Grinding removes material with an abrasive wheel rather than a defined cutting edge, taking very small amounts at a time. That gives tolerances and surface finishes beyond what turning and milling reach, and it works on hardened material that a cutting tool cannot touch, which is why so many precision parts are machined soft, hardened and then ground.
The three common configurations
Surface grinding passes a flat part under a wheel on a reciprocating table and produces flat, parallel faces to close tolerances. Cylindrical grinding holds a part between centres or in a chuck and grinds an outside or inside diameter. Centreless grinding supports the part on a blade between a grinding wheel and a regulating wheel, with no centres at all, and is extremely fast for volume production of pins and shafts.
Why hardened material goes here
Heat treatment distorts parts, so the usual sequence is to machine soft with grinding stock left on, harden, then grind to final size. Grinding does not care about hardness in the way a cutting tool does, so this order gives both the hardness and the accuracy. Trying to machine a fully hardened part with a conventional cutter is slow, expensive and hard on tooling.
Heat and burn
Grinding puts a great deal of energy into a small area, and the classic failure is thermal damage to the surface: burn, re hardening, tempering or micro cracks, sometimes invisible without an etch test. It is controlled by coolant, wheel choice, dressing and taking lighter cuts. On fatigue critical parts, grinding burn is a genuine engineering risk rather than a cosmetic one.
Specifying it sensibly
Say which features are ground and leave the rest as machined. Provide the grinding stock allowance the process needs, and a way to hold the part, since centres or a clamping face must exist. State the surface finish and any requirement about surface integrity. And remember that grinding a feature usually means the part is handled twice, so the lead time reflects two operations.
Questions people ask about precision grinding
When should a feature be ground rather than machined?
When the tolerance or surface finish is beyond what a finishing cut reaches, or when the material is hardened and a cutting tool cannot work it economically.
What is centreless grinding?
Grinding a cylindrical part supported on a blade between a grinding wheel and a regulating wheel, with no centres. It is very fast for volume production of pins and shafts.
What is grinding burn?
Thermal damage to the ground surface from excessive heat, which can temper, re harden or crack it. It is often invisible without an etch test and matters on fatigue critical parts.
Do I need to leave grinding stock?
Yes. The part is machined undersize on ground features so the grinder has material to remove. Ask the shop how much they want for the feature and material.