Whole truth about half-laps
As you can see in the photo above, a half-lap joint consists of two workpieces reduced to half their thickness where they overlap. This creates lots of face-grain-to-face-grain contact area for glue joints. By contrast, simple butt joints rely on an end-grain-to-edge-grain bond, far weaker than a half-lap. In our tests, half-lap joints outlast even a dowel-reinforced butt joint.
One potential downside to half-lap joints: They reveal end grain on the outside edges of the joint, which may create appearance problems for your project. We often use half-laps for shop-cabinet door and face frames, workbench leg frames, outdoor furniture, and internal web frames for furniture such as dressers, where strength outweighs formality.
The good news is you need only a tablesaw or router table to make a half-lap joint. On the tablesaw, we prefer a stacked dado set for fast, smooth results. If you don't own a dado set that shears cleanly across the grain and leaves the sawn surface smooth and flat, use a router table with a straight bit or down-cut spiral bit.
Here, we show how to make corner and T-joints using a tablesaw, but you easily can adapt these techniques to your router table.
Lap joints in four easy steps
1. Install your complete dado set for the widest cut possible. Raise the blade above the table—the exact height isn't important yet. Adjust the rip fence so one edge of the workpiece butts against the fence and the opposite edge aligns with the side of the dado set farthest from the fence [Photo below].
If all the workpieces are the same width, you can leave the fence in this position for all cuts. If you're working with pieces of different widths, use a workpiece's adjoining piece to set the fence for its half-lap cut. On a door frame, for example, use the horizontal workpieces (the rails) to set the fence to cut the vertical workpieces (the stiles), and vice versa.
2. Set the cutting depth of the dado set so it removes precisely one half of the workpiece thickness. This may require several test cuts in two pieces of scrap stock of the same thickness as the project workpieces.
After you rabbet one end of both scrap pieces, lay them on a flat surface and align them [Photo below]. The top and bottom faces should be flush with no gap between pieces. If not, adjust the blade height and repeat until the pieces slide firmly together.
3. Mark the mating faces of the project workpieces that you'll rabbet. Attach an auxiliary fence to your miter gauge to support the workpiece during the cut and prevent tear-out on the back edge.
Because you're not cutting through the stock, it's safe to use the rip fence together with the miter gauge. Never use these two together when cutting completely through a workpiece.
Position the workpiece with one edge against the auxiliary fence and the end butted against the rip fence. Then cut successive passes until you reach the end of the workpiece [Photo below].
4. Apply glue to all mating surfaces of each joint, clamp the assembly, and check for square [Photo below].
Make a half-lap T joint
Sometimes, you need to cut a half-lap joint in the middle of a workpiece—for the centered vertical divider of a cabinet face frame, for example. Follow this three-step method to create a T joint [Photo below].
1. Mark the mating workpiece, using a sharp pencil or knife [Photo below].
2. Brace the workpiece against a miter-gauge auxiliary fence and align a pencil mark with the edge of the dado blade. If you're cutting multiple workpieces shorter than the auxiliary fence, clamp on stops.
3. Butt a workpiece end against one stop (or align the blade with a mark) and make a cut [Photo below]. Then butt the other end of the stock against the opposite stop, make another cut, and remove the material in between.