Interpret and Prepare Drawings and Specifications

Interpreting and Preparing Drawings and Specifications

Introduction to the Role of Drawings and Specifications in Metal Fabrication

The metal fabricator works from assembly drawings, detail drawings, and specifications that define the geometric, dimensional, material, and procedural requirements of the part to be fabricated. The ability to read, interpret, and prepare these documents is a core competency assessed on the Red Seal exam. An error in interpreting a dimension or a welding symbol can result in part rejection, additional costs, and safety risks.

This chapter covers the essential elements: types of drawings, orthographic projections, dimensions and tolerances, welding symbols per CSA W59, specifications, mass and volume calculations, as well as common exam pitfalls.

Types of Drawings and Their Use

Assembly Drawing

The assembly drawing (or erection drawing) shows the complete part with all its components assembled. It indicates overall dimensions, relative positions of elements, references to detail drawings, and often the bill of materials (parts list). The metal fabricator uses it to understand the function of the assembly and the sequence of assembly.

Detail Drawing

The detail drawing represents a single part with all its dimensions, tolerances, material specifications, and finishes. It is the primary reference document for fabricating an individual component. Each part is identified by an item number that corresponds to the bill of materials on the assembly drawing.

Development Drawing

The development drawing (or flat pattern layout) shows the surface of a part unfolded flat, before bending or forming. It is essential for cutting sheet metal intended to be bent, rolled, or formed. Bend lines are indicated by thin phantom lines with the notation "bend line" or by specific symbols.

Schematics and Sketches

Schematics are simplified representations (pictorial diagrams, wiring diagrams, flow diagrams) that are not to scale but show functional relationships. Sketches are freehand drawings used in the shop to quickly communicate an idea. They are not contractual but serve as support for fabrication.

Orthographic Projections

Principle of Orthogonal Projection

Orthogonal projection (or multi-view projection) represents a three-dimensional object through a series of flat views at 90° to each other. Each view shows two dimensions: the front view (height and width), the top view (width and depth), and the side view (height and depth).

Projection System: First and Third Angle

Two conventions exist: first-angle projection (used in Europe) and third-angle projection (used in North America, including Canada). In third angle, the top view is placed above the front view, and the right-side view is placed to the right of the front view. The projection symbol (a truncated cone) is indicated in the drawing title block. On the exam, always check this symbol before interpreting the views.

Auxiliary Views and Sectional Views

Auxiliary views are used to show an inclined surface without distortion. They are projected perpendicular to the inclined surface. Sectional views (Section A-A, Section B-B) show the interior of a part by imaginarily cutting through it. Hatching (fine lines at 45°) indicates the cut surfaces. Cutting plane lines are indicated by letters and arrows on the principal view.

Dimensioning and Tolerances

Dimensioning Principles

Dimensioning must be complete, clear, and unambiguous. The fundamental rules:

Each dimension is indicated only once on the drawing.

Dimensions are placed on the view that best shows the dimension.

Dimension lines are thin, terminated with arrows or oblique strokes.

Dimensions are expressed in millimetres (mm) or inches (in.), according to the title block indication.

Functional dimensions (those that affect assembly) take priority.

Dimensional Tolerances

Tolerance is the permissible variation between the nominal dimension and the actual dimension. It can be indicated in three ways:

31.Bilateral tolerance: 50.00 ± 0.05 mm

32.Unilateral tolerance: 50.00 +0.00 / -0.05 mm

33.General tolerance: indicated in the title block (e.g., ± 0.5 mm for dimensions without specific tolerance)

Geometric Tolerances (GD&T)

Geometric tolerances (Geometric Dimensioning and Tolerancing) define the form, orientation, position, and runout of features. The main symbols:

Symbol	Meaning	Symbol	Meaning
⏣	True position	∥	Parallelism
⌓	Circularity	⊥	Perpendicularity
⌭	Cylindricity	∠	Angularity
⏥	Flatness	⌀	Diameter
◎	Concentricity	R	Radius
⌯	Symmetry	S R	Spherical radius

The feature control frame contains: the geometric symbol, the tolerance value, and the datum reference if applicable. Example: ⌖ 0.1 A means the true position of the feature must be within a circle of 0.1 mm diameter relative to datum A.

Basic Dimensions and Reference Dimensions

Basic dimensions are enclosed in a rectangular frame and represent the exact theoretical dimension. They are used with geometric tolerances. Reference dimensions are indicated in parentheses (50.0) and are not controlled; they serve for information only.

Welding Symbols per CSA W59

The CSA W59 standard (Welded Steel Construction) is the Canadian reference for welding steel structures. It is based on the AWS A2.4 welding symbols. Mastery of these symbols is essential for the exam.

Structure of the Welding Symbol

The welding symbol consists of:

The reference line (horizontal line).

The arrow that points to the joint.

The basic symbol (weld type) placed on the reference line.

The dimensions (size, length, pitch) placed to the left of the basic symbol.

The supplementary symbols (contour, process, finish) placed above or below.

Arrow Rule

If the basic symbol is below the reference line, the weld is on the same side as the arrow (arrow side).

If the basic symbol is above the reference line, the weld is on the opposite side of the arrow (other side).

If the symbol is on both sides of the line, the weld is on both sides of the joint.

Common Basic Symbols

Symbol	Weld Type	Symbol	Weld Type
⌒	Fillet weld	⌵	V-groove weld
⌓	Square-groove weld (full penetration)	⌶	U-groove weld
⌷	Single-V groove weld	⌸	Double-V groove weld
⌹	Single-U groove weld	⌺	Double-U groove weld
⌻	J-groove weld	⌼	Double-J groove weld
⌽	Spot weld	⌾	Projection weld

Fillet Weld Dimensions

For a fillet weld, the size (throat or leg) is indicated to the left of the symbol. The Canadian convention uses the throat as the primary dimension, expressed in millimetres. The weld length is indicated after the size, separated by a dash. Example: 6 × 50 means a 6 mm throat over a 50 mm length.

Intermittent Weld

The intermittent weld (segmented) is indicated by two numbers: the length of each segment and the pitch (centre-to-centre distance). Example: 6 × 75 - 150 means a 6 mm throat, 75 mm segments spaced 150 mm apart (centre to centre).

Finish and Contour Symbols

Flat contour: a horizontal line above the basic symbol.

Convex contour: a convex arc above the symbol.

Concave contour: a concave arc above the symbol.

Ground finish: the letter G (grinding).

Machined finish: the letter M (machining).

Groove Welds and Penetration

Complete joint penetration (CJP) welds are indicated by the corresponding basic symbol (V, U, J) with the notation "CJP" or by a square-groove symbol with the notation "full penetration." Partial joint penetration (PJP) welds are dimensioned by the depth of penetration indicated to the left of the symbol.

Specifications

Content of a Specification

The specification is a written document that supplements the drawings. It specifies:

Materials: steel grade (e.g., ASTM A36, CSA G40.21 350W), heat treatment, coating.

Permitted welding processes (SMAW, GMAW, FCAW, GTAW) and consumables (E7018 electrodes, ER70S-6 wire).

Quality requirements: acceptance level (CSA W59, AWS D1.1), inspection methods (visual, radiographic, ultrasonic).

Fabrication tolerances: allowable distortion, flatness, perpendicularity.

Finish requirements: pickling, primer, paint, marking.

Canadian Reference Standards

Standard	Title	Application
CSA W59	Welded Steel Construction	Structural frames, bridges, buildings
CSA W47.1	Certification of Welding Companies	Company qualification
CSA W186	Welding of Reinforcing Bars in Reinforced Concrete Construction	Reinforcing bars
CSA G40.20/G40.21	Structural Quality Steels	Steel grades
ASTM A36	Specification for Carbon Structural Steel	General steel
ASTM A500	Cold-Formed Welded and Seamless Carbon Steel Structural Tubing	Square and rectangular tubing
ASTM A53	Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless	Piping

Reading a Specification: Critical Points

On the exam, you will often be asked to identify a specific requirement from a specification. The critical points to check:

The steel grade and its mechanical properties (yield strength, tensile strength).

The type of electrode or welding wire required.

Fabrication tolerances (e.g., maximum flatness deviation of 3 mm per metre).

Preheat and interpass temperature requirements.

Testing methods and acceptance criteria.

Mass, Volume, and Dimension Calculations

Density of Common Steels

The density of carbon steel is 7,850 kg/m³ (0.284 lb/in³). For stainless steel, it is approximately 8,000 kg/m³. On the exam, these values are provided or must be known.

Calculating the Mass of a Plate

Mass (kg) = Length (m) × Width (m) × Thickness (m) × 7,850

Example: a plate of 2 m × 1 m × 12.7 mm (0.0127 m):

Mass = 2 × 1 × 0.0127 × 7,850 = 199.4 kg

Linear Mass of Structural Shapes

Structural shapes (beams, angles, tubes) are specified by their linear mass (kg/m). Example: a W200×27 beam has a mass of 27 kg/m. For a 6 m length, the mass is 162 kg.

Calculating the Volume of a Cylinder

Volume (m³) = π × (D/2)² × L

Where D is the outside diameter and L is the length. For a tube, subtract the interior volume:

Volume = π × [(D/2)² - (d/2)²] × L

Where d is the inside diameter.

Calculating the Developed Length of a Bend

For sheet metal bent at 90°, the developed length (flat length) is the sum of the straight section lengths plus the bend allowance. The simplified formula for a 90° bend:

Developed length = L1 + L2 + (π/2) × (r + k × t)

Where:

L1 and L2 are the lengths of the straight sections measured to the bend line.

r is the inside bend radius.

t is the sheet thickness.

k is the neutral axis position factor (approximately 0.33 for r/t < 2, 0.5 for r/t > 2).

Bend Reduction Factor

The K-factor is the ratio of the distance from the neutral axis to the inside face divided by the thickness. For mild steel, it ranges from 0.33 to 0.5 depending on the r/t ratio. On the exam, you will often be given the K-factor or the bend allowance directly.

Interpreting Structural Steel Drawings

Structural Symbols

Structural drawings use specific conventions:

Beams are designated by their profile (W, S, C, L, HSS) and dimensions.

Connections are indicated by symbols (bolts, welds, plates).

Marks identify each piece (e.g., B1, C2, G3 for beam, column, girt).

Erection Drawings and Shop Drawings

The erection drawing shows the complete assembly on site. The shop drawing shows individual pieces with all dimensions necessary for shop fabrication. The metal fabricator works primarily with shop drawings.

Erection Tolerances

The CSA S16 standard (Design of Steel Structures) and the National Building Code define erection tolerances. Typical tolerances:

Flatness: 3 mm per metre for bearing surfaces.

Plumbness: 1/500 of the height for columns.

Alignment: ± 3 mm for beam alignments.

Drawing Preparation Procedures

Steps for Preparing a Shop Sketch

123.Analyze the assembly drawing and specification.

124.Identify missing or ambiguous dimensions.

125.Calculate developed dimensions for bent parts.

126.Draw the sketch to scale or in isometric perspective.

127.Indicate all dimensions, tolerances, and welding symbols.

128.Verify consistency with the specification and standards.

Conformity Verification

Before starting fabrication, the metal fabricator must verify:

That the specified material is available and conforms (material certificate).

That the drawing dimensions are achievable with the available equipment.

That the tolerances are compatible with the fabrication processes.

That the welding symbols are understandable and achievable.

Pitfalls to Avoid

136.Confusing the arrow side and the other side: the welding symbol below the reference line means the weld is on the arrow side. An inverted interpretation error is common.

137.Ignoring the projection symbol: a first-angle (European) drawing interpreted as third-angle (North American) gives inverted views. Always check the title block.

138.Forgetting general tolerances: if a dimension has no specific tolerance, the general tolerance from the title block applies. Do not neglect it.

139.Confusing the throat and leg of a fillet weld: the throat is the distance from the root of the joint to the face of the weld; the leg is the dimension from the side. The throat is smaller than the leg (throat = leg × cos 45° ≈ 0.707 × leg).

140.Not accounting for the K-factor in bend calculations: the neutral axis is not at the centre of the thickness for small radii. Use the provided K-factor.

141.Using the wrong density: stainless steel (8,000 kg/m³) does not have the same mass as carbon steel (7,850 kg/m³). Check the material.

142.Interpreting an inch dimension as millimetres: check the unit indicated in the title block. An error by a factor of 25.4 is catastrophic.

143.Forgetting reference dimensions: dimensions in parentheses are not controlled. Do not use them for fabrication.

144.Confusing weld symbols: the V-groove symbol (⌵) and the U-groove symbol (⌶) are similar. A reading error completely changes the joint preparation.

145.Not checking the specification for preheat requirements: the drawing may indicate a weld, but the specification may require a specific preheat depending on the thickness and steel grade.

Summary

Assembly drawings show the complete assembly; detail drawings show each individual piece with all its dimensions.

Third-angle projection is the North American convention. Check the projection symbol in the title block.

Tolerances can be bilateral, unilateral, or general. Geometric tolerances (GD&T) control form, orientation, and position.

Welding symbols per CSA W59 follow the AWS A2.4 structure: reference line, arrow, basic symbol, dimensions.

The symbol below the reference line = arrow side; above = other side.

Specifications detail materials, processes, consumables, and quality requirements. They supplement the drawings.

The density of carbon steel is 7,850 kg/m³. Mass calculations use dimensions in metres.

The K-factor is essential for calculating the developed length of a bent part.

Erection tolerances (flatness, plumbness, alignment) are defined by CSA S16 and the National Building Code.

Systematic verification of drawings and specifications before fabrication is a mandatory step to avoid costly errors.

On the exam, read each question twice, check the units, the projection symbol, and the position of the welding symbol relative to the reference line. These simple checks will help you avoid the majority of errors.