The perfect choice for easing sharply squared edges, roundover bits are sized by radius. (Imagine that curved edge as part of a circle — the bit size corresponds with the radius of that full circle.) Sizes start at 1/16″ radius that barely breaks a square edge — in fact, unless you have a lot of edges to do, you might be better off using a sanding block for that kind of roundover — on up to a 1-1⁄2″ radius bit that is probably far larger than most shops would need. (Even larger roundover cutters exist, but they’re more suited to industrial applications.) You can buy a set with several bits, but if you’re like me, you’ll probably use the 1/4″, 3/8″ and 1/2″ radius bits most often.
Roundover bits are normally used to create a smooth, even curve on a workpiece, but they also have one sharp cutting edge at the bottom of the radius that creates a 90-degree cut at the top of the roundover if the bit is fully extended. Used in this manner, a roundover bit makes a decorative thumbnail edge. A variation on the roundover bit has these 90-degree cutting edges both top and bottom, and it is often called a beading bit. The bit creates a curve bounded on each end with a 90-degree corner.