Machining time
Cutting time is path length divided by feed rate, but real machines never hit the programmed feed everywhere because of acceleration. Expect a real job to take noticeably longer than the ideal calculation, especially on detailed toolpaths with thousands of short moves.
The simple version
Which is correct, and always optimistic.
Why the real job takes longer
The machine has to accelerate to reach the programmed feed and decelerate before it changes direction. On a long straight cut that costs almost nothing. On a toolpath made of thousands of short segments, the machine may never reach the programmed feed at all, spending the whole job accelerating and decelerating.
Things that make the gap between estimate and reality worse:
- Small detail. Lettering, texture and fine 3D work are all short segments.
- Badly traced vectors. A curve made of a thousand tiny line segments cuts slowly and looks rough. See vector troubleshooting.
- Low acceleration settings. Some machines are configured conservatively and it shows up as slow real-world cutting despite high programmed feeds.
- Lots of retracts. Every lift to safe Z and move to the next area is time not cutting.
What actually shortens a job
In rough order of effect, and each with its cost:
- A bigger bit for roughing. Removal rate goes up roughly with the square of diameter on a full slot, because the slot gets wider and the depth per pass scales with the bit. The single largest lever there is, and it costs you a tool change.
- A larger stepover on roughing only. Free if the machine can take the load. Finishing passes keep the quality.
- Deeper passes. Same, bounded by what your machine can hold.
- Restricting the finishing pass to where it is needed. Most 3D carves spend most of their time finishing flat areas that needed no such fine stepover. See 3D carve time.
- Cleaner vectors. Free, and it improves quality at the same time.
- Higher feed rate. Only if the chipload allows it, which it often does.
The time nobody counts
Cutting time is the part that is easy to measure and often the smaller half of the real total. Design, stock preparation, workholding, zeroing, tool changes, sanding and finishing all take real time, and for one-off work they usually add up to more than the machine time. If you are pricing work, this is the number that gets underestimated. See pricing CNC work.