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Lesson 5.7CutChapter 5 · Machining

Grinding

A fast wheel of hard grains shaves the surface

Grinding removes metal with a fast-spinning wheel made of countless tiny, very hard grains, each acting as a miniature cutting tool. It gives hardened steel shafts, bearing rings and drill bits their final size and smooth finish. When a part must be accurate to within a few micrometers (thousandths of a millimeter), it is usually ground.

Grinding: a real photo
Photo: CC BY-SA 2.5, via Wikimedia Commons
A fast-spinning wheel of hard grains shaves a part to exact sizeSurface grinding shown; round parts use cylindrical or centerless grindersSurface grinder, side viewClose-up: grains cut tiny chipsMagnetic tableWorkpiece (hardened steel)Grinding wheelhard grains + bondDiamond dresserSparks: hotmicro-chipsCoolantWheel spins fastLowered slightlyeach passTable slides back and forthWheel: grains held in a bondrim speed about 30 m/sChip (glows as a spark)WorkpieceGrey chunks: abrasive grains, eacha tiny cutting edge. Dull grainsbreak away, exposing sharp ones12345WorkpieceTool and machine partsCoolantHeat and sparksMotion
surface grinding · side view and close-up of the cutting grainsCircled numbers match the steps below. The figure shows surface grinding; cylindrical grinders turn a shaft against the wheel, and centerless grinders roll it between two wheels.

How it works

  1. Dress the wheel. A diamond-tipped tool skims the spinning wheel so it runs perfectly round and its grains are sharp and exposed.

  2. Spin it fast. The wheel turns at thousands of revolutions per minute; its rim moves at about 30 m/s (roughly 100 km/h).

  3. Grains cut tiny chips. Each grain that touches the part shaves off a chip far thinner than a hair. The chips get so hot that they glow: those are the sparks.

  4. Flood with coolant. A jet of water-based coolant or oil cools the part, washes chips away and keeps the wheel from clogging. Without it, the surface can burn (overheat, soften or crack).

  5. Feed in tiny steps. The table slides the part back and forth under the wheel, which is lowered slightly after each pass. Final passes with no extra lowering, called spark-out, end when the sparks die away.

Key facts

Materials
Hardened steels, tool steels, cast iron, carbide, ceramics, glass; soft metals such as aluminum tend to clog the wheel
Typical parts
Bearing rings and rollers, shafts, gauge blocks, cutting-tool edges, flat machine slideways
Part size
From needle rollers a few millimeters across to rolls and machine beds several meters long
Precision
Typically a few micrometers (about ±0.005 mm); precision work reaches about 1 µm on diameters
Surface finish
Smooth, satin to bright; typically Ra 0.2–0.8 µm (Ra is the average height of the surface roughness)
Batch size it suits
Any: one part in a toolroom to millions on centerless lines
Tooling and machine cost
Wheels (aluminum oxide, silicon carbide, or costly CBN and diamond) from tens to thousands of dollars; precision grinders from tens of thousands to over a million
Speed
Slow per volume of metal removed, often minutes per part; through-feed centerless grinders finish many small pins per minute

Strengths and limits

Strengths

  • Cuts hardened steel and other materials too hard for ordinary cutting tools
  • Very tight tolerances (a few µm) and smooth surfaces
  • Many machine types: flat faces, shafts, bores, profiles
  • Suits single parts and fully automated mass production alike

Limits

  • Removes metal slowly, so it is kept for the last few tenths of a millimeter
  • Too much heat can burn or crack the surface if the coolant or wheel choice is wrong
  • The wheel wears and must be dressed regularly
  • Precision grinders are expensive, and the swarf (grinding sludge) is messy to handle

Where it's used

  • AutomotiveCrankshaft and camshaft journals, valve stems, transmission shafts
  • BearingsInner and outer rings, balls and rollers
  • ToolingDrills, end mills and punches sharpened on tool grinders
  • AerospaceTurbine blade roots, landing-gear parts
  • Machine buildingFlat slideways, gauge blocks, precision spindles

How to recognize it

  • Very fine, straight, parallel scratch lines all running one way, or around a shaft.
  • A smooth satin-to-bright surface that feels glassy under a fingernail.
  • Rotary-ground flat parts show overlapping swirl arcs instead of straight lines.
  • Blue or brown patches on a ground surface are grinding burn (local overheating).

Compare with

Turning and milling5.1remove metal much faster with a few large cutting edges, usually before hardening; grinding then finishes the hardened part.Honing5.8also cuts with abrasive grains, but scrubs slowly inside a bore to perfect its roundness and leave a cross-hatch.Superfinishing and polishing5.9remove only a micrometer or so to make an already ground surface even smoother.

Watch it

THATLAZYMACHINIST · A beginner tour of a cylindrical grinder: the wheel, dressing and how the part is held

Metal Cutting Corp · How centerless grinding works: grinding wheel, regulating wheel and work rest

Okamoto Europe · A CNC surface grinder like the one in the figure, at work

Fritz Studer AG · Precision cylindrical grinding, with sparks and close-ups

See real photos

Remember

  • Grinding cuts with countless tiny grains, so it can shape hardened steel and ceramics.
  • It removes little material but reaches micrometer accuracy and fine finishes.
  • Coolant and regular dressing keep the wheel sharp and the part from burning.

More in Machining

All 100 processes
5.1ShortTurning (lathe)The part spins; a single-point tool cuts itOpen the lesson5.2ShortMillingA spinning multi-edge cutter moves over a fixed partOpen the lesson5.3ShortDrillingA spinning twist drill plunges into the partOpen the lesson5.4BroachingA long toothed bar is pulled through or across in one strokeOpen the lesson5.5Gear cutting (hobbing, shaping)Hobs or shaper cutters generate gear teethOpen the lesson5.6KnurlingHardened wheels press a grip pattern into a turning partOpen the lesson5.8HoningStones turn and stroke inside a boreOpen the lesson5.9Superfinishing and polishingFine stones or buffs remove the last microscopic peaksOpen the lesson

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