grind

Grinding removes material from a workpiece using a rotating wheel or belt covered with abrasive particles, typically aluminum oxide or silicon carbide bonded in a ceramic matrix. The abrasive grains cut the metal surface at high speed, generating heat and fine chips. A grinding machine holds the workpiece against this wheel with precise control, producing flat surfaces, cylindrical forms, or sharp edges depending on the wheel profile and setup.

The abrasive wheel itself comes in many forms. Bench grinders have two rigid wheels, usually 150 to 250 millimeters in diameter, mounted on a shaft for general workshop shaping and tool sharpening. Surface grinders use a large flat wheel rotating against a magnetic chuck that holds ferrous parts. Cylindrical grinders wrap the workpiece between centers and feed it across a rotating wheel to produce parallel shafts. Angle grinders are handheld tools with 100 to 180 millimeter wheels, common for cutting and rough grinding on site. Belt grinders offer more control than wheels for fine finishing work.

The grit size, bond strength, and wheel hardness determine how aggressively the grinder cuts and how long the wheel lasts before dulling. Coarse grits (24 to 60) remove material quickly but leave a rough finish; fine grits (120 to 400) produce smoother surfaces but cut more slowly and clog more easily if the metal is soft or sticky. Coolant or cutting fluid must often be applied to prevent the workpiece from overheating and losing hardness, and to wash away metal fines that would otherwise jam the abrasive surface.

Glazing occurs when the abrasive wheel becomes loaded with metal particles and no longer cuts efficiently, requiring wheel dressing to restore the cutting surface. Burning happens when excessive heat softens the metal or changes its microstructure, leaving a discolored zone that is brittle. Out-of-round wheels or poor machine rigidity cause vibration and chatter, marking the workpiece with a wavy finish. Touching the spinning wheel or getting loose clothing caught in it causes serious injury.

The term "grind" has roots in Old English and refers to the crushing action of grain between millstones, a visual parallel to how abrasive particles pulverize metal. In the machine shop, grinding stands apart from milling or turning because the abrasive action is fundamentally different from the chip-forming action of a cutting tool. A skilled grinder judges surface finish and dimensional accuracy by feel and sound, recognizing the whisper of a sharp wheel from the screech of a dull one.