Interpret and Apply Welding Drawings and Specifications

Interpreting and Applying Welding Drawings and Specifications

Chapter Introduction

This chapter covers one of the fundamental skills of the welding trade: reading and interpreting technical drawings, welding symbols, and specifications. In Canada, this skill is assessed in the "Drawings and Specifications" competency block of the Interprovincial (Red Seal) program. You must master not only reading symbols but also applying the Canadian standards that govern welding. This chapter prepares you to answer exam questions on identifying weld types, dimensions, processes, positions, and inspection requirements.

1. Technical Drawing in Welding

1.1 Orthographic Views and Projections

Technical drawing uses orthographic views to represent a three-dimensional object on a two-dimensional plane. The three principal views are:

In Canada, the third-angle projection (CSA B78.1 standard) is primarily used. In this projection, the right view is placed to the right of the front view, and the top view is placed above. A projection symbol (truncated cone) is typically indicated in the drawing title block.

Exam Point: You may be asked to identify the projection used. If the symbol is absent, assume third-angle projection unless otherwise indicated.

1.2 The Title Block

The title block contains essential information:

Golden Rule: On the exam, always read the title block first. It gives you the context needed to correctly interpret the symbols.

1.3 Scales and Dimensions

Dimensions are expressed in millimetres (metric system) or inches (imperial system). Canada officially uses the metric system, but the petroleum industry and some heavy industries still use the imperial system. A conversion table is sometimes provided.

Common Conversions:

Conversion Calculation: To convert inches to millimetres, multiply by 25.4. To convert millimetres to inches, divide by 25.4.

2. Welding Symbols According to CSA W59

The CSA W59 standard (Welded Steel Construction) is the primary reference in Canada for welding drawings. It adopts the principles of AWS A2.4 with Canadian adaptations.

2.1 Structure of the Welding Symbol

The complete welding symbol includes:

Arrow Rule: The arrow side (the side toward which the arrow points) is the side where the weld is made. The opposite side is called the other side (or opposite side).

2.2 Most Common Basic Symbols

Frequent Trap: The fillet weld symbol is a right triangle (not an equilateral triangle). The dimension indicated to the left of the symbol is the leg length, not the throat.

2.3 Dimensions and Their Placement

Dimensions are placed to the left of the basic symbol (size and length) and to the right (additional dimensions such as groove angle, root opening, etc.).

Example: A fillet weld symbol with "6" on the left and "50" after the symbol means: fillet weld with a 6 mm leg, 50 mm length.

Multiple Dimensions:

2.4 Supplementary Symbols

CSA W59 Rule: The flag (⚑) indicates a weld made in the shop. The absence of a flag indicates a weld made in the field.

3. Joint and Weld Types

3.1 Joint Classification

3.2 Groove Welds: Edge Preparation

Edge preparation depends on material thickness:

Included Angle Calculation: The included angle is the total angle between the two prepared edges. For a single V-groove with a 30° bevel angle per side, the included angle is 60°.

3.3 Fillet Welds: Dimensions and Strength

The leg is the distance from the joint root to the toe of the weld on each face. The throat is the perpendicular distance from the root to the weld face.

Throat-Leg Relationship: For a fillet weld with equal legs (45°), the theoretical throat = leg × cos(45°) = leg × 0.707.

Calculation Example: A fillet weld with a 10 mm leg has a theoretical throat of 10 × 0.707 = 7.07 mm.

CSA W59 Requirement: The minimum effective throat for a fillet weld is determined by the thickness of the thinner part. For 6 mm steel, the minimum throat is typically 4 mm (which corresponds to a leg of approximately 5.7 mm).

4. Welding Positions and Their Symbols

4.1 Positions According to CSA W59 / ISO 6947

Exam Trap: The old Canadian symbols (1G, 2G, 3G, 4G) are still used in industry. The correspondence table is as follows:

5. Reading Specifications and Standards

5.1 Typical Content of a Welding Specification

A welding specification contains:

5.2 The WPS: Essential Variables

The WPS specifies:

Exam Rule: Essential variables of the WPS cannot be changed without requalification. Non-essential variables can be changed without requalification. Supplementary essential variables (for impact testing) require requalification if changed.

5.3 Canadian Reference Standards

Key Point: CSA W59 requires that welders be qualified according to CSA W47.1. This qualification is company-specific (each company must qualify its own welders).

6. Inspection and Testing Symbols

6.1 Non-Destructive Testing (NDT) Symbols

6.2 Placement of NDT Symbols on Drawings

NDT symbols are placed on the reference line, in the same manner as welding symbols. They indicate:

Example: An RT symbol placed below the reference line with "100%" indicates complete radiographic testing of the weld.

7. Practical Calculations for the Exam

7.1 Calculating Required Weld Length

For a fillet weld subjected to a shear load:

Formula: Required length (L) = Load (P) ÷ (Allowable stress × Effective throat)

Example: Load of 100 kN, allowable stress of 124 MPa, effective throat of 7 mm.

L = 100,000 N ÷ (124 × 10⁶ N/m² × 0.007 m) = 100,000 ÷ 868,000 = 0.115 m = 115 mm

7.2 Calculating Volume of Deposited Metal

For a fillet weld with equal legs:

Volume (mm³/mm) = (leg² × 0.5) × length

Example: Leg of 8 mm, length of 500 mm.

Volume = (8² × 0.5) × 500 = (64 × 0.5) × 500 = 32 × 500 = 16,000 mm³ = 16 cm³

7.3 Calculating Dilution

Dilution is the percentage of base metal in the deposited weld metal:

Dilution (%) = (Area of melted base metal ÷ Total weld bead area) × 100

This calculation is rarely asked on the exam, but understanding the concept is essential for questions on processes and chemical composition.

8. CSA W59 Rules to Know

8.1 General Requirements

8.2 Minimum Fillet Weld Sizes (CSA W59, Table 5.1)

Trap: These dimensions are minimums. The drawing may specify larger dimensions, but never smaller than these minimums, unless otherwise indicated by the designer.

8.3 Deformation Tolerances (CSA W59, Table 8.1)

9. Interpreting Fabrication Drawings

9.1 Bill of Materials

The bill of materials lists:

9.2 General Drawing Notes

General notes contain information that applies to the entire drawing:

Example of a Typical Note: "All welds must be made in accordance with CSA W59. Fillet welds must have a minimum throat of 5 mm unless otherwise indicated."

9.3 Enlarged Weld Details

Enlarged details show complex areas at a larger scale. They include:

10. Traps to Avoid

Here are the most frequent errors on the Red Seal exam on this topic:

11. Summary

12. Self-Assessment Questions

Answer: The weld is made on the side opposite the arrow (symbol above the line). The leg is 8 mm.

Answer: 12 × 0.707 = 8.48 mm.

Answer: 30° per side (60° ÷ 2 = 30°).

Answer: 5 mm (Table 5.1, range > 6 to 12 mm).

Answer: The weld must be made in the shop, not in the field.

13. Strategic Exam Tips

This chapter covers all the knowledge required for the "Interpretation and Application of Welding Drawings and Specifications" section of the Red Seal exam. Review regularly, practice with real drawings, and consult CSA W59 for further details.