How to flatten a slab without a thicknesser
You've got a slab. It's got a twist in it like a potato chip, and every thicknesser you look up is either $3,000 or tops out at 330mm wide, which is useless because your slab is 600.
Good news: you don't need one. Three options, honest costs, honest hours.
Your three options
- Router sled. A router riding a carriage across two rails, skimming the slab flat pass by pass. Costs almost nothing if you own a router, handles any width. Most of this guide.
- Hand planes. Genuinely works, genuinely a workout. Right call for smaller slabs if you own a decent No. 5 and can sharpen it.
- Pay someone. A joinery with a wide-belt sander will run your slab through for $30-80. Sometimes that's simply the answer. More below.
Building the router sled
Three parts: rails, carriage, and a way to hold the slab still.
- Rails: two lengths of anything genuinely straight and stiff - steel box section, or a pair of checked 90x45s. One each side of the slab, packed level with each other. Level with each other matters; level with the floor doesn't.
- Carriage: a flat board spanning the rails with a slot the router sits in. Router slides along the carriage, carriage slides along the rails. That's the whole machine. Plenty of people just screw a router base to 12mm ply with a hole in it. Don't overthink it.
- Holding the slab: wedges and hot glue, or blocks screwed to the bench around it. Find the wobble, shim it, lock it. The slab must not rock.
- The bit: a surfacing bit, 25-50mm. Bigger clears faster but loads the router harder. A cheap 35mm does a slab or two fine.
Flattening, pass by pass
- Set depth to skim the highest point only. First pass might touch 20% of the slab. That's correct.
- Work across the grain, overlapping passes by a third of the bit width. Steady feed - too slow burns, too fast chatters.
- Drop 1-2mm and repeat until the whole face is machined. The router tells you when you're greedy. Listen to it.
- Flip, flat face down. Now it sits without rocking. Repeat to thickness.
- Expect ridges. A sled leaves small tramlines between passes. Normal - they're sanding work, not failure.
Honest time: a 2m x 600 slab, both faces, is a half day the first time. A couple of hours once the sled exists. Anyone claiming 40 minutes isn't counting setup, and setup is most of it.
When hand planes make sense
Under about 400mm wide and cupped rather than twisted, a coarse-set No. 5 across the grain roughs it flat surprisingly fast. Quiet, no dust, weirdly satisfying. Past that size or with real twist, the sled wins on your back alone. If your plane skills are 'owned one once', a slab is not the place to learn.
When to just pay someone
If the slab is already roughly flat and only needs cleaning up, $50 at a joinery saves you a Saturday. That's good money. The sled earns its keep when the slab is properly twisted (sanders follow twist, a sled removes it), too wide for the local machine, or when you'll do this more than once.
The short version
Twisted or wide: router sled. Small and cupped: hand plane. Roughly flat already: pay the joinery. And take shallow passes - every wrecked slab I've seen died of impatience, not technique.
After a slab to practise on, or a router someone's finished with? There's usually both on The Heap, for a lot less than new. And if you're weighing up rough sawn boards instead of slabs, read Rough sawn vs DAR: what you're actually paying for.