Bench Grinder: Cracked Wheel and Loose Clothing
A school workshop has a 150 mm bench grinder rated at 2950 rpm, bolted to a bench that rocks slightly when leaned on. A learner is sharpening a wood chisel. The tool rest is set about 6 mm from the wheel and the tongue guard about 8 mm. The wheel has a 20 mm flat worn area and a small crack near the bore; the water cup is empty. One learner is wearing loose gloves and a hoodie with a drawcord; another is grinding on the side of the wheel because the front face is worn. The grinder label states a maximum wheel speed of 32 m/s. A 200 mm wheel is stored nearby, and someone suggests fitting it because it will last longer. Goggles are on a hook; one learner has pushed them onto his forehead to see the edge better. The workshop technician is away and the class teacher asks you to decide whether work can continue.
1. Identify the hazards in this workshop and rank them by risk. Explain the reasoning behind your ranking, rather than just listing the hazards.
Read the worked answer
Ranking by risk = severity × likelihood, not by how obvious the hazard is.
- Cracked wheel (highest). A 150 mm wheel at 2950 rpm stores enough energy to cause fatal or serious injury if it bursts. Likelihood is low if the crack is seen, but severity is catastrophic, so work must stop.
- Loose gloves, drawcord and hoodie. Entanglement with a rotating wheel or spindle can pull a hand into the wheel. Likelihood is high because the learner is already working; severity is severe.
- Side grinding and the 20 mm flat. Side loading and an out-of-round wheel increase the chance of wheel fracture; the wheel is already worn, so likelihood is moderate-to-high.
- Tool rest at 6 mm and tongue guard at 8 mm. Gaps allow the workpiece or fingers to be drawn into the wheel. Likelihood moderate; severity serious.
- Goggles removed or on the forehead. High likelihood of eye injury from sparks and fragments; severity moderate but irreversible.
- Rocking bench and empty water cup. Lower immediate risk, but the bench movement affects control and the dry wheel overheats and softens.
A cracked wheel is not a 'monitor' item: it is an immediate stop. The loose clothing is also an immediate stop because the learner is at the machine.
2. Recommend control measures in the correct hierarchy of control. State clearly which items require work to stop immediately.
Read the worked answer
Apply the hierarchy in order.
Elimination/substitution: Do not use the cracked wheel; dispose of it and fit a correct replacement wheel of the same size and grade, rated at least 32 m/s. Do not fit the 200 mm wheel unless the machine guard, spindle speed and wheel rating are all checked and approved - a larger wheel at the same rpm has a higher peripheral speed and may not clear the guard.
Engineering controls: Set the tool rest to within 2 mm of the wheel and the tongue guard to within 2 mm; re-secure or replace the bench so it is rigid; fit the correct guards; provide a working water cup; ensure the wheel is dressed true. Keep the machine on a planned maintenance schedule.
Administrative controls: Supervise the class; brief learners on no gloves, no loose sleeves, hair tied back; train them to use the face of the wheel only; use a teacher check before use; arrange technician cover.
PPE: goggles or a face shield are the last line, not the first. The learner who moved his goggles must be stopped and re-briefed.
Immediate stop-work items: cracked wheel, side grinding, loose gloves or drawcord, the 200 mm wheel proposal, and goggles not worn. The class should not continue until the wheel is replaced and the guards are set.
3. Calculate the peripheral speed of the 150 mm wheel and of the 200 mm wheel at 2950 rpm. Explain what the 32 m/s marking does and does not tell you.
Read the worked answer
Peripheral speed is v = πDn/60, with D in metres and n in rev/min.
- 150 mm wheel: v = π × 0.150 × 2950 / 60 = 23.2 m/s.
- 200 mm wheel: v = π × 0.200 × 2950 / 60 = 30.9 m/s.
At first sight the 200 mm wheel is still below the 32 m/s label, so it looks acceptable. That is the trap. The 32 m/s marking applies to the wheel itself, but the machine guard clearance, spindle loading and the actual wheel rating must also be checked; a wheel rated at 30 m/s would be over-speed at 30.9 m/s. The larger wheel also increases the stored energy and the load on the spindle, and it may not fit the guard. The safe decision is not to fit it without checking the wheel marking, the machine manual and the guard. The 150 mm wheel at 23.2 m/s has margin, but that margin does not make a cracked or side-loaded wheel safe.