Keyed Miter Joint Table Saw Jig
This shop-made table saw jig makes quick work of reinforcing miter joints.
By Matthew Teague
I love the clean look of a mitered box that has continuous grain wrapping around the corners. It’s an easy detail to create, but a sure sign that the maker is paying close attention to the details. The downside of a mitered box? Miter joints are notoriously weak because they have no real mechanical strength; glue is all that holds them together.
You can reinforce miter joints in a few different ways, but my favorite method is to use exposed keys. These hardwood keys are nothing more than thin lengths of wood glued into slots that span both sides of the joint to help hold everything together. To cut the slots for these keys at the table saw you need only a small jig that runs against the saw’s fence. The jig cradles the assembled box at a 45° angle and allows you to guide it through the cut.
Keys not only add great strength to miter joints, they also provide a decorative effect. Once the jig is made, you can arrange the keys in any number of ways, using either a matching or contrasting wood. For a slightly different look, you can cut wider key slots by simply adjusting the table saw fence to take two or more passes for each slot. For angled keys, which create joints that arguably are even stronger, simply angle the blade. You can even use this same jig at the router table to create dovetailed key slots – just be sure to hog out the bulk of the waste in the jig itself before you cut the actual box.
Making the Jig
This jig can be made from plywood, MDF or whatever scrap you have on hand. If you choose sheet goods or thinner stock, you’ll need to face-laminate a few thicknesses so that the cradle of the jig is wide enough to hold the full height of the box. I simply glue and screw through all the thicknesses to create one large chunk of material. It’s a good idea to glue up a longer length of stock than necessary, just so that it is easier to hold during the next step.
Once the glue dries I remove the screws so that there’s no risk of the saw blade catching a screw when the jig is in use. You can then cut both ends of the stock at 45° angles on the miter saw or use your miter gauge at the table saw. Instead of cutting the miters to a sharp point where they meet, I like to leave about 1⁄8″ or so flat on each piece; once the jig is assembled and in use, this raises the box off the saw table and allows the jig to move more smoothly across the table.
Once the end cuts are made, cross- cut the stock into two separate pieces. Attach them point-to-point centered on a backer board. You should size the
backer board so that it will stand a few inches above your table saw fence. Use the extra height as a handle to keep your hands out of harm’s way when the jig is in use. This extra height also provides enough room for you to secure your box with clamps when necessary.
You can glue the angled blocks to the backer board, but I simply secure them with a few screws driven in close to the top of the angled blocks (where there is no risk of a blade catching them as you pass the jig across the saw). When the jig gets chewed up from use, this allows you to replace fouled parts easily.
Using the Jig
Once the jig is ready to go, start by set- ting the height of the table saw blade. Position the box in the jig and hold it alongside the blade. For both strength and aesthetics, make sure your blade is low enough so that it won’t cut into the interior of the box. I usually raise the blade so that it cuts only about three- quarters of the way through the miter joints.
Once the blade height is set, mark out the position of the keys on the box. (If you’re working with a solid box that will have the top cut loose later, re- member to locate keys with the future cutline in mind.) Hold the jig against the fence and position the box against the upright face of the jig. Then adjust the fence so that the blade aligns with your desired key location.
To cut the slots for the miter keys, I usually just hold the box in place against the upright face of the jig and then run the entire assembly against the fence. If the box is particularly large, small or otherwise awkward, you can clamp the box to the backer board of the jig before you cut the slots; just make sure to position the clamps so that they won’t interfere with the cutting procedure. You should be able to make each cut in a single pass.
Make the same cut on all four corners of the box. Relocate the fence to align with the next key location and repeat the process until all your key slots are cut. Once the slots are cut, the keys are easy to install.
I’ve come to love the look – and the strength – that hardwood keys add to a miter joint.
Installing Miter Keys
Installing keys is a simple process. Mill a long length of hardwood to fit snug in your key slot. Use a backsaw or band saw to trim it down to make triangular keys of a more manageable size. Glue the keys into place and trim them flush to the box sides with a backsaw or flush-cut saw.
This article appeared in the August 2012 issue of Popular Woodworking Magazine.
Our cover story for the August 2012 issue of Popular Woodworking Magazine is Campaign Furniture by Christopher Schwarz. In the piece you’ll discover the fascinating origins of this furniture style, which was popular in the British Empire and America for more than 150 years – plus, you’ll find the drawings and information you need to build a classic campaign chest of your own. Explore the nearly limitless aesthetic possibilities of the humble box as our editors take on 4 Boxes, 4 Ways. In A Trio of Trifids, renowned period furniture maker Charles Bender offers three high-style variations on a carved foot for a cabriole leg.