Types of Models Used in Technical & Engineering Drawings and Designs

Generally speaking, a model is a representation or hypothetical description of a complex process, entity, system, device, or theory that helps to predict its behavior.

A model can also be defined as a small-scale object that is usually built to scale to represent the details of a much larger object.

Models are used in technical and engineering drawings and designs and also preliminary works or construction, as plans from which final products are created; for example a clay model can be created for a real or an eventual casting process.

Models in technical and engineering drawings and designs and preliminary works can be used in testing, perfecting, or honing a final product after understanding and being satisfied with how it behaves; for example, a test model can be created for a solar-powered vehicle.

This article discusses the following types of models used in technical & engineering drawings and designs:

1. Descriptive models

Descriptive models are used in technical and engineering drawings and designs to represent an object, system, entity, device, or process, in either words or pictures.

A descriptive model is a group of written specifications for a design, an object, a system, an entity, a device, or a process.

The major aim of a descriptive model is to describe and provide enough details that can express the image of the final design, object, system, entity, device, process, or product.

Sometimes, descriptive models use representations that are simplified, similar, or equivalent to something that can be more easily understood.

If all the specifications in a descriptive model are adhered to, the final design, object, system, entity, device, process, or product will perform as correctly as expected.

Sketching is another type of descriptive model for the design ideas that are expressed on paper. Two-dimensional (2D) and 3D computer-aided design (CAD) drawings are also descriptive models.

Although, in certain cases, a physical model or prototype is created to be smaller in scale, they are still regarded as another type of descriptive model.

2. Analytical models

Analytical models in technical and engineering drawings and designs help to mathematically or diagrammatically (schematically) represent and predict the future behavior of an object, system, entity, device, process, or product.

For example, an electrical circuit model or simple circuit design model can help to simulate or reproduce the behavior of an actual electrical circuit, or how it would function; therefore, an electrical circuit model is an example of an analytical model.

An effective analytical model helps to determine the best aspects of a system’s, an object’s, an entity’s, a device’s, a process’, or a product’s behavior that should be modelled.

A finite element analysis (FEA) model—such as that used to calculate important properties (for e.g., stresses, temperature, etc.) during the design of a real object, system, entity, device, process, or product—helps to simplify CAD models in a similar way.

A FEA model breaks a model into smaller elements and reduces a complex or complicated system into a series of smaller systems which helps to solve a problem, or understand and estimate certain properties more easily.

Understanding and applying an analytical model requires a good understanding of the difference between the model and the actual system, entity, device, process, or product, in order to be able to interpret any results correctly.

3. Two-dimensional (2D) models

Traditional paper drawings

Two-dimensional (2D) sketches and multi-view paper drawings represent designs for technical and engineering drawings and designs.

All the information that defines an object can be shown on paper drawing through sketch details, but may require many orthographic views which could take a long or longer time to create than CAD drawings would.

Because paper drawings are difficult to modify, the labor costs involved in producing them usually outbalance or outweigh equipment savings.

Paper drawings are not always highly accurate: their accuracy is approximately plus or minus one fortieth (1/40) of the drawing scale and makes paper drawings not particularly measurable.

2D CAD Models

Although two-dimensional (2D) CAD models share the visual characteristics of paper drawings, they are much more accurate than paper drawings and easier to modify.

CAD models represent the full size of objects, unlike paper drawings which usually don’t. Also, in CAD models you can “snap” to exact locations on objects, so as to be able to determine sizes and distances.

CAD systems have standard symbols which are easy to add and change; in addition, they have many editing tools that enable users to quickly edit and reuse drawing geometry.

Two-dimensional 2D CAD drawings can be easily and quickly printed to any desired scale, and different types of information can be singled onto several layers; this gives the model an advantage and makes it more flexible than paper drawing.

Computer-aided design accurately defines the positions of lines, arcs, and other geometry. If you query the AutoCAD database, it accurately returns information to you in the form it was originally created.

2D constraint-based modelling

Constraint-based modeling was originally started as a method to create 3D models. Constraint-based 2D models provide users with technical aspects that can help them define 2D shapes based on their individual geometry.

Users can add relationships like tangency and concentricity between entities in a drawing, and once a tangential or concentric constraint is added between two shapes or drawings (for e.g., circles), they will be constrained to remain tangential or concentric, and a user will be alerted if they attempt to make a change that will violate a selected geometric constraint.

The dimensions used in drawings constrain the sizes of the features of the drawings, and the relationships defined between components of the 2D model are retained by the software that is used when making changes to any drawing.

Geometric constraints are highly valuable tools, but must be applied with a proper understanding of basic drawing geometry if any benefit is to be derived from them.

Examples of constraints in AutoCAD 2016

The following are the constraints that define 2D objects’ respective geometry in AutoCAD:

  • “Vertical”: This constrains lines or pairs of points on objects to remain parallel to only the y-axis
  • “Horizontal”: This constrains lines or pairs of points on objects to remain parallel to only the x-axis
  • “Parallel”: This constrains two selected lines to remain parallel to each other
  • “Perpendicular”: This constrains two selected lines to be at an angle of 90° to each other
  • “Tangent”: This constrains two curves to be tangent to each other or to their extensions
  • “Smooth”: This constrains a spline to be contiguous and maintain G2 curvature continuity with another entity
  • “Concentric”: This constrains two arcs, circles, or ellipses to retain or maintain the same center point
  • “Coincident”: This constrains two points to stay connected to each other
  • “Collinear”: This constrains two or more line segments to remain along the same line
  • “Symmetric”: This constrains two selected objects to remain symmetrical about a specified line
  • “Equal”: This constrains selected entities to retain or maintain the same size
  • “Fix”: This constrains points, curve points, or line endpoints to stay in fixed position on the coordinate system
  • “Angular”: This constrains the angle between two lines to be retained
  • “Linear”: This constrains the distance between two points along the x- or y-axis to be retained
  • “Aligned”: This constrains a distance between two points to be retained
  • “Diameter”: This constrains the diameter of a circle to be retained
  • “Radius”: This constrains the radius for a curve to be retained

Icons for constraints in AutoCAD 2016

4. Three-dimensional (3D) models

Two dimensional (2D) models must be interpreted in order to correctly visualize 3D objects. Three-dimensional (3D) models are used to convey technical and engineering and designs to people who are unfamiliar with orthographic projection; in addition, they (3D models) are used to evaluate properties of drawings and designs that are undefined in 2D representations.

Physical models

Physical models serve as a source of visual reference and are also called “prototypes” whenever they are created in “full-size” or used to validate a nearly last or final design for production.

Physical models are good visual representations of designs; however, if they are not created from materials that would be used in a design, their weight and other features won’t match the final product.

Physical prototypes help to discover and correct many problems in designs and enable people to interact with physical models and understand how designs would eventually look like, and how they would function.

In certain cases, due to the size of a project, a physical model is created to be smaller in scale than how the final design would be. However, physical prototypes lack flexibility, and once they have been created, it is usually difficult, expensive, and time-consuming to modify or change them.

Therefore, it is advisable to use full-sized physical prototypes late in the design process, when major design changes are less likely to be made.

3D CAD models

Three-dimensional CAD models combine the characteristics of both descriptive models and analytical models and provide the benefits of both a 2D model and a physical (prototype) model.

Three-dimensional CAD models can generate standard 2D multiview drawings for visual representation, as well as rendered and shaded views.

Because 3D CAD models accurately depict the geometry of objects or devices, they can completely describe the shape, size, and appearance of objects or devices in the same way as a physical or scaled model would.

Virtual reality

Virtual reality (also called “VR”) refers to the process of interacting with a 3D CAD model as if it were real. In virtual reality, the model simulates how a user would interact with a real object, device, or system.

The term “virtual prototype” refers to 3D CAD systems that represent objects that are adequately enough to enable people, manufacturers, or designers to acquire the same type of information they would be able to acquire from creating and studying a physical model or prototype.

When a virtual reality display is used, users can be able to immerse themselves in the model and move about or through it and view it from several points of view.

If the conditions of a virtual object are altered, it would react in a certain way and provide feedback, or a sensation of its reaction would be provided to the user or person immersed in the virtual reality they subjected themselves to.

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