Three-Way Lock-Miter

Until now, I have been focusing on hand-cut joints. But in my daily work, I use power tools to cut joints. The lock-miter is one important joint made with a special router bit on a router table:

This bit pattern provides a positive alignment as well as a lot of gluing surface. Once you set up the bit, it’s easy to glue up a very tight, strong, and good-looking joint.

Often, it’s used on the long-grain of boards, to make a box. For example, it’s really good for making boxed posts or columns - for interior or exterior use. It will stay tightly glued, and it maintains its square angle firmly. It is far superior to a butt joint with nails or screws. Those will come open and show a gap. The glued lock-miter will not come apart, as long as you use good glue properly.

Here are a few examples of bits available on Amazon.com:

::amazon2(”B000FNJ80U”):: ::amazon2(”B0000DCZLV”):: ::amazon2(”B0000DCZ88″)::

When buying a lock-miter bit, be sure that you choose one for the thickness of board you’ll be joining. There are different sizes for 1/2″ versus 3/4″ stock, for example, and you really need the right size.

Adjusting and aligning the lock-miter bit

To set up the bit, you have to set two things: the height of the bit, and the position of the fence.

The height has to be perfectly centered in the stock you’re using - as perfect as humanly possible. You can do this by cutting two test pieces on the flat, and then fitting them into each other. If they align into a perfect plane, you’re all set. If they’re not aligned, then adjust the height of the bit by half of the distance that they’re misaligned. (If the test pieces are 1/16″ off, then move the bit 1/32″.)

Perfectly Aligned Test Pieces

The fence has to be set so that when you cut the stock on the flat, it comes to a point that just grazes the outfeed side of the fence. That’s so it has something to rest on after it’s on the outfeed side.

I’ve been finding better results when I “hog out” the major part of the wood and then do the final pass removing just a small amount of wood. The cut is more reliable and finer. I do the “hogging out” by moving the fence. Some people may do it by shimming from the fence frame to the fence. Maybe that’s a more reliable method, but it takes more time to adjust. I draw a line on the tabletop before moving the fence, then set it back to the line.

Here is the router table setup. I skinned the tabletop and made a tall, continous fence. Then I slid it into the bit to cut a zero-clearance table for the smoothest operation.

Here’s the bit, spinning all by its lonesome. With the zero-clearance table, it’s less likely to bite anyone.

I’ve been experimenting with using the lock-miter joint in multiple dimensions. Instead of just a single edge, I’ve been making boxes with lock-miters on all edges. The results are very interesting. The edges intersect and make complex shapes:

Here it is glued up:

And here it is sanded a little:

The alignment doesn’t have to be exactly perfect if you’re going for slightly rounded edges like I was. But it is good for it to be almost exactly perfect, because the inside of the box will still show the alignment.

Besides, what in this world is ever exactly perfect? Except imperfection.

Links

John Lucas has a page about setting up the bit, with lots of good photos.

Gary Katz, a top-notch finish carpenter and cabinet maker, has an article about the lock-miter joint.

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