[Music] to cut the core sample we need a drill bit and a reaming shell the reaming shell has two functions it Reams the hole to its nominal diameter preventing a new drill bit from getting stuck in a slightly conical hole it stabilizes the core barrel and mitigates deviation of the bore hole the drill bit cuts out the actual core and there are three General types each corresponding to a certain ground condition tungsten carbide bits for geotechnical investigation cord Drilling in soil and in other soft formations tungsten carbide is used in the form of inserts or
chips PDC or polycrystalline Diamond composite bits for core drilling in soil soft Shale soft Limestone and other soft sedimentary formations PDC bits are used in the form of buttons or Surface set Diamond bits for core drilling in medium to extremely hard rock natural diamonds are used in surface set bits synthetic diamonds are used in impregnated bits the diamonds are embedded in a matrix since impregnated Diamond drill bits are the most common bits in Exploration we will put our Focus here first of all the diamond bits Matrix is classified according to its hardness the the general
rule is that a softer Matrix is used in a harder rock and a harder Matrix is used in a softer Rock for example in hard rock the diamonds last a shorter time therefore the Matrix needs to be softer to expose new diamonds more frequently when the old ones are worn out to maximize the productivity it is therefore important to match the Matrix hardness with the rock hardness it is also important to choose the right drilling parameters corresponding to The Matrix hardness and The Rock hardness for example if this H siiz bit is used in very
hard rock with an RPM between 750 and 950 maximum weight on bit also called wob of 50 KNN and water flow higher than 50 L per minute you can expect a penetration rate between 8 to 18 c per minute following these parameters will maximize bit life the optimal state is called free cutting and is the perfect balance between weight on bit wob RPM and flushing this will give the optimal rate of penetration Ro o and maximum bit life important providing that you have chosen the right bit Matrix for the right ground condition of course to
optimize the flushing the drill bits are manufactured with different profiles and waterways the most common is the ECF profile suitable for most rock formations another one is face discharge FD profile suitable for extremely broken loose or soft formations where there is a danger of washing out the core sample to further extend the bit life different impregnated Heights are available a height of 13 mm is the most common but both 10 mm and 16 mm are available examination of a used bit can provide the driller with useful information if the driller is using incorrect drilling or
flushing parameters this can be seen from specific wear patterns on the bit face for example too low RPM may result in a concave bit [Music]
