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This notes outlines CAD technologies and techniques which may provide benefit to all who wish to understand what CAD is and enables them to talk on that. Technologies and techniques outlined include:

Computer Aided Design (CAD)

· Two-dimensional (2-D) CAD

· Three-dimensional (3-D) CAD

· Sheetmetal CAD

Two-dimensional CAD systems are best suited to products with simple geometries that can easily be represented in two dimensions without considerable interpretation errors. In principle, sheetmetal constructions are often simple enough to warrant the use of two-dimensional drafting. However, when down stream applications such as automation of layout developments, and generation of NC code are required 2-D CAD systems severely limit what is possible.

Three-dimensional CAD systems represent the design of a product component or assembly in 3-D space, resulting in a virtual prototype. There are three levels of 3-D CAD: wireframe modelling, surface modelling, and solid modelling. Solid modelling offers the most versatility for downstream applications since it is an accurate representation of the design. Material properties can be assigned, assembly fits and clearances can be checked, automatic bill of materials generated, sheetmetal components can be folded and unfolded, NC code can be generated, and if required 2-D detail drawings can be generated. Generally, downstream applications are associative, meaning that changes to the model will result in changes to the NC code, detail drawings etcetera.

In determining the relevance of a CAD system, one must be aware of the various types of CAD and must have some clear objectives as to what benefits they expect to derive from implementing a system. Hence the purpose of this report is to outline the types of systems available as well as the benefits that companies are obtaining from effectively integrating a CAD system into their operations. Some additional information is also provided regarding Computer Aided Manufacturing technology, including a section specifically on sheetmetal applications.

Overview of CAD

CAD is an acronym for Computer Aided Design. Originally CAD systems were no more than electronic drawing boards. They simply automated the drafting stage of a product’s design. These systems were better known as Computer Aided Drafting systems. Today CAD has the capability of automating most of, if not the complete, design process. This can lead to compressed design times, which in turn will lead to reduced costs, greater quality, and improved product performance.

Despite the fact that CAD today has the potential to compress the design time, many users of CAD are still using their systems as electronic drawing boards. This may be because manufacturers are still running the old systems and that is all they are capable of doing; or they have not realised the full potential of their system and have fallen into the trap of using this tool as they had in the past.

CAD systems can be broadly classified into two main categories; these are two-dimensional (2-D) CAD and three-dimensional (3-D) CAD systems.

2-D CAD

Two-dimensional CAD systems are typically no more than electronic drawing boards. Drawing files created in these systems represent different views of the product in different two-dimensional planes. With these systems it is not possible to check that components fit together in three-dimensional space, it is not possible to determine the properties of the product such as volume, surface area, and centre of gravity, nor is it possible to unfold and fold sheetmetal components.

Often 2-D drawings can be ambiguous and are open to interpretation errors downstream. This is especially true for the more complex designs.

Some of the benefits of implementing a 2-D CAD system are that:

electronic sketches allow easier editing and copying of features
it guarantees the quality and reproducibility of drawings
it should be faster than manual drawing with a proficient operator
data can be sent to CAM software to generate 2-D tool paths

3-D CAD

Three-dimensional CAD is sometimes referred to as geometric modelling. There are three methods of modelling in three dimensions. These include wireframe modelling, surface modelling, and solid modelling.

Wireframe modelling represents the part shape with interconnected line elements that provide precise information about edges, corners, and surface discontinuities. Wireframes are the simplest geometric models. This is the original method by which three-dimensional objects were modelled by computer. Wireframes contain no information about the surfaces themselves nor can they differentiate between the inside and outside of objects. As a result, wireframes can be ambiguous in representing complex objects and often leave much individual interpretation to the user. Figure 1 shows an example of a wireframe CAD model and illustrates the difficulty in interpreting such a model.

FIGURE 1. Example of a wireframe CAD model.

Surface modelling precisely defines the outside part geometry. Surface models overcome some of the ambiguities of wireframes by precisely defining the outside part geometry. Surface models represent the next highest level of modelling sophistication and complexity. Surface models are created by connecting various types of surface elements to line segments.

Surface models appear very similar to solid models (described below). However, it is important to remember that a part modelled using a surface modeller only represents the skin of the part. If the CAD model was sectioned it would be hollow with thin surfaces defining the outside shape. Surface models are highly useful for visualising part geometry and for NC programming and other tasks where definition of structure boundaries is important.

Solid modelling utilises topology, that is, the interior volume and mass of an object is defined in the computer in addition to the exterior surface representation. Solid computer models are the most sophisticated forms of three-dimensional modelling.

The wide variety of other functions with which the solid model database can be used makes it a cornerstone of many advanced integrated systems. Cross sections may be cut through models of complex parts to expose internal details. The kinematic action of moving parts can be studied and checked for interference in three dimensions. Mass properties such as weight, moment of inertia, and centre of gravity can be directly computed. Detailed drawings can be produced accurately and quickly. Improvements in drafting productivity alone can justify solid modelling. Assemblies can be modelled which can vastly increase your product quality and reduce engineering changes. In most solid modellers, parametric relationships may be set up between dimensions to facilitate quick and easy editing of parts and assemblies. For example, a mating hole and shaft may be defined as having the same diameter (with clearance) or a sheet part’s width may be defined as a percentage of its height. After the design is refined, NC instructions can be produced from the solid model database for machining the part.

FIGURE 2. An example of a 3-D Solid Model.

With solid modelling and surface modelling it is possible to create realistic renderings to help market products. Figure 2 shows an example of a 3-D solid model. Note that on the outside a surface model would look just the same as this solid model.

Figure 3in below, portrays well, the basic difference between 2-D CAD and solid modelling.

FIGURE 3. Difference between 2-D CAD and Solid Modeling.

Sheetmetal CAD

Sheetmetal CAD packages (including modules in fully integrated packages, and add-ons) typically provide tools for automating aspects of sheetmetal design. These tools include:

Automatic filleting and notching
Unfolding and folding of sheetmetal designs
Standard and customized bending tables for a variety of materials and thicknesses (ie standard or customized bend radii and K-factor)
Other standard CAD features such as detailing.

For the folding and unfolding capability to work with sheetmetal CAD systems the model must either be a surface model or solid model. Two-dimensional drawings and three-dimensional wireframes cannot be unfolded.

Figure 4 shows an example of a 3-D-sheetmetal CAD model. Notice that this example shows both the folded and unfolded models.