Rotation: compare diameter and advance
Peripheral speed depends on bit diameter and RPM. A larger bit has a higher peripheral speed at the same rotation rate. Record penetration with RPM: revolutions per centimeter equal RPM divided by penetration in cm/min. A rising value means less advance per revolution, but the cause may be formation, load or diamond exposure.
Load: distinguish force from hydraulic pressure
Weight on bit is the effective axial force at the cutting face. String weight, buoyancy, friction and the feed mechanism all influence it. A feed-pressure gauge is not a direct measurement of downhole load. Use the rig’s load-estimation method and stay within the limits of the bit, barrel, rods and machine.
The same RPM produces different cutting speeds.
Calculated examples at 600 RPM. Set the working speed for the actual bit and formation.
Contact area: why waterways matter to load
Average contact stress is force divided by the bearing area of the cutting face. Opening the channels reduces that area. A load used with one waterway may be unsuitable for another, even at the same nominal bit diameter. Matrix type must be considered as well.
Hydraulics: separate flow from pressure
Flow cools the face and transports cuttings. Pressure reflects resistance in the whole circuit. A high pressure reading can accompany restricted circulation, and a sudden drop can indicate a leak or fluid loss. Compare pump output with returns, fluid condition and hole behavior.
Annular velocity: consider the return space
For a given flow, a larger gap between the rods and hole wall produces a lower return velocity. Washouts therefore change cleaning conditions. Cuttings size, fluid viscosity and formation stability influence the required transport capacity. Excessive flow can erode weak ground or wash the core.
Adjust one setting and record the response
Establish a baseline in a known interval. Make one controlled adjustment, then compare penetration, torque, pressure, returns and wear. If extra load produces little useful advance, investigate the cutting face and formation. Restore lost circulation before resuming loaded drilling.