Here's a sure-fire method for setting up this bit.

Lock-miter joints have a lot going for them: appearance, self-alignment for easy assembly and clamping, and ample gluing surfaces for strength. They're ideal for right-angle corners of boxes, columns, cabinets, frames, and the like. Plus, you can cut both parts of the joint with a single router-table setup.

But getting the correct router-table setup—both the bit height and fence position have to be just right—can be a challenge. Here's a sure-fire method for setting up this bit.

Drawing of Lock-miter setup gauge

Install the bit

Lock-miter bits come in different sizes, covering different material-thickness ranges; choose one that works for the thickness of your stock. Install the bit in a table-mounted router, and set the router speed to 16,000 rpm for bits 1-1⁄4–2-1⁄4" in diameter or 12,000 rpm for bits 2-1⁄4–3-1⁄2" in diameter. (Follow the bit manufacturer's recommendation if it is different.) When routing workpieces 2" or more wider than the height of your router-table fence, attach a tall auxiliary fence to ensure adequate support. (We mounted ours with double-faced tape.)

Now, make a simple gauge

From scrap stock the same thickness as the joint parts (both parts of the joint must be the same thickness), construct the lock-miter setup gauge (Drawing, above). Draw a centerline across the end of the bottom piece, and mark the material thickness on the gauge. Use it as shown as shown below.

P{photo showing how to set bit height
Set the bit height first. Place the gauge beside the bit and align the center of the bit with thecenterline on the gauge. The center of the bit lies at the midpoint of thesloped face of the tongue.
Photo showing the fence position against the gauge
Then, position the fence. With the height set, position the gauge so the top and bottom corners contact the bit's cutting edge. Then slide the fence up against the jig and lock it in place.
Photo showing first shallow cut
A spacer makes the first cut shallow; remove it for the second pass. Use afeatherboard and pushblocks for safe, accurate routing.
Photo showing the workpiece held firmly against the table and fence.
Hold the workpiece firmly against the table and fence and feed it at a steady rate as you form the complex profile on the joint parts.

Cut a test joint

Rout both parts of the joint on scrap stock the same thickness as the work-pieces. For easier routing, make shallow cuts by starting with a spacer attached to the face of the router-table fence. (We clamped a 1⁄4" plywood spacer in place to rout the joints in 3⁄4" material). Make one pass with the spacer in place, and then remove it for the final pass.

Rout one part of the joint (call it Part A) vertically (Photo A, above), with its inside face against the fence. Without changing the setup, rout the other part of the joint (Part B) flat on the table (Photo B, above) in two passes, with its inside face down.

Check the resulting joint against these examples

After you cut the test pieces, assemble the test joint and compare it with the images below. Adjust the fence position or bit height, as indicated, in small increments. Cut additional test joints and make adjustments until the parts meet precisely, like the joint at the top of this page.

Photos of 4 examples and recommended corrections

More resources

Learn more about special-duty bits:
Rabbeting bits with bearing sets:
Clean-cutting spiral bits:
Perfect angles with bird's-mouth bits: