One of the most frustrating bits to setup for many people is a lock miter bit; however, its a very effective joint especially for exterior projects like making columns, wrapping posts etc. Part of the reason its so effective is because aside from it being a self-aligning joint and having lots of glue surface, where moisture would usually weaken a regular mitered corner, the locking action in the center of the joint keeps the joint strong.

You can find instructional videos online at MLCS http://www.mlcswoodworking.com/shopsite_sc/store/html/smarthtml/pages/instruct.htmlwhere they have a lot of good online videos for setting up various bits, but I believe one of the most frustrating aspects of setting up the lock miter bit has to do with identifying the center of the router bit and aligning that with the center of the stock you're milling. I've seen several diagrams about how to set up the bit, and in most cases, they have not identified the accurate center of the router bit. My advice is to find the center of the stock you want to add the joint to, find center by marking center with a marking gauge, flip over and mark center again to ensure its dead center, and then raise the bit precisely to this point (see below)

Here's some tips I posted on another forum:

The lock miter joint is great for exterior work but I used to resist using it because of the setup until I started doing the following.

#1. Start off with material that is of uniform thickness and jointed before starting. Even for exterior work, I make sure that everything is of uniform thickness. I recently did a job with Azek that varied in thickness and it was pretty frustrating and starting with material you've jointed and planed helps.

#2. Mark the center of a piece of test stock and set the bit height to that mark. To find center, I use a marking gauge using the mark, flip piece over and mark method. Compare the two marks and if there's a pair of parallel lines, reset the marking gauge and do it again until you leave a common center line. As to bit height, the trick here is that what many think is the center of the bit is not actually the center. That's because the tenon produced by the bit has a sloping face on the top side. Center of the bit is actually the very corner of the bit.



#3. Before making any test cuts, set the fence slightly forward. To set the fence, position your test stock vertically against the fence and slide it up to the cutting edge of the router bit. When you sight across the router table surface and compare the bottom cutting edge of the bit to your stock you will see roughly where you have to position your fence to produce a crisp mitered corner. Don't go for perfect on the first cut. Move it slightly forward and lock the fence down.

At this point, you have the proper bit height and the fence roughly where it needs to be set.

#4 Make a test cut in the vertical position. Be sure to use a backer/support board if you're cutting stock that is narrow. Inspect the end of the board's mitered edge. If the mitered edge looks blunt/square, move the fence back slightly and make another cut until you can produce a crisp, sharp mitered edge.

If you've done it right, you should be able to run a piece flat and a piece vertical across the bit and produce the joint with a flush mitered corner.

Paul,

Thanks for posting this information. Providing the link to the HOW TO video was a great idea. I have a couple of these bits that I've never used, but at least now I have the idea of how they work. Now all I have to do is build my router table and get a variable speed router.


Rat.

thanks paul...i never had any luck with this bit...but seing what uou use it for i can see the value in that bit...i have a brand new one...10 years old...used it once..screwed up my piece..and never used it again...great info !

I echo that! Mine is also about 10 years old, and except for ruining a few nice pieces of white oak, its hardly been used.
Paul, thanks for the interesting information and the links to the videos at MCLS. They are a useful resource that I had never seen. I love this forum!
Clif

the video on mcls is helpful, except what is indicated as the center of the bit isn't correct. Refer to my illustration above and try for yourself after you have a test fit that works and you'll see that the center of the stock aligns with the corner of the bit-not the top of the slanted tenon.