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When a woodworker hears the words “lock miter,” they are apt to think Arts & Craft furniture and Stickley. The router bit shown below is what we use today to make a similar joint, but it’s not exactly from the period. The opening photo is the design of the joint used by L & JG Stickley for quadrilinear legs (Gustav Stickley was the smart one; he used a thick quartersawn oak veneer attached to a two-piece glued-up leg). L & JG had special shaper bits made to the profile, one for each edge design. Each pass was made with the workpiece laid flat to the table.
To work with today’s lock miter bits, there are two schools of thought. 
With both methods, you have to cut with one board flat to the router table and the second vertical, tight to the fence. One way to set up the bit is to use jigs such as the new Groove Center from Prazi (PR6000). “Groove center” doesn’t say lock miter in any way, but a lock miter is, in addition to finding the exact center for any grooves you need to make, one of the two main functions of the jig. You need to adjust the jig to work with the router bit you have , each manufacturer has a different design with different offset measurements provided by Prazi.
Once the jig is set up correctly, a pin attached to the jig slips into the router collet and the fence pulls tight to the jig. Here you set the fence first, then you adjust the bit height off of the fence location , that’s done with another setup tool where a plunger that looks like a regular screwdriver meets a specific point on the router bit. The second tool is turned upright and the height is dialed in to match. Accuracy is a must for this setup, but when done as directed, you get very good results. There is only one pass per board to make the lock miter , one heavy cut with no sneaking up on it.
The second school of thought sets the bit height first before adjusting the fence. In this setup, you match the centerline of your workpiece to the centerline of the router bit, which isn’t marked. This technique is a matter of trial and error. Get the two points close then make a cut on scraps. If you can flip one piece upside down and slide the two together, you got it right the first time. If not, raise or lower the bit and try again. Once the height is set correctly you adjust the fence. This step is also trial and error, but you can see, as you cut, when you’re getting close , move the fence until the bevel of the cut meets the top edge of the workpiece.
The joint from either method is a tight-fitting joint with plenty of glue surface, but I couldn’t get the fit perfect. If I were working with skinned plywood, the veneer would have to be sanded off before I leveled my joint. But if I were working with solid woods or painting the final surface, either setup would work. Because of this, I would like to introduce a fourth option: Not making a lock miter joint at all. Do we, as woodworkers, need this joint? Is it a functioning furniture joint, or a costly router bit that gets chucked into a drawer more often than it gets chucked in a router? Take a minute to answer a short poll and let me know if this joint is used in real-world furniture applications. Feel free to comment, too.
If you want a lock miter design without shelling out the cost of the router bit, there is another option. The photo shown here is a similar design that’s cut at a table saw. The corner of the workpiece is mitered and a short tongue slips into a groove that locks the joint together. It can be done, but I don’t think we’re going to jump at making this joint , I never have. Then again, I’ve never used a lock miter joint until I began to take a look at the Prazi Groove Center.
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So far no one seems to be commenting on the greatest shortcoming of the lock-miter, IMHO. Look at each surface as it interacts with its mating piece of wood: they are all end grain joints. The last little bit of actual mitered surface is superior to all the others. I’ve come to the conclusion that modern glues and techniques make end-grain gluing more effective than before, but no serious project should be trusted to this inadequate method of joining two boards perpendicularly.
Alf Sharp
Just spline or biscuit it. Then wrap it with old rubber tire tubes from your bike when you glue it up.
As an aside, you mentioned Gustav Stickley, well if you ever find yourself in Morris County NJ, stop by the Craftsman Farm Stickley Museum. This is the real thing, and you’ll learn about the ironic automation of the craftsman furniture movement. Here’s a link. http://www.stickleymuseum.org
The lock miter is such a specific joint.
There are spans of time when my lock miter cutter head never see’s the light of day, then wah-la its a necessary joint.
Its my opinion, jig or no jig, the real key to this joint, is that once the machine set-up is done, you machine your parts and make sure you don’t have to come back to a second machine set-up.
Increasing attrition when the lock miter is called for, has always proven valuable.
Glen –
The Lock Miter bit was the most expensive tool I have ever purchased. I bought it and have yet to make a decent joint with it.
I’m trying the Prazi GrooveCenter now to see if it will help.. but that silly bit keeps beating me at every turn!