Deep Hole Drilling Tool Wear Analysis
Tool wear is inevitable in deep hole drilling. The question is not whether tools will wear, but how quickly and whether the wear is normal or indicates an underlying problem.
This guide covers tool wear mechanisms, measurement methods, normal vs. abnormal wear patterns, and strategies for maximizing tool life across all deep hole drilling methods.
Wear Mechanisms
Five basic wear mechanisms affect deep hole drilling tools:
1. Abrasive Wear
Hard particles in the workpiece material (carbides, oxides, inclusions) mechanically remove material from the cutting edge. This is the most common wear mechanism in deep hole drilling.
Appearance: Smooth, polished wear land on the flank. Uniform across the edge.
Primary drivers: Workpiece hardness; abrasive inclusions; coolant contamination.
2. Adhesive Wear (Built-Up Edge)
Workpiece material welds to the carbide tip at elevated temperatures and pressures. The built-up material periodically breaks off, taking carbide particles with it.
Appearance: Irregular, rough wear surface; workpiece material visible on the carbide. Typically localized to the cutting edge.
Primary drivers: Low cutting speed; high pressure; reactive workpiece materials (stainless, titanium, aluminum).
3. Diffusion Wear
At high cutting temperatures, atoms from the carbide tool diffuse into the workpiece material, weakening the tool’s cutting edge.
Appearance: Crater wear on the rake face (the surface the chip slides across). The edge may be sharp but the crater behind it weakens the tool.
Primary drivers: High cutting speed (high temperature); chemical affinity between tool and workpiece.
4. Thermal Cracking
Rapid temperature changes (coolant ON → cutting heat → coolant OFF) cause thermal expansion and contraction cycles that crack the carbide.
Appearance: Cracks perpendicular to the cutting edge (comb cracks). Often invisible without magnification.
Primary drivers: Interrupted cutting; excessive coolant flow variation; thermal shock at entry.
5. Edge Chipping / Fracture
Mechanical overload causes small or large pieces of the carbide to break off.
Appearance: Missing material at the cutting edge. Irregular, jagged break surface.
Primary drivers: Excessive feed; interrupted cuts; entry technique issues; hard inclusions.
Wear Measurement
Measuring Wear Land
The wear land (flank wear) is measured on the clearance face of the cutting edge.
| Tool Type | Measurement Tool | Method |
|---|---|---|
| Gun drill | Toolmaker’s microscope (10-20×) | Measure from cutting edge to wear boundary on flank |
| BTA insert | Toolmaker’s microscope | Measure wear land at the corner and along the flank |
| DTS insert | Same as BTA | Same as BTA |
| Guide pad | Micrometer or microscope | Measure reduction in pad OD or crown height |
Wear Land Limits
| Tool Type | Normal Wear | Regrind Needed | Replace |
|---|---|---|---|
| Gun drill (carbide tip) | < 0.10 mm | 0.25 mm | > 0.38 mm |
| BTA insert (coated) | < 0.10 mm | 0.20 mm | Index when worn |
| BTA insert (uncoated) | < 0.15 mm | 0.25 mm | Index when worn |
| DTS insert | < 0.10 mm | 0.20 mm | Index when worn |
| Guide pad (all methods) | < 0.08 mm | 0.15 mm | > 0.20 mm |
Normal vs. Abnormal Wear
Normal Wear
Characteristics:
- Uniform wear land across the cutting edge
- Gradual increase over time (linear or slightly exponential)
- Light straw coloring (not blue or burned)
- Predictable tool life
- Corresponds to expected holes per regrind
Expected tool life (normal conditions):
| Tool Type | Holes Between Regrinds | Total Regrinds | Total Service Life |
|---|---|---|---|
| Gun drill (brazed) | 300-800 | 3-5 | 1,200-4,000 holes |
| Gun drill (solid carbide) | 500-1,500 | 7-10 | 4,000-15,000 holes |
| BTA head (brazed) | 200-500 | 3-5 | 600-2,500 holes |
| BTA insert | 100-500 per edge | N/A (index) | 4-6 edges per insert |
| DTS insert | 100-500 per edge | N/A (index) | 4-6 edges per insert |
Abnormal Wear
| Wear Pattern | Likely Cause | Solution |
|---|---|---|
| Rapid wear (short tool life) | Speed too high; coolant insufficient; wrong carbide grade | Reduce speed; increase coolant; check grade |
| Crater wear | Speed too high; chemical reaction | Reduce speed; use coated grade |
| Notch wear at depth-of-cut line | Workpiece has hard surface layer (scale, oxide) | Increase feed; use tougher grade |
| Chipping | Interrupted cut; feed too high; entry impact | Reduce feed; add chamfer; check entry |
| Built-up edge | Speed too low; wrong coating | Increase speed; switch coating |
| Thermal cracking | Thermal shock; interrupted cut | Improve coolant control; preheat tool |
| One-sided wear | Misalignment; spindle runout | Check alignment and runout |
| Combination of above | Multiple conditions | Systematic diagnosis |
Tool Life Curve
Tool wear follows a characteristic three-stage curve:
Wear │
Land │
(mm) │
│
0.30 │ Stage 3: Accelerated
│ /
0.25 │ /
│ /
0.20 │ Stage 2: Steady-state
│ ───────────────
0.15 │ /
0.10 │ /
│ /
0.05 │ /
│ /
0.00 │/
└──────────────────────────
Holes Drilled
Stage 1 (Running-in, 5-10% of tool life): Wear rate is higher as the sharp edge micro-chips and stabilizes. Normal.
Stage 2 (Steady-state, 70-85% of tool life): Wear rate is constant and predictable. This is where the tool should be operating.
Stage 3 (Accelerated wear, last 10% of tool life): Wear rate increases dramatically. The tool is approaching end of life. Regrind should occur at the transition from Stage 2 to Stage 3 — not during Stage 3.
Extending Tool Life
Parameter Optimization
| Parameter | Effect on Tool Life | Rule of Thumb |
|---|---|---|
| Cutting speed | Strongest effect | 20% speed increase → 50% tool life reduction |
| Feed rate | Moderate effect | 20% feed increase → 15% tool life reduction |
| Coolant pressure | Significant effect | Pressure below minimum → rapid wear |
| Coolant temperature | Moderate effect | Every 5°C above 40°C → ~10% life reduction |
Coolant Management
- Maintain proper concentration — Emulsion coolant at 8-12% is critical for tool life. Low concentration reduces lubricity and increases wear.
- Upgrade filtration — 10 micron instead of 40 micron can double tool life in abrasive materials.
- Control temperature — Keep coolant below 45°C. Above this, EP additives degrade and the coolant loses lubricity.
Edge Preparation
- Honed or chamfered edges — A small edge hone (0.02-0.05 mm) improves edge strength and prevents micro-chipping during run-in.
- Coating — The right coating can extend tool life 2-5×. TiAlN for general steel, AlTiN for high-heat applications, DLC for aluminum.
Regrinding Best Practices
- Regrind early, regrind often — Running a dull tool to push a few more holes accelerates wear and risks breakage.
- Track regrind count — A gun drill loses 0.3-0.5 mm of carbide per regrind. Track remaining carbide length.
- Verify regrind geometry — After regrinding, check concentricity (< 0.005 mm TIR) and edge condition under magnification.
Tool Life Tracking Template
Tool ID: ___________
Type: Gun drill / BTA head / DTS head
Diameter: ___ mm
Length: ___ mm
Regrind # Date Holes Material Wear at Removal Notes
─────────────────────────────────────────────────────────────
New __/__ ____ ________ _____ mm __________
1 __/__ ____ ________ _____ mm __________
2 __/__ ____ ________ _____ mm __________
...
When to Replace Instead of Regrind
| Condition | Replace |
|---|---|
| Carbide tip too short | Brazed tip: < 3 mm remaining; solid carbide: check OD |
| Body damage | Cracked shank, scored flutes, bent shaft |
| Excessive regrind count exceeded | Brazed: > 5; solid carbide: > 10 |
| Diameter cannot hold tolerance | Multiple regrinds reduced tip below min acceptable diameter |
| Thermal damage | Blue discoloration on the carbide tip (micro-cracking) |
Summary
Tool wear analysis is essential for cost-effective deep hole drilling. Wear five basic mechanisms (abrasive, adhesive, diffusion, thermal cracking, chipping), measure wear land regularly, and regrind at the transition from steady-state to accelerated wear (typically 0.20-0.25 mm). Document tool life per tool and track regrind count. The biggest tool life lever is cutting speed — a 20% speed reduction can double tool life.
For regrinding procedures, see gun drill regrinding guide. For parameter optimization, see deep hole drilling process optimization. For a complete overview, visit the troubleshooting guide.