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].

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].