Machine Setup for Lock Miters

Lock miter setup requires a lot of fiddling, so a permanent dedicated machine can be most efficient. April 6, 2011

What is the correct way to set up a lock miter to not have to use a spacer on the fence? I have used up many scrap pieces trying to get it right. Fence in and out and height changes on the bit - what else is there?

Forum Responses
(Cabinetmaking Forum)
From contributor L:
Setting up a lock miter is a pain. Once correctly setup one part is run flat on the table the other is run on edge. No spacers! Use a heavy wide board for a fence that goes all the way through. Though small parts may be run manually or with feather boards you will have the best luck with a power feed. We just leave a heavy old shaper setup with an inserted mock miter cutter and two power feeds. Two because changing a feed from horizontal to vertical takes too long. Swing one out of the way and the other into place. Reducing setup time is the goal. If you can't afford the dedicated machine, once you are setup make a set of reference jigs to allow you to come back the same location (relatively) quickly. Use the miter slot as reference for the fence by attaching a board to a stick that fits snuggly in the slot and rests against the fence. The elevation of the cutter can be done using a gage block or a sample of the lock mitered wood. A real ht. gage is the most reliable. We run a lot of both solid lumber and veneered MDF panels. We have even run laminated slatwall but that is a real challenge.

From the original questioner:
Thanks for the info the scrap is piling up.A pain is putting it mildly. We have it dialed in on our newly dedicated shaper.

From contributor C:
Try a single tongue lock miter. If you do you will never go back to the other. Both pieces are run flat. One on the shaper and the other on the saw with a dado blade. We leave one shaper set up with this setup and never change it until the bit is dull. You can even run different thickness material and the corners will line up.

From contributor C:
Run one flat and the other vertical. The trick is getting the material centered in the middle of the lock miter.