Why does my machine recut everything after a bit change, and what is rest machining?

Why this happens

Separate toolpaths don't know about each other. If you make one finishing toolpath with a 1/4" ballnose and a second, separate finishing toolpath with a 1/16" tapered ballnose, the second recuts the ENTIRE surface, because it has no idea the big bit already handled the open areas. That wastes hours and adds wear to the delicate bit.

The fix: multiple tools in ONE toolpath (rest machining)

In the 3D Finishing form, add both tools to the same toolpath (larger first, then smaller). The official documentation says the first tool removes as much material as it can, and each subsequent tool removes what it can from areas not yet machined. List the tools in the order they will run on the machine. The small tool only visits the areas the big tool couldn't reach: tight details, crevices, fine texture. It is one toolpath with separate G-code outputs per tool (see the G-code grouping entry), and no duplicated cutting.

The manual alternative: detail boundaries

For surgical control, draw closed vectors around just the detailed zones (faces, hands, lettering) and give the fine bit its own finishing toolpath limited to those boundary vectors. A community recipe for big reliefs: a 1/4" or 3/8" ball over everything, then 1/8", 1/16" and 1/32" each confined to shrinking detail boundaries. This is often faster than automatic rest machining on huge jobs, because YOU decide where detail matters.

Pro tips

• Either approach pairs with the roughing-strategy and 3D-speed entries. Together they can cut total 3D machine time substantially.
• Remember Z re-zeroing at every physical bit change regardless (see that entry). Rest machining fixes the toolpath waste, not the zeroing discipline.