Tuesday, May 14, 2013

More on sliding dovetails

The whole point of making the dovetail plane was to use it on this particular job: 4 large, fixed shelf bookcases. The carcases are single-plank wide cherry, with a finished dimension of around 16".

I used the same technique I'd used on the small footstool to do the joinery here, but with 24 joints to cut in total, (4 cases, with three shelves, 2 joints per shelf) I got a lot more practice, and a little more understanding of the process, to boot.

To make the process faster, I actually ran half of the joint with the same router I used to run the grooves. I did the final fitting of the top side with the plane. I have some funky Festool attachments that allow me to do this, but I'm sure it could easily be set up on a router table, too.

Fitting the joint, first step was to clamp the board down flat, and start working the end of the board. The goal is to have joints that don't have gaps showing at the front. Gaps can always be filled, but I like it better when I don't have to. I started the process with calipers, trying to match up the male end of the joint to the female end.

Eventually I just moved to fitting the ends of the joint to the case. That is, plane the front to fit the front of the case, and plane the back to fit the back, and then work the mid-section until the board slides home. It's important to note that the board has to be flipped for this, because the left side has to be fitted to the left side, and the right side has to be fitted to the right side, before the board is actually able to slide all the way in. The idea is to get that part fitted first, and then leave it alone.

You can see here that the fitted boards are not all the way in. This is done on purpose. Because the joint will wedge itself in place when it's glued up, wedging everything together now isn't really helpful... you'll just have to beat up the work to get that board back out. So, getting the board to slide basically to that last inch or so is pretty typical. 

 On the clamp in the photo, I encountered a couple of times where the boards had started to bow on me. It could be because a few days had gone by since I'd milled them down, cut dovetails, etc, and they'd moved a little bit. It could be because the wood reacted slightly when I ran the dovetail grooves. I'm not entirely sure. I know that some folks insist on doing carcase joinery the same day the wood is milled, because it helps to stabilize everything, and keep it in plane, but with 4 cases to do, with the width of the material, that didn't really seem like an option for me.

That said, there were a couple of the shelves that were slightly cupped. (about 1/16"- 3/32" across 15.5") I figured it would be fine, in the long run, because it would be held flat in the case, and would stabilize over time. But now, I think that doing final milling before fitting this kind of joint makes sense. Clamping a board flat while you plane it, AND fit it, AND try to do assembly is a lot of extra work. Working with flat stock is much easier. And assembling it is easier.

Last word of advice: be SURE to get those boards in to that last inch. I got sloppy on one that was slightly cupped... I figured 2-3" was fine, that the cupping would flatten out, and everything would be fine. This is the part where I confess to the fact that it took 2 I-beam clamps, with pipes slipped over the handles, to crank that shelf into the case all the way. It creaked loudly, like an old door, for about 5 minutes while it slowly shifted into place. I was sweating bullets the entire time, thinking that for sure, the thing was going to explode.  Only push in that last inch.

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