How to Flatten Boards With Just a Planer





Introduction: How to Flatten Boards With Just a Planer

About: I got an old sewing machine when I was just a kid, and I've been hooked on making stuff ever since. My name is Sam and I'm a community manager here at Instructables.

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 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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    31 Discussions

    This leveling platform is a good example of a tension torsion beam. A very good idea.

    2 replies

    I don't get why the box is better than just using say a 3/4in MDF or plywood board by itself?

    Well, I don't have the Engineering answer either. But the torsion box is (pound for pound) more rigid than a solid piece of the same material as a Steel Tube is more rigid than a Steel Rod.

    great article thanks for this one

    For all of you wanting to just put it on a flat board the longer you need it the more it will flex

    We’ve all done this pick up a piece of plywood from the end and it bends like a wet noodle. And the narrower you make it the more it flexes. but put two rails in each side and you’ve just built something you could drive you car on and it won’t flex. Just my opinion.


    2 months ago

    Where were you half a year ago when I needed exactly this? :D

    But really, I wish I had been wise enough to think of something instead of just planing it to a constant thickness and not exactly straightness.

    1 reply

    Hey there!

    I've been right here all along! ;)

    Why not just use a flat board, as the sled, such as heavy plywood?

    8 replies

    Even plywood can warp under the right circumstances... and even a slight warp will affect the results. The torsion box is still the best option. A quick second best option would be a hollow core door (hard to find in that width) or half of a hollow core bifold door (much easier to find). You RARELY see warped hollow core doors, because they're basically modified torsion boxes.

    You could do that, but it would have to be sufficiently rigid and incapable of bowing or flexing at all. This is why a torsion box is a good option - lightweight but perfectly flat and rigid.

    Okay, thanks. I wonder if 3/4 plywood would be rigid enough. I guess it depends on depth of cut and width of wood.

    I tried some 3/4in mdf yesterday and it seems to have worked well. I really wonder if the box would benefit this process significantly?

    If you shim and hot glue a twisted or cupped board down to a piece of mdf or plywood through the whole length . . as long as the base is substantial enough, perfectly flat and rigid, and does not flex at all, it should work.

    A box like I made here is a permanently rigid structure, whereas an mdf or plywood sled could potentially still flex or warp.

    Don't get that MDF near a humid environment or store it wrong! Better to use OSB Flooring than MDF if you must ignore the Torsion Box idea. His torsion box was a really simple version. He could have made one as wide as his DeWalt planer bed - 12" or so. Then he might have used six thin strips of birch plywood evenly spaced within: Look at: for one large T-box and, the way I built mine here:

    For something that is just 12" wide, all the materials can be scaled down accordingly.

    Hi there, thanks for the great comments!

    The box I made is 12 1/4" wide and incredibly rigid. I considered internal pieces, but concluded they were not needed for any level of stress this was going to be under as a simple planer sled. It's rock solid as is and working great! :)

    Very clever, as a novice compared to some of you guys this solves a problem I have often. Thanks.

    "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.

    This is very cool, I wish I had a planer.

    1 reply

    Thanks man!

    I'm 20+ years into my tool acquisition journey. Start small and stretch what you know and can do, and in time you'll have a full workshop if you want it! :)

    Some great ideas to take away and all well and good if you have a garage full of electric kit. However, you can't beat the feeling of satisfaction you get from preparing a cupped or warped piece of timber with hand planes.