and the double feedrate run time is half the cutting time.

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