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    Unlock the Secrets of the Lock Miter Joint

    Most budding woodworkers start out building boxes of one sort or another—and while miter joints are relatively easy to cut, they leave a lot to be desired in terms of strength. Of course you could build a jig to spline your miter joints for added strength but there’s another way.

    The lock miter bit, used in conjunction with a router table, produces a strong joint that’s actually quite interesting to look at. The same bit—at the same settings—is used to cut a profile into the end of one board laid flat on its face, and in the end of another board which is fed into the bit with its end grain down on the router table. The strength in this joint comes as a result of the way the bit produces a negative and positive image that interlock with another as well as the fact that the joint creates a lot of surface area for glue to adhere to. There’s one caveat however—set-up can be tedious and tricky. Once you get it dialed in however, it’s easy to cut joint after joint—super fast, super durable, and super easy! Here’s how it’s done.

    Stock Thickness and Test Pieces

    For this joint to come together correctly, your four box sides need to be of consistent thickness. All the stock should have been planed to thickness at the same time. Also, you’ll need to mill extra stock to run through the router bit as test pieces. This is a finicky joint which can require lots of test cuts, checking the fit, re-jiggering the bit or fence set-up, and re-testing. Be sure to have plenty of extra stock from your project on-hand.

    Keep in mind that—as with any router bit, you should cut a zero-clearance hole into your fence. My fence has removable inserts which I can cut custom router bit profiles into. To do this, I clamp down the right side of my fence, turn on the router, and slowly swing the left side of the fence toward myself, allowing the bit to cut its profile into the material.

    Now let’s get to the good stuff. For this joint to work, you need to set-up two positions dead accurate: bit height and fence alignment. Let’s tackle these one at a time.

    Setting Bit Height for a Lock Miter Joint

    The first thing you want to establish is where the exact center of the router bit falls. As you can see in the photo at left (below), my bit is exactly 1-in. in height. When I set my little ruler on its edge and use the marks on the end of the rule, I can locate the exact center of the bit (at the 1/2-in. mark) and either mark it with a fine pencil—or in this case just note that it falls in a specific corner of the cutting profile.

    As you can see in this photo (below-L), the exact center of my 3/4-in. thick stock is—of course—at the 3/8-in. mark. The height of the bit should be set so that the center of your router bit is in perfect alignment with the center of the stock your routing when it’s laid on its face. I used my ruler once again, to align the center point of my router bit – with the 3/8-in. mark on my ruler and bingo—bit height is now roughly set.

    Setting a Router Table Fence for a Lock Miter Joint

    The distance from the outer edge of the bit’s cutting surface to the face of your router table fence should be equal to the thickness of the stock you’re routing. To set the fence, I set my ruler on its edge and slide it towards the bit, until the ruler just hits the cutting edge of my bit (lower-L photo). Now I can set my stock against the fence and lower it until it hits my ruler. Adjust the fence until the end of the ruler just kisses the face of your stock and you should be in more-or-less perfect alignment.

    Router Pass Number 1: Stock on its Face

    Now it’s time to make your first pass. If you’re building a box, you’ll need to pass the ends of two of your four pieces across the bit in this manner. Notice how I’m using a shopmade push stick for stability. My push stick consists of a square piece of MDF with a handhold screwed to its face. The handhold is attached at a 45-degree angle so that you can push the stock into the spinning bit while at the same time providing force against the fence. This push stick helps the stock stay perfectly square against the fence and it’s a necessity for safety reasons as well.

    Here’s the result in photo 2 (lower-R). Now we can move on to pass number two.

    Router Pass Number 2: Stock on Edge

    Now it’s time to rout the other two box sides—this time laying them on edge. To do this safely, I built a simple jig that’s nothing more than a piece of plywood with a long fence screwed to it. That long fence rides atop my router table fence. I simply set the stock on it’s edge, butt it up against the plywood, and clamp the piece to my shopmade jig for added safety.

    Now you can apply some pressure on the face of the stock—for added stability against the fence—and pass it over the router bit.

    In this instance, my set-up wasn’t quite right. Notice how the corner is coming together in this photo (lower-L). It’s close, but not quite there. In this case, I had to lower the bit ever-so-slightly. The final product was spot-on (lower-R)! Just remember, there is a certain amount of trial and error in this process. Those off-cuts from your project material are crucial for dialing in your router table set-up! Now go out there and cut some lock miters!

    Not quite right.