CAD model

A CAD model (CAD stands for computer-aided design) is a digital representation of a physical object created in design software. Unlike a 2D drawing, a CAD model holds the complete geometry in three dimensions, exact dimensions, tolerance relationships and parametric constraints.

In mechanical engineering, the CAD model is the core output of the design process. Manufacturing drawings, bills of materials and production data (CNC programs, files for 3D printing) are all derived from the 3D model. Parametric CAD models are easy to modify: change one dimension and the change propagates automatically to the dependent dimensions and drawings.

The most widely used CAD systems in mechanical engineering are SolidWorks, CATIA, Inventor, NX, Creo and Fusion 360. To exchange data between different systems, engineers use the neutral STEP format.

A good CAD model should be parametric, clearly structured and built with the intended manufacturing process in mind. A model designed without regard to manufacturing can lead to unnecessarily high production costs or to parts that are difficult to make.

You need a CAD model practically any time a part is to be made other than by hand from a sketch. It is the input for CNC machining, 3D printing, mold making and structural simulation (FEA), and for requests for quote, since manufacturers price the part from it. For assemblies, the model lets you check interferences, assembly clearances and installation envelopes before the first part is made.

A CAD model also pays off for existing parts that have no data. If a part will be made repeatedly or modified, converting it into a CAD model (for example through reverse engineering) makes every later change easier, as well as any future switch to a different supplier.

A common mistake is a model that looks right but cannot be edited further: the parametric constraints are missing, the feature tree is a mess, or it is just imported "dumb" geometry. When you take over CAD data, check which format and which software version the model is in and whether it includes the feature history.

A 3D model on its own also does not replace a drawing. Tolerances, surface roughness, material and finishes must be specified either on the drawing or directly in the model as PMI (product manufacturing information). If your supplier builds parts from the model alone without a drawing, agree in advance which general tolerances apply, for example ISO 2768 or the title block tolerances you normally use.