Introduction: How to Flatten Boards With Just a Planer
In woodworking, there are often many routes to reach the same destination.
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 boardfaces 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. I have this glue gun, and it's a beast!
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.
Step 7: Plane the Other Side
Now place the newly-flat boardface downward, and plane the remaining side.
My boards were all in pretty rough shape as you can see!
Step 8: Done!
My boards were all brought to the same thickness and then were ready to be used for other projects.
Thanks again for reading!
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"The key is getting that box beam rigid enough." Look at Torsion Box Construction. I've built large (door sized) platforms using light weight 'Door Skins' over a torsion box interior with great results. The interior pieces I used were 1/4" x 1" x Length and I used 1" x 1" x Length for the 'outside' edges.
A small word of caution when milling any boards. Depending on the species and how it was dried there can be a lot of internal stresses built up between fibers. So even if you mill a perfectly square and planar board, changes in moisture, or the next step in the process like cutting, can relieve those stresses and you end up with a newly twisted board. Pay attention to grain and ensure your boards are acclimated to the conditions in which they will be put in service.
I also remember seeing in fine woodworking some time ago the trick of dropping a board from about 4 feet up. Internal stresses can be relieved to some degree with this method, prior to trueing up your piece.
When I want to temporary gluei something I glue a piece of paper in between. Holds well but a good quick blow with a hammer and the paper laminates. I then sand off the paper residue.
The key is getting that box beam rigid enough. Two ideas:
A: Since you normally run the board through centered, I'd put two more thinner boards on edge supporting the interior of the box.
B: Another way would be to make a foam core beam. Use the grade of blue board used for road construction. Use the right adhesive, and this would make the beam far more rigid and only slightly heavier. You have to make sure that the glue is even or you get glue humps and valleys. I would look for a glue that can be spread using a notched trowel.
If you are doing a lot of this, having some sort of dog system that allowed you to pin/wedge the board in place instead of glueing it would be handy.