How do I flatten a warped board or live-edge slab on the CNC?

The principle

A CNC is a giant router sled: hold the warped stock still, skim the top flat with a surfacing pass, flip, repeat. The two hard parts are holding warped stock without flattening the warp temporarily (it springs back when released) and knowing how much thickness you'll lose.

The workflow

1. Shim, don't clamp flat. Set the slab cup-down where possible, then shim every rocking point so it sits dead still WITHOUT being forced flat. Hot glue or CA the shims. Clamp or screw only where it doesn't bend the board.
2. Find the high spot: jog the bit over the surface at a fixed Z and eyeball the gap, or probe corners. Zero Z on the highest point.
3. Surfacing toolpath: use a large surfacing bit (1 to 2.5") or your biggest end mill, with a Pocket toolpath (or the spoilboard-surfacing recipe) over a rectangle just bigger than the slab. Common shop starting points: about 0.5 to 1 mm depth per pass for surfacing bits in hardwood, 40 to 50% stepover, and a raster along the grain for the final pass. Test on scrap.
4. Repeat passes until fully flat (fresh surface everywhere).
5. Flip, hold flat-face-down (now much easier, for example with tape and CA), and surface side two to final thickness.

The math to respect

A slab with 6 mm of cup loses about 6 mm or more of thickness in flattening. Measure the warp before promising a finished thickness.

Pro tips

• Let slabs acclimate in your shop for days or weeks before flattening. A wet slab re-warps as it dries. Rough-flatten, wait, then finish-flatten is the safest sequence for precious slabs.
• Surfacing-bit marks (faint scallops) sand out fast with a light final stepover (about 30%) and sharp inserts.
• This same skim-pass principle is why V-carve signs on "pretty flat" boards still get a light surfacing pass first (a 0.5 mm skim is one common choice, shop advice). See the V-carve depth entry.