it is usually because the wheels have worn out of round and are thus out of balance. Truing them by dressing usually resolves this problem. Although some consumers might imagine buying a bench grinder and then using it for many years without thinking about wheel dressing, the need for dressing is not so seldom as that. Correcting the imbalance not only reduces the annoyance of vibration (which rattles the wheel guards and shakes the workbench) but also is important to prevent premature failure of the spindle bearings, as heavy vibration beats them excessively. It is also possible to arrange the weight distribution of the wheels for better balancing. There are several methods of doing this, all being conceptually analogous to tire balancing with wheel weights. Most bench grinders never have these operations performed, because wheel dressing alone is enough to keep them sufficiently balanced, but these additional methods are not unusual for bench grinders and are quite common for machine tools that do grinding, such as surface grinders and cylindrical grinders. This is sometimes achieved by drilling a few holes in the steel flanges that hold the grinding wheels and then finding the angular orientation at which the holes' lack of weight balances out a heavy spot elsewhere on the rotating mass. Another method is a dedicated type of large flange with little weights that can be screwed down wherever needed for balancing (called a balancing flange. Wheel replacement Wheels sometimes need replacing. The grinder is unplugged, and then the outer guard is removed. Typically a large nut holds the wheel on the arbor. The handedness (sense) of each nut is assigned to avoid any loosening tendency during grinder use. Thus on a typical grinder the nut is left-handed on the left side of the grinder from the operator's viewpoint, with wheel rotation "toward" the operator (if the wheel could travel), whereas on the other side the nut is right-handed. "In other words, to remove the nut, it must be turned in the direction that the spindle revolves when the wheel is in operation." Holding the wheel against the wrench's turning action is difficult, but usually the wrench is gently tapped with a hammer instead, which neatly taps the nut loose. A large steel flange is on either side of the wheel, then a paper washer (blotter), then the wheel. The paper is conventionally held to be absolutely necessary to