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