Basic knowledge of 3D CAD

Basic knowledge of 3D CAD

Author: D2FORM Representative Minoru Enomoto
In recent years, CAD systems have been used as tools for designing and manufacturing products and parts. The CAD includes 2D CAD and 3D CAD. Since these two have different mechanisms, they are also used differently as tools. If you understand the mechanism peculiar to 3D CAD, you can see how to use each. In this series, we will introduce the basic knowledge for understanding 3D CAD over a total of 7 times. Part 1 will introduce the features of 3D CAD.

1. What is 3D CAD?
The most basic role of CAD (Computer Aided Design) is to express the design contents including the shape as digital data and facilitate the creation, modification and reuse of the design data. At this time, the difference in the expression method of shape data is two-dimensional (2D) and three-dimensional (3D). The actual product is a three-dimensional solid. Two-dimensional data is a projection drawing of a three-dimensional solid according to the drafting rules. On the other hand, 3D data is a representation of a real solid in a computer. This difference is a big difference for both computers and users, making 3D CAD difficult for engineers familiar with 2D CAD and drafting to understand.

・ What is 3D?
A three-dimensional (3D) dimension is the number of coordinates used to represent a shape. One dimension has one coordinate axis, and points on the coordinates are limited to a straight line. Two-dimensional (2D) is the position on the plane, that is, the two axes whose axes are orthogonal (Fig. 1). It is the same as the xy coordinates you see in graphs.

Figure 1: What is 3D?

Figure 1: What is 3D?
Three-dimensional (3D) is a three-dimensional axis whose coordinate axes are orthogonal, and can express the same three-dimensional space as in the real world. Set the position and orientation when the part is placed in 3D space. The orientation has a rotation angle for each of the three coordinate axes, and is called 6 axes or 6 degrees of freedom. This is necessary for the movement of objects and the placement of parts. However, since there are three coordinate axes, xyz, it is not called 6-dimensional.

・ History of 3D CAD
3D CAD has evolved and become widespread due to design and manufacturing requirements. Let's look back on the development of 3D CAD and understand the role of 3D CAD and the current situation. Surface CAD was the first to emerge in the history of 3D CAD. Surfaces represent appearance as faces. After that, solids that can also represent the contents of the surface appeared, and the current mainstream of 3D CAD is solid CAD.

Development of surface CAD

......

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2. Difference between 2D CAD and 3D CAD
2D CAD is a digitized version of drafting, and the design method is no different from handwritten drafting by drafters before the advent of 2D CAD. When moving from hand-drawn drafting to 2D CAD drafting, few changes were made to the design method.

On the other hand, 3D CAD is ...

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3. Benefits of 3D CAD
The symbol of the merit of using 3D CAD is the shortening of delivery time and the improvement of quality by digitizing and streamlining the design and manufacturing processes. The idea of ​​shortening the delivery time is to create various manufacturing processes (analysis, processing, assembly, maintenance) at the design stage using 3D CAD. It is called concurrent engineering because each process progresses in parallel from the design process. Specific benefits include the following:

Design benefits
・ Improvement of design examination speed
・ Accurately grasp the space (interference / gap)
・ Significant reduction of design mistakes
・ No mistakes in reading and writing projection drawings (compared to 2D CAD drafting)
・ Design review proceeds quickly and accurately

Use of 3D data
......

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Part 2: What is a feature?
Last time, I introduced the features of 3D CAD and its advantages in use. From this time, I will introduce the concrete mechanism of 3D CAD. This time, we will cover the basic features of solid creation. In addition, 3D CAD supports both solid modeling and surface modeling. Unless otherwise noted, this article focuses on solid modeling.

1. What is a feature?
A feature is the smallest unit for creating a shape in solid CAD. Solid CAD creates a solid shape based on the feature settings. Therefore, solid modeling can be thought of as defining feature settings. There are two types of features: sketch-type features that extrude cross-sectional shapes to create three-dimensional shapes, and edit-type features that chamfer existing three-dimensional shapes (Fig. 1).

Figure 1: Sketch and edit features

Figure 1: Sketch and edit features
2. Sketch type feature
Extruded features are typical sketched features. Extrusion feature settings can be broadly divided into sketches and extrusion settings (Figure 2).

Figure 2: Extrusion feature settings

Figure 2: Extrusion feature settings
Extruded features create features by extruding cross-sectional shapes. The drawing content of this uniform cross section is called a sketch. The features of the sketch are: 1: It is necessary to specify the plane on which the sketch is created, 2: The sketch is created in a work environment dedicated to 2D drawing, 3: The cross-sectional shape is a closed area where the material side and the outside can be distinguished. There are three things you need to do. The details of the sketch will be explained in detail next time.

Since the sketch is a closed 2D area, the extruded solid area is also a closed 3D area. This is a solid. To create a solid, set the extrusion method, such as the direction to extrude the cross section and the length to extrude (left in Figure 3). The extruded 3D area also allows you to scrape off existing solids (center and right in Figure 3).

Figure 3: Sketch extrusion method

Figure 3: Sketch extrusion method
In this way, creating a shape by combining an existing solid with another solid can be understood by the concept of set operation (Boolean operation) (depending on the CAD software, it may be a different command). Figure 4 is an example of a set operation. Figure 4 If there is no set operation at the left end, the cylinder on the left (green) exists as an existing solid. To this, when you create a new extrusion feature (yellow cylinder), you specify the set method (sum, difference, product).

Figure 4: Example of set operation

Figure 4: Example of set operation
Typical sketch-type features include ...

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3. Editable features
Editable features are features created by specifying an existing solid shape (Figure 6). There are also functions that can be associated with 2D CAD, such as patterns and mirrors. However, in solid CAD, you specify the feature as the pattern source instead of the shape element.

Figure 6: Editable features

Figure 6: Editable features
If the cross-sectional shape has fillets or chamfers ...

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4. Differences between solids, surfaces and wires
A surface is a surface element such as a plane or curved surface that has no thickness information. The surface of a solid is made up of surfaces. In other words, a solid can be said to be a model that can identify a closed space (material side) by connecting the edges of adjacent surfaces. Therefore, solid CAD has the ability to work with surfaces, including surfaces and wires as features. Typical surface features are sweep surfaces, boundary surfaces, and surface trims (Figure 8).

Figure 8: Typical surface features

Figure 8: Typical surface features
Sweep surfaces are typical sketched surfaces. Create in the same way as solid features, such as rotate, sweep, and loft. The difference from solid is ...

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Part 3: What is a sketch?
Last time, I introduced a feature that is a concept peculiar to solid CAD. This time, I will explain the 2D drawing function and sketches that are indispensable for creating features. Sketching, like features, is a unique feature of solid CAD. If you are used to 2D CAD, be careful not to get caught up in prejudice.

1. What is a sketch?
The most important role of sketching is to draw the 2D section shape needed to create sketched features. It can also be used as a wire, such as a sweep feature path. However, sketch drawing has a very different role and mechanism than 2D CAD drawing.

The usual work target in solid CAD is features. It treats one sketch as one element. 2D elements such as line segments and circles cannot be handled directly. With solid CAD ...

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2. Sketch surface and sketch shape
・ Sketch surface
To draw a 2D sketch in 3D space, you need to specify on which plane to draw in 3D space. This is the sketch surface. For example, in an extruded feature that extrudes a solid from a sketch, the sketch is the reference plane for the solid, so the sketch plane of the first feature determines the orientation of the part in 3D coordinates (Figure 1).

Figure 1: Sketch surface and part orientation

Figure 1: Sketch surface and part orientation
Unfamiliar designers tend to choose the sketch surface with intuition and operability. Considering the function and role of the feature shape, it is necessary to specify a suitable surface as a reference. Consider the shape drawing in Fig. 2 as an example. In this case, the 20mm dimension (the height of the cylinder) has a functional meaning. If the top surface of the model is a sketch plane, the extrusion distance needs to be calculated (center in Figure 2). On the other hand, if you use the reference plane (or the bottom of the model) as the sketch plane, you can model with the drawing dimensions (right in Fig. 2).

Figure 2: How to take a sketch surface

Figure 2: How to take a sketch surface
The plane to be specified is recorded as the sketch plane. When the position of the specified plane is changed, the sketch plane will follow. You can also change the sketch plane to another plane.