Appearance
Single-lip and double-lip gun drills serve different purposes in deep hole drilling. The single-lip design — one cutting edge with a V-shaped flute — is the general-purpose workhorse capable of drilling virtually any material from aluminium to Inconel, at L/D ratios up to 200:1. The double-lip design — two cutting edges with shallower flutes — achieves up to 2× higher feed rates in free-chipping materials but is limited to cast iron, cast aluminium, and similar materials that produce small chips. Selecting the wrong configuration for the material risks chip clogging, tool breakage, and scrapped parts.
Overview
Gun drills are classified by the number of cutting lips (flutes) on the drill head:
| Configuration | Common Names | Cutting Edges | Flutes | Typical Diameter Range |
|---|---|---|---|---|
| Single-lip | Single-flute, standard gun drill | 1 | 1 (deep V-shape) | 0.5–80 mm |
| Double-lip | Double-flute, double-crimp, two-flute | 2 | 2 (shallower, 180° apart) | 4.76–14.3 mm (crimped) |
| Two-flute two-hole | Milled flute gun drill | 2 | 2 (milled, internal holes) | 6.35–25.4 mm |
The distinction between these configurations affects every aspect of gun drilling performance: cutting forces, chip evacuation, coolant flow, feed rate capability, hole quality, and material suitability.
Single-Lip Gun Drill Design
Construction
A single-lip gun drill consists of three main parts:
- Carbide drill head — contains the single cutting edge, support pads, and coolant port. The cutting edge is asymmetrically positioned relative to the drill axis.
- Steel tube shank — a hollow tube through which high-pressure coolant is delivered. The tube is crimped or milled to form a single V-shaped flute (typically 110–130° opening angle).
- Driven shank (driver) — solid steel end with a Morse taper, straight shank, or other drive configuration.
Single-lip gun drills are available as three-piece (brazed carbide head + tube + driver) or two-piece solid carbide (carbide head and shank as one piece, typically for diameters under 10 mm).
Cutting Geometry
The single cutting edge is divided into two sections:
- Outer cutting edge — extends from the drill periphery toward the centre. Incidence angle 110–130° (measured from the drill axis).
- Inner cutting edge — extends from the centre toward the periphery. Incidence angle 60–75°.
The two sections meet at the drill tip, which is located at a distance from the drill circumference equal to 1/5 to 1/3 of the drill diameter. This offset tip position is fundamental to the self-guiding principle of gun drilling.
Self-Guiding Principle
The single-lip gun drill operates without a second cutting edge to balance cutting forces. Instead, it relies on:
- Support pads — typically two pads located on the drill head circumference, positioned to bear against the hole wall and react the radial cutting forces
- The V-shaped flute — the chip evacuation channel also acts as a structural element that positions the centre of mass asymmetrically
- High-pressure coolant — the coolant jet emerging from the drill head creates a hydrodynamic bearing effect that centres the drill head
The support pads are arranged on the drill head circumference at specific angles relative to the cutting edge. The primary pad is positioned approximately 90° behind the cutting edge, and the secondary pad approximately 180° behind it. This arrangement distributes the reaction forces from the single cutting edge across the hole wall.
Coolant Delivery
Coolant flows through the hollow shank and exits through a kidney-shaped or round port in the drill head, directed at the cutting edge. The high-pressure jet (80–200 bar) performs three functions:
- Cools the cutting edge
- Lubricates the support pads
- Flushes chips back along the V-shaped flute
A known limitation of the single-lip design is that coolant can become trapped on the back side of the cutting edge, reducing cooling effectiveness — particularly in small-diameter drills where the kidney-shaped channel is small.
Double-Lip Gun Drill Design
Construction Methods
Double-lip gun drills are manufactured in two ways:
Double-Crimp (Swaged Flute)
The most common method. A steel tube is crimped twice, 180° apart, creating two flute channels. The crimping action also forms the internal coolant holes directly, minimising turbulence at the head-to-body transition.
- Diameter range: 4.76–14.3 mm (0.1875–0.5625 in)
- Maximum length: Up to 1,830 mm (72 in)
- Flute depth: Shallower than single-lip design
Two-Flute Two-Hole (Milled Flute)
A solid steel body with two milled flutes and two internally drilled coolant holes. This design offers greater rigidity than the double-crimp design.
- Diameter range: 6.35–25.4 mm (0.250–1.0 in)
- Maximum length: Up to 1,220 mm (48 in)
- Construction: Solid body, not crimped tube
Cutting Geometry
The double-lip design has two cutting edges positioned 180° apart, each taking approximately half the chip load of a single-lip drill at the same feed rate. The cutting edge geometry follows similar principles to single-lip drills (outer and inner sections with different incidence angles), but the balanced arrangement eliminates the need for the offset tip.
Force Balance
The key advantage of the double-lip design:
- Radial cutting forces are balanced — two cutting edges 180° apart generate forces that largely cancel each other
- Reduced reliance on support pads — the drill tracks straighter because the net radial force is near zero
- Less friction and heat — reduced pad loading means less frictional heat generation
- Higher feed rates possible — without the radial force imbalance that limits single-lip feed rates
Coolant Dynamics
Coolant in double-lip gun drills is delivered through the two internal passages formed by the crimping process. Flow distribution between the two edges must be balanced to ensure equal cooling and chip evacuation.
Note: Because two cutting edges generate approximately twice the chip volume per revolution, double-lip gun drills require 20–30% higher coolant pressure than single-lip drills for equivalent chip evacuation.
Head-to-Head Comparison
| Parameter | Single-Lip | Double-Lip (Double-Crimp) | Two-Flute Two-Hole |
|---|---|---|---|
| Cutting edges | 1 | 2 | 2 |
| Feed rate capability | Baseline | Up to 2× baseline | Up to 2× baseline |
| Chip load per edge | Full | 50% reduction | 50% reduction |
| Radial force balance | Unbalanced (uses support pads) | Balanced | Balanced |
| Flute depth | Deep (V-shape) | Shallow | Shallow to moderate |
| Chip evacuation capacity | High | Moderate | Limited |
| Coolant pressure requirement | Baseline | 20–30% higher | 20–30% higher |
| Rigidity (torsional) | Standard | Higher | Highest |
| Rigidity (bending) | Standard | Higher | Highest |
| Hole straightness | 0.1 mm/100 mm | 0.05–0.1 mm/100 mm | Comparable |
| Surface finish | Ra 1.6–3.2 µm | Ra 0.8–2.0 µm | Ra 0.8–2.0 µm |
| Diameter range | 0.5–80 mm | 4.76–14.3 mm | 6.35–25.4 mm |
| Max L/D (practical) | 200:1 (solid carbide) | 70:1 | 50:1 |
| Material range | Virtually all | Cast iron, cast Al, free-machining | Cast iron, cast Al |
Material Suitability
Single-Lip Material Range
Single-lip gun drills are suitable for virtually all machinable materials:
| Material Class | Examples | Suitability | Notes |
|---|---|---|---|
| Low-carbon steel | 1018, 1020 | Excellent | Good chip control |
| Alloy steel | 4140, 4340, 8620 | Excellent | Reduce feed with hardness |
| Tool steel | H13, D2, M2 | Good to excellent | Reduce speed above 45 HRC |
| Stainless steel | 304, 316, 17-4 PH | Good | Requires higher coolant pressure |
| Titanium | Ti6Al4V, Ti-5553 | Good | Low speeds, high coolant pressure |
| Superalloys | Inconel 718, Waspaloy | Fair to good | CBN tooling for hard condition |
| Cast iron | Grey, ductile | Excellent | |
| Aluminium | 6061, 7075, A356 | Excellent | High feed rates possible |
| Brass / bronze | 360, 464, 932 | Excellent | |
| Copper | OFHC, ETP | Good | Long chips require attention |
| Plastics / composites | PEEK, CFRP, GFRP | Fair to good | Tool geometry optimisation needed |
Double-Lip Material Range
Double-lip gun drills are primarily suited for free-chipping materials:
| Material | Suitability | Reason |
|---|---|---|
| Grey cast iron | Excellent | Short, broken chips; ideal for shallow flutes |
| Ductile cast iron | Good | Short chips; moderate feeds |
| Cast aluminium (A356, 319) | Excellent | Small, well-broken chips |
| Wrought aluminium (6061) | Good | Can produce long chips at high feed |
| Brass (free-cutting) | Excellent | Short chips |
| Bronze | Good | Moderate chip control |
| Free-machining steel (12L14) | Fair | Short chips at optimised parameters |
| Low-carbon steel | Poor | Stringy chips clog shallow flutes |
| Stainless steel | Poor to fair | Long, stringy chips; chip evacuation failure risk |
| Titanium | Not recommended | Chip packing risk |
| Superalloys | Not recommended | Chip packing risk |
Why the Limitation?
The fundamental limitation of the double-lip design is flute depth. Two flutes in the same tube diameter means each flute is shallower than the single V-shaped flute of a single-lip drill. The shallower channel can only accommodate smaller chips. Materials that produce long, stringy, or snarled chips will clog the flute, causing coolant flow blockage, chip packing, and eventual tool breakage.
Feed Rate and Productivity
Theoretical Feed Rate Comparison
The double-lip design can achieve approximately 2× the feed rate (mm/rev) of a single-lip drill in suitable materials because:
- Chip load per edge is halved — at the same feed rate, each edge removes half the material
- Balanced forces allow higher feed — without the radial force imbalance, the drill can be pushed harder
- Reduced vibration — balanced cutting reduces chatter tendencies at higher feed rates
Practical Feed Rate Guidelines
| Drill Diameter | Single-Lip Feed (mm/rev) | Double-Lip Feed (mm/rev) |
|---|---|---|
| 5 mm | 0.015–0.030 | 0.025–0.055 |
| 10 mm | 0.025–0.050 | 0.045–0.090 |
| 15 mm | 0.035–0.070 | 0.060–0.120 |
Note: The double-lip feed rates shown apply only to suitable materials (cast iron, cast aluminium). In unsuitable materials, the double-lip feed rate must be reduced below the single-lip rate to avoid chip evacuation failure.
Productivity Trade-Off
The feed rate advantage of the double-lip design must be weighed against:
- Reduced material versatility — dedicated tooling for a narrower material range
- Higher coolant pressure requirement — 20–30% more pressure for chip evacuation
- Deeper holes require reduced feed — as L/D increases, chip evacuation becomes more challenging; the advantage diminishes beyond L/D = 50:1
- Regrinding complexity — double-lip geometry is more complex to regrind correctly
Special Designs
Double Contour Single Flute
A specialised single-lip design from Botek features a double contour on the flute geometry to improve chip control and straightness. This design achieves drift as low as 0.00003 in/in (0.03 mm/100 mm) in titanium — significantly better than standard single-lip or double-lip designs.
Botek Axial-Pulsator
The Axial-Pulsator is a coolant pulsation system that creates a controlled pressure fluctuation in the coolant flow. This improves chip breaking and evacuation, particularly in steel and long-chipping materials. The pulsator enables higher feed rates with single-lip gun drills in materials that normally produce problematic chip forms.
Step Drill Configurations
Both single-lip and double-lip gun drills can be configured as step drills with two or more diameters. The step design improves guidance in deep holes and can reduce the total number of drilling passes required.
Selection Guide
Choose Single-Lip When:
- Drilling a wide variety of materials
- Working with difficult-to-machine alloys (titanium, Inconel, stainless steel)
- Hole depth exceeds 50× diameter
- Diameter is under 5 mm
- Maximum process reliability is required across material variations
- Chip evacuation concerns exist (long-chipping materials)
- Lower coolant pressure is available
Choose Double-Lip (Double-Crimp) When:
- High production volumes in cast iron or cast aluminium
- Feed rate / cycle time is the primary productivity driver
- Coolant pressure of 100+ bar is available
- Diameter is in the 5–14 mm range
- Hole depth is under 50× diameter
- Tool rigidity is a concern (thin-walled workpieces)
Choose Two-Flute Two-Hole (Milled) When:
- Maximum rigidity is required
- Whip guides cannot be used
- Diameter is in the 6–25 mm range
- Cast iron production drilling with high feed rates
- Higher precision requirements demand the stiffest possible tool
Summary
| Selection Criterion | Single-Lip | Double-Lip (Crimped) | Two-Flute Two-Hole |
|---|---|---|---|
| Material versatility | ★★★★★ | ★★ | ★★ |
| Feed rate (suitable materials) | ★★★ | ★★★★★ | ★★★★★ |
| Chip evacuation | ★★★★★ | ★★★ | ★★ |
| Tool rigidity | ★★★ | ★★★★ | ★★★★★ |
| Hole straightness | ★★★★ | ★★★★★ | ★★★★★ |
| Surface finish | ★★★★ | ★★★★★ | ★★★★★ |
| Diameter range | ★★★★★ | ★★★ | ★★★ |
| Max L/D | ★★★★★ | ★★★ | ★★ |
| Coolant pressure requirement | ★★★ | ★★ (higher needed) | ★★ |
| Regrinding ease | ★★★★★ | ★★★ | ★★★ |
FAQ
What is the main difference between single-lip and double-lip gun drills?
The number of cutting edges. Single-lip has one cutting edge and relies on support pads to balance cutting forces. Double-lip has two cutting edges 180° apart that balance the radial forces, enabling higher feed rates but requiring shallower flutes that limit chip evacuation.
Can double-lip gun drills be used for all materials?
No. Double-lip gun drills are primarily suitable for free-chipping materials that produce small chips — cast iron, cast aluminium, brass, and similar materials. They are not recommended for materials that produce long, stringy chips such as low-carbon steel, stainless steel, titanium, or superalloys, because the shallower flutes cannot evacuate these chip forms.
What does "double-crimp" mean in gun drill manufacturing?
Double-crimp refers to the manufacturing process where a steel tube is crimped (swaged) twice, 180° apart, to form two flute channels. The crimping action also creates the internal coolant holes. This is the most common method for producing double-lip gun drills in diameters from 4.76 mm to 14.3 mm.
Do double-lip gun drills produce straighter holes than single-lip?
In suitable materials, yes. The balanced cutting forces of the double-lip design reduce the tendency to drift. However, single-lip gun drills with optimised geometry (e.g., double contour single flute designs) can achieve equivalent or better straightness in difficult materials. The choice should be based on material first, straightness requirement second.
How much higher feed rates can double-lip gun drills achieve?
In suitable materials (cast iron, cast aluminium), double-lip gun drills can achieve up to 2× the feed rate (mm/rev) of equivalent single-lip drills. In borderline materials (free-machining steel), the advantage is smaller. In unsuitable materials, feed rates must be reduced below single-lip rates to prevent chip evacuation failure.
What coolant pressure is needed for double-lip gun drills?
Double-lip gun drills require 20–30% higher coolant pressure than single-lip drills because two cutting edges generate approximately twice the chip volume per revolution. For most applications, 100–200 bar is recommended. On horizontal machines with pilot holes, operation is possible at lower pressures (approximately 10 bar) for short holes.
Can I regrind double-lip gun drills?
Yes, but regrinding is more complex than single-lip drills. Both cutting edges must be ground to precise geometry to maintain balanced cutting forces. Uneven regrinding creates unequal chip loads that can cause the drill to deviate. Many manufacturers recommend factory regrinding or dedicated CNC tool grinding equipment for double-lip drills.
Which gun drill type should I choose for aluminium?
For cast aluminium (A356, 319, 380), a double-lip (double-crimp) gun drill achieves the highest productivity. For wrought aluminium (6061, 7075), a single-lip drill is safer if hole depths exceed 30× diameter, but double-lip can work at high feed rates for shallower holes.
Are two-flute two-hole gun drills the same as double-crimp?
No. Two-flute two-hole gun drills are made from solid steel with milled flutes and internally drilled coolant holes, while double-crimp gun drills are made from crimped tube. Two-flute two-hole drills offer greater rigidity but at higher manufacturing cost and with even more limited chip evacuation capacity.
Which gun drill type has better tool life?
Single-lip gun drills generally have longer tool life in difficult materials due to more robust chip evacuation and lower cutting forces per edge. Double-lip gun drills can achieve competitive tool life in suitable materials (cast iron, cast aluminium) because the balanced forces reduce edge chipping. The regrinding interval depends primarily on the material being drilled rather than the lip configuration.