CAD ASSEMBLY DESIGN MODULE OVERVIEW

Audio: CAD assembly design module overview

It is highly unlikely that in your career you will only work with products that are single part. Normally poducts are consist anywhere from few parts to thousand (or hunder thousand) parts. In this article we are going to learn about CAD assembly design basic terminology and modeling techniques that we can utilize.

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 general idea behind CAD modeling and that you could use this knowledge on any other CAD software out there.

Introduction to CAD part design

Part or component is building block of any assembly. Assembly consist of two or more parts, and that is exactly how ISO is defining it. According to ISO 29845:2011 assembly model is model in which the product described is an assembly of two or more items. As a mechanical design engineer it is highly unlikely that you will not deal with assemblies.

In my career I created numerous assemblies and created drawings from them. I used assemblies not only for creating drawings. I used assemblies for FEA, assembly exploded views for presentations, reports, price calculations, etc. and being articulate in the assembly terminology was really helpful.

There are many availble CAD software on the market, but for purpose of this article I will use Autodesk Fusion 360. However, I will try to keep the article as generic as possible so that you could apply the knowledge to any other CAD software. In my oppinion and experience, once you learn how objects are made in 3D environment, you understand the general logic of CAD software, and you learn to work in one CAD software, you can learn with ease any other CAD software on the market.

I will repeat myself again and emphasize that CAD software is tool that mechanical design engineer use to based on mechanical engineering fundamentals build real life solutions. CAD is extension arm of our knowledge of mechanical engineering principals, not the other way around. Keep in mind that software is only smart as the person using it.

Basic terminology

Let us know look into some of the basic terms that you will encounter when creating assembly in CAD software

Assembly – subassembly

As previously mentioned assembly is model that is build from two or more parts. Subassembly is a smaller assembly that is part of larger assembly.

Part

According to ISO 29845:2011 part model is model in which the product described is one single item. We already covered this topic in depth.

Bill-of-Materials (BOM) and parts list

The bill of materials lists the subassemblies, parts, and materials required for building the assembly. The parts list or BOM can be added directly to the drawing or provided with the assembly drawing as a separate list. The difference between the parts list and BOM is that the parts list defines only one structural level.

Assembly constraints (Mates or Joints)

Assembly constrain is a type of constraint that defines the relationship between two components, specifying how they are connected or positioned relative to each other. Depending on the software they can also be called „Mates“, „Joints“, „Constrains“, etc.

When we are defining the assembly constrains basically we are manupalating the degrees of the freedom of the parts. In the 3D space we have six degrees of freedom, three linear and three rotational. When the part has all six degrees of freedom constrined, part is considered „fixed“.

Usually, in CAD software the first component that is placed in the assembly is fixed, and then we are adjusting the degrees of freedom of other components in relation to the fixed one. Of course, there is a posiblity to define any component as fixed one.

In Autodesk Fusion 360 assembly constrains are refered as joints.

Component instance

When we have a component that is used multiple times in the assembly, these multiplied component is called instance. The instance shows the same form on a new location in assembly as the orignial component, but it is not duplicated in the database.

Exploded view

The exploded view illustrates the relationships between different components that form an assembly. In other words, the exploded view shows how various components are put together to form the assembly. If you are interested about learning more about exploded views, check Projection methods on engineering drawing.

Interference and clearence

Interference occurs when two or more components within an assembly occupy the same space or overlap with each other. Clearence on the other hand is opposit.

It is important to identify and resolve interferences to ensure proper fit and functionality of the assembly. Usually, CAD software have a command that can check automatically if there is a interference between the assembly parts.

Part libraries

Part libraries are collection of predesigned and standardized components within CAD software. We also call them part catalogs. When we are working with assemblies, a lots of times we will use standardized or vendor parts like for example screws. Instead we create the screw part each time we need them, we will create them one, and then save them into our part library. Whenever we will need this part, we can just reach into our library and use it directly in our assembly. Same things goes for often used supplier parts.

Ideally, we will not create standardized part each time we need, instead we will import the part library that was previously created by some other company or individual.

There is a lots of websites on internet, but the most popular are GrabCAD, TraceParts, 3DcontentCentral, McMaster-Carr, and CADEANS PARTsolutions. Furthermore, depending on the supplier, sometimes you can find directly on their website 3D data for part you need (or you just ask them).

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 assembly data:

CAD software	Assembly file extension
NX CAD	.prt
Catia V5	.catproduct
SOLIDWORKS	.sldasm
Autodesk Inventor	.iam
Autodesk Fusion 360	.f3d
AutoCAD	.dxf
PTC Creo	.asm

Assembly modeling techniques

For assembly modeling which type of part modeling method we use is not important. Normally, usable parts ends up converted in solids anyway. Once when we need to make assembly we can use two modeling methods to do so: bottom-up and top-down assembly modeling.

Bottom-up assembly modeling

In bottom-up modeling first we create all the individual parts, and then we build subassemblies (if any) and then finaly our main assembly. The final geomtery of the product is not defined until we bring all the parts in the assembly.

Top-down assembly modeling

In top-down modeling we create overall assembly as one instance. After the assembly is completed, then we create individual parts and subassemblies that goes into the overall system.

Usually we use combination of both methods.

Closing words

As a mechanical design engineer knowledge of CAD is definetely required to land a job. Understanding and perfecting your CAD knowledge will definetely help you in a long run in your career. You will create assemblies often and from those parts you will create CAD drawings. Your drawings will be based on your assemblies created in CAD, so understanding the basics will help you in the overall design process.

Using assemblies will be part of your normal job, and it is good idea to spend some time understanding how it is properly done. Because the CAD will be the tool that you will use very often I would suggest you to spend some additional time to depeen your knowledge of it.

Now you have an excellent overview of what engineering drawing 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.

Do you like what you read? Then, share it with your friends, colleagues, and on your social media channels. And if you want to become a part of the Newtonians, make sure to subscribe to our newsletter!