Mortise & Tenon proportions

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Originally Posted by ptrott

Correct me if I am wrong, but isnt the strenght of the biscuit the strength of the joint? It sure would be a lot quicker to mitre and biscuit it, but I need strength and reliability. This door is the main one into the house so will get a LOT of use. As for using bigger wood, it is governed by the frame size (thickness) which is already there.

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I read an interesting article in a book which was a collection of articles from an American wood working magazine. In it, they constucted a wide range of joints, out of identical stock species and sizes and glued them up using identical glues. They then took them to a university engineering lab and tested them to destruction. The joint with biscuits had the highest ultimate strength, (ie took most pressure to break) however it broke without any warning. The M & T joint was the next strongest but it came loose well before it failed entirely. In practice you would pick up the loose joint and repair it before it failed.

Mick

Interesting Mick. I wonder what direction they applied the stresses for this exercise. In a door the joint would be flexing in a twisting motion every time the door is opened unless the person opening it got it dead centre every time. If they tested it by pulling straight against the joint it could well be stronger, but what if you applied a twising motion. The M&T has a much thicker "biscuit" than the biscuit itself. What are your thoughts on that?

I must say I was suprised at the results. It's been a few years since I read it, so I can't remember the exact details. I wouldn't use biscuits for any full size doors or for chairs and tables but have used them for cabinet doors in the past. Used with a good glue, they're stronger than most people give them credit for. I've never had one fail and I've been through quite a few bulk boxes of biscuits.

Mick

If every you start recycling wood from busted furniture left on verges for council collections, you will come to understand this as I did. Salvaging wood from other joints is more often than not a lot simpler, and straight forward, when you dont want to resort to a table saw or similar.

I'd go with thicker timber say 25mm and M&T joints dowel pinned. That is you drill right through the morticed section about 10 mm from the joint line, mark the position of the hole on the tenon when it's inserted, then drill through the tenon offsetting the hole about a quarter of the dowel diameter closer to the shoulder. When reinserted the hole through the tenon won't quite match the hole through the mortice. You then chamfer the end off the piece of dowel and drive it through the offset holes. This pulls the joint tightly together. The dowel is then trimmed off flush with the face of the timber. Of course glue is applied to the joint before pinning. This makes a very strong joint which is very unlikely to let go or come loose. Good Luck!

Phill,
I made and sold several wooden flyscreen doors going back 20yrs or so. There were some Pine and some Jarrah. All were 19mm x 60mm all had a centre brace and some had decorative corner braces. I used wooden dowels 2 in each joint using only white PVA glue. I would have used M/T had I known how to make them back then with the limited tools I had. The thing is I drive past the houses that I installed them into and they are still there and looking good. Never did make one for myself much to SWMBO disapointment :o

Hope this helps give you some confidence.

Kind regards Vasco

Thanks Vasco. It makes me wonder if it is worth all the effort to go M&T if a biscuit joint (which should be stronger than dowelled joint) will suffice. If I had loads of time on my hands I would go M&T just for the experience, but with several hundreds of hours of reno's in front of me I am not prepared to "overkill" at the expense of time right now.

Phill.