As the name suggests, the surface-set diamond bit differs from the impregnated diamond type in that they are set with a single layer of natural drill diamond on the working surfaces of the bit crown in a pre-determined pattern.
The diamonds are mounted in a bit crown powdered metal matrix body that is fused to a steel tool body or shank. Unlike impregnated diamond bits where the controlled erosion of the powdered metal bit crown is desirable to expose new diamond crystals, the metal bond matrix used for all IDP surface-set bits is extremely tough and wear-resistant. The function of the bit crown matrix body on a surface-set bit is to securely hold the diamonds in position throughout the operational life of the bit.
Surface-Set diamond bits are primarily recommended for use in drilling relatively soft, abrasive, unconsolidated formations that are not effectively drilled by impregnated diamond bits. They are also recommended for use in drilling harder formations where the available rotational speeds and bit loads are insufficient to use impregnated diamond bits due to drilling equipment limitations.
In most cases, surface-set diamond bits will provide higher rates of penetration than impregnated diamond bits in softer formations due largely to the greater degree of exposure of the individual diamond crystals. However, as they are set with only a single layer of diamond crystals, surface-set diamond bits will generally yield a lower overall bit life than impregnated diamond bits.