GB1229803A - - Google Patents

Abstract

1,229,803. Severing by grinding. MONSANTO CHEMICALS Ltd. 20 Feb., 1969 [7 March, 1968], No. 11106/68. Heading B3D. A process for cutting a body of material, more particularly for providing highly polished damage-free cut surfaces, comprises using a cutter made of a substance softer than the material to be cut and working in an aqueous dispersion of a particulate solid, the particles carrying hydroxyl groups or oxy anions on their surfaces, being no harder than the material to be cut and of a size not exceeding 10 microns. The cutter may be a thread or filament of a natural or synthetic fibrous material under tension; single filaments of synthetic material e.g. nylon, are generally preferred. The thread or filament may be fitted to a bow-saw frame and given a reciprocatory movement across the work but is preferably used as a continuous or reciprocatory band saw. The particle size preferably is in the colloidal range of 3-300 millimicrons. Suitable solids include the oxides and hydroxides of weakly electropositive elements, e.g. iron, chromium, aluminium, germanium, tin, zirconium and silicon. Also suitable are a variety of silicate minerals, e.g. feldspar, wollastonite, mullite, kyanite, chrysolite, cristobalite, crocidolite, spodumene and garnet. The aqueous medium may contain a proportion of a water-miscible organic liquid, e.g. an alcohol, glycol or ketone. Cutting is more efficient when the medium is alkaline. The concentration of solid may range from 0À5-50% by weight. The dispersion may be at ambient temperature, but temperatures between the freezing and boiling points of the dispersion can be used. The dispersions found most suitable are silica sols. Sols having average particle sizes of 1000 Š and upwards are preferred. Of comparable effectiveness are dispersions in aqueous alkaline media of silica normally used, e.g. as a pigment or thickening agent in paints or resins. The alkalinity of these dispersions is preferably provided by an alkali metal hydroxide, an alkaline earth metal hydroxide or ammonium hydroxide. An increase in cutting rate occurs with increase in temperature up to about 70‹ C. Increase in temperature has a greater effect where alkalinity is provided by sodium hydroxide than where potassium hydroxide is used. The dispersion can be fed to the cut line intermittently or continuously during cutting, or the work may be immersed in a bath of the dispersion. The process is of particular value in cutting slices from ingots of monocrystalline semiconductor materials. Where it is desired to cut slices from an ingot or bar of a semi-conductor material, several parallel cuts can be made simultaneously by appropriately spaced cutters. In one cutting arrangement a silicon rod 1 is clamped between the base of a block 2 of p.t.f.e. and a clamping plate 3 of the same material. A continuous filament 4 of nylon running through slots 5 in the block 2 passes round the rod and a pulley 6 on a spindle the mounting of which is adjustable to vary the vertical distance of the spindle from the rod and thus the filament tension. The clamping plate and rod are immersed in a bath containing an aqueous dispersion of colloidal silica, the static tension in the filament is adjusted to 125 grams and the pulley is set oscillating at 500 c.p.m. through an angular displacement of approximately 180 degrees. Periodic adjustment of the spindle mounting is made to maintain the tension.