Before you make this cut
As you can see by the illustration, a half-lap joint consists of two workpieces reduced to half of their thickness where they lap over each other. This provides a face-grain-to-face-grain joint with plenty of gluing surface. Simple butt joints, on the other hand, rely on an end-grain-to-edge-grain bond that can break easily. Even a dowel-reinforced butt joint won't prove as strong as a half-lap.
Half-lap joints do reveal end grain on both sides of the joint, so avoid using the joint where such an appearance proves objectionable. We often use half-laps for shop-cabinet door frames, workbench leg frames, outdoor furniture, and internal web frames for furniture such as dressers.
You need only a tablesaw or radial-arm saw to make a half-lap. We prefer to use a dado set for fast and smooth results.
If you don't own a dado set that will cleanly shear cross grain and leave the sawn surface smooth and flat, we suggest you use a router table outfitted with a straight bit. Here, we show how to make corner- and T-joints with a tablesaw, but you easily can adapt these techniques to your radial-arm saw or router table.
Four easy steps to lap-joint sucess
Install your complete dado set so you get the widest cut possible with it (typically 13/16). Raise the blade above the table (exact height isn't important yet). Adjust your rip fence so one edge of your workpiece butts against the fence and the opposite edge aligns with the side of the dado set farthest from the fence (as shown in the illustration).
If all of your workpieces are of the same width, you can leave your fence at this position for all of the following cuts. If you're working with pieces of different widths, keep in mind that you use a workpiece's adjoining piece to set the fence for its half-lap cut. For example, with a door frame use the horizontal workpieces (the rails) to set the fence for the cuts in the vertical workpieces (the stiles), and vice versa.
Step 2: Test cutting depth
Set the cutting depth of the dado set so it removes precisely one-half of the workpiece thickness. Test your cutting depth with two pieces of scrap stock of the same thickness as your workpieces. After cutting the scrap pieces, lay them on a flat surface and align them as shown. The top and bottom faces should be flush.
Step 3: Cut the joint
Mark the face sides of your workpieces so you don't get them confused. Keep in mind that you need to place the face side of one piece up, and the face side of the adjoining piece down, during this step.
Set your miter gauge for a square cut, and attach an auxiliary wooden fence to it. The auxiliary fence should come to within 1/2" of butting against the rip fence.
Now, position the workpiece with an edge against the auxiliary fence and an end butted against the rip fence. Turn on the saw, hold the workpiece firmly against the auxiliary fence, and pass the workpiece over the dado set. Make successive passes to complete the half-lap cut.
Step 4: Clamping comes next
To clamp the joint, first apply wood glue to all mating surfaces. Draw the workpieces together with bar or pipe clamps. Then, bring the glued surfaces tightly together with a small clamp. Place a scrap of wood on the joint faces to protect them from the clamp jaws.
Making a half-lap T-join
Sometimes, you may have to place a half-lap joint somewhere other than at the end of a workpiece. Then, follow these two easy steps.
First, mark the position of the overlap onto the edge of the workpiece that will be cut in its midsection as shown in illustration below right. For accuracy use a sharp pencil.
Cut the T-joint
Set the unmarked edge against the miter-gauge auxiliary fence. Align the pencil marks with the sides of the dado set, and position two handscrew clamps as stops on the auxiliary fence. (If you don't have handscrew clamps, simply clamp two blocks of wood with C- or bar clamps.)
When positioned correctly, the stops will limit the area of removed stock to the space between the pencil marks. You simply butt one end of the stock against one stop and make a cut as shown. Then, butt the other end of the stock against the remaining stop, and make another cut. Finally, remove the material between the two cuts. With the stops set up this way, you can make multiple pieces that will all turn out the same.