Interpret and Produce Engineering Drawings and Specifications

Interpreting and Producing Technical Drawings and Specifications

Introduction to the Role of Technical Drawing in Tooling

Technical drawing is the universal language of the manufacturing shop. For the tool and die maker, the ability to interpret a drawing accurately determines the quality of the final product, the safety of operations, and compliance with customer specifications. This chapter covers the full body of knowledge required for the Red Seal exam concerning the reading, interpretation, and production of technical drawings in the context of manufacturing tooling, dies, jigs, and fixtures.

A technical drawing is not simply a graphic representation — it is a contract. Every line, every dimension, every symbol has a precise meaning. An interpretation error can result in producing a part out of tolerance, high scrap costs, or risk of injury. You must master Canadian standards, drawing conventions, and tolerancing methods.

Applicable Canadian Standards

In Canada, industrial technical drawings are governed by national standards published by the CSA Group (Canadian Standards Association). The main standards you need to know for the exam are:

These standards are harmonized with international ISO standards (International Organization for Standardization). The Red Seal exam tests your knowledge of general principles, not specific paragraph numbers. However, you must know the symbols and their meanings.

Paper Formats and Scales

Standardized Formats

Technical drawings in Canada use the A-series paper formats (ISO 216):

Scales

The scale of a drawing is the ratio between the represented dimension and the actual dimension. The standard scales are:

Golden rule: The scale must always be indicated in the title block. If a drawing is printed at a different scale than indicated, the note "DO NOT SCALE" must appear. In case of conflict between a dimension and a measurement taken from the scale, the dimension always takes precedence.

Types of Lines and Their Meaning

CSA B78.1 defines the following types of lines. You must recognize them instantly:

Exam trap: Centre lines (chain lines) must never be used to represent edges. A centre line that extends beyond the part outline must end with a dash, not a dot.

Orthographic Views and Projections

Projection System

Canada uses third-angle orthographic projection (American method, also called "third-angle projection"). The projection symbol is a truncated cone viewed from the front and side, placed in the title block.

In this projection:

Frequent error: Confusing with first-angle projection (used in Europe). The symbol in the title block is mandatory and must be checked before any interpretation.

Number of Views Required

The fundamental principle: the minimum number of views sufficient to completely define the part. In practice:

Auxiliary Views

An auxiliary view is a projection onto an inclined plane that is not parallel to the principal planes. It is used to show the true size of an inclined surface. The auxiliary view is projected perpendicular to the inclined surface.

Rule: An auxiliary view shows only one surface in true size. Other surfaces appear distorted and must not be dimensioned.

Sections and Cuts

Types of Sections

Hatching

Hatching indicates cut surfaces. It is:

Important rule: Thin parts (less than 6 mm thick) may be hatched solid black. Ribs and fillets are not hatched when cut longitudinally.

Exam Traps on Sections

Dimensioning

Fundamental Principles of Dimensioning

Dimensioning must be:

Elements of a Dimension

A dimension includes:

Specific Dimensioning Rules

Dimension Chains and Common Datums

Two main methods:

Exam rule: Chain dimensioning is to be avoided for functional dimensions. Tolerance accumulation can make the part unusable. Use common datum dimensioning for critical surfaces.

Dimensional Tolerances

ISO System of Tolerances

CSA B95 adopts the ISO system of tolerances. Tolerances are designated by a letter (position of the tolerance zone relative to the zero line) and a number (tolerance grade).

Common Fits

Fit calculation: For an H7/g6 fit with hole Ø25 H7 (tolerance +0.021/0) and shaft Ø25 g6 (tolerance −0.007/−0.020):

General Tolerances

When no tolerance is indicated on the dimension, the general tolerances from the title block apply. Typically:

Exam trap: General tolerances do not apply to reference dimensions (in parentheses), hole position dimensions, or angles. Always check the title block.

Geometrical Tolerancing (GTOL)

Geometrical tolerancing defines the form, orientation, position, and runout of part features. The symbols are standardized by CSA B78.2.

The 14 Geometrical Tolerancing Symbols

Tolerance Frame

The tolerance frame is a rectangle divided into compartments:

┌──────┬────────┬────────────┐

│ ◎ │ Ø0.05 │ A │

└──────┴────────┴────────────┘

Symbol Tolerance Datum(s)

Datums

A datum is a surface, axis, or point from which measurements are taken. Datum symbols are:

Exam rule: The datum must be an actual surface of the part, not a theoretical surface. Datums are chosen based on the function of the part (bearing surfaces, axes of rotation).

Practical Interpretation

For a true position tolerance of Ø0.05 A:

Position tolerance calculation: The cylindrical tolerance zone can be converted to equivalent rectangular tolerances:

Surface Finishes

Roughness Symbols

The basic symbol is a hook (✓). The roughness value Ra (arithmetic average roughness) is indicated above the hook.

Typical Roughness Values in Tooling

Rule: Ra roughness is measured in micrometres (μm). 1 μm = 0.001 mm. Do not confuse Ra with Rz (maximum profile height) or Rt (total profile height).

Welding Symbols

Although the tool and die maker does not commonly weld, you must be able to read welding symbols on assembly drawings. The symbols follow CSA W59.

Elements of a Welding Symbol

┌──────────┐

│ Arrow │

────────┘ └────────

▲

│

Reference line

Common Basic Symbols

Weld Dimensions

Heat Treatment and Finishing Symbols

Heat Treatments

Exam rule: The treated zone is indicated by a thick chain line (long dash-dot) around the affected surface. The hardness value is placed above the dimension line or in a note.

Surface Finishes

Production of Drawings: Procedures and Best Practices

Steps for Producing a Drawing

Title Block

The title block must contain:

Modifications and Revisions

Reading Assembly Drawings

Parts List (Bill of Materials)

The parts list is a table in the upper right corner or at the bottom of the assembly drawing. It contains:

Item Numbers

Pitfalls to Avoid

Summary

Final exam tip: When analyzing a drawing during the exam, always proceed in the same order: 1) title block (scale, projection, general tolerances), 2) front view and orientation, 3) critical dimensions and tolerances, 4) GTOL symbols and datums, 5) surface finishes and treatments. This systematic method will prevent you from missing essential information and will save you time.