Introduction:

In manufacturing, conventional processes often cannot produce the required dimensional accuracy or surface finish for a part, particularly when the workpiece material is too hard or too brittle. Precision components such as ball and roller bearings, pistons, valves, cylinders, cams, gears, dies and numerous instrumentation parts demand exceptional characteristics. Abrasive machining is one of the most common methods for producing such demanding features. An abrasive is a small, hard particle with sharp edges and an irregular shape capable of removing small amounts of material through a cutting process that produces tiny chips. Grinding wheels and sandpaper are familiar examples but abrasives are also used to hone, lap, buff and polish workpieces. With computer controlled machines, abrasive processes now produce a wide variety of workpiece geometries and achieve very fine dimensional accuracy and surface finishes. Dimensional tolerances can be less than 1 micrometre and surface roughnesses can be as fine as 0.025 micrometres. Because they are hard, abrasives are used in finishing heat treated metals, alloys, ceramics and glasses as well as for cutting off bar lengths, removing weld beads and spatter and cleaning surfaces with abrasive laden jets.


Abrasives and Bonded Abrasives:

Types of Abrasives:

The most commonly used abrasives are conventional types aluminum oxide (Al₂O₃) and silicon carbide (SiC) and superabrasives cubic boron nitride (cBN) and diamond. Natural abrasives like emery, corundum, quartz, garnet and diamond contain impurities and nonuniform properties, leading to inconsistent performance so synthetic abrasives have been manufactured for many years. Aluminum oxide was first made in 1893 by fusing bauxite, iron filings and coke. Fused aluminum oxides are categorized as dark (less friable), white (very friable) and single crystal. Seeded gel introduced in 1987 is the purest unfused aluminum oxide also known as ceramic aluminum oxide. Its grain size is about 0.2 micrometres much smaller than other abrasive grains; these grains are sintered to form larger sizes. Being harder than fused alumina and relatively friable, seeded gels maintain sharpness and are used for difficult to grind materials. Silicon carbide was discovered in 1891 and made from silica sand and petroleum coke. It is divided into black (less friable) and green (more friable) generally having higher friability than aluminum oxides, thus fracturing and remaining sharp more readily. Cubic boron nitride was first developed in the 1970s. Diamond also known as synthetic or industrial diamond, was first used as an abrasive in 1955.

Abrasive Grain Size:

Abrasives are very small compared to conventional cutting tools, with sharp edges enabling removal of tiny quantities of material, yielding fine surface finishes and dimensional accuracy. Grain size is identified by a grit number based on sieve size: smaller grains have larger numbers. Grit number 10 is very coarse, 100 is fine and 500 is very fine. Sandpaper and emery cloth are identified similarly with grit numbers printed on the backing.


Abrasive Workpiece Material Compatibility:

The affinity between abrasive and workpiece material is crucial, less reactivity means less wear and dulling preventing inefficiency and surface damage. Diamond cannot grind steels because it dissolves in iron at the high grinding temperatures. Recommendations are aluminum oxide for carbon steels, ferrous alloys and alloy steels, silicon carbide for nonferrous metals, cast irons, carbides, ceramics, glass and marble, cubic boron nitride for steels and cast irons above 50 HRC hardness and high temperature alloys and diamond for ceramics, cemented carbides and some hardened steels.

Grinding Wheels:

Since each abrasive grain removes very little material, high removal rates require many grains acting together achieved through bonded abrasives like grinding wheels where grains are randomly distributed and oriented. A bonding material holds the grains acting as supports and porosity is essential to provide chip clearance and cooling without porosity, chips would interfere severely. Porosity is visible on any grinding wheel surface, and a fully dense wheel would have no room for chips. A wide variety of wheel types and sizes are manufactured, marked with a standardized system indicating abrasive type, grain size, grade, structure and bond type.

Bond Types:

Vitrified bonds essentially glass are the most common. Raw feldspar and clays are mixed with abrasives, moistened, molded under pressure and fired slowly to about 1250°C to fuse the glass and develop strength then cooled slowly to avoid cracking, finished, inspected and tested. Vitrified wheels are strong, stiff, porous and resistant to oils, acids and water, but brittle and lacking thermal and mechanical shock resistance. They are sometimes made with steel backing plates for support. Color can be modified for coding to specific workpiece materials.

Resinoid bonds are thermosetting resins in various compositions. Being organic, these wheels are also called organic wheels. Manufacturing mixes abrasive with liquid or powdered phenolic resins and additives, presses the mixture into shape, and cures at about 175°C. Their lower elastic modulus makes them more flexible than vitrified wheels. Polyimide is a newer substitute for phenolic offering toughness and higher temperature resistance. Injection molding is another manufacturing method.

Reinforced wheels have one or more layers of fibreglass mats of various mesh sizes. The fibreglass retards disintegration if the wheel breaks during use rather than improving strength. Large diameter resinoid wheels can include internal steel bar rings inserted during molding.

Thermoplastic bonds are also used including wheels with sol gel abrasives bonded with thermoplastics.

Rubber bonds provide the most flexible matrix. Manufacturing mixes crude rubber, sulfur, and abrasive grains, rolls into sheets, cuts disks and heats under pressure to vulcanize. Thin wheels made this way serve as cutoff blades.

Metal bonds use powder metallurgy to bond abrasive grains usually diamond or cBN to the wheel periphery to depths of 6 mm or less under high pressure and temperature. The core can be aluminum, bronze, steel, ceramics or composites depending on strength, stiffness and dimensional stability requirements. Superabrasive wheels can be layered with a single abrasive layer plated or brazed to a metal wheel, layered wheels are lower cost and used for small production quantities.


The Grinding Process:

Grinding uses individual abrasive grains as cutting tools. Key differences from single point tools: grains have irregular shapes and random spacing, average rake angle is highly negative, typically -60° or less, causing chips to undergo much larger plastic deformation; radial positions vary, so not all grains are active and surface speeds are very high typically 20 to 30 m/s up to 150 m/s in high speed grinding with specially designed wheels.


Grinding Operations and Machines:

Grinding traditionally finishes parts whose geometries are already created. Machines grind flat surfaces, cylinders and contours often using formed wheels with the reverse of the desired contour. Grinding is also used in tool rooms for cutting tool geometries. Applications now expand to high speed, high removal operations.

Surface Grinding:

Surface grinding grinds flat surfaces using either the wheel periphery or flat face. With horizontal workpiece orientation, peripheral grinding uses a horizontal spindle axis and face grinding uses a vertical axis. Work motion is reciprocating or rotating. The horizontal spindle machine with reciprocating worktable is most common. Grinding reciprocates the work longitudinally at very small depth feeding the wheel transversely between strokes, wheel width is usually less than workpiece width. This machine can form contoured surfaces by plunge feeding a formed wheel vertically instead of transversely. Vertical spindle reciprocating table machines have wheel diameter greater than work width, so no transverse feed is needed, grinding reciprocates the work and feeds the wheel vertically to dimension achieving very flat surfaces. Vertical spindle rotary table machines are more common among rotary types capable of high removal rates due to large contact area.

Cylindrical Grinding:

Used for rotational parts, cylindrical grinding is external or internal. External cylindrical grinding (center type) resembles turning; machines are like lathes with a high-speed motor for the wheel. The workpiece rotates between centers at 18–30 m/min, and the wheel rotates at 1200–2000 m/min. Two feed motions: traverse feed (wheel fed parallel to work axis, infeed 0.0075–0.075 mm, sometimes with reciprocating motion for finish) and plunge cut (wheel fed radially used with formed wheels). It finishes parts machined to approximate size and heat treated axles, crankshafts, spindles, bearings, bushings and rolling mill rolls producing final size and finish on hardened parts. Internal cylindrical grinding resembles boring. The workpiece is held in a chuck and rotated at 20–60 m/min, wheel speeds similar to external grinding. Feed can be traverse or plunge. The wheel diameter must be smaller than the bore often requiring very high rotational speeds for desired surface speed. It finishes hardened inside surfaces of bearing races and bushings.

Centerless Grinding:

An alternative for external and internal cylindrical surfaces, the workpiece is not held between centers reducing handling time and making it suitable for high production. External centerless grinding setup has a grinding wheel and a regulating wheel. Workpieces individual short pieces or long rods up to 3–4 m are supported by a rest blade and fed between the wheels. The grinding wheel rotates at 1200–1800 m/min, and the regulating wheel rotates much slower, inclined at a slight angle to control throughfeed. Typical parts are roller bearings, piston pins, engine valves, camshafts, etc. diameters as small as 0.1 mm can be ground. Centerless grinders achieve wheel speeds around 10,000 m/min, often with cBN wheels. In through feed grinding, the larger wheel grinds while the smaller rubber bonded regulating wheel controls axial movement. Variable diameter parts like bolts, tappets, multi diameter shafts are ground by infeed or plunge grinding. Tapered pieces use end feed grinding and high production thread grinding uses specially dressed wheels. Internal centerless grinding uses two support rolls instead of a rest blade to position the work with the regulating wheel tilted for feed. Throughfeed is not possible so production rates are lower than external centerless but it provides very close concentricity between internal and external diameters on tubular parts like roller bearing races.

Creep Feed Grinding:

Developed around 1958, creep feed grinding uses very high depths of cut and very low feed rates. Depths of cut are 1000 to 10,000 times greater than conventional surface grinding with feed rates reduced proportionally. Material removal rate and productivity increase because the wheel cuts continuously avoiding lost reciprocating stroke time. It applies to surface and external cylindrical grinding. Surface applications include slots and profiles especially for high depth to width ratios. Cylindrical applications include threads, formed gear shapes and other components. In Europe, deep grinding describes these cylindrical applications. Special machines feature high static and dynamic stability, accurate slides, 2–3 times the spindle power of conventional grinders consistent low table speeds, high pressure fluid delivery and in process dressing systems. Depth of cut can be up to 6 mm with low workpiece speed, softer resin bonded, open structure wheels keep temperatures low and finish good. Machines have power up to 225 kW, high stiffness for high forces, high damping, variable speeds and ample fluid capacity with diamond roll continuous dressing. Advantages include high removal rates, improved formed surface accuracy and reduced work surface temperatures. It competes with milling, broaching and planing and is economical for shaped punches, key seats, twist drill flutes, turbine blade roots and complex superalloy parts. Since the wheel is dressed to the workpiece shape, prior milling, shaping or broaching is unnecessary making near net shape castings and forgings suitable. A single pass generally suffices though a second may improve finish.


Other Grinding Operations:

Tool grinding:

Uses special machines to sharpen and recondition hardened tool steel tools. Devices position and orient tools for specified angles and radii. General-purpose tool and cutter grinders use attachments for various geometries, single purpose types include gear cutter sharpeners, milling cutter grinders, broach sharpeners and drill point grinders.

Jig grinders:

Traditionally grind holes in hardened steel parts to high accuracies originally for pressworking dies. They are now used broadly for high accuracy, good finish hardened components with numerical control available.

Disk grinders:

They have large abrasive disks on horizontal spindle ends, work is held against the flat wheel surface. Double opposing spindles allow automatic feeding between disks, grinding opposite sides simultaneously providing good flatness and parallelism at high rates.

Snag grinders:

They are similar but grind on the outside wheel periphery, wheels differ in design. Manual snag grinding removes flash from castings and forgings and smooths welds.

Abrasive belt grinding:

They uses abrasives bonded to a flexible cloth belt. Support by a roll or platen is needed behind the belt, a flat platen for flat surfaces or a soft platen to conform to contours. Belt speed depends on material typically 750–1700 m/min. Improvements in abrasives and bonding have increased its use for heavy stock removal rather than just light grinding. Belt sanding refers to light hand applications to remove burrs and high spots and improve finish quickly.


Design Considerations for Grinding:

Design considerations mirror those for general machining with additional specifics. Parts must be mounted securely in chucks, magnetic tables or fixtures thin or tubular workpieces may distort. Interrupted surfaces like holes and keyways should be avoided for high accuracy to prevent vibration and chatter. Cylindrical parts should be balanced with long slender designs avoided to minimize deflections, fillets and corner radii should be as large as possible, or relief provided. In centerless grinding, short pieces may be difficult to grind accurately due to lack of blade support in through feed, only the largest diameter can be ground. Accurate form grinding designs should be simple to avoid frequent form dressing. Deep, small or blind holes for internal grinding should be avoided or provided with relief. Designs should require minimum material removal except in creep feed grinding and ideally allow all grinding without repositioning the workpiece.



Finishing Operations:

Fine abrasive grain processes are used for final finishing but can increase time and cost so they should be specified with cost benefit consideration.

Coated Abrasives:

Examples are sandpaper and emery cloth. Most are aluminum oxide, with silicon carbide and zirconia alumina making up the rest. They have more open structure than grinding wheels with more pointed, aggressive grains. Grains are electrostatically deposited on flexible backings like paper, cotton, rayon, polyester, polynylon, blends. The resin bond first applies a make coat then grains are bonded with a size coat with long axes perpendicular to the backing to improve cutting. Coated abrasives are available as sheets, belts, and disks used for flat or curved metallic and nonmetallic surfaces, metallographic specimens and woodworking, finish precision depends on grit size.

Belt Grinding:

Belts with grit numbers 16 to 1500 run at 700–1800 m/min for high rate material removal with good finish replacing conventional grinding in some cases. Machines require proper support and rigid construction. Conventional coated abrasives have random single or multiple layers. Microreplication uses tiny aluminum oxide pyramids in a predetermined orderly arrangement, performing more consistently with lower temperatures on stainless steels and superalloys. Applications include surgical implants, golf clubs, firearms, turbine blades and medical/dental instruments.

Wire Brushing:

Power brushing holds the workpiece against a circular wire brush rotating at 1750 rpm for large wheels to 3500 rpm for small wheels. Wire tips produce longitudinal scratches creating fine or controlled texture. Under proper conditions, it is a light material removal process. Polymeric wires with embedded abrasives are also used.

Honing:

Primarily improves hole surface finish after boring, drilling or internal grinding. The tool has aluminum oxide or silicon carbide bonded abrasive stones mounted on a mandrel that rotates at 45–90 m/min with radial force. The reciprocating axial motion produces a crosshatched pattern. Stones are radially adjustable for different sizes, oil or water based fluids flush debris. Honing also applies to external cylindrical or flat surfaces and for manually removing sharp edges. Finish quality is controlled by abrasive type and size, pressure and speed, fluid removes chips and lowers temperature. Improper honing can produce bell mouthed, wavy, barrel shaped or tapered holes.


Superfinishing:

Uses very light pressure and short stone stroke, with controlled motion so grains do not follow the same path.

Lapping:

Finishes flat, cylindrical, or curved surfaces. The lap, relatively soft and porous (cast iron, copper, leather, cloth) has abrasive embedded or carried in a slurry. Spherical objects and glass lenses use specially shaped laps, gear lapping applies to hypoid gears for rear axles. Pressures range from 7 to 140 kPa depending on material. Dimensional tolerances of ±0.0004 mm can be achieved with fine abrasives up to grit size 900 and surface finish as smooth as 0.025–0.1 µm. Production lapping uses machines similar to those for honing.

Polishing:

Produces a smooth, lustrous finish through softening and smearing of surface layers by frictional heating and very fine abrasive removal. The shiny appearance results from smearing. Disks or belts of fabric, leather or felt are coated with fine aluminum oxide or diamond powder. Double sided polishing uses pairs of pads on horizontally rotating platens in opposite directions. Irregular shapes, sharp corners, deep recesses and sharp projections can be difficult to polish.


Chemical Mechanical Polishing (CMP):

Extremely important in the semiconductor industry using a suspension of abrasive particles in a water based solution with controlled corrosion chemistry. Material removal combines abrasion and corrosion, yielding exceptionally fine finish and flatness often called chemical mechanical planarization. A major application is silicon wafer polishing: the wafer is held face down on a rotating carrier and pressed against a polishing pad on a rotating disk. Carrier and pad angular velocities are selected for uniform wear resulting in constant relative velocity on the axis between centers. Pad grooves uniformly supply slurry and rotation prevents a linear lay. Specific abrasive/solution combinations exist for copper, silicon, silicon dioxide, aluminum, tungsten and other metals. For silicon dioxide or silicon polishing, an alkaline slurry of colloidal silica (SiO₂ particles in KOH or NH₄OH) is fed continuously to the pad-wafer interface.

Electropolishing:

Mirrorlike finishes on metals are obtained by electropolishing, the reverse of electroplating. With no mechanical contact, it suits irregular shapes. The electrolyte attacks projections and peaks faster than the rest of the surface, producing smoothness and also deburring.

Magnetic Field Polishing:

Two methods exist. First, magnetic float polishing of ceramic balls: a magnetic fluid with abrasive grains and ferromagnetic particles in water or kerosene is in a chamber with a guide ring. Ceramic balls are between a driveshaft and a nonmagnetic float, abrasive grains, balls and float are suspended by magnetic forces. Balls are pressed against the rotating driveshaft and polished with extremely small, controllable forces, yielding very fine polishing. Times are much lower than other methods making it economical with few surface defects. Second, magnetic field assisted polishing of ceramic rollers: a ceramic or steel roller is clamped and rotated on a spindle, magnetic poles oscillate giving vibratory motion to the magnetic abrasive conglomerate polishing the cylindrical surface. Bearing steels of 63 HRC have been mirror finished in 30 seconds.

Buffing:

Similar to polishing but produces an even finer finish, using very fine abrasives on soft cloth or hide disks with abrasive supplied externally from a stick of compound.



Economics of Abrasive Machining and Finishing Operations:

These operations are necessary when forming and machining alone cannot achieve sufficient accuracy or finish. Abrasive processes serve both finishing and large scale removal, creep feed grinding is economical versus milling even with high wheel wear, and grinding has become competitive with hard turning for certain applications. Automation computer controls, optimization, robotic handling has reduced labor costs and production times though capital investment is significant. Because these are additional, relatively slow operations, they affect product cost as surface finish requirements increase, more operations are needed, and costs rise rapidly. Total cost depends on part size, shape, finish, accuracy, machinery, tooling and labor. Machinery costs for grinding can be high, while other finishing costs are low. Grinding wheel costs are generally low relative to overall operation but can be hundreds or thousands of dollars depending on composition and size. Finishing tool costs for honing and lapping vary widely. Labor and skill requirements depend greatly on automation. If finishing is important, design stages should analyze required finish and accuracy and whether they can be relaxed and all preceding processes should be assessed for their ability to produce acceptable surface characteristics through proper tool selection, process parameters, metalworking fluids, machine tool characteristics and work holding devices.