How do i create a really long box joint?

Question

The Sane Man's Approach

As I'm about to remove a wall I have to add a steel I-Beam as a post to support the ceiling. My plan is to cover all four sides so that it looks like a wooden post. Normally I would cut mitered angles and try to make the seams as unobtrusive as possible or just glue and brad nail boards together in place as there is only exposed long grain and I think that wouldn't look too bad either.

My Delusions Of Grandeur

While mulling this idea around in my head I came up with the idea of joining the boards with box joints. Although they wouldn't need any strength to speak of, I think this would give the post a nice look. I'm even considering using two or four different types of wood to maximize the effect.

There are a ton of guides, tutorials, discussions out there on how to make box joints with the table saw or a router table and some kind of jig. By now it feels like I watched and read ALL of them :). While being excellent for cutting box joints for their intended size and application, none of them suit me and my idea. The room is somewhere under three meters (ten feet) high and so would the boards be.

My Questions

• Is the grain direction a problem for me? Box joints are usually cut into end-grain, I would cut them into long-grain.
• If I use different wood(species), would the joints explode all over my living room while they probably expand at different rates/times?
• What is the best tool to use?
• What would be the best jig to use?
• Is this completely loony or is there already an established way of doing this that I simply didn't find?

Answer 1

Is the grain direction a problem for me? Box joints are usually cut into end-grain, I would cut them into long-grain

In terms of cutting the joint initially, possibly depending on the method chosen. In terms of the finished joint, very much yes (see final point).

If I use different wood(species), would the joints explode all over my living room while they probably expand at different rates/times?

Expansion and contraction is across the width of boards, not along their length, so I don't think you have to worry about it.

Even without using quarter-sawn wood the expansion across the width of single boards is usually small enough that it can safely be ignored.

What is the best tool to use?

So I need to say it? There is no best. This sort of thing is so highly dependent on what's available and the experience of the user anyway that it's always better to ask this sort of thing differently, e.g. "What tools would be good choices to cut this joint?"

What would be the best jig to use?

The jig you envisage in your Answer is probably the perfect solution here if cutting this joint by machine. And while I'm a big fan of jigs perhaps no jig at all is worth a moment's consideration, because one clear option that was overlooked above is doing this using hand tools.

I'm not going to actually recommend this because the amount of work would seem so overwhelming (and anyway it's trumped by my final point) but you asked for options so......

Hand tools
There's technically no limit to how long you could make a joint like this by hand.

This might seem contradictory given the known lower accuracy of hand processes compared to machine-cut joints, but just as in dovetailing any irregularities are transferred to the mating piece. So yes, you will get irregularity but the process factors it in.

Also these joints would not require the level of tight fit that you'd want on the dovetails on fine furniture. Sloppy fit would be much more acceptable here, even desirable given the difficulty in interlocking over such lengths, and as you say it's not like this needs any strength to speak of.

You could use a router to hog out the centres, for speed and to reduce the effort needed, but an additional benefit of using hand tools for almost all the wood removal is that it gives almost total control over tearout if you do it right as all the cutting processes go into the wood, never exiting on long grain.

Why this isn't a viable idea

The elephant in the room: the little cubes of the cut joint are all short grain. This isn't just weak theoretically it's weak in practice. You will find if you do a quick test that even modest pressure against the fingers can be enough to snap them free.

I can't find any images but people have cut box joints/comb joints/finger joints as you're proposing here, 90° to the normal orientation, and almost invariably they get one or more breakages just when dry-fitting the joints. If you did manage to make it through that stage and complete glueing up without incident manoeuvring the piece into position you might get failures along the corners.

Answer 2

I'll try to answer the question as good as possible myself, but on a few points I'm in over my head.

Is the grain direction a problem for me?

As far as the cutting goes it shouldn't be a problem, one should use sacrificial boards in the places where the router-bit/saw-blade comes out though, as the long grain will probably tear out otherwise.

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If I use different wood(species), would the joints explode all over my living room while they probably expand at different rates/times?

While the wood probably won't explode I think the expansion/shrinkage can cause the glue to fail or produce cracks in the boards. I think investigating and using species that have similar expansion rates would help mitigate this issue.

This could be a question on it's own.

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What is the best tool to use?

The following listing is not sorted by viability.

Table Saw

• (+) produces clean cuts
• (+) jigs can be built to cut very reliable
• (-) expansion tables are seldom wide enough and handling such long stock can be dangerous
• (-) space - hobby workshops are seldom big enough for tasks like this on the saw.

Miter Saw

As I don't have one and never used one it's hard to imagine how this would work with one, I'd go with NAH!

Router Table

I think the advantages and disadvantages align pretty much with those of the table saw.

Handheld Router

In my opinion the handheld router would be suited best for this task.

• (+/-) as reliable in terms of cutting exact recesses as the jig one is using
• (+) routers are cheap and many have one
• (+) can be done wherever there is power
• (-) tear-out is a big risk

Clamp the four boards with a bit of excess length and a sacrificial piece on the side where the router comes out. The excess length should be about the length of at least two joint recesses so you can omit calculating for offset. Cut the recesses with the handheld router and a comb like jig (more about that below). After cutting the joints dry-fit it and mark the length on the dry-fitted "box". Then cut the boards to length and install them.

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What would be the best jig to use?

When using the handheld router method described above, a comb-like jig like the one from Leigh Industries would probably work best. I'd build the jig from some scrap and insert a locking piece in the first recess so that the jig may be moved along the boards precisely. Something along the lines of this:

The Jig

The First Cuts

The Next Few Cuts

Answer 3

A simpler option may be to fake it.

Instead of an actual box joint you could make a fake looking one by making the cuts in one board and then filling the voids with wood in the other grain direction. This works really well with contrasting woods.

The actual joint will be a butjoint like the second image. But it will look like a box joint.

That way you don't need to be as precise with the long box joint and risk a misalignment building up until it doesn't fit any more.