Flattening Large Wood Slabs with a Router

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Duration: 6:20

Wood slabs can make great tables. The problem is getting them flat. The coolest slabs are usually too big to go through a planer or sander, and end grain slabs shouldn’t go through a planer anyway. Fortunately, there is an easy way to flatten any oversized board using a shop-made jig and a router. Here’s how.

Slab flattening router jig

Details of building and using the shop-made jig are in this article, but you can watch the video above to get a feel for the jig and the process.

Build the Router Sled

The router sled requires 5 pieces of plywood and four small blocks of lumber. The router sled should be twice as long as your bench is wide. My bench is 34″ wide, so I made my router sled 68″ wide. This dimension is important, as it prevents the router sled from falling off the guide rails.

Parts for slab flattening router jig

Build the Router Sled

1/4″ plywood is best for the base of the sled because the router bit must pass through the base before the cutter reaches the wood. If the base is too thick, the router bit might not be long enough to extend past the bottom of the jig. The base must be wide enough to fit your router plus 1-1/2″ to accommodate the 3/4″ sides. Most router bases are about 6″-7″ in diameter so I made my router sled base 8-1/2″ wide.

router template wo annotation

Make a template of your router base by wrapping a piece of paper around the base. Mark the screw holes and collet location by poking holes through the paper with a pencil.

Position the template on the sled base so that the collet location is at the center of the sled. Drill and countersink the holes for the router mounting screws and drill a 1-5/8″ hole at the collet location for the router bit to pass through.

Next cut the two side pieces for the sled and glue them to the edges of the base piece. The sides of the sled should be about 2-1/2″ high, and should be made from 3/4″ plywood.

Dust Collection

I take dust collection pretty seriously. Not fanatically, but seriously. Flattening a large board creates a lot of dust and chips.

Router jig dust collection

I decided to add a dust hood (see Sources) to the sled. Before I screwed the dust hood in place I made a shallow relief cut in the side rail to help move the dust through to the hood.

Shop made dust collection

I fastened the dust hood to the router sled with a few old brackets that I had been saving for “someday”. I am fortunate to have a nice dust collection system in my shop, but using even a shop vacuum will help a lot!

Secure and Level the Top Surface of the Wood

Your lumber might be cupped, twisted, or bowed. Your objective is to remove the least amount of wood necessary to get a flat surface.

Levelling large slab

Place a level on the piece (assuming your bench is level) and look at the gap between the level and the low spots on the work piece. Then adjust the height of the work piece with shims until that gap has been minimized. After you have minimized that gap you are ready to secure the piece to your workbench.

Securing slab to bench

You can secure the wood to your bench with three pieces of scrap wood. Clamp a piece of scrap on one end of your bench and then butt the wood up to that first scrap. Then clamp another small piece of scrap to the opposite end of your bench. Now place a third piece in the space between the wood and the end of your bench. Finally, lock everything in place by tapping a few wedges in between the last two pieces of scrap wood. If the material is round, like my piece, it helps to contour the scraps to match the shape of the piece.

Securing your work will be idiosyncratic to your particular piece. Just keep in mind that you need horizontal holds, not vertical clamps, because you’ll be routing the top face.

Guide Rails

The guide rails are simply two straight boards that the router sled rides on. If the sides are too tall then your router bit might not be long enough to reach your board. It is easy to add shims to raise the sides up if needed, so it is better to make the sides too short than too tall, and shim under them as needed. My slab was about 5-3/4? thick and I made the rails 6? high. Glue small blocks on the ends of the guide rails to give yourself another place to clamp the rails down.

Router Sled on Bench Annotated

Here’s a great look at the overall setup. Note the shims being used to level the guide rails, and the “housekeeping” being used to keep cords and hoses out of the way.

The router and router bit

Router Bit for flattening slabs

You can use any router with a 1/2? collet, but it will be easier to adjust the depth of cut with a plunge router. If you use a bit that is over 3/4″ in diameter then you must also use a variable speed router because it is dangerous to run large diameter bits at full speed.

The most important consideration with the router bit is its length. If the bit is too short, it won’t extend very far past the bottom of the sled. In that case you might have to shorten the height of the side rails to complete your project. Use a bit that is a minimum of 2-1/4? long and preferably one that is over 2-1/2″. See Sources

You can use any straight bit between 1/2″ in diameter and 1-1/2″ in diameter. A larger bit will get the job done faster and will likely leave a cleaner surface. Just remember that a 1-1/2″ diameter bit should only run at about 15,000 RPM. (Download a Router Bit Speed Chart) I have used standard mortising bits to flatten boards and they worked just fine. This time I used a Dado/Planer bit with a 1-1/2″ diameter. It did work a little better than a regular bit, but don’t worry about it if you don’t have one.


Adding wax to the bottom of the sled and to the top of the guide rails will make it easier to move the router sled.

Now for the easy part!

  • Hook up your vacuum hose, or hoses
  • Double check to make sure that your router is set to the proper speed for the bit you are using
  • Set the depth of cut so that you will remove about 1/4″ from the high spots on the work piece
  • Remember to wear your safety glasses and hearing protection
  • Slowly pass the router and sled back and forth across the board until all the high spots have been removed. Watch the video to see this in action
  • Re-adjust the depth of cut and repeat until the whole board has been flattened
  • Adjust depth of cut to 1/8″ or less for a final pass across board

Finishing up

Flattened slab

The work piece will have tracks from the router bit when you are done flattening it. These are easily removed with either a belt sander or a random orbit sander. If you want both faces of the slab parallel to each other, flip the slab over and repeat this process on the other face. You won’t have to shim it, since this side is already flat.

Dust hood
Part #151293
Woodcraft Supply
(800) 225-1153

Router bit
CMT #352.504.11

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17 Responses to “Flattening Large Wood Slabs with a Router”

  1. Mick Brice

    Excellent. Just what I needed to square off the ends of pieces of telegraph pole. They are too heavy for the lathe and too tall-awkward for the thicknesser.

  2. Arty

    This has made life a lot easier and saved so much time any recommendations on a strong brand of router bits cause I’m going through them due to the timber being so dense and hard

  3. Hossein

    I was really interested to watch how the operation can be carried on, but video could not opened.

    • Customer Service

      Hi Hossein. The images from this video have now been fixed. We are sorry for any inconvenience. If you are still having problems please give our customer service team a call at 1-855-253-0822.
      Jean-WWGOA Video Membership

  4. Jerame

    Great idea. I wonder if a small window (cut out a square and cover with plexiglass) on each side of the router would give you a better view of that has been cut and what remains?? I also see this being useful on more than a cookie. What about a rough-cut mantel piece from a log?

    • Customer Service

      Yes a window might help but you can develop a good feel for where you are at when using this setup even without having that level of visibility.
      This technique can be used for slabs of all sizes and is an effective way to flatten unruly stock for any number of different projects.

  5. GARY

    This is the best info on a router sled i have seen so far. The router bit and speed info was excellent.

  6. Martin Schwartz

    I would use a chain saw to level the piece and then sand it smooth. I would use an Festool MFT table to secure the piece to the table and not require the elaborate plywood jig to secure to the table, which seems far too nice to mess up. All in all, I suspect doing it my way would save a lot of time and plywood.

  7. Bob

    How is the “gap” between the level and the work surface reduced by shimming the bottom of the workpiece?

    • Customer Service

      Hello Bob,

      The shim on the bottom serves to stabilize the work piece so that you are working on a constant plane as you flatten. Once the work piece is stabilized, the router is used to flatten.

      Paul, Woodworkers Guild of America

  8. Carlos Alvarado

    That was a very good way of doing the flatting of a round of wood. My problem with that is that I do not use power tools only hand tools. How would you all suggest that I do the flatting using say a hand plane would you suggest that I go in a circle so as not to gouge the wood. I understand that you are most power tool advocates and only use hand tools on occasions, well I am the other way around and only use power tools occasionally. So, how would you suggest I tackle the problem.

    • Customer Service

      Hello Carlos,

      You could do this with hand planes. Use winding sticks to be sure that you are keeping things flat. Start with a #4, going 45 degrees to the grain. Then switch to a #5, going 45 degrees in the other direction so that you can see where you’ve been. Then, finish with a #6, going with the grain.

      Paul, Woodworkers Guild of America

  9. David Colella

    Such a simple cheap alternative to expensive commercially made sleds. Well done.