Lock Miters: Setup or Not
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.