Because of hardness and tolerance. Parts that are heat treated distort during treatment and finish hard, and hardened steel cannot be turned economically to a fine finish. Grinding removes the distortion and brings the part to final size at tolerances and surface finishes an order better than turning. Where a part is soft and tolerances are moderate, turning is faster and cheaper and grinding adds nothing.

A plain machine grinds straight external diameters with the work between centres and is rigid, simple and productive at that. A universal machine adds a swivelling wheelhead and table plus an internal grinding attachment, so tapers, shoulders and bores can be ground without moving the part to another machine. Universal machines are more versatile and less rigid; plain machines are preferred where the work is repetitive external grinding.

The workpiece is not held at all. It rests on a support blade between a grinding wheel and a slower regulating wheel, which controls both rotation and, when angled, axial feed. In through-feed mode parts pass continuously between the wheels; in in-feed mode the part is stopped against an end stop for shouldered or profiled work. Setup is more involved, but output for plain cylindrical parts is very high.

When concentricity to another feature matters. Because the process references the part's own outside surface rather than centres or a datum, it produces excellent roundness and size but cannot control where that ground diameter sits relative to a bore, a flange or a second journal. For assemblies where features must run true to one another, grind between centres. For plain pins, rollers, bar and dowels, centreless is faster and cheaper.

It is central rather than routine maintenance. Dressing restores the wheel's form and exposes fresh abrasive, and a wheel that has loaded or dulled will burn the work, generate heat, produce chatter and lose size control. Dressing frequency, infeed and traverse rate all change how aggressively the wheel cuts, so dressing parameters are part of the process specification and not simply a housekeeping task.

Excess heat at the contact, which alters the metallurgy of the surface and can leave a softened or re-hardened layer with residual tensile stress, sometimes with visible discoloration but often not. Causes are dull or loaded wheels, too much stock removed per pass, inadequate coolant, or coolant not reaching the contact. Correct wheel specification, regular dressing and coolant delivered into the grinding zone are the controls.

More than cooling. It carries heat away from a process that converts most of its energy into heat, flushes swarf and abrasive from the wheel face to stop it loading, and lubricates the contact. Delivery matters as much as volume: coolant must penetrate the air barrier the wheel carries around with it and reach the actual contact, which is why nozzle design and pressure are part of the process rather than an afterthought.