Gun Drill Sharpening: Angles, Equipment, and Quality Control

A gun drill is the only cutting tool where a regrind error of 0.1 mm at the cutting edge produces a hole that is 0.5 mm off-center at 500 mm depth. The geometry of the cutting head is not just about cutting — it is about the delicate balance of forces that keeps the drill tracking straight.

Overview ​

The gun drill cutting head combines multiple functions in a small space: cutting the workpiece material, guiding the tool through the bore, and directing high-pressure coolant to the cutting zone. The ground faces on the cutting head determine how well each function is performed. Incorrect sharpening angles cause poor surface finish, oversize or undersize holes, rapid tool wear, and drill breakage.

Gun Drill Cutting Geometry ​

The gun drill cutting head has a single cutting edge divided into an outer (peripheral) section and an inner (center) section. The head typically has five or six ground faces:

Critical Angles ​

The Point Configuration ​

The gun drill point has two critical dimensions:

The point offset creates the unbalanced cutting force that keeps the guide pads in contact with the bore wall. An offset that is too small reduces pad contact pressure, causing the drill to wander. An offset that is too large increases the bending moment on the drill, causing oversize holes and rapid guide pad wear.

Land width by diameter:

Sharpening Sequence ​

The standard gun drill sharpening sequence consists of six steps, performed in order:

Step 1: Primary Relief (Outer Cutting Edge) ​

The primary relief forms the main cutting edge at the outer diameter. The land width (Y) is the most critical dimension in this step — too narrow and the edge will chip; too wide and the drill will rub.

Step 2: Inner Relief (Center Cutting Edge) ​

The inner relief forms the cutting edge from the center of the drill outward to the point. The intersection of the inner and primary relief faces defines the point tip and its offset from center.

Step 3: Secondary Relief ​

The secondary relief provides additional clearance behind the primary relief. It reduces friction between the cutting head and the bore surface.

Step 4: Front Clearance ​

The front clearance prevents the forward face of the drill from rubbing against the bottom of the hole.

Step 5: Oil Clearance ​

The oil clearance provides a channel for coolant to flow between the inner and outer cutting edges.

Step 6: Chamfer (Peripheral Corner) ​

The chamfer protects the vulnerable outer corner of the cutting edge from chipping during entry and exit.

Sharpening Equipment ​

Manual Fixtures ​

Manual gun drill sharpening fixtures mount on a standard tool and cutter grinder:

Manual sharpening requires significant operator skill. The angles are set using graduated scales on the fixture, and the grinding feed is controlled manually. Typical cycle time: 5–15 minutes per drill.

Dedicated Gun Drill Sharpeners ​

CNC Grinding Machine Considerations ​

Wheel Selection ​

Quality Control ​

In-Process Inspection ​

Optical Measurement Systems ​

Automated Regrind Verification ​

Advanced regrind centers use automated inspection integrated with the grinding cycle:

Regrind cycle with integrated QC:
1. Measure incoming drill (optical) → compare to nominal geometry
2. Calculate stock removal required
3. Rough grind with compensation for wear
4. Intermediate optical measurement
5. Finish grind with fine compensation
6. Final optical inspection → generate measurement report
7. Pass/fail decision based on geometric tolerances

Regrind Documentation ​

For production operations, each regrind should be documented:

Common Sharpening Errors ​

Regrind Management ​

When to Regrind ​

Regrind Allowance ​

Removing more material than the recommended allowance per regrind reduces the total number of regrinds achievable over the tool life. The goal is to remove only enough material to reach clean, undamaged carbide.

Summary ​

FAQ ​

What are the critical angles for gun drill sharpening? ​

The four critical angles are: (1) primary relief angle (8–15°) — controls cutting edge strength and clearance; (2) inner relief angle (10–15°) — controls point offset; (3) secondary relief angle (20–30°) — provides additional clearance; and (4) the point offset (D/4) — controls the force balance that keeps the drill tracking straight. The point offset is arguably the most critical single dimension — an error of 0.01 mm in point offset can produce a measurable change in hole diameter and straightness.

How often should a gun drill be sharpened? ​

Sharpen the gun drill when flank wear reaches 0.2 mm, when surface finish degrades below specification, or when hole diameter shifts by more than 0.02 mm from nominal. In production, track the number of holes per regrind and establish a scheduled replacement interval at 70–80% of the expected wear life. A typical carbide gun drill in steel achieves 500–3,000 holes per regrind depending on material and parameters.

Can gun drills be sharpened on a standard tool grinder? ​

Yes — most gun drills are sharpened on universal tool and cutter grinders using a specialized gun drill sharpening fixture. The fixture provides the multi-axis adjustment needed to set the primary relief, inner relief, secondary relief, front clearance, and oil clearance angles. Dedicated gun drill sharpeners (such as the GD-600 or RT-205) integrate the fixture with a suitable grinder in a single package.

What is the point offset and why is it important? ​

The point offset is the distance from the drill centerline to the tip of the cutting point, typically set at D/4 (25% of the drill diameter). This offset creates the unbalanced radial cutting force that presses the guide pads against the bore wall, providing the self-piloting action of the gun drill. If the offset is too small, the drill loses guidance and wanders. If too large, the lateral force increases, causing oversize holes and accelerated guide pad wear.

How do I check gun drill sharpening quality? ​

Use an optical comparator or tool microscope to measure: (1) point offset — should be D/4 ± 0.01 mm; (2) land width — compare to diameter-dependent table; (3) primary relief angle; (4) edge condition — check for chips, cracks, or burrs at 10–50× magnification. The surface finish of ground faces should be Ra ≤ 0.8 µm. Advanced shops use automated optical inspection systems (ZOLLER, Alicona, Walter Helicheck) for full geometry verification.

What causes a gun drill to chip at the outer corner? ​

Outer corner chipping is most often caused by: (1) insufficient chamfer — the sharp corner is vulnerable to impact during entry and exit; (2) excessive feed rate — the mechanical load exceeds the edge strength; (3) interrupted cuts (keyways, cross-holes) that impact the corner; or (4) excessive primary relief angle that leaves insufficient support behind the cutting edge.

Can a gun drill be sharpened too many times? ​

Yes. Each regrind removes material from the cutting head, reducing the diameter slightly and moving the guide pads closer to the cutting edge. When the cutting head diameter has been reduced by 0.2–0.5 mm (diameter-dependent), or when the remaining carbide thickness at the cutting edge is insufficient for structural support, the drill should be retired. The total number of regrinds ranges from 3–5 for small drills (< 6 mm) to 10–15 for larger drills (> 20 mm).

What is the difference between sharpening carbide and HSS gun drills? ​

Carbide gun drills require diamond grinding wheels (resin-bond diamond for fine finish); HSS drills use CBN wheels. Carbide is more brittle and requires sharper angles with better edge support to prevent chipping. HSS is tougher and can tolerate higher relief angles. The grinding feed rate should be lower for carbide to prevent thermal cracking — carbide is sensitive to thermal shock from aggressive grinding.

How do I know if a gun drill sharpening error is causing my drilling problem? ​

Compare the defect pattern to the error table: a wandering drill that produces oversize holes likely has insufficient point offset. Rapid edge wear suggests insufficient primary relief angle (the edge is rubbing). Edge chipping suggests excessive relief angle or insufficient chamfer. Inconsistent chip form across the cutting edge suggests uneven inner/outer edge intersection. The defect pattern almost always points to the specific sharpening error.

What equipment do I need to start sharpening gun drills in-house? ​

Minimum equipment: a universal tool and cutter grinder (or bench grinder with suitable speed range), a gun drill sharpening fixture with angle adjustment (V-block type or 50Q-type), diamond grinding wheels (120–180# rough, 240–400# finish), and an optical comparator or tool microscope for inspection. Total investment for a basic setup: $5,000–$15,000. For a CNC solution: $50,000–$200,000.

Gun drill sharpening parameters depend on drill diameter, carbide grade, workpiece material, and coating. The angles in this article are typical starting points for general-purpose steel drilling. Always verify with the drill manufacturer's recommendations. This article reflects industry knowledge as of 2026.