What are the basics of solid modelling?
Each solid modelling software offers unique methods for creating and working with solid models. Simple solid modelling programs allow you to build models using solid primitives, which are objects such as boxes, cones, spheres, and cylinders that you combine, subtract, and edit to produce a final model. The process of adding and subtracting primitive shapes is known as a Boolean operation in geometry (see Figure). Boolean operations also apply to more complex solid models defined by features and surfaces.
In contrast to modelling with solid primitives, feature-based solid modelling programs allow you to construct solid models using more intuitive feature tools. A feature often begins with a 2-D sketch, followed by a sketched feature such as extrusion or revolution created from the sketch. Additional features include adding or subtracting solid material to generate a final model. Many feature-based solid modelling programs are highly sophisticated and include many advanced tools and functions that significantly automate the design and documentation process. Parametric solid models are the most common models created using feature-based solid modelling software. Parametric refers to the method of using parameters and constraints to drive object size and location to produce designs with features that adapt to changes made to other features. Some solid modelling programs generate nonparametric solids, known as basic solids or dumb solids.
Feature-based solid modelling programs often maintain a history of the modelling process, which typically appears in a feature tree or history tree (see Figure). History-based solid modelling is most often associated with parametric solid modelling. The software stores and manages all model data, including calculations, sketches, features, dimensions, geometric parameters, the sequence in which each piece of the model was created, and all other model history and properties.
Parametric Solid Modeling
One of the most common 3-D solid modelling techniques is feature-based, parametric solid modelling. Autodesk Inventor, Pro/Engineer, NX, and SolidWorks are examples of feature-based, parametric solid modeling. Many parametric solid modeling programs are surprisingly similar in the way they function. In fact, once you learn the basic process of creating a model using specific software, you can usually transition to different parametric solid modeling software.
Parametric design tools allow you to assign parameters or constraints to objects. Parameters are geometric characteristics and dimensions that control model geometry’s size, shape, and position. A database stores and allows you to manage all parameters. Parametric design is also possible with some 2-D CADD programs. The parametric concept, also known as intelligence, provides a way to associate objects and limit design changes. You cannot change a constraint so that it conflicts with other parametric geometry. Parameters aid the design and revision process, place limits on geometry to preserve design intent, maintain relationships between objects, and help form geometric constructions.
Parameters are added by using geometric constraints and dimensional constraints. Geometric constraints, also known as relations, are characteristics applied to restrict the size or location of geometry. Dimensional constraints are measurements that numerically control the size or location of geometry. Well-defined constraints allow you to incorporate and preserve specific design intentions and increase revision efficiency. For example, if the two holes through the bracket shown in the Figure must always be the same size, then geometric constraints must be used to make the holes equal, and dimensional constraints must be used to size one of the holes. The size of both holes changes when you modify the dimensional constraint values.
Model Work Environments
Parametric solid modelling software often includes several work environments and unique file types for different applications. A part file allows you to create a part model, such as the engine block shown in Figure. A part is an item, product, or element of an assembly. Some systems include separate files or work environments for specialized part modelling and related applications, such as sheet metal part design, surface modelling, analysis and simulation, and rendering.
An assembly file allows you to reference component files to build an assembly model. Components are the parts and subassemblies used to create an assembly. A subassembly is an assembly that is added to another assembly. The figure shows an engine subassembly that references the engine block part shown in Figure.
Part Model Elements
Part models allow you to design parts, build assembly models, and prepare part drawings. A part model begins as a sketch or group of sketches used to construct a feature. Add features as necessary to create the final part model. Primary part model features include sketched, placed, work, catalogue, and patterned features. Develop additional model elements, such as surfaces, as needed to build a part model.
Every part model usually contains at least one sketch and at least one sketched feature. A sketch is a 2-D or 3-D geometry that provides the profile or guide for developing sketched features (see Figure). A parametric sketch includes geometric constraints that define common geometric constructions such as two perpendicular lines, concentric circles, equal-sized objects, or a line tangent to a circle. Dimensional constraints specify the size and location of sketch objects. Examples of sketched features built from a sketch include extrusions, revolutions, sweeps, and lofts. Normally, the initial feature on which all other features are built, known as the base feature, is a sketched feature, such as the extrusion shown in Figure.
Adding placed features requires specifying size dimensions and characteristics and selecting a location, such as a point or an edge. No sketch is necessary. You typically use a dialogue box or other on-screen tool to describe size data. The figure shows two of the most commonly placed features: chamfers and fillets. Placed features are also known as built-in, added, or automated features. Shells, threads, and face drafts are other examples of placed features.
Assembly Modeling
One option for developing an assembly is to insert existing components into an assembly file and then assemble the components with constraints or mates. This is an example of a process that some designers refer to as bottom-up design, and it is appropriate if all or most components already exist. Depending on your approach and the complexity of the assembly, you can insert all components before applying constraints, as shown in Figure. A common alternative is to insert and constrain one or two components at a time.
Another option is to create new components within an assembly file or in place. This is an example of a process that some designers refer to as top-down design. Both assembly techniques are effective, and a combination of methods is common. However, for some applications, developing components in place is faster, easier, and more productive. Developing components in an assembly file usually creates an assembly and a separate part or assembly file for each component.
Once you insert or create assembly components, the typical next step is to add assembly constraints, also known as mates. Assembly constraints establish geometric relationships and positions between components, define the desired movement between components, and identify relationships between the transitioning path of a fixed component and a component moving along the path. There are multiple types of assembly constraints, such as a mate or similar constraint that mates two or a combination of component faces, planes, axes, edges, or points. Component geometry and design requirements determine the required constraints.
Editing Parametric Solid Models
The parametric nature of parametric solid modelling software allows you to edit model parameters anytime during the design process. You can manipulate parameters assigned to sketch and feature geometry, parts, and assemblies to explore alternative design options or to adjust a model according to new or different information. The model stores all of the data used to build the model. Often, modifying a single parameter is all that is required to revise a model. Other times, a completely different product design is built by editing several existing model parameters. The example in Figure shows how changing a few model parameters can significantly alter a product design. In most cases, the tools and options used to edit models are similar or identical to the tools used to create the model originally.
Parametric geometry allows you to make any necessary changes to the design of a model, allowing you to assess design alternatives almost immediately by changing, adding, or deleting sketches, features, dimensions, and geometric controls. Parameter-driven assemblies allow changes made to individual parts to reproduce automatically as changes in the assembly and assembly drawing. Adaptive parts in assemblies are effective when you may not know the exact dimensions of a part or you may not fully understand the relationship between assembly components. Adaptive parts modify automatically if another part changes. Paramedic geometry also allows you to develop equations that drive your models, allowing a few dimensions to define the entire model or even create a family of related parts.
Extracting Drawing Content
Some parametric solid modelling CADD systems, such as Autodesk Inventor, Pro/Engineer, NX, and SolidWorks, combine 3-D solid modelling with 2-D drawing capabilities. You can create any type of part, assembly, or weldment drawing from existing models. The figure shows an example of a part drawing extracted from a part model. When you edit a model, the corresponding drawing adjusts to reflect the new design. You can also edit a model by modifying parametric model dimensions inside a drawing.
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