Can I pause a 40-hour carve overnight, and how do I split a huge 3D carve into sessions?

The short answers

Pausing mid-toolpath overnight: avoid it. There are two real risks: (1) wood moves. Overnight humidity and temperature shifts change the surface enough that the resumed cut can leave a visible seam or depth step. (2) Many hobby controllers don't resume mid-file reliably after a full power-down. (Many spindles are rated for continuous duty, so heat worries are often misplaced, but check your spindle's rating.)

What to do instead, in order

1. Shrink the time first. Many 40-hour estimates can be cut dramatically with better settings: roughing strategy, stepover, rest machining, boundaries, plunge rate (see the 3D-speed, roughing-strategy, and rest-machining entries). Do this before any splitting.
2. Split by TOOLPATH, not mid-file. Structure the job as several complete toolpaths (rough, big-tool finish, detail zones; see rest machining) and run one or two per session. Each file finishes cleanly, and between sessions the machine sits idle with the work still fixtured. Leave the workpiece clamped and untouched between sessions and don't re-zero, because the coordinate frame is the continuity.
3. For genuinely enormous carves (the 200-hour map): split the MODEL into regions with vector boundaries, overlapping each neighbor by about 1/8 to 1/4" (shop advice, test it) so the overlap blends the seams. Run one region per session, with the same fixturing rules.

The fixturing discipline that makes it work

A homed machine plus a saved work zero (see the coordinates entry) means even a power cycle between sessions is safe, because the machine finds the same zero tomorrow. What breaks multi-session carving is moving the stock, or re-zeroing Z on a surface that has already been carved away.

Pro tips

• Run the riskiest sessions (deep roughing) when you can be nearby. Quiet finish passes are the safest to run long, but never truly unattended (see the safety entry).
• Humidity-stable shops (or stable MDF or HDU stock) tolerate multi-day jobs far better than a garage in a rainstorm with pine.