Why 3D carves take so long, and what actually helps
Finishing is almost always the majority of the time, and most of it is spent on areas that did not need a fine stepover. The biggest wins are a bigger roughing tool, restricting the fine finishing pass to detailed areas, and not using a ball nose on flat backgrounds.
Where the time goes
Finishing time is roughly the surface area divided by the stepover, divided by the feed rate. Three things drive it:
A 6 by 6 inch relief at a 0.01 inch stepover is about 3,600 inches of path. At 120 in/min that is roughly half an hour at full feed, and considerably more in practice because a detailed model never reaches the programmed feed.
Halve the stepover and the path doubles. Double the area and the path doubles. This is why a large detailed relief can genuinely take all day.
What helps, ranked
| Change | Time saved | Quality cost |
|---|---|---|
| Restrict the fine finishing pass to detailed areas only | Very large | None, if the boundary is chosen sensibly |
| Use a flat end mill on flat backgrounds instead of the ball nose | Very large | None. A flat bit leaves a better flat surface anyway. |
| Bigger roughing tool | Large | None. Finishing removes the evidence. |
| Larger finishing ball nose where detail allows | Large | Detail is limited by tool radius |
| Slightly larger stepover, sand the difference | Moderate to large | A few minutes of sanding |
| Higher feed rate | Moderate | None if the chipload stays sane; the machine often cannot go faster on detail anyway |
| Reduce the model's physical size | Large | It is a smaller piece |
| Higher RPM | None | It does nothing for time and can hurt the surface |
The single biggest one, in detail
Take a typical relief: a detailed subject in the middle, a smooth background around it, and a border. If you finish the whole thing with a 1/16 inch ball nose at a fine stepover, you are sweeping that tiny tool over the entire panel, and most of the panel is a flat background with nothing in it.
Instead:
- Level the flat background with a flat end mill. Minutes, and a better flat surface.
- Finish the general form with a 1/8 inch ball nose at a moderate stepover.
- Finish just the detailed subject with a 1/16 inch tapered ball nose at a fine stepover, using a boundary vector drawn around it.
The finished piece looks identical to the all-day version, because the fine tool went exactly where fine detail exists. This is the technique that separates people who carve 3D regularly from people who tried it once and gave up.
Long jobs, practically
- Start with a fresh bit on a long carve. A bit that dulls at hour two ruins everything after it.
- Empty the dust collector first. A full one part way through a six hour carve is how chips pack into the cut.
- Check the workholding before you start rather than hoping.
- Cut a small test region first at the real settings, so you find out about the surface quality in ten minutes rather than six hours.