Appearance
A gun drilling operation producing 12 mm × 1,200 mm bores in stainless steel experiences inconsistent tool life and poor surface finish, with tools averaging only 15 m drilled before failure. Analysis of the gun drill geometry reveals a 118° point angle with a 0.3 mm tip offset, producing excessive cutting forces at the drill centre and uneven chip formation. Optimising the geometry to a 130° point angle with 0.4 mm tip offset, increasing the coolant channel cross-section by 25%, and adding a chip breaker groove on the outer cutting edge increases average tool life to 45 m and improves surface finish from Ra 2.0 µm to Ra 0.8 µm.
Gun Drill Geometric Parameters Overview
The single-lip gun drill is the most geometrically complex tool in deep hole drilling. Its asymmetric design — a single cutting edge offset from the tool axis — creates the characteristic cutting action that enables deep hole drilling at extreme L/D ratios but also makes geometry optimisation critical for performance.
The key geometric parameters of a gun drill can be grouped into five functional categories:
| Category | Parameters | Function |
|---|---|---|
| Point geometry | Point angle, tip offset, chisel edge | Centring, penetration, force balance |
| Cutting edge | Rake angle, relief/clearance angles, edge radius | Chip formation, cutting forces, edge strength |
| Chip control | Chip breaker groove, land width | Chip curl, fracture, and evacuation |
| Coolant delivery | Coolant hole shape, area, position | Cooling, lubrication, chip ejection |
| Flute and shank | Flute shape, core thickness, back taper | Chip transport, torsional stiffness |
Point Angle Selection
The point angle is the primary geometric parameter controlling the cutting action at the drill centre. It determines the effective rake angle distribution along the cutting edge and the direction of chip flow.
| Material | Recommended Point Angle | Effect |
|---|---|---|
| Carbon and alloy steel (200–350 HB) | 125–135° | Good centring; balanced edge strength |
| Stainless steel (austenitic) | 130–140° | Reduced work hardening at centre |
| Titanium alloys | 120–130° | Lower cutting forces; reduced heat |
| Inconel / superalloys | 130–145° | Maximum edge strength; reduced chipping |
| Cast iron | 110–120° | Reduced edge pressure; prevents edge break-out |
| Aluminium | 118–125° | Sharp edge for clean cutting |
| Hardened steel (> 400 HB) | 135–145° | Reinforced edge for high cutting forces |
The point angle also affects the chip thickness distribution. A larger point angle produces a longer cutting edge with thinner chip cross-section at the same feed rate, reducing chip load per unit edge length but increasing cutting forces in the radial direction.
Tip Offset and Centring Action
The tip offset — the distance between the cutting edge and the drill axis — is the defining geometric feature of the single-lip gun drill. It creates the asymmetric cutting action that produces the characteristic V-shaped chip and enables self-centring.
| Tip Offset (fraction of diameter) | Effect on Cutting Action | Application |
|---|---|---|
| 0.20–0.25 D | Moderate centring; lower cutting forces | Soft materials, small diameters |
| 0.25–0.30 D | Standard centring; balanced forces | General purpose steel |
| 0.30–0.35 D | Strong centring; higher cutting forces | Hard materials, large diameters |
| > 0.35 D | Very strong centring; risk of vibration | Specialised applications only |
The tip offset creates a non-cutting zone at the drill centre. The material at the centre is not cut but is pushed outward and sheared by the inner cutting edge as it advances. The width of this non-cutting zone is approximately 2 × tip offset and must be carefully controlled — too large produces high thrust forces, too small produces inadequate centring.
Rake Angle Distribution
The rake angle in a gun drill is not constant along the cutting edge. It varies from the inner edge (near centre) to the outer edge (bore diameter) due to the geometry of the cutting face relative to the drill axis.
| Position Along Edge | Typical Rake Angle | Cutting Condition |
|---|---|---|
| Inner edge (centre) | −5° to +5° | Low cutting speed; squeezing action |
| Mid-point | +5° to +10° | Moderate cutting speed; normal chip flow |
| Outer edge (periphery) | +8° to +15° | Highest cutting speed; best chip flow |
The variation in rake angle along the cutting edge means that the inner portion of the edge operates with less efficient cutting geometry. This is compensated by the higher chip load at the inner edge (thicker chip at lower speed).
Optimisation guidelines:
- For ductile materials (low-carbon steel, aluminium): Use higher positive rake angles (+8° to +15°) to reduce cutting forces
- For hard materials (hardened steel, Inconel): Use lower rake angles (0° to +8°) for edge strength
- For stainless steel: Use moderate positive rake (+5° to +10°) to reduce work hardening
- Negative rake angles should be avoided in gun drills — they increase cutting forces and promote built-up edge
Clearance and Relief Angles
The clearance angles on the flank face prevent the cutting edge from rubbing against the machined surface. Gun drills have two clearance angles:
| Clearance Angle | Location | Typical Value | Function |
|---|---|---|---|
| Primary clearance | Adjacent to cutting edge | 6–12° | Immediate edge relief; strongest effect on wear |
| Secondary clearance | Behind primary clearance | 15–25° | Chip clearance; reduces friction on flank |
Optimisation guidelines:
- Smaller clearance angles (6–8°) for hard materials — provides maximum edge support
- Larger clearance angles (10–12°) for soft/ductile materials — reduces friction and BUE
- The clearance angle must be at least 2–3° larger than the helix angle of the flute to ensure chip clearance
- Insufficient clearance (< 5°) causes rapid flank wear from rubbing
- Excessive clearance (> 15°) weakens the cutting edge and can cause chipping
Chip Breaker Geometry
Chip breaking in gun drilling is essential because the V-shaped flute provides limited space for chip transport. The chip breaker groove on the rake face curls the chip to a smaller radius, inducing fracture.
| Chip Breaker Parameter | Typical Range | Effect |
|---|---|---|
| Groove width | 0.5–2.0 mm | Narrower = tighter curl = better breaking |
| Groove depth | 0.2–0.6 mm | Deeper = more deflection = higher strain |
| Groove distance from edge | 0.1–0.4 mm | Closer = earlier chip contact with backwall |
| Land width (before groove) | 0.05–0.20 mm | Controls initial chip curl direction |
The chip breaker geometry must be matched to the feed rate. For a given material, each feed rate has an optimal groove width — too wide and chips do not curl enough to break, too narrow and chips jam in the groove.
For gun drills in stainless steel (work-hardening material), the chip breaker groove should be positioned closer to the cutting edge (0.10–0.15 mm land) with a narrower groove width compared to carbon steel at the same feed rate.
TIP
The chip breaker geometry on a gun drill must be optimised for the specific chip load at each point along the cutting edge. Unlike BTA inserts where the chip breaker is moulded into the insert, gun drill chip breakers are ground into the tool. This allows customisation for specific materials and feed rates. A common optimisation strategy is to use a two-stage chip breaker: a shallow groove for the inner portion of the edge (where chip thickness is highest) and a deeper groove for the outer portion (where chip thickness is lower). This ensures consistent chip breaking along the entire cutting edge. The chip breaker should be inspected under a microscope every 5–10 regrinds — wear at the backwall reduces chip breaking effectiveness even when the cutting edge itself appears sharp.
Coolant Hole Geometry
The internal coolant passage in a gun drill delivers high-pressure coolant directly to the cutting tip. The geometry of this passage — its cross-sectional shape, area, and position — directly affects coolant velocity, flow rate, and cutting edge cooling.
| Coolant Hole Type | Diameter Range | Cross-sectional Area | Coolant Velocity | Application |
|---|---|---|---|---|
| Single kidney-shaped | < 10 mm | 5–15% of tool area | Highest | Standard for small diameters |
| Two round holes | 10–30 mm | 8–20% of tool area | High | Standard for medium diameters |
| Kidney + round | > 30 mm | 10–25% of tool area | Moderate | Large diameters |
Kidney-Shaped Channel Design
For gun drills below 10 mm diameter, the single kidney-shaped cooling channel is the standard design. Key optimisation parameters:
- Channel position: Centered behind the cutting edge — closer to the cutting edge improves cooling but reduces edge support
- Channel width: 40–60% of the tool diameter
- Channel height: 30–50% of the channel width
- Channel area: 5–15% of the total cross-sectional area
- Edge distance (wall thickness): Minimum 0.15–0.25 mm from channel to cutting edge
Research by Oezkaya et al. (2022) demonstrated that increasing the coolant channel cross-sectional area by 25% and simultaneously modifying the outer and inner cutting edge angles increased cutting fluid velocity at the drill tip by 40% and improved chip evacuation by 60%.
Flute Shape and Core Thickness
The V-shaped flute of the gun drill serves as the chip evacuation passage. Its geometry represents a trade-off between chip transport capacity and torsional stiffness.
| Flute Parameter | Typical Value | Trade-off |
|---|---|---|
| Flute included angle | 100–130° | Larger angle = more chip space but weaker core |
| Flute depth | 0.4–0.6 × diameter | Deeper = more chip space but lower stiffness |
| Core thickness | 0.4–0.6 × diameter | Thicker core = stronger but less chip space |
| Flute length | Up to 200 × diameter | Limited by buckling; longer = more chip transport distance |
| Back taper | 0.01–0.05 mm per 100 mm | Reduces friction; prevents seizure |
Optimisation guidelines:
- For high-feed operations (higher chip volume): Increase flute included angle to 120–130°
- For deep holes (L/D > 100): Maintain core thickness at ≥ 0.5 × diameter for torsional stiffness
- For soft materials that produce large chips: Increase flute depth to 0.55–0.60 × diameter
- For hard materials: Core thickness can be reduced to 0.40–0.45 × diameter as chip volume is lower
Lip Height and Edge Preparation
Lip height — the radial distance from the drill axis to the inner end of the cutting edge — determines the size of the non-cutting zone at the drill centre.
| Lip Height (mm) | Effect | Typical Application |
|---|---|---|
| 0.05–0.10 | Small non-cutting zone | Small diameters (< 3 mm) |
| 0.10–0.30 | Standard non-cutting zone | General purpose (3–20 mm) |
| 0.30–0.50 | Large non-cutting zone | Large diameters (> 20 mm) |
Edge preparation (edge honing) is applied to the cutting edge to improve strength and consistency:
| Edge Radius | Application | Effect |
|---|---|---|
| 0.005–0.015 mm | Sharp edge — aluminium, plastics | Low cutting forces; sharpest cut |
| 0.015–0.030 mm | Standard — carbon steel, alloy steel | Balanced edge strength and sharpness |
| 0.030–0.050 mm | Reinforced — stainless, Inconel | Maximum edge strength; reduced chipping |
Coating Selection
| Coating | Typical Thickness | Max Operating Temperature | Application |
|---|---|---|---|
| TiN (titanium nitride) | 2–4 µm | 600°C | General purpose steel; low cost |
| TiCN (titanium carbonitride) | 2–4 µm | 700°C | Harder than TiN; steel and cast iron |
| TiAlN (titanium aluminium nitride) | 2–5 µm | 900°C | High-temperature; stainless, titanium, Inconel |
| AlTiN (aluminium titanium nitride) | 2–4 µm | 900°C | Better oxidation resistance than TiAlN |
| AlCrN (aluminium chromium nitride) | 2–4 µm | 1,100°C | Maximum hot hardness; difficult materials |
| Uncoated (micro-grain carbide) | N/A | N/A | Small diameters (< 2 mm); sharpest edge |
Material-Specific Geometry Summary
| Material | Point Angle | Tip Offset | Rake Angle | Clearance | Chip Breaker | Coating |
|---|---|---|---|---|---|---|
| Carbon steel | 125–135° | 0.25–0.30 D | +5° to +10° | 8–10° | Standard | TiN or TiCN |
| Alloy steel | 130–135° | 0.25–0.30 D | +5° to +8° | 8–10° | Standard | TiCN or TiAlN |
| Stainless steel | 130–140° | 0.25–0.30 D | +5° to +10° | 10–12° | Aggressive (narrow groove) | TiAlN |
| Titanium | 120–130° | 0.20–0.25 D | +8° to +12° | 10–12° | Standard | TiAlN or uncoated |
| Inconel | 130–145° | 0.30–0.35 D | 0° to +5° | 6–8° | Aggressive | AlCrN or TiAlN |
| Cast iron | 110–120° | 0.20–0.25 D | +3° to +8° | 8–10° | Minimal | Uncoated or TiN |
| Aluminium | 118–125° | 0.20–0.25 D | +10° to +15° | 10–14° | Standard | Uncoated or DLC |
FAQ
What is the most important geometric parameter in a gun drill?
The tip offset is the most defining geometric parameter of a gun drill — it creates the asymmetric single-lip cutting action that distinguishes gun drilling from all other drilling processes. The tip offset determines the centring action, cutting force balance, and chip formation at the drill centre. After tip offset, the point angle is the second most important parameter, determining edge strength and chip flow direction. For production optimisation, however, the chip breaker geometry is often the most impactful adjustment because it directly determines chip breaking reliability.
How does point angle affect gun drilling performance?
The point angle controls the effective rake angle distribution along the cutting edge, the direction of chip flow, and the edge strength. A larger point angle (130–145°) produces a longer cutting edge with thinner chip cross-section, better edge strength, and higher radial forces. A smaller point angle (118–125°) produces a shorter cutting edge, lower cutting forces, and better penetration. For hard materials, a larger point angle is preferred for edge strength. For soft materials, a smaller point angle reduces cutting forces.
What is the function of the kidney-shaped coolant channel?
The kidney-shaped coolant channel in a gun drill delivers high-pressure coolant directly to the cutting tip. Its asymmetric shape concentrates coolant flow toward the cutting edge while maintaining structural support for the tool. The channel cross-section is typically 5–15% of the total tool area and must be positioned to direct coolant precisely at the cutting zone. Increasing the channel cross-sectional area improves coolant flow and chip evacuation but reduces tool stiffness and must be balanced against the strength requirements.
How is the chip breaker designed in a gun drill?
The chip breaker in a gun drill is a groove ground into the rake face behind the cutting edge. It induces chip curl by deflecting the chip against the backwall of the groove. The groove width, depth, and distance from the cutting edge determine the chip curvature radius and the strain on the chip surface. For a given material and feed rate, the chip breaker must produce sufficient strain to exceed the material's fracture strain. In gun drills, the chip breaker geometry can be customised along the cutting edge to match the varying chip load from inner to outer edge.
What clearance angles are used in gun drills?
Gun drills use two clearance angles: a primary clearance of 6–12° immediately behind the cutting edge, and a secondary clearance of 15–25° behind the primary clearance. The primary clearance provides immediate edge relief and is the most critical for tool life. The secondary clearance provides additional chip clearance and reduces friction. Smaller clearance angles (6–8°) are used for hard materials to support the edge, while larger angles (10–12°) are used for soft materials to reduce friction and built-up edge.
How does flute geometry affect gun drilling performance?
The V-shaped flute provides the chip evacuation passage from the cutting tip back along the tool. The flute included angle (100–130°) and depth (0.4–0.6 × diameter) determine the chip transport capacity. A larger included angle provides more chip space but reduces the core thickness and torsional stiffness. The core thickness (0.4–0.6 × diameter) is the primary determinant of torsional stiffness — a thicker core resists twisting but leaves less space for chips. The flute geometry must be optimised for the specific chip volume and material being drilled.
What coating is best for gun drilling stainless steel?
TiAlN (titanium aluminium nitride) is the best coating for gun drilling stainless steel. It provides high oxidation resistance (up to 900°C), high hot hardness, and low thermal conductivity that insulates the tool from cutting heat. Stainless steel's work-hardening tendency and low thermal conductivity make it particularly demanding on coatings — TiAlN's combination of wear resistance and thermal protection provides the best tool life. For severe applications, AlCrN provides even higher temperature resistance.
How many times can a gun drill be reground?
A gun drill can typically be reground 5–15 times depending on the diameter and the amount of material removed per regrind. Each regrind removes 0.10–0.20 mm of carbide from the drill face. The limiting factor is the remaining margin for the chip breaker geometry — after multiple regrinds, the chip breaker groove becomes too shallow to be effective. Small-diameter gun drills (< 3 mm) have less margin and may only achieve 3–8 regrinds. Large-diameter gun drills (> 20 mm) can achieve 10–20 regrinds.
What causes a gun drill to produce oversized bores?
Oversized bores from gun drilling are most commonly caused by incorrect tip offset or excessive lip height. If the tip offset is too large for the material, the drill walks outward during cutting, producing a bore larger than the tool diameter. Other causes include: worn guide pads (reduced centring action), incorrect point angle (excessive radial force component), or spindle misalignment. Measuring the tip offset and comparing it to the recommended range for the material is the first diagnostic step.
What is the most common gun drill geometry mistake?
The most common mistake is using a point angle that is too small for the material being drilled. A 118° point angle (standard for twist drills) is often applied to gun drills by default, but gun drills require a larger point angle (125–145° depending on material) because the single-lip design concentrates cutting forces at one side of the centre. The insufficient edge strength from a small point angle causes chipping at the inner cutting edge, which is the most common failure mode for gun drills. The second most common mistake is incorrect clearance angles — too small causes rapid flank wear, too large causes edge chipping.
Summary
Gun drill geometry optimisation requires balancing multiple interdependent parameters — point angle, tip offset, rake angle distribution, clearance angles, chip breaker geometry, coolant hole design, and flute shape — to achieve the best combination of tool life, hole quality, and chip evacuation. The tip offset (0.20–0.35 × diameter) defines the gun drill's unique cutting action, while the point angle (118–145°) must be selected for the specific material. The chip breaker groove must be matched to the feed rate for reliable chip breaking. The coolant hole geometry — kidney-shaped for small diameters — should maximise flow area while maintaining structural integrity. The flute shape represents a fundamental trade-off between chip transport capacity and torsional stiffness. Edge preparation, coating selection, and regrinding practice complete the optimisation. Material-specific geometry selection and coolant channel design are the key factors distinguishing successful gun drilling from operations plagued by poor tool life and inconsistent bore quality.