How to Flatten Boards With Just a Planer

In woodworking, there are often many routes to reach the same destination.

This is one way to flatten* large boards using primarily an electric thickness planer (without the help of a jointer).

In order to flatten a warped, twisted, or cupped board, a common approach is to first use a jointer to create one perfectly flat face.

Then you run the board through a thickness planer with the flat face downward, and the planer makes the top face parallel to the bottom.

However, for boards or slabs that are too wide for a jointer (but still narrow enough to fit through a thickness planer), the approach demonstrated in this Instructable is one way to perform both steps needed to flatten boards using just a thickness planer, rather than using a jointer AND a planer.

*The term "flatten" as I'm using it here means: make two board faces perfectly parallel to one another by removing warps, twists or cupping.

Step 1: Lumber and Overview

I picked up a bunch of old oak beams recently, and have been using the wood for various projects.

In order to use the material, I've been splitting the beams in half using my bandsaw.

However, the resulting boards all had twists and/or cupping, so they were flattened using the process shown in this Instructable. I'm not sure what these boards were used for previously, but they have a lot of character!

Here's the gist of this Instructable:

To flatten a board with just a thickness planer, secure the board to a perfectly flat and rigid support structure (I show how I made mine in the following steps)
Run it through your planer until the topside of the board is flat
Remove the board from the support structure
Run the board through the planer with the now-flat side down

This Instructable is my version of a pretty common board-flattening solution; if you do a bit of search-engine-ing, you'll find several great versions.

Step 2: Support Sled

I built a support box using baltic birch plywood.

A simple box like this is guaranteed to stay perfectly flat and will not bow or warp, and creates a lightweight rigid sled to attach boards for planing. Alternately, a plain piece of plywood or MDF could be used as long as it stays perfectly flat and becomes sufficiently rigid when the board to be flattened is affixed to it.

Using a table saw, I cut two 60" lengths of 3/8" plywood that were just narrower than the opening of my planer (in my case, the width capacity is 12 1/2", so I made these pieces 12 1/4").

Two 2" wide strips were then cut from 3/4" plywood.

I used glue and pneumatic brads to fasten all these pieces together as shown.

This created a lightweight, but perfectly flat box.

To finish the box, I sprayed it with a few coats of spray lacquer, followed by a light sanding with 220 grit sandpaper. I then waxed the two larger faces with furniture wax (so either side could be used face down, but this is also helpful to remove masking tape that's used as well - which is shown later).

Step 3: Level the Workpiece

The board to be flattened is placed on the box and made as level as possible using shims to fully support it. I use sample laminate pieces which you can typically get for free from the kitchen section at home centers and hardware stores.

This was all done on top of a few strips of painters masking tape.

Since this board was so beefy, I felt comfortable only supporting it at the ends. For a thinner board where it is likely to flex in the middle under the pressure from the planer's cutter head, you will want to shim and secure the entire length.

The goal is to affix the imperfect board firmly to the support structure so it cannot move or flex at all.

Step 4: Hot Glue the Board Down

The board is now fastened securely in place using hot glue.

The glue holds the board firmly in place while planing, but is easy to remove once the top face has been planed.

Step 5: Plane the Top Face

The top face of the board is now planed using the thickness planer.

You only want to remove a tiny fraction at a time. In my case this was especially important because the middle section of the board was not shimmed, so it was still possible that any extra pressure from the cutter head could have made the board bow downward, resulting in an unflat cupped board.

Step 6: Remove Glue

The hot glue can be removed with the help of a chisel. You can pull the glue off of the taped surface, or just remove the tape and the glue should come with it.