Cutting time isn’t cycle time
Machining time splits in two. Cutting time is the stretch where the tool is engaged and moving at feed, actual metal coming off. Non-cutting time is everything else: rapid positioning, retracts and approaches, dwells and tool changes. Both are on the clock from cycle start to program end, but only the first one shows up if you estimate from cutting moves alone. That’s the gap that turns a tidy quote into a long afternoon.
Where the non-cutting time hides
Rapids (G0). Repositioning moves aren’t instant. They’re bounded by your machine’s rapid rate and, just like feed moves, by acceleration. The axis ramps up to rapid speed and back down to a stop. A program that hops all over the part racks up a surprising amount of rapid time, and short rapids never reach full speed at all. The motion behind that is the same trapezoidal profile covered in feed rate vs. cycle time.
Retracts and approaches. Every feature has an entry and an exit: rapid up to clearance, rapid across, plunge in, peck or cut, retract out. On a part with dozens of holes or pockets, those approach and clearance moves repeat dozens of times. None of it cuts. All of it counts.
Dwells (G04). Programmed pauses for letting the spindle settle, breaking a chip, or clearing a bore. Each one is small, but a dwell inside a loop that runs on every hole adds up. They’re easy to forget precisely because no axis moves. UltraNC counts them, holding the tool in place for the programmed dwell so those seconds land in the cycle time.
Tool changes (M06). The big one on multi-tool work, and the one most worth getting right, so it gets its own section below.
Tool changes are real seconds
An automatic tool change isn’t free and it isn’t instant. The spindle stops, the changer unloads the old tool, indexes, loads the new one and spins back up. That whole stop-swap-restart is a single figure, the tool change time you set in UltraNC, not three separate numbers. A fast arm-type ATC does that in a couple of seconds. A mid-range machining centre is commonly six to twelve seconds chip-to-chip. Call it three to ten seconds per change for planning. Now count the M06s in a ten-tool job: that’s a minute or two of pure non-cutting time before you’ve added a single rapid.
UltraNC holds the tool stationary for the real change duration and steps through the sequence, so the seconds land where they belong: in the cycle time, not stretched across the toolpath. Most simulators either ignore the change or smear it across the path, so their on-screen clock never actually stops.
How much it really is
There’s no single number, because non-cutting time depends on how the part is programmed. A roughing job that hogs out one pocket with one tool is nearly all cutting. A detailed part with many small features, several tools, and lots of repositioning can spend a large fraction of its cycle not cutting at all. The only honest way to know is to run the actual file and watch the split, rather than assume the cutting time is the whole story.
This is also why cutting time-only estimates underbid in a predictable direction: every item above adds time. Miss them and you’re always short, never long. It is the same one-way error that makes CAM cycle time estimates read optimistic.
Putting a number on it
Run the posted G-code through a simulator that models real motion, set to your machine. UltraNC times every rapid with your rapid rate and acceleration, ramping up and back down, the way the machine really moves. Every M06 tool change lands at your tool change time. Every dwell counts at its programmed length (G04 and the hole-bottom dwells of G82/G89 alike), and every tapping reversal is on the clock too. Set three numbers, drop the file in, and the estimate arrives with the non-cutting overhead already inside it. Nothing is uploaded, and there’s no sign up.
To see it on the exact file from the video, open the viewer and hit Demo · Tool change. It loads the three-tool job so you can scrub through each change and watch the clock hold.
Common questions
What counts as non-cutting time on a CNC?
Non-cutting time is every part of the program where the tool isn't removing material: rapid positioning moves, retracts and approach moves, dwells and tool changes. On parts with lots of features or several tools it can be a large share of the total cycle, so an estimate built only from cutting moves comes out short.
How much time does a tool change add?
It depends on the machine. A fast arm-type changer swaps in a couple of seconds; a mid-range machining centre is often six to twelve seconds chip-to-chip. Three to ten seconds per change is a fair planning figure. Multiply by the number of M06s in the program and it stacks up quickly on a multi-tool job.
Does cycle time include rapids and tool changes?
It should. True cycle time runs from cycle start to program end and includes the non-cutting moves (rapids, retracts and tool changes), not just the cutting passes. A figure built from cutting time alone understates the real run, which is exactly how quotes end up short.
How do I estimate non-cutting time?
Run the posted G-code through a simulator that models real motion, set to your machine's rapid rate, acceleration and tool change time. It times the rapids the way your machine actually moves, ramping up and back down, and folds each M06 tool change, G04 and hole-bottom dwell, and tapping reversal into the total, so the non-cutting overhead lands in the number instead of being assumed away.
Part of the guide: How to estimate CNC machining time from G-code
Why estimates read low: Why your CAM software’s cycle time estimate is wrong: the non-cutting time it quietly skips.
The motion behind rapids: Feed rate vs. cycle time: why even rapids ramp up and down instead of jumping to speed.