Unlocking the Essence of Engineering Drawing: A Comprehensive Overview
Basics of Engineering Drawings: In the realm of technical communication, especially within the context of machinery and its intricate components, engineering drawings stand as a pivotal medium. These drawings, characterized by their technical precision, serve to meticulously outline the specifications essential for the manufacturing process.
Crafted in adherence to fundamental principles and standardized conventions governing layout, nomenclature, and symbolic representations, engineering drawings transcend geographical barriers. They form the bedrock of seamless communication among engineers worldwide, earning them the moniker of the universal language of engineering.
Mastering the creation and interpretation of these drawings holds paramount significance for aspiring engineers. In India, adherence to basic principles, standardized conventions, and recommended symbolic representations outlined by the Bureau of Indian Standards (BIS) governs the preparation of engineering drawings.
Essential Tools for Engineering Drawings: A Comprehensive Guide
In the realm of engineering drawing, precision is paramount, whether crafted manually using drafting instruments or through computer-aided drafting (CAD). While this book primarily focuses on manual drafting techniques, it’s crucial to understand that the quality and efficiency of drawings hinge on the choice and condition of drawing instruments.
To ensure optimal accuracy and speed, students are encouraged to invest in high-quality drawing instruments. Here’s a breakdown of commonly used drawing tools:
- Drawing board
- Minidrafter
- Precision instrument box
- 45° and 30°–60° set squares
- Engineers’ scales
- Protractor
- Irregular or French curves
- Drawing pins or clips
- Drawing papers
- Pencils
- Erasers
- Erasing shields
- Templates
- Dusters
- Flexible curves
- Tracing paper
- T-square
In the subsequent sections, we’ll delve into the specifics of each of these drawing instruments.”
Drawing board
Crafted from high-quality materials such as well-seasoned teak, blue pine, oak, or red cedar, a drawing board serves as the foundation for manual drafting. It features a straight edge, typically made of sturdy wood, affixed along one edge to facilitate the guidance of the T-square.
The surface of the drawing board must be flawlessly smooth and devoid of any cracks. To prevent warping, it’s constructed from narrow strips of wood meticulously glued together, complemented by two battens affixed to the underside. Standard sizes of drawing boards are readily available in the market.
Interestingly, in the British system, drawing board sizes were termed Imperial and half-Imperial, measuring 31″ x 23″ and 23″ x 16″ respectively.
Minidrafter
The Minidrafter is ingeniously designed with two scales permanently positioned at right angles to each other, as depicted in Figure 1.2. Its functionality allows for the adjustment and clamping of working edges to any angle relative to the drawing board, facilitated by a scale-fixing knob.
This device ensures that once affixed to the drawing board, the working edges remain parallel to their original positions even when moved. To utilize a minidrafter effectively:
- Adjust the angular scale by aligning it to zero using the scale-fixing knob.
- Align one working edge with the horizontal edge of the drawing sheet and the other with the vertical edge.
- Secure the drafter onto the board using the clamping knob.
When drawing parallel lines at an angle to the horizontal or vertical axis, adjust one of the working edges to the desired angle using the scale-fixing knob. Once fixed, these edges maintain their parallel orientation regardless of their movement on the drawing sheet. Hence, the minidrafter proves invaluable for drawing multiple parallel lines with precision.”
Precision instrument box
A well-equipped instrument box comprises the following essential tools:
- Large compass
- Small spring compass
- Large divider
- Small spring divider
- Inking pen
- Extension bar for large compass
- Inking attachments for compasses
The large compass is ideal for drawing circles and arcs with sizable radii. Its sharpened bevel serves as the lead, while the needle point, slightly longer than the lead by about 1 mm, facilitates insertion into the paper, enabling the creation of circles with large radii. To ensure precision, adjust the needle point and lead (or ink pen) to remain perpendicular to the drawing sheet during circle and arc drawings. If drawing circles of exceptionally large radii, attach an extension bar to the large compass.
On the other hand, the small spring compass is designed for drawing circles and small arcs, typically up to 25 mm in radius. Its screw-and-nut arrangement enables precise radius adjustment without risk of disturbance. Type-I and Type-II small spring bow compasses have differently positioned nuts: Type-I has the nut on one leg, while Type-II has it between the legs. This illustration demonstrates how a spring bow compass is slightly inclined when used to draw a circle.
Dividers serve a function similar to that of large and small compasses, albeit with a needle point replacing the pencil lead. They are utilized for measuring and marking distances from scales to drawings or between different parts of a drawing.
Inking ruling pens and inking attachments for compasses offer the flexibility to adjust line thickness while drawing. An inking ruling pen typically features two nibs separated by a gap, adjustable via a screw mechanism to vary line thickness in the drawing. A variety of pens are available to cater to different line thickness requirements, with users able to employ multiple pens as needed.
45° and 30°–60° set squares
Set squares, commonly found in the form of right-angled triangles, feature angles of 45° each or 30° and 60° respectively. Typically crafted from transparent plastic, they may come in a solid pattern with a central hole or an open-center pattern, with square or beveled edges.
Primarily, set squares are employed for drawing lines inclined at 30°, 45°, or 60° to the horizontal axis. Additionally, they can be used in conjunction with a drafter’s scale to draw lines at various other angles, such as 15°, 75°, and 105° to the horizontal.
Set squares are denoted by their angle measurement (either 45° or 60°) and the length (in mm) of the longer edge containing the right angle. For instance, the set squares depicted in Figure would be designated as 45° – 150 and 60° – 200.”
Engineers’ scales
Engineers’ scales play a crucial role in marking necessary measurements on lines within engineering drawings. Depending on the dimensions of the object and the paper used, individuals often create drawings to full, reduced, or enlarged sizes. These scales adjust the lengths for drawings, whether they reduce or enlarge them.
For instance, on a 1:2 scale, a length marked as 2 cm equates to the length marked as 1 cm on a 1:1 scale. Engineers commonly utilize a variety of scales in the preparation of engineering drawings, as detailed in the table.
Protractor
A protractor serves the purpose of measuring angles. Typically crafted from transparent plastic, it comes in either a semicircular or circular shape, featuring square or beveled edges for added precision. Protractors with diameters of 100, 150, or 200 mm are commonly employed for accurate angle measurement.
To utilize a protractor effectively, its center is positioned at the intersection of two straight lines, with one of these lines aligned with the zero line of the protractor. The other straight line indicates the measurement of the angle between the two lines.”
Irregular or French curves
A diverse array of irregular curves is readily available. These curves serve the purpose of drawing non-circular arcs. To utilize them effectively, the curve’s trajectory begins with plotting several points. Subsequently, the desired curve is sketched by hand. Align the irregular curve with the sketched curve to achieve the closest possible match.
Continuing the curve drawing process necessitates ensuring that the irregular curve aligns with the sketched curve at each end, extending beyond the segment to be drawn. This practice ensures a seamless transition in the line curvature, eliminating any abrupt changes.
Drawing pins or clips
Spring clips serve the purpose of securing the drawing sheet onto the drawing board. Alternatively, you can utilize adhesive tape for this purpose.
To affix a drawing sheet onto a drawing board:
- Position the drawing sheet on the board, ensuring its edges align with those of the board.
- Insert the spring clip with the flat part over the drawing sheet and the curved part beneath the board.
- Typically, users insert four clips, one at each corner, to securely hold the sheet in place.
Drawing papers
Drawing paper should possess certain qualities to ensure optimal performance. It should be thick, smooth, strong, tough, and maintain uniform thickness throughout. Additionally, the paper fibers should not disintegrate when erased with a good-quality eraser. The table outlines commonly used and available sizes of drawing papers.
Pencils
Clutch pencils offer convenience as they eliminate the need for sharpening. Typically, medium-hard (HB), firm (F), moderately hard (H), and hard (2H) leads are ideal for engineering drawing purposes. HB and F leads excel in freehand sketching and lettering, while H and 2H leads are preferable when guiding by the scale of a drafter or set square.
Erasers
Employ a soft eraser to erase unwanted lines. Using a hard eraser can damage the paper’s surface.
Erasing shields
An erasing shield serves to safeguard the drawing from accidental erasure while removing unwanted portions. Typically crafted from either plastic or metal, it features openings of various shapes and sizes.
Templates
Crafters typically create templates as flat sheets from plastic, featuring cutouts of circles, triangles, squares, and ellipses of various sizes.These templates also include markings for the positions of mutually perpendicular diameters of circles and major and minor axes of ellipses.
To utilize a template effectively, one must align the specific shape on the template with the corresponding features of the required shape. For example, when drawing a circle (or an ellipse), align the template for circles with the mutually perpendicular diameters of the required circle (or the major and minor axes of the ellipse).
Dusters
A duster, ideally, should be a clean, soft piece of cloth. Its purpose is to gently flick off eraser crumbs that accumulate while erasing lines. Additionally, it is essential to use the duster to clean the set squares, protractor, and minidrafter before starting work and periodically during the task.
Flexible curves
A flexible curve typically consists of a rubber rod that users can bend to align with various plotted points that do not fall on a straight line. Once bent, it serves as a guide for a pencil or ink pen to draw the desired curve. To enhance its durability, the flexible rod contains a centrally inserted soft metallic rod for reinforcement.
Tracing paper
Users often employ tracing paper, a semi-transparent type of paper, to reproduce existing pencil or ink drawings by tracing over them. They frequently use these traced copies as the basis for creating blueprints.
T-square
The T-square comprises a wooden blade and a stock, firmly attached to each other with their working edges forming right angles. It serves as a substitute for a minidrafter when drawing horizontal lines. To use it, keep the working edge of the stock in contact with the drawing board’s working edge, while the blade lays over the drawing sheet with its working edge positioned horizontally.
Now that we’ve explored the different drawing instruments, let’s delve into an example demonstrating their usage.