4.2 Cutting, Edge Preparation & Mechanisation

4.2 Cutting, Edge Preparation & Mechanisation

Quick Answer: Good welds start with good edges. Thermal cutting can leave HAZ, dross, and out-of-square bevels; mechanical prep must restore the geometry the WPS assumes. Mechanised and robotic welding demand fixtures, path accuracy, and logged parameters—inspectors verify these before and during production, not only after the bead looks smooth.

Modules WT1.11 and 1.13 connect cutting, edge preparation, and mechanisation to weld quality. Many imperfections that later appear as lack of fusion, incomplete penetration, or slag traps actually begin as bad prep or unstable automation setup. IWI-S competence includes knowing what to look for at the cut edge and what changes when a human hand no longer guides the torch.

Why Edge Preparation Matters

A WPS describes a joint design: bevel angle, root face (land), root gap, and sometimes backing. That design assumes a clean, dimensionally correct preparation. If the cut edge is ragged, oxidised, or wrong angle:

• Root passes may bridge or burn through unpredictably.
• Fusion lines may sit on oxide or dross, causing LOF.
• Required throat or penetration may never form even when current looks “normal.”
• Excessive grinding to “fix” a bad cut can thin the member or create notches that act as fatigue starters.

Pre-weld inspection (visual of prep, fit-up gauges, ITP hold points) is therefore process control, not bureaucracy.

Thermal Cutting Processes — Effects on Edges

Oxy-fuel cutting

Oxy-fuel uses a preheat flame and pure oxygen jet to oxidise and blow away steel. Typical issues:

• Heat-affected zone along the cut face—hardened or altered microstructure on carbon/low-alloy steels if cooling is severe.
• Dross (resolidified oxide/metal) clinging to the bottom edge.
• Bevel angle error and drag lines if travel speed or tip size is wrong.
• Notch-like gouges from unstable torch motion.

For weld prep, many procedures require grinding or machining off the as-cut face until bright metal and correct geometry are achieved, especially for fracture-critical or pressure applications.

Plasma cutting

Plasma arc cutting melts and ejects metal with a high-velocity plasma jet. Compared with oxy-fuel on thin/medium plate it is often faster and usable on more alloys, but edges may show:

• Nitrogen or oxygen pickup depending on plasma/shield gases—surface chemistry and porosity risk if welded without cleanup.
• Top-edge rounding and bevel from torch standoff error.
• Dross and HAZ, usually narrower than heavy oxy-fuel cuts but still real.
• Squareness out of tolerance on thick sections without bevel units or multi-pass cutting strategies.

Laser cutting

Laser cutting offers high precision and narrow kerf when parameters are correct. Residual risks still include:

• Recast layer and oxide on the cut face.
• Micro-HAZ and, on some alloys, microcracking tendency if heat input or assist gas is wrong.
• Taper and squareness errors that affect root gap consistency on precision fit-ups (critical for laser or automated GMAW root passes).

Inspector checklist after thermal cutting

1. Remove loose dross and heavy oxide before fit-up.
2. Confirm bevel angle, root face, and land within drawing/WPS tolerances (use gauges, templates, or measurement as defined).
3. Check for gouges, notches, and plate-edge laminations opened by cutting.
4. Verify any required grind depth or machining was done—not just wire-brushed paint.
5. For alloy steels, confirm preheat or hydrogen control still applies after heavy grinding if the procedure says so.
6. Document nonconforming prep before welding starts; repairing a welded bad edge is far more expensive.

Mechanical Preparation

Mechanical methods include shearing, machining, milling, grinding, and chipping. Benefits and risks:

Inspectors should treat preparation cleanliness as a consumable-adjacent control: oil, paint, moisture, and zinc coatings in the joint zone are classic porosity and crack contributors. Where standards require removal of coatings to a defined distance from the joint, measure that distance.

Fit-Up and Joint Geometry Verification

After edges are prepared, fit-up creates the actual welding geometry:

• Root gap too tight → incomplete penetration risk.
• Root gap too wide → burn-through, excessive reinforcement, or need for buttering not allowed by WPS.
• Misalignment (hi-lo) → stress concentration and incomplete root fusion.
• Excessive root face → lack of penetration on single-sided welds.
• Insufficient root face → melt-through and internal concavity.

Use the project’s acceptance standard and WPS tolerances. Record measurements when the ITP requires them. Tack welds must not introduce cracks or reduce the root opening below minimum; tacks are often ground or incorporated per procedure.

Mechanised and Robotic Welding — Inspection Implications

Mechanised welding uses equipment that moves the torch or workpiece with limited manual control (tractors, column-and-boom, positioners). Robotic welding uses programmed paths with higher degrees of freedom. Quality advantages (repeatability, parameter stability) only appear when the system is set up correctly.

Parameter logging

Automated cells often record current, voltage, wire-feed speed, travel speed, gas flow, and timestamps. For IWI-S:

• Confirm logging is enabled and calibrated where the QC plan requires objective evidence.
• Compare sampled records to WPS ranges—not to operator preference.
• Investigate spikes, stoppages, and torch collisions as potential defect locations for NDT focus.
• Remember that a logger proves electrical/motion history; it does not replace visual and NDT of the actual weld.

Path, TCP, and torch attitude

Robot path errors (wrong tool centre point, worn contact tips, bent goosenecks) produce systematic LOF, undercut, or poor toe blend along entire seams. Inspectors should:

• Witness first-article or start-of-shift trial beads when required.
• Verify torch angle, stick-out, and weave amplitude match the qualified procedure.
• Check that seam tracking sensors (if used) are functioning—failed tracking can walk the arc off the joint.

Fixtures and positioners

Clamps, jigs, and positioners control gap and distortion. Loose fixtures create variable root openings that a robot cannot “feel” like a skilled welder. Verify:

• Fixture identification matches the job.
• Clamping sequence does not introduce buckling or closed roots.
• Positioner rotation/tilt achieves the welding position stated on the WPS (position is often an essential variable).

Human factors still apply

Mechanisation does not eliminate consumable changes, liner wear, gas purity, or base-metal prep. Operators who clear torch spatter, change tips, and load wire still influence quality. Training and work instructions remain part of the quality system under ISO 3834-type regimes.

Cutting, Prep, and Mechanisation in the Inspection Plan

A practical ITP flow:

1. Material ID and cutting plan vs approved drawings.
2. Post-cut edge inspection (dross, squareness, HAZ removal as required).
3. Final prep dimensions and cleanliness hold point.
4. Fit-up and tack inspection.
5. Mechanised setup verification (WPS parameters loaded, fixture OK, trial run if specified).
6. In-process monitoring (logging review, visual of beads).
7. Final VT/NDT.

Skipping steps 2–5 and relying only on final NDT is a common shop failure mode—and a common exam scenario.

Exam Traps

• Believing plasma or laser cut edges are always weld-ready without cleanup.
• Ignoring root face and gap because “the robot will fix it.”
• Treating data-logger printouts as acceptance of the weld rather than process evidence.
• Confusing back-gouging profile quality with finished weld acceptance.
• Forgetting that positioner position must match the qualified welding position.

Bottom line: edges and automation setup are process parameters. Inspect them with the same seriousness as voltage and current.