Study guide: Mechanical Technical Drawing

📋 Course Outline

1. Purpose and Standard Drawing Formats
2. Drawing Layout and Documentation
3. Orthographic Projection
4. Line Types and Drafting Conventions
5. Specific Views and Breaks
6. Cross-Section Methods
7. Threads and Fasteners
8. Intersections and Dimensioning

📖 1. Purpose and Standard Drawing Formats

🔑 Key Concepts & Definitions

• Engineering Drawing: A technical drawing that clearly defines and communicates a design to interested parties.

★ Must-know

• The standard A-series sizes and dimensions are:
  • A5: 210 × 148 mm
  • A4: 297 × 210 mm
  • A3: 420 × 297 mm
  • A2: 594 × 420 mm
  • A1: 841 × 594 mm
  • A0: 1189 × 841 mm

Further detail

• Technical drawings communicate designs to colleagues, suppliers, and customers; support part manufacture and mechanism assembly; and record new ideas.

📖 2. Drawing Layout and Documentation

🔑 Key Concepts & Definitions

• Nomenclature: A complete list of the elements that constitute a mechanism.

★ Must-know

• A title block contains:
  • project or part name or reference
  • description
  • scale and unit
  • projection type
  • drafter’s name
  • company information
  • material for machined parts

Further detail

• The title block is situated in the lower-right corner of the drawing sheet.

• Grid references start at the top left of the sheet, with letters running vertically downward and numbers running horizontally from left to right.

• A nomenclature lists:
  • element number
  • description, name, or reference
  • quantity
  • sometimes the supplier

📖 3. Orthographic Projection

🔑 Key Concepts & Definitions

• Orthographic Projection: A geometric projection onto a plane using projectors that are parallel to one another and perpendicular to the projection plane.

★ Must-know

• 🔄 To project a part:

  1. Choose the most informative view as the front view
  2. Draw the general shape of this view
  3. Project the different views using straight lines at 45°
  4. Complete the views with all secondary elements

• A single projection is insufficient to describe an object completely, so multiple views are used to define it without ambiguity with a minimal number of views.

📌 In first-angle projection, the left view is placed to the right of the front view; in third-angle projection, the left view is placed to its left.

Further detail

• First-angle projection is mainly used in Europe and is described as the most used worldwide, while third-angle projection is mainly used in the USA.

• The projection planes produce the front view on P1, the top view on P2, and the right-hand view on P3.

💡 Memory Hook

First angle places the left view on the right; third angle places it on the left.

📖 4. Line Types and Drafting Conventions

★ Must-know

• Line precedence is: object lines over hidden lines, object lines over centrelines, hidden lines over centrelines, cutting-plane lines over centrelines in sectioning

• Continuous thick lines show:

  • visible edges and outlines
  • section outlines
  • arrows indicating observation directions

• Dashed thin lines show hidden edges and outlines, including hidden internal and external threads.

Further detail

• Continuous thin lines show:

  • imaginary edges
  • hatching
  • dimension and extension lines
  • folded-up section outlines
  • visible bottoms of internal threads
  • local hand-drawn sections

• Mixed line types show: axes, symmetry planes, primitive gear lines, nearby-part outlines with two-dash mixed lines, extreme positions of moving parts with two-dash mixed lines

📖 5. Specific Views and Breaks

★ Must-know

• An auxiliary view shows the true size and shape of an inclined surface that is not parallel to any of the six principal views, using a viewing plane perpendicular to that surface.

Further detail

• A partial view replaces a complete view to save space or simplify a drawing, and a break line marks where the view is interrupted.

• An enlarged view shows a feature in greater detail, usually at an enlarged scale, with the enlarged portion enclosed by a sketch such as a circle or another closed contour.

• A local view shows only features from a main view that need clarification.

📌 Wavy break lines represent short breaks, while jagged break lines represent long breaks; other styles are used for breaks in rods and tubes.

• Phantom lines indicate a part’s range of movement or repetitive detail, while thin continuous imaginary lines that stop before the part edges represent tangent edges.

📖 6. Cross-Section Methods

🔑 Key Concepts & Definitions

• Sectional View: A view showing the interior construction of an object as if a portion had been removed by an imaginary cutting plane.

★ Must-know

📌 Section hatching uses thin, equally spaced lines, normally inclined at 45°; the spacing is 1.5–2 mm, and 30° or 60° hatching is used when 45° would run parallel to an outline or axis.

📌 A full section cuts completely through an object, whereas a half section is used for a symmetrical object.

Further detail

• A section is indicated with thick lines, a thick cutting-plane line, arrows showing the projected direction, and the same two-letter name on both the section view and the view showing the cutting plane.

• A local or broken-out section exposes a small area to clarify an interior feature using a short break line.

• Revolved sections are turned into position and interposed on the same view, while removed sections are placed clear of the main outline and show only the cut surface.

📖 7. Threads and Fasteners

🔑 Key Concepts & Definitions

• Thread: A helical groove formed on the outside or inside of a cylinder and derived from an inclined plane, used to hold parts together, move parts relative to one another, or transmit power.

★ Must-know

📌 Thread pitch is the axial distance between corresponding points on adjacent threads, whereas lead is the axial distance moved by a nut or bolt in one complete revolution.

📌 A right-hand thread screws onto a bolt when the nut is turned clockwise, whereas a left-hand thread screws off when turned clockwise.

Further detail

• An internal thread is made by first drilling a hole and then tapping it with a machining tool; an external thread is cut using a die or a lathe.

• Common thread types include:

  • V thread
  • square thread
  • Acme thread
  • buttress thread
  • rolled thread

• Thread terminology includes:

  • crest: outermost part
  • root: innermost part
  • flank: surface between crest and root
  • thread angle: angle between flanks in an axial plane
  • depth: crest-to-root distance measured perpendicular to the axis
  • nominal diameter: diameter of the cylinder on which the thread is cut
  • major diameter: diameter at the crest
  • minor diameter: diameter at the root or core

• ISO 965-1 defines metric threads, which have a 60° thread angle and are designated by nominal outside diameter and pitch.

💡 Memory Hook

External threads are on bolts or screws; internal threads are in nuts or tapped holes.

📖 8. Intersections and Dimensioning

★ Must-know

📌 Dimension lines must not serve as dimension features or coincide with other dimension lines; place short dimensions close to the model and long dimensions farther away, avoiding crossings where possible.

• Dimensioning rules include:
  • Do not dimension to hidden or dashed lines
  • Use cross-sections or local sections when needed
  • Distribute dimensions around the drawing at a constant distance
  • Group dimensions for the same detail
  • Avoid redundant dimensions

📌 Always show the part’s total external dimensions, dimension hole position from its axis rather than its edges, and indicate hole diameters.

Further detail

📌 For small dimensions, place the dimension number outside the extension lines and invert the arrows.