First use a real ruler to draw the upper right corner area of a 90-degree orthogonal coordinate system
Assuming that the diameter of the tube we use is 400, then we can determine that the diameter of the tube used is 200.
Assuming that the number of half-sections we use is 4, then we can divide the equal points on the pitch diameter, the number of equal points N=(the number of elbow sections A-1)*2, where A=4, so N =6, draw 6 equal points on the map.
Then make a semicircle with a radius of 54.5, and divide the semicircle into 6 equal points, and make a vertical line through these 6 equal points.
Then expand according to the circumference of the tube and make equal points, because the figure is symmetrical, just draw half of it.
Intercept the corresponding projection line distance on the half-section projection diagram on the bisector in the 5 steps, connect these intercept points to become a smooth curve, and then fold it symmetrically