The production of vitrified bonds also begins with mixing the components. By covering the grits with a temporary binder (e. g. organic adhesive and water), sufficient form stability can be achieved in the blank. The pourable mixture is transferred to the mould and compressed via pressing and shaking methods.
In the casting method, the vitrified bond and the abrasive grits are mixed with water in mixing machines. Before the mass becomes homogeneous and pourable, this mixture requires a stirring time of a half an hour to six hours. Then it is poured into moulds with porous bottoms and allowed to dry very slowly in drying chambers in order to prevent the formation of cracks. After drying, the rough grinding component has enough fracture resistance to be transported and premachined to furnace dimensions.
The grinding wheels can only be cast with very clayey bonds as well as with finer granulations (from about F 80), since only these emulsify sufficiently and can be held in suspension. The casting process is very costly and is being increasingly replaced with the pressing method. However, it yields very homogeneous grinding components with consistent hardness and edge stability and is thus still used today in the manufacture of fine-grit grinding wheels.
After drying the grinding component, profiled grinding wheels, e. g. cup grinding wheels, are prepared in accordance with the furnace dimensions. An intermediate inspection follows this, which essentially comprises an assessment of the geometry and structure, as well as an examination into crack formation during drying [FRAN68].
To harden the rough grinding wheels, they are burned at temperatures from 800 to 1350 °C. The lower temperatures used here apply to the fused bonds. Sintered bonds are burned at temperatures in excess of 1250 °C. The exact temperature is based upon the fluxing agents (frits) or leaning agents (e. g. clay) contained in the respective bond. Burning takes place in electric, gas or oil furnaces, either in continuously operating tunnel kilns or in periodically burning charge furnaces, e. g. hood kilns.
Tunnel kilns function with constant and stationary heating and erratic engagement. They reach a length of up to 70 m. The rough grinding wheels are placed upon carts, which pass through, one after another, the preheating, burning and cooling zones of the furnace in about 8 days. The grinding wheels heated up slowly in the preheating zone lose any remaining mixture as well as organic auxiliaries like adhesives or burnout material. In the burning zone, the ceramic bond is sintered and finally fused into stoneware to glassy mass, which encloses and bonds the abrasive grits to each other. In the subsequent cooling zone, which takes up about half of the length of the furnace, occurs a slow cooling down to room temperature, so that residual stresses and cracks in the grinding wheels can be avoided.
The burning/hardening of the abrasive tools is usually followed by finishing with special machines. This serves to produce the prescribed shape and dimensions as well as to remove the hard sintering crust. For particular grinding and honing processes, vitrified bonded grinding wheels are imbued with synthetic resin, small grinding components and honing stones with sulphur. The grinding wheels are also partially cemented on metal foundations. Steel shafts are cemented into mounted points.
Finally, it should be mentioned that, among bonded grinding wheels, there are also the so-called “chips”, which are used for free abrasive grinding. We chiefly find paraboloids, stars, triangles, tetrahedra, and cylinders with edge lengths of 3 to 30 mm. The grits in free abrasive grinders consist of conventional abrasives like Al2O3 and SiC and are extremely fine. Free abrasive grinding components are, as a rule, have a vitrified bond. Due to their relatively high density, vitrified bonded free abrasive grinding components can generate a high grinding pressure. For this reason, for gentler working cycles, lighter chips with plastic bonds have also been developed.