In the groove. Maneuvering a large piece of wood across a stationary machine can be inefficient and yield less-than-perfect results. Take the easy way out and cut dados with a router and this simple jig.

The quest for accuracy in woodworking is often like trying to find your way out of a maze. Make the wrong choice early on, and you’ll find yourself going in circles and not getting any closer to your goal. Often the entrance to a path looks promising but soon becomes an uphill journey.

Making dados seems like a simple task, and it is. The difficulty is that there are so many different ways to go about it that it isn’t always clear which choice makes the most sense. On the surface it would seem that setting up an accurate stationary machine, such as a table saw or router table, would be the best way to go. This is true if the pieces are small enough to be manageable all the way across the machine’s table.

When the work gets too large, however, it makes more sense to move the machine over the work than to move the work over the machine. This simple jig and a router will make dados that are square, straight, a predictable width and depth and, most important, exactly where you want them to be.

The two parts of the jig will likely come from your scrap bin. A piece of plywood with a straight edge guides the base of the router. Its thickness and width allow you to clamp it to your work without interfering with the handles of your router. The second piece registers the jig at a right angle (or any other angle if you’re so inclined) and locates the exact position of the router bit. I make this about 1⁄8" thinner than the workpiece, 11⁄2"-2" wide and about 12" long.

Pivot to Perfect Alignment
Two screws and a dab of glue hold the parts together. Spread glue where the parts overlap and drive one screw. Use a square to align the parts at a right angle and drive the second screw. Set the jig aside and let the glue dry. It’s important to let the glue dry completely. If you don’t wait, the two parts of the jig can slip out of square.

Install a straight bit in your router that matches the width of the dado you want to cut. Adjust the depth of cut by measuring from the router base to the tip of the bit. In most cases, the dado will be 1⁄4" deep or less so you can make the cut in one pass. Clamp the jig to a piece of scrap and make a cut, keeping the base of the router firmly against the fence, cutting a notch in the other part of the jig. This notch will be used to align the jig with your layout marks.

On the Straight and Narrow
A router with a straight edge on its base will work better than one with a round base. The advantage of the straight edge is that it keeps the router in the same location on the work. A round base may not be perfectly centered and if so, holding a different part of the base against the fence will change the distance from the fence to the bit.

If your router has a round base, you can either make a new baseplate with a straight edge, or you can make a mark on the router base to ensure the same point is held against the straightedge of the jig.

To use the jig, lay out the location of the dado on the work, line up the notch in the jig to the layout marks, then clamp the jig in place. Use another clamp to hold the work to your bench if you need to. Turn on the router and run it across the work, holding it firmly against the fence.

If you have numerous pieces, you don’t need to measure each and every one. You can lay out one piece, stack up the parts and use a square to transfer the marks to the front edge of all the pieces. Marking both sides of the dado will prevent you from cutting on the wrong side of your layout line.

If you want to make an odd-width dado or stopped dado, you can easily add a second straightedge, or a block to limit the router’s forward progress. You can also use a combination of bits. After routing a shallow dado, you can come back with a dovetail bit to make a housed-dovetail joint. The cuts will be centered because the distance from the mid-point of the cutter and the jig is constant.

The Other Part of the Equation
All that remains is to fit the piece that goes in the dado. The fit should be snug, taking a little effort to push it together by hand, but not so tight that you need to beat on it or crank down unreasonably with a clamp. How to achieve this fit depends largely on the material you’re using. Solid wood is easy; plywood can be a nightmare.

Now is the time to get a good finicky measurement of the thickness, and a pair of dial calipers will deliver that easily. If you’re planing your own solid wood, measure the width of the dado, and compare that to the thickness. If you mill the wood just slightly oversize, a few strokes with a handplane allow you to sneak up on a perfect fit.

Plywood is a different matter. Most plywood varies in thickness within a sheet and that deviation is enough that you won’t be able to match it. If you try to match a thin portion, the rest of the shelf won’t seat in the joint, and if you match a thick portion you will have a visible gap somewhere along the joint line.

Don’t waste your money on router bits that claim to be undersized to match plywood thicknesses or believe someone who tells you the plywood is a metric size. What you really need to do is match the wavy edge on the plywood to the straight dado you cut.

If you’re working with nominal 3⁄4"-thick plywood, rout the dado with a 5⁄8"-diameter bit. At the router table, set up a slot-cutting bit so the bottom of the bit is 5⁄8" above the table and outside the fence the same distance as the depth of the dado. Run the end of the plywood under the bit, producing a small rabbet. Because you’re capturing the plywood between the table and the bit, you’ll produce a consistent thickness that will fit the way it ought to. PW



Bob is a senior editor of Popular Woodworking and author of several books on Craftsman-style furniture.
His web site is craftsmanplans.com.