Balsa is a very light and soft wood used primarily for scale modelling and other crafts and hobbies. Due to its light weight and open grain, balsa is easily damaged and can be challenging to cut. There are several hand tools that work well, but for cutting accurate circles a band or scroll saw works best. You will need a thin blade with very fine teeth to make accurate cuts. Both band and scroll saws function in much the same way. There are specific details for different brands, but much of the process is the same.
Make a template by using a cup, can or other round object of the correct diameter to trace your circle on the surface of the balsa wood with a felt tip pen. Set the template on the balsa wood and trace the outline. Draw precise circles of a particular size with a compass. To draw a circle with a compass, open the legs of the compass to the correct diameter and tighten the nut to hold it firmly. Set the metal scribe point where you want the centre of your circle to be and drag the compass pencil around the scribe to draw the circle.
Adjust the blade guide on the saw. Place the balsa on the saw table. Turn the wing nut on the guide counterclockwise and drop the guide down onto the balsa. Lift the guide slightly to allow the balsa to move freely and then turn the nut clockwise to tighten it in place.
Start the saw and allow the blade to come up to full speed. Guide the piece through the blade, turning it as it goes. Remember that the blade is stationary and you are driving the piece, not the blade as with hand-held power saws. Keep the piece moving steadily and stay on the line. Continue turning the piece through the blade until the circle is completed. Sand the edges of the cut circle with 200-grit sandpaper to smooth the edge.