Solid modelling

Solid modelling (or modeling) studies unambiguous representations of the solid parts of an object, that is, models of solid objects suitable for computer processing. It is also known as volume modelling. Other modelling methods include surface models (used extensively in animation) and wire frame models (which can be ambiguous about solid volume).

Primary uses of solid modelling are for CAD, engineering analysis, computer graphics and animation, rapid prototyping, medical testing, and visualization of scientific research.


Basic concepts

  • Sweeping
    • An area feature is "swept out" by moving a primitive along a path to form a solid feature. These volumes either add to the object ("extrusion") or remove material ("cutter path").
    • Also known as 'sketcher based modelling'.
    • Analogous to various manufacturing techniques such as extrusion, milling, lathe and others.
  • Boundary representation
    • A solid object is represented by boundary surfaces and then filled to make solid.
    • Also knowing as 'surfacing'.
    • Analogous to various manufacturing techniques; Injection moulding, casting, forging, thermoforming, etc.
  • Parameterized primitive instancing.
    • An object is specified by reference to a library of parameterized primitives.
    • For example, a bolt is modelled for a library, this model is used for all bolt sizes by modifying a set of its parameters.
  • Spatial occupancy
    • The whole space is subdivided into regular cells, and the object is specified by the set of cells it occupies.
    • Models described this way lend themselves to Finite difference analysis.
    • This is usually done after a model is made, as part of automated pre-processing for analysis software.
  • Decomposition
    • Similar to "spatial occupancy", but the cells are neither regular, nor "prefabricated".
    • Models described this way lend themselves to FEA.
    • This is usually done after a model is made, as part of automated pre-processing for analysis software.
  • Feature based modelling
    • Complex combinations of objects and operators are considered together as a unit which can be modified or duplicated.
    • Order of operations is kept in a tree, and parametric changes can propagate through the tree.
  • Parameteric modelling
    • Attributes of features are parameterized, giving them labels rather than only giving them fixed numeric dimensions, and relationships between parameters in the entire model are tracked, to make changing numeric values of parameters easier.
    • Almost always combined with features, giving parametric feature based modelling.

Solid modelling CAD

Solid modellers have become commonplace in engineering departments in the last ten years due to faster PCs and competitive software pricing. They are the work horse of machine designers.

Solid modelling software creates a Virtual Reality for machine design and analysis. Interface with the human operator is highly optimized and includes programmable macros, keyboard shortcuts and dynamic model manipulation. The ability to dynamically re-orient the model, in Real-time shaded 3-D, is emphasized and helps the designer maintain a mental 3-D image.

The designer generally has access to models that others are working on concurrently. For example, several people may be designing one machine that has many parts. New parts are added to an assembly model as they are created. Each designer has access to the assembly model, while a work in progress, and while working in their own parts. The design evolution is visible to everyone involved.

A solid model generally consists of a group of features, added one at a time, until the model is complete. Engineering solid models are built mostly with sketcher-based features; 2-D sketches that are swept along a path to become 3-D. These may be cuts, or extrusions for example.

Another type of modelling technique is 'surfacing'. Here, surfaces are defined, trimmed and merged, and filled to make solid. The surfaces are usually defined with datum curves in space and a variety of complex commands. Surfacing is more difficult, but better applicable to some manufacturing techniques, like injection molding. Solid models for injection molded parts usually have both surfacing and sketcher based features.

Another example of where surfacing excels is automotive body panels. If two curved areas of the panel have different radii of curvature and are blended together, maintaining tangential continuity (meaning that the blended surface doesn't change direction suddenly, but smoothly) won't be enough. They need to have a continuous rate of curvature change between the two sections, or else their reflections will appear disconnected.

Engineering drawings are created semi-automatically and reference the solid models.

The learning curve for these software packages is steep, but a fluent machine designer who can master these software packages is highly productive.

Solid Modelling software packages do the following (at a minimum);

  • model solid parts
  • model assemblies of parts
  • maintain libraries of parts and assemblies
  • calculate mass properties of parts and assemblies
  • reflect the 'bill of material' required to build the product
  • create Engineering drawings from the solid models
  • aid visualization with shading, rotating, hidden line removal, etc...

Most modern software packages also have additional capabilities;

  • Bi-directional parametric associatively (modification of any feature is reflected in all information relying on that feature; drawings, mass properties, assemblies, etc... and counter wise)
  • kinematic, interference and clearance checking of assemblies
  • specialized add-in modules (FEA, sheet metal, surfacing, hose/cable routing, electrical component packaging...)
  • management of controlled documents and revision levels
  • inclusion of programming code in a model to control and relate desired attributes of the model
  • automated programming of CNC machines
  • Photo realistic rendering
  • Import/Export routines to exchange data with other software packages
  • Programmable design studies and optimization
  • Automatic shell feature creation
  • Automatic draft feature creation
  • Sophisticated visual analysis routines, for draft, curvature, curvature continuity...

Parametric solid modelling

A revolution in 3-D cad began in 1989 when the first parametric modeller, T-FLEX was released for the pc. Pro/ENGINEER was released the same year. Most solid modellers are now parametric.

Parametric modelling uses parameters to define a model (dimensions, for example). The parameter may be modified later, and the model will update to reflect the modification.

Example: A shaft is created by extruding a circle 100mm. A hub is assembled to the end of the shaft. Later, the shaft is modified to be 200mm long (click on the shaft, select the length dimension, modify to 200). When the model is updated the shaft will be 200mm long, the hub will relocate to the end of the shaft to which it was assembled, the engineering drawings and mass properties will reflect all changes automatically.

Examples of parameters are; dimensions used to create model features, material density, formulas to describe swept features, imported data (that describe a reference surface, for example).

Parametric modelling is obvious and intuitive. But for the first three decades of cad this was not the case. Modification meant re-draw, or add a new cut or protrusion on top of old ones. Dimensions on engineering drawings were created, instead of shown.

Parametric modelling is very powerful, but requires more skill in model creation. A complicated model for an injection molded part may have a thousand features, and modifying an early feature may cause later features to fail. Skillfully created parametric models are easier to maintain and modify.

Parametric modelling also lends itself to data re-use. A whole family of capscrews can be contained in one model, for example.

Animation of a computer generated character is an example of parametric modelling. Jar Jar Binks is described by parameters which locate key body positions. The model is then built off these locations. The parameters are modified, and the model rebuilt, for each frame to create animation.

Medical solid modelling

Modern computed axial tomography and magnetic resonance imaging scanners can construct solid models of interior body features.

Uses of medical solid modelling;

  • Visualization
  • Visualization of specific body tissues (just blood vessels and tumor, for example)
  • Creating solid model data for rapid prototyping (to aid surgeons preparing for difficult surgeries, for example)
  • Combining medical solid models with CAD solid modelling (design of hip replacement parts, for example)

See also

External links

Commercial solid modellers

Free solid modellers


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