Abrasion Tools

Abrasion Tools

 

tools fabricated from abrasive materials for machining metal, leather, wood, glass, rock, plastics, and so on. Abrasives began to be made by industrial methods in the second half of the 19th century, at the time grinding machines made their appearance. Abrasion tools are subdivided into two types: rigid (grinding wheels, grinding heads, segments, and sticks) and flexible (abrasive cloth and products made from it, such as belts, and disks). Materials used in the production of abrasion tools include electrocorundum (normal, white, in alloy with titanium and chromium oxide additives, monocorundum); silicon carbide (green and black); and synthetic and natural diamonds. Abrasion tools are made from ceramic, bakelite, and vulcanite rubber bonds and less frequently with silicate, glyphthalic, and magnesial bonds holding the individual abrasive grains together. Oleanite and shellac bonds are also used in some countries.

The list of standard manufactured abrasives covers about 750 distinct types and sizes and about 12,000 varieties. Grinding wheels of electrocorundum and silicon carbide are manufactured in diameters from 3 to 1,100 mm and in thicknesses from 0.5–200 mm, with the diameter of the fitting hole from 1 to 305 mm. Grinding wheels of diamond grains are made (using bakelite, metallic, and ceramic bonds) with diameters from 6 to 300 mm and thicknesses of grinding face of 1.5–5 mm. Overall width will range from 3 to 20 mm. One important indicator of the quality of an abrasion tool is the concentration of diamond—that is, the content of diamond grit per cu mm of the diamond-bearing layer (0.878 mg diamond grains are contained in 1 cu mm at 100 percent concentration). The diamond concentration in the diamond-bearing layer in grinding wheels ranges from 25 to 200 percent.

Grinding cloth and products made from it are fabricated on bases of tissue and paper, with cutting grains of electrocorundum, silicon carbide, glass, or silicon. The cloth is used for manual and mechanized grinding work and in particular for belt grinding. Depending on the strength required, the cloth is made on bases of coarse calico, serge, a special semibifilar fabric, or paper. Serge provides the greatest tensile strength.

Abrasion tools differ from other cutting tools in the large number of random grains—of cutters with pores between them—and also in the shape and intermittent patterns of the cutting edges. The chips and waste they produce are usually short in length. Abrasion tools can be used to machine parts made of materials of any hardness and can operate at cutting speeds far above those used in other cutting and metal-removing processes, while removing from the work a layer of metal as much as several millimeters thick down to fractions of a micron.

Abrasive grains chip and break off as they become dull during grinding, leaving bare the layer of undulled grains beneath them. This property of abrasion tools is referred to as its self-sharpening ability. The more intensive the chipping and breaking away of the grit grains, the more completely the tools sharpen themselves. The cutting ability of the abrasion tool is only partially restored when the tool sharpens itself. The abrasion tool is completely restored by dressing it, removing the surface layer of grain. The shape of the tool is restored in the process.

Abrasion tools are dressed by using mounted diamonds, diamond pencils, diamond rollers, and various diamond substitutes, hard alloys and steel rollers, milling cutters, grinding wheels of high hardness, abrasive sticks, and the like. The abrasive ability of the tool is greater the longer its service life between dressings, and the service life is longer the thinner the layer of abrasive removed in each dressing operation.

To a significant extent the fabrication technology of abrasion tools determines their working properties, such as homogeneity of composition, hardness, resistance to wear, and dimensional accuracy. To achieve stability in these properties, the technological process specifies the shape and quantity of the bond, the volume and quantity of the grinding media, the pressure and method used for molding, the quantity of adhesive added to the bond to improve the ductility of the grinding medium, the temperature, and the heat-treatment time. The production of abrasion tools consists of preparation of the bond, mixing of the abrasive mass, molding, heat treatment, mechanical finishing, and testing for strength and hardness. Ceramic bonds are prepared from finely pulverized refractory clays of different compositions, fusing agents (such as talcum and common potash feldspar), pearlite, and quartz. Bonds are mixed in mixers with the abrasive grains and adhesive (dextrin or water-soluble glass) and are run through a vibration-aided screen or a loosening machine. The grinding medium so prepared is molded in hydraulic presses. The material emerging from the drying ovens is inserted in tunnel kilns, where it is gradually heated to temperatures of 1240–1320° C and then allowed to cool slowly. Bakelite-bonded abrasive tools undergo bakelitization at 180° C. The temperature conditions and the heat-treatment time for abrasive tools determine their tensile strength, bending strength, compressive strength, impact strength, and accordingly their operational properties. After firing, abrasion tools are finished mechanically; the required dimensions are imparted to them and they are balanced. Abrasion tools are tested for tensile strength at loads 50 percent above the working load and are branded after the hardness has been determined. The fabrication of vulcanite-bonded abrasion tools is distinguished by the fact that the material is mixed on mixing rollers, while the required thickness of the material is achieved by rolling.

Diamond-grinding wheels are molded under high pressures (as high as 200 meganewtons per m2, or 2,000 kilograms-force per cm2). Metallic-bonded diamond wheels are heat-treated at temperatures of 600–650° C, while ceramic-bonded diamond wheels are heat-treated at 800–850° C. The tensile strength of abrasion tools allows cutting speeds of 25–80 m/sec.

The grinding cloth is fabricated on semiautomatic conveyor-equipped machines, where the process is continuous. The abrasive grit applied to the fabric or paper is bonded by hide glue or synthetic varnish. The grains are applied in an electrostatic field in order to enhance the cutting properties of the cloth.

Abrasion tools are widely used in all branches of metal-working machinery and particularly in the fabrication of bearings. In the automotive, tractor, and toolmaking industries, as much as 30–35 percent of the total inventory of machine tools consists of grinders, tool sharpeners, lapping machines, and buffing machines. Abrasion tools are used on these machine tools in snagging work for removing large machining allowances and in finishing and precision operations for fabricating high-precision parts and parts with high surface finish.

Production is being intensively developed of such promising types of abrasion tools as wheels for high-power grinding, roll plate scarfing, shaped contoured grinding, and other operations associated with heavy metal removal and high precision machining.

REFERENCES

Ippolitov, G. M. Abrazivnye instrumenty i ikh ekspluatatsiia. Moscow, 1959.
Kudasov, G. F. Abrazivnye materialy i instrumenty, 2nd ed. Leningrad, 1967.

G. M. IPPOLITOV

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