Plane

Plane

Friday, November 25, 2016

Bowties


Bowtie joints go by many names, butterfly keys, dovetail keys, bowties, or Dutchman joints. They have two big uses: First, they look awesome. Always err on the side of awesome. Secondly, they provide mechanical stability to a joint, and, especially in situations where wood is checking, splitting, or cracking, a butterfly joint can inhibit the expansion of that problem.
So in order to talk about why the work and how to do them, let's briefly talk about wood movement.


This is a picture of a slice out of an elm tree from my parents' family friends yard. We had the tree sliced into a series of these discs, and covered them in green wood sealer, Anchorseal 2 Green Wood Sealer Gallon, which is a waxy fluid that is supposed to moderate the rate at which the wood dries. If it dries too quickly, the wood can pull apart due to the stresses in the wood exceeding the strength of the wood fibers' ability to resist those stresses. Moderating the rate of drying can only be sort of successful because when wood dries it shrinks. The circumference of a disk may want to shrink 8%, but unless the diameter of the disc shrinks proportionally (it doesn't, it wants to shrink roughly half of as much), then you've created then you've created a great deal of internal stress. That stress gets relieved when the stress overwhelms the wood's strength, and then you get the development of the cracking and checking that's pictured. So, if we concede that cracking and checking is bound to happen, and can spread, then we need a method of stabilizing the cracks and preventing them from propagating to ruin the entirety of the piece.

Enter, the bowtie key:


Bowtie keys put the long grain of an inserted piece of wood across the crack. The long grain does not want to stretch or pull apart, so that, combined with the angle, provides a mechanical resistance to the crack widening.

So now that we know what it is and why we use them, there are a couple ways to to make them.

First, the classic, and perhaps "proper" way, which is to cut out a bowtie, trace it on your work piece, rout out most of the waste, chisel to the line, and then slot the key in place. Do this right, and the key will fit tightly.

Second, use a router template (I used this one from Rockler: Butterfly Key Inlay Template Set) to rout out the slot. Clean up the corners with a chisel, trace the pattern onto the material you are using for the bowtie, and then cut out the bowtie.

I went with the second method. It allowed me to first, rout the slots quickly and en masse (I had to make a lot of these). It also allowed me to practice cutting the keys with a dovetail saw. I use the Gyokucho Razor Saw, which is a rip saw because to cut the keys you are cutting with the grain. Then I cleaned up the edges using a chisel, putting a chamfer on the bottom corners in case there is any scrap left in the slot that I didn't clear out, put in some glue, and rubber malleted that sucker home.


Cutting the dovetail line

After routing the slot but before squaring the corners

Squaring the corners



Slotted in place

When making the bowtie key, use material that is a little thicker than the depth of the slot. This leaves the key proud of the surface of the piece, so you can plane/sand the key down to the surface, rather than needing to plane the piece down to the key. I used my Lie-Nielsen No. 102 Block Plane, but it was pretty obvious that a larger plane would have been better suited for the task.

Note, this was my first attempt and the larger bowtie is oriented the wrong way. The long grain won't provide positing force to the wood movement. Not ideal.


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