Engineering Drawing Fundamentals

Engineering drawing is a graphical language used by engineers and designers to communicate their ideas and designs. It is a precise and standardized way of representing three-dimensional objects in two dimensions. This language is crucial for manufacturing, construction, and many other fields. Understanding its fundamentals is the first step towards mastering this essential engineering skill.

Projections

Projections are the process of transferring points or objects from their three-dimensional space onto a two-dimensional plane (the drawing surface). This is achieved by drawing lines of sight from the object to the plane. The type of projection used depends on the desired view and the relationship between the object and the plane.

Orthographic Projection

Orthographic projection is the most common type of projection used in engineering drawings. In this method, lines of sight are perpendicular (orthogonal) to the projection plane. This results in views that show the true shape and size of the object's surfaces when viewed directly.

Imagine an object placed in a glass box. If you were to look at the object from the front, top, and side, and draw what you see on the walls of the box, you would be creating orthographic views. The key principle is that the projection lines are parallel and perpendicular to the plane.

There are two main systems of orthographic projection:

• First Angle Projection: The object is placed in the first quadrant, between the observer and the plane of projection. The views are arranged such that the front view is at the bottom, the top view is above the front view, the left-hand view is to the right of the front view, and so on. This system is commonly used in European countries.
• Third Angle Projection: The object is placed in the third quadrant, behind the plane of projection. The plane of projection is between the observer and the object. The views are arranged such that the front view is at the top, the top view is below the front view, the left-hand view is to the left of the front view, and so on. This system is commonly used in the United States and Canada.

Isometric Projection

Isometric projection is a type of pictorial projection where all three dimensions of an object (length, width, and height) are represented. It shows the object in a three-dimensional view, but it's a single view rather than multiple orthographic views. In an isometric view, the angles between the axes are 120 degrees, and the scale along each axis is the same. This gives a visually balanced representation of the object.

Imagine looking at a cube from a corner. The three visible faces appear foreshortened, and the edges meeting at that corner are drawn at angles of 120 degrees to each other.

Perspective Projection

Perspective projection aims to represent an object as it appears to the human eye. It uses vanishing points to create a sense of depth. Parallel lines in the object appear to converge at these vanishing points on the horizon line. This method is often used for architectural drawings and illustrations where realism is important, but it's less common for detailed manufacturing drawings due to its complexity in dimensioning.

There are three types of perspective projection:

• One-point perspective: Used when the observer is looking at the object face-on.
• Two-point perspective: Used when the observer is looking at the object at an angle.
• Three-point perspective: Used when the observer is looking up or down at the object.

Views

Views are the two-dimensional representations of an object created through projection. The primary goal of engineering drawings is to convey complete information about the object's shape, size, and features. This is achieved by presenting multiple views.

Orthographic Views

These are the standard views used in orthographic projection. The most common orthographic views are:

• Front View (Elevation): This view shows the object as seen from the front. It is usually the most descriptive view and determines the orientation of other views.
• Top View (Plan): This view shows the object as seen from directly above. It is projected onto a plane above the front view (in third angle) or below the front view (in first angle).
• Side View (End View): This view shows the object as seen from either the left or the right. The Right-Hand Side View is projected to the right of the front view (in third angle) or to the left of the front view (in first angle). The Left-Hand Side View is projected to the left of the front view (in third angle) or to the right of the front view (in first angle).

Depending on the complexity of the object, additional views like the bottom view, rear view, or auxiliary views might be necessary.

Auxiliary Views

Auxiliary views are special orthographic views projected onto planes that are not parallel to the principal projection planes (horizontal, vertical, or profile). They are used to show the true shape and size of inclined or oblique surfaces that are foreshortened in the principal views.

For example, if a part has a slanted face, an auxiliary view projected perpendicular to that face will show its true shape.

Sectional Views

Sectional views are used to reveal internal features of an object that are not visible in external views. The object is imagined to be cut by a cutting plane, and the part of the object between the observer and the cutting plane is removed. The view then shows the internal shape as if looking at the cut surface.

The cut surface is typically shown with a shading pattern called "section lines" or "hatching lines" to distinguish it from external surfaces.

Common types of sections include:

• Full Section: The cutting plane passes through the entire object.
• Half Section: Used for symmetrical objects, showing half of the object in section and the other half in external view.
• Revolved Section: Sectional view lines are revolved 90 degrees and drawn within the outline of the view.
• Removed Section: Sectional view is removed from the main view and shown separately.

Drawing Instruments

Accurate and neat engineering drawings rely on the use of specific instruments. Each instrument has a particular function to ensure precision and clarity.

Essential Instruments:

• Drawing Board: A flat, smooth surface, usually made of wood or plastic, on which the drawing paper is fastened. It must be perfectly flat and have a straight edge for guiding the T-square.
• T-Square: Used for drawing horizontal lines and as a guide for setting squares to draw vertical and inclined lines. It consists of a blade and a stock. The stock is placed against the working edge of the drawing board.
• Set Squares (Triangles): Usually come in pairs: a 45-degree triangle and a 30-60-90 degree triangle. They are used with the T-square to draw vertical, horizontal, and inclined lines at specific angles (30°, 45°, 60°, 75°, 90°, 15°, 105°, 120°, 135°, 150°).
• Drawing Pencils: Used for drawing lines. Pencils are graded from hard (H) to soft (B). For engineering drawings, medium-hard pencils like 2H, H, HB, and B are typically used. Harder pencils (H, 2H) are used for construction lines, dimensions, and lighter marks, while softer pencils (HB, B) are used for darker lines like outlines and lettering.
• Compass: Used for drawing circles and arcs. It has a needle point and a pencil or lead holder. Different sizes are available for drawing circles of various diameters.
• Dividers: Similar to a compass but with two needle points. Used for transferring distances, dividing lines into equal parts, or marking points.
• Rulers (Scales): Used for measuring and drawing straight lines. Engineering scales are graduated in specific ratios (e.g., 1:1, 1:2, 1:5, 2:1) to represent drawings at a reduced or enlarged size while maintaining accurate proportions.
• French Curves: Used for drawing smooth, irregular curves.
• Protractor: Used for measuring or drawing angles.
• Eraser: A good quality eraser is essential for removing lines cleanly without damaging the paper.
• Drawing Paper: Available in various sizes (e.g., A4, A3, A2, A1, A0) and weights. It should be smooth and strong.

Symbolic Representation

Symbolic representation, also known as conventional representation or standard symbols, is a shorthand used in engineering drawings to represent common features, materials, or components concisely. This standardization ensures that drawings are easily understood by anyone familiar with the conventions, regardless of their specific location or company.

Common Symbols:

• Materials: Different hatching patterns are used to represent various materials in sectional views. For example, a general-purpose material might have parallel lines, while cast iron might have a specific pattern of circles and lines.
• Welds: Symbols indicate the type, size, and location of welds.
• Threads: Standard symbols represent external and internal threads (e.g., V-thread, square thread).
• Fasteners: Symbols for bolts, nuts, screws, rivets, etc.
• Electrical and Electronic Components: Standard symbols represent resistors, capacitors, transistors, switches, motors, generators, etc.
• Surface Finish: Symbols indicate the required surface texture (roughness) of a machined surface.
• Tolerances: Symbols indicate allowable variations in dimensions and form.
• Geometric Dimensioning and Tolerancing (GD&T): A system of symbolic representations used to define the permissible variation in form, orientation, location, and profile of individual features on a part.

Standardization Bodies:

Several international and national bodies develop and maintain standards for engineering drawings and symbols. Key among them are:

• ISO (International Organization for Standardization): Develops international standards for a wide range of products and services, including engineering drawings.
• ANSI (American National Standards Institute): Develops standards in the United States.
• BIS (Bureau of Indian Standards): Develops standards in India.

Using these standard symbols ensures that a drawing produced in one country can be understood in another, facilitating global trade and collaboration. For example, a symbol for a resistor in an electrical schematic will be universally recognized.

Drawing Sheet Layout

A standard drawing sheet has a defined layout to ensure consistency and legibility. This includes the drawing space, border lines, title block, and revision block.

Key Components of Sheet Layout:

• Border Lines: Typically, a border is drawn around the sheet. A thick outer border and a thinner inner border are common. The inner border defines the main drawing area.
• Title Block: Located in the bottom-right corner of the drawing sheet, it contains essential information about the drawing.

Information typically found in a title block includes:

Information	Description
Drawing Title	Name of the part or assembly.
Drawing Number	Unique identification number for the drawing.
Scale	The ratio of the drawing size to the actual object size (e.g., 1:1, 1:2).
Drawn By	Name of the person who created the drawing.
Checked By	Name of the person who reviewed the drawing.
Date Drawn	Date the drawing was completed.
Material	The material of the part.
Projection System	Indicates whether First Angle or Third Angle projection is used (often with a symbol).

• Revision Block: Usually located above or beside the title block, it records changes made to the drawing over time, including the revision number, date, description of changes, and who approved them.
• Zones: The borders of the drawing sheet are often divided into zones (like a grid reference on a map) to easily locate specific details on a large drawing.