How to Flatten Boards With Just a Planer

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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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    44 Discussions

    1
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    Chuck666

    9 months ago

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

    3 replies
    0
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    onkelhugoChuck666

    Reply 9 months ago

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

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    DennisT33onkelhugo

    Reply 7 weeks ago

    A torsion box that is 2" thick is roughly as rigid as a 2" thick board but much lighter. The reason is that almost all of a board's rigidity derives from the surfaces and is increased by the distances between them.

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    charlessenf-gmonkelhugo

    Reply 9 months ago

    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.

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    G.1

    9 months ago

    How does this technique achieve parallel surfaces? I assume that the gap to the box allows the oak to flex, but how does this help? I seem to be having a simple misunderstanding of the concept.

    I need to do this soon, so nice timing!

    4 replies
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    seamsterG.1

    Reply 9 months ago

    Good question. The pieces of oak shown here were beefy enough that I was comfortable not shimming along the middle section of the board. If there's any chance the board would flex under the planer's cutter head, that would require full support of the board with shims in order to produce a flat top surface coming out of the planer.

    The concept is to support the piece well enough in a fixed position that we can send it through the planer and incrementally remove the high spots from the imperfect top face of the board, to create a top face that is one perfectly flat surface. This step is typically done with a separate machine (a jointer), but for boards that are too wide for your jointer (or if you don't have one), the approach shown here can be used.

    Then this new flat face can be placed downward on the planer bed and the planer will do it's job on the remaining face, making two boardfaces that are perfectly parallel.

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    edsoboseamster

    Reply 5 weeks ago

    Is it completely necessary to shim the board so the whole surface is lifted off of the backing? It seems like if you left part of it touching the support and shimmed the high spots, it would be less likely to flex during planing.

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    seamsteredsobo

    Reply 5 weeks ago

    You are correct. It all depends on the shape of the board in question, and what twisting and various thicknesses you're trying to overcome. Unless you're dealing with some crazy cupping, when removing board-twist at least one corner of the board will likely be able to rest directly on the sled.

    In this case - the board was shimmed in such a way to get the top side as level as possible to begin with, to reduce the amount of material that needed to be removed. In the first photo in step 3, the furthest back corner of the board is resting on the sled box, with all other corners shimmed. The board end closest to the camera was thinner, so it was propped high enough to avoid wasting time and losing tons of material to get the top side flat.

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    edsoboseamster

    Reply 5 weeks ago

    Ah, I see. That makes more sense now. Thanks!

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    seamsterjbtech2

    Reply 6 weeks ago

    I've never had a big issue with snipe, honestly. But there doesn't seem to be any snipe when I use this setup.

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    seamstermsoomro

    Answer 7 weeks ago

    There are several ways one could check for flatness - my worktable is flat, so if a board lays flat on my table with no wobble or gaps, and the top surface is parallel to the table, I know it's flat. A separate board with a straight edge could be laid on top of a newly planed board, and this will tell you if it's flat. Some people use the winding stick method to see if there is any twist in a board too.

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    msoomroseamster

    Reply 7 weeks ago

    Thanks for the quick reply. I mis-stated. I meant initial flatness of top surface when you shimming the board up with laminates on the sled you made. Does that top surface has to be level and if so how you check for that level? Or does need gaps at bottom between the sled and board bottom filled so it doesn't get pushed down with planner rollers.

    Appreciate it.

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    seamstermsoomro

    Reply 7 weeks ago

    Ah, got it. The board is shimmed to both support the board fully, and to make the top face as level as possible, simply to avoid removing more material than needed to in the process of making it flat when it runs through the planer. You can just eye-ball it to get it generally parallel with the sled. The shims keep the board from deflecting under the pressure of the rollers, just as you have said. Good questions, I hope my answer makes sense!

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    msoomroseamster

    Reply 7 weeks ago

    Awesome. Your answers make perfect sense. I'll try the technique next time I have wide board to flatten instead of ripping it to 5" or so to fit my jointer.
    I appreciate the help.
    Happy new year.

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    Kinnishian

    Tip 7 weeks ago

    Ooh! I have a tip: Try putting isopropyl alchohol on the hot glue when you want to debond it. It's not guarenteed to work on wood (grains and mechanical bonding) but in the past I've found it works like seeming literal magic to delaminate hot glue joints. If it does prove effective on wood you can go more ham on hot glue knowing you can easily remove it with isopropyl alchohol :)

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    DennisT33

    Tip 7 weeks ago

    Great 'ible and a great method. I've been doing this for a while. Two comments to enhance, possibly:

    1) Once in a while the glue will break free, usually by pushing the board backwards on the sled. I added a thin cleat at the back end of the sled to keep the board from moving backwards to prevent this.

    2) you can make a simple torsion box sled simply by gluing thin sheets of plywood to front and back of a scrap piece of foam insulation board. Glue them on your flattest surface.

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    uncletj

    7 months ago

    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.

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    Raitis

    9 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.