Adjustments in rotating speeds and pulldown pressures cannot be set into hard and fast rules. The same formation may require different drilling techniques in holes spaced only a short distance apart. A good driller knows from experience why each thing happens or why an operation is done in a certain way. He is able to visualise conditions at the bottom of the hole and knows the reaction of the bit in different formations. For example, a bit which becomes balled up when drilling in soft sticky formations creates stresses and vibrations in the drill rod similar to those caused by drilling in shattered, consolidated formations. A good driller can recognise formation changes and keep an accurate log of them. Experience is the only way to gain these qualifications.
For down-the-hole hammer drilling, proper rotation speeds are important in achieving optimum penetration and bit life. 10 - 40 RPM is the recommended range. The slower rotation should be used in harder, more abrasive formations. Excessive rotational speeds wear out the carbide without a corresponding gain in penetration. Applied bit weight should not be less than 350 Kg, but should not exceed 1000 Kg
