Patent GB-1188678-A (GB)
Title: Peripheral Abrasive Wheels with Composite Rims
IPC: B24D18/00
Filed: 1967-11-01
Abstract: 1,188,678. Grinding wheel. E. I. DU PONT DE NEMOURS & CO. 1 Nov., 1967 [2 Nov., 1966], No. 49721/67. Heading B3D. [Also in Division C4] A grinding wheel includes a composite rim having an outer layer and an adjoining inner layer; the outer layer comprises a mixture of 90-50% by volume of a solid coalesced aromatic polyimide, 10-35% of diamond grains and 0-40% of inert filler; the inner layer is a mixture comprising 10-60% by volume of inert filler and substantially all of the remainder being resin having a second order glass transition temperature of at least 300‹ C. and a modulus of elasticity of at least 300,000 p.s.i. (as measured by ASTM D-790), the inner layer having a modulus of elasticity no more than twice the modulus of elasticity of the outer layer; the composite rim is mounted or formed on a core. In the outer layer, the diamond grains are 10-200 microns; suitable fillers are glass, metal fibres, metal powders, silicon carbide, alundum, molybdenum sulphide and silicon dioxide. In the inner layer, suitable resins are aromatic polyimides coalescible by high temperatures and pressures, novolac phenolic resins, polybenzimidazoles, polyamide imides, polyimines and polybenzoxazoles; suitable fillers are aluminium or other metal particles, glass, metal fibres, silicon carbide, alundum and silicon dioxide. Where a novolac phenolic resin is used it is desirable to use a filler having a modulus of elasticity of under ¥ million p.s.i., e.g. aromatic polyimides and other high temperature resins, to reduce the modulus of elasticity of the composition. The core material should have a modulus of elasticity of at least 6 million p.s.i.; resin may be used, but metal, e.g. aluminium, is preferred. Example I.-An essentially linear aromatic polyimide resin having a second order transition temperature greater than 500‹ C. and a modulus above 350,000 p.s.i., prepared by reacting 4,4<SP>1</SP>-oxydianiline with pyromellitic dianhydride, is mixed with 25% by volume of 180 grit diamond; the composition is lightly compacted into the cavity between a 6 in. diameter mould and a 5¥ in. core. The same resin is mixed with 25% by volume of 120 grit silicon carbide, and the composition lightly compacted into the cavity between the preform and a 5¢ in. core. The composite preform is further compacted at about 3000 p.s.i., heated to about 270‹ C. and subjected to a pressure of about 50,000 p.s.i. The preform is removed from the mould and heated for approximately 16 hrs. in a vacuum oven at about 300‹ C., followed by heating in a substantially inert atmosphere at about 450‹ C. for about 30 min. The rim is then shrink-fitted around an aluminium core coated with epoxy resin, which is then cured. Example III.-The resin of Example I is mixed with 25% by volume of 120 grit diamond and the composition compacted at about 4000 p.s.i. into the cavity between a 6 in. diameter mould and a 5¥ in. core. A composition of 6À5 gm. of 240 grit SiC, 1À7 gm. aluminium powder and 7À92 gm. of polyimide powder is compacted at about 4000 p.s.i. into the cavity between the preform and a 5¢ in. core. The mould and its contents are heated to about 300‹ C. and further compacted at about 5000 p.s.i. The preform is heated for about 16 hrs. in a vacuum oven at about 300‹ C., followed by heating in a substantially inert atmosphere at about 450‹ C. for about 25 mins. The rim is shrink-fitted over an aluminium core coated with epoxy resin. Pressure is applied to the opposite faces of the core, using a pair of indenting dies having raised ring-shaped bearing faces with a semi-circular cross section to expand the core to give a total permanent rim strain of 0À60%. The adhesive is cured at 70‹ C. for 4 hrs. Example V.-A diamond outer layer is produced as in Example III and placed in a 6 in. mould containing a 5¢ in. aluminium core coated with cured epoxy resin containing 60% by weight of aluminium powder. A composition is prepared by mixing 30% by volume of 200 atomized aluminium powder, 30% of a particulate polyimide resin made from pyromellitic dianhydride and 4,4<SP>1</SP>-oxydianiline and passing a 40 U.S. Standard sieve and 40% of a medium flow novolac phenolic resin; the composition is compacted into the cavity between the outer layer and the core, further compacted at 5-8000 p.s.i. and heated to 160‹ C. for about 15 mins. The wheel is removed from the mould and aged in an air oven at 100‹ C. for 16 hrs.