QA Procedures For The Design Of CAD Data Models

Background

The creation of CAD (Computer Aided Design) models is an often complex and confusing procedure. To reduce long-term manageability and interoperability problems, the designer should establish procedures that will monitor and guide system checks.

Establish CAD Layout Standards

Interoperability problems are often caused by poorly understood or non-existent operating procedures for CAD. It is wise to establish and document your own CAD procedures, or adopt one of the national standards developed by the BSI (British Standards Institution) or NIBS (National Institute of Building Sciences). These may be used to train new members in the house-style of a project, provide essential information when sharing CAD data among different users, or provide background material when depositing the designs with a preservation repository. Particular areas to standardize include:

• Drawing sheet templates
• Paper layouts
• Text fonts, dimensions, line types and line weights
• Layer naming conventions
• File naming conventions

Procedures on constructing your own CAD standard can be found in the Construct IT guidelines (see references).

Be Consistent With Layers And Naming Conventions

When creating CAD data models, a consistent approach to layer creation and naming conventions is useful. This will avoid confusion and increases the likelihood that the designer will be able to manipulate and search the data model at a later date.

The designer has two options to ensure interoperability:

• Create layers that divide the object according to pre-defined criteria. E.g. a model building may be divided into building part, building phase, site stratum, material, chronological standing, etc. The placement of too many objects on a single layer will increase the computational requirements to process the model and cause unexpected problems when moving objects between layers.
• Establish a layer name convention that is consistent and appropriate to avoid confusion in complex CAD model. Many users use 'wall', 'wall1', 'door', etc. to describe specific objects. This is likely to become confusing and difficult to identify when the design becomes complex. Layer names should be short and descriptive. A possible solution is the CSA layer-naming convention that uses each character in the layer name to describe its position within the model.

Ensure Tolerances Are Consistent

When exporting designs between different CAD applications it is common for model relationships to disintegrate, causing entities to appear disconnected or disappear from the design altogether. A common cause is the use of different tolerance levels - a method of placing limits on gaps between geometric entities. The method of calculating tolerance often varies in different applications: some use absolute tolerance levels (e.g. 0.005mm), others work to a tolerance level relative to the model size (e.g. 10-4 the size), while others have different tolerances according to the units used. When considering moving a design between different applications it is useful to ensure the tolerance level can be set to the same value and identify potential problem areas that may be corrupted when the data model is reopened.

Check For Illegal Geometry Definitions

Interoperability problems are also caused by differences in how the system identifies invalid geometry definitions, such as the three-sided degenerate NURBS surfaces. Some systems allow the creation of such entities, others will reject them, whereas some systems know that they are not permissible and in an effort to prevent them from being created, generate twisted four sided surfaces.

Further Information

• AHDS Guides to Good Practice: CAD, AHDS,
  <http://ads.ahds.ac.uk/project/goodguides/cad/>
• National CAD Standard,
  <http://www.nationalcadstandard.org/>
• CAD Standards: Develop and Document,
  <http://www.arch.vuw.ac.nz/papers/bbsc303/assign2/198mc.htm>
• Construct I.T: Construct Your Own CAD Standard, (Note direct URL to document is not available)
  <http://www.construct-it.org.uk/>
• Common Interoperability Problems in CAD,
  <http://www.caduser.com/reviews/reviews.asp?a_id=148>