Core drilling bits
1. With what type of cutting devices can a core bit be armed?
a. TC (Tungsten Carbide)
b. PDC (Poly-Crystalline Diamonds Composite)
c. Natural diamonds
d. Synthetic diamonds
2. Which types of core bits are primarily used for geotechnical investigation drilling and for which types of geological formations are these drill bits intended?
ü Bits armed with TC or PDC are usually used for geotechnical core drilling in soft formations such as soil, softy shale, clay and very soft sedimentary formations.
3. Mention two characteristics of a diamond which make the diamond suitable for cutting hard rock?
ü The hardness of the diamond. A diamond is the hardest known naturally occurring mineral.
ü The very high coefficient of thermal conductivity.
4.Why can a TC bit not be used for drilling in granite?
ü The coefficient of thermal conductivity is too low.
5. What types of diamond bits are used in core drilling? Describe the difference between these diamond core bits
ü Surface set diamond bits and impregnated diamond bits.
- A surface set diamond bit is dressed with one layer of natural diamonds
imbedded in the envelope surface of a hard matrix.
- On a impregnated diamond bit the matrix is thoroughly impregnated with small diamonds, much smaller than the diamonds used in a surface set bit.
6. Which are the advantages with an artificial diamond as compared to a natural diamond?
ü The technique for manufacturing synthetic diamonds has had a fast development during the last decades. Today the synthetic diamonds are superior to the natural diamonds both in terms of life length and cutting ability.
ü Another advantage with the synthetic diamonds is that these can be given special qualities required for core drilling. The synthetic diamonds can thus be “tailor made” to suit drill bits intended for different types of rock. Brittleness, grain size and geometry are three important characteristics that can be “tailor made”.
7. What does a gradual graphitization of a diamond mean?
ü The diamonds looses their hardness and thus their ability to cut the rock.
8. In which units do we measure the weight and the size of a diamond?
ü The weight is measured in Carats. One carat is 0,2 grams
ü The size of a diamond is measured in stones per carat (SPC).
9. For which geological formations are the surface set diamond bits primarily intended?
ü Soft to medium hard formations
10. When is a surface set diamond bit consumed?
ü When all the protruding diamonds are blunt and worn.
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