Troubleshooting for Rock Drilling Tools

There is one simple fact about carbide tools — they eventually fail during use, no matter how high their quality.  The extreme cutting forces and high temperatures generated by a drilling operation take a tremendous toll on the insert. The key to troubleshooting is to recognize the type of failure so appropriate corrective measures may be taken.

Wear

Wear is a natural occurrence and is not usually an indication that the insert has failed. Wear results in the flattening of the insert caused by the erosion of material due to the cutting action. In situations where
premature wear is a problem, it is recommended that a harder, more wear-resistant grade is used, which will typically have a lower cobalt.

Thermal Shock

Failure from thermal shock is identified by distinctive bi-axial cracks intersecting at an approximate 90° angle. The cracks may vary from a few to several hundred on each insert. The cause of thermal shock is the rapid cooling of the insert due to water or other fluids. The incidence of thermal cracking may be reduced by using carbide with a lower coefficient of thermal expansion, but changes to bit design and drilling methods are more likely to improve the situation than changing grades of carbide.

Flaw Breakage

This problem usually becomes apparent early in the drilling operation. Failure is caused by an imperfection in the carbide which often results in fractures. The fracture will exhibit both a smooth portion and a separate
rough portion. The smooth portion will exhibit little or no evidence of cracks, while the rough portion will have a sunburst pattern of fracture lines. Inserts failing in this manner should be returned for analysis and
replacement.

Surface Cracks

This is probably the most common cause of insert failure but is often a function of normal wear and tear. Cracks originate in the area of highest compressive loading and rock crushing. Cracks manifest themselves in many forms and when the crack reaches a critical size, the insert will suffer catastrophic failure. The propensity to cracking may be reduced by selecting an insert manufactured from a tougher grade of carbide.

Spalling

Spalling is the flaking of small particles of carbide from the insert. This condition often results from excessive cold working that leads to cracking, which spreads in carbide that is insufficiently tough. The
correction is to use a carbide insert with higher fracture toughness and a higher percentage of cobalt.

Sub-Surface Breakage

This is characterized by a clean, unmarked fracture of the insert, perpendicular to the axis of the insert. The fracture usually occurs at or below the junction of the carbide with the drill head. This failure is associated with the drilling of out of round holes. This may be caused by an insert that is off center due to either a fault in its geometry or incorrect fitting. Harder grades of carbide are more susceptible to sub-surface breakage. Remedies include inspection of inserts to ensure they are ground concentrically and ensuring that the insert holes are round and without taper.