I’ve written before about end mills, but so far all we’ve discussed are the straight ones, for cutting square corners. Those are real workhorses, and I use them most of the time. But once in a while I need to make a curved shape. What then? Corner rounding end mills, that’s what!

Rounding Outside Corners

Perhaps I’m making a tool, and I know I will hold it in my hands for long periods. It would make sense to round over the sharp corners to make it more comfortable to handle. But doing that by hand, with a file takes a long time. Moreover, it takes a lot of skill and attention to keep the rounding consistent. Such an approach might make sense for a small part, if you’re only making one, but if it’s a big part, or you’re making a bunch, you need a better approach.

A corner rounding end mill.

Enter the corner rounding end mill! Compared to a straight end mill, the cutting edges are very small. You use other cutters to make the basic shape of the part. Then you switch to this cutter just to round over the sharp edges. The rounded edges of the cutter leave the reverse shape in the metal.

Varieties of Corner Rounding End Mills

Corner rounding end mills are sized by the radius of the curve in the cutting edge, and thus the radius of the corner the cutter creates. As usual, there’s a vast array, as you can see from this lot at Travers Tools (as usual, this isn’t an affiliate link. I’m just one of their customers.)

Like straight end mills, you can buy corner rounding end mills in several different materials. Mine are high speed steel or HSS, an alloy of iron, carbon, tungsten and molybdenum). But cobalt steel and solid tungsten carbide are also available, at increased cost.

They’re sized according to the radius of the curve they make. Suppliers sell them in fractional inch and decimal inch sizes. They range from 1/100″ to a full inch radius. You can also get them in metric sizes.

But with a small machine like mine there’s a limit to how big of a radius I can cut. As the radius increases, the length of the cutting edge that’s engaged with the metal increases. As that increases the cutter needs more power to turn. Eventually you run out of horsepower.

Worse yet, the cutter exerts just as much force against the frame of the machine as it does against the metal. If the cut gets too big, the frame will flex, which causes a bunch of problems. For me, anything much bigger than 1/8″ is really pushing my luck.

I hope that gave you a little insight into corner rounding end mills. Thanks for reading!