What Feed Pressure Should I Use for Hard Rock Drilling?

Feed pressure is one of the three primary controllable parameters in rock drilling, alongside impact pressure and rotational speed. Yet it is perhaps the most misunderstood. Get it wrong, and you will pay for it—in slower penetration, accelerated bit wear, broken rods, and unnecessary downtime.

The right feed pressure keeps the bit in solid contact with the rock, ensuring that every impact from the piston transfers energy into fracturing the rock rather than dissipating as heat or vibration. The wrong feed pressure—whether too high or too low—creates a cascade of problems that can turn a routine drilling operation into a costly headache.

This article explains what feed pressure is, how it affects hard rock drilling performance, and how to determine the right setting for your specific rock conditions.

What Is Feed Pressure?

Feed pressure (also called thrust pressure or feed force) is the force that pushes the drill bit against the rock face. In hydraulic and pneumatic rock drills, this force is generated by a feed motor or cylinder and is typically measured in bar, MPa, or kN.

Feed pressure serves two essential functions:

• Maintains bit-rock contact: Keeps the bit firmly pressed against the rock so that impact energy transfers efficiently

• Controls penetration rate: Influences how fast the bit advances into the rock

Modern drill rigs are equipped with remote-controlled feed pressure adjusting systems that allow operators to adjust the feed force according to drilling conditions such as rock hardness and ground stratum. The appropriate feed force varies depending on the ease of penetration of the rock by the particular combination of drifter model and drill bit used.

Why Feed Pressure Matters in Hard Rock

Hard rock—granite, quartzite, basalt, and other formations with high uniaxial compressive strength (UCS)—presents a unique challenge. The rock is dense, abrasive, and resistant to fracture. To drill efficiently, you need sufficient feed pressure to keep the bit engaged, but not so much that you stall the bit or accelerate wear.

Research has demonstrated the critical nature of this balance. In a study at Rössing Uranium Mine, where UCS values ranged from 90 to 180 MPa for layered marble-quartzite and banded gneiss, increases in feed pressure, weight on the bit, and rotary speed beyond the optimum level led to a decrease in penetration rate and caused the drill bit to stall. The study found that penetration rate increases with an increase in feed pressure—but only up to a point. After reaching a maximum value, the penetration rate begins to decrease despite increasing feed pressure.

This relationship has been confirmed in other research. Experimental results demonstrate a negative linear correlation between penetration rate and feed pressure beyond the optimum point. In other words, more feed pressure is not always better.

The Consequences of Incorrect Feed Pressure

Feed Pressure Too Low

When feed pressure is too low, the bit loses solid contact with the rock. This creates a condition known as "air blows" or ineffective impacts—the piston strikes, but the energy is not transferred into the rock because the bit is bouncing or gapping. The result is:

• Wasted impact energy

• Slower penetration

• Increased vibration, which can damage drill string components

• Premature fatigue failure of threads and connections

Feed Pressure Too High

Excessive feed pressure forces the bit too deeply into the rock, creating a different set of problems:

• Bit stalling: The bit stops rotating because the torque required exceeds the rig's capacity

• Accelerated wear: Buttons and bit body wear faster under excessive load

• Hole deviation: The bit may wander off course

• Rod damage: Bending stress increases, risking rod fracture

• Poor cuttings removal: If air pressure is insufficient to remove cuttings at maximum bailing velocities, the bit can stall

Recommended Feed Pressure Ranges

Feed pressure recommendations vary by equipment type, rock hardness, and drilling method. The following table summarizes typical ranges from industry sources:

Important: These are general ranges only. Always consult your specific drill rig's manual for manufacturer-recommended feed pressure settings.

How to Find the Optimal Feed Pressure

Finding the right feed pressure is not a set-it-and-forget-it exercise. It requires observation, adjustment, and experience. Here is a practical approach:

Step 1: Start with Manufacturer Recommendations

Begin with the feed pressure range specified in your drill rig's operator manual. For light rock drills, a feed force of 1.4 kN is recommended on each rock drill cradle. For larger hydraulic drills, the range may be significantly higher.

Step 2: Adjust Based on Rock Hardness

• Hard rock: Slightly increase feed pressure from the baseline. The dense structure requires stronger contact to transfer impact energy effectively.

• Soft or broken ground: Reduce feed pressure. The feed rate will tend to increase under these conditions, so you may need to back off to maintain control.

Step 3: Monitor Rotation Smoothness

The most reliable indicator of correct feed pressure is smooth, stable rotation. If the rotation is jerky or stalls, feed pressure is too high. If rotation is too easy and penetration is slow, feed pressure may be too low.

Step 4: Check Temperature

After adjustments, monitor the temperature of the bit shank or coupling sleeve. This is one of the most practical ways to verify whether your settings suit the rock and flushing method:

• Water flushing: Target connection-sleeve temperature ≈ 40°C

• Air flushing: Target connection-sleeve temperature ≈ 60°C

If temperature is too high, impact pressure or feed pressure may be excessive, causing increased friction and wear. If temperature is too low, impact energy may be insufficient—revisit and increase relevant parameters.

Step 5: Adjust for Hole Orientation

Feed pressure requirements differ by hole direction:

• Vertical (downward) holes: Typically require less feed pressure because gravity assists the feed

• Horizontal or inclined holes: Require more feed pressure to overcome gravity and maintain bit contact

Feed Pressure and Bit Wear

Feed pressure directly affects button bit wear. Excessive feed pressure accelerates wear on both the bit body and the tungsten carbide buttons. In one study of WC drill bit buttons, a rotary/percussive drill was found to hit the rock approximately 50 times per second with hydraulic impact pressure of about 170–200 bar and feed pressure of about 90–100 bar, while rotating at 75–200 rpm. Operating outside these parameters—particularly with excessive feed pressure—accelerates wear and shortens bit life.

Abnormal spline wear, for example, is usually caused by a worn chuck, excessive feed force, or high RPM operation. Regular inspection of the bit for wear patterns can help identify feed pressure issues before they cause catastrophic failure.

Common Mistakes to Avoid

Final Thoughts

Feed pressure is not a "one size fits all" setting. It depends on rock hardness, hole orientation, equipment type, and flushing method. The goal is to find the sweet spot where:

• The bit maintains solid contact with the rock

• Rotation is smooth and stable

• Penetration rate is maximized without stalling

• Temperature stays within normal operating ranges

The research is clear: penetration rate increases with feed pressure up to an optimum point, then decreases. Finding that optimum point requires observation, adjustment, and a willingness to adapt as conditions change.

Start with manufacturer recommendations, adjust based on rock hardness, monitor rotation and temperature, and always err on the side of caution—a slightly lower feed pressure that keeps the bit turning is always better than a higher pressure that stalls the bit and damages your tools.

Frequently Asked Questions

Q1: What is the difference between feed pressure and impact pressure?

Feed pressure is the force that pushes the bit against the rock, keeping it in contact so that impact energy can transfer. Impact pressure is the force of the piston striking the shank adapter, which creates the stress wave that fractures the rock. Both must be coordinated for efficient drilling. Impact pressure determines breaking force and must match rock hardness, while feed pressure ensures that breaking force is actually delivered to the rock.

Read more: Important Parts And Working Principle Of Hydraulic Rock Drill

Q2: How do I know if my feed pressure is too high?

Key indicators include: rotation becomes jerky or stalls completely; penetration rate decreases despite increasing feed pressure; the bit shows signs of accelerated or abnormal wear; and the shank or coupling sleeve temperature exceeds normal operating ranges. If you observe any of these signs, reduce feed pressure gradually until operation stabilizes.

Read more: Button Bit Wear Patterns and What They Mean

Q3: Should I use the same feed pressure for vertical and horizontal holes?

No. Feed pressure requirements differ by hole orientation. Vertical (downward) holes typically require less feed pressure because gravity assists the feed. Horizontal holes require more feed pressure to overcome gravity and maintain solid bit-rock contact. Always adjust feed pressure based on hole direction.

Read more: T38 vs T45 vs T51 Drill Rod: How to Choose the Right Thread for Your Rock Drilling Application

Q4: How does rock hardness affect the feed pressure I should use?

Hard rock (e.g., granite, quartzite) generally requires slightly higher feed pressure to keep the bit in solid contact. Soft or broken ground requires less feed pressure—excessive feed forces the bit too deep into weak rock, distorting hole shape or causing stuck rods. The appropriate feed force varies depending on the ease of penetration of the rock.

Read more: What Are the Best Stone Drilling Tools for Limestone Quarries?

Q5: What is the best way to verify that my feed pressure setting is correct?

Monitor three things: rotation smoothness, penetration rate, and temperature. If rotation is smooth, penetration is optimal, and shank/sleeve temperature stays within target ranges (≈40°C for water flushing, ≈60°C for air flushing), your feed pressure is likely correct. If rotation stalls, reduce feed pressure. If penetration is slow and temperature is low, increase feed pressure slightly.

Read more: Best Practices for Button Bit Maintenance and Regrinding in Hard Rock Drilling

References

1. Adebayo, B. and Mukoya, J.G.M. "Rock properties and machine parameters evaluation at Rössing Uranium Mine for optimum drill performance." Journal of the Southern African Institute of Mining and Metallurgy, Vol. 119, No. 5, May 2019. http://www.scielo.org.za/scielo.php?pid=S2225-62532019000500009&script=sci_arttext&tlng=en

2. "Assessment of rock strength by a new drillability index in percussive-rotary drilling." Bulletin of Engineering Geology and the Environment, Vol. 84, Article 328, 2025. https://link.springer.com/article/10.1007/s10064-025-04282-3

3. Gupta, Anurag; Chattopadhyaya, Somnath; and Hloch, Sergej. "Critical Investigation of Wear Behaviour of WC Drill Bit Buttons." Rock Mechanics and Rock Engineering, Vol. 46, 2012, pp. 169-177. https://www.semanticscholar.org/paper/Critical-Investigation-of-Wear-Behaviour-of-WC-Bit-Gupta-Chattopadhyaya/39b06c62e59f2fe3acc447f89aa9d5bd3e5ef547

4. "Low rock-drilling efficiency? Start with rock properties. Match equipment parameters in 4 steps to solve rock-breaking problems." Stone Demolition. https://www.stonedemolition.com/news/low-rock-drilling-efficiency-start-with-rock-properties-match-equipment-parameters-in-4-steps-to-solve-rock-breaking-problems

5. Atlas Copco ECM 660IV Instruction Manual – Feed Pressure Adjustment (Page 223). https://www.manualslib.com/manual/4183787/Atlas-Copco-Ecm-660iv.html?page=223

6. "Spindle Type Drill Rig Operation Points And Precautions." China Xizuan. https://m.chinaxizuan.com

7. "Key Parameter Optimization Study of Composite Rod Drill in Gas Extraction Borehole Drilling in Soft, Medium, and Hard Coal Seams." OUCI. https://ouci.dntb.gov.ua