It's very easy to create rounded corners for simple Rectangles like shown above using the Object Inspector palette. Using the units of measure that the document is set at, you enter in the amount and it's automatically applied.

But what about other polygon shapes or rectangles that are ungrouped? Check out the solution below.

Above is a triangle shape used to represent a metal part. Where the screws are set, I'd like to make the angles rounded. Normally, you could just add points with the Pen or Bezigon tools and adjust the point's curve handles. This could take quite a while, and it's not very accurate.

What I'd like to do is set the roundness or corner radius to 20 points for the angles. First I select the polygon shape.

Then I go under the Modify menu -> Alter Path -> Inset Path or you can click the Inset Path Operation button (shown above at left).

In the dialog shown above, I enter the amount of the corner radius I want in the Inset field. Keep the Steps at 1, and make sure the Join option is set at Squared.

When you hit OK, the polygon will shrink down in size 20 points from the original like shown above. Keeping the shape selected, choose the Inset Path Operation again.

Now in this dialog, enter the same value as before, only negative like shown above. Also, make sure that the Join option is set at Rounded.

When you hit OK, the polygon will now have perfectly rounded corners like shown above. Very easy, very fast and very accurate.

To show you how accurate this technique is, compare the two rectangles above. The one on the left has the corner radius set at 10 points. The one on the right was modified like shown above, using 10 points as the Inset amount.

Try this technique on other types of polygons too. Shown above I applied it to a pentagon and a simple star shape that gives some pretty interesting results.