Tuesday, August 20, 2013

Asymmetrial Offset Dovetails

A popular woodworking project recently has been a Classic Step Stool designed by Tommy MacDonald and published in Fine Woodworking magazine. Tommy designed and built the project with hand cut dovetails, and a lot of builders will go this way. Years ago I spent a lot of money on a very nice Leigh D4 dovetail jig (now out of production and replaced by the new Leigh D4 Pro), that has only gotten minimal use, so I decided to take this project in a different direction by machine cutting the joints with my jig. What makes it a different kind of challenge is that the joints on a project like this are asymmetrical and to complete the tail board cuts the pieces need to be offset. Doing this by hand would be fairly "easy" if you're skilled at hand cut dovetails. Unfortunately getting skilled at hand cutting dovetails takes time and practice. Machine cut dovetails are a little less flexible in terms of design, but the final outcome can be near perfect. It's the setup that is often time consuming and sometimes equally difficult. Both methods require a degree of care to get right.

On this project I opted for the perfect fit (yeah, full disclosure, I don't have hundreds of perfectly done hand cut joints in my arsenal) so the decision to expend time on the jig setup finessing part was easy. It's also a chance to learn the procedure of cutting asymmetrical joints with the jig too... something I've never done before.

So over the last couple of nights I've been working on the layout and test fitting joints using scrap material (poplar). To cut Tommy's design on the jig requires doing the layout on a test piece, transferring it to a scrap board, and then mirroring it as carefully as possible on both the left and right sides of the jig. It becomes critically important to keep track of which piece is which when doing this, so the markings in chalk denote the right and left, and inside and outside faces of the pieces. To cut the tails on an offset I inserted a filler piece of scrap against the boards being cut. As seen in the shot above, the joints fit near perfectly, but the spacer still needs to be adjusted by about 1/16" to avoid having to trim any stock. That part will be tested with more scrap tomorrow night. In my quiet neighborhood I don't use any power tools much past about 7:30 p.m.... it helps keep the "peace" part of peace and quiet we enjoy around here. :)

The internet is filled with people who have had trouble with the Leigh dovetail jig, and one of the biggest issues seems to be tear out. Honestly I never have that trouble, but I always do two things to avoid it: First, I use a backer board when cutting pins to support the fibers behind the cuts completely. And secondly, when cutting the tails I will route from both the front and the back sides of the piece to avoid tearout. It takes a little more time but the result is worth it.

My final project will be built using African Mahagony. Those pieces have already been milled to size so they're waiting and ready for the dovetail jig once I'm happy with the setup. The project also requires three stretchers that I will cut and fit by hand.

More as I get further along. By the way, this test piece fit so tight and so well that it easily supported my full weight unglued and without racking.

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