Why cant you create through dovetails with a single pass on a jig? – quora
You do indeed need to make two individual passes with different setups (or with different templates) in order to make through dovetails with a router, you can’t use a half-blind dovetail jig to get the job done, even if you’re using specially thin workpieces. That’s because router-made half-blind tails (the bits that taper outward toward the tips) aren’t fully tail-shaped, they’re rounded on one side to fit the sockets between their matching router-made pins (the bits that taper toward one side). The sockets are rounded because a router bit can’t cut into sharp corners – it’s a rotary cutter, so the sockets it cuts will always be rounded.
A half-blind dovetail template or jig has the fingers in its “comb” spaced a very critical distance apart, and you adjust the tightness of the joint by cutting slightly deeper or shallower; if you were to just extend the router bit enough to cut fully through a 3/4″ workpiece, the tails would end up being much too thick and the sockets would be much too small for the joint to fit together. That’s because the dovetail bit is narrower near the shank end and wider near the tip. You’ll need to experiment to get the correct depth to make a snug-fitting joint, and once you find that magical depth you should make a little jig to match that depth so you can set it easily again, later.
Note that the magical “correct” depth will be specific to that particular router bit, and won’t work correctly for any other router bit – not even an identical one from the same manufacturer. There’s enough variation between side angles and diameters that no two bits will ever cut dovetails that fit exactly the same; dovetails are highly critical joints, especially with a 7-degree bit.
To get full through dovetails, you’ll need either another – much more expensive – dovetail jig (and two router bits to use it) or a dovetail saw and a couple of narrow chisels. Making secret dovetails is even more challenging, and making really good sliding dovetails is extremely difficult.
There are jigs built for through dovetails available on the market… the Leigh and the Keller are arguably the best examples, although there are a few competitors these days – MLCS makes a decent one, for example. They’re all very expensive compared to a half-blind dovetail jig, though, and they’re also very picky about which router bits you use with them.