Definition

Grinding is defined as machining applying tools with a large number of geometrically undefined cutting edges with negative rake angles, which are composed of natural or synthetic abrasive material retained by a bonding material. The chip formation is characterized by a noncontinuous contact and a high relative velocity between the abrasive grains and the workpiece. The cutting motion is either linear or rotating.

Theory and Application

Introduction

Grinding is a manufacturing process that belongs to the group of material removal processes. Material removal processes where a chip is formed can be subdivided into the groups of cutting processes and abrasive processes. Grinding differs from other abrasive processes such as honing, lapping, polishing, and blasting by the tools that are used, the depth of cut, and the kinematics during chip formation. The tools that are used for grinding are grinding wheels, pins, and belts, where the abrasive grains are retained in a bonding material. In the German DIN 8589-13 Standard (2003), the process of “honing by linear cutting motion” is also defined as a grinding process. Honing, in contrast to “honing by linear motion,” is characterized by a cutting motion composed by two components, as defined in DIN 8589-14 Standard (2003). At least one of the motion components in honing is reciprocating. For both honing and “honing by linear motion” stones are used as tools.

The main advantages of grinding are:

  • The good machinability of hard and brittle materials

  • The high shape and dimensional accuracy

  • The excellent achievable surface quality

Figure 1 shows an exemplary grinding process without the use of coolant (dry grinding). As illustrated by the fire sparks, most of the energy in the material removal process is being dissipated into thermal energy. A consequence of improper process design due to the high thermal energy can be thermal damages (Brinksmeier et al. 1982).

Fig. 1
figure 1

Fire sparks while surface grinding without coolant (Reprinted with permission)

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Grinding Processes

Regarding to the DIN standards, the different grinding processes can be classified by several specific attributes (See Table 1).

Table 1 Attributes and resulting grinding processes (according to DIN 8589–11 Standard 2003)
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Grinding Tools

Common grinding tools (also grinding wheels) have a round wheel shape. The standard wheel shapes and sizes are defined in the FEPA standard (Féderation Européenne des Fabricants de Produits Abrasifs). Special wheel shapes beyond the standard for specific grinding tasks are also common in the grinding industry. Grinding wheels are divided into two main categories, conventional and superabrasive grinding wheels. This separation results from the used abrasive, such as aluminum oxide (Al2O3) or silicon carbide (SiC) for conventional grinding wheels and diamond (natural or synthetic) or cubic boron nitride (cBN) for superabrasive grinding wheels (see also “Grinding Wheel”).

Other grinding tools are belts for belt grinding or stones for “honing by linear cutting motion.” Belts are composed by conventional abrasives that are commonly bonded via synthetic resin. Stones are comparable to conventional grinding wheels with regard to abrasives and bond.

Grinding Wheel Preparation

Before a grinding wheel can be used or after it is worn, it needs to be conditioned. Conditioning includes the subcategories dressing and cleaning. Dressing is subdivided into profiling (regeneration of the macro geometry) and sharpening (regeneration of the micro geometry). Cleaning in this case means removing chips out of the bond with a high-pressure coolant jet, aimed directly at the bond (see, e.g., Heinzel and Antsupov (2012)). More about conditioning can be found in Dressing or Wegener et al. (2011). Recent research aims to structure wheels via dressing to increase the grinding performance (see, e.g., da Silva et al. (2016) or Warhanek et al. (2015).

Coolants/Metal Working Fluids

Grinding is a thermodynamically dominated process due to the high friction in the contact area. Hence, coolants are always necessary for high-performance grinding processes. Their three main tasks are (VDI 3397 Part 1 2007):

  • Lubricate the contact area for reducing the friction

  • Dissipate the heat from the contact area, e.g., by cooling

  • Flushing chips out of the contact area and transport them to the coolant filter equipment

A distinction for coolants in grinding is drawn between oil-based (base oils with and without additives) and water-based coolants (emulsions or solutions). In general, oil-based coolants have much better lubricating properties than water-based coolants but comparatively bad cooling properties. The coolant supply to the contact area has to be set up for every specific grinding task with the right quantity, velocity, and direction (see Heinzel et al. (2015)). For further information see “Grinding Fluids” (coolants) and Brinksmeier et al. (2015).

Grinding Machines

A wide variety of machines are used for grinding, e.g.,

  • Sanders mostly for belt grinding

  • Handheld power tools such as angle grinders

  • Bench grinders

  • Various machine tools, here grinding machines in a narrow sense

Widely used grinding machines are flat and profile grinding machines, such as those shown in Fig. 2. Those machines are able to grind a wide range of workpieces starting with geometric simple parts like linear guideways or profiled parts like broachs. Typical grinding operations with this kind of machines are surface and form grinding done as peripheral grinding. If the machine is also equipped with an index table or additional spindle (coordinate grinders), it is also possible to perform internal circle grinding, gear grinding, etc. Standard are three linear axes and one spindle for the grinding wheel.

Fig. 2
figure 2

Schematic view of a flat grinding machine (Reprinted with permission)

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Another popular kind of grinding machines are tool grinders (Fig. 3). Those machines are used for production or resharpening of drills, mills, tread cutters, inserts, etc. To fulfill these operations, CNC tool grinders have at least five full CNC-controlled axes (in the example of Fig.3, three translational and two rotational axes).

Fig. 3
figure 3

Schematic view of a 5-axis CNC tool grinding machine (Reprinted with permission)

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Application

Grinding, as a key technology for manufacturing advanced products and surfaces, can be found where one or more of the following factors apply (Marinescu et al. 2007):

  • High accuracy

  • High removal rate

  • Machining of hard/brittle materials

High Accuracy

Due to its high accuracy, grinding is used to produce parts with high requirements for accuracy and tolerance as well as surface roughness and performance. The range varies from large parts like hardened machine tool slideways to small parts such as medical injection needles.

High Material Removal Rate

Grinding is recommended particularly for materials that are hard to machine (like tungsten carbide or some kinds of nickel-base alloys). The achievable material removal rates are much higher compared to other machining processes. Examples are the flute grinding of mills and drills out of solid tungsten carbide rods in one pass with superabrasive grinding wheels or fir tree slotting on turbine vans (nickel-base alloys) with conventional grinding wheels in one pass.

Machining of Hard/Brittle Materials

Grinding is the predominate process while machining brittle materials such as glass, ceramics, or even diamond at high accuracy. The ability to machine those materials makes grinding also the first choice for finishing tasks. Examples are grinding of hardened bearing seats or machining engineering ceramics. Figure 4 shows a crank shaft with ground bearing seats.

Fig. 4
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Crank shaft for a 2-cylinder motorbike engine with ground bearing seats (Reprinted with permission)

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Cross-References