Appearance
The regrind decision determines whether a deep hole drilling tool delivers consistent quality for another production run or fails catastrophically on the next hole. Measuring the right wear parameter and acting at the right threshold separates controlled tool management from guesswork.
Tool Wear Types in Deep Hole Drilling
Five distinct wear patterns affect gun drills and BTA tools. Each has a different wear mechanism, different measurement method, and different replacement threshold.
Flank Wear (VB)
Flank wear occurs on the relief face of the cutting edges due to abrasive and adhesive wear against the workpiece. It is the primary wear mode that determines tool life.
| Drill Diameter | Max VB (Area 1) | Max VB (Area 2) | Max VB (Area 3) |
|---|---|---|---|
| 3.00 – 6.00 mm | 0.20 mm | 0.20 mm | 0.20 mm |
| 6.01 – 10.00 mm | 0.25 mm | 0.20 mm | 0.20 mm |
| 10.01 – 14.00 mm | 0.25 mm | 0.25 mm | 0.25 mm |
| 14.01 – 17.00 mm | 0.30 mm | 0.25 mm | 0.25 mm |
| 17.01 – 20.00 mm | 0.35 mm | 0.30 mm | 0.30 mm |
The three area designations correspond to different zones of the cutting edge: Area 1 is the outer diameter region, Area 2 is the mid-radius, and Area 3 is near the centre. The outer edge typically wears fastest because it has the highest cutting speed.
Crater Wear (KB)
Crater wear forms on the rake face from chip friction and thermal effects. It weakens the cutting edge and changes the effective geometry.
| Drill Diameter | Max KB (All Areas) |
|---|---|
| 3.00 – 6.00 mm | 0.20 mm |
| 6.01 – 10.00 mm | 0.25 mm |
| 10.01 – 14.00 mm | 0.30 mm |
| 14.01 – 20.00 mm | 0.30–0.35 mm |
Guide Pad Wear
Guide pads wear from the continuous burnishing contact with the bore wall. The wear mechanism is primarily adhesive, with carbide particles transferring to the workpiece and vice versa.
ISCAR's guide pad management rule: reverse the pad when the first corner wears to approximately 70% of its width. Replace the pad when the second corner is worn. For high-accuracy applications, L/D > 50:1, or when drilling into tailstock holes, replace the top guide pad at shorter intervals.
Margin / Circular Land Wear
The cylindrical margin of the drill wears from continuous contact with the bore wall. This wear affects hole diameter and surface finish but is typically slower than flank or crater wear. Measured at the outer corner, the margin wear limit follows the same VB values as flank wear.
Chisel Edge Wear
Chisel edge wear occurs at the centre of the drill where cutting speed approaches zero and the material is extruded rather than cut. It increases thrust force and can cause workpiece deformation.
Three-Zone Wear Classification
Industry practice classifies tool wear into three condition zones:
| Zone | VBmax Range | Status | Action |
|---|---|---|---|
| Green | 0 – 0.15 mm | Normal wear | Continue production |
| Yellow | 0.15 – 0.30 mm | Monitor | Plan regrind, inspect every Nth hole |
| Red | > 0.30 mm | Replace | Regrind or replace immediately |
The yellow zone is where most regrinding decisions are made. Planning regrinds while in the yellow zone prevents unplanned downtime from wear-related failures.
Practical Indicators for Regrinding
Beyond direct wear measurement, these production indicators signal that the tool needs regrinding:
- Surface roughness increasing: Ra rises by more than 50% from baseline for the same material and parameters
- Hole diameter drifting: Progressive undersize (gun drilling) or oversize (BTA) indicates outer corner wear
- Torque or feed force increasing: A sustained rise of 15–20% above baseline signals advanced wear
- Chip colour changing: Blue or burned chips indicate excessive heat from a dull cutting edge
- Chip shape changing: Chips become thicker, more irregular, or start stringing
- Chatter or vibration appearing: Worn geometry loses the stable cutting edge condition
Research by Sihvo and Varis (2010) demonstrated that feed force signal patterns can estimate mean flank wear with sufficient accuracy for production monitoring, enabling condition-based rather than interval-based regrinding.
Regrinding Guidelines
The critical rule of regrinding: the entire flank wear zone must be completely removed. Partial regrinding that leaves residual wear in the wear zone accelerates subsequent wear progression because the remaining wear zone concentrates stress.
Regrinding should restore:
- Original point geometry (apex offset, inner and outer angles)
- Clearance angles on all cutting edges
- Coolant hole opening geometry (changes affect hydraulic resistance)
- Guide pad clearance (for BTA tools)
Each regrind removes approximately 0.3–0.5 mm from the tool length. A gun drill can typically be reground 15–25 times before the tool length is exhausted. BTA heads with indexable inserts are replaced rather than reground.
Tool Life Monitoring Methods
| Method | Measurement | Accuracy | Implementation |
|---|---|---|---|
| Direct optical | VB measurement under microscope | High | Offline, requires tool removal |
| Feed force monitoring | Force signal pattern analysis | Medium | In-process, sensor integration |
| Vibration/acoustic emission | High-frequency signal analysis | Medium | In-process, sensor integration |
| Surface finish trending | Ra measurement on produced holes | Low-medium | Post-process, statistical |
| Torque monitoring | Spindle load signal | Low | In-process, machine-integrated |
AI-based tool condition monitoring systems combining force signals and acoustic emission can predict VBmax with R² = 0.83 and estimate remaining useful life within 15–20% accuracy.
FAQ
When should a gun drill be reground?
When flank wear (VB) reaches 0.20–0.35 mm depending on drill diameter, or when crater wear (KB) reaches 0.20–0.35 mm. Smaller diameter tools have tighter wear limits.
What is flank wear VB?
Flank wear (VB) is the width of wear land on the relief face of the cutting edge, measured in millimetres. It is the standard parameter for determining tool life in drilling.
How many times can a gun drill be reground?
Typically 15–25 regrinds per tool, depending on the original tool length and the amount of material removed per regrind (0.3–0.5 mm).
When should BTA guide pads be reversed?
Reverse the guide pad when the first corner wears to approximately 70% of its width. Replace the pad when the second corner is worn.
What is the difference between flank wear and crater wear?
Flank wear occurs on the relief face (clearance side) from abrasion against the workpiece. Crater wear occurs on the rake face from chip friction and heat. Both must be measured independently.
How do I know a tool is worn without measuring VB?
Production indicators: surface roughness increases >50%, hole diameter drifts, torque rises 15–20% above baseline, chip colour changes to blue, or chatter appears.
What is the green-yellow-red wear classification?
Green (VBmax < 0.15 mm) = normal operation. Yellow (0.15–0.30 mm) = monitor and plan regrind. Red (> 0.30 mm) = regrind immediately.
Does tool wear affect hole quality progressively?
Yes. Wear degrades surface finish first, then diameter tolerance, then straightness. The first measurable change is usually an increase in Ra.
Can feed force monitoring detect tool wear?
Yes. Research shows feed force signal patterns correlate with mean flank wear and can be used for real-time wear estimation without removing the tool.
What happens if I regrind without removing all the wear?
Partial regrinding that leaves residual wear concentrates stress in the remaining wear zone, accelerating subsequent wear progression and reducing tool life.
Conclusion
Tool wear management in deep hole drilling is a measurement-driven discipline. Flank wear (VB) of 0.20–0.35 mm and crater wear (KB) of 0.20–0.35 mm are the standard regrind thresholds, with smaller diameters at the lower end of the range. The three-zone classification (green-yellow-red) provides a practical framework for production decision-making. Guide pads follow a two-corner lifecycle per ISCAR guidelines. Modern monitoring methods — feed force signal analysis, acoustic emission, and AI-based condition monitoring — enable condition-based maintenance that maximises tool utilisation while preventing catastrophic failure.