A router sled is combined with a router to flatten stock that has twisting, warping, bowing or other issues with uneven material. It works by making successive passes over the workpiece taking off a little bit at a time. After each complete pass you lower the router bit down a little until everything is level. A plunge router is best for this application because you can easily adjust the router down and set stops.
The sleds themselves are easy to make with some plywood, glue and screws. For mine I decided to use 1/2" baltic birch which can be found at a lumber dealer. I started with figuring out what the average width of board I would be flattening is to get myself within the ballpark for sizing. Once I had my size set I then cut the bottom board a few inches wider than the footprint of my router and whatever length is desired. The router I use is a Festool OF1400 plunge router which you can find a link to at the bottom of this page. This router has a flat spot on the edge of its base, which works well up against a guide fence. Next I cut four pieces about 3" tall by the desired length and then glued them together leaving myself with two 1" fences. The fences were then glued and screwed to the edge of the sled bottom. The inside width allows my OF 1400 to easily slide between the two fences. On the underside everything that was screwed has been countersunk so that it will slide smoothly when operated.
Once the glue dried and been cleaned up I used a candle wax on the inside surface to help the router slide smoothly. I use a 1-1/2" planer bit on my router for this next step. Since the router now glides along the fences smoothly I plunge the router down about 1/8" at a time and slide it down the length of the sled stopping 4-6" shy of the ends. I keep making passes lowering by 1/8" each time until the bit works its way through the bottom. I remove the router and sand up the edges and the sled is now finished.
The sled is really simple to operate. First you will need a surface to mount your work piece onto. I usually use a piece of plywood on top of my table saw. If the work piece is fairly flat on one side I can get away securing it with 2-way carpet tape. Sometimes the work piece is really bowed and you kinda have to get creative using shims and clamps. I usually use my fences and wedge the piece in between them and doesn’t move on me. You can place in some end stops as well as seen below.
The fence I use is nothing more than scraps of plywood about half an inch taller than the work piece. If the slab is 3” thick and 60” long then my fence is 3” to 3-½ tall finished size. Usually I will just make a t-shaped fence and then screw directly to the bottom plywood. As seen above this allows you to pinch the piece with the fences as a method for securing it.
Once the fences are setup then all that is left is to set your newly made sled on top and set your depth. I will set my depth so that it takes off about ⅛” of material at the highest point. Take controlled passes from one end to the other and then move the fence over left or right allowing a little overlap in the cut each time. Repeat, repeat, repeat. Lower the router bit so you take about ⅛” passes until you get close to your desired thickness. The last pass should be a lighter pass to give yourself a cleaner surface. This isn’t mandatory since you will be sanding afterward.
Congratulations! Now you can make some awesome slabs in your shop! I have since added in stops on either end of my sled so that the router doesn’t overshoot the sled. These pieces just screw into the end and stop the router by butting up against the base. Below you can find a few links to the tools used. If you decide to purchase them please use the links here, as they support these blog tips and my shop.