Make jigs for template routing

I'm making speaker cabinets that require several round and rectangular holes. I plan to use templates, guide bushings, and my router to do the work. There's only one problem: How do I figure out what size to make the templates?
—James Sullivan, Columbus, Ohio

Get a pattern-cutting bit with a top-mounted bearing, James, and you'll eliminate the need for any math. This bit will duplicate the same-size hole in your stock as that in your template. After cutting the opening to rough size, just clamp your pattern in its precise location and let the bearing ride the inside edges of the pattern while the bit does the cutting. If you still want to use a bushing and straight bit, subtract the bit diameter from the outside diameter of the bushing, and add half the difference to each dimension of the template. For example, if you have a ¹⁄₂ "-diameter bushing and a ³⁄₈ " bit, half the difference is ¹⁄₁₆ ". So you'd make a template opening 5¹⁄₁₆ ×7¹⁄₁₆ " for a 5×7" opening.

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