Orthographic Projection Principles

• Orthographic projections use 2D drawings to accurately represent an object.

Defining the 6 Principal Views (Orthographic Views)

• Picking a descriptive Front View is a general guideline
• The longest dimension is usually the width or depth
• Front, Top, and Side Views are the most common view combinations
• The Glass Box Approach involves placing the object in a glass box, freezing the view from each side, and unfolding the box.

Line Thickness

• Most engineering drawings need both thick and thin lines
• Thick lines should be double the thickness of thin lines
• Thick continuous lines are for visible edges and outlines
• Thin lines are for hatching, leader lines, short center lines, dimensions, and projections

Line Styles

• Thin chain lines indicate center lines, which identify the center of circles, cylindrical shapes, or symmetry lines.
• Dashed lines reveal important hidden details like wall thickness and holes

Precedence of Lines

• Visible lines take precedence over all other lines
• Hidden and cutting plane lines take precedence over center lines
• Center lines have the lowest precedence

Dimensioning

• Dimensioned drawings provide all the necessary information to manufacture a finished product

• Dimensions use continuous thin lines

• Projection lines should not touch the object

• Projection lines are perpendicular to what is being dimensioned

• Any dimension less than 1 must have a leading zero

Types of Dimensioning

• Parallel dimensioning uses several dimensions from one projection line
• Superimposed Running Dimensions are a simplified version of parallel dimensions using a small circle at the intersection of the first dimensions and the projection line
• Chain dimensioning

Dimensioning of Circles

• Dimensioning a circle can be done between two lines from diametrically opposite points or internally
• Figure B is used when the circle is too small for the dimension to be easily read if it was placed inside the circle

Dimensioning Radii

• All radial dimensions are preceded by the capital R
• Figure A shows a radius dimensioned with the centre of the radius located on the drawing
• Figure B shows how the dimension radii which do not need their centres locating

Tolerancing

• Tolerance values indicate the maximum permissible variation from a dimension because products cannot be manufactured to exact measurements
• Dimensions must include a tolerance value, as a general tolerance for several dimensions or specific to that dimension

Drawing Layout

• Engineering drawings should have a title block
• The title block includes the title, name of the creator, checker, version, date, notes, scale, company name, and projection system

Dimensioning

• Expressing size features with lines, symbols, figures, and notes is called dimensioning

Principles of Dimensioning

• All necessary dimensions to describe a component clearly must be written directly on drawings

• Each feature should only be dimensioned once per drawing

• Dimensions should be placed on the view where the shape is best seen

• Dimensions should be expressed in one unit (preferably mm) without showing the unit symbol

• Dimensions should be placed outside the view when possible

• Dimensions should be taken from visible outlines rather than hidden lines.

• No gap should be left between the feature and the start of the extension line

• Crossing of center lines should be done with a long dash, not a short dash

Principles of Dimensioning Methods

• With the aligned method, dimensions should be placed parallel to and above their dimension line, preferably at the center
• With the unidirectional method, dimensions should be placed so they can be read from the bottom of the drawing only
• Non-horizontal lines are interrupted, preferably in the middle, for the insertion of the dimension

Dimensioning Symbols

• Diameter - Ø
• Radius - R
• Square - ☐
• SR - Sphere Radius
• SØ - Sphere diameter

Arrangements of Dimensions

• Chain Dimensioning

• Parallel Dimensioning

• Combined Dimensioning

• Staggered Numerals

Drawing Sheets

• A4: 210 x 297
• A3: 297 x 420
• A2: 420 x 594
• A1: 594 x 841
• A0: 841 x 1189
• All sizes are measured in millimeters

How to Begin a Drawing

• Clean the drawing board and instruments with a duster
• Fix the drawing sheet on the drawing board
• Position the mini-drafter
• Draw border lines using an HB pencil
• Complete the title box using an HB pencil
• Plan the spacing of drawings/views beforehand
• Print the problem number and begin drawing

Scales

• Scales in engineering drawings are the ratio/proportion representing object size relative to its actual size
• Engineering drawings use scales to show accurate size and proportion, allowing engineers, architects, and fabricators to understand, measure, and ensure accuracy/compatibility in the final product
• The choice of scale is based on drawing sheet size, detail level, and purpose
• Scales are expressed as Representative Fraction (RF) or Ratio Scale or as Graphic Scale or Bar Scale

Metric Measurements - Length

• The base unit of length is the meter (m)
• Smaller units: centimeter (cm) and millimeter (mm)
• Larger units: kilometer (km)
• Conversions: 1 m = 100 cm, 1 m = 1,000 mm, 1 km = 1,000 m

Metric Measurements - Area

• The base unit of area is the square meter (m²)
• Smaller unit: square centimeter (cm²)
• Larger unit: hectare (ha)
• Conversions: 1 m² = 10,000 cm², 1 ha = 10,000 m²

Metric Scale

• Metric scales are used in engineering drawings following the metric system; units are meters, centimeters, millimeters, etc
• Metric scales are globally used

Inch Scale

• Inch scales are used in the imperial system, mainly in the USA; units are inches, fractions of an inch, and feet

Bar Scale or Graphic Scale

• The bar scale, or graphic scale, is printed on an engineering drawing
• It is a line/bar divided into equal segments
• Bar scales allow direct measurement without conversions or calculations

Diagonal Scale

• Diagonal scales are used for precise measurement and conversion of distances
• They accurately convert measurements between different scales/units

Constructing a Polygon Inscribed in a Circle

• A polygon with all its vertices on the circle
• Draw Circle: Use a compass with a center(OOO)
• Mark the Center: Label as OOO
• Choose Number of Sides: triangle, pentagon, hexagon, etc
• Divide the Circle: Using a protractor, divide into n equal parts
• The angle between each adjacent point is 360° / n.
• Mark Points: Mark the n points
• Connect Points: Draw straight lines between consecutive points

Constructing a Polygon Circumscribed Around a Circle

• A polygon with all its sides tangent to the circle
• Draw Circle: Use a compass with center OOO
• Mark the Center: Label the center OOO
• Choose Number of Sides
• Draw Tangents: Find the Tangency points and divide the circle into equal parts
• Construct Perpendiculars: To the circle's radius at each tangency point
• Connect Perpendiculars: This forms the polygon around the circle