Here's one simple jig that does the work of two. Use it to locate the center of round stock, or use the increment holes to mark different sizes of circles and arcs.
Cut the body to the size and shape noted on the drawing at right from 3/4" stock. Using a hacksaw, crosscut a piece of 1/8x1" aluminum bar stock to 12" in length for the blade. Mark the screwhole centerpoints and increment hole centerpoints on the blade, where dimensioned on the Hole Location Detail. Using your drill press, drill three countersunk screwholes through the blade. Then drill the increment holes and the pivot hole. Drilling through the aluminum will create rough surfaces, so sand the aluminum smooth with 220-grit sandpaper.
Use a square to positon the blade perpendicular to the end of the body at the apex of the V-shaped notch. Using the screw holes in the blade as guides, drill pilot holes into the wood body. Screw the two pieces together.
Using the tool
To find the center of round stock, such as the bowl blank shown above, rotate the body of the tool around the diameter of the stock and mark a pair of roughly perpendicular intersecting lines as shown. The intersection marks the centerpoint.
To use as a trammel, fit an awl, nail, or other pointed object through the pivot hole in the body and blade. Using one of the previously drilled increment holes in the bar, rotate the tool around the trammel pivot point to mark a circle or arc, as shown, at right.