In diamond drilling, bit load (weight on bit), rotational speed (RPM), and penetration rate (ROP) form an interdependent system. Increasing one parameter without considering the others often leads to accelerated diamond wear, reduced footage, or sudden bit failure.

Decades of laboratory testing and field experience, summarized in classic references such as the Diamond Drill Handbook[1], show that drilling efficiency is not achieved by force alone, but by balance. This article translates those proven principles into practical guidance for modern diamond drilling operations.

1. Understanding Bit Load: More Pressure Is Not Always Better

Critical Pressure Explained

Each diamond cutting point requires a minimum effective load, commonly referred to as critical pressure, to penetrate rock.

· Below critical pressure: diamonds slide and polish the rock surface without cutting

· Above critical pressure: effective cutting begins

Operating below this threshold causes rapid diamond polishing, rendering the bit ineffective even though the diamonds may appear undamaged.

Total Bit Load vs. Load per Diamond

A common misconception is that increasing total bit load always increases cutting force. In reality, unit load per diamond is what matters.

For example:

· 120 diamonds with a total load of 900 lbs → ~7.5 lbs per diamond

· 250 diamonds with a total load of 1250 lbs → ~5 lbs per diamond

This demonstrates why diamond count, size, and distribution are as important as total applied load. Excessive load on a densely set bit may actually reduce cutting efficiency.

2. How Bit Load and RPM Work Together

Load Establishes Cutting, Speed Controls Productivity

Field experience consistently shows that:

· At a fixed RPM, penetration increases with load up to an optimum level

· Beyond that point, increasing load does not improve penetration

· Further gains in ROP can only be achieved by increasing rotational speed

In simple terms:

Load enables cutting; speed determines how fast cutting occurs.

Cuttings as a Diagnostic Tool

Cuttings provide immediate feedback:

· Coarser cuttings generally indicate effective penetration

· Very fine cuttings may signal polishing, insufficient load, or excessive RPM

Experienced drillers often adjust parameters based on cuttings before visible bit damage occurs.

3.Cooling and Flushing: The Hidden Performance Drivers

The Role of Coolant Beyond Temperature Control

Water or drilling fluid serves multiple purposes:

· Cooling diamonds and matrix

· Transporting cuttings away from the bit face

· Preventing regrinding of cuttings

When flushing capacity is insufficient, the bit begins to grind its own cuttings, leading to accelerated diamond and matrix wear—even under “reasonable” load and speed.

Overloading and Heat Generation

Excessive load generates heat in two ways:

1. Increased friction at the diamond–rock interface

2. Larger cuttings that exceed flushing capacity

The result is often premature bit failure without obvious visual damage, misleading operators into continuing with incorrect parameters.

4.Bit Performance in Different Rock Types

Very Hard, Fine-Grained Siliceous Rock

· Diamonds may polish after only a few feet

· Penetration may stop entirely despite intact diamonds

· A slight change in formation can restore cutting ability

This explains why a bit that appears “dead” in one zone may perform well in another.

Highly Abrasive Formations

· Matrix wears rapidly, exposing diamonds

· When diamonds protrude more than ~1/3 of their size, retention becomes unstable

· Continued drilling risks diamond loss

In such cases, the bit should be withdrawn and reset before further use.

Deep Holes and Large Diameters

For holes larger than NX and at significant depths, many bits fail shortly after drilling resumes. Common causes include:

· Air trapped in drill rods

· Temporary loss of coolant circulation

· Brief dry running at startup

Best practice is to:

Lift off bottom → circulate fluid fully → confirm stable return → restart with light load.

 5.Rotational Speed and Diamond Wear

Classic testing shows:

· Diamond wear decreases as RPM increases up to ~700 RPM

· Between 700 and 2000 RPM, wear remains relatively constant

However, these figures:

· Do not apply to oil well bits

· Differ from underground or blast-hole drilling, where RPMs may exceed 3000

Speed alone is not the problem,mismatch between speed, load, and flushing is.

6.Drilling Is a System, Not a Single Setting

Bit load cannot be considered in isolation. Rock removal results from the combined vertical load and horizontal cutting force generated by rotation. Changes in speed alter force direction and magnitude, affecting both penetration and diamond retention.

Diamonds can withstand extremely high loads when properly supported, which often leads to overloading. Because damage is not always immediately visible, improper parameters may go unnoticed until performance collapses.

→ For more information about ROCKCODE’s Products, please visit: https://www.rockcodebit.com/drill-bits-products 

→ Email us at: info@rockcodebit.com

→ Information in this article is for general reference only. For specific drilling projects and drilling bits, please consult qualified professionals. Thank you.

Source

【1】Cumming, J. D. (1956). Diamond drill handbook. (2nd ed.). Smit.

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