the wheel and the tool rest. The spark arrestors catch stray sparks. Grinding is usually done with the workpiece resting on the tool rest. This prevents sudden slips in which the wheel grabs the work momentarily and yanks it out of the operator's hand or pulls the hand toward the wheel. Wheel guards The wheel guards serve to intercept the fragments of an exploding wheel, keeping them from killing people or damaging surroundings. This is why running a grinder without the guards is prohibited. Coolant Grinding metal on a power-driven grinding wheel quickly heats up the workpiece. Most bench grinders are of the dry type, in which no cutting fluid (coolant) is used at the grinding interface, but often the workpiece is recurrently dunked into a pan or pot of water for cooling so as to keep it from getting hot enough to lose its temper, burn the operator's hand, or both. Such pots are often mounted just below the grinder for safe and easy reach. For medium-carbon or high-carbon workpieces that are already hardened and tempered (such as tool bits and drill bits), the dipping of the piece into the water must be frequent enough to avoid very high heat followed by substantial quenching, which can easily destroy the existing heat treatment. Some grinders for knife sharpening duty are of the wet type, in which the bottom of the wheel runs within a pan of water or other coolant. A tube may also deliver a stream of coolant near the top of the wheel. These grinders are not always called bench grinders, but they are among the class of benchtop grinding machines. Wheel dressing Bench grinder wheels sometimes need to be restored to roundness and given a fresh grinding face with newly exposed grains. New wheels have suitable balance when first made, but whether the balanced state will last after the wheel has been mounted, and (especially) after it has been in use for a while, is "quite uncertain". It can seem counterintuitive that they could wear as unevenly as they sometimes do, but a slow spin of the resting wheel by hand confirms it by showing the gap between wheel and tool rest varying substantially as the wheel turns. Thus it is normal that wheels sometimes need to be dressed (neatly trimmed), which is achieved by any of several types of wheel dresser. When bench grinders vibrate excessively,