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Half-lap joint jig

Half-lap jig diagram
Cutting on-the-money rabbets for half-lap joints with a portable circular saw and handheld router is a breeze with this two-in-one jig.

Build the jig

You can make a simple half-lap joint on your tablesaw with a dado blade, miter-gauge extension, and a workpiece stop. But when working with very long parts, cutting them on a tablesaw proves awkward at best, and maybe unsafe. And what if you don't own a tablesaw? Here's how to form the rabbets that make up a half-lap joint with portable tools. With this jig, you'll save time by cutting several parts with one setup.

To make the jig base, measure from the motor edge of your circular-saw base to the blade, and add 1/8". Then chuck a 1/2" straight bit into your router, measure from the edge of the router subbase to the bit, and add 1/8". To these two dimensions, add 1 1/2" for the guide, and cut an 18"-long piece of 1/2" medium-density fiberboard to this width.

Cut the guide to the size shown right. Glue and clamp it to the base. Then, with the glue dry, clamp the base to your workbench with the saw side overhanging. With the saw base against the guide, trim the saw side to width. Now with the router side overhanging the workbench, use your router with the 1/2" bit to trim the router side to width.

Measure the trimmed width of the base, and cut two cleats to this length. Then glue and clamp the cleats to the bottom of the base, flush at the ends and edges.

Put the jig to work

Clamp together, edge to edge, the parts to be rabbeted. Secure them to your workbench with the ends and faces flush. Mark a line across the parts at the rabbet shoulders. Then, to prevent chip-out when the router bit exits the last part, adjust the cut depth of your circular-saw blade to one-half the thickness of the parts. Now cut a saw kerf at the rabbet shoulders, as shown top photo.

Chuck a 1/2" straight bit into your router, and adjust the cut depth to one-half the thickness of the parts. Then rotate the jig and clamp it to the parts, pushing the infeed end cleat against the first part to keep the jig square. Now clean out the rabbets, as shown second photo. Move the jig in from the part ends until the cut meets the saw kerf.

Half-lap jig
Align the saw edge of the jig with the rabbet layout line, clamp the jig in place, and cut a saw kerf to define the rabbet shoulders.

Making a half-lap jig
To clean out the rabbets, start with the router edge of the jig 3/8" from the stile ends, and make repeat router cuts to the saw kerf.

Half-lap jig diagram
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