7.2 Solid primitives and extrusions

3D solid primitives are the building blocks of complex 3D models in CAD. Cubes, spheres, cylinders, cones, and tori are common primitives defined by specific dimensions. These can be created, modified, and scaled using CAD software tools.

Extrusion is a key 3D modeling technique that creates 3D solids from 2D profiles. By extending a 2D shape along a path, you can create various objects with constant cross-sections. Advanced options like tapering and thin-walling add versatility to this method.

Creating Basic 3D Primitives

Common Primitives and Their Characteristics

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• 3D solid primitives are the basic building blocks used to create more complex 3D models in CAD software
• Common primitives include cubes, spheres, cylinders, cones, and tori
• Cubes are defined by their width, depth, and height dimensions (example: a 10x10x10 unit cube)
  • They have six square faces, eight vertices, and twelve edges
• Spheres are defined by their center point and radius (example: a sphere with a radius of 5 units)
  • They have a smooth, continuous surface with no edges or vertices
• Cylinders are defined by their radius and height (example: a cylinder with a radius of 3 units and a height of 8 units)
  • They have two circular bases connected by a curved lateral surface

Creating and Modifying Primitives

• Primitives can be created using specific tools or commands within the CAD software interface, typically found in the modeling or geometry toolset
• The size and proportions of primitives are controlled by inputting precise numeric values or using interactive handles and manipulators
• Primitives form the starting point for building 3D models and can be further modified, combined, or used in Boolean operations to create more complex geometry
• Scaling allows resizing solid primitives proportionally by applying a scale factor to the object's dimensions
  • It can be performed uniformly in all directions or non-uniformly along specific axes
  • Scaling can be used to create variations of a primitive shape with different sizes while maintaining its proportions (example: scaling a cube with a factor of 0.5 will result in a cube half its original size)

Extruding 2D Profiles into 3D

Extrusion Techniques and Parameters

• Extrusion is a 3D modeling operation that creates a 3D solid or surface by extending a 2D profile along a straight path perpendicular to the profile plane
• The 2D profile can be a sketch, curve, or set of edges that defines the cross-sectional shape of the extruded object (example: a circular profile to create a cylinder)
  • It is typically created on a work plane or face
• The extrusion distance determines the height or depth of the resulting 3D object
  • It can be a positive value to create a solid protrusion or a negative value to create a cut or cavity
• Extrusion can be performed using a single profile or multiple profiles to create more complex shapes with varying cross-sections along the extrusion path

Advanced Extrusion Options

• Additional options for extrusion include tapering the extruded shape at an angle, creating thin-walled parts with a specified wall thickness, or capping open ends of the profile
• Tapering allows creating angled or sloped extrusions by specifying different start and end dimensions for the profile (example: a tapered rectangular prism)
• Thin-walled extrusions create hollow objects with a specified wall thickness, reducing material usage and weight (example: a thin-walled tube or container)
• Capping options determine whether the open ends of the extruded profile are closed off or left open (example: a capped cylinder vs. an open-ended pipe)
• Extrusion is a versatile technique used to create common 3D objects such as walls, posts, beams, brackets, or any shape with a constant cross-section along one axis

Transforming Solid Primitives

Rotation and Positioning

• Rotating enables changing the orientation of solid primitives around a specified axis or reference point
  • Rotation angles can be input precisely in degrees or manipulated interactively using handles
  • Rotation can be performed around the X, Y, or Z axis or a custom axis defined by two points (example: rotating a cube 45 degrees around its vertical axis)
• Positioning tools allow moving solid primitives to a specific location within the 3D modeling space
  • Objects can be translated along the X, Y, or Z axes or moved freely in 3D space
  • Positioning can be done by specifying exact coordinates, snapping to reference points or geometry, or using interactive dragging methods (example: moving a sphere 10 units along the positive X-axis)

Alignment and Combined Transformations

• Alignment tools help position primitives relative to other objects or reference planes, ensuring precise placement and orientation in the overall model
• Primitives can be aligned to faces, edges, or vertices of existing objects or to predefined reference planes such as the XY, YZ, or XZ planes (example: aligning the base of a cylinder to the top face of a cube)
• Combining multiple transformation operations, such as scaling, rotating, and positioning, allows for precise control over the size, orientation, and location of solid primitives in the 3D model
• Transformations can be applied in a specific order to achieve the desired result, and they can be easily modified or undone if needed (example: scaling an object, then rotating it, and finally positioning it in the desired location)

Combining and Subtracting Solids

Boolean Operations

• Boolean operations, such as union, subtraction, and intersection, enable combining or subtracting solid primitives to create more complex 3D shapes
• Union (or add) operation merges two or more solid primitives into a single unified solid, creating a shape that encompasses the volume of all the original primitives (example: merging a cylinder and a cube to create a composite shape)
• Subtraction (or difference) operation removes the volume of one solid primitive from another, creating a shape with the overlapping portion carved out (example: subtracting a smaller cylinder from a larger cube to create a hole or cavity)
• Intersection operation creates a new solid shape that represents only the overlapping volume between two or more solid primitives (example: finding the common volume between intersecting spheres)

Creating Complex Shapes

• The resulting shape from Boolean operations maintains the properties and attributes of the original primitives, such as material, color, or visibility settings
• Combining primitives allows creating objects with more intricate geometries, such as holes, pockets, or interlocking features, that would be difficult to achieve with a single primitive (example: creating a gear by subtracting cylinders from a disk)
• When subtracting primitives, the order of selection determines which object acts as the base and which acts as the cutting tool
  • The base object is modified by subtracting the volume of the cutting object
• Boolean operations can be performed iteratively, combining or subtracting multiple primitives in a sequence to build up complex shapes incrementally (example: creating a complex mechanical part by unioning and subtracting various primitives step by step)