Appearance
A manufacturer drilling a 25 mm diameter hole at 100× depth faces a choice between three fundamentally different technologies. The wrong selection can mean 7× slower production, 10× higher tool cost, or a machine investment that exceeds the part margin for the life of the contract.
Gun drilling, BTA, and ejector drilling are the three standard processes for producing deep holes with depth-to-diameter ratios exceeding 10:1. While all three can produce deep, straight holes with good surface finish, they differ fundamentally in their operating principle, application range, productivity, and machine and tooling cost structure.
Selecting the correct process for a given application requires understanding these differences across multiple dimensions simultaneously — diameter, depth, precision, material, volume, and capital available. This article provides a complete technical and economic comparison to support that decision.
Operating Principles
The three processes are distinguished by their coolant delivery and chip evacuation paths.
Gun Drilling
Gun drilling is the oldest and most widely used deep hole drilling process, particularly for diameters below 20 mm.
The gun drill is a single-lip tool with a kidney-shaped cross-section. High-pressure coolant (30–120 bar) is delivered through the drill shank to the cutting tip, where it cools the cutting edge and flushes chips along the external V-groove on the drill body. The chip returns along this groove to the outside of the bore, carried by the coolant flow.
The single cutting edge produces a characteristic hole geometry and requires a guide bushing or pilot hole at entry to stabilise the tool. The drill body is supported by the bore wall through carbide guide pads that burnish the surface as drilling progresses.
BTA Drilling
BTA (Boring and Trepanning Association) drilling, also known as STS (Single Tube System), was developed for larger diameters and higher productivity.
The BTA drill head carries multiple cutting inserts (typically 2–4 depending on diameter) mounted on a thick-walled cylindrical tube. Coolant is pumped at high pressure (50–200 bar) through the annulus between the outside of the drill tube and the bore wall. Chips and coolant return through the hollow interior of the drill tube.
The BTA system requires a pressure head assembly that seals against the workpiece face and guides the drill tube at the entry point. The pressure head also provides the chip-tight seal that forces coolant to flow down the external annulus and return internally.
Ejector Drilling
Ejector drilling, also known as DTS (Double Tube System), is a variant of BTA that eliminates the need for a pressure head seal.
The ejector drill uses a double-tube assembly — an inner tube inside an outer drill tube. Coolant is pumped between the inner and outer tubes to the cutting head. The coolant then returns through the inner tube, carrying chips with it. Approximately two-thirds of the coolant flows to the cutting zone; the remaining one-third is diverted through angled slots in the ejector head to create a Venturi effect that draws the chip-laden coolant through the inner tube.
The key advantage is that no external pressure head seal is required — the coolant circuit is entirely contained within the tool assembly. This makes ejector drilling suitable for conventional machine tools where a pressure head cannot be fitted.
Diameter and Depth Capability Comparison
| Parameter | Gun Drilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Typical diameter range | 1–50 mm | 20–630 mm | 20–200 mm |
| Micro-drilling diameter | 0.2–1 mm | Not available | Not available |
| Practical minimum diameter | 0.2 mm (micro) | 19 mm | 18 mm |
| Maximum diameter (standard) | 50 mm | 250 mm (solid) / 630 mm (trepanning) | 200 mm |
| Depth-to-diameter ratio (standard) | Up to 400:1 | Up to 100:1 (400:1 with specialised setups) | Up to 100:1 |
| Maximum practical depth | 15,000 mm | 20,000 mm | 2,000 mm |
Gun drilling is the only option for diameters below 19 mm. BTA covers the widest diameter range at the top end. Ejector drilling occupies the middle band with depth limitations.
Surface Finish and Tolerance
| Parameter | Gun Drilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Surface finish (Ra) | 0.2–0.8 µm | 0.4–6.3 µm | 1.6–6.3 µm |
| Typical tolerance (IT grade) | IT7–IT8 | IT7–IT10 | IT8–IT11 |
| Hole straightness | 0.5/1000 of depth | 0.5–1.0/1000 of depth | 1.0/1000 of depth |
Gun drilling produces the best surface finish and dimensional accuracy of any deep hole drilling process. The single-lip design with carbide guide pads burnishes the bore surface as the tool advances, creating a characteristic smooth finish that often eliminates the need for secondary honing or reaming.
BTA drilling produces acceptable surface finishes for most engineering applications but generally requires secondary operations (honing, roller burnishing, or skiving) when surface roughness below Ra 0.8 µm is specified. The multi-insert cutting action leaves a characteristic surface pattern with fine feed marks.
Ejector drilling produces finishes comparable to the lower end of BTA quality. The Venturi-based chip evacuation system can affect process stability, leading to greater surface finish variation than controlled BTA drilling.
Coolant Systems and Chip Evacuation
Coolant system requirements differ substantially between the three processes and directly affect machine design and cost.
| Parameter | Gun Drilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Coolant pressure | 30–120 bar | 50–200 bar | 20–80 bar |
| Coolant flow rate | Moderate | High | Moderate |
| Coolant path | Internal through tool | External annulus → internal return | Between tubes → internal return |
| Chip exit path | External V-groove | Internal through tube | Internal through inner tube |
| Filtration requirement | 25–50 µm | 25–50 µm | 25–100 µm |
| Pressure head required | No (guide bushing) | Yes | No |
Chip evacuation reliability is the single most important operational difference. BTA (STS) has the most reliable chip evacuation because the full coolant flow is available to transport chips through the large-diameter interior of the drill tube. The high coolant pressure also assists in breaking chips at the cutting edge, which is essential for materials that form long ribbons.
Gun drilling chip evacuation depends on the V-groove being unobstructed. In deep holes (beyond 200× diameter), chip accumulation in the groove can cause friction and tool binding. Chip morphology control through parameter selection is more critical for gun drilling than for BTA.
Ejector drilling chip evacuation is the least robust. The Venturi effect that drives chip return is sensitive to chip size and shape — large or stringy chips can block the ejector nozzles or bridge inside the inner tube. For this reason, ejector drilling is less suitable for materials that form difficult-to-break chips.
Key industry reference: Sandvik Coromant experts state that "the high cutting fluid pressure makes the STS-system more reliable than the Ejector system, especially when drilling materials where good chipbreaking is difficult to obtain."
Feed Rates and Productivity
The productivity difference between gun drilling and BTA is the largest single factor in process selection.
| Metric | Gun Drilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Relative feed rate | Baseline (1×) | 5–7× | 3–5× |
| Typical feed (steel, 20 mm) | 0.02–0.04 mm/rev | 0.10–0.20 mm/rev | 0.08–0.15 mm/rev |
| Production efficiency | Baseline (1×) | 3×+ | 2–3× |
The BTA process achieves 5–7× higher feed rates than gun drilling at the same diameter because of three factors:
- Multiple cutting edges: A BTA head with 3 inserts removes material 3× faster per revolution than a single-lip gun drill
- Rigid tool support: The full-round BTA drill tube is torsionally stiffer than the kidney-shaped gun drill shank, allowing higher torque transmission without vibration
- Efficient chip evacuation: Internal chip return through the tube centre prevents chip congestion that limits gun drilling feed rates
For a 25 mm diameter hole in 4140 alloy steel at 80 m/min cutting speed, a gun drill at 0.03 mm/rev achieves approximately 0.85 cm³/min/mm of hole depth, while a BTA drill at 0.15 mm/rev achieves 4.25 cm³/min/mm — a 5:1 productivity advantage.
Tooling Configurations and Cost
| Factor | Gun Drilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Tool design | Single-lip brazed carbide | Multiple indexable inserts | Multiple indexable inserts |
| Tool body | Solid carbide or carbide-tipped steel shank | Steel drill tube with threaded head | Double-wall steel tube |
| Regrinding | Regrindable (2–3 cycles) | Indexable inserts replaced | Indexable inserts replaced |
| Tool cost (small dia) | Moderate | Higher | Higher |
| Tool cost (large dia) | Impractical | Cost-effective | Moderate |
| Tool change | Remove entire tool | Change head only (quick thread) | Change head only |
Gun drilling tooling: For diameters below 20 mm, gun drills are the most economical choice because BTA tooling is not available. The gun drill body can be reground 2–3 times, reducing per-hole tool cost. Above 20 mm, gun drill cost increases non-linearly with diameter due to carbide volume and shank complexity.
BTA tooling: BTA heads with indexable inserts have higher initial cost but lower per-edge cost in high-volume production. A single BTA tool body can be used for dozens of head changes. The indexable insert approach eliminates regrinding cost and provides consistent cutting geometry throughout tool life.
Brazed vs. indexable BTA: Brazed BTA heads have lower initial cost and are ground to a precise diameter (better tolerance), but must be discarded when worn. Indexable heads have higher initial cost but inserts and guide pads can be replaced, extending tool body life significantly. For production volumes above 100 parts per year, indexable BTA tooling typically has lower total cost.
Ejector tooling: Ejector tooling costs are comparable to BTA, but the dual-tube construction adds complexity and cost to the tool assembly. The replaceable head design provides similar per-edge economics to indexable BTA.
Machine Requirements
| Requirement | Gun Drilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Machine type | Dedicated gun drill or CNC with HPC | Dedicated BTA machine | Conventional lathe/MC with HPC or dedicated |
| Pressure head | No | Yes | No |
| Guide bushing | Yes | Yes (in pressure head) | Yes |
| Coolant system | Medium pressure (30–120 bar) | High pressure (50–200 bar) | Medium pressure (20–80 bar) |
| Filtration | 25–50 µm | 25–50 µm | 25–100 µm |
| Spindle power | Low to moderate | High | Moderate |
| Workpiece rotation capability | Common | Common | Common |
| Retrofit feasibility | Good (CNC lathe with through-coolant) | Poor (requires pressure head) | Good (no pressure head needed) |
The machine investment gap between the processes is significant. A dedicated BTA drilling machine with pressure head, high-pressure coolant system, and filtration can cost 2–3× an equivalent-capacity gun drilling machine. Ejector drilling equipment bridges this gap — it requires less investment than dedicated BTA because it eliminates the pressure head, but more than simple gun drilling because of the dual-tube tooling and supporting equipment.
Ejector drilling's ability to retrofit conventional machine tools is its primary market advantage. A standard CNC lathe or machining centre with through-spindle coolant can be equipped for ejector drilling with a toolholder modification, avoiding the capital cost of a dedicated deep hole drilling machine.
Process Selection Framework
Primary Constraint: Diameter
Diameter is the hard constraint that eliminates processes:
- Below 19 mm: Gun drilling is the only option. BTA and ejector tooling are not manufactured below this diameter range.
- 19–20 mm: Overlap zone. All three processes can theoretically operate. Gun drilling offers best precision; BTA offers highest productivity; ejector offers retrofit flexibility.
- 20–50 mm: All three processes available. Selection depends on depth, volume, and machine investment.
- 50–200 mm: BTA and ejector available. BTA preferred for high-volume or difficult materials; ejector for retrofit applications.
- Above 200 mm: BTA only. Ejector tooling maxes out at approximately 200 mm.
Secondary Constraints and Preferences
Depth ratio >100:1: Gun drilling or BTA only. Ejector drilling depth is limited to approximately 100:1.
Ultra-precision (IT7 or Ra <0.8 µm): Gun drilling preferred. BTA may require secondary finishing.
Production volume >1,000 parts/year in 20–50 mm range: BTA typically has lowest total cost per hole due to 5–7× higher feed rate and indexable tooling cost structure.
Small batch / prototype (1–50 parts): Gun drilling or ejector drilling preferred to avoid the setup complexity and machine investment of dedicated BTA.
Retrofit to existing conventional lathe: Ejector drilling (DTS) is the most practical choice. BTA pressure head installation is generally not feasible on conventional machines.
Difficult chip-forming materials (low-carbon steel, stainless, titanium): BTA (STS) has the most reliable chip evacuation. Ejector drilling's Venturi system is vulnerable to chip congestion in these materials.
Maximum hole depth >2,000 mm: Gun drilling or BTA required. Ejector drilling depth is practically limited to 2,000 mm.
Summary Comparison Table
| Factor | Gun Drilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Diameter range | 0.2–50 mm | 19–630 mm | 18–200 mm |
| Max depth ratio | 400:1+ | 400:1 | 100:1 |
| Surface finish (Ra) | 0.2–0.8 µm | 0.4–6.3 µm | 1.6–6.3 µm |
| Tolerance | IT7–IT8 | IT7–IT10 | IT8–IT11 |
| Feed rate (relative) | 1× (baseline) | 5–7× | 3–5× |
| Coolant pressure | 30–120 bar | 50–200 bar | 20–80 bar |
| Chip evacuation | External V-groove | Internal tube | Internal tube (venturi) |
| Pressure head | No | Yes | No |
| Retrofit capability | Good | Poor | Good |
| Machine investment | Low–moderate | High | Moderate |
| Tool cost (small dia) | Low | Not available | Not available |
| Tool cost (large dia) | Impractical | Low per edge | Moderate per edge |
| Regrind capability | Yes (2–3×) | No (indexable inserts) | No (indexable inserts) |
| Best for precision | ✅ | — | — |
| Best for productivity | — | ✅ | — |
| Best for flexibility | — | — | ✅ |
FAQ
What is the main difference between gun drilling and BTA drilling?
The coolant and chip evacuation path. Gun drilling delivers coolant through the tool and returns chips externally along a V-groove. BTA delivers coolant externally (between the drill tube and bore wall) and returns chips internally through the tube. This fundamental difference gives BTA higher feed rates (5–7×) and larger diameter capability, while gun drilling provides better surface finish and operates at smaller diameters.
Which deep hole drilling process has the best surface finish?
Gun drilling produces the best surface finish, typically Ra 0.2–0.8 µm, because the single-lip design with carbide guide pads burnishes the bore surface during drilling. BTA surfaces are typically Ra 0.4–6.3 µm, and ejector surfaces are in the 1.6–6.3 µm range.
Can ejector drilling replace BTA?
Not completely. Ejector drilling (DTS) is suitable for diameters 18–200 mm at depths up to 100:1, particularly when retrofitting conventional machine tools. However, BTA (STS) has more reliable chip evacuation for difficult materials (poor chip-breaking steels, stainless, titanium), higher depth capability (400:1), and higher maximum diameters (630 mm). BTA is preferred for long-series production, while ejector is preferred for retrofit flexibility.
What diameter range does each process cover?
Gun drilling covers 0.2–50 mm (micro-drilling from 0.2 mm). BTA covers 19–630 mm (solid drilling to 250 mm, trepanning to 630 mm). Ejector covers 18–200 mm. The overlap zone is approximately 19–50 mm where all three processes can be applied.
Do I need a dedicated machine for each process?
Gun drilling can be retrofitted to CNC lathes and machining centres with through-spindle coolant. BTA requires a dedicated machine with a pressure head assembly. Ejector drilling can be retrofitted to conventional machines but requires specialised toolholders and coolant delivery systems. Most high-volume production facilities use dedicated machines for each process.
Which deep hole drilling process is most cost-effective?
The most cost-effective process depends on diameter, volume, and quality requirements. For diameters below 19 mm, gun drilling is the only option. For 20–50 mm at high volume, BTA typically has the lowest cost per hole due to 5–7× higher feed rates. For low-volume work on existing conventional machines, ejector drilling avoids the capital investment of dedicated BTA.
What is the difference between BTA and STS?
BTA and STS (Single Tube System) refer to the same process. BTA is the historical name from the Boring and Trepanning Association that standardised the tooling system. STS is the more descriptive technical name for the single-tube coolant delivery design.
Can I use BTA tooling on a gun drilling machine?
Generally no. BTA requires a pressure head to seal coolant at the workpiece entry and redirect the return flow through the drill tube interior. Gun drilling machines lack this pressure head assembly. However, some modern machines (e.g., UNISIG UNI-50BTA) are designed for rapid changeover between gun drilling and BTA tooling.
What is the depth limit for each process?
Gun drilling and BTA can both exceed 400:1 depth-to-diameter ratios, with maximum practical depths of approximately 15,000 mm (gun) and 20,000 mm (BTA). Ejector drilling is limited to approximately 100:1 and 2,000 mm maximum depth, limited by the Venturi chip evacuation system's effectiveness at extended lengths.
How do I select the right deep hole drilling process for my application?
Start with diameter: below 19 mm → gun drilling only. Then depth ratio: above 100:1 → gun drilling or BTA. Then precision: IT7 or Ra <0.8 µm → gun drilling. Then volume: high volume in 20–50 mm → BTA has lowest per-hole cost. Then machine: existing conventional machine without retrofit capability → ejector drilling. Each dimension eliminates at least one process until the appropriate selection remains.
Conclusion
Gun drilling, BTA, and ejector drilling are complementary rather than competing processes, each occupying a distinct region in the deep hole drilling parameter space. Gun drilling dominates below 20 mm diameter and where maximum precision is required. BTA provides the highest productivity for medium-to-large diameters in high-volume production. Ejector drilling offers a practical compromise for conventional machine retrofitting and medium-depth applications.
The selection between these processes is not a matter of technological superiority but of matching process characteristics to application requirements across the dimensions of diameter, depth, precision, material, volume, and capital investment. For the majority of applications, the choice is straightforward: gun drilling for small precision holes, BTA for high-volume production above 20 mm, and ejector drilling for flexible retrofitted applications.
When the selection is not clear — typically in the 19–50 mm overlap zone — the decision should be validated through process testing with representative parts, as the interaction of material-specific chip formation with each process's chip evacuation system is difficult to predict theoretically and has a determining effect on process reliability and cost.