Wood magazine joint testing

[RichG]

I've always thought of a biscuit joint as sort of a mortise and loose tenon joint. It's too bad they didn't include loose tenons in the test. I've often wondered if a loose tenon is similar in strength to M&T. Based on the biscuit results I would guess not.

[drumpriest]

Different test was run about a month earlier with loose M&T vs integral M&T. There was a 5% strength difference. Loose M&T is about 5% weaker than integral M&T.

You can't overlook volume of the tenon, a biscuit is 1/8" thick. (slightly smaller, but swells with glue). A 1/4" tenon is more than 2* stronger than a 1/8" tenon. Also, depth of the mortises. A biscuit is football shaped, and doesn't go all that deep into either side of the joint. A loose tenon is square, and can go as deep as you want.

Basically, there is a diminishing return on mortise depth. That's why loose tenons are almost as strong. Once you go deep enough, it's as if the tenon was integral.

[Jeffrey Schronce]

You can't overlook volume of the tenon, a biscuit is 1/8" thick. (slightly smaller, but swells with glue). A 1/4" tenon is more than 2* stronger than a 1/8" tenon. Also, depth of the mortises. A biscuit is football shaped, and doesn't go all that deep into either side of the joint. A loose tenon is square, and can go as deep as you want.

In addition to those valid points, a biscuit is composed of pressed together wood fibers versus solid, long grain wood which a tenon is typically composed. Even in a loose tenon versus biscuit situation, the loose tenon is a much stronger material.

[JimD]

The compressed wood fibers in decent biscuits is solid hardwood. It could be better material than a loose tenon or worse depending on your species. The big advantage of biscuit joints from a strength standpoint is a good fit is easy to obtain. You get very little strength from the smallest size of biscuits as Wood illustrated. The other thing you can usefully say about their results is that a biscuit is not a great joint for a 2 inch wide face frame. In wider material with a #20 biscuit, you would have a lot more strength - as FWW showed in a previous test.

Jim