Marking corner radiiWhen marking a corner for cutting it round, you can use a compass to mark the arc, or you can use any round object that you might have handy.
The advantage of just using any old round object is that you don't have to fiddle around to figure out where the center of your arc needs to be. A more significant advantage though is that you can visualize how the arc is going to look before you even mark it on the piece of wood. When I do this sort of thing with a compass, I usually end up marking one size arc, look at it, and then decide to go bigger or smaller. If I trace the edge of a round object I can see how it's going to look before I even mark it.
Many objects can be used for this sort of thing - the base of any can, plastic containers, mugs, CDs, or in this case, an old hard drive platter.
See also: Marking large arcs
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