A table saw and dado blade will make quick work of this eye-catching, interlocking corner joint.
Corner lap joints aren't as strong as mortise-and-tenons or as quick to assemble
as pocket screw joints, but their interlocking crossover pattern is quite attractive
when a decorative joint is what you want. These simple joints are excellent choices
for picture frames, light-duty cabinet doors or face frame construction. It's a
good idea to reinforce the crossgrain glue bond by driving a pair of pegs, dowels
or screws through the parts to create a mechanical connection.
STEP 1: Since most of the geometry of a corner lap joint is exposed, it's important
that your dado cuts are crisp and clean. A good way to minimize tearout is to outfit
your table saw with a zero-clearance throatplate made of scrap. Use the saw's factory
throatplate as a template for cutting and shaping the new throatplate blank, and
plane it to match the thickness of the original. Then, install a wide dado blade
in the saw and clamp the rip fence partially over the new throatplate to hold it
securely. Make sure the fence is still clear of the blade. Start the saw and raise
the blade slowly up through the new throatplate to create a zero-clearance blade
opening (see Photo 1).
STEP 2: A corner lap joint is really nothing more than a pair of rabbets with elongated
tongues that fit together. Usually, the joint parts are the same thickness and size.
Once assembled, you'll want the faces of both parts to meet flush. The trick is
milling these rabbets so they mesh together perfectly. To find the correct blade
height, cut a rabbet into both ends of a piece of scrap material that's the same
thickness as your joint parts. Set the blade height to about half the thickness
of the scrap and make the test cuts, backing them up with a miter gauge outfitted
with a sacrificial fence (see Photo 2).
STEP 3: Cut the test scrap in two and fit the rabbets together. If the part faces
and tongues meet flush, like the sample shown in photo 3, your blade height is perfect.
Raise the blade slightly if there's an offset between the faces - the tongues are
too thick. Lower the blade a tad if there's a gap in the middle between the parts
- the tongues are too thin.
STEP 4: If your joint parts are the same width, you can use the rip fence to establish
the length of both tongues easily and without cutting to layout lines. You'll need
to clamp a scrap block to the rip fence to serve as a step-off block during cutting.
Here's how to find the tongue length: set the step-off block against the rip fence,
and place the edge of one workpiece against the scrap block. Adjust the rip fence
until the outer edge of the workpiece lines up with the outside edge of the dado
blade (see Photo 4). Lock the fence.
STEP 5: Position the step-off block toward the front of the rip fence, far enough
so the workpieces will clear it before making contact with the blade. Clamp the
block to the fence. Then, set the joint parts together to determine the relationship
of the parts, and mark the inside tongue areas you'll cut away (see Photo 5).
STEP 6: Cutting the tongues to shape will take several passes, starting with the
workpiece pressed against the step-off block. Make this first pass, sliding off
the step-off block and holding the workpiece tightly against the miter gauge fence
(see Photo 6). For large or long workpieces, it's a good idea to clamp the workpiece
to the miter gauge to keep it from accidentally shifting.
STEP 7: Now, slide the workpiece about one blade width away from the step-off block
and make a second pass to hog out more waste (see Photo 7).
STEP 8: Repeat the process until you reach the end of the tongue, then run the full
tongue over the blade again to remove any bits of waste that may still remain (see
Photo 8). Cut the other workpiece tongue the same way.
STEP 9: Slip the joint parts together and check their fit. If your blade height
and rip fence settings were accurate, you'll have a square, flush and neatly machined
lap joint that's ready for glue up.