Secure the Work and Fix the Location
Two clamps are needed to keep the jig from shifting. I place a clamp on the guide that clamps both the guide and the work down to my bench, as shown at right. A sliding bar clamp will securely hold the guide bar to the edge of the workpiece. Making the bar as long as possible provides plenty of room for your clamps. An alternative location for the second clamp is to clamp down on an additional piece on the opposite side of the router from the guide.
When making a jig, keep in mind that it may need to be placed near the end of a board on either the right- or left-hand side, or in the middle of a board. Carefully plan your clamp locations before you put the jig together. It’s no fun to spend an hour making a jig only to find that it isn’t possible to securely clamp it down, or that the only locations available for clamping interfere with some part of the router.
If you want to locate the cut a specific distance from the end of a board, a block of wood can be added to the underside of the plywood guide, to butt against the end of the piece. Plus, if you’re making a series of regularly spaced dados, you simply can adapt the jig by making a wider guide, and attaching a piece to the bottom of the guide that is the exact width of the dado. After you cut the first dado, place the piece on the bottom of the guide in it and then clamp the jig down. Then you can cut the next one.
Blocks can be added on top of the plywood guide to limit the travel of the router and make a stopped dado at one or both ends of the piece. These can be located by drawing the dado directly on the workpiece, setting the depth of cut of the router to zero, and locating the router in place. The blocks can then be placed to catch the ends of the router base plate at the beginning and end of the cut, as shown on the next page.
In use, the router needs to be pushed firmly along the plywood guide. Often, especially when starting or stopping the cut, there is a tendency to let the router drift away from the guide. A second guide can be located on the opposite side of the router that will trap it in place (shown at right), preventing it from drifting away from its intended path.
The second guide bar can also be used to make dados of different widths. If you’re routing a dado for a piece of plywood that is slightly less than its nominal dimension, you can use a smaller diameter cutter, set the second guide the right distance from the first, and make the cut in two passes. Again you can set the distance by clamping the guide to the work, setting the depth of the router bit so it just touches the surface, then locate the guides by positioning the router on your layout marks. This second guide is also the easiest way I have found to place a second clamp.
You’re not limited to square cuts, or to any profile of router bit for that matter. If you need to make an angled cut, simply fasten the guide to the bar at the angle you want. If you’re using two guides, one on each side of the router, you can even make dados that taper in width. This is a good trick if making a relatively wide sliding dovetail joint. You can make one end slightly wider than the other so that the joint slides easily together, and then locks in place the last inch or two. This jig offers many possibilities.
Make Your Cut
Once you have the jig clamped in place, cutting the dado is simply a matter of turning the router on, and pushing it against the guide as you move it across the board. You need to be careful as you start the router that it doesn’t jump back towards you. When you hit the edge of the workpiece, you’ll encounter some resistance, and you don’t want the leading edge of your cut to be away from where it is supposed to be. A small block, can be placed on the opposite side of the guide to prevent the router from wandering at the start of the cut. You may also need to clamp a piece of scrap on the edge of the board where the cutter exits to prevent tear-out. The alternative is to back in to the far edge of the dado for the same reason.
A T-square jig is the key to perfectly precise dados for all your casework needs.
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Better Dados for Casework