Angle Grinder Near-Miss in the Fitting Shop
At 10:40 a.m. in a 12-student fitting shop, a second-year student is asked to clean weld beads on a bench-mounted bracket before the 4 p.m. dispatch. The 180 mm angle grinder is fitted with a 230 mm abrasive disc; the disc is marked MAX 6,600 rpm, while the grinder nameplate reads 8,500 rpm. The wheel guard has been removed because it 'fouled the job', and the side handle is missing. The student wears safety glasses but no face shield; a paint-thinner tin is open 1.5 m away, and the grinder cable crosses the walkway. The regular instructor is at a meeting, and a senior student tells the second-year student to continue because 'it is only a five-minute job'. You are the workshop trainee in charge of housekeeping and see the situation from the doorway.
1. List the hazards in order of how soon they could cause harm, and state the first three actions you would take before any grinding resumes.
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Rank by severity and immediacy, not by housekeeping convenience:
- Overspeed 230 mm disc on an 8,500 rpm grinder - disc burst risk, potentially fatal.
- Missing guard and side handle - fragments and kickback.
- Open paint thinner 1.5 m away - vapour ignition from sparks.
- Cable across the walkway - trip and cable damage.
- No face shield - eye and face injury.
First actions: (a) stop the grinder and unplug or isolate it, not just switch off; (b) move the student away, ventilate the area and close the thinner tin; (c) inform the instructor or supervisor and tag the grinder 'out of service'. Only then plan the job. A 4 p.m. deadline does not override a fatal-risk hazard. I would not restart until the disc, guard, handle and supervision are corrected.
2. Explain why fitting a 230 mm disc to a 180 mm grinder is unsafe even if the disc is undamaged, using the rpm and diameter figures.
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Peripheral speed is v = πDN. At 8,500 rpm with a 230 mm disc: v = π × 0.230 × 8,500/60 ≈ 102 m/s. The disc is rated 6,600 rpm, i.e. π × 0.230 × 6,600/60 ≈ 79 m/s. Running at 102/79 ≈ 1.29 times the rated speed raises centrifugal stress roughly with v², about 1.66 times the design value. 'Undamaged' only means no visible crack; the burst margin has been eaten by overspeed. The guard is also sized for a 180 mm disc and may not contain fragments from a larger one. The correct action is to fit a 180 mm disc rated at least 8,500 rpm and refit the guard and side handle.
3. Write the corrective actions you would record in a near-miss report and explain how you would decide whether to resume the job today.
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Report: date/time, location, people, machine ID, disc size and speed rating, guard/handle status, ignition sources, immediate actions, root causes and corrective actions. Root causes include no pre-use check, guard removed, missing supervision and production pressure. Corrective actions: quarantine the oversize disc; verify every grinder disc matches spindle speed and wheel size; refit guards and handles; issue face shields; move flammables to a closed cabinet; route cables overhead; require instructor sign-off before grinding; hold a toolbox talk.
Resume decision: only after a competent person confirms the machine is correct, the area is clear, PPE is worn and supervision is present. If the dispatch cannot be met safely, use a file or belt sander or renegotiate the deadline. Safety is not traded for schedule.