Rock Drilling Technical Information
There are many Factors & Variables when it comes to Drilling that contribute to the ‘drillability’ of different rock formations.
Rock UCS hardness (Unconfined Compressive Strength) is only one factor that contributes to establishing the optimum operating parameters for each Bit selection. They can include:
- Fracture toughness
- Shear strength
- Young’s modulus of elasticity
- Poisson’s ratio of stress vs strain
- Internal angle of friction
These rock variables must be considered when attempting to determine the best bit for overall life at optimum penetration rates.
Weight On Bit (WOB) and Rotation are the key to achieving that optimum performance.
WOB (pull down) is required to adequately push teeth into the rock to break in efficiently. It is the primary factor in rate of penetration. The UCS of the rock determines how much pull down is to be applied. High UCS rocks may need time for the indenter to break it and Lower RPM is often required for efficient rock breakage.
Rotation is required to move the cutting teeth to the next rock cutting position. The faster you move the teeth to the next position, the faster you will drill. If the rock resists indentation by the teeth, there will be minimal rock breakage, and rate of penetration will not increase proportionally with higher rotation. For softer rock high RPM is required as Soft rock responds to frequency and for harder rock low RPM is required as Hard rock responds to time.
Other equally important factors affecting drilling performance are specific to the drilling equipment and operator. Notable items include:
- Driller experience and training
- Compressor size and condition
- Mechanical condition of the drill
- Pipe size and straightness
- Drill sub / stabilizer size and condition
- Deck bushing size and condition
- Surface condition and overburden
- Sub surface blast fracturing
- Angle drilling
- Revolutions per minutes utilized
- Weight applied to bit
- Hhole cleaning
- Water applied for dust control
The amount of internal air pressure is regulated by nozzle size.
- Increase the nozzle size to reduce pressure inside the bit
- Decrease the nozzle size to increase pressure
- Nozzles sizes are determined per rig
Consult with your Tricon representative or bit manufacturer prior to changing nozzle sizes. Tricon will organize an air test to confirm the correct nozzle size for your specific application.
Nozzle the bit to achieve pressures of approximately 35-40 psi (2.4-2.8 bar) in the drill bit.
Note that cab pressure gauges can read significantly higher than this value. As a rule-of-thumb: the difference between cab and at the bit is 8-10 psi (0.5-0.7 bar).
Air / Bailing Requirements
In rotary blasthole drilling, delivery of air in sufficient volume and at proper pressure is very essential to assure optimum bit performance.
For the most effective hole cleaning a minimum bailing velocity of 5,000-7,000 feet per minute is considered desirable for dry cuttings; and 7,000-10,000 ft./min. for wet or more dense cuttings. Right amount of air ensures efficient cuttings removal from the hole bottom to the surface and it also reduces the cutting structure wear and bearing erosive wear by means of efficient bottom hole cleaning.
This volume is calculated for the most common bit and pipe sizes. For dense materials, this volume should be increased in order to yield optimum cleaning efficiency.
The simple formula Q=AV can be applied to determine the volume flow of air (Q) necessary to give the desired velocity (V). (A) is the area of the hole through which the cuttings-laden air flows. This area is the area of the hole less the area filled by the drill pipe.
BV = 183.3 x ACFM / D2 _ d2
BV = Bailing velocity (feet per minute)
ACFM = Actual cubic feet of free air per minute delivered
183.3 = Factor for conversion
D = Diameter of the hole being drilled (inches)
d = Outside diameter of the drill pipe being utilized (inches)
Helpful Drilling Tips
Galling On Threads
Q: There are galling on bit threads, how do I prevent that?
A: Use quality thread grease and ensure that threads are coated properly.
Q: The bit threads are often cross-threaded, What can I do?
A: When making up the rotary bit, use slow rotation to ensure that it mates shoulder to shoulder properly with the drill string. Monitor the process carefully to avoid cross-threading.
Q: What is the recommended air pressure in a rotary bit?
A: The minimum recommended air pressure that should be maintained in the bit is 35 – 40 PSI (2.4 – 2.8 bar). Note that the Drill Cab pressure gauges can read significantly higher than this value, generally 8 – 10 PSI (0.5 – 0.7 bar) higher.
Q: The air pressure suddenly increased/decreased, what does it mean?
A: A sudden increase in air pressure may indicate unwanted restrictions in the air system. A sudden decrease in air pressure may indicate leakage in the air system.
Rotary Bit Air Supply
Q: Why does a rotary bit require adequate air?
A: Adequate air supply would ensure trouble-free bearing performance and reduced abrasion wear on the bit cones and shirttails.
Bearing Related Problems
Q: How can I avoid bearing-related problems?
A: Ensure that the bits are maintained by observing the best practice to break-in a new bit, maintenance, and storage.
Breaking In Rotary Bits
Q: How should new rotary bits be broken-in?
A: New bits should be broken-in at reduced weight and rotation to ensure that all working surfaces are run-in and work-hardened prior to applying full operating load – following this practice would maximize bit life. Generally, 50% of normal weight and rotation for the first 10 minutes, then 75% of normal weight and rotation for the next 10 minutes prior to drilling as normal.
Prolong Life of Rotary bit
Q: What are the best practices to prolong the life of a rotary bit?
A: There are numerous practices to be observed but here are some that are commonly missed:
- Periodically check the bit cones;
- Use minimum amount of water to reduce dusting;
- Ensure that air supply is kept on while drilling and until 1 – 2 metres before the rotary bit is pulled out of the hole;
- Blow the hole clean (for approximately 60s) if the drill bit is to stay in the whole while adding drill string;
- Keep the bit rotating when moving in or out of a hole;
- Use the highest possible air-pressure and flow rate in holes that contain a lot of water to avoid bearing problems.
Storing Rotary Bit
Q: How should the rotary bits be stored when not in use?
A: Open Air Bearing bits should be stored away from dust. In the event of extended storage during hole size change or extended drill servicing, these bits should be stored in a sealed container with enough diesel to cover the bearings. Sealed bearing bits should be cleaned and dried with air prior to storage. They should be stored in a sealed container and should NOT be soaked in diesel or other fluids.
Rotary Bit Air Pressure
Q: How can I achieve the recommended air pressure in a rotary bit?
A: This is achieved by choosing the correct nozzle size. Incorrect nozzle size would not only affect the air pressure inside the bit but also the bailing velocity.
Q: What is bailing velocity?
A: Bailing velocity is the velocity of the air that cleans a hole. Generally, the higher the bailing velocity, the better the hole is cleaned. This, in turn, would increase the penetration rate as well as the bit life.
Q: How can I tell if a hole was properly cleaned?
A: If a hole was properly cleaned, there should be no increase in torque (no increase in hydraulic pressure), no increase in air pressure, and there should not be heavy wear or damage on shirttails. If any of the opposites occurred, it may indicate that the hole was not properly cleaned.
Recommendations for Bailing Velocity
Q: What is the recommended bailing velocity?
A: For light to medium materials (typically coal overburdens): 6,000 –7,000 feet per minute (fpm) or 30 – 36 m/s. For denser materials (hard rock mines, iron ore, etc.) or when encountering groundwater: 7,000 – 10,000 fpm or 36 – 50 m/s.
Bit Cone Heating
Q: One of the bit cones is hotter than the others, what does that mean?
A: Uneven cone temperature indicates that the air passage to that bearing is obstructed. The bit should be cleaned with water prior to resuming drilling.