Silicon carbide grinding

Abrasive sheet grinding Silicon carbide Coarse (600 pm) to fine (120 pm)... 

In the Premier Mill the rotor is shaped hke the frustrum of a cone, similar to that in Fig. 20-53. Surfaces are smooth, and adjustment of the clearance can be made from 25 [Lm (0.001 in) upward. A small impeller helps to feed material into the rotor gap. The mill is jacketed for temperature control. Direct-connected hquid-type mills are available with 15- to 38-cm (6- to 15-in) rotors. These mills operate at 3600 r/min at capacities up to 2 mVh (500 gal/h). They are powered with up to 28 kW (40 hp). Working parts are made of Invar alloy, which does not expand enough to change the grinding gap if heating occurs. The rotor is faced with Stellite or silicon carbide tor wear resistance. For pilot-plant operations, the Premier Mill is available with 7.5- and 10-cm (3- and 4-in) rotors. These mills are belt-driven and operate at 7200 to 17,000 r/min with capacities of 0,02 to 2 mVh (5 to 50 gal/h). 

Silicon carbide, SiC or carborundum, has the diamond structure, and it is widely used as an abrasive in grinding wheels. These are made by crushing the SiC, adding clay, then heating the material in molds. Silicon carbide is prepared by the reaction... 

Silicon carbide (SiC), nearly as hard as diamonds, is used as an abrasive in grinding wheels and metal-cutting tools, for lining furnaces, and as a refractory in producing nonferrous metals. 

Composites. Another type of electro deposit in commercial use is the composite form, in which insoluble materials are codeposited along with the electro-deposited metal or alloy to produce particular desirable properties. Polytetrafluoroethylene (PTFE) particles are codeposited with nickel to improve lubricity (see LUBRICATION AND LUBRICANTS). Silicon carbide and other hard particles including diamond are co-deposited with nickel to improve wear properties or to make cutting and grinding tools (see Carbides Tool materials). 

SiC, like boron carbide, silicon carbide is nearly completely covalent and is e.g. used as grinding powder, in the manufacture of ball bearings, balls, outlet casings and jet nozzles, as well as in packing rings of pumps used to transport materials which are likely to cause wear. 

Silicon combines with carbon to form silicon carbide or carborundum, SiC, and forms are known that have the wurtzite and zinc blende structures. It is a very hard, tough material that is used as an abrasive and a refractory. The powdered material is crushed after mixing with clay and heated in molds to make grinding wheels. It is prepared by the reaction... 

There are many types of mechanical grinding equipment available ball mills, ball and mortar sets, planetary mortars, impact mills, etc. These are available in a variety of materials steels, porcelain, agate, mullite, silicon carbide, tungsten carbide and others. Prices range from 200 to about 2,000. Unfortunately, the cost increases rather markedly as one goes to the harder materials, especially the carbides. 

A casehardened steel-grinding tool of precise taper (Fig. 11.7) is fitted to the chuck of an otherwise redundant lathe headstock and rotated at about 60 rev/min. A suspension of No. 400 silicon carbide (carborundum) in a mixture of glycerine and water is applied to the tool and the socket blank held in place with the right hand. 

The raw silicon carbide is processed by crushing in jaw crushers or hammer mills and subsequent fine grinding in ball mills. Very pure SiC qualities are obtained by ehemical treatment with sulfuric acid, sodium hydroxide or hydrofluoric acid. 

Refractoriness (Melting Temperature). Instantaneous grinding temperatures may exceed 3500°C at the interface between an abrasive and the workpiece being ground (14). Hence melting temperature is an important property. Additionally, for alumina, silicon carbide, B4C, and many other materials, hardness decreases rapidly with increasing temperature (7). Fortunately, ferrous metals also soften with increasing temperatures and do so even more rapidly than abrasives (15). 

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