Whether using a shaper or a router table, setting up a locking miter bit is the same. While it’s a simple 3 step process, you should take this set up seriously. Because like its cousin the miter joint, there’s no room for error with the locking miter joint–if you don’t get it dead-on, it won’t look good. But unlike the miter joint, it can be fixed, if it’s a little off.
The process for setting up the locking miter cutter is straight forward, but can often take several attempts to fine tune the adjustment. First, set up the cutter height; then set the fence position. I’ll illustrate on a shaper.
One word of caution, when buying lock miter bits for your router table. Most manufacturers offer 2 sizes of bits; the design and construction of the bit will determine the thickness of the stock it can cut. I’ve included links for both Freud and CMT bits. Normally the smaller size bit can cut thicknesses of 3/8″ – 3/4″ and the larger bits 5/8″ — 1 1/8″.
Begin by setting the cutter height, and fence depth so the cutter is approximately centered on the thickness, and the width of the stock to be cut (photo 3).
A starting point can also be set by using stock from a previous milling operation. But remember, unless the stock being milled is EXACTLY the same dimensions as the previous run, the set up will require fine tuning.
To begin, make a trail cut of the bit height, by feeding a piece of stock through horizontally (Photo 4 and 5).
To test the initial set up, cut a piece of stock off the end of the test piece. Flip it around, and over, than mate the locking miter joints. If the bit height is properly set the two pieces will be flush long Line “A” illustrated in photo 7. Otherwise you will need to adjust the bit height and make test cuts until you have found the correct set up.
Photo 8 shows the results of the cutter being set too high; and Photo 9, shows what happens if the cutter is too low.
Once you’ve set the bit height, you can move on to the fence position. And essentially, you’re going to use the same process you did when adjusting the bit height. Feed a board through, cut a piece of stock off the end of the test piece. Flip it around, and over, and mate it with the other piece of stock. If the fence setting is correct, the two pieces will be flush along the top where they join. If the fence setting needs adjusting, you get the results of photo 12 or 13. Simple adjust the fence, and test again.
That all there is to it! You’re now ready to start cutting locking miter joints!