To work out the double groove spiral for the magazine holder illustrated, proceed as follows:
1. Square up the stock to 1⅜". Center carefully and turn the design on both ends as shown, in the upper diagram (Plate B-V--3-a’). Turn the cylinder between the top and bottom, making it 5½" long and 1⅜" in diameter.
2. Divide the cylinder into two equal parts. Each part represents one revolution of the spiral.