The surface roughness of a casting is influenced by the manufacturing process (moulding, grinding, finishing, etc.), the moulding materials used (sand, coating, etc.), the equipment available and the alloy cast.
Since cast surfaces do not exhibit the same cyclic character as machined surfaces it is difficult to evaluate their roughness using conventional mechanical, optical, or pneumatic devices.
The use of visual tactile comparators is therefore preferred in these circumstances.
Moreover, in order to take account of the irregularities on as-cast surfaces, ground surfaces or other means of finishing of castings, comparators should have relatively large dimensions [greater than or equal to 15 000 mm2] in order to make them more reliable and their results repeatable and consistent.
Two sets of comparators are in widespread use:
- 'SCRATA comparators for the definition of surface quality of steel castings', available from Steel Castings Technology International, 7 East Bank Road, Sheffield S2 3PT, United Kingdom;
- 'BNIF 359 Recomrnandation technique du Bureau de Nonnalisation des Industries de la Fonderie. Caracterisation d'etats de surface des pieces moulees - Utilisation des echantillons types de 110 mm x 160 mm', available fronl Editions Techniques des Industries de la Fonderie, 44 avenue de la Division Leclerc, 92310 sevres, France.
This standard describes a method for the evaluation of casting roughness by means of visual tactile comparators.
The method is applicable to all casting processes (except pressure die casting) for all cast materials. The method can be used for the assessment of castings or areas of castings in the as-cast condition (beyond removal of casting appendages, such as gates, risers and flash and removal of residues of the moulding material).
This method does not give a means of determining the quality of castings by visual examination2).
However, since non-destructive testing (ultrasonic, dye penetrant etc.) is influenced by surface condition, preliminary roughness evaluation may be made using this method as an aid to subsequent non-destructive testing examination.
