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Half lap joint jig

My two test pieces with lap joints cut in them, being held together to make sure the depth of cut is accurate.

I had to cut eight 4″ x 6″ half lap joints for the corners of my barn doors, so I made a shop-built jig.

Lay out the boards

Once I chose and cut all my frame members with grain movement in mind, I laid them out on the floor and marked the joints. You can mark them A, B, C, or 1, 2, 3, or Blue, Purple, Pink — whatever you prefer. Just label them. You don’t want to have to think about which board goes where while you’re cutting joints.

The frame boards laid out on the floor, ready to be labelled.

You can see one of my panels gluing up in the background. To read about that see my post, Gluing up resawn panels.

The half lap joint

In this joint, half of the thickness of each board is cut away so that when the joint is assembled, the resulting thickness is equal to that of one board.

A drawing of the lap joint a seen from plan, top, and side views

Building the jig

When building the jig, test it with pieces that are the same size as your project pieces are. As you’re milling, cut extra just for this purpose.

Because my joint isn’t square, I actually had to build two jigs for this – one for the 4″ wide cuts and one for the 6″ wide cuts. Below are both jigs mounted to the bench. I find it’s easier and safer to mount jigs to the bench by screwing them down. They don’t slip around and I don’t have to dedicate a hand to holding them down. Why not use clamps? They get in the way. Yes, it means that I have to sand the screw holes flat after I’m done and once in a while I have to replace the bench top. I figure it’s easier to replace a bench top every few years than my fingers at any time.

Both of my lap joint jigs mounted to a bench top. My drill and router are sitting on the bench between the jigs, and there's sawdust everywhere.

Here’s my jig with a board in it and router on it. The jig frames are exactly the same depth as the boards they’re cutting, and they support the router as I cut. The fence is screwed down to the frame. I started cutting at the end of the piece and move toward the fence, holding the plastic router shield down with my fingers to keep the router flat.

My lap joint jig set up with lumber in it and the router placed where it will start cutting.
My lap joint jig with the frame, fence, and direction of cut shaded and labelled

When setting up the router, sneak in on depth of cut. That is, cut slightly less than you know you’ll need on the first pass, test it, then hone your depth with an adjustment for the second cut. Sometimes you may need to use three test cuts, but you shouldn’t need more than that. If you do, you’re being too timid. Trust yourself – you’ve got this.

Two maple boards with lap joints cut in them, being held together to make sure the depth of cut is accurate.

Checking depth of cut I

As you sneak in on depth of cut, fit the pieces together between each cut to see how close you are.

Two maple boards with lap joints cut in them, being held together to make sure the depth of cut is accurate.

Checking depth of cut II

Slide your fingers across the top and bottoms of the joint to feel how closely the boards line up. You shouldn’t feel the transition from one board to the next

Once you’ve perfected the jig, you’re ready to cut all the joints. Check each one as you cut. There’s theory (they’re all exactly the same size) then there’s reality (each one is 1/32″ different than the others). Why does this happen? Maybe because table saw fences aren’t perfect. Maybe because you’re fitting them up three days after you cut them and they’ve swollen or shrunk since you cut them. It doesn’t matter why, what matters is that you cut them to fit what you actually have at the moment.

As you work, clean sawdust off the jig between each cut. If there’s dust caught in the corners and edges, your cut won’t be exactly right.

My lap joint jig cleaned up and ready to cut joints

Finished lap joint

And here it is! Both boards are cut and they fit together perfectly. This joint is marked with hash tags for easy reference later. It’s also marked for slots, which I’ll get into in the next post.

My lap joint cut and perfect.