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]
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
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Fig. 3.16 Manufacturing of electroplated tools |
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].
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].
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].
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Grit Nickel |
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”).