10.1 Engineering Drawings and Its Concepts
Engineering drawings are a crucial part of the design and manufacturing process. They communicate ideas, specifications, and details about a product or system. These drawings provide visual instructions and technical specifications to engineers, architects, and manufacturers to create a product with accuracy and precision.
1. Fundamentals of Standard Drawing Sheets
Standard drawing sheets are predefined formats used in engineering drawings. They have specific dimensions and a structured layout to ensure consistency and ease of understanding. These sheets typically follow international standards such as ISO or ANSI.
Size:
The most common sizes are A0, A1, A2, A3, and A4, with A0 being the largest.
A0: The largest standard size, measuring 841 mm x 1189 mm (or 84.1 cm x 118.9 cm). The area of an A0 sheet is 1 square meter (approximately).
A1: Half of A0, measuring 594 mm x 841 mm (or 59.4 cm x 84.1 cm).
A2: Half of A1, measuring 420 mm x 594 mm (or 42 cm x 59.4 cm).
A3: Half of A2, measuring 297 mm x 420 mm (or 29.7 cm x 42 cm).
A4: Half of A3, measuring 210 mm x 297 mm (or 21 cm x 29.7 cm).
Tips
To get A2 from A1, you halve the dimensions of A1 in one direction (either length or width).
The width of A1 (594 mm) becomes the height of A2.
The height of A1 (841 mm) is halved to get the width of A2.
Border and Title Block:
Each drawing sheet has a border that defines the usable area, and a title block that contains key information such as the drawing number, title, scale, and name of the designer.
Scale:
Scale is a ratio that represents the relationship between the size of an object and its representation in a drawing. It is used when the object is too large or too small to fit on a standard-sized sheet.
Scale Example: A drawing scale of 1:2 means that each unit on the drawing represents 2 units in real life (i.e., half the size).
Common Scales:
Full scale: 1:1
Half scale: 1:2
Double scale: 2:1
Reduced scale: 1:5, 1:10, etc.
Representative Factor (RF)
The Representative Factor (RF) is the ratio between the dimensions on a map, drawing, or model and the actual dimensions of the object it represents. It is a way to express the scale of a drawing or map.
Formula:
Usage: Widely used in maps, models, and scaled drawings to understand the relationship between the representation and real-world dimensions.
Example:
If 1 cm on a map represents 1 km in reality:
2. Line Diagram
A line diagram is a simplified drawing that represents an object's geometry or conceptual design using lines. It is commonly used in engineering, architecture, and technical illustrations to highlight features and relationships without excessive detail.
Types of Lines and Their Roles
Continuous Thick Line (Object/Visible Line):
Representation: Thick, solid line.
Purpose: Depicts the visible edges and contours of an object in a given view.
Example: The outer edges of a cube or visible features on a mechanical part.
Appearance:
__________
Dashed Line (Hidden Line):
Representation: Short, evenly spaced dashes.
Purpose: Represents hidden or invisible edges of an object that are not visible in the current view.
Example: Internal features like holes or slots.
Appearance:
- - - - - -
Long-Short-Long Line (Center Line):
Representation: Alternating long and short dashes.
Purpose: Indicates the geometric center or axis of symmetry for circular or symmetrical features.
Example: Center of a hole, circle, or axis of a shaft.
Appearance:
— · — · —
Thin Continuous Line (Extension Line):
Representation: Thin, solid line.
Purpose: Extends from the object to indicate the limits for dimensioning.
Example: Shows the dimensions of a block’s width, height, or feature position.
Appearance:
________
Thin Line with Thick Edges (Sectional Plane):
Representation: Thin line bordered by thick edges.
Purpose: Marks a sectional plane where the object is cut to reveal internal features.
Example: A vertical or horizontal cut through an object to expose hidden details.
Appearance: Thin with thick borders.
Hatching Line:
Representation: Thin, uniformly spaced, diagonal lines.
Purpose: Used in sectional views to indicate areas of the object cut by the sectional plane.
Example: Cross-section of a solid block or material.
Appearance:
Chain Line:
Representation: Long dashes alternating with short dashes.
Purpose: Used to indicate a special requirement or boundary on the object.
Example: Identifying a heat-treated zone, surface finish limits, or inspection areas.
Importance: Highlights specific areas that need attention or additional processing.
Appearance:
— — · — —
Phantom Line:
Representation: Alternating long dash, two short dashes.
Purpose: Represents alternate positions of a part or motion paths.
Example: A door's swinging motion or a part's alternate placement.
Appearance:
— · · —
3. Dimensions
Dimensions provide the necessary information for the size and shape of the object being drawn. They include:
Linear Dimensions: Indicate lengths, widths, and heights.
Angular Dimensions: Represent angles between lines or surfaces.
Radial and Diametric Dimensions: Used for circles and arcs, showing the radius or diameter.
Tolerance: Defines the allowable variation in size for manufacturing purposes, ensuring parts fit together correctly.
Types of Projection
Projections are methods used in technical drawing to represent three-dimensional objects on a two-dimensional surface. They are classified into parallel projection and angular (perspective) projection based on how the projection rays interact with the object and the plane.
1. Parallel Projection
In parallel projection, the projection rays (lines) are parallel to each other and perpendicular (or oblique) to the projection plane. This method preserves the true dimensions of the object, making it ideal for technical drawings where accurate measurements are required.
Types of Parallel Projection:
Orthographic Projection:
The projection rays are perpendicular to the projection plane.
Usage: Used for front, top, side views in technical drawings.
Example: Engineering blueprints.
Axonometric Projection:
The object is tilted relative to the projection plane, showing multiple faces simultaneously.
Subtypes:
Isometric Projection: Equal angles (120°) between the axes; preserves true dimensions along all three axes.
Dimetric Projection: Two axes have equal angles; dimensions along these axes are preserved.
Trimetric Projection: Angles between all axes are different; dimensions are scaled independently.
Oblique Projection:
The projection rays are at an oblique angle to the projection plane.
Usage: Used for showing one face in true size while the depth is distorted.
Example: Cabinet projection (depth scaled by half).
Key Characteristics:
Maintains the true size and shape of the object.
Objects are not distorted by perspective.
Lines parallel in the object remain parallel in the drawing.
2. Angular (Perspective) Projection
In perspective projection, the projection rays converge at a single point called the projection center or station point. This creates a realistic representation of how objects appear to the human eye, with objects farther away appearing smaller.
Types of Perspective Projection:
One-Point Perspective:
All projection lines converge at a single vanishing point.
Usage: Often used for straight-on views of objects like hallways or roads.
Two-Point Perspective:
Projection lines converge at two vanishing points, usually on the horizon.
Usage: Used for objects at an angle to the viewer, like buildings or furniture.
Three-Point Perspective:
Projection lines converge at three vanishing points, including one for height.
Usage: Used for dramatic or complex viewpoints, like looking up at a tall building.
Key Characteristics:
Creates a realistic, life-like view of objects.
Lines parallel in the object appear to converge in the drawing.
Distorts sizes and angles based on distance from the viewer.
4. Orthographic Projection
In orthographic projection, the goal is to represent a three-dimensional object in two dimensions by projecting its views from different directions.
Some standards view of the orthographic projections:
Front View (Elevation):
Description: This view shows the object as seen from the front, providing a clear look at the height and width of the object.
Plane: The front view is projected onto a vertical plane.
Position: Located at the center of the drawing to give the primary understanding of the object.
Key features: The height and width of the object are visible in this view.
Top View (Plan):
Description: This view displays the object from above, providing a clear view of the width and depth.
Plane: The top view is projected onto a horizontal plane above the front view.
Position: Placed directly above the front view, with the center aligned.
Key features: Shows how wide and deep the object is when looking from the top.
Side View (Profile):
Description: This view shows the object from one side, typically the right side.
Plane: The side view is projected onto a vertical plane parallel to the side of the object.
Position: Placed beside the front view, often to the right.
Key features: This view provides information about the height and depth of the object.
Additional Views:
Rear View: Sometimes, a rear view (opposite of the front) is included to show details that are only visible from the back.
Bottom View: A bottom view can be included to show the underside of the object.
Isometric View: Occasionally, an isometric or 3D view might be included to provide a more realistic perspective.
Types of Projection
First-Angle Projection: The object is placed between the observer and the projection plane. Common in Europe and Asia.
Third-Angle Projection: The projection plane is between the observer and the object. Common in the USA and Canada.
Orthographic projections use projection lines at right angles to the object, which ensures that the dimensions and shapes are true to scale.
5. Isometric View & Isometric Projection
Isometric View:
A drawing technique used to represent 3D objects on a 2D surface.
Key Features:
All three axes are drawn at equal angles of 120°.
No foreshortening of dimensions; objects are scaled equally along all axes.
Commonly used in sketches and CAD designs for clear visualization.
Purpose: Provides a realistic yet simplified way to represent objects, making it practical for engineering and technical drawings.
Isometric Projection:
A formal projection method where 3D objects are represented with precise geometric scaling.
Key Features:
Similar to the isometric view but involves mathematical foreshortening (approx. 82% of true size) to correct distortions.
It ensures angles and dimensions are accurate for technical standards.
Purpose: Used when strict adherence to geometric proportions is required, typically in detailed technical documentation.
6. Pictorial Views
Pictorial views are 3D representations of objects that show how the object appears in real life. They provide a more comprehensive view than orthographic projections by presenting the object in perspective.
Types of Pictorial Views:
Isometric View: As discussed earlier, the object is viewed at equal angles on all three axes.
Perspective View: Represents an object as it would appear to the human eye, with lines converging at vanishing points.
Axonometric View: A type of projection where the object is viewed at an angle, typically with unequal scales along each axis.
7. Sectional Drawing
Sectional drawings are used to show the internal features of an object. A section is created by imagining that the object is cut along a plane to reveal the internal structure. The cut surface is then represented in a detailed view.
Types of Sectional Views:
Full Section: The object is cut completely through the center to show the internal features.
Half Section: Only half of the object is cut to reveal internal features, the other half is shown in an external view.
Broken-Out Section: A portion of the object is "broken out" to show internal details without cutting the entire object.
Revolved Section: A part of the object is rotated to display its features.
In sectional drawings, the cutting plane is indicated by a line marked with arrows, and the internal features are drawn with different line types to distinguish them from external features.
8. Polygon
Interior Angles of a Polygon
The sum of the interior angles of a polygon is given by:
Where:
( n ) = Number of sides in the polygon.
Individual Interior Angle (for Regular Polygons):
If the polygon is regular (all sides and angles are equal), the measure of each interior angle can be calculated as:
Conclusion
Engineering drawings are essential for effectively communicating the design of an object or system. They provide clear instructions and specifications that help transform ideas into tangible products. Understanding concepts such as dimensions, scale, orthographic and isometric projections, pictorial views, and sectional drawings is fundamental for anyone involved in design and manufacturing. These drawings ensure that products are built according to the correct specifications and quality standards.
Last updated
