Most of the really cool carpentry tricks that I know how to do I’ve learned from other carpenters, like my brother and the guys at Roadshows and JLC Live events, and I’ve learned a bunch from Jed Dixon. One thing Jed taught me is how to cut lock-miter joints. He uses lock miters on all of his newel posts and columns—whether they’re installed indoors or out and whether they’re tapered or not. He believes the lock-miter joint is simply the strongest corner joint possible.
Lock miters can be cut quickly—they can cut assembly time in half—and all the pieces are easy to keep track of, so it’s tough to make a mistake. Jed cuts his lock miters with a shaper, using a power feeder so all the pieces are cut precisely the same way. All four sides of every column are cut the same width—there’s no wide side or narrow side, as with butt joints or rabbet joints, to confuse you. Cutting different widths can really complicate matters when you’re making tapered columns.
LOCK-MITER JOINTS ON TAPERED COLUMNS?
I’ve had folks at my demonstrations argue that lock-miter joints cannot be used on a tapered column. One attendee wouldn’t believe me even after I cut and assembled the joints right in front of him. OK, the joint isn’t perfect—the outside edge stays slightly open—but it’s close. For a column with a taper angle of 2 to 3 degrees, the angle on a lock-miter bit works just fine. But be aware that if the taper angle of a column is much more than 7 degrees, the mating surfaces of the lock miter misalign to the point that the joints don’t come together. Here’s why: Once a column starts to taper, its sides no longer meet at a 90-degree angle. And the greater the taper angle, the further away from 90 degrees the corner gets. Roof cutters call this the “dihedral” angle.
Most tapered columns angle at just a few degrees, so lock miters are absolutely the best joint for the job because the angle across the joint is so close to 90 degrees. With a 2-degree pitch, that angle is 90.07 degrees (for readers who only trust math). That’s pretty darn close to a perfect 90-degree corner.
LOCK-MITER SETUP
The biggest drawback to lock miters is the time it takes—especially the first time around—to set up your router or shaper. The bit and the fence must be set up precisely. However, there are a few things you can do to work more efficiently. First cut a bunch of set-up blocks and save them. You’ll probably need between six and eight pieces of 1x6 about 8 inches long. These blocks are for making test cuts as you slowly adjust the height of the bit and the location of the fence. Once you’ve set up the bit and the fence perfectly, label the final test blocks and keep them in a safe place. In the future, using those test blocks to dial in the settings will be a lot easier than starting from scratch.
A lock miter is a good example of something that looks complicated but is pretty simple once you understand how it works. One thing that makes it simple is that the same setup is used for cutting all the corner joints, but you cut two of the boards lying flat on the router table (face miters), and you cut the other two boards standing on edge against the fence (edge miters). Once the bit height and the fence location are set, you don’t need to touch the router table except to push material through.
Let’s start by setting the height of the router bit for cutting the face miters. In the simplest terms, the lock-miter profile consists of two short miters with a tongue-and-groove section in between. For the joint to fit perfectly, the cutter must be centered vertically in the workpiece and the two miters must be exactly the same length (see Lock-Miter Intersection, left). If you’re starting from scratch—that is, without a precut test block—you can lay out the length of the miters with a sharp pencil or pen and get pretty close on the first few tries. I like to make a few quick adjustments to get the setup close, and then micro-adjust the final height with a router lift. A sensitive lift mechanism on your router makes this part much easier to do.
The next step is positioning the fence. This time, the miters of the bit profile have to be perfectly centered on the workpiece when it’s run through on edge (vertically). Again, I get it close and then dial the fence to the precise location using test blocks. At this point, the machine should be ready for both the edge cuts and the face cuts.
SAFE CUTTING
When making cuts like these on a router table, I always use hold-down clamps and feather boards to apply consistent pressure against the fence and the table. For cutting the face miters—when the board is lying flat on the table as it’s cut—the hold-down device on my CMS stand works perfectly. The CMS hold-down applies plenty of pressure to secure the material, and it’s also easy to flip up out of the way if you have to make minor adjustments. But I still use push blocks to move the work while holding it tight against the fence.
When you cut the edge miters, the board must be standing on edge against the fence, so that the miters are cut into the bottom edge of the board. The CMS hold-down device won’t work in this situation, so I use a custom feather board. I first make a mounting board from 3/4-inch plywood for my CMS router table, and secure the board to the router table with a couple of Fast Clamps. A dado in the mounting board allows me to adjust the feather board. I’ve even used two feather boards in the same mounting board. Trust me, when you’re cutting lock-miter joints, you don’t want anything to move once you start up the router.
When I run a board on edge through the router, I hold on to the top of the board. Moving my hands as they get near the bit, I apply consistent pressure down toward the table top, while the feather boards apply pressure against the fence. Although I didn’t use work gloves in these photos, I recommend wearing a thin pair, such as FastCap Skins, to protect your hands from getting cut up by the sharp miters.
Cutting lock miters generates a lot of saw dust, so good dust collection is very important, both to keep the cutter sharp and to let you run boards through the machine more quickly without dust being trapped between the fence or table and the boards. The CMS router table has a great dust extraction port, but it doesn’t work once you lift the hold-down to cut edge miters. So I made a small baffle out of 3/4-inch stock that lets the port work even when the hold-down is lifted out of the way.
ASSEMBLY
Over the years, I’ve tried just about every technique for joining the corners of newel posts. And lock miters are not only the strongest, but they are the easiest to assemble as well. (Dominos, Festool’s version of a mortise and tenon, also work extremely well).
I start by laying one of the face boards on my workbench, and then I run a bead of glue in the lock miter. It isn’t necessary to apply glue to both boards. Glue applied to one side spreads easily onto the mating piece, and I get less squeeze-out with glue on just one side. (With PVC, it’s especially important to glue only one side for the strongest and longest-lasting glue joint.)
Next I set the two edge-mitered boards into the face board, letting their miters interlock. Then I run beads of glue in the lock miters of the last face board, flip the board over, and drop it onto the edge boards.
Lock-miter joints seem to close up nicely no matter what direction you clamp from, but for the best results, I apply clamping pressure to the face boards—the boards that are milled flat in the shaper or router table—rather than to the edge boards. However, I’ve found that the joints close even if I clamp on the edge boards instead. Most often, I secure the edges with 23-gauge or 21-gauge pins.
Photos by Gary Katz
