A Lap Joint for Curved Members

Ideas for tools and jigs to dado out for half-lap joinery where two curved pieces meet. December 7, 2007

I am looking for ideas to create a half lap joint where 2 curved (laminated) members cross by each other. Members are 1 3/8" thick x 5" wide, radius is 42", and they lay flat on each other's wide side.

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
I would use a long shank cutter in the bench router with a limit fence jig (with desired radius) on each side of the cut, and have at it.

From the original questioner:
Thanks. By router, do you mean mounted? The piece is large and I think the tool cannot be stationary. I was looking into a router with brass bushings and using the actual pieces for a template, but pieces are too thick for router bit to reach depth needed. Don't want to make a template because the radius may vary.

From contributor D:
I would use a trammel swing arm attached to a handheld router with a mortise bit. The trammel has to be fixed with a brad at the center of the radius and measured to cutter. If you don't want to attempt this on an actual piece, use the trammel to make a pattern in plywood or MDF and then use a top bearing pattern bit to make dado off template clamped to finished piece. Fairly foolproof, I think.

From contributor I:
I would use the second half of contributor D's method. Just remember that you will need two templates per beam. You are going to want the templates to fasten to the beam on the outside of the half lap line you draw on the beams, if you are using a flush cutting top bearing bit.

From the original questioner:
Thanks to all. Looks like I will try the router plate and a 1/2" shank bit. I need to remember to clamp waste stock on the entry and exit of cut to prevent chip out.

From contributor W:
I am currently doing a similar thing - using straight bit (3/8" ex.) with roller bearing to cut rabbet in scrap using curved stile as guide. This produces 3/8" rabbets on either side, total width 3/4" over stile. Use the created scrap as pattern on top of vertical rail with matching roller to offset the 3/8" and cut rabbet in rail, and then clean out material between the two cuts. See Whiteside catalog for bits with sets of graduated roller bearings.