It’s hard to think of a woodworking joint you can cut faster than a half-blind dovetail — both parts of the joint are cut at the same time with a hand-held router and a half-blind dovetail jig (photo above). The real challenge is setting up a dovetail jig so that routing the joint is fast, easy, and accurate. And doing that depends on the style of dovetail jig you have (or might be thinking about getting). A tale of two jigs. Page through a number of woodworking catalogs and it might seem like there are over a dozen different dovetail jigs. But if you look closely, you’ll notice that most of them resemble one of the half-blind dovetail jigs below. What’s the difference? On the jig at the far left, the depth of the socket that accepts the tail is established when a bushing reaches the end of the slot on the template. On the other jig, the base of the router contacts an adjustable fence to limit the cutting depth. The bushing simply guides the router in and out of the template slots. I know this doesn’t sound like much of a difference. But to be honest, the dovetail jig with the fence took me a little longer to “tweak” before I was able to rout perfect half-blind dovetails. The template. Another difference worth mentioning is the template that comes with the jig. A jig with a metal template will probably be more expensive (around $100). While a jig with the plastic template can be found for $50. I prefer a metal template since it’s less likely to sag. As you’re routing, it just feels more secure. To provide solid support on the plastic template, I had to slip a scrap piece in place to support the opposite end. You can see this in the main photo at the beginning of this post. Note: A couple scrap pieces are a good idea with either jig since they allow the clamp bars to hold the workpieces more securely. A more important consideration regarding the template is how it controls the spacing of the dovetails. Most templates are designed to cut 1⁄2"-wide dovetails spaced 7⁄8" apart. Other jigs space the dovetails 1" or 1 1⁄8" apart. (We use the 7⁄8" spacing for Woodsmith projects.) Why is that important? The spacing determines the width (height) of the drawers. The drawer heights should be a multiple of the spacing. This way, you’ll be sure to end up with a joint that’s symmetrical — a perfect half-pin on both the top and the bottom, as in the photo above.
Setting Up The Jig
There are a few key steps you’ll need to be concerned with when you initially set up a half-blind dovetail jig. You can see how I did this for a dovetail jig with a fence in the How-To below. Note: When you’re making a drawer or box, half the joints are cut on the left side of the jig and half are cut on the right. So you’ll need to repeat the setup for the stop at the other end of the jig once you’ve completed the initial setup. The process for setting up the other dovetail jig is virtually identical. Since there’s no fence, the template simply has to be positioned according to the instructions that come with the jig. Most often this is just a matter of measuring from the back of a template slot to the end of the upper workpiece. Jig instructions. There is one thing to mention about the instructions that come with some of the fence-style dovetail jigs. While the jigs work great once they’re set up, I felt that the instructions that came with the dovetail jig weren’t as clear as I thought they could be. It’s always best to think through each step of the process and do a little “common-sense check” before making any changes. And always make a “dry run” with the router off before actually routing any workpieces. This way, you won’t inadvertently cut into the workpiece, template, or jig. (Believe me, it’s easy to do.) Cutting depth. Finally, the last thing to do before making any test cuts is to install the bushing and dovetail bit recommended by the manufacturer. For the depth of cut, most manufacturers suggest an initial setting around 1⁄2" to 9⁄16". I’ve found that to be a good starting point.
Routing The Dovetails
Now that the jig settings are “in the ballpark,” you’re ready to start making some test cuts and tweaking the settings one by one to get a perfect fit. test cuts. The template may control the spacing of the dovetails, but the settings you’ve made are what really control the fit of the joint. The fit probably won’t be perfect the first time, so it’s best to practice on test pieces before you rout your actual workpieces. The test pieces should be the same width and thickness as your drawer pieces. (You can use them several times by just cutting off the ends.) Once you get a perfect fit, then you can rout the dovetails in the actual workpieces. With the test pieces clamped in place, the first step is to minimize any chipout by making a light scoring pass from right to left, as in Figure 1 below. I know. It’s not usually a good idea to rout in this direction (backrouting), but since you’re making a light pass, it’s okay. After the initial scoring pass, you can rout the tails by moving the router in and out of the slots in the template, working from left to right (Figure 2). Before removing the pieces from the jig, make sure that every pin and socket is routed cleanly. You can remove the template to check this. Next, remove the test pieces and see how they fit (photo below). Don’t worry if the fit isn’t perfect on the first try. It rarely is. To see how to get everything just right, check out the tips in the box below. Once you have a good fit, you’re ready to rout the actual workpieces. The only thing to worry about is keeping all the drawer pieces organized as you do this. To help keep things straight, I like to label all my pieces like you see in the drawing below.
Troubleshooting:Tips for Getting a Perfect Fit
Setting up to rout machine-cut dovetails is always a trial and error task. So don’t be surprised if you have to spend a little time adjusting the jig to get a perfect fit. A few of the “problems” you might run into are shown in the drawings at right. As you make adjustments, it’s best to make a small change to one setting and make another test cut. Then move on once you have that just right.
If the top and bottom edges don’t align, the workpieces may not have been tight against the stops.
If the joint is too loose, increase the depth of cut.
If the joint is too tight, decrease the depth of cut.
If the tails go too deep move the fence forward.
If the tails are proud, move the fence back.