If you don't have a compass on hand and need to draw a large circle, you can turn a few scrap pieces into a beam compass.
A speed square can replace a compass for drawing small circles, but a beam compass is the preferred tool for drawing large circles. Attach trammel points to a shop-made beam like a 1x1 piece of wood. This provides unlimited radius lengths for your circle or curve. Then, you can turn it into a compass with two small clamps, a deck screw, and a small pencil. The size of your circle is only limited by the length of beam you cut (as well as your arm wingspan). Check out the video above for the detailed instructions.
Draw Large Circles or Curves without a compass or trammel [Make Something (YouTube)]