Glass Cutting and Fabrication

Glass Cutting and Fabrication

Chapter Introduction

Glass cutting and fabrication are at the heart of the glazier trade. This chapter covers all the technical skills required for the "Cutting and Fabrication" component of the Red Seal exam. You will find the physical principles of controlled fracture, tools and equipment, cutting procedures for different types of glass, dimension calculations, and applicable Canadian standards. Each section is designed to be directly applicable in the workshop and on the job site.

Fundamental Principles of Glass Cutting

Controlled Fracture: The Score Line

Cutting glass is not "cutting" in the literal sense, but rather creating a surface tension crack using a carbide tungsten or diamond wheel. This crack, called a score line, locally weakens the glass. Separation then occurs through flexing or tapping along this line.

The physical principle is based on Griffith's law: the crack propagates when the energy released by deformation exceeds the surface energy of the material. For standard 6 mm float glass, the score depth should be approximately 0.1 to 0.2 mm. A score that is too deep weakens the edge; a score that is too shallow does not allow for a clean break.

Factors Affecting Score Quality

Rule of thumb: for 3 to 6 mm glass, pressure should be approximately 2 to 3 kg. For 10 to 12 mm glass, increase to 4 to 5 kg. The wheel must be held perpendicular to the cutting plane, with a slight 5° inclination in the direction of travel.

Types of Glass and Cutting Behaviour

Cutting Tools and Equipment

The Cutting Wheel

The cutting wheel is the primary tool. It comes in several diameters and angles:

The wheel must be mounted on a cutting handle (pencil style) or on a cutting straightedge with a wheel head. Professional cutters often use a handle with an integrated oil reservoir to automatically lubricate the score line.

The Cutting Straightedge

The straightedge must be made of aluminum or steel, with a strip of cork or rubber on the underside to prevent slipping and protect the glass. The width of the straightedge must be deducted from measurements: if the wheel is offset by 10 mm from the edge of the straightedge, you must add or subtract this value when marking out.

Positioning formula:

Straightedge position = desired dimension − wheel offset

Example: for a 500 mm piece with a 10 mm offset, the straightedge is placed at 490 mm from the reference edge.

Cutting Tables and Work Surfaces

The cutting table must be perfectly flat, covered with a felt or cork mat. Common dimensions are 2.5 m × 3.0 m for fixed tables, with portable tables for job sites. The table must be equipped with a stop (fence) to align the glass and ensure parallel cuts.

Other Essential Tools

Cutting Procedures

Straight Cut of Float Glass

Cutting Laminated Glass

Cutting Shaped Glass (Arc, Circle)

Cutting Tempered Glass: Strictly Prohibited

Tempered glass cannot be cut, drilled, or ground after tempering. Any attempt results in fragmentation into small pieces. Cutting and machining must be done on annealed glass before heat treatment. This rule is absolute and is the subject of frequent exam questions.

Dimension and Clearance Calculations

Installation Clearance (Fabrication Tolerance)

Glass must be cut smaller than the frame opening to allow for thermal expansion and fabrication tolerances. Standard values are:

General formula:

Glass dimension = opening dimension − (2 × clearance per side)

Example: 1000 mm × 800 mm opening, 3 mm clearance per side → glass = 994 mm × 794 mm.

Area and Weight Calculation

Area (m²) = length (m) × width (m)

Weight (kg) = area (m²) × thickness (mm) × 2.5

The factor 2.5 represents the density of glass (2,500 kg/m³).

Example: 1.2 m × 0.8 m glass, 6 mm thick → area = 0.96 m² → weight = 0.96 × 6 × 2.5 = 14.4 kg.

Calculation for Laminated Glass

The weight of laminated glass is the sum of the weights of the panes + the weight of the PVB (approximately 0.5 kg/m² per mm of PVB thickness).

Example: 6 mm + 6 mm + 0.76 mm PVB → weight = (6 + 6) × 2.5 + 0.76 × 0.5 ≈ 30.4 kg/m².

Cutting Tolerances

According to industry standards (CAN/CGSB-12.1), cutting tolerances for float glass are:

These tolerances apply to the finished dimension, after edge smoothing.

Glass Fabrication and Processing

Cutting and Machining

Fabrication includes cutting, beveling, drilling, and edge shaping. Glass drilling is done using carbide or diamond bits, with water lubrication. Holes must be at a minimum distance of 6 times the glass thickness from the edge (rule of thumb). For 6 mm glass, a 10 mm hole must be at least 36 mm from the edge.

Edges and Finishes

Thermal Tempering

Tempering involves heating glass to approximately 620–650 °C, then rapidly cooling it with air jets. This creates compressive stress on the surface and tensile stress in the core. Tempered glass resists approximately 4 to 5 times more than annealed glass of the same thickness. It fragments into small, non-cutting pieces (CAN/CGSB-12.1 standard).

Lamination

Laminated glass is composed of two or more glass panes bonded together by one or more PVB (polyvinyl butyral) films. Assembly is done under pressure and heat (autoclave at 130–150 °C, pressure of 10–12 bars). Laminated glass retains its fragments upon breakage, making it mandatory for skylights, guardrails, and safety glazing.

Solar Control and Coated Glass

Coated glasses (low-E, solar) are manufactured by sputter coating or pyrolysis. The coating is deposited on a specific face. When cutting, the wheel must always be applied to the untreated face. The coated face can be identified using a coating detector or by its tint. The coating must face toward the inside of the air space in an insulating glass unit.

Applicable Canadian Standards

CAN/CGSB-12.1 — Flat Glass

This standard defines safety requirements for flat glass in buildings. It specifies the types of glass permitted for various applications (doors, windows, showers, guardrails). Safety glass (tempered, laminated) is mandatory in hazard areas: doors, side panels, showers, large partitions.

CAN/CGSB-12.2 — Safety Glass

This standard specifically covers tempered and laminated safety glass. It defines impact resistance criteria and test methods (ball drop test, sandbag test).

National Building Code (NBC)

The NBC, in Part 9 (housing) and Part 4 (structures), requires the use of safety glass in hazardous areas. Glazing located less than 900 mm from the floor, doors, and shower enclosures must be tempered or laminated glass.

CSA B149.1 — Natural Gas and Propane Code

Although this code does not directly concern glass cutting, it applies to glazing adjacent to gas appliances. Minimum distances between a burner and glazing are specified. The glazier must verify these distances when installing glass near gas equipment.

Rule 8-200 of the Canadian Electrical Code, Part I (CE Code)

This rule concerns minimum distances between electrical conductors and glazed surfaces. It applies to glazing located near power lines. The glazier must ensure that installed glass does not reduce safety clearances.

Workshop Fabrication Procedures

Reading Plans and Specifications

The glazier must read architectural and structural plans to determine exact glazing dimensions. Plans indicate glass types, thicknesses, treatments (tempering, lamination, coating), and fastening systems. Any discrepancy between the plan and actual site conditions must be reported before cutting.

Templates and Gauges

For complex shapes, templates made of plywood or rigid cardboard are used. The template is traced onto the glass with a fine-point marker. The cut follows the template with a 1 to 2 mm offset for installation clearance.

Quality Control

After cutting, each piece must be checked:

Safety Glass Marking

Tempered and laminated glass must bear a permanent mark indicating the manufacturer, the standard (CAN/CGSB-12.2), and the glass type. This mark is applied by silkscreening or etching before tempering. It cannot be removed.

Pitfalls to Avoid

Summary

Exam Tips