A comprehensive educational community devoted to trim carpentry, finish carpentry and architectural millwork. Hosted by nationally recognized author and finish carpentry specialist Gary M. Katz.
  Lock Miter Joints  
by Jed Dixon & Gary Katz
For nearly a year I've been meaning to shoot some photographs for a short article on cutting lock miter joints. A couple days before the Providence JLC Live! show, I was visiting Jed Dixon in his shop and he suddenly pulled this huge bit out of a drawer and headed for his shaper. While I watched, he started adjusting the height and fence to cut boards for about a half-dozen newel posts. I asked him why he used a lock miter, thinking that most carpenters used them only for exterior columns. Well, for those of you who know Jed, you might have guessed the answer wasn't short or simple: "The thought of building a newel without lock miters...that's never entered my mind. If I'm going to make a newel post, I want it to be around a hundred or even two hundred years after I'm gone. That's the attitude carpenters had in the 18th century. It's a shame more carpenters don't feel that way today. I guess it's more than just the carpenters. The clients are at fault, too. It's our responsiblity to try and education them...if we can. If we can just help one of them understand, if we can just make one small contribution...."
I always suspected there was an easy way to set up a lock miter bit. Though I've done it several times in my table-mounted router, I've never been able to figure out a simple and accurate setup system. Jed made the whole process look easy. He starts by raising the bit until it's centered in the material (more on that in a moment). Then he moves the fence until the top miter is whisker thin.
Jed free-hands the first few boards through the shaper. The shaper must be centered
perfectly, because the same setup is used to make both the vertical and the horizontal
cuts. One board is fed through vertically on both edges, and the next board is fed
through horizontally.


The joints cut by his shaper are much tighter than my router. To close the miter requires a weighted mallet. Opening those joints afterward isn't easy.
If the pieces fit snugly, he assembles a complete box, still using test pieces.  

With the shaper adjusted, Jed sets up his power feeder. The feed rate isn't fast. That's not the purpose of the feeder. Rather, the power feeder applies consistently even pressure along the entire cut, which is critical if the pieces are going to fit properly.
  The powerfeeder is angled slightly toward the table at the outfeed end, which keeps the boards down tight against the shaper table.

Switching this imported power feeder from vertical to horizontal requires the
deft touch of a fine craftsman....
because the crank won't raise the arm unless the weight of the feeder is removed,
and even then, cranking this monster up can't be done from the ground!
Not only THAT, but sometimes the dang thing won't start on it's own. Jed removes the cover and flicks the flywheel with his thumb (standing well back just in case). Fine woodworking does require some mechanical knowledge.

The boards are fed through and stacked on the outfeed side
of the shaper.

In the horizontal position, the shaper is tweaked slightly toward the fence, so the
boards are always tight against the fence.
A large shaper and powerfeeder makes this operation easy, reliable, and safe--your fingers never get near the cutter, especially while you're moving boards quickly through the machine. But if you don't have a shaper or a power feeder, read on...
I've cut lock miters with a 2 hp. and 3 hp. router, mounted in a router table. Here you can see that the joint really breaks into four sections, with each miter equaling about one quarter the width of the board.


Assembling two horizontal cuts should result in a perfectly flush joint.



Corners assemble quickly, too, and provide an uncommon amount of gluing surface. Additionally, the miter doesn't run completely through the edge of the board, so seasonal miter movement is minimalized, just the thing for exterior trim work like pilasters and columns. If you must miter exterior trim, this is definitely the way to go.

These joints can not be cut free hand. At minimum, a good hold down system
is a necessity.


Be sure to make and use a good pushblock, so your hands will stay well away from
the bit.


In the vertical position, a featherboard will help hold the board flat against the fence.


Smaller power feeders are also available. For my shop, I've found this Delta model extremely useful, on my router table, my band saw (re-sawing), and my table saw.


You might think that a smaller power feeder would be easier to adjust and switch from
vertical to horizontal. Not true. This model isn't equiped with a geared riser. Rather, ball
joints provide all the movement. Twisting the feeder into the right position requires patience.
Each of the four directional movements must be adjusted
individually, and then locked down tightly.


Forward and reverse switches on the front of the motor control the direction of cut.
the small red knob controls the feed rate. This feeder doesn't have nearly the feed rate
that Jed's has, but for delicate moldings, this machine is a gem--you can adjust the rate
to a real crawl, which virtually elminates tear out, a nice feature when raising panels.


With the feeder covering the bit, it's pretty hard to put your fingers in harm's way.
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