Abrasive Tools Types

Abrasive Tools

It is difficult to say when abrasive technology had a beginning. Abrasives were used by man many thousands of years before he learned to write. Primitive man used abrasives for the sharpening of tools of wood, bone or flint. […] The beginning of the science of abrasives, however, may be taken as that time when man began to select certain rocks for their peculiar properties, and to fashion these into tools for grinding. [COES71].

Grits and bonding systems have been introduced in the previous two chapters and their impact on technological, environmental, economic and social sustainability has been discussed. Yet not only the ingredients, but also the design and type of tools define sustainable grinding process performance. Therefore, this chapter discusses abrasive tools distinguished by the cutting edge engagement (Fig. 1.1). In grinding with bonded or coated tools, the grit penetrates the workpiece bound to a path. Room-bound interaction takes place in lapping and polishing, where the grits roll between workpiece and a counterbody, often within slurry. In energy-bound abrasive processes, the grits are accelerated towards the workpiece material and dash particles out of the surface. The fourth active mechanism occurs when the tool is pressed with higher force but constant pressure onto the workpiece. Vibratory grinding and honing are considered to be mostly force-bound processes.

1.1  Grinding Wheels

Grinding wheels are also known as bonded tools. The German Norm DIN 8589 specifies grinding processes by the machined workpiece area, wheel contact area, and feed direction [DIN03]. Most conventional grinding wheels consist of the conventional abrasives Al₂O₃ or SiC in a monolithic wheel body (Fig. 4.1 left). Superabrasive grits are more wear-resistant and expensive, so it is established that they are used in a thin abrasive layer on a wheel body. Depending on the bonding system, the abrasive layer is multi-layered and can be dressed (Fig. 4.1 middle) or mono-layered (Fig. 4.1 right).

© Springer International Publishing Switzerland

B. Linke, Life Cycle and Sustainability of Abrasive Tools, RWTHedition, DOI 10.1007/978-3-319-28346-3_4

For high-speed machining, vitrified bonds have higher safety requirements on grinding tool toughness. High efficiency deep grinding requires tools with appro- priate porosity and high toughness. Conventional grinding tools have gained competitiveness through low-temperature sintering bonds and enhanced abrasive grits. [KLOM86, p. 9]

1.1.1   Shapes

Tools are characterized by tool shape and abrasive layer composition. The tool shape specification follows DIN ISO 525 and DIN ISO 603, FEPA standards, ANSI B74.2 (conventional tools), or ANSI B74.3 (superabrasive tools) [DIN00a, DIN00b]. In addition, the wheel specification often includes company specific terms as well as the used standard. Table 4.1 shows typical categories in tool specifications.

Fig. 4.1 Bonded abrasive tools

The wheel dimensions are expressed in the form of “(outer diameter) × (thick- ness) × (inner diameter)”; for superabrasive wheels the layer thickness is added [MARI07, p. 46]. Many examples of tool shapes are shown by Marinescu et al. [MARI07, p. 46–48].

Layer width is important for grinding forces and achievable surface roughness. Cup wheels with a higher layer width can achieve significantly lower workpiece roughness than cup wheels with a small layer width due to the higher number of passes [JUCH78].

The abrasive layer is composed of abrasive grits, bonding, and pores (see Sect. 6.1 “Abrasive Layer Composition”). The volumetric composition of grits, bonding, and pores tailors the grinding tool application. The grits have to perform the cutting action. The bonding holds the grits together and releases blunt grits. Furthermore, the bond transfers the forces from the tool rotation and conducts process heat. The pores bring cooling lubricant to the cutting zone and transport chips away from it. The pores act as chip and cooling lubricant space and can be varied in number, size and total volume in a wide range via artificial pore builders.

Table 4.2 describes typical information contained in the tool designation. Marinescu et al. summarize examples of conventional wheel specifications [MARI07, p. 109] and of vitrified CBN wheel specifications [MARI07, p. 117].

“Structure” can be defined as a characteristic proportional to the distance between grits, but this definition just works for constant grit sizes [RAMM74]. These structure grades do not necessarily provide information about the porosity of the abrasive layer [RAMM74]. The abrasive layer structure is discussed in Sect. 6.1 “Abrasive Layer Composition”.

Table 4.2 Typical specifications of grinding tool layers [KLOC09, MARI07, p. 109, 117, BORK92, p. 33, 40, DIN00a]

1.1.2  Special Grinding Wheel Types

• Centerless Grinding Wheels

Centerless grinding is commonly applied for large batch and mass production. In this circumferential grinding variant, the workpiece is not fixed along its axis between centers, but is supported on its circumference. Centerless grinding can be external or internal.

In external grinding, the workpiece lies between grinding wheel, workrest plate and control wheel.

In internal centerless grinding, the workpiece lies between rolls or shoes and is driven by a control wheel or a face- plate.

In centerless grinding wheels, the structural density along the wheel width has to be monitored carefully.

During manufacturing and pressing of large width wheels, the pressure needs to be higher and applied longer than for thinner wheels [TYRO03]. For centerless grinding wheels, wear is highest in the transition zone from grinding to spark-out zone. This is however not connected to the wheel density, but to the profile wear in transverse grinding operations as explained in Sect. 6.3.1 “Macro Effect—Tool Profile Loss”.

The contact properties between workpiece, grinding wheel, control wheel, and workrest plate define process stability [SCHR71]. The grinding wheel elasticity affects the grinding forces and depth of cut [SCHR71, p. 28 f.].

The control wheel, also known as regulating wheel, regulates the speed of the workpiece. It is a conventional grinding wheel, often rubber bonded corundum, or a steel body with a cemented carbide coating. The control wheel regulates the workpiece speed by accelerating or decelerating through friction. Control wheels have complex shapes to realize a linear support of the workpiece. Control wheel profile is influenced by control wheel inclination angle, workpiece center height, and workpiece diameter [MEIS81]. New calculation models for control wheel profile enable shorter dressing times and lower wheel wear [MEYR12].

• Gear Grinding Wheels

Gear grinding is distinguished into generating grinding and profile grinding as well as continuous and discontinuous grinding [KARP08]. Generating gear grinding generates the gear shape mainly by the complex process kinematics, profile gear grinding mainly through the grinding wheel profile (Fig. 4.2).

The grinding tools have to withstand long contact lengths. The contact length in tooth flank profile grinding can be as long as in creep feed grinding [SCHL04]. Karpuschewski et al. [KARP08] describe recent developments in gear grinding applications.

Fig. 4.2 Gear grinding tools, with permission from the Gear department, WZL of RWTH Aachen University

• Cylindrical Peel Grinding Wheels

External cylindrical form grinding processes or so called peel grinding procedures are defined by a transverse feed and an inclined wheel. As consequence, the machining zone is small and approaches a punctiform contact in theory. This procedure is very flexible regarding workpiece shape and the grinding forces are comparatively small. Due to the punctiform contact zone, however, the grinding forces result in high pressure and high load on the single abrasive grit. Often  superabrasive grits are used and they have to be fixed well within the binding matrix. In general, the abrasive layer exhibits a high hardness [BLAN07]. Today, mostly vitrified bonded CBN grinding wheels are used with wheel circumferential speeds of up to 160 m/s for peel grinding applications [BLAN07]. Specific material removal rates of several hundred mm³/mms are possible [BLAN07].

The variant Quick-Point grinding was patented by Sir Erwin Junker in 1994 and integrates high wheel speeds, CBN grits, and CNC controlled paths [WANG12]. Consequently, tool layout has to be adapted [BLAN07]. Wang et al. [WANG12] suggest that future research focusses on new matrix materials with low density, thermal expansion coefficient, and high specific strength.

• Tool Grinding Wheels

Tool steels and high-speed steels are machined with corundum or CBN wheels [LINK09, KÖNI80]. Carbides and ceramic tools are ground mainly with silicon carbide or diamond wheels. Diamond grit sizes for carbide tool grinding wheels lie commonly between D46 and D181 [FRIE02, p. 46]. Diamond grits with low toughness enable self-sharpening during the grinding process, whereas an irregular shape facilitates good grit retention in the bond [TOML78a]. For the machining of tough carbides and cermets, tendency goes to applying high diamond grit con- centrations to have small single grit forces [FRIE02, p. 46]. As consequence, the

grinding wheel wear is low and dimensional tolerances as well as form tolerances of the machined workpieces can be kept constant. Tool grinding operations demand for grinding wheels of complex shapes. Resin bonds have advantages in easy dressability and good damping capabilities because of their soft bonding [FRIE02, p. 53].

• Surface Grinding Wheels for Turbine Materials

Turbine materials such as Ni- or Al-alloys are highly ductile and produce long chips. The danger of wheel clogging is given and chip formation is characterized by smearing and burr generation. The low heat conductivity of turbine materials leads to heat induced damage of the surface layer in the grinding process. As conse- quence, grinding wheels for turbine materials stand out by their high porosity. In industry, continuous dressing is applied to sharpen and clean the grinding tool simultaneously to the grinding process.

The so-called Viper grinding method uses high pressure coolant flow to con- tinuously clean and dress the grinding wheel [HILL00, HYAT10]. Highly porous wheels are used. In addition, the carbides and intermetallic phases in turbine materials object the grinding tool to abrasive wear, so that self-sharpening occurs.

• Grinding Pins

Grinding pins, also called mounted wheels or mounted points, are small wheels to which a mandrel is cemented, molded or die casted into one end [LEWI76, p. 44]. These tools are often used in hand-held operations for deburring, finishing of welds, chamfering, or dental operating procedures. A large variety of shapes exists (see DIN ISO 525) [BORK92, p. 32, DIN00a]. The long shafts of grinding pins act as cantilever und grinding forces lead to deformation of the tool. An open wheel structure with soft bonding enables favorable self-sharpening.

• Long Needle Diamond Grinding Wheels

In the late 1970s, long needle diamonds were offered on the market [METZ86,

p. 43, DYER79]. These long needle diamonds have a proportion of length to thickness of between 2:1 and 5:1 [TOML78a]. They are synthesized with growth direction in diamond main axis direction, i.e. crystallographic [100]-direction [TOML78a]. The growth mechanism of constant buildup on (111)-areas leads to a stepwise surface structure. Diamond cleaves parallel to the octahedric planes, so that long needle diamonds are weak in rectangular direction to their main axis. In grinding, the grits can easily break along the stepwise growth planes and a con- trolled grit wear is likely [JUCH78, TOML78a].

Fig. 4.3 Electromagnetic system to orientate long needle diamonds in cylindrical 1A1-wheels after [TOML78a]

The grit size classification of long needle diamonds cannot be done via con- ventional sieve procedures, therefore grit manufacturers have to do physical mea- surements with projectory microscopes [TOML78a].

To attain the theoretical breakage behavior as well as optimum grit adherence in the bond, needle grits with ferromagnetic coating can be arranged with electro- magnetic systems. The example in Fig. 4.3 works with radial magnetic field lines in cylindrical 1A1 grinding wheel die forms. The grits orientate themselves parallel to the magnetic field lines when they are filled into the die form with the resin bond mixture. Pressing and curing processes are similar to other resin bonded wheels. [TOML78a]

In grinding applications, grinding wheels with needle diamonds have a higher number of active abrasive particles on the surface than similar abrasives of the same grit size. Therefore, resin bonded long needle diamond tools showed a better per- formance for brittle workpiece materials [TOML78b]. However, the long needle diamond grinding wheels was not popular in the market [METZ86, p. 43 f.]. In contrast to grinding, needle type diamonds find wide application in dressing of vitrified bonded grinding wheels.

1.2  Coated Abrasive Tools

Coated abrasive tools are composed of abrasive grits that are held by a bond on backing material (Fig. 4.4). Coated abrasive tools are belts, pads, or discs. Abrasive discs are used for preparing car bodies before painting [BORK92, p. 42 ff.]. Deburring, roughing, and finishing are important operations with coated tools in the metal grinding field [KÖNI86]. Coated abrasive tools cover a bigger market volume than grinding wheels [ASAM10, p. 312].

Figure 4.4 shows the abrasive layer structure and common designs of coated abrasive tools. Grinding engagement between grinding belt and workpiece is

Fig. 4.4 Coated abrasive tools

established by contact shoes or wheels or the engagement can take place on the free part of the grinding belt without a contact element [KÖNI86].

Coated abrasive tools consist of the components in Table 4.3, namely grits, backing material, make the coating, size coating, and splice. In addition, abrasive grit size, shape, and size of the abrasive tool are important characteristics [BORK92,  p. 42 ff.].

1.2.1           Manufacturing

Grinding belts are manufactured as displayed in Fig. 4.5. The manufacturing route starts with the backing material, which can be paper, cloth of natural or synthetic fiber, or metal (in the case of diamond coated abrasive tools) [BORK92, p. 47]. In addition, the backing can be wet-proofed or reinforced with wire. Strength and flexibility are main characteristics of the backing material.

The bond material to hold the abrasive grits can be applied in one or more layers, with the make coat as base bond and size coatings as upper bonds [BORK92, p. 47, KÖNI86]. Additional coatings can have dust-repellent effects [KLOC09, p. 66]. First, the make coating is applied and will form a gel after cooling or drying [KÖNI86]. This gel fixes the abrasive grits, which are applied in the so called mineral coating step either by gravitational force (gravity scattering) or by elec- trostatic scattering. [KLOC09, p. 69 f.]. In the gravity scattering method, a distri- bution device applies the grits on the coated backing material. In the electrostatic scattering method, the coated backing material is moved upside down over a transport belt, on which the abrasive grits are oriented by an electrostatic field [KLOC09, p. 69]. The advantages of the electrostatic scattering method are an even grit distribution, a higher reproducibility, and higher grit protrusion (Fig. 4.6).

After positioning the grits, the size coating is applied. It supports the retention of the grits amongst each other. After drying and hardening, the belt is rolled, cut, and possibly joined. The type of belt splice is crucial for the process stability and can be reinforced with foil [BORK92, p. 45]. Overlapping splices can be used for all

Table 4.3 Components of coated abrasive tools [BORK92, p. 47, KÖNI86, KLOC09, p. 67 ff., CHRI96, COLL88, p. 911, UAMA09]

1.2.2   Abrasive Grits

The single crystal grits used in coated abrasives are similar to the ones used in grinding wheels. Grit sizes are usually graded in larger intervals than for grinding wheels [BORK92, p. 46]. The grits are fully embedded in the bond material, which leads to high resistance against grit-breakout [KÖNI86, KLOC09, p. 217]. Therefore, the wear mechanisms grit splintering and attritious wear are more common than grit-breakout in belt grinding [KÖNI86, KLOC09, p. 217].

Besides single crystal abrasives, polycrystalline abrasives are standard for coated abrasive tools. Hollow corundum is a sintered abrasive in the form of a hollow sphere and has proven to be a long-lasting abrasive material for belts [BORK92,

p. 46]. Even if the spheres wear, they expose abrasives on their hull and perform uniform cutting action.

Compact bonds are aggregated abrasives in a bond, which can form special shapes. Recently developed examples are pyramid-shaped conglomerates of dia- monds in vitrified bond, which are advantageous in lapping [ZHEN10]. The dia- mond particles are held in a vitreous bond and are shaped like truncated pyramids, which allows a stationary wear [ZHEN10]. The pyramids are fixed with a resin matrix to the backing material [ZHEN10].

Fig. 4.7 Honing tools, with permission from the Gear department, WZL of RWTH Aachen University

1.3        Honing Tools

Honing tools are similar in bonding and grit material to grinding wheels. Prominent tools are honing sticks, profiled tools, and gear honing tools (Fig. 4.7). The shape is defined by DIN 69186.

In particular, vitrified, resin, sintered, and electro-deposited bonds are used [KOPP81]. Grit sizes and grit concentration are similar to grinding tools with relatively high concentrations. Fine grit honing sticks are used for sizing and sur- face finishing, not for form correction [KOPP81]. Unlike grinding operations, diamond and CBN can have similar honing results for the same material machined. In honing tools, the bond hardness is selected in relation to abrasive grit size.

Harder bonds are generally taken for coarse diamond and CBN. For honing sticks, ferrous bonds are frequently used for coarser grits (bigger than D151 or B151), softer bronze bonds are the norm for middle size grits (D64–D126 or B64–B126) and resin bond is used for finer grits to micron size. [KOPP81]

According to the German standard DIN 8589 honing is defined by the contin- uous contact between tool and workpiece. This does not apply to gear honing. Per definition and in consequence, gear honing processes belong to the category of grinding processes. In gear honing, the honing tool is profiled with an internal gear toothing. The honing tool is dressed with a gear dressing roller and a cylindrical dressing roller for setting back the honing wheel teeth [VUCE08]. Vucetic com- pared vitrified and resin bonded gear honing tools in single grit scratch tests and honing tests and found consistently that the vitrified bond can hold the grits better than resin bonds [VUCE08].

1.4   Polishing Tools

In the polishing process, the abrasive particles are finely dispersed in a liquid medium or binder and are directed over the workpiece surface by a counterbody [KLOC05a, p. 404]. Therefore, the polishing tool is rather a paste or suspension of abrasive grits in a medium or binder [MARI04, p. 442].

Fig. 4.8 Lapping and polishing after [KASA90]

In plate polishing processes, a binder retains the abrasive particles on the plate surface. The binder needs to fuse on light contact with the polishing plate and adhere well to the surface. The quality of the paste depends on fusion temperature and vaporization temperature of the binder. The greater the difference between these temperatures, the better the quality of the compound will be. [MARI04, p. 442]

Polishing processes can be classified according to abrasive grit size (fine or coarse) and counterpart (hard or soft) (Fig. 4.8) [KASA90]. Optical polishing of glass works with fine grit and a soft counterpart. This classification, however, does not give an indication of the active mechanisms for material removal [HAMB01,

p. 6]. In general, an interaction of chemical and mechanical mechanisms achieves the material removal [KLOC05a, p. 404].

Polishing of steel with diamond grits in a water-alcohol suspension can be explained by the local loads between polishing grits and workpiece which can lead to plastic deformation of the steel [DAMB05, p. 125]. Due to the high thermal conductivity of the diamond grits, heat effects and chemical reactions are unlikely [DAMB05, p. 126]. Material removal takes place because of micro plowing and micro chipping [DAMB05, p. 126].

1.4.1  Abrasives for Polishing

Abrasives for polishing processes tend to be chosen according to the desired material removal rates. Common grit sizes are around 1 µm [MARI04, p. 442]. In rough abrasive pastes, the following abrasives with higher hardness are used [MARI04, p. 381, BORK92, p. 11, 13, 20, DAMB05, p. 33]:

• Diamond (C),
• Magnesia (MgO),
• Pumice (vitreous spongy compound formed after drying of volcanic lava),
• Beryllium oxide (BeO),
• Chrome oxide (Cr₂O₃),
• Iron oxide (Fe₂O₃),

• 4.4      Polishing Tools                                                                                                            109
• Garnet (class of minerals with the formula (Me^″Me^′″SiO ) , with Me″ being Fe,

3         2           4 3

Mg, Co, or Mn, and Me′″ being Fe, Al, or Cr),

• Corundum (60–90 % Al₂O₃),
• Emery (60 % Al₂O₃ and Fe₃O₄, Fe₂O₃, SiO₂),
• Quartz (SiO₂ with CO₂, H₂O, NaCl and CaCo₃ inclusions),
• Silica carbide (SiC), and
• Glass (Processed recycled glass has been sold as an abrasive blasting medium under several brand names in the United States).

Soft abrasives as the following are employed for fine polishing paste compo- sitions [MARI04, p. 381 f., BORK92, p. 20]:

• Kaolin,
• Chalk (obtained from crushed, rinsed and washed limestone),
• Barite (barium sulfate),
• Talc (hydrous silicate of magnesium),
• Tripoli (white sedimentary mineral obtained after coagulation of silica gels in laminae compacted into soft rock mass), and
• Vienna lime (white powdery composite of fine-crystalline calcium and mag- nesium oxides, obtained after burning dolomite).

1.4.2 Binding Materials for Pastes

Binders for pastes include the following [MARI04, p. 442 f., BORK92, p. 24]:

• Stearin—stearic acid, CH₃(CH₂)₁₆COOH, white, solid, crystalline substance, melting point of 140 °C; Stearin is a good binder and brings cohesion and hardness to the paste.
• Oleic acid—olein, unsaturated fatty acid, melting point of 15 °C; Oleic acid  accelerates the polishing process by dissolving metal oxides.
• Paraffin—waxy, crystalline mixture of fatty hydrocarbons, melting point of 44 ° C; Paraffin does not likely convert into a resin or carbonize. It brings cohesion, elasticity, hardness, and adhesion to polishing pastes.
• Fats—Organic, fusible glycerides of saturated fats and unsaturated oils; Fats are often used instead of stearin.
• Wax—Solid, unctuous or liquid fatty acid esters with higher fatty alcohols; Examples are carnauba plant wax (melting point 353–359 K), beeswax (melting point 333–340 K), and montan wax (melting point 351–353 K). Waxes provide hardness and cohesion to a paste.
• Petroleum jelly—petrolatum, obtained from asphalt less paraffin-base crude oils; This substance reduces the hardness of a paste.

Surface-active substances and emulsifiers are sometimes added to polishing pastes. Their tasks are to intensify the machining operation and to increase

durability of the abrasive compound. Thixotropic substances (including aluminum soaps, aluminum alcoholates, complex bentonite, and fine talc powder below 1 μm) are added to fluid pastes to increase viscosity [MARI04, p. 443]. For example, in glass polishing the fluid has a large effect on the tribological effects and material removal process [HAMB01, p. 127].

Polishing with pitch is used to finish high value optical components. Pitch is a viscoelastic material and primarily derived from either pine tree resins or petroleum-based resins [MULL08].

1.4.3     Counterparts

A large variety of counterparts or polishing pads is available with a lack of application based models [DAMB05, p. 30]. Three main types for counterbodies can be distinguished [KHAL79, DAMB05, p. 30]:

• Deformable polishing pads, such as pitch or cast polyurethane,
• Soft polishing pads, such as cloths and synthetic felt with porous structure,
• Hard polishing pads, such as hard felt, filled or not filled polyurethane foam, impregnated cloths, fine laminates.

The choice of the pad affects the material removal rate. Hambuecker [HAMB01,

p. 126], for example, showed that polyurethane foam is superior to pitch pads in automated glass polishing.

1.5   Lapping

Lapping is a mainly room-bound process with geometrically undefined cutting edges. It is defined as a cutting process with loose grits distributed in a fluid or paste, so called lapping slurry, guided by a counterpart, which is usually shape-transferring (also called lapping tool). The cutting paths of the individual grits are ideally undirected [KLOC09, p. 338]. Effective mechanisms are complex and assumed to be a superposition of chip formation, micro-grooving, and brittle machining by micro-cracks and particle break-out [KLOC05a, p. 384 f.].

Known process variants are planar lapping with fixed or freely moving work- pieces, external or internal cylindrical lapping, profile lapping and ultrasonic lap- ping amongst others. Karpuschewski [KARP01, p. 173 ff.] summarizes sensors for lapping operations.

1.5.1  Abrasives for Lapping

Common abrasives for lapping processes include silicon carbide, corundum, boron carbide (B₄C), and diamond [KLOC05a, p. 394, STAD62, p. 24]. Boron nitride (B₄N) is used for lapping of carbides [MARI04, p. 394]. Regular grit dimensions are as follows [MARI04, p. 442]:

• Silicon carbide and corundum: 5–1 μm
• Boron carbide:                              60–5 μm
• Diamond:                                      5–0.5 μm
• Chromium oxide:                        2–1 μm

Diamond slurrys are designed so that even coarse grits remain homogeneously dispersed in the fluid. The abrasives will not settle out, even if the slurry binder has a low viscosity [KLOC05a, p. 393]. Conventional abrasives are continually fed onto the lapping plate to restore broken down particles. Recharge of diamond grits occurs less frequently [DAVI74].

In abrasive processes with loose abrasives, shape, grit size, size distribution, and grit breakdown characteristics are important variables for process control [DAVI74]. As for grit shape, blocky isometric grits are considered the best, because they lie between the extremes, sphere and needle [DAVI74]. Spherical grits tend to roll between lapping plate and workpiece instead of performing machining action; Needle particles, in contrary, can cause deep scratches. The presence of a few oversized abrasive grits can result in serious workpiece damage [DAVI74].

1.6 Tools for Abrasive Sawing

The biggest volume of diamond saws is used in cutting stone and refractory materials. Abrasive wires and saws are also used in the electronics and solar industry to cut silicon and quartz crystals into wafers. In Multi-Wire-Slicing, a wire is led by several coils to form several wire lines, which cut simultaneously [KLOC09, p. 386]. The wire can either contain grits or work within a grit con- taining slurry (Fig. 4.9 left).

Fig. 4.9 Abrasive wires and hole saw

1.6.1   Wires with Bonded Grits

Wires with bonded grits consist of a core coated with abrasive grits, mostly dia- mond. The diamonds can be bonded through resin bonds or by electroplating [CHIB03, KLOC09, p. 385]. Chiba et al. [CHIB03] presented a new high-speed electroplating process to manufacture the wire saws faster and cheaper.

1.6.2   Wires with Loose Abrasives

In the wire lapping procedure, a lapping medium of grits (commonly SiC) and medium (commonly  oil  or  glycol)  is sprayed  on  a  non-coated  wire [KLOC09,

p. 386]. Because of the high costs and expensive disposal of the slurry, researchers have developed strategies for slurry refreshment and recycling [KLOC09, p.  387].

1.6.3  Inner Diameter Saw

The inner diameter saw, also called inside hole saw or ID-saw, consists of a rotating steel blade, whose inner diameter is electroplated with diamond grits (Fig. 4.9 right) [KLOC09, p. 387]. The steel blade is clamped at its outer diameter and either the saw blade or the workpiece are moved radially.

1.7 Other Methods with Free Abrasives

1.7.1   Crushing

In the materials processing terminology, particle size reduction by mechanical means is called grinding [HOGG01, LYNC05]. Other terms are crushing, milling, or attrition milling. The term crushing is used for bigger end size particles (e.g. about 13–19 mm in diameter); the term milling refers to low micron sizes or below.

Machines used for these processes are often called grinding mills [SCHI10]. The grinding machines can be media mills (tumbling mill, centrifugal mill, stirred-media mill, vibratory mill), impact mills (hammer mill), or fluid energy mills [HOGG01, LYNC05]. Size reduction in crushing processes results from particle fracture under stress [HOGG01].

For size reduction processes, a huge variety of abrasive materials is used such as steel, corundum, glass, nylon, SiC, SiN, tungsten carbide, zirconium oxide, zirco- nium silicate, etc. [NN12]. Several factors such as abrasive grit size, hardness, specific gravity, shapes, and chemical reactions have to be considered when selecting the grinding media [NN12, SCHI10].

Fig. 4.10 Example shapes of the abrasive particles for vibratory grinding after [ROES12, WALT12]

1.7.2   Free Abrasive Machining

In barrel finishing or tumbling, workpieces, abrasive particles, and a fluid are tumbled in a slowly rotating container. In vibratory grinding, also known by the trade name “trowalizing”, the container vibrates. The fluid can be water, acid, or alkaline compound [MENA00]. Barrel finishing and vibratory grinding are applied in die and mold manufacturing, medical and aerospace engineering for deburring, degreasing, polishing, or derusting [BROC11].

The abrasive particles, also called “chips”, can be ceramic, plastic or metallic materials and have various shapes (Fig. 4.10). Sizes range from edge lengths of 3–25 mm or more. Vitrified bond is the dominant bond type for chips with abrasive action [KLOC09, p. 57]. Steel particles are used for ball burnishing, i.e. inducing compressive stresses. Coarse chips are sized through sieving according to ANSI B74.11 [ANSI03].

The liquid carrier compound ranges from acidic to basic pH values and has additional tasks such as removing contaminants from the process, keeping the machined parts clean, etc. The compound type is chosen under environmental and economic considerations.

1.7.3     Abrasive Blasting

In abrasive blasting, the abrasive grits are energized with compressed air, centrifugal force, or pressurized water and aimed at the workpiece material. Corundum, silicon carbide and quartz are typical abrasive grits for blasting [KLOC09, p. 371]. Emitted particles [particulate matter (PM)] and particulate hazardous air pollutants (HAP) are the major concerns relative to abrasive blasting [EPA97]. Several methods exist to control the air emissions, such as blast enclosures, vacuum blasters, drapes, water curtains, wet blasting, and reclaim systems [EPA97].

1.8   Tool End of Life

Product life is defined by several causes, such as physical life (break-down beyond repair), functional life (need for the product ceases), economic life (new products offer the same functionality at lower operating costs), legal life (regulations make

the product illegal) [ASHB09, p. 66]. The most important causes for end of grinding tool life are tool wear to the minimum abrasive layer dimensions or tool degradation at the end of shelf life. Options for end of life are landfill, combustion, recycling, reengineering, or re-use [ASHB09, p. 67]. Reengineering and re-use are not feasible for most used abrasive tools and are not considered here.

1.8.1   Shelf Life and Transport

Grinding tools should be stored in dry, evenly tempered, frost-free rooms [BGI10]. Direct sunlight, uneven heating, bending, and vibrations during transport might lead to dangerous cracks [BGI10]. Large grinding wheels need to be moved by a crane with specific fixtures [BGI10].

Coated abrasive tools may contain organic material and may degrade with time [UAMA09]. One recommendation is to use grinding belts and discs within 10 years from the date of manufactured if stored under ideal conditions [UAMA09]. Before use, coated abrasives should be inspected and must not be used, if they appear brittle, curled, damaged, discolored, or of the joint can be pulled apart [UAMA09]. Tools with magnesite bonding age faster in humid environments [BGI10]. Vitrified grinding wheels have an almost infinite shelf life, but the chance of tool damage during storage still applies [UAMA09].

1.8.2    Disposal

The European Waste Catalogue (EWC) [or Verordnung ueber das Europaeische Abfallverzeichnis (AVV)] classifies used honing and grinding tools with the code 12 01 21 or 12 01 20, if they contain hazardous materials [BGBI01]. The main category 12 includes waste of material removal processes and physical and chemical surface alteration. The waste generator has to determine toxicity and physical characteristics to identify the waste correctly and dispose in compliance with the applicable federal, state, and local regulations [UNIT12a]. For example, metal bonds for diamonds are often considered hazardous waste and have to be disposed accordingly [MCCL10b]. Coated abrasives can produce a huge amount of waste.

In 2010, Germany disposed 7400 t of honing and grinding tools with hazardous materials and 13,600 t of other honing and grinding tools [DEST10]. 200 t of the honing and grinding tools with hazardous materials were combusted and 5700 t got chemical-physical treatment; 1000 t of the honing and grinding tools without hazardous materials went to landfill, 200 t to combustion and 7100 to chemical-physical treatment [DEST10].

The intrinsic energy of materials can be turned into heat through combustion. Heat recovery efficiency is at best 50 % and the efficiency to generate electricity

Table 4.4 Material data on combustion (*estimated values) [GRAN10]

from recovered heat at best 35 % [ASHB09, p. 68]. Material for combustion has to be separated from non-combustible material, and the combustion process needs careful control so that the emissions are not toxic [ASHB09, p. 67 f.]. Table 4.4 shows heat and CO₂ of combusting epoxy and phenol, which are resin bond ingredients.

1.8.3   Recycling of Abrasive Tools

There is little information available about the re-use of abrasive grits. Especially for the expensive superabrasives, recycling is important under the growing awareness of material and energy efficiency. McClarence [MCCL10b] estimated in 2010 that only between 8–10 % of new diamond is reclaimed.

• Conventional Tools

Conventional grinding wheels can be crushed and backfilled in roadworks. Re-use of the abrasive layer is difficult, because inhomogeneous density and heterogenous particles with variable diameter decrease tool toughness and increase the danger of cracks and tool breakage in vitrified bonded tools [BEHR11b]. Because of high  safety and liability requirements for rotating abrasive tools, grinding wheel man- ufacturers have refrained from using recycled raw materials [BEHR11b]. There are also some attempts to regain abrasives from conventional grinding wheels for refractories, but this is a down-cycling of abrasives.

Recently, a new manufacturing procedure for vitrified bonded tools from partly recycled tool material was invented [BEHR11b]. The tools consist of an abrasive layer made from new materials and a body material made with recycled materials in the form of particles with a minimum diameter of two times the grit diameter.

• Single Layer Plated Tools

Electroplated diamond grinding tools exhibit potential for recycling. In particular, the body can be reused. Electroplated grinding wheels are generally returned to the manufacturer, who will strip off the abrasive layer and re-plate the body.

Usually chemical and electrochemical stripping methods are used [BULJ99]. Chemical methods such as acid baths corrode the bonding [BULJ99, YU11].

Electrical voltage might induce reverse electroplating in addition [BULJ99]. Electrochemical approaches have lower recovery efficiency and bear the danger of corroding the tool body [YU11]. To fulfill stricter requirements on environmental friendly production, new chemical baths are cyanide-free [GEBH99]. Yu et al. [YU11] investigated a thermal shock procedure to induce stress between the abrasive layer and the tool body. The electrobonded tools are heated and then quickly cooled down in water, so that the abrasive layer peels off from the body. Other approaches aim at inventing metallic bonds that are easier to remove, such as bronze alloys with Sn, Cu, and Ti [BULJ99].

In 2010, the cost to recover diamond from metal bonds was roughly 0.25–

0.40 USD/ct and the cost of new diamond grits has fallen dramatically in the last years [MCCL10b]. Recovery costs often offset the value of the reclaimed material [SAIT11].

• Multi Layer Bonded Tools

Grinding wheel bodies of steel are manufactured with high dimensional quality and can easily be re-plated or re-layered. In multi-layer tools, the old abrasive layer is removed by etching of the adhesives that connect abrasive layer and body with  chemical baths. A steel body can be reused up to six times [MARI07, p. 107]. Wheel bodies of resin, however, are more often disposed than re-layered. Effenberger [EFFE01] invented a grinding disc with a releasable abrasive layer, which can be disposed separately.

• Other Abrasive Processes

In abrasive blasting operations, magnetic separation, rotary brooms, or mechanical conveyors can recover grits [DREN97]. Several methods are able to recover grits from coated abrasive tools depending on the bond type [ANSI84]. The recovery is, however, mostly applied for grading and quality control. The abrasive sludge from glass processing is composed of SiO₂ and Al₂O₃ from the abrasive grits and can be reutilized as cement mixture [KWON03].

1.8.4  Conclusion and Sustainability Model for Tool End of Life

In general, the recycling or re-use of grinding tool components takes place in very few instances, although it is technically feasible in many cases. The reason lies in cost considerations. Today, the costs of recycling and re-use are higher than the benefits. This might change in the future because the decision is volatile and

depends on the constraints and limiting factors. This chapter cannot quantify the potentials for re-using grinding wheel components, but show options qualitatively.

Tool manufactures offer

• collecting and recycling of abrasive tools as take-back service.
• Re-layering of steel bodies reduces the costs and material needed for superabrasive tools.

Tool users are concerned with tool life and disposal of tools.

• Abrasive tools with organic and resin bonds have a limited shelf life.
• The life of all other tools is mainly limited by tool wear.
• Tool disposal has to follow waste regulations and can have high disposal costs. Special care has to be taken for tools with hazardous bond ingredients, such as metal bonds.
• Tool users are interested in the lower costs and shorter processing times when

re-layering steel bodies.

The whole society takes care about

• the reduction of waste and recycling in general, because a lot of waste in form of grinding wheels and bonded tools appears each year.

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