INTRODUCTION TO ENGINEERING DRAWING & DRAWING INSTRUMENTS

ENGINEERING DRAWING - LECTURE 1

Introduction to Engineering Drawing

Engineering and Engineers

• Engineering Definition: The profession applying knowledge of mathematical and natural sciences, gained by study, experience, and practice, to develop and utilize economically the materials and forces of nature for the benefit of humanity.
• Informal Definition: "the art of making things work."
• Origin of "Engineer": From the Latin word "ingenerare," meaning "to create."
• Engineer Expectations: Required knowledge, planning skills, and the ability to create.

What is Engineering Drawing?

• It is the language of manufacturing.
• A document that communicates a precise description of a part.
• Typically consists of pictures, words, numbers, and symbols.
• A designer uses drawings to communicate their intentions.
• Contains all necessary information to get products made, manufactured, modified, or assembled.

Why Engineering Drawing?

• Often referred to as Engineering Graphics due to CAD software.
• Three historical roles:
  • Communication: Conveys concepts and ideas quickly and accurately without words.
  • Record keeping: Documents designs and features for future applications.
  • Analysis: A tool for determining critical shapes, sizes, and other variables in engineered systems.
• Provides a background for courses like Computer Aided Drawing (CAD).

Types of Drawing

• Artistic Drawing:
  • Used to express feelings, beliefs, philosophies, or abstract ideas of the artist.
  • Often difficult for a layperson to understand.
• Engineering Drawing:
  • Not abstract; requires understanding of technical drawing.
  • Communicates information clearly and concisely to transform an idea into reality.
  • Contains dimensions, notes, and specifications.

The Design Process

• Understanding the design process is crucial to appreciate the need for engineering drawings.
• An organized, step-by-step procedure applying mathematical and scientific principles with experience to solve problems or meet needs.
• Key Principle: Any product to be manufactured, fabricated, assembled, constructed, built, or converted must first be designed.
• Design Process Flow:
  1. Problem Identification
  2. Initial Ideas
  3. Proposed Solution
  4. Testing Models or Prototypes / Testing 3D Computer Models
  5. Working Drawing

Application of Engineering Drawings

Engineering drawings are essential in any setting involving design and subsequent conversion processes, including:

• Manufacturing
• Engineering (Civil, Electrical, Mechanical, etc.)
• Architecture
• Construction

Regulation of Engineering Drawing

• Ensures that communication through graphic language is standardized.
• Standards: Sets of rules that govern how technical drawings are represented.
• Widely used standards:
  • U.S Department of Defense (DOD)
  • U.S Military (MIL)
  • The American National Standard Institute (ANSI)
  • American Society of Mechanical Engineers (ASME)
  • Japanese Standard (JIS)
  • International Standards Organization (ISO)
  • British Standard (BS)

Engineering Drawing and Quality/Competitiveness

• Promotes quality by ensuring it's designed from the outset and drawings accurately communicate the design.
• Examples of errors:
  • Incorrect dimension: part made with wrong size.
  • Improperly drawn view: part manufactured in the wrong configuration.

Principle of Projection

• Engineering drawing is based on the fundamental principle of projection.
• Projection: A drawing or representation of an entity on an imaginary plane or planes.
• Four components of projection:
  1. The actual object being represented.
  2. The eye of the viewer.
  3. The imaginary projection plane (the viewer's drawing paper).
  4. The imaginary lines of sight called projectors.

Types of Projection

The two broad types of projection are:

• Parallel Projection:
  • Orthographic projection: Multi-view drawings showing flat representations of principal views.
  • Oblique projection: Shows the full size of one view. Varieties include cabinet (half-scale), cavalier (full-scale), and general (between half and full-scale).
  • Axonometric projections: Three-dimensional drawings. Varieties include isometric, diametric, and trimetric.
• Perspective Projection:
  • Attempts to replicate what the human eye actually sees when viewing an object.
  • Types: one-point, two-point, and three-point projection.

Drawing Instruments and Their Proper Use

General Rules

• "A Design is only as good as instruments used!!!"
• No line is made freehand; every line is drawn using specific tools.
• Use the correct instruments for the task (e.g., rulers cannot perform the function of a T-square).

Drawing Surface

• Drawing Board: Available in various styles and sizes, often adjustable and tiltable.
• Must be clean, flat, smooth, and large enough for the drawing and equipment.
• At least one edge must be absolutely true/straight to accommodate the Tee square.

Tee Square

• Provides a parallel straight edge for drafting.
• Composed of two parts: the head and the blade.
• Rule 1: While drawing, the head *must always* lay on the true/straight edge of the drawing board (preferably the left side) to guarantee parallel lines.
• Rule 2: All horizontal lines *must* be drawn with the aid of a Tee square.
• Used to draw horizontal lines and, with additional equipment (triangles/set squares), vertical and slanted lines.
• Use only the upper edge of the blade for drawing lines.
• Rule: Before drawing, align the paper to the drawing board using the Tee square.

Compass

• Two main types: friction-joint and spring-bow.
• Friction-joint: For lightly laying out pencil drawings to be inked. Setting may slip under strong pressure.
• Spring-bow: Best for pencil drawing and tracing; retains setting even with strong pressure for dark lines.
• Used to make circles and arcs.
• Extension bars are used for large diameter circles.

Divider

• Similar to a compass but with a metal point on each leg.
• Pivot-joint divider: For measurements of approximately 20mm or more.
• Bow divider: For measurements less than 20mm.
• Proportional divider: Used to enlarge or reduce an object in scale, with a sliding, adjustable pivot.
• Primarily used to lay off distances and transfer measurements.

French Curves

• Thin, plastic tools with an assortment of curved surfaces.
• Used to produce irregular curved lines that cannot be made with a compass.
• Segments of geometric curves such as ellipses, parabolas, and hyperbolas.

Protractor

• Used to measure and lay out angles.
• Can be used in place of a drafting machine or an adjustable triangle.
• Usage: Place the center point on the angle's corner, align the base along one side, and read the degree along the semi-circular edge.

Triangles/Set Squares

• Two standard types: 30-60 degree triangle and 45 degree triangle.
• Made of plastic and come in various sizes.
• Used to make common lines at common angles.
• An adjustable triangle can be used in place of either standard triangle.

Pencils

• Engineering instrument for drawing directly on vellum sheets.
• Hardness scale for drafting purposes:
  • 3H or 4H: Recommended for layout work.
  • 2H, H: For most line work.
  • HB: For outlines.

Eraser and Erasing Shield

• Eraser: Soft, white block-type erasers are commonly used.
• Erasing Shield: Restricts the erasing area to prevent disturbing correctly drawn lines. Made from a thin metal piece with various cut-outs. Placed over the line to be erased, and erasing is done through the cut-out.

Ruler

• The easiest measuring tool to use.
• Used for measuring the distance between two points.
• The metric system uses the meter (m) as its basic dimension.

Templates

• Thin, flat pieces of plastic containing various cut-out shapes.
• Designed to speed up drafting work and improve drawing accuracy.
• Used for drawing circles, ellipses, plumbing fixtures, bolts and nuts, screw threads, electronic symbols, springs, gears, and structural metals.
• Available in many sizes to fit the scale used on the drawing.
• Used wherever possible to increase accuracy and speed.

Mechanical Lettering Sets

• Consist of a scriber, various pen sizes, and templates.
• Scriber Templates: Laminated strips with engraved grooves used to form letters. Scribers can be adjusted for vertical or slanted letters of several sizes.
• Standard Templates: Require practice to form mechanical letters and typically include sets of uppercase and lowercase letters.

Drawing Media

• Drawings can be made on paper, vellum, or polyester film.
• When selecting drawing sheets, consider:
  • Size of objects drawn.
  • Drawing scale.
  • Amount of additional content (border, title block, item list, notes, etc.).
  • Drafting standards.
• Standard Sizes of Drawing Sheets (ISO A-series):
  • A0: 841 mm x 1189 mm
  • A1: 594 mm x 841 mm
  • A2: 420 mm x 594 mm
  • A3: 297 mm x 420 mm
  • A4: 210 mm x 297 mm

Linework

• Linework is the generic term for techniques used to create graphics data on technical drawings.
• For pencil drawing: Thick lines should be approximately twice as wide as thin lines.
• Standard line thicknesses:
  • 0.6 mm minimum for thick lines.
  • 0.3 mm minimum for thin lines.
• Rule: All lines of the same type should have uniform thickness throughout the drawing.

Line Types

Assignment

Reproduce the shapes shown in the lecture slides, where the square grids have an area of 100 mm².