the natural diamond grains on surface set core bit are large and self-sharpening easily, in order to protect the exposed diamond grains, the rotary speed of surface set core bit should be lower than the impregnated core bit
in order to reach a proper linear velocity, the rotary speed of the small diameter bit should be higher than the big diameter bit
from the formula of rotating speed, we can find the liner velocity is proportionate to the rotating speed. It means the higher speed of the liner velocity, rotating speed is higher accordingly
high rotary speed is suitable for medium hard, complete rock formations, in broken, fractured mixed formations, with high vibration during drilling, drillers should slow down the rotary speed according to the broken level of the rock, in the soft formation with high drilling efficiency, in order to keep cooling and carry out the cuttings, the penetration speed needs to be limited, as well as the rotary speed
the larger diamond size, the quicker the self-sharpening. To avoid the bit face chipped or cracked the rotary speed of the bits with big diamonds should lower than the bits with small diamonds
when the drilling machine is with poor stability and the drill rods have a low intensity, correspondingly, the rotary speed should be slowed down. If the lubricants or other similarities for decreasing the vibration are adopted, the rotary speed could be raised
when the depth of drilling hole become deep, the weight of the core barrels will be larger, the pressure situation is more complex, it takes later powder while rotating the core barrels, hence, in the deep hole, because of limit of the power and intensity of the core barrels, the rotary speed should be decreased, in shallow hole, conversely.
high rotary speed can be used in the condition that the borehole strcture is simple and the clearance between the drill rods and borehole wall is small. On the contrary, the drilling hole with a complex situation, lots of changeable diameters, big space between the drill rods and borehole wall, it causes poor stability and can’t use the high rotary speed.