Gun Drill Geometry and Tool Types
Gun drills are precision cutting tools with a distinctive single-lip design that sets them apart from conventional twist drills. Understanding the different tool types and their geometry is essential for selecting the right drill for your application and getting the best performance from it.
This guide covers the three main gun drill configurations, the critical geometric features that determine cutting performance, and how to match tool selection to your material and hole requirements.
The Three Gun Drill Types
Gun drills are manufactured in three main configurations, each suited to different diameter ranges and production requirements.
Brazed Tip Gun Drills
Brazed tip is the most common gun drill type, covering approximately 80% of all gun drilling applications. A solid tungsten carbide cutting tip is silver-brazed to a steel shank, combining the wear resistance of carbide with the economy of a steel body.
| Property | Typical Range |
|---|---|
| Diameter range | 1.0 mm to 30 mm (0.040" to 1.25") |
| Tip material | Micro-grain tungsten carbide (6–10% cobalt) |
| Shank material | Alloy steel (4340, 4140) or high-speed steel |
| Regrinds possible | 3–5 |
| Cost | Moderate |
The brazed joint is a critical quality feature. A properly brazed tip has a uniform bond line with no voids, cracks, or excessive filler metal. Poor brazing is a common cause of tip detachment in service—one of the most catastrophic gun drilling failures.
Brazed tip drills offer the best balance of cost and performance for general-purpose gun drilling. When the tip dulls, the entire tool is reground; the steel shank lasts through multiple regrind cycles.
Solid Carbide Gun Drills
Solid carbide gun drills are made from a single piece of tungsten carbide with no brazed joint. They offer maximum rigidity and are the preferred choice for small diameters where tool strength is critical.
| Property | Typical Range |
|---|---|
| Diameter range | 0.5 mm to 12 mm (0.020" to 0.50") |
| Carbide grade | Fine to ultra-fine grain (6–12% cobalt) |
| Shank | Same carbide as tip (monolithic) |
| Regrinds possible | 7–10 |
| Cost | Higher than brazed |
The absence of a brazed joint means there is no risk of tip separation and no heat-affected zone from the brazing process. Solid carbide tools can run at higher penetration rates than equivalent brazed tools because the entire tool is rigid.
Solid carbide drills are more expensive to purchase but can be reground more times (7–10 regrinds versus 3–5 for brazed), which can offset the initial cost over the tool’s lifetime.
Indexable Insert Gun Drills
Indexable insert gun drills use replaceable carbide inserts and guide pads mounted on a steel body. When the cutting edge wears, the insert is simply indexed or replaced—no regrinding needed.
| Property | Typical Range |
|---|---|
| Diameter range | 16 mm to 65 mm (0.625" to 2.5") |
| Insert type | Standard or custom carbide inserts with chipbreakers |
| Guide pads | Replaceable carbide pads |
| Regrinding | None (replace inserts) |
| Cost | Highest initial cost, lowest per-edge cost |
Indexable insert tools eliminate the downtime and cost of regrinding. They are the most economical choice for high-volume production at larger diameters where multiple cutting edges per insert provide good tool life.
Key Geometric Features
Single-Lip Cutting Edge
The defining feature of any gun drill is its single cutting edge (one-lip design). Unlike a twist drill with two cutting edges, the gun drill’s single lip removes material along one side of the hole.
The single-lip design creates an unbalanced cutting force that pushes the drill toward the side of the hole. This force is precisely countered by the guide pads, which bear against the bore wall and keep the tool centered—the self-piloting action that makes gun drilling possible.
Nose Grind Types
The nose grind (the shape of the carbide tip’s cutting face) must be matched to the workpiece material. The grind determines how the cutting edge engages the material, how chips form, and how cutting forces are directed to the guide pads.
| Nose Grind | Profile | Best For |
|---|---|---|
| N-8 (R1 relief) | Standard point with 8° relief angle | Steel, stainless steel, Inconel, general purpose |
| N-4 (R4 relief) | Wider point angle with 4° relief | Aluminum, brass, copper, soft non-ferrous |
| N-73 | Modified point with specific clearance | Cast iron, brittle materials, powdered metals |
| Facet grind | Multi-facet cutting edge | General purpose, most steels, good chip control |
The nose grind geometry affects:
- Chip formation — proper grind produces short, C-shaped chips
- Cutting forces — grind angle directs force to guide pads for self-piloting
- Surface finish — proper clearance prevents rubbing and burnishing damage
- Tool life — incorrect grind causes edge chipping or accelerated wear
Coolant Hole Design
The internal coolant hole runs the full length of the gun drill. Its size and shape directly affect coolant flow rate and pressure at the cutting tip.
| Drill Diameter | Typical Coolant Hole Diameter | Coolant Exit Area |
|---|---|---|
| 3 mm | 0.8–1.2 mm | Single crescent or kidney-shaped exit |
| 6 mm | 1.5–2.5 mm | Single or dual exit ports |
| 12 mm | 3.0–5.0 mm | Dual exit ports typical |
| 25 mm | 6.0–10.0 mm | Dual or triple ports |
The coolant exit geometry at the tip is designed to direct coolant precisely to the cutting edge and guide pads. Proper coolant flow is critical for lubrication, cooling, and chip evacuation.
Guide Pads
Two carbide guide pads are located immediately behind the cutting tip on most gun drills. These pads serve three functions:
- Self-piloting — the pads bear against the freshly cut bore wall, continuously steering the drill on-axis
- Burnishing — the pads smooth the bore surface during cutting, producing the characteristic Ra 0.4–0.8 µm finish
- Stabilization — pads dampen vibration and prevent the drill from chattering at depth
Pad geometry (width, length, position relative to the cutting edge) is precisely calculated based on drill diameter and the expected cutting forces.
For detailed information on guide pad function and maintenance, see our guide pads guide.
Manufacturing Quality Indicators
When selecting gun drills, these quality indicators distinguish premium tools:
- Concentricity: Tip concentricity with the shank should be within 0.005 mm (0.0002") for precision work
- Brazed joint inspection: Ultrasonic or dye-penetrant inspection for voids
- Carbide grade: Micro-grain carbide (0.5–1.0 µm grain size) for best edge retention
- Flute surface finish: Smooth V-flute surface (Ra < 0.4 µm) for efficient chip evacuation
- Relief angle consistency: ±0.5° tolerance on primary and secondary relief angles
Major Manufacturers
| Manufacturer | Series | Key Strengths |
|---|---|---|
| Guhring | EB 100, EB 80, EB 800 | Wide range, excellent quality control, global availability |
| Hartner | E 100, E 800 | Good value for standard applications |
| Star SU | Custom-engineered | Special geometries for difficult materials |
| Botek | Standard and custom | Premium carbide grades, long tool life |
| UNISIG | Machine-integrated | Optimized for their machines, technical support |
Selecting the Right Gun Drill
| Your Requirements | Recommended Tool Type |
|---|---|
| Diameter under 3 mm | Solid carbide (maximum rigidity) |
| Diameter 3–16 mm, general production | Brazed tip (best value) |
| Diameter 16–65 mm, high volume | Indexable insert (no regrinding) |
| Maximum penetration rate | Solid carbide (most rigid) |
| Difficult material (titanium, Inconel) | Brazed or solid carbide with proper nose grind |
| Budget-sensitive | Brazed tip (lower initial cost) |
Summary
Gun drill geometry directly determines cutting performance, tool life, and hole quality. Brazed tip drills offer the best value for most applications; solid carbide provides maximum rigidity for small diameters; and indexable inserts eliminate regrinding for large-diameter, high-volume work. Match the nose grind to the workpiece material, and ensure concentricity and coolant hole design meet your tolerance requirements.
For tool maintenance and regrinding information, see our gun drill regrinding guide. For detailed parameters for different material types, see our gun drilling speeds and feeds guide. For a complete overview, visit the gun drilling guide.
For the practical buying decision — spec sheet, selection steps, and delivery checks — see carbide gun drill bits: how to choose and order.