Hi Richard,
Good luck with your mantle project. Below is a link to a plan I build quite frequently for one of my customers. The columns feature raised panel elements, the architrave is radiused, and the top is built up using stock crown molding. The whole thing is made from MDF - you'll need a sheet of 3/4" mdf, a quarter sheet of 1/2" mdf (for the raised panels), and a small piece of 1/8" hardboard. When I build this I use particle board for the blocking and non-exposed pieces to help cut costs.
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's my disclaimer: I just moved my plan for it from a spiral bound notebook page to cad, and haven't built it off this redrawn version of the plan yet - so you may want to double check the cut list (I've been know to juxtaposition numbers occasionally). The dimensions for the rails and stiles are sized to the raised panel bit set that I use - your bit set will cut differently so these dimensions will probably need to be adjusted. Also, the dimensions on the top piece and top blocking will be dependent upon the crown molding that you use.
Here's a hint: make the raised panels first, make the columns out of the raised panels second, make the center architrave third. Then, assemble these components and make/attach the top pieces.
This drawing doesn't reference any of the profiles that are routed on the edges - just about anything will look good, but I suggest using profiles that are easy to sand after they are primed and not too delicate as to avoid future damage.
To make the radiused architrave, I made a template from 5/8" particle board, carefully cut the template out with a jig saw, then laminated a
5/8" wide strip of 1/8" hardboard to the cut surface using glue and brads. This results in a great template for pattern routing (for cutting multiples, this saves a load of time and $$$). I was going to have my template cut on a CNC machine so it would be very accurate, but this method gave me close to perfect results using pieces from my scrap bin (read: kept $$$ in my pocket).
Here's a link to the offsets to make your own template. It's easy to loft the template - just transfer the offsets to both sides of a piece of particle board then you are ready to go using finish nails at the offset marks and a flexible batton (I like to use a piece of 1/8" hardboard that is about 1 1/2 inches wide as a batton when laying out radius cuts like this).
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this mantle is designed to fit a 36" x 36" zero clearance gas fireplace - remember to adjust the dimenisions to meet your area's building codes (and to keep your house from burning down!!!).
Good luck,
Scott