Bonding Systems

Abstract

The grit material performs the main abrasive cutting action and has been discussed in the previous chapter. However, grits alone cannot maintain a sustainable process with sufficient workpiece quality.

After thousands of years of manual grinding with stones, the first synthetic grinding tools were known in the beginning of the 19th century. [COLL88, p. 894]

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The grit material performs the main abrasive cutting action and has been discussed in the previous chapter. However, grits alone cannot maintain a sustainable process with sufficient workpiece quality. One method is to bond the grits together by a bonding system that has to fulfil several tasks as follows [KLOC05a, MARI04]:

• Provide sufficient grit retention without untimely grit pullout,

• Provide sufficient strength to transfer machining forces and centrifugal forces,

• Allow controlled bond erosion to expose new cutting edges,

• Offer enough pore space to transport chips and cooling lubricant,

• Provide adequate heat conductivity and thermo-shock resistance,

• Provide chemical resistance against cooling lubricant.

Bonded grits form either grinding wheels or grinding belts (see Sects. 4.1 “Grinding Wheels” and 4.2 “Coated Abrasive Tools”). As consequence of the complex requirement profile on the tools, maximum grit retention capability is not synonymous with high performance in the abrasive process [MARI04].

Several bonding systems have evolved for grinding tools and will be discussed in this chapter. As contrasting method, the abrasive grits can also be applied as loose abrasives, which will be described in Sects. 4.4 “Polishing Tools”, 4.5 “Lapping”, and 4.7 “Other Methods with Free Abrasives”.

The most important bonding systems for grinding wheels are resin bonds, vitrified bonds, and metallic bonds (multi-layer or single-layer) (Fig. 3.1). Besides the primary ingredients, there are fillers, separator agents, auxiliary components, or even secondary abrasives in the bonding composition to modify the abrasive tool. All ingredients need to be considered in their life cycle from raw material extraction to end of life.

Fig. 3.1

Grinding tool bonds after [KLOC09]

Grinding tools with resin, vitrified and multi-layer metallic bond systems pass through similar manufacturing steps, such as mixing, forming, pressing, heat treatment, and post-processing [TYRO03]. All tool manufacturing processes include additional auxiliary steps such as raw material quality control, weighing, intermediate control steps, sieving, stocking, etc.

The homogeneous distribution of abrasive grits, bonding material and pores inside the abrasive layer is crucial for a constant process performance. A non-uniform distribution of the abrasive material leads to an uneven material removal process and respectively to a change in average chip thickness. This results in varying loads affecting the generated workpiece surface as well as the wear behavior of the grinding wheel that decreases process sustainability [KLOC05c].

Variations in the manufacturing process of tools cannot be totally avoided. However, it is important to control the deviation of the quality parameters per single production step so that the final product stays within the acceptable quality ranges [KLOM86, p. 12].

In contrast to conventional grinding tools, superabrasive tools are commonly built from an abrasive layer applied to a carrier, so called body (see Sect. 4.1 “Grinding Wheels”). The abrasive layer is fixed to the body either as ring or as straight or curved segments for larger diameters (commonly above 200 mm wheel diameter). Like conventional grits, superabrasives can be held by resin and vitrified bonds. Resin and vitrified bonds, however, have to be adapted to the chemistry and performance of superabrasives. Metallic bonds have particular importance for superabrasive grits and are nearly exclusively used for this grit type.

3.1 Resin Bonds

3.1.1 Chemistry and Types of Resin Bonds

A resin bonded grinding wheel consists of abrasive grits in a resin bond with or without fillers (Fig. 3.2). This wheel type has commonly a low porosity compared to vitrified bonded tools.

Fig. 3.2

Structure of a resin bonded tool

Resins are viscous liquids capable of hardening. They are polymers, i.e. large molecule chains composed of monomers. Monomers are substances with the elements C, H, O, N, Cl, S, or F, from which oligomers (“resins”) are synthesized [GARZ00, p. 109]. These are then transformed to crosslinked, insoluble polymers in a second step, called curing, which optionally involves heat, catalysts, fillers, or pressure [GARZ00, p. 109].

Resin bonds for abrasive tools consist of single resins or a resin combination with or without fillers (see Sect 3.1.3 “Fillers in Resin Bonds”). The resin itself is typically manufactured by esterification or soaping of organic compounds. Filler material has not only the task to reinforce the bonding in toughness, heat resistance, strength, and breakage safety, but also to support the grinding process as secondary abrasive [COLL88]. Silicates, dulfides, halogenides increase the bonding strength and wear resistance and hinder the oxidative degradation of the resin [KREB06]. In cut-off wheels, the resin bond is additionally enforced by body materials of glass fiber, linen cloth, etc. (see Sect. 4.2 “Coated Abrasive Tools”).

Resin bonds can be divided into three classes based on strength and temperature resistance [KLOC05a, p. 43, MARI07, p. 119, COLL88, p. 896 f., ROWE09, p. 41, MENA00]:

• Phenolic resin,

• Polyimide and polyamide resin, and

• Epoxy or urethane resin, often called plastic bonds.

3.1.1.1 Phenolic Resin

Phenolic resin bonds, in particular phenol-formaldehyde resin bonds, are the most common resin bonds; tools made of this bonding type represent the largest market segment for conventional wheels after vitrified tools [KREB06, KLOC05a, p. 65, MARI07, p. 119]. Originally, this bond type was known as bakelite and for this reason retains the letter “B” in many wheel specifications [MARI04, p. 413, GARZ00, p. 318]. In comparison to other resins, phenolic resin is less expensive and easier to mold [ASAM10, p. 310].

Phenolic resins are obtained by the reaction of phenol and aldehyde [GARZ00, p. 3, 111]. Phenols are aromatic compounds with the hydroxyl group bonded to the aromatic nucleus (Fig. 3.3) [GARZ00, p. 3]. The phenol synthesis is commonly done by the cumene process, an oxidation process of cumene (isopropylbenzene) and air to cumene hydroperoxide, which is cleaved to phenol and acetone [GARZ00, p. 5, 8]. Safety is a critical aspect for plant design and operation, because the oxidation takes place close the flammability limit and cumene hydroperoxide is an unstable material [GARZ00, p. 8].

Fig. 3.3

Structures of phenol, relevant aldehydes, and phenolic resin [GARZ00, p. 8, 14, 21, HARR89]

Relevant aldehydes for bond production of abrasive tools are formaldehyde, furfural, and hexamethylenetetramine. Formaldehyde is a hazardous chemical with potential eye, nose and throat irritation above a certain concentration (Fig. 3.3) [GARZ00, p. 14]. Hexamethylenetetramine, also called hexamine, is a common hardener in phenolic resin bonds [KREB06, HARR89] (Fig. 3.3). Basic oxides such as calcium oxide or magnesium oxide are curing accelerators in phenolic resins [GARZ00, p. 321].

The mode of catalysis and molar ratio of phenol and aldehyde result in a resin that is either of resole type or novolak type [GARZ00, p. 24]. Resoles are easily cured by acid, base, or thermal conditions; novolaks are cured with formaldehyde from hexamethylenetetramine, solid resoles, or other methods [GARZ00, p. 61]. Resoles occur either as solid resole, resole solution or aqueous resole; novolaks appear as solid resins, novolak solutions, aquenous novolak dispersions, and powder resins with hexamethylenetetramine [GARZ00, p. 122]. For the manufacturing of abrasive tools, aquenous resole and powder novolak are the most important forms [GARZ00, p. 122].

The phenolic resin bond for grinding wheels contains liquid (resole) and powdered (novolak), straight and modified phenolic resins, powdered resins with wetting agents, or low melting phenolic resin combined with powdered phenolic resin [GARZ00, p. 323]. Various modifications with epoxy resins, rubber, polyvinyl butyral, etc. are possible [GARZ00, p. 325]. Furthermore, phenolic resin bonds for superabrasives are enhanced by SiC grits and solid lubricants [METZ86, p. 54].

Phenolic resins are cured at around 150–200 °C through polycondensation [KREB06, MARI04, p. 413]. Gardziella et al. give detailed compositions of the single liquid and powdered resins used for the production of abrasive tools [GARZ00, p. 324 and 326].

3.1.1.2 Polyamide and Polyimide Resin

Polyimides are polymers with a noncarbon atom of nitrogen in one of the rings in the molecular chain (Fig. 3.4) [HARP03, p. 432 f.]. Polyamide-imide are members of the same polymer family and contain aromatic rings and nitrogen linkages (Fig. 3.4) [HARP03, p. 432 f.].

Fig. 3.4

Structures of polyimide and polyamide-imide [HARP03, p. 432 f.]

Polyamid and polyimide bondings have a higher toughness, thermal resistance and elasticity than phenol resin bondings [KLOC05a, p. 66]. Polyimide bonds have 5 to 10 times the toughness of phenolic bonds and can withstand temperatures of 300 °C for 20 times longer [MARI07, p. 121]. However, the higher price reduces the use of this bonding system type to special applications and to superabrasives [MARI07, p. 121]. Polyimide resin is a dominant bond type for high-production carbide grinding especially for flute grinding [MARI07, p. 121] or for cut-off grinding under cooling lubricant.

3.1.1.3 Epoxy or Urethane Resin

Epoxy or urethane wheels are the softest of resin bonded wheels. With conventional abrasives, they are popular for double disc and cylindrical grinding. However, for superabrasives, epoxy or urethane bonds appear to be limited to micron diamond grain applications for the glass and ceramics industries [MARI04, p. 413].

Epoxy resins are characterized by their epoxide group which consists of oxirane rings (–CH₂–O–CH₂–) [HARP03, p. 161 f., 388]. The epoxy resin then is hardened with a fluidic hardener to a polymer [KLOC05a, p. 63, COLL88].

3.1.2 Manufacturing of Resin Bonds

Resin bonded grinding tools are manufactured via mixing, pressing and hardening at temperatures up to 200 °C (Fig. 3.5) [COLL88]. Resins consist normally of the two main components resin and hardener. Mixing both parts results in the reactive resin material. During hardening, the resin’s viscosity rises and a duroplastic material is generated.

Fig. 3.5

Manufacturing of resin bonded tools after [GARZ00, ASAM10, KLOC09, TYRO03]

3.1.2.1 Mixing and Molding

Mixing is often done in several steps, wet mixing, dry mixing, and final mixing (Fig. 3.5). In the wet mixing step, abrasive grits and liquid resins or furfural, a wetting agent, are combined [COLL88, GARZ00, p. 328 f.]. The abrasive grits will be coated so that the powder resins and fillers will cleave easier onto the grit surface. Wetting agents also improve the aggregation of grits. In addition, the abrasive tool can be handled in its raw, so called “green” state [TYRO03, A2, p. 6, COLL88].

Powder resin on phenol basis and fillers are mixed dry before they are combined with the wet mixture composed of abrasives and wet resins. Additives are included to support the mixing process by improving the pourability and storage life of the mixture and reducing the clumping tendency [GARZ00, p. 326 f.]. Examples for additives are powdered additives, silica and derivatives [GARZ00, p. 327]. The mixing process is continued until a homogenous, pourable mixture emerges [COLL88]. Dust is a safety issue during handling of powder resins. Therefore, antidusting agents can be useful additives to minimize dust [GARZ00, p. 327]. Before pressing, body material such as glass cloth for cut-off wheels or aluminum bodies for superabrasive wheels can be imbedded [KREB06].

3.1.2.2 Pressing

The bond and grit mixture can be either cold pressed and hardened in a furnace or hot pressed and hardened on a press with a heating plate. Figure 3.6 shows an example setup for pressing an abrasive layer onto a grinding wheel body. Conventional wheels are either hot or cold pressed and hardened at 140–200 °C [COLL88, p. 896 f., MENA00]. Most superabrasive wheels and dense, low porosity wheels are produced by hot pressing at 160–175 °C [MENA00, GARZ00, p. 338]. In the case of superabrasive wheels with a diameter below 200 mm, the abrasive layer can be pressed directly onto the tool body [KLOC09]. Epoxy or urethane bonds are casted or joggled in molds where they are hardened at temperatures of 20–80 °C [KLOC05a, p. 63, COLL88].

Fig. 3.6

Pressing of resin bonded tools

Cold pressing is done on hydraulic presses with compression strength of 15–30 N/mm² [GARZ00, p. 330, KLOC09, p. 54]. Pressing time ranges from 5–50 s and depends on the dimensions and shape of the abrasive tool, grit size, mixture plasticity and distribution [GARZ00, p. 330]. In hot pressing, the pressing times are determined to be about 30–60 s per millimeter of wheel thickness [GARZ00, p. 338].

The pressing process works either on a defined volume or with a defined pressure [COLL88]. The internal friction and friction with the mold walls lead to a deviation in particle density and therefore tool hardness (Fig. 3.7) [TYRO03b]. Density deviations can be overcome by two-anvil presses or superimposed oscillations in the pressing process, which was successfully proven for vitrified bonded tools [BEHR11].

Fig. 3.7

Schematic theoretical hardness deviation in a pressed grinding wheel [TYRO03b]

3.1.2.3 Curing

The hardening process has to follow a defined temperature program (examples given in Fig. 3.8). Several chemical processes happen during curing depending on the actual temperature [COLL88, GARZ00, p. 331]:

Fig. 3.8

Example temperature profiles for curing of resin bonded tools after [COLL88], for two wheel widths, b_s, and similar wheel diameters, d_s

• 70–80 °C: The resin bond starts to flow and to transform into a fused mass. Water in the phenol resin evaporates and the resin hardens under this separation of water [COLL88]. Water can drain off well in porous tools [ESCH05].

• 110–120 °C: The hexamethylenetetramine decomposes and induces the hardening process of the fused powder resin (Fig. 3.9). Gas is liberated, particularly ammonia (NH₃). [COLL88, GARZ00, p. 331]

  Fig. 3.9

  

  Temperature profile and emissions for curing of a resin bonded tool [COLL88, GARZ00]

• 170–180 °C: The structure finally hardens and the crosslinking of the phenol resin takes place [COLL88]. Overcuring should be avoided because overcured tools exhibit reduced strength [GARZ00, p. 331].

• 180–200 °C: Benzylamin structures split and result in new ammonia generation (Fig. 3.9). The resin bond becomes brittle, but also thermally more stable [COLL88].

• The final temperature level (165–170, 175–180, or 185–195 °C) affects the final tool properties (hardness, toughness, brittleness) considerably [GARZ00, p. 332].

Curing can also happen dielectrically with radio frequency and microwave heating [MENA00, HARI83]. These methods need the existence of a significant electrical loss factor, which is common with phenolformaldehyde resin [HARI83].

3.1.3 Fillers in Resin Bonds

Fillers in resin bonds have several tasks in both manufacturing phase and grinding operation: They induce porosity, reinforce bond properties, change aesthetics, and more [FRAC10]. Common fillers in resin bonded grinding tools are cryolite (Na₃AlF₆), pyrite (FeS₂), zinc sulfide (ZnS), lithopone (ZnSBaSO₄), potassium fluoroborate and potassium chloride (KAlF₄, K₃AlF₆), potassium sulphate (K₂SO₄), and mixtures of these materials (KCl) [COLL88, p. 897, GARZ00, p. 321, HICK91]. The toxic materials antimony trisulfide (Sb₂S₃) and lead chloride (PbCl₂) were used in the past, but are substituted by special iron halides and others [GARZ00, p. 321]. The percentage of fillers and resin bond varies with the grinding tool hardness and density (Table 3.1).

Table 3.1 Common resin and filler content percentage in grinding wheels [COLL88, p. 897, GARZ00, p. 323]

In the manufacturing phase, fillers can induce porosity [FRAC10]. Basic oxides, such as CaO and MgO, are fillers that accelerate the hardening process [COLL88, p. 897]. However, CaO should only be applied to grinding tools for dry grinding operations, because CaO fillers can hydrate and transform into CaCO₃ in contact with cooling lubricant [COLL88, p. 897 f.].

In the grinding process, fillers reinforce the bonding in toughness, heat resistance, strength, and burst resistance or they support the grinding process as secondary abrasive [COLL88, p. 897, GARZ00, p. 321]. Glass chips reinforce wheels around the inner diameter [ASAM10, p. 317]. Fine metal powder of high thermal conductivity can be introduced into the bond to improve the tool’s heat absorption in the machining process [HERB80]. An example is fine silver powder, mesh size 325 or finer [NN74]. In addition, fine silicon carbide grits act as bond strengtheners [HERB80].

Fillers in the form of solid lubricants introduced into the bond formulation resulted in the following functional characteristics [HERB80]:

• Reduced friction at the wheel/workpiece interface,

• Fewer loading of the wheel by grinding swarf,

• Preserved sharpness of the abrasive grits.

An example is provided by organic dryfilm lubricant of polytetrafluoroethylene (PTFE) type for wheels with a mesh size of 325 or finer [NN74]. In other applications, the lubricants FeS₂ and K₂SO₄ improved grinding quality and versatility [HICK91]. Finely divided graphite can improve the performance in dry grinding [NN74]. Cryolite (Na₃AlF₆) melts at about 950 °C and prevents wheel clogging by metal [COES71, p. 158 f.]. However, it acts as solvent for Al₂O₃ grits [COES71, p. 159]. The resin bond of grinding belts that will be used in very basic media has to be additionally stabilized [COLL88, p. 916]. Fillers can change the tool aesthetics when they act as coloring agents [FRAC10].

3.1.4 Performance of Resin Bonds

Resin bonds have comparably high elasticity. Therefore, this bond type is selected for wheels that are subject to impacts, sideways load, sudden loads, or high cutting speeds. Typical applications are cut-off or roughing operations. In addition, resin bonds work well for finishing processes to achieve high surface quality. Bond elasticity, however, might have a negative effect on dimensional accuracy [COLL88].

Resin bonded wheels are easy to profile, but do not achieve a high enough grit protrusion through profiling. Therefore, they need an extra sharpening process. In the grinding process, only workpiece materials that set back the bond will lead to a sufficient self-sharpening effect. Appropriate materials are brittle materials, for example in the application of carbide tool grinding.

Dense grinding tool surfaces are achieved with fine abrasive grit sizes. Large grits and metallic grit coatings improve grit retention in resin bonding systems. Some filler materials, such as CaO, should not be exposed to water [COLL88, p. 897 f.].

Furthermore, resin bonds are sensitive to heat. They start to degrade at temperatures above 200 °C and grit coatings help to dissipate the grinding heat. Resin bonded wheels have limited shelf lifetime and should be used within two years [KREB06]. The polycondensation process that hardens the bond resins does not lead to complete hardening so that the strength of resin bonds can change due to atmospheric or chemical exposure [KREB06].

3.2 Vitrified Bonds

3.2.1 Chemistry and Types of Vitrified Bonds

Vitrified bonds consist of silicates (red and white clay), kaolin (also known as white clay, Al₂Si₂O₅(OH)₄), field spar (KAlSi₃O₈–NaAlSi₃O₈–CaAl₂Si₂O₈), quartz (also known as silicon oxide, SiO₂), and frits, i.e. pre-molten bonding components [BEYE04, HADE66, PADB93, JACK11, p. 92]. The bond is sintered at temperatures above 800 °C and results in a structure where the grits are enclosed by the bond (Fig. 3.10). The bond bridges between grits leave room for pores. Due to the high sintering temperatures, reactions between grits and bond are likely (Sect. 3.2.3 “Performance/Grit Retention”). In contrast to resin and metallic bonds, vitrified bonds have a significant amount of pores.

Fig. 3.10

Structure of a vitrified bonded tool

Vitrified bonds vary in their appearance due to the proportion of ingredients and sintering conditions. The two bond type extremes are melted bonds and sintered bonds [KLOC09, KREB06, TYRO03b]:

• Melted or fusible bonds are glassy bonds with a high amount of glass phase [JACK95]. They result from a high proportion of clay and frits as raw materials. Melted bonds melt totally, flow around the grits and react with the grit surface [HELL94]. The frits enhance the melting properties of the bond resulting in higher grit retention at lower firing temperatures [KREB06].

• Sintered bonds occur as porcelain bond type with a small amount of the glass phase. A high proportion of fieldspar and a low amount of frits leads to this bond type. The grits are only “glued” together by the partially molten bonding. The proportion of bond and the firing temperatures are comparatively high [KREB06]. Nevertheless, this bond type is used for critical grinding operations with low forces and for silicon carbide tools [KREB06]. High glass contents can decompose the silicon carbide grits [KREB06].

3.2.2 Manufacturing of Vitrified Bonds

First, the bond components and abrasives are mixed and filled into a mold, either by a casting process in the case of clayey bonds or by a molding process (Fig. 3.11). Pressing compacts the material and produces the so-called “green body” which can be handled. The green body is dried and sintered. After that, the grinding wheel will be finished and undergoes quality control.

Fig. 3.11

Manufacturing of vitrified bonded tools after [KLOC09, TYRO03]

3.2.2.1 Mixing

The proportion of grit, bond and pore volume defines the structure and hardness of a grinding tool (see Sect. 6.1 “Abrasive Layer Composition”). The right amount of bond components and grits are weighted and mixed. Even if the various ingredients have different size, density, form, and weight it has to be controlled that the mixture is homogenous and without agglomerations, so that the final abrasive tool has a homogeneous cutting edge distribution [BOTS05, p. 23]. Furthermore, the mixing process must not demix the components, induce changes in the mixture, or produce heat [TYRO03].

In the first step, the coarse particles are made sticky by mixing them in for example a water dextrose solution [MESP91, TYRO03]. The fine components are added afterwards and stick to the prepared coarse components [MESP91, TYRO03]. The lubrication has to be controlled to avoid aggregation [BOTS05, p. 23]. Aqueous phenolic resin binders are one example for temporary binders [HUZI00].

A common mixer type is the planetary mixer [MESP91]. As ingredients might change their viscosity and react chemically during this process an adequate mixing time is also of great importance [MESP91]. Standard mixing times of up to one hour are therefore quite common. To enhance tool quality, vitrified bondings can be granulated and sieved after mixing.

3.2.2.2 Casting

Casting is applied for bonds with high clay content. The casting process is more expensive and therefore being replaced by molding processes [KLOC09].

3.2.2.3 Molding and Pressing

Most vitrified bonded tools are manufactured by molding. In the past, the mixture was formed into the mold manually; today fully or partially automated systems supply and disperse the mixture into steel molds. The mixture has to be filled into the mold homogeneously and without demixing, which is done by conveyor belts, vibrating trough, or other supply systems. The filling height is leveled with sheets. Molding gets more complicated for complex grinding tools such as wheels with different layers for crankshaft grinding, different hardness areas for raceway grinding, or large width for centerless grinding [TYRO03].

The pressing process defines the density distribution and thus the specific characteristics of the component [BEHR11]. The pressure is an important factor for tool quality and commonly not published. Today, computer controlled hydraulic presses between 500 kN to 45 MN are in use [TYRO03]. Commonly, the target volume, not the target pressure, controls the pressing process. Segments for superabrasive wheels are pressed with hydraulic or pneumatic presses or by hand at comparatively low pressures (max. 50 N/mm²).

Inevitable friction between the dies, molds, and powder leads to density gradients, (Fig. 3.7), which can result in sintering distortions and inhomogeneous material properties [BEHR11]. Additives like paraffin, polysaccharide, silicon oil, wax, and polyethylene emulsions lubricate the mixture and therefore support the pressing process [BOTS05, p. 25]. Oscillations of about 60 Hz can be superimposed to the pressing process and enhance the density distribution [BEHR11]. Expensive two-die presses can be substituted.

Hot pressing is used for high density tools [KLOC09, p. 59]. Hot pressing of diamonds in vitrified bond produces wheels of low porosity. Using graphite molds enables higher sintering temperatures in the hot pressing process. However, as the graphite molds are of low strength, the process has to be heated above regular sintering temperatures to limit pressing pressures [MARI04, p. 418]. As consequence, the bonds are densified with less than 2 % of open porosity.

3.2.2.4 Drying

In the drying or debinding process, the water from the temporary binders is expelled [TYRO03]. The drying process is conducted in a special drying furnace or in the sintering furnace during the general heat-up period. The drying temperature depends on tool ingredients, composition, and application and ranges usually between 200 to 600 °C.

3.2.2.5 Sintering Process

In the sintering process, the bond components melt and flow around the abrasive grits. Bonds with a large amount of clay melt at higher temperatures; bonds with more frit content melt at lower temperatures [MALK08, p. 27]. The mechanisms of melting, wetting, resolidification, and forming of bonding bridges between grits are highly complex [BOTS05, MOSE80].

The bond mixture is formed into non-uniform glass of complex composition including several ceramic bond minerals that form during the sintering process [JACK11, p. 83, 85]. Jackson studied the vitrification behavior of sintered and fusible bonds for conventional grinding wheels [JACK95]. Bot-Schulz analyzed the chemical reactions and sintering mechanisms for varying bonding composition for white corundum and Sol-Gel-corundum [BOTS05]. If the bond contains MgO, mullite (3Al₂O₃·2SiO₂), cordierite (2MgO·2Al₂O₃ ⁻5SiO₂), and spinel (MgO·Al₂O₃) are formed during devitrification [JACK11, p. 83 ff]. If the bond contains CaO, anorthite (CaO·Al₂O₃·2SiO₂) and mullite (3Al₂O₃·2SiO₂) can form [JACK11, p. 83 ff].

One problem in manufacturing of vitrified bonded tools is the shrinkage after melting and sintering. A higher amount of abrasives and secondary abrasives reduces the shrinkage, but might have a negative effect on the grinding tool performance. Pore builders that burn leave voids, which sometimes collapse during the sintering shrinkage and counter the purpose. New approaches to reduce sintering shrinkage are to use fired clay (“grog”) or crushed firebricks in sizes between 30–200 μm. Cerium oxide, andalusite, sillimanite, willemite, or a combination thereof serves the same purpose. [HUZI12]

Several chemical processes happen during sintering [BOTS05, 25 f.]:

• 20–600 °C: Free and bound water dissolves. The temporary binders burn out. Quartz (SiO₂) changes volume at 573 °C. Gas pressure and volumetric shrinkage lead to high internal stresses in the tool.

• 600–900 °C: Carbonates, organic material, chemically combined water in the clay, and other volatile materials drive out.

• Furthermore, chemical reactions between grits and bond are possible.

The sintering temperature must not harm the abrasive grits by inducing unwanted oxidization or other chemical reactions. The bondings for CBN and diamond have to fuse together under lower forming pressures and vitrification temperatures than for conventional grits [JACK07]. Because CBN is thermally more stable than diamond, CBN grits can be used with a wider range of vitrified bonds [MALK08, p. 26]. Special coatings protect CBN grits from chemical reactions above 800 °C with the water and alkali in most frits [MALK08, p. 27]. For diamond wheels, a non-oxidizing atmosphere is applied to enable a sintering temperature higher than the diamond oxidation temperature of around 700 °C.

Different furnace types are used depending on application and batch size. Continuous furnaces, also called tunnel furnaces or traveling kiln, are often used for the mass production of conventional grinding wheels and can be 70 m long. Periodic hood-type furnaces are used for smaller batch sizes. In general, about 80 % of the inner furnace volume can be used for producing products. Additonal accessories enable special gas atmospheres in the furnace or pressure control. Furnaces are heated by electrical energy, with gas or oil [KLOC09]. Gas furnaces are usually cheaper but they cannot be used for all processes.

The temperature distribution is not constant along the furnace cross-section. Temperature fluctuations inside the furnace should not exceed ±10 °C. A smaller range enhances process capability. Tools properties can depend on their position on the kiln car [RAMM74].

The temperature curve has to be controlled precisely. The rate of heating, peak temperature, and isothermal soaking time define the grinding wheel properties [JACK95, p. 82]. The temperature profile consists of a heating, soaking and cooling phase. The total time can be 100 h [TYRO03]. For example, corundum wheels in a tunnel furnace are heated up to 1260 °C over 1 to 2 days, held for about 12 h at the maximum temperature, then slowly cooled down [MALK08, p. 27]. The cooling process has to be carefully controlled to avoid thermal stresses or cracking of the wheels [MALK08, p. 27]. The cooling process can take weeks for very large wheels [MALK08, p. 27].

3.2.2.6 Frits

Frit is a generic term for a material that is generated by blending minerals, oxides, and other inorganic compounds, heating to at least melting temperature, cooling, and pulverizing [HAY90]. An example frit production process is given as follows: Melting of the ingredients above 1150 °C, holding at the temperature for 4 h, cooling down in water, crushing, and pulverizing of the material. A glass is an amorphous substance without significant crystal formation [HOLL95]. In thermodynamics, glass is defined as frozen, undercooled fluid. All substances that were molten and cooled down appropriately follow this definition. The quick cooling results in the formation of crystal seeds during solidification, but the time is too short for the crystallization process.

Frits for the manufacturing of grinding wheels consist commonly of boron silicates or magnesium glass [BOTS05]. In addition, frits also contain TiO-₂, fieldspar, borax, quartz, soda ash, red lead, zinc oxide, whiting, antimony trioxide, sodium silicofluoride, flint, cryolit, and boric acid [HAY90]. In producing grinding tools, frits act as flux agents and change the properties of the vitrified bond, for example by lowering the sintering temperature [JACK11, p. 92].

Frit manufacturers characterize their products by melting behavior (temperatures for beginning of fusion, melting, and sintering) and thermal expansion. A hot stage microscope analyzes the melting behavior.

3.2.2.7 Flux Agents

Siliceous clay with low melting point is considered as ‘flux agent’ and minimizes the surface tension at the interface between bond and abrasive grit [JACK07]. Lithium for example is one of the most expensive flux agents, but can decrease softening and melting point, viscosity, as well as heat expansion coefficient [BOTS05].

3.2.2.8 Porosity Builders

In vitrified bonded wheels, the natural packing of the abrasive particles leaves certain porosity [DAVS04]. Additional pore builders produce higher porosity when they sublimate or burn off during the sintering process. Pore builders are typically either hollow particles or fugitive materials [MARI07]. Hollow particles such as hollow ball corundum, glass beads, or mullite maintain a stronger wheel structure [MARI07]. Conventional fugitive pore builders include nut shell powder, sugar, starches, polymeric materials, plast granulat, naphthalene, ceralith (from rye groats), wax balls, etc. [DAVS04, HUZI12]. Fugitative pore formers allow flexible pore shapes and sizes [MARI07].

Naphthalene is a fugitive pore builder, which boils at 218 °C [GEST12]. Because naphthalene is regarded as carcinogenic, tool manufacturers attempt to substitute it with natural components, supercritical CO₂, liquid CO₂, or other materials [GEST12, DAVS04].

Other substances that build pores during the tool production are often organic such as wood shavings, salt, etc. Variations in pore distribution can lead to non-uniform tool shrinkage in the manufacturing process, especially for tools produced at high temperatures. This is a result of two factors. On the one hand, the effective thermal expansion coefficient of a composite body depends on the relative contents of the various components. On the other hand, denser zones with fewer pores undergo a more thorough sintering action with stronger contraction. To minimize discontinuities in the abrasive layer, the mixture of pore builder and the other ingredients should be well blended. [YARN69]

One problem of pore builders can be carbon residues [DAVS04]. Possible defects in the grinding tool arise from swelling, slumping, off-gassing, or collapsing of pores during sintering shrinkage [DAVS04, HUZI12].

3.2.2.9 Finishing

Connecting body and abrasive layer for superabrasives is explained in Sect. 5.1 “Body Concepts”. Conventional grinding wheels are machined to achieve the final dimensions and tolerances. Superabrasive tools are near-net-shape and need less effort if any.

3.2.3 Performance/Grit Retention

The first vitrified bonded wheels were considered as inappropriate for large temperature variations because the bond was not elastic enough to withstand thermal expansion differences within the tool [KING86, p. 87]. Certain bond elasticity is important to equalize the volumetric expansion of the abrasive grits induced by the grinding process heat [STAD62, p. 51]. Vitrified bonds are thermally highly stable. However, Stade reports about molten beads of vitrified bond in the grinding debris [STAD62, p. 51].

Grit retention in vitrified bonds is partially mechanical because the bond encloses the grits. In addition, the bond often reacts chemically with the grit surface. For example, the vitrified bond dissolves the grit surface in corundum wheels [JACK11, p. 62]. For CBN, the early vitrified bonds were just transferred from conventional applications and happened to be so reactive that they dissolved the CBN into the bond and converted it into boric oxide [MARI04, p. 419]. However, the bond composition was changed over the years so that well dressable CBN bond systems became state of the art [MARI04, p. 419]. CBN grits can be coated to avoid chemical reactions between CBN grits and alkali or water present in most glass frits at temperatures above 800 °C [MALK08, p. 26].

Diamond, however, does not show significant chemical bonding with the components of a vitreous bond [MARI04, p. 418]. Retention of diamond grits is mostly mechanical. However, diamond is reactive with oxygen at temperatures above 650 °C. Therefore, diamond wheels must be fired at low temperatures or in inert or reducing atmosphere [MARI04, p. 418]. However, titanium based coatings on diamond or CBN grits have a protective function during the sintering process.

Reaction layers between the vitrified bond and corundum white or sol-gel corundum consists from bond and grit material and can be adjusted by the sintering process. The characteristics of the layer affects grit retention. Sol-gel corundum grits expose thicker reaction layers than white corundum due to microcrystalline structure and higher number of grit boundaries. [KLOC06a]

The first vitrified bonded CBN grinding wheels needed separate profiling and sharpening processes [STUC88]. However, this problem has been overcome by new bonding compositions. Today, vitrified bonded grinding wheels are easy to profile and sharpen in one step. The dressing mechanisms are mainly grit breakage, grit splintering, bonding breakage, grit break-out off the bonding, and grit deformation [MARI04, WIMM95, MINK88, MESS83, KLOC08b]. Linke found that the dressing forces likely induce cracks or support crack propagation in the tool bond, so that the abrasive layer is weakend by the dressing process [LINK07]. However, hot pressed vitrified bonded wheels are difficult to dress und need a separate sharpening process similar to resin and metal bonded wheels [MARI04, p. 418].

3.3 Metallic Multi-layer Bonds

Metallic bonds are either multi-layered (produced by sintering or infiltration) or single-layered (produced by electroplating or brazing) [MARI07]. They are only applied to superabrasive grits because conventional grits wear too quickly to use the bonding strength to full capacity.

3.3.1 Chemistry and Types of Metallic Bonds for Multi-layer Abrasive Tools

Metallic multi-layered bondings consist of various alloys such as copper/tin-bronze (Cu/Sn), cobalt-bronze (Co/Cu), tungsten carbides (W/WC) or alloys from the iron-copper-tin-system (Fe/Cu/Sn) [KLOC09, STOC86]. Common Cu/Sn alloys have a ratio of 85:15 or 80:20 Cu to Sn with fillers and additional alloys [MARI07, p. 122]. The binder needs to have good wettability for the abrasive grits.

Iron and iron consisting metals may cause undesired reactions with diamond grits [STOC86]. Diamond tools for stone drilling and sawing use bond systems based on tungsten, tungsten carbides, or cobalt alloys [STOC86, WEIS08]. Besides high wear resistance and toughness, tungsten builds an advantageous, small interface layer of tungsten carbide with the diamond grits [STOC86]. Therefore, the bond retention is not only mechanical but also chemical. Moreover, auxiliary metal powders on the basis of cobalt, nickel, copper, iron, and mixtures of copper-nickel-zinc, copper-zinc and copper-manganese optimize bond retention [STOC86].

Metallic bonds have strong grit retention and are wear resistant against the chips of brittle, short-chipping materials. The high hardness becomes a disadvantage in tool conditioning. Nevertheless, metallic bonds can be modified to be brittle and dressable, for example increasing the Sn content or adding Ag embrittles bronze bondings [MARI07, p. 122]. Co increases wear resistance.

3.3.2 Manufacturing of Metallic Bonds by Infiltration

Metallic bonding by infiltration is used mainly for dressing rollers and special applications [KLOC09]. Infiltrated bonds are also used for superhard honing sticks (bronze, Co-bonding), stone drilling crowns, or saws.

Grits, bond, and fillers are either mixed and filled into the die or the grits are fixed first onto the bottom of the die form and the matrix powder is added afterwards (Figs. 3.12 and 3.13). After then adding a defined amount of hard solder and flux (often copper, nickel or zinc alloys), the die form is heated in continuous ovens with inert gas or by induction [KLAU76, STOC86]. The solder has a relatively low melting point. Therefore, the molten solder penetrates the matrix powder by capillary forces. The resulting matrix is strong and highly wear-resistant [KLAU76]. The tool body can be connected to the abrasive layer simultaneously by brazing. Subsequent to the heating process, pressure can be applied to further influence the tool characteristics [STOC86] (Fig. 3.12).

Fig. 3.12

Manufacturing of metal bonded tools by infiltration after [KLOC09, STOC86, KLAU76]

Fig. 3.13

Manufacturing of diamond crowns with infiltration method [YOUN66]

The melting point of the solder defines the temperature of the infiltration process. Infiltration of copper-based solder takes place at 1000–1250 °C [YOUN66]. Die forms are made of graphite. This material has several tasks [KLAU76]:

• Prohibiting wetting of the die form with solder by its non-adhesive behavior,

• Building an air gap between die form and produced tool by different thermal expansion,

• Being multi-usable because of the two above described properties,

• Binding oxygen from the atmosphere and building of CO/CO₂-atmosphere as oxidization protection for diamond grits.

The appropriate choice of the graphite die material is important for a stable manufacturing process as reactions between tool bonding ingredients and graphite matrix can occur [KLAU76]. Graphite for the infiltration process should be easily machinable, be free of cracks and have low porosity, so no solder flows into the die material itself and destroys it. The generation of carbides between graphite and matrix powder ingredients can be suppressed by coating of the die form before each infiltration process. Coating materials can be e.g. Al₂O₃-powder and alcohol or natural graphite and glycol [KLAU76].

3.3.3 Manufacturing of Metallic Bonds by Sintering

Sintered bonds are produced by mixing of metal powder and abrasive grits, molding, either hot pressing or cold pressing, and sintering (Fig. 3.14). In exceptional cases, the metallic powder is mixed with an organic binder (paraffin oil and/or wax granule). This binder helps to produce a green compact, which can be handled, and vaporizes in the sintering process [STOC86]. The binder also leaves pore space, which might be infiltrated with a soldering agent.

Fig. 3.14

Manufacturing of sintered metal bonded tools after [KLOC09, STOC86]

In the cold pressing method, the green compact is taken out and sintered in a melting oven under inert gas atmosphere [STOC86]. Sintering takes place with or without form [STOC86]. Iron or steel compacts are sintered at 1100 °C [YOUN66]. Bronze bondings are easy to process and press and they are sintered at temperatures between 500–700 °C gesintert.

In the hot pressing method, the mixture is compressed under additional heat directly in the graphite form. Three heating types, namely inductive heating, indirect and direct resistance heating, can be used in hot pressing [DRFR05].

The cold pressing procedure allows the application of fast, mechanical pressing methods as well as sintering at neutral or reduced atmosphere in melting ovens. Hydrogen or other inert gases provide an oxygen-free surrounding, which is important for the protection of diamond grits at higher sintering temperatures. The hot pressing procedure is a one step operation and, therefore, a time-effective process with good quality control [STOC86].

Hot isostatic pressing is a competing method for traditional hot pressing [DRFR05]. Onishi et al. produced highly porous wheels from hot isostatic pressing (HIP) of 73 w% of cast-iron or iron powder, 25 w% of diamond grits and 2 w% of wax [ONIS97]. In this procedure, the material was pressed first, presintered to remove the wax, and HIP processed.

Porosity in sintered metal bonded tools is achieved by vaporizing fillers and adjusted pressing pressure. Porosity in bronze bonding is gained from adding carbon, e.g. up to 20 V% in diamond cup grinding wheels [BÜTT68, p. 74].

3.3.4 Performance of Metallic Multi-layered Bonds

Metallic bonds provide high grit retention and low wear during grinding. However, a copper bonding might smear during grinding. Metallic bonds might have low porosity, so that fillers in the bond are added for lubrication during grinding.

Metallic bonded tools are difficult to condition mechanically [WEGE11]. Electro physical and electro chemical processes can be applied due to the electrical conductivity of the metallic bond and are used in many different set-ups [WEGE11]. Klink examined the electro discharge and electro chemical dressing of different metallic bond compositions (Cu-bronze, Fe-bronze, Co-bronze) [KLIN09]. Ohmori and Nakagawa invented the Electrolytic In-process Dressing (ELID) method [OHMO90]. The cast iron fiber bonding of a diamond grinding wheel is anodized to generate grit protrusion [OHMO90]. ELID has advanced a lot and is possible when applied to grinding wheels with cast iron bonding, hybrid metal-resin bonding, and bronze bonding [BIFA99, ITOH98, WEGE11, KLIN09].

Porous metallic wheels can be conditioned by crushing, i.e. inducing bond breakage by high dressing forces [HESS03]. Crushing commonly takes place at dressing speed ratios of q_d = +1, which means there is no relative speed between the dressing tool and the grinding wheel in the contact point. For example, crushing replicates fine threads into brittle bronze bonding [KLOC82].

3.4 Metallic Single-layer Bonds

3.4.1 Chemistry and Types of Metallic Bonds

Single layer metallic bonds have a layer height of only the average grit size and a high grit protrusion of 20–70 % of the grit diameter. Because of the high grit performance, single layered wheels are limited to superabrasives. The metallic single-layer bonds are produced by electroplating, brazing, or electroless plating processes:

• Electroplated wheels—The production takes place at room temperature. The most common bond type for single layer bonds is nickel bonding deposited in an electroplating process. Electroplated CBN wheels were important for the successful development of high efficiency deep grinding (HEDG) [ROWE09].

• Brazed wheels—The production happens at temperatures of up to 1000 °C [MARI04, p. 415]. Strongly wetting solders like titanium containing material, Ni–Cr–Bo–Si-alloys or others are used.

• Electroless plating process—Nickel/phosphor-alloys can be chemically depositioned [KLOC09, p. 61]. Chemically precipitated bonds are of higher strength than electroplated bonds and can have a more even coating thickness, but the equipment is more expensive, processing temperatures are higher and the bond is more brittle [KLOC09, p. 61].

3.4.2 Manufacturing of Electroplated Bonds

Electroplating is based on the cathodic metal deposition from a watery electrolyte (Fig. 3.15). A metallic layer can only be deposited on a workpiece, if there are enough electrons to discharge the metal ions within the watery solution. Depending on the origin of the electrons, a distinction is drawn between chemical metal deposition (without external voltage source) and electrochemical metal deposition (with external voltage source) [KLOC07, p. 187]

Fig. 3.15

Electroplating process after [KLOC07, p. 188, BOLD02]

The body material of grinding tools needs to be electrically conductive, at least in the area to be coated [KLOC09, p. 60]. Common materials are steel, e.g. C15, C45 or alloyed steel, hardened ball bearing steel (100Cr6), aluminum, or bronze/brass if the application does not allow for a magnetic material [BOLD02]. Before electroplating, the body has to be prepared carefully and the areas that should not be plated need to be painted (Fig. 3.16) [METZ86, p. 63]. The surface needs to be degreased and oxide layers need to be removed [KLOC07, p. 196]. Aluminum alloys need special treatment to remove oxide layers and activate the surface layer for better chemical bonding [KLOC07, p. 196].

Fig. 3.16

Manufacturing of electroplated tools

The body is covered with superabrasive grits and placed into the electrolytic bath [KLOC09, p. 60]. The area to be coated needs to be surrounded by a sufficient amount of grits, which can present a big amount of fixed capital [METZ86, p. 63]. The bath consists of a watery solution of metal salts from the deposited metal, such as Ag, Co, Cu, Ni, Au salts [BOLD02, KRAF08]. In general, the anode consists of the bond material and the tool body acts as cathode. The direct current (DC) voltage leads to precipitation of Ni at the tool body. After the initial bonding of the grits, the excessive grits are removed and the process is continued until the desired plating depth is reached [KLOC09, p. 60]. The first bonding phase needs a motionless bath; the second phase of layer growth can work with higher power and bath circulation [KLOC07, p. 197]. The plating depth leaves about 50 % of the grit exposed (Fig. 3.17) [MARI04, p. 415].

Fig. 3.17

Structure of an electroplated layer after [MARI04, p. 416]

Typical superabrasive grits are strong, well-formed and blocky with well-defined cutting edges [NOTT80]. Disadvantageous process parameters or grit choice can lead to faulty tools. If the operating current density is too high, overplating, spikes, or nodules in the space between the grits occur and the grit protrusion shrinks [NOTT80, CHAT90]. Handling and disposal of the electrolytic baths and metals used underlies strict regulations (see Sect. 3.6.3 “Environmental Dimension”).

Profile accuracy of electroplated tools with a single layer of grits depends on the grit size distribution as well as concentricity and profile precision of the body [KLOC07, p. 196]. The grit size defines the minimum concave profiles [KLOC07, p. 197]. Because of electric field concentration, edges and corners can be hard to coat evenly [KLOC07, p. 197]. To get a more even precipitation of Ni the throwing power of the electrolyte might be modified or the anode shape might bear the reverse profile of the wheel profile.

The metallic body of the abrasive tools can be re-used and re-coated, if the abrasive layer is removed. This can be done by an unsoldering process, so called stripping (see Sect. 4.8.3 “Recycling of Abrasive Tools”).

3.4.2.1 Manufacturing of Dressing Rollers

Diamond dressing rollers are important electroplated tools. They are used for profiling and sharpening of grinding tools (Sect. 6.5 “Tool Conditioning”). Diamond dressing rollers are either produced with a direct method or a reverse method [MINK99, YEGE86, KLOC09, p. 140]. In the direct method, the diamond grits are fixed stochastically on a profiled body. Therefore, the size deviation of the grits defines the geometrical envelope [KLOC09, p. 140].

The reverse method works with lost molds that have the negative profile of the dressing roller [LIER01]. The diamonds are either scattered or hand-set onto the profile area. The grits are then bonded together by electroplating or infiltration methods and the abrasive layer is fixed on the body.

Figure 3.18 displays the reverse centrifugal method for electroplating. The reverse centrifugal method works with a Ni-rich electrode to improve the coverage of the grits and a graphite mold. The process is relatively slow. The geometrical envelope of the dressing tool is not defined by the grits but by the mold. The concave profile elements of the mold define the maximum grit diamond size. Diamond rollers with highest packing density manufactured by the reverse method have the highest importance in industrial applications [KLOC09].

Fig. 3.18

Reverse centrifugal method for manufacturing dressing tools

The German term “Diamantierung” summarizes diamond pattern, grit type, and bonding method. Manufacturers specify dressing tools by either the concentration of diamond grits in carat per cm³ or by the number of diamond grits per cm² [MERZ94]. Profile rollers with a large diamond grit size or small diamond grit concentration generate high effective grinding wheel surface roughnesses [MERZ94, SCHM68]. Similar tendencies are obtained for form rollers [MERZ94, STUF96]. High grit concentration decreases the load on the single dressing grit, increasing dressing tool life [WIMM95]. However, the higher dressing tool costs might be unprofitable.

Hand-set diamond patterns can vary along the profile of a dressing roller. For example, the shoulder of a profile roller is equipped with bigger diamond grits in a smaller concentration than the face area [KLOC87]. The profile roller can then generate the grinding wheel shoulder with a higher surface roughness and the danger of thermally induced workpiece damage is minimized.

3.4.3 Manufacturing of Brazed Bonds

Brazing is a soldering process at higher temperatures. The grits are held mechanically and chemically in the bond. Therefore, the grit exposure can be higher than for electroplated wheels leading to bigger chip space (Fig. 3.19) [MARI04, p. 416 f.]. Chemically bonded grits allow for even thinner bonding layers than electroplated grits [CHAT90]. This has advantages of higher chip storage space and achievable material removal rate.

Fig. 3.19

Structure of a brazed layer after [MARI04, p. 416, DING05]

The wheel body material acts as substrate for the brazing process and has to be cleaned before [DING05]. The solder, also known as braze material, is deposited on the body (Fig. 3.20). This can be done by spreading the solder as powder. The abrasive grits are either deposited along with the solder or fixed temporarily before the brazing takes place. Brazing is done at temperatures of up to 1000 °C; the solder builds up around the grits and puts them under tensile stress when cooling [MARI04, p. 416 f.]. The mechanisms within the brazing process include complex steps, such as dissolution of the substrate, diffusion of elements, formation of reaction products, and wetting phenomena [DING05].

Fig. 3.20

Manufacturing of brazed bonded tools after [CHAT94, DING05]

In the case of diamond, conventional brazing often results only in mechanical retention. Therefore, vacuum soldering is applied [BENZ91]. Using metals such as titanium, zircon, niob, or tantal in the solder enable carbide forming and therefore chemical grit retention. Because of the high oxygen affinity of these metals, the soldering process must be conducted in inert gas atmosphere or in high vacuum [BENZ91].

For CBN, high temperature metal alloys, for example based on Ni/Cr, are used [MARI04, p. 416]. Density and distribution pattern of the CBN grits define the performance of a brazed grinding tool [CHAT94]. Research on Ag–Cu–Ti alloy solders for CBN grinding wheels shows that reaction layers of TiN and TiB₂ can form [DING05]. These layers are key factors in achieving a strong bond between the CBN grits and the steel body of the grinding wheel.

3.4.4 Performance of Metallic Single-layered Bonds

In general, electroplated wheels have just one layer of abrasive grits. Therefore, tool life can be a major determining factor and grit strength is of major relevance. The range of characteristics available in diamond grit types is considerably wider for metal bonds than for use in other bonding systems. Often high strength crystalline diamond types are used. Abrasives that are more friable at lower loads are chosen for surface finish operations. [BAIL99]

The typical wear behavior of an electroplated grinding tool is not based on grit pull-out. Preferrably, the grits undergo progressive chipping which maintains sufficient cutting action of the tool [BAIL99]. The worn surface, however, has minimal grit protrusion and the top surfaces of the grits are rough [BAIL99]. The achieved surface roughness Rt amounts to around 10 % of the abrasive grit size for a new electro-plated tool; with tool wear (running-in) Rt amounts to around 5 % [BOLD02].

Electroplated grits are held mechanically, brazed grits are held mechanically and chemically. In comparison to an electroplated CBN grinding wheel, a brazed CBN tool may offer higher grit protrusion and a stronger grit-bond adhesion [CHAT94]. The solder is able to establish a strong joint between the grit, solder, and the metal substrate of the wheel body as a result of chemical affinities [DING05]. Regular grit distribution can overcome loading problems from clustered grits in a random grit distribution [CHAT94].

A major improvement in performance for single-layered metallic bonds was achieved by the so called “Touch dressing” method in the mid 1980s [FERL92, p. 9]. Small depths of dressing cut level the protruded grit tops, so that the surface roughness of the grinding wheel is decreased, its profile accuracy improves, and the grinding process stability rises [STUC88]. Electroplated tools can be cleaned with a sharpening stone or ultrasonic bath [BOLD02].

3.5 Other Bonding Types and Hybrid Bonds

3.5.1 Rubber

Rubber bonds are another type of organic bonds [ROWE09, p. 42]. Rubber bonds were once prominent for grinding of bearings and cutting tools, but today are mainly used for cut-off wheels and control wheels in centerless grinding [MALK08, p. 29]. Rubber bonded tools are manufactured by mixing of grits with synthetic rubber or vulcanized natural rubber and sulphur, then rolled into sheets with the required thickness, and cutting out the desired shape (Fig. 3.21). Thin wheels are then directly vulcanized under pressure at 150–275 °C. In the case of thick wheels, the sheets are first stacked to the final wheel width and then vulcanized [MALK08, p. 29].

Fig. 3.21

Manufacturing of rubber bonded tools after [KLOC09]

3.5.2 Shellac Bonds

Shellac bonding is another type of organic bonds [ROWE09, p. 42]. The first commercial use dates back to watchmaking applications in England in 1880, but it is also said that shellac bonds have been in use by Tamils longer before [LEWI76, p. 23]. Shellac bonds were once used for flexible cut-off wheels, but today are applied commonly to fine finishing of mill rolls, camshafts, and cutlery [MALK08, p. 29]. A further application is razor blade grinding [COLL88, p. 869 f.].

Shellac is a natural resin based on abietic acid derivates [COLL88, p. 869 f.] Insects swarming Cassum or lac trees in India exude shellac and its availability and properties depend on the weather conditions and species [MARI04, p. 413]. Shellac bonded tools are made by mixing grits with shellac, shaping under pressure in heated molds, and baking at temperatures of up to ca. 150 °C [MALK08, p. 29, COLL88, p. 869 f]. Thinner wheels are consolidated on the mold by a steel roller; thicker wheels are baked several hours in quartz sand [LEWI76, p. 23].

3.5.3 Other Bonds

Oxychloride bonds were popular about a hundred years ago, but are used today only for disk grinding [MALK08, p. 29]. Oxychloride bonded wheels have excellent cool cutting abilities under dry grinding conditions [LEWI76, p. 23]. Oxychloride is formed as cement by cold-setting from the oxide and chloride of magnesium, i.e. magnesium oxide and an aquaeous solution of magnesium chloride [MALK08, p. 29, LEWI76, p. 23].

Silicate bonds consist mainly of liquid glass and enable a grinding process with low temperatures, but silicate bonded tools wear quickly [BORK92, p. 38]. This bond type can be manufactured at lower temperatures (about 600 K) and in shorter cycles (10 - 30 h) than vitrified bonds [MALK08, p. 29, BORK92, p. 38]. Silicate bonded tools are produced by mixing grits with soldium silicate, compacting in a mold, drying, and baking [MALK08, p. 29, BORK92, p. 38].

Magnesite bonds consist of magnesium oxychloride from MgO and MgCl₂ solution [COLL88, p. 896]. The tools are formed by pounding or pressing, then dryed and hardened in the mold at room temperature via formation of Mg(OH)₂ with stored MgCl₂. This bond type is only seldomly used, for example for the grinding of knives and files [COLL88, p. 896]. Magnesite bonds have to be marked with an expiration date, which is at maximum one year after the manufacturing date [DIN07].

Hybrid bonds of a resin and a metallic phase are generally used for superabrasive tools [UPAD09]. The metallic phase can be either a filler or a binding component to enhance grit retention depending on specification or manufacturing conditions [METZ86, p. 54]. In the 1970s, a hybrid bond from interpenetrating bronze and epoxy resin was invented and is used for the grinding of carbide cutting tools [CHAL72, UPAD09]. The porous metal bond is manufactured by cold pressing and sintering; the resin component is vacuum casted to infiltrate the pores [CHAL72, UPAD09]. Another composition of a hybrid bonding consists of bronze and polyimide phases [UPAD09].

3.6 Sustainability Dimensions to the Bonding System

3.6.1 Technological Dimension

Bond composition and structure define the self-sharpening ability of the tool and process stability. The main bonding systems for grinding tools are resin, vitrified and metallic bonds (Table 3.2). Each type has a huge variety in its specifications, manufacturing, and ingredients. For polishing and lapping processes also different kinds of slurry or pasteous binders exist, described in Sects. 4.4 and 4.5.

Table 3.2 Overview on bonds after [COLL88, p. 869 f.]

The bond composition has to enable strong grit retention, optimum grit protrusion, and optimum self-sharpening abilities. The tool manufacturer has to find the appropriate compromise between hardness and erosion resistance [STOC86]. In the past, resin bonded wheels were preferred over vitrified bonded tools for working at higher speeds. This was due to the higher tensile strength of resin bonded wheels, but this limitation has been overcome [WHIT72]. Example mechanical properties of resin, vitrified and metallic bonds are shown in Table 3.3.

Table 3.3 Mechanical properties for most utilized bonds after [MARI04, METZ86, p. 51]

3.6.2 Economic Dimension

The type of bonding is decided by the application and technological performance and not so much by tool price. Nevertheless, the costs for a grinding tool are mainly impacted by the costs of the ingredients and processing costs. The ingredients’ prices result from raw material extraction and material processing. Many raw materials in vitrified bonding systems are still obtained from natural resources because of the lower price, although synthetic materials ensure better constant quality [BOTS05, p. 14].

The heat treatment processes in tool manufacturing are major cost factors within the processing costs. Table 3.4 shows examples for different heat treatment processes indicating the varying energy demands for producing different bond types.

Table 3.4 Examples for heat treatment processes [TYRO03]

Reproducibility of bonded abrasive tools not only depends on the process stability influenced by machines and workers but also on environmental factors like air humidity, raw material wetness, etc. On a larger scale, this affects the number of faulty grinding tools and variability in grinding performance.

In grinding cost calculations, the user not only considers the acquisition costs of abrasives tools, but in addition, the costs for auxiliary processes such as conditioning and tool change (see Sect. 7.1.3 “Life Cycle Costing (LCC)”). The conditioning costs can vary widely depending on the choice of the bonding system.

3.6.3 Environmental Dimension

In Europe, the chemicals in the grinding tools have to be classified by REACH, the European Community Regulation on Chemicals and their Safe Use standards [ECHA12]. In the U.S., OSHA (Occupational Safety and Health Administration) and EPA (Environmental Protection Agency) have developed the OSHA Occupational Chemical Database [OSHA12b].

Bond ingredients might be health endangering, such as Li, CaO, B₂O₃ in vitrified bonds (Table 3.5) or heavy metals in metallic bonds (Table 3.6). However, substituting these risky ingredients might reduce the residuals intensity and direct hazards from these materials, but indirectly this might lead to higher tool manufacturing costs.

Table 3.5 Ecological and safety information on ingredients for vitrified bonding; hazardous materials identification system (HMIS) from 0 (no/minimum hazard)—4 (severe risk) [GEST12, SIGM12, NPCA02]

Table 3.6 Ecological and safety information on ingredients for metallic bondings [GEST12, UNIT09b]

Pore builders are important fillers in bonded grinding tools. The prominent chemical naphthalene (Table 3.5) is hazardous to workers and the environment. Ongoing attempts to substitute naphthalene, e.g. with renewable material, indicate the problem to find a solution with technological similar results [DAVS04].

Particulate matter (PM) might occur from handling of loose abrasive grits and bond components [EPA94]. Heat treatment processes such as curing of resin bonded tools or sintering of vitrified bonded tools emit the main emissions in grinding tool production, in particular volatile organic compounds (VOC) or products of combustion, such as CO, CO₂, NO_x, SO_x, and PM [EPA94].

During the manufacturing of resin bonded tools, emissions form from formaldehyde and phenol. These emissions can be reduced to follow air standards by thermal after-burning [COLL88, p. 916]. However, investment and energy costs for the after-burning systems are relatively high and not feasible for small and medium sized companies, who will likely use absorption methods with activated-carbon filters [COLL88, p. 917].

Metal bonded abrasive tools are produced by infiltration, sintering, brazing, chemical or electroplating processes. The following basic principles can be applied for disposing the electrolytic baths [KLOC07, p. 192 f.]:

• Transformation of toxic substances in non-toxic chemical compounds,

• Transformation of all organic products into CO₂, water, and N₂,

• Transformation of all heavy metal compounds into an insoluble form,

• Neutralization of all solutions, or

• Avoidance of additional saline load from the waste water treatment.

Table 3.7 shows data on embodied energy, CO₂ and water usage in the primary production of epoxies and phenolics for resin bonds and Ni, Cu, and WC for metallic bonds. Reserves and recycliability are important for the future availability of these materials.

Table 3.7 Environmental material data (*estimated values) [GRAN10]

3.6.4 Social Dimension

The workers need to be protected during handling of potentially dangerous bond ingredients (Tables 3.5 and 3.6). Furthermore, the electrolytic baths in producing single layer metal bonded tools need to be maintained carefully to keep workers and local communities safe. Naphthalene as pore builder is discredited and might also affect worker health.

3.6.5 Sustainability Model for Bonding Systems

The bond material is chosen with technological, economic, environmental and social considerations. The raw material supplier is mostly concerned with the following aspects:

• Bond and raw material price depend on the raw material availability.

• Furthermore, energy and labor costs affect the price.

• The location of the production affects logistics and energy availability.

The tool manufacturer needs to consider additional aspects:

• Bond price affects the manufacturer’s profit margin. Raw material availability is important for a consistent tool quality.

• Furthermore, worker health and protection measures depend on the bond type and ingredients.

• Legislation might demand for disclosing all bond ingredients.

The tool user is mainly interested in

• Dressability of the bonding because it affects tool use and process quality.

Society and local communities are worried about

• Emissions from bonding ingredients and tool manufacturing processes. For example, the artificial pore builder naphthalene is under strict observation.