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Making Radius Trim for an Arched Pediment

Making Radius Trim for an Arched Pediment

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    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.

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    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.

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    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.

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    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.

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    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.

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.

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. 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 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 1 1/16-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.

These days I can find prehung raised panel mahogany doors like the ones here for less than $2,000. I spent another $200 in materials to trim the doors, which took about 20 hours of additional labor. Robert finished other trim work while on the site, but estimates that he spent about six hours sanding, staining, and finishing the doors and trim. His materials cost less than $100, and included a gallon each of sealer and topcoat and about one quart's worth of stains.

Gary Striegler is a custom builder in Fayetteville, Ark.