Grinding is an abrasive machining process in which hard abrasive grains remove small amounts of material from a workpiece. The abrasive grains are commonly contained within a rotating grinding wheel, with each exposed grain acting as a small cutting edge.
Grinding is widely used to produce accurate geometry and high-quality surface finishes and to machine hardened materials which may be difficult to finish economically using conventional cutting tools. Grinding can also be used for efficient material removal where the process, wheel and machine are selected for that purpose.
Typical reasons for using a grinding process include:
A conventional grinding wheel consists of abrasive grains held within a bonded structure. Spaces or pores between the grains provide room for grinding debris and contribute to the cutting behaviour of the wheel.
Important grinding-wheel characteristics include:
| Characteristic | Engineering significance |
|---|---|
| Abrasive | The cutting material. Common abrasive families include aluminium oxide, silicon carbide, cubic boron nitride (CBN) and diamond. |
| Grain size | Describes the size of the abrasive particles and influences material removal, surface finish and grinding behaviour. |
| Grade | Describes the ability of the bond to retain the abrasive grains. Wheel grade should not be confused with the hardness of the abrasive itself. |
| Structure | Describes the relative spacing of the abrasive grains and the amount of open space within the wheel. |
| Bond | Holds the abrasive grains together and gives the wheel its mechanical structure. Common bond systems include vitrified and resinoid bonds. |
The abrasive material is selected according to the workpiece material, required material-removal behaviour, surface finish and grinding conditions. The traditional distinction between aluminium oxide and silicon carbide remains useful, but modern grinding also makes extensive use of specialised abrasives and superabrasives.
Aluminium oxide abrasives are widely used for grinding steels and many other ferrous materials. Different forms and compositions of aluminium oxide are available to provide different cutting and wear characteristics.
Silicon carbide is a hard, sharp and relatively brittle abrasive. It is commonly used for materials including cast iron, some non-ferrous materials, carbides and certain non-metallic materials, depending on the application.
Cubic boron nitride is a superabrasive used particularly for grinding hardened ferrous materials. Its high hardness and wear resistance make it valuable in production and precision grinding applications.
Diamond abrasives are used for very hard and abrasive non-ferrous and non-metallic materials, including cemented carbides, ceramics and composites. Diamond is generally not selected for conventional grinding of steels because of its chemical interaction with iron at elevated grinding temperatures.
Grinding machines are available in many configurations according to the surface being produced, the way the workpiece is supported and the relative motions of the grinding wheel and component.
Surface grinding is primarily used to produce accurate flat surfaces. The workpiece is supported on a reciprocating or rotary table while the grinding wheel removes material from the surface.
Ferromagnetic components are commonly held using a magnetic chuck. Mechanical clamps, vacuum systems or specialised fixtures may be required for other materials and component geometries.
Surface grinding may use the periphery or face of the grinding wheel, depending on the machine arrangement. Workholding, machine condition, wheel condition and thermal effects all influence the resulting flatness, accuracy and surface finish.
External cylindrical grinding is used to produce accurate cylindrical and tapered surfaces. The workpiece normally rotates while the grinding wheel rotates independently and is fed relative to the workpiece.
Components may be supported between centres, held in a chuck or located using specialised workholding. Typical applications include shafts, bearing seats, journals, spindles and other precision rotational features.
Cylindrical grinding is particularly useful where a component has been hardened before its final diameter, geometry and surface finish are produced.
Internal grinding is used to finish bores and other internal cylindrical surfaces. A relatively small grinding wheel operates within the workpiece while relative rotation and feed generate the required surface.
Because the grinding wheel must fit within the bore, internal grinding often requires relatively small wheels and high spindle rotational speeds to obtain the required wheel surface speed. Spindle stiffness and wheel overhang can therefore be important considerations.
Vertical grinding machines use a vertically orientated grinding spindle and are available in a range of configurations. Depending on the machine, they may perform surface, internal and external grinding operations.
A vertical arrangement can be particularly useful for large or heavy components because the workpiece can be supported on a horizontal table. Modern vertical grinding machines may combine several grinding operations within a single setup.
Centreless grinding produces external cylindrical surfaces without supporting the workpiece between centres or holding it in a conventional chuck. The workpiece is supported on a work-rest blade between a grinding wheel and a regulating wheel.
The grinding wheel performs the principal material removal while the regulating wheel controls the rotation of the workpiece. The geometry and setting of the regulating wheel and work-rest system also influence the movement and geometry of the component.
In through-feed grinding the workpiece passes continuously through the grinding zone. This method is particularly suitable for components having a substantially constant external diameter over the length being ground.
In in-feed grinding the workpiece is positioned in the grinding zone and the wheel is fed radially into the component. This permits the grinding of components containing shoulders, steps or formed features which cannot simply pass through the wheels.
In end-feed grinding the workpiece is fed axially into the grinding zone to a controlled position. The method can be used for certain tapered and formed components.
Grinding-wheel operating speed is commonly expressed as wheel surface speed. For a grinding wheel having diameter D in millimetres and rotational speed N in revolutions per minute:
Vs = πDN / 60000
where:
The corresponding rotational speed is:
N = 60000Vs / (πD)
As a grinding wheel wears and its diameter decreases, its surface speed also decreases if the rotational speed remains unchanged.
The cutting surface and geometry of a grinding wheel change during use. Abrasive grains become worn, workpiece material can load the wheel surface, and wheel wear can alter the required profile.
Dressing conditions the working surface of the wheel. It can remove loaded material and worn abrasive and establish the wheel face condition required for the grinding operation.
Truing restores or generates the required wheel geometry and ensures that the grinding surface runs correctly relative to the wheel axis.
Dressing and truing are often performed as part of the same machine operation, but they perform different functions.
A grinding wheel can become loaded when workpiece material accumulates in the spaces between abrasive grains. Loading can reduce the ability of the abrasive grains to engage the workpiece effectively.
A wheel is commonly described as glazed when the exposed abrasive cutting points have become dull while remaining strongly held by the bond. The wheel can then rub rather than cut efficiently, increasing grinding forces and heat generation.
Correct wheel selection and dressing are therefore important parts of maintaining an effective grinding process.
Grinding can generate considerable heat within a relatively small contact area. Excessive grinding temperature can affect the surface and near-surface condition of the workpiece.
Depending on the material and severity of the process, excessive heat may contribute to:
Grinding temperature is influenced by wheel specification and condition, material-removal rate, contact area, wheel and workpiece speeds, dressing condition and grinding-fluid delivery.
Grinding fluids may provide cooling, lubrication and removal of grinding debris from the contact zone. Effective delivery of the fluid to the grinding zone can be an important part of the process.
The appropriate fluid and delivery method depend on the grinding wheel, workpiece material, machine and operation. Some grinding processes are performed dry while others depend strongly on controlled fluid delivery for process stability and surface integrity.
See also: Cutting Fluids.
Grinding is capable of producing close dimensional control and fine surface finishes, but these are not automatic consequences of using a grinding machine.
The resulting component is affected by factors including:
Grinding-wheel selection should consider the complete grinding operation rather than workpiece material alone. Important factors include:
Where grinding is required as a finishing operation, the component should be designed with the grinding process in mind. Important considerations include:
The following external resources were included on the original RoyMech page and provide additional information relating to grinding and machining.