Gun Drill Regrinding Guide
A gun drill is a precision cutting tool, and like all cutting tools, it wears. Regular regrinding restores the cutting geometry and extends the tool’s useful life—often by a factor of 5–10× compared to running a single edge to failure.
Proper regrinding is one of the most cost-effective practices in gun drilling. A $120 gun drill that can be resharpened 8 times and lasts 500 holes per regrind delivers 4,500 holes of service, bringing the tool cost per hole to under $0.03. Neglecting regrinding, on the other hand, leads to poor hole quality, reduced tool life, and catastrophic breakage.
This guide covers when to regrind, how many times you can regrind, the procedures used, and when it makes more sense to replace.
When to Regrind a Gun Drill
Gun drills wear gradually, and the signs are visible to an experienced operator. The general rule: the best time to regrind is early—before wear affects hole quality or tool integrity.
Visual Inspection Criteria
The most reliable indicator is a wear land on the cutting edge corner. When this wear land reaches 0.25 mm (0.010") to 0.38 mm (0.015"), the tool needs regrinding.
The wear land is measured on the outer corner of the carbide tip—the point that does the most cutting and therefore wears fastest. A worn tip loses its sharp edge and develops a flat, shiny area at the corner.
| Wear Land Width | Assessment | Action |
|---|---|---|
| < 0.10 mm (0.004") | Normal running-in wear | Continue drilling |
| 0.10–0.25 mm (0.004–0.010") | Moderate wear | Monitor; plan regrind soon |
| 0.25–0.38 mm (0.010–0.015") | Regrind needed | Send for regrinding |
| > 0.38 mm (0.015") | Excessive wear | Regrind urgently; increased risk of breakage |
Performance Indicators
Even without measuring the wear land directly, several operating signals indicate a dull tool:
Increased thrust and torque. The machine’s load meters will show higher values for the same material and parameters. A worn tool requires more force to penetrate.
Hole quality changes. The most noticeable signs are:
- Diameter trending toward the high side of tolerance
- Rougher surface finish than normal
- Increased runout or drift (loss of straightness)
Irregular chip formation. A sharp gun drill produces consistent, well-formed chips. As the tool dulls, chips become irregular—frayed edges, inconsistent thickness, or a mix of long and powdery chips in the same cut.
Coolant pressure spikes. Chips that do not form cleanly can begin to pack in the V-flute, causing intermittent coolant pressure fluctuations. This is often a sign that regrinding is overdue and breakage risk is elevated.
Cumulative drilling length. As a rule of thumb, most gun drills deliver 500–1,000 linear inches (12,700–25,400 mm) of drilling between regrinds under normal conditions. Track length drilled per tool and use it as a regrind scheduling baseline, adjusted for your specific material and parameters.
How Many Regrinds Can a Gun Drill Take?
The number of regrinds depends on the gun drill type and the amount of material removed per regrind.
| Gun Drill Type | Typical Regrinds Possible | Total Tool Life Before Replacement |
|---|---|---|
| Brazed carbide tip | 3–5 regrinds | 4–6× the life of the original grind |
| Solid carbide | 7–10 regrinds | 8–11× the original grind life |
| Indexable insert | N/A (index inserts instead) | Replace insert when worn |
Important: A properly reground gun drill performs at 80–100% of original tool capability—the first regrind is nearly as good as new. Performance degrades gradually with each subsequent regrind as the carbide tip shortens and the tool’s back taper increases.
What Limits Regrind Count
Three physical factors limit how many regrinds a gun drill can accept:
Carbide tip length. Each regrind removes 0.3–0.5 mm of carbide from the tip face. A brazed tip is typically 6–10 mm long, giving 3–5 regrinds before the carbide is too short to anchor.
Back taper. The drill body has a slight taper (typically 0.002–0.003 mm per 100 mm of length). Each regrind reduces the effective diameter at the corner. Eventually, the drill can no longer produce a hole within the required diameter tolerance.
Shank and flute condition. After multiple regrinds, the steel shank and V-shaped flute may accumulate scoring, wear, or galling that affects chip evacuation. Deep flute damage cannot be repaired by regrinding the tip.
Coated Gun Drills
Coated gun drills (AlTiN, TiAlN, diamond-like carbon) can still be reground multiple times. The coating is removed from the cutting face during regrinding but remains on the guide pads and flank areas, maintaining the benefits of reduced friction and wear resistance where they matter most. Coated drills typically achieve the same number of regrinds as uncoated drills of the same type.
The Regrinding Procedure
Modern gun drill regrinding is performed on specialized grinding machines with optical measurement systems. The process takes 5–10 minutes per tool on modern equipment.
Preparation
The gun drill is thoroughly cleaned to remove cutting oil and debris, then inspected under magnification to assess the wear pattern. The regrind fixture is set up based on the drill’s original geometry specification.
Step-by-Step Regrind Sequence
1. Primary angle grind. The tip face is ground at the tool’s specified primary angle (typically +30° horizontal, +15° vertical, with +5° rotation). Material is removed in light passes (0.05 mm per pass) to avoid heat damage to the carbide.
2. Inner relief facet. The fixture is indexed to create the secondary relief angle (typically −20° horizontal), forming a clearance facet behind the cutting edge. The point is positioned at approximately D/4 (one-quarter of the drill diameter) from the center.
3. Front clearance grind. Clearance behind the cutting edge is restored. The front clearance angle depends on the material being drilled—for example, N-8 geometry for steel, N-4 for aluminum.
4. Outer corner radius. The outer corner is radiused or chamfered as specified by the tool design. This edge is critical for finish quality and must be precisely controlled.
5. Oil dub-off (flute clearance). The V-flute edge behind the cutting tip is relieved to ensure smooth chip flow. The fixture is set at approximately −30° horizontal with +25° vertical rotation.
Quality Check
After grinding, the tool is inspected under a toolmaker’s microscope or camera system:
- Cutting edge condition (no chips, cracks, or burrs)
- Primary and secondary angles within specification (±0.5°)
- Corner radius and position
- Tip concentricity with the drill shank
Tools that fail inspection are either re-ground or scrapped.
Regrinding Cost vs. Replacement Cost
| Gun Drill Size | Replacement Cost | Regrind Cost | Regrinds Possible | Cost Savings per Tool Lifetime |
|---|---|---|---|---|
| Ø3 mm (0.125") | $50–80 | $15–25 | 5–8 | $250–500 |
| Ø6 mm (0.250") | $80–120 | $20–30 | 5–8 | $350–550 |
| Ø12 mm (0.500") | $100–180 | $20–35 | 5–8 | $450–700 |
| Ø25 mm (1.000") | $150–300 | $25–40 | 4–6 | $500–1,000 |
Regrinding a gun drill typically costs 15–25% of the replacement price, making it one of the highest-ROI maintenance practices in precision machining.
When to Replace Instead of Regrind
Do not regrind a gun drill when:
- The drill has already reached its maximum regrind count. After 5× regrinds for brazed tips or 10× for solid carbide, the tip carbide or diameter tolerance is likely exhausted.
- There is visible body damage. Cracks in the shank, deep flute scoring, or bent shanks cannot be corrected by tip regrinding. Continued use risks catastrophic breakage down-hole.
- The tool has been thermally damaged. Blue discoloration on the carbide tip indicates overheating, which can cause micro-cracking. Grinding away the damaged material may not fully restore integrity.
- There is extensive edge chipping. If the cutting edge has large chips (over 0.5 mm) that would require excessive material removal to clear, replacement is more economical.
- The drill can no longer hold diameter tolerance. After multiple regrinds, the back taper reduces the effective diameter below the minimum acceptable size. At this point the tool has reached the end of its useful life.
In-House vs. OEM Regrinding
| Factor | In-House Regrinding | OEM or Regrind Service |
|---|---|---|
| Equipment cost | High (CNC tool grinder: $30,000–100,000+) | None (paid per regrind) |
| Turnaround | Same day | 1–3 days plus shipping |
| Quality control | Dependent on operator skill | Consistent, specification-controlled |
| Best for | High-volume production, common geometries | Low volume, complex geometries, coated drills |
| Geometry restoration | Good for standard shapes | Precise to OEM print for complex grinds |
Recommendation: For shops running fewer than 10 gun drilling jobs per day, OEM or specialty regrind services are more cost-effective. High-volume producers should consider in-house capability for faster turnaround and lower per-regrind cost at scale.
Best Practices for Maximizing Tool Life
- Regrind early, regrind often. Running a dull tool to maximize time between regrinds is false economy—it degrades hole quality and risks breakage.
- Track regrind count per tool. Mark each regrind on the tool shank (or track in your tool management system) so you know when it’s time to replace.
- Use proper coolant filtration. Contaminated coolant causes abrasive wear that accelerates edge breakdown. Filtration to 10–20 microns is recommended.
- Inspect every reground tool. Never assume a regrind is correct—confirmation takes two minutes under a microscope and prevents a scrapped part.
- Match regrind geometry to material. A regrind that restores general-purpose geometry may not be optimal for the specific material you’re drilling. Specify the nose grind type (N-8, N-4, facet, etc.) when sending tools for service.
Summary
Gun drill regrinding is a high-ROI practice that extends tool life 4–10× and maintains consistent hole quality. Regrind at the first sign of wear (0.25 mm wear land), track regrind count per tool, and replace when the tool reaches its maximum regrinds or shows body damage. For most shops, a specialty regrind service offers the best balance of cost and quality.
For troubleshooting gun drilling issues related to tool wear, see our common gun drilling problems and solutions guide. For breakage prevention, see how to prevent gun drill breakage. For a complete overview, visit the gun drilling guide.