Machining Lock Miters on Large Pieces

A millwork fabricator gets advice on how to cut lock-miter profiles into the long edge of very long and wide MDF panels. November 10, 2006

We fabricate architectural millwork and store fixtures, and find ourselves using lock mitres on 3/4'' MDF columns for a paint finish. Many of the panels are 12 feet long, and at least 24'' wide, so the first cut is simple, on the flat with our shaper/power feed. The second cut must be done on edge, and it is at best a 3 man job and 2 passes, featherboards, etc. to get results. If only I had a shaper turned sideways? I have been looking at every conceivable machine, and at this point, a double end tenoner seems like the best fit. The cope head can be set level, and the machine is power fed. Does anybody have other thoughts? We have tried mitres with splines, rebates, biscuits, dowels, and the lock mitre works best for us, if we can do the machining. On smaller panels, perfect; large panels, who knows?

Forum Responses
(Architectural Woodworking Forum)
From contributor P:
You might consider the lock miter that you run both sides of the corner through the shaper, then you create the groove on the table saw with a dado blade. Or just glue and tape like a miter fold. I would look into a miter fold machine before the tenoner.

From contributor M:
If you're looking for a shaper turned on its side, take a look at the Bridgewood BWM 605 single spindle moulder. If you were to mount your tooling and run it with the door open, you could get your 24" piece through it. Although you would have to temporarily defeat the safety switch in order to do it that way, it could be done. It has a 40mm arbor, so it would require something different than shaper tooling, but hey, if you're desperate enough or do it quite often, it may be an option.

From contributor C:
I have been working on the same problem in order to join corners of tall, plywood wardrobes. I use two routers, each with a lock miter cutter and each with its own custom base, or "sled". The fences on my sleds provide lateral adjustment, and the routers have micro height adjustment (Bosche).

The sled with the router spindle perpendicular to the panel (the "easy" cut) is small. It simply helps me start and exit the edge correctly. The sled for the router with its spindle parallel to the panel is wider. As I slide the sled down the edge of the panel, I hold down the "inboard" end to offset the weight of the router as it hangs off the "outboard" end of the sled.

With these, I move the tool instead of the panel. Because I am handling something small and controllable, I can make either cut by myself. I clamp a panel to my bench, run the appropriate sled down the edge, and I'm done.

From contributor J:
I would suggest buying the Freeborn cutter suggested. I have been using that one for about 10 years and have been very happy with it. One shaper setup, one dado setup for the table saw. Everything run flat, and you control the depth of the groove.

From contributor G:
I use the Freeborn double t&g miter lock and a power feed. If I have a board that is too wide to fit vertically under the powerfeed's arm, then I bolt the feeder's base out in front of the fence to run those parts. It's a pain, but all you need is 4 more holes in your shaper... not that expensive. The double t&g miter lock is a great joint... once it is set up!