Straightness is an index to limit the deviation between actual straight line and ideal straight line. This is a requirement for straightness.
Roundness is an index to limit the deviation between the actual circle and the ideal circle. It is a requirement for the circular outline of parts with cylindrical surfaces (including conical surfaces and spherical surfaces) on the normal section (the surface perpendicular to the axis).
Cylindricity is an index to limit the deviation between the actual cylindrical surface and the ideal cylindrical surface. Control various shape errors of cylinder section and shaft section, such as roundness, element line straightness, axis straightness, etc. Cylindricity is a comprehensive index of various shape errors of cylinders.
Various graphics
The figure expressing the shape of mechanical structure is a figure obtained by orthographic projection (that is, the projection of machine parts to the projection plane). According to the projection direction and the position of the corresponding projection plane, the common views are divided into main view, top view, left view and sectional view (previously called sectional view). (Other views are rear view, bottom view and right view. But not commonly used) view is mainly used to express the external shape of machine parts. The invisible contour lines in the figure are indicated by dotted lines. When a machine part is projected onto the projection plane, there are two relative positions among the observer, the machine part and the projection plane.