When a woodworker hears the words lock miter, they are apt to think Arts & Craft furniture and Stickley. Although the router bit in the opening photo is what we use to make similar joints, it’s not from the period. L & JG Stickley used the following joint to make quadrilinear leg joints. Gustav Stickley was smarter than anyone else; he used a thick quartersawn Oak veneer and a two-piece glued up leg. L & JG had special shaper bits made to the profile, one for each edge design. Each pass was done with the workpiece flat on the table.
There are two schools that can be used to work with lock miter bits today.
Both methods require you to cut one board flat to your router table and the other vertically 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 doesnt 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. First, you need to set the fence. Next, adjust the bit height by using another setup tool. This is where a plunger, which looks similar to a regular screwdriver, meets a point on the router bit. Once the second tool has been turned up, the height can be adjusted to match. Accuracy is a must for this setup, but when done as directed, you get very good results. To make the lock miter, you only need to do one pass on each board. This is one large cut that requires no sneaking.
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 isnt marked. This is best done by trial and error. Get the two points close then make a cut on scraps. You will be able to flip the one piece upside down, then slide the two together. If not, raise or lower the bit and try again. After you have set the fence correctly, adjust it. This step is also trial and error, but you can see, as you cut, when youre 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 couldnt get the fit perfect. If I was working with plywood skinned, I would need to sand the veneer before I could level my joint. If I was working with solid woods, or painting the final surface of the building, any 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? It is a furniture joint or a more expensive router bit that gets chucked in drawers more than it gets chucked at a router. Take a minute to answer a short poll and let me know if this joint is used in real-world furniture applications. Comment, as well.
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 thats 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 dont think were going to jump at making this joint , I never have. Then again, Ive never used a lock miter joint until I began to take a look at the Prazi Groove Center.
