CAD PART DESIGN MODULE OVERVIEW

Audio: CAD part design module overview

Part or component CAD modeling is a module that you will use daily as a mechanical design engineer. In the introductory article, we covered the theory behind the different CAD modeling types, and now we are going to look into different possibilities and modules that we can use to create part CAD models.

For this article, I am using Autodesk Fusion 360 to show you general concepts of CAD software. I will keep it as generic as possible so that you learn the general idea behind CAD modeling and can use this knowledge on any other CAD software.

Introduction to CAD part design

As a mechanical design engineer, you will create different mechanical solutions for different purposes. From product design to assembly fixtures, you will usually use CAD software to achieve the desired outcome. The main building block of any assembly is the part or component. According to ISO 29845:2011, part model is a model in which the product described is one single item. In this article, we will focus on modeling one single item as standalone creation. Once we learn about part, in the next article, we will move to assemblies built from two or more parts.

For this purpose, I am going to use Autodesk Fusion 360. There are many other CAD software on the market, but the logic behind using it is the same. Once when you internalize the logic behind CAD software, you understand how to create objects in a 3D environment, and you learn to work in one CAD software, it is easy to learn how to work in any other CAD software.

Keep in mind that for mechanical design engineers, CAD software is a tool that we use to give our ideas shape and then to communicate our design intent through engineering drawing creation. CAD is the extension arm of our knowledge of mechanical engineering principles, not the other way around. Keep in mind that software is only smart as the person using it.

You can be the best CAD modeler in the world, but if you cannot design things that can actually be manufactured, assembled, verified, validated, and in the end, sold, you are not of any use to the companies that are planning to make profit out of the product you designed. However, if you are working as a CAD modeler under engineering supervision, then being the best is the way to go.

Basic terminology

Let us know to look into some of the basic terms that you will encounter when creating a part of CAD software.

Model tree (feature tree or timeline)

One of the most powerful options available in CAD software is that you can track the history of used features for creating a model. Not only that you can track it, but you can go back in history and change the parameters of your design, and it will automatically update the whole model in the current status. In Fusion 360, this is called “Timeline. “

Sketches

We can divide sketches into 2D and 3D sketches. 2D sketches are used for creating a profile that drives the shape of the model. With 2D sketches, we are creating two dimensions of our model and adding the third one with the use of the features.

When we want to create a complex shape, we can use 3D sketches. Usage depends from software to software and what features you can use with 3D sketches.

It is important to note that your sketches should always be “fully defined. ” What is meant with the fully defined is that you define enough constraints to your sketch that there are no degrees of freedom or ambiguity in the sketch, and its shape and size are precisely determined.

Parameter tables

Usually, you are defining the component’s size with the use of dimensions. However, another way to use this is to define the parameters in parameter tables instead of the numerical values. Instead of having a fixed value of dimensions, we can change the parameter value as we want in the parameter table, and the part’s size will automatically update. Furthermore, we can connect different parameters with equations. This possibility is useful when creating components with the same geometrical attributes but varying sizes (the parameters table is usually visible on tabular drawings).

Part file formats

Usually, each CAD software has its own file formats to store relevant design and manufacturing data. Let us now look into some of the most known CAD software on the market and their file extensions used to store part data:

CAD software	Part file extension
NX CAD	.prt
Catia V5	.catpart
SOLIDWORKS	.sldprt
Autodesk Inventor	.ipt
Autodesk Fusion 360	.f3d
AutoCAD	.dxf
PTC Creo	.prt

Part created with surface modeling

Usually, parts created with surface modeling are created to explore different design options. With surfaces, we can create complex organic shapes that are ergonomic and aesthetically pleasing. Usually, industrial designers are focused on the aesthetics, ergonomics, and in general user-centered aspects of a product.

But suppose you are working in a small company. In that case, there is a possibility that as a mechanical design engineer, you will have to wear multiple hats, including the industrial designer hat.

Surfaces are usually used for designing consumer products, automotive parts, home appliances, industrial machinery, airplanes, ships, helmets, medical devices, or anything else with a complex surface that is hard to create with standard solid modeling features.

In my experience, surface models that I would create are in the final step transformed into solid models so that I could keep working on preparing the model for the choosen manufacturing technique.

Direct surface module

The direct surface module enables the use of primitive shapes to create the final part. Usually, predefined commands enable one to immediately create a box, cylinder, sphere, torus, etc. The final shape is created by manipulating surface position and/or adjusting control points of different curves. In addition, some software can add and manipulate surfaces directly without creating primitive shapes.

In Autodesk Fusion 360, this is called the “Form” environment. Depending on software and context, this is also known as freeform modeling, digital sculpting, explicit modeling, etc.

I use this type of modeling for quick concept creation when working on complex shape parts where complete control over dimension is not crucial. Also, this modeling approach is really useful when you need “artistic “freedom, for example, creating Star Wars characters.

Procedural surface module

The procedural surface module enables the creation of complex surfaces from sketches. It allows us to use different sketches as boundaries and create a surface shape with predefined commands. These commands can be extrude, revolve, sweep, loft, patch, etc. In Autodesk Fusion 360, this environment is called “surface. “

Personally, I use this type of modeling once when the concept of the product is clear, and my next step afterward is preparation for manufacturing. Using sketches to create a complex shape gives me control over the dimensions, and in case changes are needed, I can easily control those changes. As I mentioned before, once when I am done with the surface modeling, I would transform the surface model into a solid and finish the model.

Generally, the final shape is an iterative process. When I would finish version one, I would create a prototype and see how it feels in hand. I would add weights to simulate the device weight, etc. Finally, I would update my design, create a prototype again, test it, etc., until I am satisfied with the final shape.

Zebra stripes

Zebra stripes are used to analyze and visually asses the continuity and smoothness of the surfaces. Using zebra stripes, we can identify potential irregularities or defects.

Part created with the solid module

Solid modeling is the most represented modeling type among mechanical design engineers. The reason for this is that solid models represent a digital replica of the real-world model. We can assign material properties to it, we can analyze its mass, and it can be used for analyzing the model’s response to the real-life effect. The ability to create solid models is crucial for every mechanical design engineer.

I would say that as a mechanical design engineer, you will mostly use solid models.

As discussed in the introductory article, there are two types of solid modeling techniques we can use:

Constructive Solid Geometry, where we use the solid primitives with Boolean operators to create the final shape. We will mention this only in the context of hybrid modeling.
Feature-based modeling is based on creating 2D sketches and using different features to add the third dimension. We can further divide feature-based modeling in:
- Solid or feature module
- Sheet metal module
- Plastic module,
- Structural members, welding, or profile module (the name depends on the software).

Keep in mind that all feature-based modules are based on 2D sketches, but they differ in predefined features specific to certain tasks.

Solid or feature modeling

When using a Solid or feature module, we are usually referring to the standard set of solid modeling features as a solid module (we are talking about simple geometries created with features like extrude, extrude-cut, revolve, fillet, chamfer, etc.). We do have other modules that are feature-based solid modeling modules, so do not get confused with the naming. We will look into these later in the article.

These commands can be extrude, extrude-cut, revolve, sweep, loft, hole, fillet, chamfer, etc. In Autodesk Fusion 360, this environment is called “solid. “

As you can see, these are basic features that are usually the same or use the same logic in most CAD software. Learning how to utilize these features will already make you a powerful CAD user.

Sheet metal module

Sheet metal is a module for creating a part from flat metal pieces with a constant thickness. This is also feature-based solid modeling, but the features are specially created for sheet metal.

An important feature that is available within the sheet metal module is called “flat pattern.” The sheet metal part is formed from a flat piece of metal using different manufacturing operations. A flat pattern is useful for creating a final part geometry and then “flattening” the sheet to the cutting shape. This shape can be exported in, for example, a .dxf file used for generating G-code for laser cutting machines.

Furthermore, a flat pattern is used for dimensioning the flattened sheet and specifying the bending and other information required to manufacture a sheet metal part. When you finish your design, you will usually send to your supplier .dxf file, .step file, and an engineering drawing.

Plastic module

The plastic module is another feature-based solid modeling module, but with features that are specific for designing with plastic.

Structural members, welding, or profiles module

Feature-based solid modeling module where you can use a predefined set of sketches (profiles) to create different constructions, depending on the software, is called structural members, welding, or profiles module. The basic idea behind this module is to create line sketches of your construction, then use the feature with predefined sketches to extrude the final structural member shape.

Furthermore, there are a set of predefined features for cutting, trimming, connecting different profiles, etc. There is also the powerful possibility of creating “cutting lists, “where you can generate a list of the different profile types and their cutting lengths with a few clicks.

Autodesk Fusion 360 does not have a standard option for this, but I did find a plugin that works quite well called “Structural Members. ” It is a paid plugin that you pay once a year, and it is not expensive.

Hybrid modeling

Sometimes, the best way to quickly and efficiently create a new part is to use a combination of the few modules mentioned above. It is common practice to use one part created as a surface model, transform it into a solid model and finalize the part using feature-based solid modeling. Furthermore, it is also common practice to use an existing part created with whatever module and use boolean operators (constructive solid geometry) to create a new part that is similar to the existing part.

In the introductory article, we showed you an example of creating a nest for pad printing.

Closing words

As a mechanical design engineer, you will use CAD software for your design daily. Moreover, to land a job, you will need CAD skills. For the tools you will use every day, I advise you to spend additional time getting proficient as possible with them. In this case, CAD software is a tool that will help you use your mechanical engineering knowledge and turn it into working products.

Part is a fundamental model that you will often create. Every CAD software has its own set of features and rules on how to use them, but the overall logic stays the same. I believe that once you understand these three things, the theory on how to create three-dimensional objects, the logic behind CAD modeling, and once you learn to work with one CAD software, every other software will not be a problem for you to learn.

Now you have an excellent overview of the CAD part design module. 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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