Building a Router Flattening Jig from Scratch

When you get your hands on a beautiful live edge oak slab — rough, uneven, and full of character — a drum sander or hand plane only gets you so far. I needed a better solution, so I built one.

Why I Built It

It started with a piece of oak. Specifically, a rough-sawn live edge slab with all the charm you’d want — open grain, natural edge, a dramatic check running down the centre — and absolutely none of the flatness. The slab had a twist to it, and getting that face flat and true was the first challenge standing between raw timber and a finished project.

A drum sander would have worked for smaller pieces, but slabs this size need something more capable. A hand plane is the traditional answer, but it’s slow, demands real skill, and leaves room for error on a board I didn’t want to ruin. What I really needed was a way to mount a router and have it travel across the full surface of the slab in a perfectly flat, repeatable plane — that’s a router flattening sled.

Beyond this one slab, I also wanted something I could use for general workshop flattening — cutting boards that have warped, and glued-up panels. Once you have a reliable jig like this, you end up reaching for it constantly.

How It Works

The concept is simple: the router travels along two axes — left-to-right on a set of parallel guide rails, and forward-back along stainless steel linear shaft rods — while being held at a fixed height above the spoilboard. As long as the rails are level with each other and sitting higher than the tallest point of the workpiece, every pass the router makes cuts to the same flat plane.

The router itself sits in a carrier plate that slides smoothly along the steel rods using linear bearing blocks. Those rods are mounted to a crossbeam that rides along the linear guide rails. It’s essentially a manual two-axis gantry — a poor man’s CNC, if you like.

The Build

I kept the materials list tight. The frame uses a linear guide rail comprised of a supported stainless steel shaft — the same stuff people build 3D printers and CNC machines from — which is rigid, straight, and relatively affordable. The cross-axis rods are smooth steel shafting with off-the-shelf linear bearing blocks, so the movement is precise with zero slop. The spoilboard is standard MDF sheet, which sits flat and is easy to replace once it gets chewed up.

The DeWalt DCW600B is a fantastic router for this application — compact, powerful, and cordless so there’s no cable to manage as you sweep across a big slab. The plunge depth is easy to set precisely, which matters when you’re taking light finishing passes.

Assembly was mostly a case of screwing the guide rails to the spoilboard, squaring everything up, and making sure the two long rails were dead parallel and at the same height. Getting that setup right is the most important step — if your rails aren’t level with each other, every piece you flatten will have a taper. A decent spirit level and some patience goes a long way here.

Pro Tip: Before running the first pass, always check your rails are at exactly the same height using a reliable straight edge or level — even a millimetre of difference will translate directly into the surface of your workpiece.

The Results

The difference on that first slab was immediately obvious. What came off the jig was flat, consistent, and — once I followed up with some finishing sanding — genuinely beautiful. The open grain of the oak caught the light perfectly, and the live edge was completely preserved. No material lost to hand-planing mistakes, no twist remaining.

For general workshop use it’s proven just as useful. Warped cutting board blanks, glued-up panels, thick slabs that won’t fit anywhere else — the jig handles all of it. At approximately £130 all in, it’s one of the best value tools I’ve built.

What’s Coming Next

This jig was built for a reason — there are projects on the bench that need flat, true surfaces to start from. The oak slab you can see in these photos is destined for something, and I’ll be documenting that build here as it comes together. Stay tuned.

If you’re thinking about building something similar and have questions about materials, dimensions, or setup, drop a comment below. Happy to share what worked and what I’d do differently next time.