Mechanical drawing is a type of technical drawing which shows information about mechanical systems like heating, ventilation and air conditioning. Technical drawing, also called "drafting," is a particular form of drawing used by designers, architects and engineers. There are a number of standards and conventions for technical drawing, as technical drawings are meant to show information and are interpreted as such. Elements like layout, text, symbols, types of view projections, dimensioning, descriptive geometry and line thickness are all standardized in technical drawing.
Manual Drawing and Instruments
For years, architects and engineers made technical drawings by hand, with the aid of a few drafting instruments, like drafting compasses.
To draft a mechanical system, an architect or engineer will place a piece of paper (of a set size and with straight sides and 90-degree angles) on a drafting table, which is typically slanted up toward the draftsman. Drafting tables include a sliding straight-edge called a T-square, which can slide across the table to form vertical parallel lines. Draftsmen can also use the T-square as a tool to hold instruments of set squares or triangles with known angles, to create even and straight-edged shapes with proper angles.
To draw curves and circles, the drafter can use compasses for simple arcs and circles or a "French curve," a piece of plastic with a series of complex curves.
CAD (Computer Aided Design)
The first Computer Aided Design (CAD) program, Sketchpad, was invented in the early 1960s, and by the late 60s and early 70s computers became an important tool for mechanical drawing and technical drawing in general--for those with access to large computer spaces. CAD use became even more prevalent with the invention of the personal computer in the 1980s.
CAD software began as a tool for mechanical drawings--it was developed in the 1960s and 70s for aerospace engineers and airplane designers and engineers. There are two types of CAD program used for technical drawings: "two dimensions" and "three dimensions." 2D CAD systems like AutoCAD (which started in 1982, one year after the first IBM PC) largely replaced the discipline of drawing on paper. AutoCAD creates circles, arcs, straight lines and curves--and makes it much easier to revise mistakes.
3D CAD for Mechanical Drawing
3D CAD systems like Pro/ENGINEER effectively render mechanical parts and larger mechanical systems; the end image is always in two dimensions, but can be moved around on the computer screen to show multiple perspectives. 3D CAD systems begin by defining the geometry of a part. Then the technical drawing component comes from the user defining the various views that the computer creates. The software creates projected views, section views, and orthographic projections (all standardized formats in the technical drawing tradition).
Many use the terms "engineering drawing" and "mechanical drawing" interchangeably. Often engineers refer to mechanical drawings as "blueprints," though the the term is not necessarily accurate any more--the name refers to the blue carbon papers that engineers used for drawings before such materials were taken over by more modern reproduction technologies like CAD.
Engineering drawings must convey geometry; that is the shape of the object as seen from various angles. Drawings must also represent the dimensions of the object, the material the object is made of and the finish (the surface quality of the object: shiny, matte, textured and so forth).