11. Sections of Solids

The working parts of a machine are generally made of a number of geometric shapes assembled to produce the desired form. The details of such parts are hidden in an outside view of the machine. The machine is assumed to be cut through by a section plane. Sectional views are obtained to show the true forms of hidden or internal parts. In solid geometry, the solid is cut through by a section plane. The portion of the solid between the section plane and the observer is removed. If the portion of solid between the section plane and the plane of projection is projected, as well as the section, the projection is called a sectional plan or sectional elevation. The true shape of the section is obtained in sectional plan if the section plane is parallel to the horizontal plane. The sectional elevation shows the true shape of the section if the section plane is parallel to the vertical plane. If the section plane is not parallel to any one of the principal planes, an auxiliary projection is necessary on a plane parallel to the section plane to show the true shape of section. The projection of section is distinguished by drawing evenly spaced thin lines, called section lines, drawn at 45° to the horizontal or the reference line xy.