Ah the compass. I’m sure everyone used one of these back in school for geometry class. Great for making circles and arcs with an appropriate radius.
However, there is another function it can perform to help in your prop making (or anything else [I picked this up from woodworking, the wood whisperer is a great resource]). That is it can make a border around your object with the desired thickness. Here I am setting it to do a 1/4″ thickness. My compass is a little hokey so I had to tape my pencil to it, but it still functions just fine.
With the thickness set, I can go around the edge of my object without worry.
As you can see in the above picture, the needle helps to guide the pencil along the edge. And here is the end result:
As you may have guessed, I am working on Kirito’s Elucidator from Sword Art Online. I needed to make sure my sword edge was drawn correctly for the entire blade as it has points and curves near the hilt. Using the compass saved me quite a bit of time rather than having to spend time trying to align my ruler correctly or worse, free handing the whole thing. A compass is a very cheap tool and can be very useful for circles, arcs, and borders.