By Robert Lang
From the Spring 2005 issue of Woodworking Magazine, pages 25-27
Dados are a “bread and butter” kind of joint. They’re simple and strong, and a router with a straight bit and a way to guide it are all you need to make them quickly and accurately. Of course, dados can be made with the table saw and a dado set, but more often than not, this is an unwieldy operation if you are working with long pieces. Much of the time it’s easier to move the tool across the wood, than to move the wood over the tool.
The “T-square” jig is an ideal way to guide the router for cutting dados. It can be put together quickly, and adapted to many different circumstances. In its simplest form, there are only two pieces: The guide that goes on top of the work and the bar that goes against the edge of the work, which provides a place to clamp and to accurately locate the cut. Additional pieces can be added to the jig to make clamping easier and more secure, to allow you to stop and start the cut, to make an odd width and to keep your router from “jumping” out of place when you switch it on.
I’ve made a lot of these jigs. While they are simple in form, making a good one requires attention to detail and a few tricks. Too often, these T-square jigs aren’t quite at 90°. Or there is no easy way to clamp it to your workpiece. Both problems lead to sloppy work. Here’s how to avoid those common pitfalls.
Select the materials for your jig with care. I like to use 3⁄8″- or 1⁄2″-thick Baltic birch plywood for the guide – it’s a stable and strong material, and the multi-ply edges wear well. I’ll make a couple of cuts on the table saw and check the edge against a reliable reference (such as a machinist’s straightedge or my table saw’s fence face) to ensure I have a straight edge to run the base of the router against. The other advantage of plywood is that it is thin enough to be out of the way of the handles of the router, and can be easily cut wide enough to provide plenty of surface area for clamping.
I usually make the bar from solid wood that is about 1⁄8″ thinner than the workpiece (i.e., I use a 5⁄8″-thick bar to cut dados in 3⁄4″-thick wood). This ensures that the guide part of the jig lays flat on the surface to be dadoed, and that the jig and the workpiece can be firmly clamped to the bench. Before putting the jig together, I like to run the guide bar stock over the jointer to make sure it also is straight and true. With both parts straight, it’s time to put this jig together.
Square the Square; Choose a Router
Of course, if the jig isn’t square it won’t be worth using, so here’s how I make sure that it goes together precisely. With a bit of glue and one screw I fasten the guide and bar together, as shown below. Using only one screw at first lets me adjust the angle between the guide and the bar to a perfect 90°. Then I clamp a speed square across the two parts. Once the square is in place, I drive a second screw through the two pieces to make the attachment permanent, and then I wait for the glue to dry.
While the glue is drying, I set up a router with the appropriate bit. A router with a flat area on its base plate (as shown below) is the one to use, because a round-base router is likely to have the bit off center. If the bit isn’t perfectly centered, and the exact same spot on the router base isn’t run against the edge of the guide, the cut will be off the mark. If a round-base router is all that is available, I replace the factory base plate with a square or rectangular shop-made one. Even with a square-base router, it’s prudent to always run the same edge of the router base against the guide, as there can be some variation in the distance from the bit to the edge. Some routers will have a different distance from the bit to the end of the base plate, and this may cause problems if you’re making a stopped cut. Draw or mark on the router base plate, and on the jig, an indication of the proper orientation.
With the depth of the router set to the correct dimension, I make a test cut, letting the bit cut through the bar. This cut through the guide bar can now be used to locate the cut exactly on the work, as shown below – no adding the distance from the base plate edge to the bit or any other fussiness. Lay out the location on the workpiece, locate the cut in the guide bar against the marks and clamp the jig in place.