Patent GB-899021-A (GB)
Title: Improvements in or relating to the abrading of optical glass workpieces
IPC: B24B31/14
Filed: 1960-10-14
Abstract: 899,021. Tumbling. U.K. OPTICAL BAUSCH & LOMB Ltd. Oct. 14, 1960 [Oct. 15, 1959], No. 35269/60. Class 60. The surfaces of optical glass workpieces are abraded by placing the workpieces in spaced relationship in an abrading slurry of such composition that the workpieces have at most only small tendency to float or sink therein and agitating the slurry to cause relative movement between the slurry and the workpieces. The composition of the abrading slurry is preferably such that, immediately following agitation thereof, a workpiece placed on the slurry will, due to its own weight sink down at least until its top surface is approximately level with the surface of the slurry but will not sink down through the slurry at a rate exceeding one foot in ten seconds; this avoids the accumulation of workpieces at the surface or at the bottom of the slurry during agitation, so minimizing damage to the workpieces. The slurry comprises a liquid and media, i.e. a large or relatively large number of particles or pellets of solid material insoluble in the liquid and chemically inert thereto. The slurry preferably also includes an abrasive (also inert to the liquid) and may include a thickening material for increasing the effective viscosity of the slurry. The liquid is preferably water but may be glycerine, kerosene, light mineral oil or various other organic liquids alone or in combination with water. The media is in the form of particles or pellets of mean size in any one dimension greater than the mean size of the abrasive particles; the media may consist of pellets of ceramic material or nylon or a mixture of both or of cellulose acetate in powder form, pellets of various plastics materials other than nylon, various plastics moulding powders, finelydivided limestone or pellets thereof, aluminium oxide chips, pellets of soft steel or maple wood, corn cobs, pellets of cork, pellets of wax and pieces of leather. The abrasive is in the form of particles of mean size in any one dimension less than 0.004 in. but greater than 0.0008 in. when the slurry is formed; the abrasive may consist of powdered emery, finely divided crystalline quartz, garnet, corundum, boron carbide, aluminium oxide or silicon carbide. The thickening material may be colloidal clays, vegetable gums or synthetic gums. Moulded lenses, e.g. those used in the projection of photographic transparencies, are abraded, to remove the flashing and dirt or other impurity embedded in the surface, to polish the optical surfaces and to impart a smooth finish to the edge, in an octagonal tumbling barrel B of wood, metal or plastic, lined with neoprene rubber, and rotated through chain C<SP>1</SP> by a variable speed electric motor C. The slurry comprises water, sufficient abrasive to cover the exposed surfaces of the media and workpieces with a coating of suspended particles and sufficient media to envelope individually each of the workpieces. The operation is performed in four stages, viz. (1) 300 1b. of media are loaded into a 7 cu. ft. barrell, 2/3 by weight constituted by ceramic cones of fused aluminium oxide land 1/3by weight by nylon bi-cones. 500 spherical or aspherical lenses are then placed in the media in spaced relationship by pushing one half of the lenses below the surface layer of media in spaced relationship, rotating the barrel through 180 degrees and pushing the remainder of the lenses below the new surface layer of media in spaced relationship. 35 1bs. of No. 500 emery in 7 gallons of water are then added. The barrel which is now 60 % full is sealed and rotated at about 5¢ r.p.m. for 24 hours. The No. 500 emery is removed by filling the barrel with water, and rotating it 4 or 5 times and draining the water out, the rinsing being repeated 3 or 4 times. (2) 35 1b. of equal amounts of No. 1100 and No. 1200 emery in 7 gallons of water are added, the barrel sealed and rotated at 5¢ r.p.m. for 24 hours. Rinsing is carried out as before and the barrel emptied. (3) 300 1b. of nylon bi-cones are loaded into the barrel and the lenses placed in the media in spaced relation as before. 75 1b. of finely-divided crystalline quartz in 5 gallons of water are added, and the barrel sealed and rotated at 5¢ r.p.m. for 24 hours. After rinsing, the barrel is emptied. (4) The barrel is loaded with 144 1b. of cellulose acetate extracting powder to act as media, along with 70 1b. of finely-divided crystalline quartz and 8 gallons of water. The lenses are placed in the slurry in spaced relationship, the barrel rotated at 5 r.p.m. for 24 hours and then the lenses are removed and rinsed. The lenses are stated to have surfaces which are harder and more resistant to chemical staining than those conventionally polished. A tumbling barrel of larger capacity may be used, divided into two or more compartments so that successive stages of the treatment can be carried out simultaneously with two or more batches of workpieces. The treatment can be effected in a single stage. A vibrating pan may be used instead of a tumbling barrel.