tenon too wide?

[Bernie Kopfer]

I am building a bed frame and plan on using a 3/4 X 5in tenon fully glued into a same size mortise. The mortise will be in a 3" square post and the rail is 6" x1 1/8. The wood is popular. I am concerned that that width of tenon could fail due to wood movement from humidity swings. Oh, depth of tenon will be 1 1/2 ". Does anyone have any experience with this size M&T joint? I live in an area where humidity changes are minimal. ( Eastern Wa) Thanks.

[Dave Gaul]

Bernie,

I have yet to make a M&T of that size, as I have just recently starting making this type of joint on a recent project. In my research, I don't think the 5" wide tenon would pose any issues. You may want to increase the thickness of the tenon from 3/4" to 1", since it is going in to 3" stock.. much of what I read says that your tenon should be 1/3 the size of the stock.. nvm.. just read that your rail is on 1 1/8" thick... 3/4" sounds good!

[david brum]

I've used wider tenons using the following method:

1)drill a relief hole in the middle of the tenon at the 2 1/2" mark. The hole will be close to the tenon shoulder. I think I drilled a 1/4" hole. This will allow the rail and it's tenon to expand independently of each other.
2) Cut a kerf from the end of the tenon to the hole. You now have a double tenon which has plenty of glue surface but shouldn't expand enough to break the joint.

[Frank Drew]

Bernie,

I wouldn't worry about a 5" wide tenon, but since your bed posts are 3" square I might increase the tenon length to 2" or 2-1/2" for more glue surface thus a stronger joint.

For a really wide rail, like the bottom rail on a door, making a double tenon, for the wood expansion reasons you cite, is considered standard practice.

[John Thompson]

Double tenon as mentioned...

[Bernie Kopfer]

Perhaps I'm missing something, but a double tenon would still expand just as much since the rail itself is doing the expanding and not just the tenon.
In other words the expansinon or contraction of the rail is going to try to move the tenon in the mortise.
Thank you for confirming my thinking that 5" should be ok and safe. But the conversation about how big is too big and how to compensate is important info.

[Bill Davis]

I am building a bed frame and plan on using a 3/4 X 5in tenon fully glued into a same size mortise. The mortise will be in a 3" square post and the rail is 6" x1 1/8. The wood is popular. I am concerned that that width of tenon could fail due to wood movement from humidity swings. Oh, depth of tenon will be 1 1/2 ". Does anyone have any experience with this size M&T joint? I live in an area where humidity changes are minimal. ( Eastern Wa) Thanks.

Yellow Poplar is moderate as far as Radial and Tangential % shrinkage/expansion (8.2T & 4.6R in the chart I'm looking at) when compared to other hard woods. You state "I live in an area where humidity changes are minimal" (what ever that means - a min/max RH range would be more helpful). I also presume you will apply a finish to the wood which does help mitigate shrinkage/swelling to various degrees depending on finish you use. If you make sure the wood is properly seasoned i.e. appropriately dry for your average humidity and make appropriate design allowances, as already suggested, for the expansion/contraction that will occur, and use quarter sawn wood for the rails since radial shrinkage is about half the tangential, you'll not have any problem (I hope).

[Tony Bilello]

The width of the tenon should not fail due to wood movement from humidity because there will be virtually no movement.The outside humidity should not affect the tenon snugly inside the mortise because the tenon is 'insulated' by the thickness of the mortise cheeks as is the mortise itself.

The real question is........why are you glueing? Bed rail hardware is readily available and cheap. If you glue, you will lose the ability to disassemble. There are also other choices.
Since there will be lots of side play (so to speak), I would make the tenon 7/8" thick - size does mater.. That would still give you a 1/8" shoulder on each side.

[Bernie Kopfer]

The width of the tenon should not fail due to wood movement from humidity because there will be virtually no movement.The outside humidity should not affect the tenon snugly inside the mortise because the tenon is 'insulated' by the thickness of the mortise cheeks as is the mortise itself.

The real question is........why are you glueing? Bed rail hardware is readily available and cheap. If you glue, you will lose the ability to disassemble. There are also other choices.
Since there will be lots of side play (so to speak), I would make the tenon 7/8" thick - size does mater.. That would still give you a 1/8" shoulder on each side.
Good Question. The side of bed rails will be attached with hardware. The question more correctly was about the foot and headboard rails. Thanks for the advice

[Frank Drew]

Bernie,

I'd assumed you meant the head and foot board assemblies, and those I'd glue unless one or both are too tall to make up up some stairways.

What I should have said about a double tenon is that the purpose is to keep the mortise cheeks from bowing outwards, which they might with the really long mortise that you'd have with a door bottom and even middle rails.