Slab flattening on a CNC router
Short answer
Shim the slab so it cannot rock, flatten the top, flip it onto the flat face, and flatten the other side. Take roughly equal amounts from each face and give the slab time between operations, because removing material releases stress and it will move.
The setup is the whole job
The machine can only reference its own plane of motion. So flattening works by holding the slab so it cannot move, then skimming whatever is proud. If the slab rocks or flexes, it springs back after the cut and you have flattened nothing.
- Find the high points. Put the slab on a flat surface and see where it rocks or where light shows underneath.
- Shim under every gap until it sits without rocking. Wedges, shims, hot glue, anything that fills the space rigidly.
- Check it again. Press on each corner in turn. Nothing should move.
- Fix it down so it cannot shift sideways: screws into waste areas, hot glue at the edges, cleats around it.
- Check your Z clearance at the highest point of the slab before you start.
The mistake that wastes a slab
Clamping a twisted slab flat before flattening it. The clamps hold it flat, you machine a flat face, you release the clamps, and it springs back twisted with a face that is now neither flat nor original. Shim to support, do not clamp to deform.The toolpath
- A surfacing bit as wide as your machine will drive comfortably.
- Light depth per pass. You are skimming, and a wide bit at depth stalls routers.
- Generous stepover overlap, around 40 to 60 percent, which hides any residual tram error.
- Lower RPM, brisk feed. Slow feeds on a wide bit burn.
- Multiple light passes rather than one heavy one.
- A final very light pass at a slower feed to leave an even surface.
See surfacing toolpaths for reading the result and diagnosing ridges.
Both faces, and why
Wood holds internal stress. Remove material from one face only and you unbalance it, and it moves. Take roughly equal amounts from both faces and it stays much closer to flat.
- Flatten the first face just enough to get a reference plane.
- Flip it onto that face, which now sits flat on the spoilboard.
- Flatten the second face.
- If it still needs thicknessing, alternate faces rather than taking it all from one side.
Give it time
Leave the slab a day or more between operations if you can. Stress release is not instant, and a slab that looks perfect coming off the machine can show a small bow the next morning. Finding that out before you cut joinery is much better than after.Practical notes
- Bark and voids. Decide before you start whether bark stays. Loose bark under a surfacing bit becomes projectiles.
- Check for metal. Reclaimed and urban slabs hide nails, fence wire and bullets. A metal detector pass is cheap insurance for an expensive bit.
- Dust volume is enormous. Flattening produces far more waste than any other operation. Empty the collector first.
- Tiling a slab larger than the machine means two setups with an accurate reference between them. Doable, and it demands care with registration.
- Moisture. A slab that is not dry will move regardless of how well you flatten it. Check it with a moisture meter before spending the time.