(We’re talking about mechanical CAD programs here rather than the architectural ones.)
Here are the three things: 1. they have a part modelling environment, 2. an assembly modelling environment and 3. a drafting environment.
A lot of these mechanical CAD programs are built on the same underlying kernel. That is the foundational code that runs the software. The two kernel alternatives that are available are sold to all the CAD companies who really only build the user interfaces for the competing brands. You’ll find that all the CAD companies have many features in common, its just their menus and workflows that take a bit of getting used to if you have to learn a different program when you change jobs for instance.
Lets talk more about the three things.
First the part environment.
Most manufactured products are assemblies of a number of components, a car for example is a collection of thousands of separate parts and the part environment in the CAD program is the place where each individual part is designed and drawn in three dimensions. You will very quickly recognise the part sketching function in whichever CAD program you open.
A few lines form a closed 2D profile which is then extruded into a 3D shape. Most engineered part components are consequently blocky, simple prismatic shapes with perhaps some extra features such as holes and rounded edges. They may be hollowed out to save weight and whatever manufacturing process has been chosen may also have an affect on the design. There are a couple of variations on this procedure; when rotating parts are needed in motors and pumps, the sketch we referred to earlier can be rotated around an axis instead of extruded in one direction. These revolved shapes will likely be manufactured by turning on a lathe, and probably one that is controlled by a computer with the CAD model loaded into its memory.
The designer will also save the part file with a note specifying the material each part is made from. A built-in database has the density of all the common materials is able then to calculate the weight of the part since the designer has given it a precise size and shape.
Two other common features of the part environment are Equation driven design and Configurations. Equation driven design means that the designer can specify mathematical relationships between dimensions in the model. Configurations are useful for those situations where there is a family of products of varying size. Complexity can be involved by linking a spreadsheet to the model.
Second, the assembly environment.
Once we have a collection of components we can assemble these in the assembly environment of the CAD program, the second thing all CAD programs have in common.
Rather than creating geometry, the assembly environment exists for the purpose of arranging the parts as they will be after the parts are made. Think of a bicycle or motorcycle. A lot of parts are needed and they can be assembled one at a time on screen. The designer specifies a fixed position for the axle in the forks, then an axial relationship between the axle and the hub of the wheel. The software is able to understand that the axial relationship will allow the wheel to turn around the axis and therefore realistic animations are possible with assembled products depending on the program.
People will be familiar with the idea of exploded diagrams. These are assembly drawings where the parts are moved apart but kept basically in the order of assembly. Often a line indicates the direction of assembly. These exploded diagrams are useful in product manuals for maintenance tasks and the replacement of parts. CAD software usually has this functionality built into the assembly environment and these exploded views can be translated in the next environment where printed pages are prepared.
Just before moving on, its worth noting that configurations as described in the previous section are also possible in the assembly environment.
And Finally, the Drafting Environment
The production of plans and blueprints for engineered parts and products is a whole discipline of its own with a host of traditions and conventions that have remained largely unchanged despite the radical changes in the way these plans are produced. An older friend of mine did an apprenticeship and the first decade of his career on a drawing board, he then switched to CAD on the earliest computers and eventually to 3D CAD later on. The book he produced at the end of his career had hand drawn plans, vector CAD plans and 3D plans but they were all plans produced on paper with the same views, symbols and annotations.
Earlier two dimensional CAD involved the painstaking drawing of every line of every view, line by line for every view; front, top, side and section. The advantage of 3D mechanical CAD is that the three dimensional model is created just once and the drafting environment shows the model on the page from any angle that is required. Section views automatically reveal the internal features of the model and all of the views are arranged on a page of the desired standard size and dimensions and notes are added and there is usually a title panel in the bottom right with the company name and version tracking information.
Exceptions
These three common environments are easily recognisable as soon as you’ve seen them once. I have necessarily partly simplified this in order to keep the article short and readable however I’ll mention a couple of exceptions here. It is a well established practice that software has the ability to accept extensions or plugins and in this way many CAD programs have additional functions. In some cases the functions are built in, in others you pay extra. Some of the better known extensions allow surface modelling, for example the curved surfaces of the exterior panels of a car or an electric kettle or a hair dryer.
These parts are anything but blocky and prismatic and the CAD software needs special tools. Another is sheet metal design where components will be cut out on a special press in the workshop and then folded into shape. CAD can design these parts and then unfold the part in the software showing the profile to be cut out. Similarly welded fabrications can be designed in a weldments extension which makes it convenient to work with the standard square steel tubes and plates. There are many others including all kinds of simulation and the planning of toolpaths for CNC machine tools but we have to stop somewhere, I hope to cover many of these in future articles and podcasts.