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I want to match the existing radius on my stairs trim, but I'm unsure what size rounding over bit I need to get. I measured the arc of the radius the best I could with a flexible tape measure and it's about 9/16". I did the math and that equates to a 0.6" radius, which is close to 5/8". Perhaps I'm not measuring correctly. Is there a common radius for 3/4" board?

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There are a brazillion router bit roundover sizes. From somewhere with a gentle return policy, buy the one you think you need plus sizes above and below.

Or if you're going to a brick+mortar, you could create a profile template (stiff cardboard) of your stair and compare it with what's on the shelf.

  • This is the solution I went with.. cutting profile out of cardboard. Great suggestion – Andrew Nov 15 '15 at 14:52
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First, setup a lamp shinning on the rounded edge.

Get two rulers, place one perpendicular the vertical side of the stair and the other perpendicular the horizontal side. Then line rulers up flush to each other, say at the 4” mark, you should be able to get a measurement of where the gap starts (from both directions). That’s the radius!

The idea is you are making a small carpenters square (image below), so you see the radius of the circle. The light shinning from behind should make it easier to see where the gap between a ruler and the stair starts.

Carpenters Square

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You could get a better measurement with a profile tool.

Profile tool. Image does not imply endorsement.

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I used a compass and traced a 1/2" radius on a cereal box and cut it out. It matches almost exactly so it's 1/2" radius.

  • Hi! I edited out the first part of your post because it read like a reply to another answer, probably this one. – Niall C. Nov 15 '15 at 4:10
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Well, what I would do is grab my collection of router bits and hold a likely suspect next to the step to see if that bit matches.

If it was routed in the U.S. by a U.S. craftsman, it is probably one of these four bits which can be obtained for $20 (for the set).

  • I was really surprised how much some of these bits cost (> $30 for freud). But carbide isn't cheap and I'm cutting hard maple. I only need one so went with a Yonico (good reviews) for $14 shipped. The reviews on the cheapest bits all seem to indicate the bearing will fail quickly or fail on hard woods and wreck the workpiece – Andrew Nov 15 '15 at 14:50
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    Cheap router bits are fine for rough work and making "proof of concept" type things, but you'll be served well by a good bit, especially on maple. I suggest you cut in multiple passes (ie, progressively deeper each time) with the last pass taking off a tiny amount and moving relatively fast. (Maple can burn if you're too slow with the tool.) – Aloysius Defenestrate Nov 15 '15 at 15:09

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