I had a very nice slab that I wanted to make into a coffee table. I flattened this slab right on my workbench! I am happy to say that flattening a slab of wood with a router was even easier that I thought it would be! Not only was it easy, but the results were great!
Step 1: Make a Simple Jig to Hold the Router
The first thing I did was build a simple jig to hold the router. The jig is just 3 pieces of plywood or MDF with a router bolted in the center. I also added a dust chute.
I cut a piece of 1/2" MDF about 6" wide for the base of the jig. This needs to be twice as long as the width of your workbench. My bench is 32" wide so I made the jig about 65" long.
I wrapped a piece of paper around the base of the router and used a pencil to mark where the mounting holes were. I also marked where the router bit would come through the jig.
Next I transferred the hole locations to the piece of MDF and drilled two countersunk holes to mount the router to the jig. After that I drilled a hole in the center where the bit would come through.
The sides of the jig are two pieces of 3/4" plywood that are about 2" wide. I simply clamped and glued them on to the edges of the base piece.
I happened to have a 4" dust chute laying around, but I think you could probably find something similar in the sheet metal area at Home Depot.
I used a jigsaw to make a little relief cut where the dust chute would be. Then I used a few angle brackets to mount the dust chute to the jig.
Step 2: Make Side Rails and Clamp Them to the Workbench
The side rails are just two 6" wide pieces of 3/4" plywood that are about the same length as your workbench.
I tested clamping them on edge right down on my bench and it "looked" OK, but I was worried that I might bump them and knock them loose while routing the slab. So, just for insurance I glued a small wooden block at each end so that I could clamp them more securely to the bench top.
I also rubbed some canning wax along the top edges of the rails after they were clamped down to my bench. This ensured that the jig would slide freely across the rails while routing.
Step 3: Setting Up the Router Jig
It is important that the piece you are routing on is secure and that it won't move while you are routing. In my case the slab was 42" long and 5" thick, so I didn't worry about it moving! If I were working with a smaller piece of wood I would clamp it in place in some way that didn't interfere with the flattening process.
It is also important that the slab is as level as possible. This will ensure that you don't remove more wood than necessary to achieve a flat surface.
In this case the slab had a high spot in the middle. I adjusted it so that the gap under my level was about equal at each end.
From a safety perspective, the most important thing is to adjust the depth of cut so that you don't take off more than about 1/4" on each pass. If you are working with some especially hard or knotty wood then maybe even less than 1/4" would be better.
The only thing left to do is to attach the dust hose and start routing!
Step 4: Using the Router Jig
I am fortunate that I have a nice dust collection system and was able to hook the jig directly into it. But, a good vacuum cleaner mounted behind the router would help immensely.
Using the jig is really easy. The mass of the jig makes it much less likely that the router will get away from you. That said, you are still dealing with a very large router bit, so be careful!
I had heard that using a climb cut would leave a cleaner surface and help to direct the dust toward the dust chute, but in actual practice it didn't seem to make much difference. I just carefully and methodically moved the jig back and forth across the slab until I got to the end. In my case it took two passes to get it perfectly flat.
Step 5: Finishing Up
Even though the surface was perfectly flat when I was done the surface still needed a bit more work. The surface of the slab had little "tracks" from the router bit when I was done flattening with the router. Initially, I tried cleaning these up with my random orbit sander, but it left the surface kind of wavy. I had much better luck when I started using a belt sander.
All in all I am very happy with this process. It was much easier than I expected it to be and I would not hesitate to use it again. In fact, I already did!