The graphics obtained by flattening the surface of spatial shape on a plane is one of the contents of descriptive geometry research. For the parts made of sheet metal, it is necessary to represent the shape of the part by polyhedral orthographic drawing, and also to represent the shape of the sheet metal by unfolded drawing before the part is made. According to the multi-plane orthogonal projection of the part, the development drawing is essentially to get the real shape of the surface. The surfaces that make up the body can be divided into two categories: plane, cylinder, cone and other planes are developable surfaces; Surfaces that cannot be flattened, such as spheres and torus, are undevelopable surfaces. For developable surfaces, cylinders are drawn by parallel lines and cones by rays. For undevelopable surfaces, such as ruled surfaces, the river line method is often used to expand; If it is not a ruled surface, it will often be divided into several parts, and each part is regarded as a cylinder or a conical surface to approximate expansion. Figure 1 is an expanded drawing of oblique truncated cylinder by parallel line method. Firstly, the cylindrical surface is divided into several equal parts, and the real length of the prime line at each equal part is determined. Then, the circumference of the bottom surface of the cylinder is expanded into a straight line, the real length of each element line is drawn at each corresponding bisector of the straight line, and the upper end point of each element line is connected with a curve to obtain the expanded diagram of the oblique cylinder. The drawing principle of ray method is similar to that of parallel line method. After the cone is unfolded, all prime lines intersect at one point, so it is called radial method. The method of river line is to regard the surface of an object as a triangle with many adjacent sides, find out the true shape of each triangle, and then splice them together. Fig. 2 is the development diagram of the hemisphere, which is divided into several equal parts, and each equal part is approximately regarded as a cylindrical surface development. The development diagram can also be drawn by calculation method: the equation of contour line or the coordinates of a series of points on the development diagram can be obtained by mathematical calculation, and the development diagram can be drawn according to them or the plate can be directly cut by numerical control automatic cutting machine. As shown in figure 1, the equation of the curve on the expansion diagram is z = d/(2tg β cos θ)+h.