A practical guide to identifying wear, connection damage and flushing problems in small-hole rock drilling

Introduction

Small-diameter rock drill bits in the Ø26–45 mm range include R25, R28 and R32 threaded top hammer bits, as well as 4°46′, 7°, 11° and 12° tapered button, chisel and cross bits. They are commonly used with handheld and air-leg rock drills in small mines, quarries, tunnelling, secondary breaking and construction.

Bit life is not determined by product quality alone. Rock hardness and abrasivity, jointing, bit design, rod compatibility, percussion and feed settings, flushing performance and regrinding practice can all affect the final result. A reliable diagnosis therefore combines the failure location, wear pattern and operating conditions.

Quick Failure Diagnosis

1. Premature Carbide Wear

The most common signs are flattened button tops, rapid gauge-button wear, blunt chisel or cross-bit edges, and a gradual reduction in bit diameter. Continuing to drill after excessive wear reduces penetration and increases the risk of button breakage and gauge loss.

Common causes

• Highly abrasive rock;
  
• Carbide grade or button profile unsuitable for the formation;
  
• Excessive rotation speed;
  
• Insufficient flushing and poor chip evacuation;
  
• Over-drilling beyond the appropriate regrinding point.
  

Recommended action

Select spherical or ballistic buttons according to the formation and establish a consistent inspection and regrinding interval. Regrinding should restore the button profile while maintaining the gauge diameter and correct button-to-body clearance.

Figure 1. Premature wear of carbide buttons

2. Carbide Button Breakage or Chipped Cutting Edges

Button bits may develop cracked or partially fractured inserts. Chisel and cross bits more commonly show chipped cutting edges or broken carbide corners.

Common causes

• Jointed or broken formations producing uneven impact loads;
  
• Insufficient carbide toughness or an unsuitable insert profile;
  
• Excessive percussion energy or feed pressure;
  
• Bit deflection, jamming or blank firing;
  
• Continued drilling after severe carbide wear.
  

Recommended action

Determine whether the failure is limited to one insert or affects several inserts. Repeated breakage across multiple buttons within a short drilling interval requires a review of carbide selection, impact loading, operating practice and batch consistency.

3. Carbide Button Loss

Button loss means that the complete carbide insert has come out of the bit body, leaving an empty insert hole. This differs from button breakage, where part of the carbide normally remains inside the hole.

Common causes

• Poor insert-hole accuracy;
  
• Incorrect interference between the carbide insert and the hole;
  
• Insufficient bit-body strength or severe body wash;
  
• Unstable heat-treatment or button-insertion process;
  
• Abnormal impact caused by jamming or side loading.
  

Recommended action

Inspect the empty hole for enlargement, deformation and cracking. If several bits from the same batch lose buttons in a similar pattern, the insertion process and bit-body condition should be reviewed.

Figure 3. Complete carbide button loss

4. Uneven Gauge Wear and Loss of Bit Diameter

Gauge buttons maintain the hole diameter. Rapid or uneven gauge wear reduces the bit diameter and may prevent a new bit from entering an existing hole. It also increases the risk of jamming and hole deviation.

Common causes

• Hard or highly abrasive rock;
  
• Unstable bit rotation;
  
• A bent drill rod or deviated hole;
  
• Face design or gauge-button profile unsuitable for the formation;
  
• Delayed regrinding.
  

Recommended action

Measure the gauge diameter regularly and compare the height of all peripheral buttons. Regrind or replace the bit before the diameter falls below the acceptable limit.

Figure 4. Uneven gauge-button wear and reduced bit diameter

5. Bit Body Wash

Rock cuttings continuously flow across the bit face and flushing grooves. In some formations, the steel body wears faster than the carbide inserts. Excessive carbide protrusion then reduces steel support and can lead to button breakage or button loss.

Common causes

‍

• Insufficient wear resistance of the bit-body steel;
  
• High-velocity abrasive cuttings in the flushing grooves;
  
• Excessive service interval;
  
• Poor flushing that allows cuttings to repeatedly abrade the body;
  
• Bit design unsuitable for the formation.
  

Recommended action

Inspect the flushing grooves, the steel around the carbide inserts and the shoulder area. The bit should be retired when the remaining steel can no longer support the inserts securely, even if usable carbide remains.

Figure 5. Bit body wash and erosion channel

6. Cracks in the Bit Body or Skirt

Cracks commonly start at the shoulder, skirt or connection area. Continued drilling can allow the crack to propagate through the body, increasing the risk of a complete break and costly recovery from the hole.

Common causes

• Insufficient toughness after heat treatment;
  
• Misalignment between the bit and drill rod;
  
• Extended blank firing;
  
• Forced rotation or percussion after jamming;
  
• Defects in material, geometry or machining.
  

Recommended action

Stop using the bit when a visible crack is found. Record the crack origin, propagation direction and drilled metres, and check the drill rod for straightness and connection wear.

Figure 6. Cracks in the bit body and skirt

7. Premature Thread Wear or Thread Cracking

This failure mainly affects R25, R28 and R32 small-diameter threaded top hammer bits. Typical signs include thin or rounded thread flanks, chipped crests, cracks at the thread root and difficulty uncoupling the drill string.

Common causes

• Mismatched bit and rod threads;
  
• Poor thread accuracy;
  
• Insufficient thread lubrication or contamination by rock dust;
  
• Excessively worn rod threads;
  
• Percussion power above the load capacity of the connection.
  

Recommended action

Confirm the complete thread standard rather than relying on the thread name alone. Inspect both rod and bit threads regularly, and do not combine a new bit with a severely worn drill rod.

Figure 7. Premature thread wear and cracking

8. Loose Taper Fit, Taper Socket Cracking or Difficult Removal

These problems occur mainly in tapered drill bits. A loose fit reduces energy transfer and may allow the bit to detach. An excessively tight or deformed taper can make removal difficult and may crack the socket or skirt.

Common causes

• Different taper angles on the bit and drill rod;
  
• An excessively worn or deformed rod tip;
  
• Incorrect taper-socket dimensions or surface finish;
  
• Rock dust, corrosion or debris inside the socket;
  
• Loose or excessive installation and prolonged side loading.
  

Figure 8. Loose taper fit, socket cracking and difficult removal

9. Blocked Flushing Holes and Poor Chip Evacuation

Flushing holes and grooves remove rock cuttings from the borehole. Poor evacuation causes repeated crushing, reduced penetration, higher bit temperature and a greater risk of jamming.

Typical signs

• A clear reduction in penetration rate;
  
• Less cuttings return at the collar;
  
• Higher bit temperature;
  
• Repeated crushing and abnormal vibration in the hole;
  
• More frequent jamming.
  

Common causes and action

Check the flushing holes for rust and debris, and verify that air or water flow is sufficient. The flushing passages in the bit and rod must align, and the groove design must provide enough capacity for the actual chip size.

Taper Bits	Thread Bits