Torsion box joinery question

I’m starting to build a torsion box for my LR4, it’ll be 3/4 MDF ribs 4” tall, joined with an edge joint exactly half the height of the ribs (like the pic below).

. The top and bottom will be a 1/4” underlayment type plywood, glued and stapled to the ribs.

(Yes, I know it’s overbuilt, but this is what I’m going with…)

The dado for the joints will be a friction fit - loose enough to join by hand, no hammering needed.

My question is whether I should glue the joints or not, given that the top and bottom will be glued and stapled - my concern is whether I’ll be able to make the box flat during assembly if I glue the joints - lots of potential for something to move a little bit. If I rely instead on the top and bottom sheets to hold everything square and flat, that seems like a better option.

So based on my research during making my torsion box table, apparently the goal is to have an opening for the cross lap joint that is exactly the same width as the thickness of the material.

I glued my joints, but for some reason did not glue my skins. Also my skins were too thick. If I ever redo it, I will use thinner skins, and I will glue both the joints and the skins.

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Thanks, @DougJoseph - I’ll tune the dado blade to make a perfect (but not too tight) cut (I’ve got a range of shims to let me get to 0.005” adjustments if needed). An earlier attempt saw me using an exact width dado, which ended up needing a lot of hammering (which, in turn, broke a lot of MDF ribs).

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Makes sense!

The videos I was watching were wood working not CNC so that concept of “exactly the same width” is not as precise.

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typically, these joints are made with an allowance that makes them a nice friction fit when dry fit, but allow for expansion after glue addition to really tighten them up. Honestly, if its a good friction fit, and you glue the skins, you’ll get a great torsion table. I doubt you would see any additional benefit to gluing the half laps, and may even introduce (probably irrelevant) variance due to unequal glue volume addition and squeeze out.

personally- wouldnt glue the half laps, would glue to skins

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The strength of the torsion box comes from the skins being held to the ribs, and the ribs being unable to move. A tiny amount of movement reduces the strength of the box greatly.

As a result, I glue and screw everything.

However, if you glue the skins, it will be fine, because that will hold the ribs and spars in place. Look at Ikeaboard as an example. That is expanded corrugated paper, glued to the skins inside, and it’s plenty rigid.

I would insist on the skins being glued, as that bond is far superior to any reasonable number of screws or nails holding the skin in place, and will result is a far superior torsion box, although screws will still result in one good enough for our purposes.

Remember, we aren’t doing high precision machining. Or if we are, the CNC isn’t doing everything. I’ve seen luthiers doing guitar and violin necks going for 0.001" tolerances, but not from a raw CNC piece. I’m not going to machine a cylinder head on my LowRider either, nor would I suggest it to anyone.

We can easily go way overboard on the table here. It’s nice to have something that’s good, but there is such a thing as “good enough.”

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I mostly agree with Dan, it comes down to how the skins are connected and whether you have a top skin only or both top and bottom.

Think about the notch that is cut in that rib, it basically makes it 1/4 as stiff at that one point and any load will either try to pinch the notch closed or bend it open. If you make it a tight fit then it can’t pinch closed but can open easily. If you glue/screw/cleat/whatever it to the crossing rib then it can’t open or close easily.

If you glue/screw the skin on the other side then you’ve braced the open part of the notch and it can’t close or open.

So with no bottom skin or poorly attached skins then that joint is important.

With bottom skin well attached, I doubt those joints even need to touch.

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My thought is put together the grid and bottom, glue and brad those and then surface the top and then glue the top

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Progress is being made - this is the first half, just loosely assembled to verify everything fits. (you can see my LR4 patiently waiting in the background…)

Second half looks just like it and joins at the obvious location. Overall size is about 64” wide by 116” long.

The plan is to skin both sides with 1/4” underlayment plywood (glue and 18ga staples), then add MDF “runners” on the left and right edges for the LR4 to ride on and a full sheet of MDF as a spoil board. Ought to weight about a metric ton when I’m finished… :wink:

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