Tool Failure Analysis in Deep Hole Drilling

A broken gun drill does not give a warning. One second it is cutting, the next it is a fragment embedded in the hole. The cost is not just the tool — it is the scrapped part, the hours of extraction or rework, and the lost production while the root cause is investigated. In deep hole drilling, the tool is the most stressed component in the system. It operates at the limit of its material properties while hidden from view, making failure analysis a forensic discipline. Every chip, every wear mark, every fracture surface tells a story — the analyst's job is to read it before the same failure repeats on the next part.

Tool Failure Modes Overview ​

Classification ​

Relative Frequency ​

Normal Wear Modes ​

Flank Wear ​

Flank wear is the progressive loss of tool material from the flank (relief) face of the cutting edge. It is the expected failure mode for a tool reaching the end of its useful life.

Normal flank wear progression in gun drilling:

Primary causes of accelerated flank wear:

Crater Wear ​

Crater wear forms on the rake face of the cutting edge, behind the cutting edge line, from diffusion of tool material into the chip.

Causes and corrections:

Guide Pad Wear ​

Guide pad wear is unique to deep hole drilling — the guide pads slide against the bore wall under high pressure, producing a characteristic wear pattern.

Abnormal Wear Modes ​

Chipping ​

Chipping is the loss of small fragments from the cutting edge — not yet catastrophic but a precursor to rapid failure.

Built-Up Edge (BUE) ​

Built-up edge is workpiece material that welds to the cutting edge, altering the effective geometry.

Causes: Low cutting speed, insufficient coolant lubricity, chemical affinity between tool and workpiece, sharp cutting edge (too sharp).

Corrections:

• Increase cutting speed — BUE decreases above 25 m/min for most materials
• Use coated tool (TiAlN reduces adhesion)
• Increase coolant EP additives
• Use polished rake face

Thermal Cracking ​

Thermal cracks form perpendicular to the cutting edge from cyclic thermal expansion and contraction.

Thermal cracking is most common in:

• Interrupted cutting (cross-holes)
• Dry-wet cycling (coolant applied intermittently)
• High-speed machining of superalloys

Catastrophic Failure Modes ​

Torsional Fracture ​

The gun drill shank or BTA drill tube twists apart from excessive torque:

Torsional fracture sequence:

1. Chip packs in the flute or inner tube
2. Torque increases rapidly (2–5× normal)
3. Drill tube twists at the point of highest stress (typically at the bushing or collet)
4. Fracture occurs — tool separates into two pieces

Tip Separation ​

The carbide tip separates from the steel shank at the brazed joint:

Causes of braze failure:

Bending Fatigue ​

Bending fatigue occurs in the drill shank from cyclic lateral loading:

Causes:

• Excessive drill overhang
• Missing or worn steady rest
• Misaligned guide bushing
• Chatter vibration during cutting
• Overspeed (critical speed excitation)

BTA Drill Tube Fracture ​

BTA drill tube fractures typically occur at threaded connections:

Troubleshooting Guides ​

By Symptom — Gun Drilling ​

By Symptom — BTA Drilling ​

By Wear Pattern ​

Failure Analysis Protocol ​

Step 1: Secure the Evidence ​

When a tool fails catastrophically:

Step 2: Visual Examination ​

Step 3: Measurement ​

Step 4: Process Data Correlation ​

Step 5: Root Cause Determination ​

Step 6: Corrective Action and Verification ​

Preventing Recurrence ​

Tool Life Management ​

Tool Exchange Protocol ​

FAQ ​

Q: What is the most common cause of gun drill breakage? Chip packing is the most common cause, accounting for approximately 60% of catastrophic gun drill failures. Chips lodge in the flute, torque spikes, and the shank fractures torsionally.

Q: How can I tell if a gun drill failed from chip packing vs. a material defect? A torsional fracture with a 45° spiral pattern and chip debris in the flute indicates chip packing. A clean, flat fracture with no chip debris suggests a material defect or overload.

Q: What does uneven guide pad wear indicate? Uneven guide pad wear — where one pad shows significantly more wear than the other — indicates misalignment between the guide bushing and spindle centreline. The tool is being forced sideways.

Q: How is flank wear measured on a gun drill? Flank wear (VB) is measured as the width of the wear band on the flank face behind the cutting edge, using a metallurgical microscope at 50–100× magnification.

Q: What causes built-up edge in deep hole drilling? BUE forms when workpiece material welds to the cutting edge at low cutting speeds (typically below 25 m/min). Increasing speed, using coated tools, and improving coolant lubricity eliminate BUE.

Q: What is the difference between chipping and edge fracture? Chipping is the loss of small fragments (< 0.1 mm) from the cutting edge. Edge fracture is the loss of a larger section (> 0.1 mm). Chipping is progressive; edge fracture is typically a single overload event.

Q: How often should guide bushings be replaced? Guide bushing life depends on production volume. A rule of thumb: replace the bushing every 50th tool change or when hole diameter at entry exceeds the upper tolerance limit.

Q: Can a broken gun drill be removed from the workpiece? Removal is possible but difficult. Methods include EDM (for conductive materials), drilling around the broken tool, or dissolving with acid (tungsten carbide dissolves in hydrogen peroxide + ammonia). Prevention is far more cost-effective than removal.

Q: What information does a fracture surface reveal? The fracture surface reveals the failure mode: torsional (45° spiral), bending (beach marks, ratchet marks), tensile (flat, perpendicular), or fatigue (smooth region + rough final fracture). Each pattern points to a different root cause.

Q: How do I set up a tool failure reporting system? Record for each failure: tool ID, hole number at failure, material, parameters, fracture appearance, coolant pressure and spindle load trends, and corrective action. Analyse the data monthly to identify the most common failure modes and address the top three.