Needs tips for making/inserting structural butterfly keys

1. Will adding a key to stop this crack simply mean another crack will start elsewhere?
Yep. As wood dries it shrinks across the growth rings, so the outer edges of the wood are going to shrink more than the heart, and you are going to get cracks.

2. Is there a right or wrong time to add the key? I assume the sooner the better.
Given (1), I would wait until the thing is dry. But, if you want to limit the size of any one crack, then you could throw one in at any time.

3. The cookie in question is 2.5” to 3” thick, and has a diameter ranging from 39” to 49” (thus a circumference of about 140”), so even 2% shrinkage could result in over 2” of gaps in cracks —how thick should the key be to provide the necessary support for that much wood/force? I’ve seen decorative butterflies discussed that are 1/8” to 3/16” thick. I can’t image that would be enough to hold this piece of wood. I’ve some posts talk about 1” deep mortises. And do I need butterlfies on top and bottom?
I’d probably use one that is a bit beefier than 1/8” ... 3/4” or 1” would probably hold fine, but if the crack is big enough, you might want one that is full thickness just for aesthetic reasons.

4. The whole cookie is end grain—I assume the key should have the grain running the length of the key, so that the forces pulling it appart will be pulling on the strong, long grain.
Yep.

butterflies on both sides of the crack, both sides of the slab. This will help avoid warping the slab.
I have no experience with ‘predictive” butterflies, but from UUFIZE comment, maybe a good idea to circle the whole thing with butterfly-joints an the non-show side? Strictly an “idea”, not a solution based on anything.
Bon chance!
CMc

The crack will stop on it’s own once it reaches the center. I wouldn’t try to stop it or other cracks will form somewhere else. You can’t really prevent it. It occurs because wood shrinkage in the tangential direction is roughly double the shrinkage in the radial direction. I’d wait until it is dry before butterflying it.

-- Bondo Gaposis

Actually, I did use Pentacryl. It’s my first time, both drying wood myself and using pentacryl, but it does seem to work well. The entire slab is dry now, with only a total of 1/2” of gaps when all cracks are combined. And none of the cracks reaches the center of the slab. I did add two Walnut butterflies soon after I posted this thread, about 1” thick (or halfway through the thickness of the slab). As Chris warned above (although too late for me, :) ) the top did shift a bit after I added the butterflies. The bottom, unbutterflied side seems like it expanded a bit more than the butterflied side, so I have about 1/8” to 1/4” of curve across the 49” length. Not terrible, and actually, when screwed to it’s base it comes flat again. But it does seem like a full-thickness butterfly, or matching keys on both sides, would have been advisable.

The sad thing, in looking back at this thread, which I posted… 712 days ago? Can that be right??... is that I’m STILL working on this project! AAAAA! Time in the workshop is rare, and I’m doing a lot of thinking, planning and research so that I hopefully don’t screw up these unique pieces of wood. But I’m close! The base is just about done—needs a couple more coats of oil/varnish, and a bit of polishing. And the top has been grain-filled and is drying, waiting for more sanding, and the oil/varnish finish. I’ll post pics when I’m done.

-- Pete in Wilmette, IL