Purchasers of castings can specify and assess surface quality and surface texture more accurately and definitively using tactile comparators developed by Castings Technology International (Cti). These provide a practical and functional alternative to photographic representation of casting surfaces in standards such as MSS SP-55-2001.
More than 1000 of these sets of comparators are used around the world by multi-national companies and by small enterprises. The durable plastic comparator plates, actual replicas of relevant characteristics of surface finish, have proved to be a more attractive way of reaching agreement with casting suppliers on the required surface quality during contract negotiations. They also provide a basis for acceptance of the castings after order placement.
The comparators form the basis of ASTM A802/A802M – 95 (2006) Standard Practice for Steel Castings, Surface Acceptance Standards, Visual Examination. This standard comprises 31 comparators that define features such as surface roughness, surface inclusions, laps and cold shuts, surface finish after thermal and mechanical dressing, etc.
A larger, 48 comparator set applies to ISO 11971:2008 Steel and Iron Castings – Visual examination of surface quality; BS EN 1370:1997 Founding – Surface roughness inspection by visual tactile comparators; BS EN 12454:1998 Founding. Visual examination of surface discontinuities. Steel sand castings. (Companies that already have the ‘short’ ASTM A802 set can upgrade to the larger set by purchasing the 17 additional comparator plates).
A third set of 14 comparators, the basis of BS 7900:1998 Specification for examination of the surface texture of precision steel castings, is applicable to the procurement of steel castings manufactured by precision casting techniques such as resin-shell moulding, lost wax, Shaw process, etc.