Hell Tenons - Jigoku hozo

[Chris Hall]

The partitions attach to the floor panel using some floating hardwood keys, which are wedge on both ends. The end of the key with protrudes through the floor is wedges externally, however the end of the key which attaches to the partition is internally wedged. This form of joint is called fox-wedged, or foxtailed in English. The Japanese call it the hell tenon since once it goes together there is no coming apart.

You can just see the protruding ends of the wedged keys on the underside of the floor in this drawing:

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Once you have the partitions done, they are assembled to the floor and the whole works is flipped upside-down:

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Since the mortising for the keys in the floor has already been completed, you can use the existing mortises to transfer marks to the partition edges:

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Marking complete, separate the parts:

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Here's the marked partition edge:

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The mortise is 1" deep, though this dimension is not critical. The width of the mortise is 3/8" on a 1/2" board, however the mortise could be slightly skinnier if it had to be (or the partition a bit thicker).

I used a hollow chisel mortiser to rough out the opening, using a underside chisel:

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Then I used my router table to enlarge the opening to size, and finished with some chisel work:

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A mostly-completed mortise:

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All that remains is to internally taper the mortise. I chose to flare it out 1/16" over the 1" depth, a 1:16 ratio, about 4.5˚. I made up a paring block, and clamped it into place on the edge of the board:

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It is critical to get a smoothly pared and clean mortise.

The keys and wedges are then prepared:

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At this point, the key is about 1/2" (12.7mm) longer than required. Notice that I've marked a line on the key showing the maximum insertion depth (1" in this case).

It is important to work accurately and gauge the sizes of your parts carefully! One cannot overemphasize this point. If you make the wedges too skinny, then the key will be driven down and will not fully expand the tenon at the bottom, making the joint loose. If the wedges are too fat, the joint will bind before it is all the way down. In either case, the only recourse will be to drill it out and excavate the remains of the key and wedges. Do not try to pull it back out or you will have a mess on your hands.

If you've never done one of these before, it would be wise to execute a few practice ones first.

The wedge ends are chamfered as is the end of the key, to ease in fitting down the mortise. The assembly is then offered up to the mortise:

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Cross your fingers and down it goes:

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All the way down:

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As noted above, the key was left long. Now I mark a cut line 1/2" away from the partition edge:

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The excess is sawn off and the trimmed end of the key chamfered. Next step is to kerf the end of the key for the other set of wedges, which run 90˚ opposite to the internal hell-tenoned ones:

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Done:

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Then, just to confirm alignments, I re-assemble:

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And flipping the floor upside down again, we can see the two kerfed keys poking through:

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A closer look:

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Later, two wedges will be driven in and that will lock the partition assembly to the floor panel. Pictures to follow when that stage is reached.

There are another 10 of these jigoku-hozo keyed joints on this toolbox, so we'll get lots of practice making them. They're tricky, but if you are careful in your work and measuring, and are willing to do some practice runs, the obstacles can be overcome. Worst case is you cut the tenon off and drill it back out. And if the tenons really seem too difficult, the option remains for glue in certain cases and places.

[Jack_Ervin.]

Chris,

Since you first published the views of this design I have been pondering what would happen here so now we know. You stated that the floating keys are hardwood. What is the wood that you are using? My partitions and floor are basswood and I have soft maple and cherry that I can use for floating keys. Would either of these be suitable? I think the cherry would be a nice contrast for the caps. As for a starting point for the wedges would the taper that is pared into the mortises a good starting point?

I also have determined that I will work trays in. I have found that the local Big Blue Box has aspen in s4s sizes in thickness of from 1/4" to 3/4" and in a variety of lengths and widths up to 8' and 8" respectively. For the small amount I will need wont break the bank and this will blend in both color and hardness with the basswood that I am using.

Jack

[Chris Hall]

Jack,

I used Jatoba for the keys. In the drawings I colored than as Purpleheart. I think that with basswood parts, that either the Cherry or Maple would work fine for keys. The cherry would contrast, but the maple would be stealth.

The taper that was pared for the carcase boards mortises was 1:8. You could do that, but on a 1" long mortise, that means each side would flare 0.125", which seems like a lot. That's why I cut that slope in half, making the slope 1:16, requiring that the key flare out 1/16" each side. I imagine somewhere in that zone should be good, 1:10, 1:12, 1:14, etc. also fine. 1:16 is convenient with 16ths to an inch. If working in metric I might look at flaring 1~2mm/side for a 25mm deep mortise, something like that.

[timoore]

Chris,
How do you prepare the wedge? Does it have a 1:32 slope on each side, or 1:16 on one and flat on the other? Anything else special, like grain orientation, choice of wood, etc.?

You need to spell it all out for some of us

[Chris Hall]

Tim,

the grain needs to run along the length of the wedge and be aligned to the length of the wedge. The most perfect way to do it is 1:32 slope each side. By hand, you can set up a sawing jig (box) to cut the wedges, which has a kerf in at at the 1:32 angle. Cut, flip, cut, and repeat...

[Chris Hall]

See also the thread "Cap in Hand" for more jigoku hozo fitting...

[djwong]

Hi Chris,
I am preparing the floating tenons on the floor of the toolbox, and I am working off the original 2 tenon per interior partition model. Could the two tenons restricting the seasonal movement of the floor piece? Should one of the mortises on each interior partition be slightly elongated to accommodate this movement? I already cut the mortises on the floor piece, but not the partitions. Should I go to a single tenon as you have done?
Thanks, David

[Chris Hall]

David,

good questions! In my toolbox the floor panel is entirely vertical grain, and in a wood which doesn't move much, so I think that two floating tenons should be alright. A slight elongation in one or both of the mortises would be a wise precaution however.

If your floor panel has some flat grain in it or is in a species which you expect to move significantly, then the centralized single floating tenon is the best choice.

~C

[djwong]

My floor panel is a vertical grain douglas fir, so it should not move much, but I will take your advice and elongate the mortises slightly (sticking with 2 per partition).
Thanks, David

[Chris Hall]

You'll be fine.