Hell Tenons - Jigoku hozo

[djwong]

Chris,
Your toolbox looks great. I am not as far along as doing the lid yet. It took me a while to get up the courage to execute the hell tenons. I finally got the tenons done for the inner partitions, but had some difficulty with the tenons splitting. I had to redo 3 of the tenons due to splits. The split occurred across the growth rings, which run lengthwise on the tenons. I used a 1:16 ratio for the inner slopes of the mortise, and a 1:32 slope for the wedges. I think my wedges may have been a bit too fat and caused the split.

Experiments with an "open" mortise.

The splits...

Finished partitions

[Chris Hall]

DJ,

thanks for posting and good of you to tackle the hell tenons and do some experiments. They're a bit tricky to be sure.

The split occurred across the growth rings, which run lengthwise on the tenons.

Do you mean that the keys were in a flatsawn orientation? If so, that direction would be more prone to splitting with the wedges going 90˚ to the growth rings. You always want to have the growth rings oriented in line with the narrower dimension of a rectangular piece if possible. Also make sure that the grain runs straight in the keys. Looking at the splits shown in your keys, I am seeing a bit of sloped grain there. It looks like you used mahogany for the keys, yes? Normally mahogany is fairly complaint. The best choices are woods with long fibers and good elasticity, like Yew, Ipe, etc..

If the wedges are too fat, then the result would normally be that the key cannot insert far enough down before binding. However, if the keys are a bit fat and the fit of the key in the mortise is too loose lengthwise, then the fat wedges could more readily split the key.

You could also try leaving the key longer in length, fitting it in with the wedges, and so long as a crack isn't propagated in the key by the wedges, you trim the key back down in length afterward. That's what I did - not sure if that was your method.

Anyway, excellent job and the thing is even if a given hell tenon doesn't come out right, you can drill out the thing and remove it and try again. I probably wrote it up with a bit too much in the way of dire warnings in there. Keep up the good work!

[djwong]

Do you mean that the keys were in a flatsawn orientation?

I am using tenons in a flat sawn orientation. I thought this would be best since the longer wedges would be 90 degrees to the growth rings. I guess this is a confusing point for me since the since these tenons are wedged at both ends in perpendicular orientations. For the lid tenons, I will try aligning the grain to the narrow dimension.

It looks like you used mahogany for the keys

The wood is actually claro walnut. I was going to use it for the lid tenons also. I will make sure it is straight grained first.

Aside from the splits, then tenons were straightforward. I paid attention to the wedge thickness, and the experiments helped. The most time was spent cutting all the wedges by hand. I had to cut a lot of wedges to get some suitably sized ones.