How do I cut a blind half-lap T-joint?

Question

(Apologies if this has been answered before. This probably has a really obvious answer.)

I need to connect two 4x4s in a "T". (These are one leg and a center / longitudinal support for a long table.)

I decided on a half lap joint since it looked relatively easy to do, but my wife doesn't want to see the back of the joint on the table feet. I thought: easy enough, I'll just make it a blind joint so that the end of the long board doesn't go all the way through, leaving about 3/4" on the end of the lap.

However, I'm not sure what the best way is to cut this into the table foot (about 32" piece of 4x4). My initial attempt on a piece of scrap was to drill a few holes in the corners of the waste area, hit it with the jig saw and chisel out the rest. This works, mostly, but is something of a pain and doesn't get me a smooth surface at the bottom of the cut for the long board to mate with.

I'm not super-comfortable with the idea of putting a 1.5" bit on my router as I'm a bit afraid of snapping the shaft if the teeth catch on a knot.

Suggestions? I considered a mortise-and-tenon joint, but that leaves me with the problem of cutting a 1.5-1.75"-deep mortise, which is basically the same issue I'm having here.

I could probably just get a large bit or hole saw, use my neglected drill press and clean up the excess with the jig saw or chisel, but is there a better / more accepted / safer way to do this?

Answer 1

Kudos for doing a test run first and identifying the problem ahead of time!

Standard advice from older woodworking guides (or any modern handtool book) for doing many things like this: cut the bulk of the material out with the chisel (if you like drilling out some waste first), then smooth the bottom of the recess/channel/dado with a router. In the old days this would have been a hand router but you can do the same thing with a powered router of course.

Suggestions? I considered a mortise-and-tenon joint, but that leaves me with the problem of cutting a 1.5-1.75"-deep mortise, which is basically the same issue I'm having here.

Not really, since the bottom of a mortise doesn't need to be clean at all. In fact the bottoms of most mortises probably look like they were chewed by a beaver. Long as the side walls of the mortise are clean though that's all that matters.

The M&T is the stronger joint, but the half lap (UK: housing joint) is plenty strong enough for what you're doing here if it's cut well and the parts are glued together properly*.

I'm not super-comfortable with the idea of putting a 1.5" bit on my router as I'm a bit afraid of snapping the shaft if the teeth catch on a knot.

You can use a long bit with safety to cut deeply even into very hard wood if you just take a little off at a time. If you had to you could just nibble away 1/8" or less at a time.

A slower feed rate helps reduce the strain on the bit as well, although this can increase the chances of scorching.

Just thought I'd tack this on for anyone with a table saw who might like to try an alternative......

'Cheating' using a table saw
If you have a table saw and the raw material for the legs is thick enough that you can afford to lose some there is a workaround method.

First you saw a thin strip from the leg (you'll lose about 1/8" or so to the kerf), then you cut the lap through on the thicker portion, which is straightforward to do and it's easy to get a nice flat surface using a sharp chisel with fine paring cuts to finish off. Then you simply glue the strip back on the leg, closing the back of the joint.

This can create a seamless joint on the leg if the grain is nice and straight or if the wood is a tight-grained species without obvious grain lines or figure that would be interrupted by the cut.

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*Use enough glue that some squeezes out, use plenty of clamp pressure.