INTRODUCTION TO COMPUTER-AIDED DESIGN (CAD)

Audio: Introduction to computer-aided design (CAD)

Introducing computers in design and drafting was probably the biggest breakthrough in recent engineering history. Computer-aided design and Computer-aided drafting (CADD or just CAD) expedited the new product development process and enhanced the collaboration between different departments resulting in shortening the time from idea to market and lowering the cost of NPD.

CAD is a tool that every mechanical design engineer should be proficient with, so in this article series, we are going to talk about CAD in great detail to further increase your knowledge in this area. In this article, we are going to talk about what CAD is, what benefits and applications of CAD are, and look into the CAD modeling types.

Introduction to CAD

We already discussed in the introduction to engineering drawings that engineering drawing is one of the most represented ways of communicating design intent. In the past, engineering drawings were made with hands, using pen, paper, and different tools for a more efficient drawing process. With the development of computers, the design and drawing practices have been revolutionized.

Drawings made on a drawing board were replaced with drawing creation on computers, and CAD was initially the term for Computer Aided Drafting. With further development of computers and software, 3D modeling was introduced to the market. This significantly changed the approach to design, and a new term was born, Computer-Aided Design. With these software capabilities, you can also often hear the term CADD – Computer Aided Design and Drafting. However, the acronym CAD is generally used and accepted in the industry. In a nutshell, Computer-aided design and Computer-aided drafting terms are used to describe the use of Computer with CAD software for design and drafting applications.

During my studies, we had to create drawings with pen on paper as part of the exercise to get familiarized with drawing creation rules. It was not easy, it took a lot of time, and from time to time, it was highly frustrating. However, the exercise really helped with understanding the knowledge required to fully specify drawing specifications. Furthermore, once when I transitioned to CAD, I appreciated all the help and convenience that CAD software provides.

In my professional life, I did not make one engineering drawing with a pen. I made many sketches but never official drawings. Using CAD software, I created thousands and thousands of 3D models and drawings. Based on my experience with hand-made drawings, I am pretty sure that I would not have made one-tenth of the drawings I made if there was no CAD software.

Furthermore, I keep in mind that knowledge of CAD is an industry standard for mechanical design engineers, and this skill will definitely be needed to get a job as a mechanical design engineer. Considering that CAD software enables you to create exact virtual replicas of physical artifacts relatively fast and cost-effective, most of the companies hiring mechanical design engineers require CAD skills in their job vacancies.

I strongly believe that to move on to the more complex issues; the basics must be firmly adopted. That is the reason why in this article and the next few that are coming, we will cover the basics so that we can finally move to the more complex topics that will lay on the foundations of introductory topics.

Benefits of using CAD

There are multiple reasons to use CAD software in the design and drafting process. Some of the benefits are:

Virtual prototyping
Ability to create complex geometry parts
Ability to create complex assemblies
Collision check between different parts
Mass properties analysis
Tolerance analysis
Kinematic analysis
Design evaluation and review
2D and 3D drawings can be produced
Photorealistic rendered product pictures
Ability to create animations
Virtual reality
Accurate drawings
The fast and efficient drawing creation process
Drawings are neat
Parts of drawings can be used on other drawings
Settings inside of the CAD software are easily adjustable to industry standards,
Consistency between different people or companies (usage of predefined templates)
Automated drafting
File revision control
CAD data can be used to extract machine codes (CAM – Computer-aided manufacturing)
Easy to export and share design files and drawings
Predefined libraries of 3D model
Faster time to the market
Easier collaboration between different departments
Cloud collaboration, and many more.

Benefits of using CAD

As a mechanical design engineer, you might use CAD software for:

Different concepts development
Industrial design and aesthetic mockups
System-level design (preliminary component design)
Creating virtual prototypes
Detailed design of different components and assemblies
Optimizing design for functionality, manufacturing, assembly, quality, testing, cost, serviceability, maintenance, usability, and environment.
The motion analysis of the components
Engineering drawing creation, tolerance definition, and tolerance analysis (and reports)
Creating assembly instructions or guidelines
System data creation and maintenance
Product updates (engineering change requests)
Working in a team with other mechanical design, manufacturing, electrical and quality engineers, programmers, scientists, etc.
Photorealistic rendered product pictures.
Animations, etc.

CAD modeling types

As a mechanical design engineer, you will mostly use CAD software for geometric modeling as a starting point for further steps in the design process. Geometric modeling refers to using CAD software to create a mathematical description of the geometry of an object. Once you develop the geometric model, you can proceed with following steps like finite element analysis (FEA), rendering, drafting, etc.

We will focus on three-dimensional modeling (3-D) as a primary modeling approach, and we will use two-dimensional modeling (2-D) in the following step after the 3-D model has been created. While only 2-D modeling is present today, we will not focus on it except when we are talking about drafting from existing 3-D models.

As the name alludes, a 3-D model is described with three dimensions: width, height, and depth dimensions. However, in a 2-D model, we are missing the depth dimension. I prefer to work with 3-D models and then use them for drawing creation, and I avoid only 2-D as plague. I find the 3-D modeling and drafting much more efficient, it is easier to check if the design is good, and any change made to a 3-D model is automatically visible on the drawing.

Let us now look into the most common types of CAD modeling approaches.

Wireframe CAD modeling

A wireframe CAD model is the simplest form of CAD model. It is a 3-D line drawing of an object showing only the edges and vertices without any side surface in between.

Wireframe model does not provide the volume information of the various regions of the part. They have been replaced with surface and solid modeling because they are hard to visualize, create uncertainty about design intent, and do not provide a true representation of a product. However, they are still used as a way to create a pictorial view for a 2-D drawing.

Surface CAD modeling

In addition to defining edges and vertices, surface models define surface features as well. The surface is an outer boundary of an object connected to edges and vertices. With surface modeling, it is possible to create complex curves and forms that are normal in the industry. Using only surface modeling is interesting to industrial designers and other product designers that are interested in the product’s external shape and appearance.

However, for mechanical design engineers, surface models without the possibility of transforming into solid models are almost useless. In the real world, mechanical design engineers would receive external shapes from industrial designers. The mechanical design engineer’s responsible for ensuring that the product is manufacturable with the minimum intrusion on industrial design. In some cases, a mechanical design engineer is responsible for both, but that depends on the company and the engineer’s skills.

Surfaces can be created using different techniques, and each CAD software has unique methods for creating and working with surfaces. One of the most important concepts for modeling surfaces is polygonal modeling. Polygonal modeling is a basic form of surface modeling that produces lower-quality surfaces without precise curvature control. Polygonal modeling creates quick and easy-to-modify surfaces, and most CAD systems use non-uniform rational B-spline (NURBS) mathematics to produce accurate curves. A spline is a curve that uses a series of control points and other mathematical principles to define the location and form of the curve.

I wanted to explain different splines in detail, but I found this video, and it is explained extremely well. If you want to learn more about splines, I strongly suggest checking the following video:

Surface creation

There is a number of different techniques how to create a surface model. The techniques mostly depend on the shape to be created and the available tools in CAD software. You need to have detailed knowledge of your CAD software tools and processes to know when to use which tool in order to get the desired result.

Direct surface modeling

Direct surface modeling is when the desired shape is created from primitive shapes by manipulating surface position and/or adjusting control points of different curves. Depending on the CAD software, we can start our design process with primitives like box, cylinder, sphere, torus, pipe, plane, etc.

Procedural surface modeling

Procedural surface modeling is when the desired shape is created from curves using different tools available in CAD software. Basically, we are creating surfaces from sketches. Depending on the CAD software, we can start our design process with different tools like extrude, revolve, sweep, loft, etc.

Furthermore, most CAD software provides additional surface construction tools from existing surface models. Depending on the CAD software, we can edit our surface with different tools like patch, ruled surface, offset, trim, split face, merge, stitch, extend, etc.

Solid CAD modeling

In addition to defining edges, vertices, and surfaces, solid models also define the model’s mass. It is an exact digital replica of a physical, real-world model. The solid model can be assigned a material that closely replicates the material used to manufacture the product. Assigning a material to a solid model allows for analyzing and testing physical and inertial properties. The result is a solid model that acts as a digital prototype of a product.

Once when a solid model is finished, the process of creating an engineering drawing is not hard. Creating new views, sections, and detailed views is a straightforward process. Furthermore, dimensioning and tolerancing are semy automatic, basically at a click of a button. However, rules for creating engineering drawings are still something that CAD software cannot do for you.

Let us now look into the two main solid modeling techniques:

Constructive Solid Geometry (CSG)

Constructive solid geometry modeling basically uses solid primitives (like, boxes, cones, spheres, etc.) with Boolean operators to form the desired shape. Boolean operators are union, difference, and intersection.

Feature-based modeling

Feature-based modeling, in contrast to CSG, allows the construction of models from parametric shapes. In this case, the process of creating 3-D models starts with a 2-D sketch that defines two dimensions and uses features to add the third dimension in space.

Features that can be used to create a solid model from a 2-D sketch are extrude, revolve, sweep, loft, etc. Using feature-based modelers allows different features to be associated with specific edges and faces. These features are extrude-cut, holes, fillets, chamfers, pockets, etc.

In addition, different software can have different modules for modeling solid models with a predefined set of features. For example, there can be a solid module, sheet metal module, welding profiles module, plastic parts module, etc.

Characteristics of good feature-based solid modeling are:

Constraint-based modeling

Geometry can be constrained using geometric or dimensional constraints. Geometric constraints are characteristics applied to restrict the size or location of geometry. Examples are vertical/horizontal, coincident, tangent, equal, parallel, perpendicular, midpoint, concentric, collinear, etc. Dimensional constraints are used to numerically control the size or location of geometry. Constrains-based modeling enables engineers to introduce intelligence into the design. When an object has no freedom of movement, a design is considered fully constrained.

Parametric modeling

Parametric modeling means that parameters of the model, like length, width, depth, position, orientation, tolerances, material properties, reference to other features, etc., may be modified to change the geometry and properties of the model. For example, geometry is updated when the numerical value of the dimension is changed in CAD software.

Parametric modeling tools allow parameters to reference other parameters through relations or equations. The power of this approach is that when one dimension is modified, all linked dimensions are updated according to specified mathematical relations, instead of having to update all related dimensions individually. Furthermore, parameter values are stored in a database (for example, an Excel sheet), and changes made in the database will automatically update the geometry of the object.

History-based modeling

History-based modeling enables designers to track and change the features used to create an object. That means the order in which part is built is critical and upfront planning is required. When modeling, we should plan ahead where and how our model could be used.

If we are making a model for exploratory purposes, the order in which features are used is not critical. If we are making a model that may be used as a representative design, then we have to construct our model in a way that it behaves as expected when features are modified. The best way is to use sketches to visualize the desired model and the relationship between features and possibly other models.

Associative modeling

Associative modeling means that modification of one geometry feature applies to every associated object. For example, if we change the diameter of the hole on the solid model, it will automatically change that diameter in the associated assembly and drawing.

In the picture below, you can see how we changed the hook’s length, and it automatically was upgraded in the beam trolley assembly

Hybrid modeling

Hybrid modeling is a combination of two or more modeling techniques for creating desired geometry. For example, in the picture below, the surface modeling techniques were used to create utility knife housing, and it was transformed into a solid object.

Let us assume that AutoDesk wants to put its logo on the knife handle with a pad printing technique. We created a simple block created using feature-based modeling. We subtracted the utility knife housing from the block with the use of the Boolean operator (Constructive Solid Geometry modeling technique).

The result is a negative shape of the complex geometry of the utility knife housing in the block, which with some additional steps, we can use for nesting the housing during the pad printing.

Commercially available CAD software

Now you know what the benefits of CAD are, where you could use it, and how models can be made, let us look at some of the commercially available CAD software interesting to mechanical design engineers:

AutoDesk AutoCAD
Autodesk Fusion 360
Autodesk Inventor
Dassault Systemes CATIA
Dassault Systemes SOLIDWORKS
CAD NX
CAD Solid Edge
PTC Creo
Alibre design
IronCAD, and many others.

All these listed software has the same purpose and similar characteristic. However, they all differ in price, complexity, and available features. It is important to choose the proper CAD software that will fit your company’s product complexity and internal processes.

Closing words

Computer-aided design and drafting completely changed the world of engineering. CAD enabled a faster new product development process and enhanced the collaboration between different departments, shortening the time from idea to market and lowering the cost of NPD.

As a mechanical design engineer, the chances of not using CAD software in your professional life are slim to zero. You will use modules like part, assembly, and drawing daily. CAD software is used as a tool, and as such, the better your proficiency with it is, the better and faster you will be able to do your job. Your knowledge of mechanical engineering principles, combined with excellent handling of CAD software, will give you almost infinite possibilities for finding good and stable employment or maybe starting your own company.

Now you have an excellent overview of what computer-aided design is all about. However, I suggest you go through the text once more and identify areas you think need more understanding and clarity. Then, once you have identified those areas, start building up your knowledge in those areas.

To make it easier for you to find related posts, check the “Further reading” chapter below. Do you have any questions or need something to be clarified better? Leave a comment below, and I will give my best to adjust the post accordingly.

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