Patent GB-637090-A (GB)
Title: Improvements in and relating to diamond grinding wheels
IPC: B24D5/04
Filed: 1946-10-21
Abstract: 637,090. Grinding - wheels. NORTON GRINDING WHEEL CO., Ltd. Oct. 21, 1946, No. 31270. Convention date, Oct. 20, 1945. [Class 60]  A grinding-wheel comprises an outer abrasive annulus of diamond grains, in a vitrified bond integrally united with an inner part which in turn is united with a hub portion or portions projecting laterally therefrom, the inner part  and hub portion or portions each being composed of granular material distributed in a vitrified bond. The inner part may be held under radially inward compressive stress by the hub portion or portions to reduce the tensile stress in the inner part caused by centrifugal force. The grinding-wheel shown in Fig. 2 is made up of a wheel part 10 and hub elements 15 and 16 which are preformed prior to assembly to the required dimensions. The part 10 comprises an outer annulus 12 composed of diamond grains in a vitrified bond, and a centre portion 11  composed of a vitrified bonded granular material such as vitreous silica. The portion 11 may be subjected initially to a low-pressure moulding operation and the diamond-containing mix for the annulus 12 distributed around its periphery, the whole then being subjected to a high-pressure moulding operation. The elements 15, 16 are moulded from a mix comprising a granular material and vitrifiable bond. The three parts are then aligned by a mandrel 18 passed through their central holes 13 and placed on a support 23. Weights W are placed on the element 15 and the whole is then subjected to heat to fuze the bond. If desired, layers of vitrifiable bond 24 may be interposed, as by spraying, painting or dusting, between the three parts. Examples given for the bonding materials are: (1) a mixture of borax, boric acid, silica, hydrate of alumina, whiting and ball clay; or (2) Kentucky ball clay, ground flint and borax. A temporary binder Glutoline may be used to plasticize the mix, and protective agents such as aluminium, silicon or boron, or compounds thereof, may be employed. These agents may serve as body materials as described in U.S.A. Specifications 2,356,937 and 2,356,938. Mixed glasses may be employed in the bond as described in U.S.A. Specification 2,343,218. In Fig. 3, the wheel part 10 and hub element 16 are made up of preformed discs 11a-12a, 11b-12b, 11c-12c and 16a-16b, united by layers of vitrified bond 24a, 24b, 24c and 24e, the structure being fuzed together as described above. The layers 24a-24c are stated to improve the resistance of the wheel to compressive stress, and in both constructions the laminations minimize bending of the wheel due to axial stresses, e.g. when grinding with the side-faces of the annuli 12. The hub element or elements may have the same coefficient of thermal expansion as the centre part 10, or by adding fused alumina in greater amounts to the mix for the hub elements their coefficient of expansion may be rendered greater after fusing than that of the centre part, so that on subsequent cooling the centre part is placed under compression and the hub elements under tension. In consequence, when the wheel is rotating, the centrifugal force has to overcome this initial compressive stress before the centre part is placed under tension so that the risk of cracking is decreased. The tensile strength of the hub elements may be increased relative to that of'the wheel part by applying a greater pressure to these elements during moulding. Body material of a smaller grit size than that used for the wheel part may be used in the hub elements.