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I need to make MANY 6" holes using a router. If I use a 5/16" template guide and a 1/4" straight bit, what size hole must I make the template? I don't have much scrap to experiment with. My gut tells me I need to divide the difference of 5/16"-1/4" in 2 and then add that to the radius of the circle... but that sounds wrong.

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From the link below...

Figuring template offset
Offset is the distance from the outside of the guide bushing tube to the cutting edge of the router bit. This dimension determines where the bit cuts the workpiece.

To figure the offset, measure the outside diameter (OD) of the guide bushing tube and subtract the diameter of the bit. Next, divide this figure by 2 to determine the offset. In the example at right, subtract the 1⁄2 " bit diameter from the bushing's 3⁄4 " OD. You get 1⁄4 ". Now divide this number by two and you arrive at a 1⁄8 " offset.

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Understanding Guide Bushings - WoodMagazine.Com

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