Technical Drawing: Projections, Cuts, and Views
Overview
Cuts are used in technical drawings to clarify the representation of an object by removing a portion. A section is the intersection of a cutting plane with the part (indicated in red). Sections are preferred over cuts as they offer a clearer and simpler representation.
Cuts are essential when the interior parts of an object need to be shown. The surface revealed by the cutting plane is called the cut surface. To create a cut:
- Determine the cutting plane, which should be parallel to the projection plane.
- Clearly identify the cut surfaces using a hatched pattern. This pattern, known as chitterlings, indicates the cut area and varies depending on the material.
Classification of Cuts
There are three basic types of cuts:
- Cuts along the principal axes:
- Total Cut
- Quarter Cut
- Off-axis Cuts
- Staggered Cuts
- Special Cuts
Projection Methods
Graphic Projection
Graphic projection represents an object on a surface using auxiliary lines that project from a point (focus) and reflect the object onto a plane, similar to a shadow. Projections can be:
- Cone Projection (central or perspective)
- Parallel Projection
Diédrico System
The Diédrico system uses orthogonal (perpendicular) projection lines. It provides elevation, plan, and profile views instead of a perspective view. These views can be used to create a three-dimensional representation in the axonometric system, which utilizes both orthogonal and oblique projection lines. Cavalier perspective is a specific case of the axonometric system.
Dimensioned Drawing
Dimensioned drawing, a variant of the dihedral system, uses orthogonal projection to represent parallel sections of an object. It provides better definition and reproduction of complex surfaces, such as building sections, ship hulls, and terrain profiles. It is widely used in architecture, engineering, and surveying.
Octagonal Projection
Octagonal projection uses auxiliary lines perpendicular to the projection plane, establishing a relationship between all points of the projected element.
Isometric Projection
Isometric projection is an axonometric, cylindrical, and orthogonal graphical method. It represents a three-dimensional object in two dimensions, with the three main orthogonal axes forming 120-degree angles. Dimensions parallel to the axes are measured on the same scale.
Isometry allows for scaled representation but doesn’t reflect the apparent decrease in size with distance, as perceived by the human eye.
Conventional Projections
Conventional projections don’t follow a real projection system. An example is the Polycentric projection used by the National Map of Spain. It divides the peninsula into a grid of meridians and parallels. Each trapezoidal grid cell is projected onto a tangent plane, creating a polyhedral surface. This method minimizes anamorphosis (distortion).
Projection of Points and Lines
Projection of a Point
A point in space is represented by its horizontal and vertical projections on the principal planes.
Projection of a Line
A line is defined by its horizontal and vertical projections. Projecting a line onto a plane creates another line formed by the projections of all its points. Knowing the projections of two points on a line allows for the construction of the line’s projection.
Projection of a Plane
A plane is defined by its vertical and horizontal traces (lines of intersection with the principal planes). A line belongs to a plane if its vertical projection lies on the plane’s vertical trace and its horizontal projection lies on the plane’s horizontal trace.
Isometric View
Isometric view uses a rotated grid, resulting in a diamond shape. It provides a three-dimensional representation without perspective and is suitable for engineering models.