Sliding Dovetails Are the Epitome of Precision

So what good are some pretty French feet without a case to sit upon them? I had already started the casework before the feet but took a detour from it to build the feet because my set up for the sliding dovetail that secures the center shelf was being fussy. Rather than obsess over it, I moved on to another facet of the build and let the dovetail problem simmer.

So here is my case, dry fit and clamped. Before starting the feet I had routed the sliding dovetail pin groove. With the case assembled I can now get an accurate measure of how long the shelf needs to be. Let’s talk about the process and then I can explain where I was having problems. My router table hangs off my table saw. I like this arrangement because I can use the table saw fence from time to time as a fence for the router. This works great for case work because you can set the fence far away from the bit to make grooves in the center of a side like in this case. My groove runs 14.5″ up from the bottom of the side so using the table saw fence makes this job easy. So first you need to run the dovetail grooves. Make sure you do NOT touch the bit height setting that that when you cut the tails you can ensure a good fit. Then you set up a sacrificial fence and bury the bit into it exposing only the tiniest piece of the bit. You then need to run your shelf board across the bit vertically so it helps if your fence is tall to provide extra support. I always take a piece of scrap and sneak up on the fit making minuscule adjustments until I have a sound fit. With sliding dovetails you want the fit a little looser or you will kill yourself trying to get them seated properly especially once the glue has swollen the joint.

So I started on my test piece sneaking up on the fit when I realized that my tail was about 1/8″ longer than the pin groove and I would have a gap at the shoulder.

I knew I had not touched the bit setting so something else must be wrong. My first thought was that the router table wasn’t flat and maybe the top had sagged. Using a straight edge and feeler gauges I determined that the top is flat to within .002 so that is not the problem. I double checked the bit height against my groove and then is when I realized that my case board was just the slightest bit bowed (curved along the length) and must have risen up as I made the cross cutting rout. The bow was so slight that I could flatten the board with very little downward pressure and the way the case is constructed would pull everything in line so I wasn’t worried about that. That slight variation however had essentially shortened the depth of the pin groove in a few places. I had to set up again for the cross cut and making sure of solid downward pressure was able to fix the depth of the groove in that one board. Of course the way a dovetail bit cuts means that that particular groove would be slightly wider on that edge. That is not a problem because it is the back edge of the case and I usually slightly taper my tails to ease the fit. In this case I had the opposite with an every so slightly flared groove. If you do have a board that isn’t exactly flat you can either make sure it is flat or route the groove using a hand held router that will track the curves like a shorter hand plane would.

So now it was back to cutting the tails on the shelf. Using a 12″ wide board makes for plenty of registration against the fence, but you must make sure that this board is flat or risk ending up with a similar problem that I had with the groove. So I double checked the flatness and ran the tails.

As I said above, I like to slightly taper my tails to ease the fit into the case so using a shoulder plane set for a very light cut, I trimmed a 128th or so off each side of the back half of the tails. I am aiming for a fit that I can press into place without a mallet but one that is very tight right at the front to seal up any gaps on that show face. Here you can see the back of the shelf as it enters the front of the case and the slight gap created by tapering the tail.

So I set the shelf a little more than halfway in to make sure I have a good fit that will still work once glue is has swollen the joint and I stop there. Working with a soft wood like Pine means that you don’t want to spend too much time dry fitting a joint like this or it won’t be so tight anymore. One great thing about this joint is that although it takes the highest precision to fit well, it really helps to correct a lot of errors in the base by pulling all the side together and flattening out any cup or bow and this is common with cheap home center pine like this. It will be flat one moment and potato chip the next. The other thing is that properly fit, you only need glue the very front edge and the rest of the joint will allow the case side to expand and contract while keeping the all important show face nice and flush.

By the way, here are the glue blocks in place on the inside of the feet as well. This will really strengthen that miter joint nicely.

Next I will take everything apart again and start cutting rabbets for the applied integral case beading. Time to put away the power tools and pull out the rabbet plane. Now I’m going to turn up the music, turn off the dust collector and galoot out for a bit.

3 Responses to “Sliding Dovetails Are the Epitome of Precision”

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  1. Tico Vogt says:

    Hi Shannon,

    The bowed board problem you’re dealing with can be dealt with if you use a stout ninety degree crosscut guide for the router that you clamp to the board. That way the depth of cut stays consistent. It does mean the extra step and set up. Okay, another step: use a straight bit to hog out most of the wood before one clean pass with the dovetail bit.
    When you’re running the vertical piece, a raised feather board that makes contact just above the groove shoulder can further help keep it pressed tight to the fence.
    I love the smell of pine and the friendly shavings it gives. Nice project.

    Tico

    • Shannon says:

      Using that crosscut guide is a great tip. You can tell I am normally a hand tool guy who turns to power every now and then. I don’t have much in the way of jigs or set ups. I’m used to marking and cutting. I didn’t write it but I did plow the initial groove using a straight bit before switching to the dovetail bit. Great comments, thanks Tico

  2. Mark Mazzo says:

    Hey Shannon,

    Nice work on the sliding dovetail.

    You are right, they can be pretty fussy – especially one as long as the one that you have for a shelf in the case.

    I remember doing one similar for a shelf a while back and I had the same problem as you – slight bow causing a big headache. Sounds like you solved it nicely.

    By the way, if you ever use one of those drawer joint bits or a lock miter bit they can exhibit the same issue when the boards are even slightly bowed – I used a drawer joint bit on a very early set of drawers and had such a time getting tight joints my next set of drawers was with hand cut dovetails as have been all the rest!

    –Mark
    The Craftsman’s Path

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