[hallamjeff]

Just finished watching the latest episode of "Rough Cut" and it reminded me of something I still haven't grasped fully. I don't understand why you need a haunched tenon. Why not just have a tenon with four shoulders? seems like it would make for easier machine setup.

Can someone enlighten me?

 

[Brit]

In a table for example, the haunch is to stop the tenon blowing out the end grain of the mortise, since wood expands across its width. Rather than have a big shoulder to at the top of a rail, you have a haunch which keeps the top of the tenon far enough away from the end grain of the mortise, and also prevents the rail with the tenon from cupping.

 

[araldite]

I don't know what Tommy was doing because I didn't see the show, but if he was making raised panel doors with simple mortise and tenon joints on the frame, the grove for the panel would show on the end unless the haunched part of the tenon filled it in.

 

[Eli]

Hello,

The haunch does a couple of things. First, it lets you leave more wood at the top of the leg. A shoulder accomplishes the same thing, though. Second, The haunch still provides a bit of support that a shoulder wouldn't. As Andy pointed out, on a shouldered tenon, nothing prevents the board from cupping at the top. If the rails are set flush to the legs, like on the show, this is a noticeable problem.

Also, we don't generally shoulder the bottom because we're trying to get the maximum strength out of the joint. It probably doesn't matter, but that extra bit of tenon, and the haunch, do add some strength and glue surface. Also, we find it easier to get consistency by using the bottom of the rail as a reference edge. Extra cuts introduce extra uncertainty. Additionally, if the joint is completely hidden with shoulders, we can't see if it's sitting tight to the bottom of the mortise.

Wow. That answer got longer than I meant for it to be. Oops.

Eli

 

[hallamjeff]

Thanks for the answers folks. The cupping issue is one I hadn't thought about. Now I am going to have to try one on my next project just for kicks.

That's what I love about these message boards, instant knowledge!

 

[Broglea]

Jeff - Great question. I always wondered the same thing.

Guys - Thanks for taking the time to explain this. I learned a lot by reading your responses. It makes a lot of sense.