3.1 Understanding Technical Drawings
Key Concepts
1. Orthographic Projections
Orthographic projections are a method of representing a three-dimensional object on a two-dimensional plane. This is typically done using multiple views, such as front, top, and side views. Each view provides a different perspective of the object, allowing the machinist to understand its shape and dimensions fully.
Example: Imagine a cube. An orthographic projection would show three views: the front view (a square), the top view (a square), and the side view (a square). Each view helps in understanding the cube's dimensions and structure.
2. Dimensions and Tolerances
Dimensions specify the size and location of features on a part, while tolerances define the allowable variation from these specified dimensions. Tolerances are crucial in ensuring that parts fit together correctly and function as intended. They are often expressed in fractions or decimals.
Example: A shaft might be specified to have a diameter of 20mm ± 0.05mm. This means the shaft's diameter can be anywhere between 19.95mm and 20.05mm. If the shaft is outside this range, it may not fit properly into the corresponding hole.
3. Section Views
Section views are used to show the internal features of an object. A section view is created by "cutting" through the object at a specific location and showing the "cut" surface. This helps in understanding the internal structure and assembly of complex parts.
Example: Consider a pipe with a complex internal structure. A section view might show the internal threads and the material's thickness at different points, providing a clear understanding of how the pipe is constructed internally.
4. Symbols and Annotations
Technical drawings often include various symbols and annotations to convey additional information. These can include surface finish symbols, material specifications, and notes about the manufacturing process. Understanding these symbols is essential for correctly interpreting the drawing.
Example: A surface finish symbol might indicate that a part's surface must be smooth to a certain roughness level. Without understanding this symbol, the machinist might not know that additional finishing steps are required.
5. Geometric Dimensioning and Tolerancing (GD&T)
GD&T is a system for defining and communicating engineering tolerances. It uses a symbolic language to describe the size, form, orientation, and location of features. GD&T ensures that parts are manufactured to the correct specifications, even when the part is viewed from different angles.
Example: A GD&T symbol might specify that two holes must be aligned within a certain positional tolerance. This ensures that the holes are correctly positioned relative to each other, regardless of the part's orientation.
By mastering these key concepts, machinists can accurately interpret technical drawings, ensuring that parts are manufactured to the required specifications and tolerances.