The Four Deep Hole Drilling Methods Explained: Gun Drilling, BTA, Ejector, and Trepanning

Not all deep holes are drilled the same way. In fact, the question “which method should I use?” is one of the first an engineer faces when a part requires a bore deeper than about 10× its diameter.

The answer depends on four variables: diameter, depth-to-diameter ratio, production volume, and tolerance requirements. Each of the four mainstream methods — gun drilling, BTA (single-tube system), ejector (double-tube system), and trepanning — occupies a distinct part of the application space.

This guide covers each method in detail, then provides a decision framework for selecting the right one.

Method 1: Gun Drilling (Single-Lip Drilling)

Gun drilling is the oldest deep hole drilling method, developed in the late 19th century for manufacturing firearm barrels. It remains the standard for small-diameter precision holes with extreme depth-to-diameter ratios.

How It Works

A gun drill is a single-lip cutting tool with an internal coolant channel running through its length. High-pressure coolant (typically 40–150 bar, up to 200 bar for small diameters) is pumped through this channel and exits at the cutting tip. The coolant lubricates and cools the cutting edge, then forces chips back along a V-shaped external flute (groove) that runs the length of the tool.

The tool has three structural components (UNISIG; Silmax):

1. Carbide tip — contains the cutting edge and guide pads, available in brazed or indexable configurations
2. Hardened alloy steel shank — a hollow tube with a V-shaped flute formed to the shank’s center, typically 110°–120° included angle
3. Clamping sleeve (driver) — cylindrical steel element for retention in the spindle

The carbide tip is slightly larger than the shank diameter so only the tip contacts the bore wall — the shank rotates freely inside the hole.

Key Specifications

Sources: UNISIG “What is Gundrilling”; Ingersoll brazed gundrill specs; Silmax catalog.

Tool Configurations

Brazed carbide tips: The most common configuration for diameters from 1.9 mm to 50 mm. The carbide head is silver-brazed to the steel shank. The tip’s nose grind features two basic angles (e.g., 30°–15° or 25°–20°) that balance cutting forces and distribute them to the guide pads. Brazed tools can be reground 5–15 times before replacement, giving them a lower per-edge cost in high-volume production (Machinery’s Handbook econometrics; UNISIG).

Indexable tip: Mechanically clamped replaceable inserts, available for diameters ≥12 mm. Eliminates regrinding; the insert is simply replaced when worn. Advantages include no sharpening labor, no length adjustment needed, and easy inventory management (Silmax; Estool).

Solid carbide: One-piece carbide head and shank for diameters 0.8–12 mm, offering maximum rigidity for the smallest diameters.

Guide Pad Function

The gun drill’s guide pads serve two critical functions: they support the tool against the bore wall to maintain alignment (the self-piloting effect), and they burnish the bore surface through plastic deformation — which is the primary reason gun drilling produces Ra 0.4–1.6 μm finishes in a single pass. Research by Weinert and Bruchhaus (Wear, 1999) demonstrated that guide pad condition has a greater influence on final surface quality than cutting edge sharpness.

When to Use Gun Drilling

• Hole diameters below 20 mm (where BTA tooling becomes impractical)
• L/D ratios exceeding 30:1
• Applications where surface finish must be good enough to eliminate reaming
• Low to medium production volumes (tooling cost per edge is lower than BTA at small diameters)
• Materials requiring extreme precision: medical implants, fuel injectors, firearm barrels

Method 2: BTA Drilling (Single-Tube System / STS)

BTA drilling — named after the Boring and Trepanning Association that standardized it in 1930s Germany — uses a fundamentally different coolant and chip flow path that enables much higher material removal rates than gun drilling.

How It Works

The BTA system uses a multi-edge cutting head mounted on a single hollow drill tube. High-pressure coolant is delivered externally — pumped through a pressure head (BOZA — Bohrölzuführapparat) that seals against the workpiece face. The coolant flows around the outside of the drill tube to the cutting zone, then chips and coolant are drawn back through the hollow center of the drill tube (UNISIG; Botek; Engineer Live).

The internal chip evacuation channel occupies more than 60% of the hole cross-section — roughly three times the clearance area of a gun drill. This large flow area is the fundamental reason BTA drilling can run at 5–10× the penetration rate of gun drilling at the same diameter.

The Pressure Head (BOZA)

The pressure head is the most distinctive component of the BTA system. It has three jobs (Botek; DeepHoleMachines):

1. Direct coolant — channels filtered, high-volume coolant through the annular space between the drill tube and the bore wall, delivering it to the cutting head
2. Seal the system — creates a high-pressure seal against the workpiece entry face and around the drill tube, containing the coolant pressure (typically 20–100 bar) during operation
3. Guide the tool — houses the drill bushing that provides initial tool guidance and alignment

The pressure head design requires the workpiece face to be flat and perpendicular to the bore axis, or a starting surface must be machined first. This is the main practical limitation of BTA compared to ejector drilling.

Key Specifications

Sources: UNISIG “What is BTA Drilling”; Botek BTA catalog; VDI 3209 Blatt 1; HTT machine specs.

Tool Configurations

Brazed BTA heads: Used for smaller diameters (12–20 mm). Carbide inserts and pads are brazed onto a steel body and ground to a precise diameter. Advantages: tightest tolerances, suitable for small diameters where indexable inserts cannot fit.

Indexable BTA heads: Used for diameters ≥20 mm. Inserts are mounted with retention screws or in replaceable cartridges. Key advantages (Engineer Live; UNISIG):

• Quick insert replacement — no brazing or regrinding
• Consistent cutting geometry — inserts are precision-ground by the manufacturer
• Indexable — multiple cutting edges per insert
• Chip breaker geometries optimized for specific materials (SP 1, SP 5, SP 2 from Botek)

Chip Formation and Evacuation

In BTA drilling, chip breakers play a decisive role. Chips must be broken “just short enough to ensure that there is no chip congestion,” but not so short that they strain the cutting edges (Botek). For long drill tubes and large diameters, a chip tube is used just behind the drill head to increase coolant flow velocity locally, ensuring chips are flushed out effectively.

The vibration damper is another critical component — it supports the drill tube and reduces longitudinal and torsional vibrations, improving hole surface quality and reducing cutting edge wear.

When to Use BTA Drilling

• Hole diameters above 20 mm (where the productivity advantage over gun drilling is significant)
• High production volumes (10,000+ parts/year) where faster cycle times justify higher machine investment
• Applications where the workpiece face can be prepared flat and perpendicular
• Materials that benefit from higher feed rates and robust chip evacuation

Method 3: Ejector Drilling (Double-Tube System / DTS)

Ejector drilling was developed by Sandvik Coromant in the 1970s to solve a specific practical problem: how to perform deep hole drilling on standard machine tools that lack a pressure head (Sandvik Coromant; UNISIG).

How It Works

The ejector system uses two concentric tubes — an inner tube and an outer tube — that create a Venturi suction effect for chip evacuation:

1. Coolant is introduced through a rotary union at the spindle and flows down the annular gap between the outer and inner tubes
2. At the drill head, the coolant splits into two paths:
   • Ejector flow (Qf₁): passes through Venturi slots (annular slits) that create a high-velocity jet
   • Working flow (Qf₂): passes through radial holes to the cutting edges for cooling and lubrication
3. The Venturi effect creates a partial vacuum (up to -0.06 MPa) in the inner tube
4. This suction draws the working coolant and chips back through the inner tube

The critical advantage: no pressure seal is required at the workpiece entry. This makes ejector drilling retrofittable onto standard CNC lathes, horizontal boring mills, and machining centers (Baucor; Botek).

The Venturi Effect in Detail

From fluid dynamics analysis (Sandvik; TU Dortmund research), the ejector’s performance is governed by three dimensionless numbers:

• Relative flow ratio (q_e): Qf₂ / Qf₁ — the ratio of working flow to ejector flow
• Relative hydraulic head (h_e): the head ratio across the ejector nozzle
• Ejector modulus (m_e): A_en / A_mc — nozzle area divided by mixing chamber area

The hydraulic efficiency is:

[ \eta_e = q_e \cdot \frac{h_e}{1 - h_e} ]

Maximum efficiency is approximately 20%, which is an inherent physical limitation of the ejector principle. Commercial ejector drills operate at lower efficiency (m_e > 10) to maximize suction rather than efficiency.

Key Specifications

Sources: Sandvik Coromant; Botek ejector drilling; Baucor deep hole drills; TU Dortmund research.

Design Configurations

Nozzle at drill head (original Faber patent): The ejector nozzle is located in the drill head. All coolant must travel to the drill head through the narrow annular gap before being split. This requires higher inlet pressure.

Nozzle at adapter (Sandvik standard design): The ejector nozzle is located at the adapter near the spindle. Only the working flow (Qf₂) travels to the drill head through the annular gap. This significantly reduces inlet pressure requirements but limits maximum drill length to approximately 1 meter due to pressure drop in the return line.

When to Use Ejector Drilling

• Retrofitting deep hole drilling capability onto existing CNC machines
• Mid-range diameters (18–200 mm) where dedicated BTA investment is not justified
• Workpiece faces that are irregular or non-square (cannot seal a BTA pressure head)
• Low to medium production volumes
• Shops entering deep hole drilling without capital investment in dedicated machines

Limitation to Consider

Ejector drilling requires higher coolant flow rates than BTA for the same diameter, and the maximum drill length is limited to approximately 1 meter with the standard Sandvik design. For deeper holes or higher efficiency, BTA remains the superior choice (TU Dortmund research; Botek).

Method 4: Trepanning

Trepanning is not a competing method to the three above — it is a specialized variant for very large diameters where preserving the center core has economic value.

How It Works

Instead of cutting the entire cross-section into chips, a trepanning head cuts an annular (ring-shaped) groove at the periphery of the hole, leaving a solid cylindrical core (slug) intact (Sandvik Coromant; Coastal Metals; TAES).

The trepanning head is essentially a hollow BTA-style tool with cutting inserts distributed around its circumference. Coolant and chip evacuation follow the BTA principle: coolant flows externally, chips exit through the hollow tool body, and the solid core passes through the center of the tool.

Key Specifications

Sources: Sandvik Coromant trepanning guide; Coastal Metals; TAES Trepanning; Hole Specialists Inc.

Applications for Trepanning

• High-cost materials (nickel alloys, titanium, stainless steel) — the preserved core has significant scrap value
• Large diameters where spindle power is limited — removing only the annular groove requires substantially less torque than drilling the full cross-section
• Core sample extraction for material testing and analysis
• Energy and heavy equipment — turbine shafts, valve bodies, pressure vessels, heat exchanger tube sheets

Economic Advantage

For a 300 mm diameter hole in Inconel 718, trepanning removes approximately 40% of the material that solid drilling would remove. The preserved 250 mm diameter core can be used to produce another part or sold as high-value scrap. At current nickel alloy scrap prices, this can offset 15–30% of the drilling cost.

Direct Comparison: All Four Methods

Sources: Consolidated from UNISIG, Botek, Sandvik Coromant, Baucor, AGrade Carbide, VDI standards, Coastal Metals, TAES.

Selection Decision Matrix

Step 1: Filter by Diameter

Step 2: Filter by Production Volume

Step 3: Filter by Tolerance

Step 4: Filter by Machine Available

Economic Comparison: Tooling Cost per Edge

The cost of cutting edges differs significantly between gun drilling (reground brazed carbide) and BTA/ejector (indexable inserts). Using the Machinery’s Handbook econometric formulas:

Gun drill (reground): [ C_E = \frac{\text{tool cost} + (\text{regrinds} \times \text{cost per regrind})}{1 + \text{regrinds}} ]

Example — $120 gun drill, 10 regrinds at $15 each:

• Cost per edge = ($120 + 10 × $15) / (1 + 10) = $24.55

BTA indexable insert: [ C_E = \frac{\text{insert cost}}{\text{edges per insert}} \times \frac{4}{3} + \frac{\text{cutter body}}{\text{body life in edges}} ]

Example — 4 inserts at $12 each, 3 edges per insert, $400 head, 2,000-edge body life:

• Cost per edge = 4 × ($12/3 × 4/3) + $400/2,000 = $21.53 + $0.20 = $21.73

At these costs, gun drilling and BTA tooling are remarkably close per edge. The economic differentiator is cycle time: BTA’s 5–10× faster penetration rate means lower machine cost per hole, which at high volumes dominates total cost.

Operating Mode Considerations

All four methods can be operated in three configurations, which affect straightness and productivity:

Source: UNISIG; Tiefbohrbär; HTT machine specifications.

Counter-rotation cancels the radial component of the cutting force, which is the primary cause of drift. Aerospace landing gear components (requiring straightness of 0.05 mm/m) are almost always counter-rotated.

Summary

Key Sources

1. UNISIG, “What is Gundrilling,” “What is BTA Drilling,” “Deep Hole Processes” — manufacturer technical reference library
2. Botek, “BTA System” and BTA catalog (111-204-Katalog_BTA_E_2021) — BTA/STS technical specifications
3. Sandvik Coromant, “Innovations Have Changed the Use of Coolants” and Trepanning Guide — ejector drill history and trepanning process
4. Silmax, single-lip drill product catalog — gun drill geometry specifications
5. Baucor, “Deep Hole Drills: Gundrills, BTA & Ejector Drills” — product and process comparison
6. VDI 3209 Blatt 1, “Deep hole boring systems with external supply of coolant” — BTA coolant reference
7. Coastal Metals, “Trepan vs. Gun-Drill: Choosing the Right Deep-Hole Technique” — trepanning comparison
8. TAES, “Trepanning” — trepanning process specifications
9. Engineer Live, “Brazed versus indexable tooling” — BTA tooling comparison
10. AGrade Carbide, “Types of Deep Hole Drilling Machines” — machine classification
11. Machinery’s Handbook (27th/29th editions) — tooling econometric formulas
12. Weinert, K., Bruchhaus, T., “Tribological investigations into the operational behavior of self-piloting drilling tools,” Wear, 1999 — guide pad research
13. TU Dortmund, ISF research on ejector drilling — SPH simulation and Venturi analysis
14. UNISIG, “Gunning for Better Results” and “BTA: Brazed vs Indexable Tooling” — technical articles
15. Accurate Edge UAE, “Gun Drilling UAE: Deep Hole Drilling for Oil & Gas” — coolant pressure data
16. HTT BTA machine specifications — straightness and surface finish data
17. Tiefbohrbär GmbH — precision deep hole drilling quality specifications
18. Hole Specialists Inc., “Trepanning Services” — trepanning diameter and depth capabilities
19. Rapid-protos, “Deep Hole Drilling: Methods, Tolerance & Cost Guide” — cost estimation
20. ScientificDirect, matrix selection for drilling methods — decision framework
21. Estool Carbide, “What Are Gun Drilling Inserts and How Do They Work?” — indexable gun drill design
22. Sandvik Coromant, “Recommendations for Correct Trepanning” — trepanning best practices