How to Flatten a Slab Without Buying a CNC

Every live-edge slab thread online eventually turns into someone showing off a CNC flattening a table for them while they narrate from a lawn chair. Great content. Not a requirement. A router sled costs about as much as a nice dinner and does the same job, just with more of your own attention involved.

What you actually need

• A router with enough depth of cut for your slab's cupping (a 1/2" plunge base helps)
• A flat sled that rides on two straight rails spanning the slab's width
• Two rails longer than the slab, shimmed dead level relative to each other
• A big straight bit -- a 1.5" surfacing bit clears more material per pass than a spiral upcut and leaves a cleaner floor

The actual process

1. Level the rails, not the slab. The slab can be cupped, twisted, whatever wood does when it dries unevenly -- the rails define your reference plane, so get those flat and coplanar with shims and a straightedge before you touch the router.
2. Rough pass first. Set the bit to just kiss the highest point and run the sled across the whole surface. You're not trying to finish anything yet, just establishing a flat reference face.
3. Flip and repeat. Flatten one face, flip the slab, and use that new flat face as your reference for the second side. This is where a lot of people skip a step and end up with a slab that's flat on one face and still a wedge overall.
4. Step down in shallow passes. Don't try to remove 3/8" of cup in one go -- you'll tear out grain and possibly stall the router. A few light passes beat one heroic one.
5. Sand out the router tracks. Even a good sled leaves faint scalloping. Start around 80 grit and work up; a random orbital sander earns its keep here.

Where people get burned

Slabs move as internal stress releases during flattening -- a board that was flat when you finished the first face can cup slightly by the time you flip it. That's not a mistake, that's wood being wood. Flatten, let it sit overnight, check it again before you commit to final thickness.

And wear a respirator. A router chewing through a slab at speed puts a genuinely alarming amount of fine dust in the air, and your lungs don't get a router sled of their own to fix the damage later.