CNC Cutting (Computer Numerical Control) represents the bridge between digital architectural models and physical glazing elements. While standard cutting tables are optimized for rectilinear execution, CNC fabrication allows for the production of non-orthogonal shapes, complex curves, and intricate internal cutouts with sub-millimeter repeatability.

Architectural Application

For the façade consultant and architect, CNC cutting is the enabling technology for frameless glass assemblies, spider-fittings requiring countersunk holes, and interlocking glass fins. It ensures that the perimeter of the glass and any internal penetrations align perfectly with hardware tolerances, eliminating the need for hazardous and inaccurate on-site adjustments.

Technical Principles

CAD/CAM Integration

The process begins with the direct import of CAD data (typically .DXF or .DWG files) into Computer-Aided Manufacturing (CAM) software. This software generates the G-code that controls the movement of the cutting head along multiple axes (X, Y, Z, and often C for rotation). This eliminates manual translation errors and ensures the physical pane matches the digital model.

Subtractive Manufacturing & Tooling

Unlike simple scoring wheels used on float lines, heavy-fabrication CNC glass centers often utilize diamond-impregnated tooling. The process involves:

1. Milling/Grinding: Using rotating diamond bits to grind away material for notches or internal cutouts.
2. Drilling: Creating precise holes for point-fixed glazing systems.
3. Cooling & Lubrication: A continuous stream of coolant prevents thermal shock and flushes away glass fines (swarf), which is critical to maintaining edge strength.

Stress Management

A critical function of the CNC process is the management of surface tension. When glass is cut or drilled, micro-fractures are introduced. Advanced CNC machinery includes polishing stages that refine these raw edges. By smoothing the edge profile, the manufacturer reduces the potential for stress concentrations (stress risers) that could lead to spontaneous breakage during the tempering process or under wind load.

Specification Notes

Managing Re-entrant Corners

A frequent failure point in CNC-fabricated glass is the specification of sharp, 90-degree internal corners (re-entrant corners) for notches or hinges.

• The Risk: Sharp internal corners create severe stress concentrations where cracks will initiate during the tempering quench or under thermal load.
• The Specification: Always specify a fillet radius for internal corners. At Hexad, we require a minimum radius equal to the thickness of the glass (e.g., 10mm glass requires a ≥10mm radius) to distribute stress, though a larger radius is always structurally superior.

Edge Finish & Tolerances

Inaccurate CNC setups can lead to “stepped” edges or rough grinding marks that weaken the panel.

• The Risk: A rough ground edge has a significantly lower allowable design strength than a polished edge due to micro-flaws.
• The Specification: Clearly define the required edge finish (e.g., ASTM C1048 flat polish). Require that CNC tolerances for hole locations and overall dimensions be held tighter than standard industry allowances—typically ±1.0mm for high-end structural applications, rather than the standard ±1.6mm or ±3.0mm often found in generic commodity glass.

Anisotropy and Heat Soak Implications

While CNC cutting occurs prior to heat treatment, the quality of the cut dictates the success of the tempering process.

• The Risk: Poorly finished CNC cutouts can cause breakage in the heat soak oven. While this filters out bad glass, it impacts project lead times.
• The Specification: Ensure the manufacturer conducts an edge quality audit prior to tempering. Hexad mitigates lead-time risks by utilizing automated optical scanning to detect edge chips or hairline fissures at the CNC stage, rejecting the glass before value-added processing (coating/laminating) occurs.