Making Radius Trim for an Arched Pediment

The mahogany doors shown in this article were flanked with frame-and-panel pilasters and capped with an arched pediment. I assembled the pilasters with pocket screws, sizing them so that the pediment would be no wider than 8 feet and could be cut from a single sheet of birch plywood. The pediment consists of three basic parts: a plywood body, which is a segmental arch with an 8-foot radius, laid out so that the arch starts about 4 inches up each edge; a transitional molding that covers the joint between the plywood and the casing below; and a curved molding at the top.

To look right, the crown molding capping the pediment needed to have a beefy profile. I don't have a shaper or molder that could accommodate the size I wanted, so I strip-laminated three wide curved blanks in thicknesses that could be profiled with a router, then assembled them into a built-up crown. Using 3/16-inch poplar strips 2 1/2 inches wide, I glued up the three blanks at the same time, separating the individual blanks with dry joints and tightening the radius slightly to allow for springback.

Since I oversized each section, the bearing on my router bit had plenty of clearance while I profiled each piece; to prevent the router from tipping, I screwed a block of wood to the router base.

I then profiled the blanks separately and combined them to make one large piece of trim. The flat top cap measures about 5/16 inch thick and has a square edge, while the other two laminations measure about 13/16 inch thick. I cut the profile on the middle section with a 3/4-inch roundover bit and used a 5/8-inch cove bit to cut the profile on the lower section.

I also profiled short sections of matching straight material to use for the mitered returns. When installing the three-part molding, I started with the middle section, then fit the bottom and top sections and added the returns. Finally, I added the transitional molding, a combination of 11/16-inch-by-2 1/8-inch and 3/8-inch-by-1/2-inch panel mold profiles, to hide the joint between the casing trim and the plywood.

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