Goggles or safety glasses need to be worn to protect the operator's eyes from the sparks and metal filings that result. Depending on the workpiece and time of exposure, hearing protection may also be required, the dust produced is also potentially hazardous. The toolrest (or workrest) should be mounted slightly below the center of the grinding wheel with less than 3 mm clearance from the wheel. This prevents the work from jamming between the toolrest and the wheel. Grinding wheels designed for steel should not be used for grinding softer metals, like aluminium. The soft metal gets lodged in the pores of the wheel and expand with the heat of grinding. This can dislodge pieces of the grinding wheel. Wire brushes require particular attention for the safety of the operator and bystanders as the metal wires may become dangerous projectiles as they could bind with the object being brushed and so throw it away with great energy. The machine needs to be securely mounted to a pillar or a bench to be used safely and effectively, often with an emergency stop switch or pedal fitted close by the machine, for use in emergencies.