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.
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.
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.