Appearance
Deep hole drilling is not simply "drilling a deep hole." It is a distinct family of machining processes defined by specialized tool geometry, high-pressure coolant systems, and self-piloting mechanics that together enable hole geometries impossible with conventional twist drills.
What Qualifies as a Deep Hole?
The most commonly accepted definition comes from VDI Standard 3210: deep hole drilling refers to processes where the hole depth exceeds three times the diameter (L/D > 3:1). In practice, however, the term applies to applications where conventional drilling methods become impractical — typically at L/D ratios of 10:1 or greater.
| L/D Ratio | Classification | Typical Method |
|---|---|---|
| 3:1 – 10:1 | Shallow deep hole | Conventional twist drill (with pecking) |
| 10:1 – 50:1 | Moderate deep hole | Gun drilling or BTA |
| 50:1 – 100:1 | Deep hole | Gun drilling (dedicated machine) |
| 100:1 – 400:1+ | Extreme deep hole | BTA or gun drilling (specialized equipment) |
What truly distinguishes deep hole drilling is not just the depth ratio but the self-piloting mechanism. Deep hole drilling tools use guide pads that bear against the machined bore wall, creating a self-centering effect that maintains straightness without pre-drilled pilot holes or follow-bushings (though bushings are often used at entry).
TIP
The self-piloting action means that once the tool enters the workpiece, it follows the existing hole axis rather than wandering. This is why deep hole drilling can achieve straightness tolerances of 0.1–0.5 mm per meter of depth — far better than a conventional twist drill at equivalent depths.
The Three Classic Methods
Deep hole drilling divides into three primary methods, each suited to different diameter ranges and production requirements.
Gun Drilling
Gun drilling is the oldest and most widely recognized deep hole drilling method. Originally developed for manufacturing firearm barrels in the 19th century, it uses a single-lip, single-flute drill with an internal coolant channel.
How it works: High-pressure coolant (30–150 bar) is pumped through the drill shank's internal hole, exits at the cutting edge, and returns along the external V-shaped flute, carrying chips out of the hole. The tool is guided by two support pads that ride along the bore wall.
| Parameter | Range |
|---|---|
| Diameter | 0.5 – 50 mm (typical 2–20 mm) |
| Max L/D ratio | > 100:1 (up to 200:1 in special cases) |
| Coolant pressure | 30 – 150 bar |
| Surface finish (Ra) | 0.4 – 1.6 µm |
| Tolerance (IT grade) | IT7 – IT10 |
Gun drilling is the preferred method for small-diameter, high-precision holes. It is widely used in fuel injection systems, hydraulic components, medical implants, and mold cooling channels.
WARNING
The external chip evacuation path means that chips travel along the finished bore surface, which can cause scoring in soft or gummy materials. For such materials, BTA drilling — where chips exit internally — may be preferable.
BTA Drilling
The Boring and Trepanning Association (BTA) method, also known as STS (Single Tube System), was developed in the 1940s for higher material removal rates. It uses a multi-edge cutting head attached to a thick-walled hollow tube.
How it works: Coolant is pumped through the annular gap between the drill tube and the bore wall. Chips are carried back through the hollow center of the tube and out through the machine spindle. A pressure head seals the coolant at the workpiece entry point.
| Parameter | Range |
|---|---|
| Diameter | 12 – 850 mm (typical 20–200 mm) |
| Max L/D ratio | Up to 400:1 |
| Coolant pressure | 15 – 80 bar |
| Surface finish (Ra) | 0.8 – 3.2 µm |
| Tolerance (IT grade) | IT7 – IT10 |
BTA drilling offers 5–7 times higher feed rates than gun drilling at comparable diameters, making it the high-productivity choice for medium-to-large holes. Because chips exit internally, the finished bore surface is not damaged by chip flow.
Ejector Drilling
Ejector drilling, also called DTS (Double Tube System), was developed by Sandvik Coromant in the 1970s. It uses two concentric tubes to deliver coolant and evacuate chips.
How it works: Coolant enters through the annular gap between the inner and outer tubes. About two-thirds flows to the cutting head, while one-third is directed through angled slots at the rear of the inner tube, creating a Venturi (ejector) effect that draws chips out through the inner tube.
| Parameter | Range |
|---|---|
| Diameter | 18 – 200 mm |
| Max L/D ratio | < 100:1 |
| Coolant pressure | 10 – 50 bar |
| Surface finish (Ra) | 0.8 – 3.2 µm |
| Tolerance (IT grade) | IT8 – IT11 |
The key advantage of ejector drilling is that it requires no pressure head — only a simple guide bush. This makes it suitable for retrofitting onto conventional lathes and machining centers.
TIP
Because ejector drilling operates at lower coolant pressures and does not require a dedicated pressure head, it is often the most economical entry point for shops adding deep hole drilling capability to existing equipment.
Method Selection Guide
| Condition | Recommended Method |
|---|---|
| Diameter < 20 mm, high precision | Gun drilling |
| Diameter 20–200 mm, high volume | BTA drilling |
| Diameter 20–200 mm, retrofit/CNC | Ejector drilling |
| Diameter > 200 mm | BTA drilling or trepanning |
| L/D > 100:1 | BTA or gun drilling |
| Sensitive to bore surface damage | BTA or ejector (internal chip exit) |
Key Process Parameters
Successful deep hole drilling depends on four interlinked parameters:
Coolant pressure and flow — This is the single most critical parameter. Insufficient pressure means chips are not evacuated, leading to jamming and tool breakage. Required pressure increases with depth and decreases with diameter.
Cutting speed — Typically lower than conventional drilling due to the extended cutting edge engagement. Speeds range from 30–120 m/min depending on workpiece material.
Feed rate — Feed per revolution is determined by the cutting edge geometry and desired surface finish. BTA heads with multiple cutting edges can achieve significantly higher feed rates than single-lip gun drills.
Tool geometry — Point angle, edge preparation, and guide pad positioning all affect chip formation, cutting forces, and bore straightness. These are tailored to the workpiece material.
Industries and Applications
Deep hole drilling is essential across a wide range of manufacturing industries:
| Industry | Typical Components | Common Method |
|---|---|---|
| Aerospace | Landing gear, turbine shafts, engine cooling holes | Gun drilling, BTA |
| Automotive | Crankshafts, camshafts, fuel injectors, transmission shafts | Gun drilling |
| Oil & gas | Drill collars, downhole tools, valve bodies | BTA |
| Medical | Bone screws, intramedullary nails, surgical instruments | Gun drilling (micro) |
| Hydraulics | Cylinder barrels, valve blocks, piston rods | BTA, skiving & burnishing |
| Mold & die | Cooling channels, ejector pin holes | Gun drilling |
| Defense | Gun barrels, missile components, naval hardware | Gun drilling, BTA |
| Energy | Turbine shafts, heat exchanger tubes, generator rotors | BTA |
Advantages and Limitations
Advantages:
- Achieves L/D ratios impossible with conventional drilling
- Excellent straightness (self-piloting tools)
- Good surface finish in a single pass (Ra 0.4–3.2 µm)
- No pecking required — continuous feed from entry to exit
- Can machine a wide range of materials from aluminum to superalloys
Limitations:
- Requires specialized tooling and machinery (high capital investment)
- High-pressure coolant systems add cost and complexity
- Limited to cylindrical or near-cylindrical hole geometries
- Setup and tooling selection require specialized knowledge
- Not economical for short, shallow holes where conventional drilling suffices
FAQ
What L/D ratio qualifies as deep hole drilling?
There is no single standard. VDI 3210 defines deep hole drilling as L/D > 3:1, but most industry professionals consider L/D > 10:1 as the practical threshold where dedicated deep hole drilling methods become necessary.
Can deep hole drilling be done on a conventional CNC machine?
Yes — with appropriate tooling and high-pressure coolant systems. Ejector drilling was specifically designed for retrofitting onto conventional machines. However, for L/D ratios beyond 50:1 or production volumes requiring maximum efficiency, dedicated deep hole drilling machines with whip guides, through-spindle coolant, and chip handling systems are recommended.
What is the difference between gun drilling and BTA drilling?
The fundamental difference is chip evacuation. Gun drilling delivers coolant internally through the tool and evacuates chips externally along a V-groove. BTA drilling delivers coolant externally through the annular gap between the tool and bore wall, evacuating chips internally through the hollow tool shank. BTA offers higher feed rates (5–7×) and cleaner bore surfaces but requires higher coolant pressure and a pressure head seal.
What is the smallest hole that can be gun drilled?
Production gun drilling can reliably produce holes as small as 0.5 mm diameter. Micro gun drilling extends to approximately 0.2 mm with specialized equipment. Below this range, EDM or laser drilling become the practical alternatives.
How straight is a deep hole drilled bore?
With proper setup and stable conditions, deep hole drilling achieves straightness of 0.1–0.5 mm per meter of depth. High-end BTA drilling on dedicated machines can reach 0.05 mm/m or better. Counter-rotation (workpiece and tool rotating in opposite directions) further improves straightness by canceling out deflection forces.
Specifications and parameters vary by machine builder, tool manufacturer, and workpiece material. Consult your equipment and tooling supplier for application-specific recommendations.