Gun drill selection determines the upper limit of what your process can achieve — hole straightness, surface finish, tool life, and productivity. No amount of parameter optimization compensates for a poorly chosen drill. Matching the drill construction, geometry, grade, and coating to the specific workpiece material and hole specification is the first and most important step in process design.
Gun Drill Construction Types
Comparison of Construction Styles
| Construction Type | Diameter Range | Depth Capability | Cost per Tool | Typical Applications |
|---|
| Solid carbide | 1–20 mm | Up to 100×D | Medium | Small diameters, precision, high production |
| Brazed tip (carbide head, steel shank) | 6–40 mm | Up to 150×D | Low to medium | General purpose, most common type |
| Replaceable tip | 12–40 mm | Up to 100×D | Medium (tip only) | High production, quick changeover |
| Indexable insert | 20–65 mm | Up to 80×D | Higher | Large diameters, roughing |
Tip: Brazed tip gun drills account for approximately 80% of deep hole drilling applications. They offer the best balance of cost, performance, and flexibility. Solid carbide drills are preferred for small diameters (< 6 mm) where shank rigidity is critical.
Brazed Tip Gun Drill Components
| Component | Material | Function |
|---|
| Carbide tip | Micrograin or sub-micrograin carbide | Cutting edge, chip forming |
| Braze joint | Copper-silver alloy | Joins tip to shank |
| Steel shank | Alloy steel (4140, 4340) | Provides length and rigidity |
| Coolant hole | Brass or steel tube | Delivers coolant to cutting edge |
| Driver pad | Hardened steel or carbide | Drives the drill from the holder |
Tip Geometry Selection
Geometry Options by Material
| Workpiece Material | Point Angle | Clearance Angle | Rake Angle | Edge Preparation |
|---|
| Low-carbon steel | 30–35° | 8–12° | 0–3° positive | Honed edge |
| Medium-carbon steel | 25–30° | 8–10° | 0–2° positive | Slight hone |
| Alloy steel (4140, 4340) | 25–30° | 8–10° | 0° neutral | T-land + hone |
| Stainless steel (304, 316) | 20–25° | 10–14° | 3–5° positive | Polished rake face |
| Stainless steel (precipitation-hardened) | 22–28° | 8–12° | 2–4° positive | Polished + hone |
| Aluminum | 30–35° | 12–15° | 5–8° positive | Sharp edge |
| Cast iron | 25–30° | 6–8° | −2–0° negative | Chamfered edge |
| Titanium | 20–25° | 10–14° | 3–5° positive | Polished + hone |
| Inconel / superalloys | 18–22° | 12–16° | 5–8° positive | Polished + large hone |
Point Geometry Variations
| Geometry Type | Characteristic | Best For |
|---|
| Standard single-angle | One grinding angle on each facet | General purpose, most materials |
| Double-angle | Primary + secondary angle on OD | Improved edge strength on hard materials |
| Radius point | Rounded tip profile | Soft materials, interrupted cuts |
| Split point | Secondary grind on chisel edge | Reduced thrust force, better centering |
| Stepped point | Multiple diameters | Rough and finish in one pass |
Carbide Grade Selection
Grade Characteristics
| Grade Type | Grain Size (µm) | Hardness (HRA) | Toughness | Wear Resistance | Best For |
|---|
| Micrograin | 0.5–0.8 | 91–93 | Good | Excellent | General purpose, steel |
| Sub-micrograin | 0.2–0.5 | 92–94 | Very good | Very good | Hard materials, abrasive |
| Nanograin | < 0.2 | 93–95 | Fair | Superior | High-speed finishing |
| Coarse grain | 1.0–3.0 | 89–91 | Excellent | Good | Interrupted cuts, heavy feeds |
Recommended Grades by Material
| Material Group | Recommended ISO Grade | Recommended Carbide Type | Cobalt Content |
|---|
| Steel (low-carbon) | K10–K20 | Micrograin | 6–8% |
| Steel (alloy) | K15–K25 | Micrograin | 7–9% |
| Stainless steel | K20–K30 | Sub-micrograin | 9–11% |
| Cast iron | K05–K15 | Micrograin | 5–7% |
| Aluminum | K10–K20 | Micrograin | 6–8% |
| Titanium | K25–K35 | Sub-micrograin | 10–12% |
| Inconel / superalloys | K30–K40 | Sub-micrograin | 11–14% |
Coating Selection
Coating Types and Benefits
| Coating Type | Hardness (HV) | Max Temperature | Primary Benefit | Best Material Match |
|---|
| TiN (Titanium Nitride) | 2,300 | 600°C | General purpose, low cost | Steel, cast iron |
| TiCN (Titanium Carbonitride) | 3,000 | 700°C | Higher wear resistance | Alloy steel, stainless |
| TiAlN (Titanium Aluminum Nitride) | 3,300 | 900°C | High-temperature performance | Titanium, Inconel, hard steel |
| AlTiN (Aluminum Titanium Nitride) | 3,500 | 900°C | Oxidation resistance, hard materials | Superalloys, hardened steel |
| AlCrN (Aluminum Chromium Nitride) | 3,200 | 1,100°C | Maximum heat resistance | Inconel, titanium |
| Diamond (CVD) | 8,000 | 600°C | Non-ferrous, abrasive materials | Aluminum (high Si), composites |
| No coating | — | — | Low cost, simple regrind | General steel, cast iron |
Coating Decision Matrix
| Application | Recommended Coating | Why |
|---|
| Production steel, general | TiN or TiCN | Good wear resistance, economical |
| Stainless steel | TiAlN or AlTiN | High-temperature stability prevents BUE |
| Titanium and superalloys | AlTiN or AlCrN | Maximum thermal protection |
| High-silicon aluminum | Diamond (CVD) | Resistance to abrasive silicon |
| Cast iron, low-volume | No coating or TiN | Cost-effective, easy to regrind |
| Interrupted cuts | TiCN | Tough coating with good adhesion |
Shank Types and Drive Configurations
Shank Styles
| Shank Type | Diameter Range | Features | Best For |
|---|
| Cylindrical (straight) | 3–40 mm | Simple, low cost | Standard holder |
| Whistle notch | 6–40 mm | Notch for set screw retention | Positive drive, most common |
| Flatted (single or double) | 10–40 mm | Ground flats for drive | High torque, large diameters |
| Threaded | 15–65 mm | Screw-in connection | Heavy-duty, high feed |
| Captive (quick-change) | 10–40 mm | Quick-release mechanism | High-production, frequent changes |
Drive Considerations
| Factor | Recommendation |
|---|
| Torque < 10 N·m | Whistle notch or cylindrical with set screw |
| Torque 10–50 N·m | Double-flatted shank |
| Torque > 50 N·m | Threaded connection |
| Speed > 5,000 RPM | Precision-ground cylindrical, balanced |
| Quick-change needed | Captive shank system |
Diameter-to-Length Ratio
Selection Impact
| L/D Ratio | Construction Recommendation | Special Considerations |
|---|
| < 20:1 | Any construction type | Standard selection |
| 20:1–50:1 | Brazed tip, steel shank | Check shank straightness, use steady rests |
| 50:1–100:1 | Brazed tip, heavy-wall shank | Reduced feed, critical alignment |
| 100:1–150:1 | Brazed tip, special straightness | Very low feed, premium shank material |
| > 150:1 | Special order only | Drastically reduced parameters |
Selection Workflow
Step-by-Step Selection Process
| Step | Action | Key Questions |
|---|
| 1 | Define hole specifications | Diameter, depth, tolerance, surface finish |
| 2 | Identify workpiece material | Material group, hardness, inclusion content |
| 3 | Select construction type | Brazed tip for general, solid carbide for small |
| 4 | Choose tip geometry | Point angle and clearance per material |
| 5 | Select carbide grade | Match ISO grade to material group |
| 6 | Choose coating | Based on material and production volume |
| 7 | Determine shank type | Based on torque and drive system |
| 8 | Verify L/D ratio | Confirm construction supports depth |
| 9 | Select coolant hole size | Match to pressure and flow requirements |
| 10 | Confirm with supplier | Validate selection before ordering |
FAQ
What is the most common gun drill construction type?
The brazed tip gun drill (carbide tip brazed onto a steel shank) is the most common, accounting for approximately 80% of deep hole drilling applications. It offers the best balance of cost and performance, with the carbide tip providing a sharp cutting edge and the steel shank providing rigidity and lower cost.
How do I choose between TiN, TiAlN, and AlTiN coating?
Choose TiN for general steel drilling at moderate speeds. Choose TiAlN for stainless steel and when running at higher speeds where heat generation is greater. Choose AlTiN (higher aluminum content) for titanium, Inconel, and superalloys where cutting edge temperature exceeds 800°C. AlTiN's aluminum oxide layer that forms at high temperature provides thermal barrier protection.
What point angle should I use for stainless steel gun drilling?
Use a flatter point angle of 20–25° for stainless steel. The shallower angle reduces thrust force and allows the cutting edge to shear cleanly, reducing work hardening and built-up edge formation. Combine with a positive rake angle (3–5°) and a polished rake face for best results.
When should I use a replaceable tip gun drill instead of brazed tip?
Use replaceable tip drills when: production volume is high enough to justify the higher initial tool holder cost (typically > 500 holes per setup), fast tip changes are needed to minimize downtime, or you want to eliminate the need for in-house regrinding. The tip itself costs more than regrinding a brazed drill, so the economics favor replaceable tips only at higher volumes.
What carbide grade is best for drilling Inconel?
Use a sub-micrograin carbide grade (ISO K30–K40) with 11–14% cobalt content for Inconel and superalloys. The higher cobalt content provides the toughness needed to resist the extreme edge pressure and notch wear that occurs when machining these materials. Pair with an AlTiN or AlCrN coating for thermal protection.
Gun drill selection is a systematic process — match the tool to the material, the coating to the heat, and the shank to the torque. Time spent on selection pays back in tool life and hole quality. This article reflects industry practice as of 2026.
