I had some heavy timber benches, made from four-by-twelve Douglas fir headers, that stood on paired stacks of cinder blocks. These actually worked fairly well, but I got tired of complaints about barked ankles, so I had a bright idea. One of the benches was warped, making it a little wobbly and uncomfortable to sit on. I decided to cut this warped beam into short lengths, and use the pieces to make actual legs for the benches. For the legs I just tapered the pieces slightly and mounted them in slots cut with a circular saw. A tight fit and pocket hole screws on each side keep the legs secure.

Of course, using a bench to make legs meant I'd need a new header for the second bench. I was pretty confident I could pick just the right end grain to avoid getting a header that would warp, I dug through a big stack of those heavy beams at the lumber store, and found just the right one. My confidence was not rewarded. As soon as the beam started to dry I noticed it was beging to twist. I put heavy counterweights on the ends of long two-by-fours, clamped to the ends of the bench, but that did'n't discourage the warping at all. Eventually there was about an inch of difference between the corners of the bench, and the opposite legs had lifted off the ground.

A couple of years have passed, and I'm feeling pretty confident that the plank is finished twisting. I decided to fix this wobbly bench by making a jig so I could "plane" down the high corners of the bench with a router. Since this is a one-time job (I'm pretty confident about that), I'm just making a jury-rigged setup that will be cheap and easy to make.

Leveling the bench is a two step process: first, the legs need to be reset so they sit flat on the ground; and second, I need to build a jig so I can run the router back and forth across the entire top of the bench to make it level. I know some people build dedicated router sleds, but I barely have room for a router. This jig is mostly made from scraps I had lying around. It's not fancy, but it works

Before I begin, let me state that I considered just buying another beam to re-make the bench, but that would be a waste of wood, and it's a lot of work cutting the slots for the legs. Also, I'm not at all confident that a new board wouldn't warp just like this one. Planing off the surface seemed like the best solution.

Supplies

We need

2 10-foot 2X4s

2 2-foot 2X3 (scraps)

2 1-foot 2X4 (scraps)

1 1-foot by 2-foot piece of 1/4-inch plywood

Screws

Router

Belt Sander

Orbital Sander

Level

Measuring tape

Urethane Sealer and Paint Brush

Step 1: Re-Set the Legs

Looking under the bench, you can see how the legs are held in place with screws in pocket holes. All these screws need to be backed out so we can move the legs in their slots to make them sit flat on the ground. Before re-setting the screws, use a tape measure to check the high points to make sure they're more or less even.

Once the legs are flat on the ground, and the top corners are more or less the same height, re-tighten the screws to hold the legs securely in place. This leaves some gaps in the slots the legs are mounted in. I'll fill the gaps with wedges later to keep from creating spider habitat.

Step 2: Build the Bench Jig

The legs are re-set, and the bench doesn't wobble, but it is still pretty uncomfortable to sit on. I bought two ten-foot 2X4s to mount on each side. I'm using scrap pieces of 3/4-inch plywood as spacers between the bench sides and these rails. Mostly, this is to leave room to sweep away the shavings.

To position the rails, I used clamps to keep them in place while leveling them in all directions. You need to be sure that the tops of the rails are at least as high as the high corners on the bench and that both rails are level, side-to-side, and end-to-end. Looking back, I wish I had added more supporting blocks mid-way along the length of the rails to keep them from sagging while moving the router along the bench.

These are kind of rustic benches, so I don't mind the screw holes in the sides. If it mattered you could figure out a way to mount the rails with screws on the undersides, or just have faith in your clamps. If you do, you have a better grip than I do. When you're satisfied that the rails are level, screw them down tight, I needed to counter-sink the screw holes because my longest screws were only 2 1/2-inches.

Once the rails are in place we need to make a little slide, just wide enough for the router, and long enough for the router bit to cut all the way to the edges of the bench. I'm making the sides of the sled just a bit wider than the width of the router (the added width is slightly less the width of the cutting bit (3/8-inch), so you can make two passes with the router before moving the slide). The slide also needs some cross members on the underside to keep it snug to the rails. I used a staple gun to assemble all the sled parts to the 1/4-inch thick piece of plywood. I also added a couple of scrap pieces of 2x4 on top of the sled as "stops" to keep the router from digging into the rails (I'll reuse these long 2x4s in another project). This is a one-time-use jig, so I'm not making it very tidy.

That's the set up, just make sure the rig slides smoothly up and down the rails without sagging (I wish I had been more careful about that part) or binding anywhere.

Step 3: Cut the New Bench Top

Use an appropriate router bit, mine is 3/8-inch wide, with a quarter-inch shank. Adjust the router so it cuts through the thin plywood and about a quarter-inch into the bench material you want to remove. The first cut will create a slot for the router bit in the slide, in addition to removing bench material, so go slowly on the first cuts. Don't try to cut more than a quarter-inch deep on each successive pass, less would probably be better, that seasoned Douglass fir is tough wood.

I went back and forth with the router, cross grain, moving the slide along and going back to pick up any missed places. Go over all the wood you can reach before lowering the bit for the next pass. Take your time and don't try to hog out too much material at once.

Once the surface was more or less level all the way across, I lowered the router bit just a couple of millimeters (about a 16th of an inch) and went over the entire surface again, this time sliding the sled along the rails, cutting the top of the bench with the grain.

The final routed surface is far from smooth, but it's fairly flat. Note, I left a little bit of the old surface on the back corner of the bench. I just didn't feel like taking the whole surface down another notch, and this little corner won't be too noticeable to somebody sitting on the bench.

Looking back, I wish I had put another pair of blocks between the bench and the rails, to keep the rails from sagging while working the surface down. It really isn't a big deal, but it seems like the center of the bench is a little lower due to the rails sagging. The main idea is to make the bench pretty flat to sit on.

Step 4: Finish the Bench Top

Finally, I removed the rails and went over the surface with a belt sander (sorry, no photos of the sanding) to make the top smooth. I used a coarse grit sanding belt, followed by a medium grit sanding belt. The last step was a quick going over with an orbital sander and finer sand paper to be sure no one would get a splinter.

The freshly sanded wood receives a couple of coats of urethane sealer to finish the job. The new bench feels nice and level. And with a foam pad it will be even more comfortable for diners. I may fill in the screw holes with some wood putty later, if I get around to that.

For a minimal investment (two long 2x4s, and some parts from the scrap bin) this jig was reasonably effective, and the bench doesn't wobble and is actually comfortable to sit on.