Grinding

Grinding on hardened steel: role and sequence

Grinding cuts with thousands of small abrasive grains bonded into a wheel. Each grain removes a chip a few microns thick, so the process is indifferent to workpiece hardness. On hardened steel it is the final sizing step for faces, diameters, bores and profiles.

The usual sequence is rough machine → harden and temper → grind. The part arrives with 0.1-0.3 mm per side of grinding stock. The heat treat allowance table lists typical stock by part size.

Grinding always runs with flood coolant. Hard turning runs dry because it needs heat in the shear zone. Grinding needs the opposite, because heat at the grinding contact damages the surface.

Grinding types and what each holds

Type	Typical parts	Size or position	Geometry	Ra
Surface	Plates, die blocks, gauge blocks, punch heads	±0.005 mm thickness, ±0.002 mm best case	Flatness 0.002-0.005 mm per 100 mm; parallelism 0.003-0.005 mm	0.2-0.8 µm
Cylindrical OD	Shafts, pins, punches, bearing seats	±0.002-0.005 mm diameter	Roundness 1-2 µm	0.1-0.4 µm
Internal (ID)	Bushings, die buttons, bores	±0.003-0.008 mm	Roundness 1-3 µm	0.2-0.5 µm
Centerless	Pins, ejector pins, rollers, plain shafts	±0.002-0.005 mm	Roundness 0.5-2 µm	0.1-0.4 µm
Profile and form	Punch profiles, form tools, guide rails	±0.002-0.005 mm on profile	Inside corner radius down to about 0.05-0.1 mm	0.2-0.4 µm
Jig	Dowel holes, guide bores, die openings	±0.002-0.005 mm hole position	Roundness 2 µm or better	0.2-0.4 µm

Typical production values on a well-maintained machine. Confirm per part and per drawing.

Surface grinding

The part sits on a magnetic chuck and the table reciprocates under the wheel. The wheel cross-feeds a fraction of its width per stroke and downfeeds at each pass.

Surface grinding sets thickness, flatness and parallelism on plates and blocks. Details on plates are in the blog post on surface grinding hardened plates.

Cylindrical grinding (OD and ID)

The part turns between centres or in a chuck while the wheel infeeds against it. OD grinding holds IT5 on diameters, about ±0.004 mm on a 30 mm shaft.

ID grinding uses a small wheel on a long quill. Quill deflection limits ID work to slightly wider tolerances than OD work.

Centerless grinding

The part runs on a work rest blade between a grinding wheel and a regulating wheel, with no centres. Through-feed grinds plain pins continuously; in-feed (plunge) grinds parts with shoulders.

Centerless grinding can produce odd-lobed out-of-roundness that a 2-point micrometer does not detect. Roundness on centerless parts is checked on a roundness tester or in a V-block.

Profile and form grinding

Profile grinding uses a wheel dressed to the required shape or a small wheel following a CNC path. Typical parts are punch profiles, form tools and rails.

Optical profile grinders compare the part to a magnified drawing during the cut. Outside corners can be sharp; inside corners are limited by the dressed wheel radius.

Jig grinding

A small high-speed spindle orbits inside a hole while the machine positions it in X and Y. Jig grinding finishes holes to a few microns of position and roundness. The blog post on jig grinding hardened die plates covers when hole tolerances justify it.

Grinding wheels and dressing

Abrasive	Bond	Application
White or pink aluminium oxide	Vitrified	General tool steels up to about 62 HRC: O1, A2, S7, H13, 4140, 52100
Ceramic (sintered) aluminium oxide	Vitrified	Higher stock removal, tougher grades, lower burn risk than standard oxide
CBN	Vitrified	High-vanadium and high-speed steels: M2, M42, powder steels; long production runs
CBN	Resin	Tool and cutter grinding, some profile work; needs separate truing and sharpening
Diamond	Any	Not used on steel. Carbon diffuses into iron at grinding temperature and the grain wears quickly

Vanadium carbides are harder than aluminium oxide grain. On high-vanadium grades an oxide wheel dulls fast and generates heat. CBN wheels cut these grades cooler and hold their form longer.

Wheel grade rule for hard steel: a softer bond. Dull grains must break out before they rub. A wheel too hard for the job glazes and burns the part.

Dressing restores sharp grains and the wheel’s shape:

• Single-point or multi-point diamond | standard for vitrified aluminium oxide wheels
• Rotary diamond disc or roll | form dressing and vitrified CBN
• Coarse dress | freer cutting and less heat, rougher finish
• Fine dress | finer finish, more heat; used for the last passes only

Grinding burn and grinding cracks

Grinding burn is thermal damage at the ground surface. It occurs in 2 forms:

Type	Cause	Result
Temper burn	Surface heated above the tempering temperature	Softer over-tempered layer, lower wear resistance
Re-hardening burn	Surface heated above the austenitising temperature, then quenched by coolant	Brittle untempered martensite over a soft zone; tensile residual stress

Grinding cracks usually start in burned areas. They are fine cracks, often at right angles to the grinding direction or in a network pattern. They may open later in service under load.

Common causes:

• Dull or loaded wheel, or a wheel grade too hard for the steel
• Downfeed too large on the final passes
• Coolant not reaching the contact zone
• Parts with high retained austenite, for example D2 or carburised parts without a full temper cycle

Discolouration is a warning sign, but its absence proves nothing; coolant can hide it. Inspection methods:

• Nital temper etch | shows temper and re-hardening burn
• Magnetic particle or dye penetrant | shows cracks
• Barkhausen noise | non-destructive burn check, see Barkhausen effect

Magnetic chuck and demagnetising

The magnetic chuck holds steel parts without clamps. On thin plates it also pulls the part flat. A plate with heat-treat bow is ground flat while clamped, then springs back after release.

Shop methods for thin plates:

• Grinding both sides alternately, with several flips
• Reduced chuck force on finishing passes
• Shims under the gap of a bowed plate
• Vacuum or mechanical clamping for very thin parts
• Stress relief between roughing and finishing, where the grade allows

Parts leave the chuck with residual magnetism. It holds swarf on sliding faces, bearing seats and gauge surfaces. Demagnetising after grinding is a separate step; the drawing states it with a measurable limit.

Cases where grinding is required

• Size below ±0.005 mm | or IT5 and finer on diameters
• Roundness below 2 µm | cylindricity on bearing and seal seats
• Flatness or parallelism of a few microns | over large plates and die blocks
• Ra below 0.2 µm | on functional surfaces, before lapping
• Long slender shafts | length to diameter above about 6-8:1, beyond the reach of hard turning
• Hole position ±0.005 mm or tighter | jig grinding
• “Ground” called out on the drawing | some aerospace and automotive standards name the process

Above these limits, hard milling and hard turning usually cost less. Below Ra 0.1 µm or for flatness in the sub-micron range, lapping and polishing follow grinding.

Grinding vs other finishing routes

Feature	Grinding	Hard turning	Hard milling	Wire EDM
Flat face to 0.005 mm	First choice	Faces only on round parts	Marginal over large areas	Not applicable
Diameter IT5	First choice	IT6 typical	Not applicable	Not applicable
3D cavity	Not applicable	Not applicable	First choice	Not applicable
Sharp inside corner, through profile	Not possible	Not applicable	Limited by tool radius	First choice
Surface layer	Clean if burn-free	Clean with fresh edge	Clean	Recast layer
Setups	1 per face or diameter	1 for several features	1-2	1

Through profiles with sharp inside corners go to wire EDM. A common route: hard mill or hard turn most features → grind the 2-3 critical faces.

Drawing and RFQ requirements

• Hardness | as a band after heat treatment, e.g. 58-62 HRC.
• Grade | by standard designation. D2 and M2 change wheel choice and cycle time.
• Ground surfaces | mark each surface that must be ground. Leave the rest to the shop’s routing.
• Geometric tolerances per surface | flatness, parallelism, roundness or position, each on the feature that needs it.
• Finish per surface | e.g. Ra 0.2 µm on a bearing seat and Ra 0.8 µm on other faces.
• Burn and crack inspection | method and sample size, e.g. nital etch per batch, for fatigue-loaded parts.
• Demagnetising | required or not, with a residual field limit.
• Grinding stock | if the shop receives a pre-machined blank, the stock left per side.