Appearance
A manufacturer of hydraulic cylinder components for construction equipment was producing Ø80 mm × 1,800 mm bores in 4140 alloy steel (28–32 HRC) at a volume of 12,000 units per year. The existing process used gun drilling at Vc = 75 m/min, f = 0.08 mm/rev, yielding a penetration rate of 40 mm/min and cycle time of 45 minutes per bore. Ten single-spindle gun drilling machines running three shifts were required to meet production volume. Tool cost was $4.20 per bore. Surface finish was Ra 0.6 µm — well within the Ra 1.6 µm specification. The bottleneck was cycle time. A method evaluation compared: gun drilling (baseline), BTA single-tube drilling with indexable carbide inserts, BTA with brazed carbide heads, and ejector double-tube drilling. BTA drilling with indexable inserts at Vc = 140 m/min, f = 0.35 mm/rev achieved a penetration rate of 280 mm/min — a 7× improvement — reducing cycle time to 6.4 minutes per bore. The capital investment was $420,000 for two BTA machines plus $180,000 for a skiving and burnishing machine. The total cost per bore including tooling, labor, and machine amortization was $3.80 for BTA + burnishing, versus $6.20 for gun drilling — a 39% cost reduction. Payback period was 14 months. The surface finish after BTA drilling was Ra 1.8 µm, which required a subsequent skiving and burnishing operation to meet the Ra 0.3 µm specification for hydraulic cylinder sealing surfaces. The combined BTA + burnishing process produced bore straightness of 0.03 mm/m and surface finish of Ra 0.2 µm.
The Three Deep Hole Drilling Methods
Deep hole drilling — defined as hole depth exceeding 10× diameter — is performed by three primary methods: gun drilling, BTA (single tube system, STS), and ejector drilling (double tube system, DTS). Each method uses a fundamentally different chip evacuation mechanism, which determines its achievable diameter range, penetration rate, coolant pressure requirements, and surface finish capability.
Comparison of Methods
| Parameter | Gun Drilling | BTA Drilling (STS) | Ejector Drilling (DTS) |
|---|---|---|---|
| Diameter range | 0.5–50 mm (typical 1–30 mm) | 18–1,250 mm (typical 20–200 mm) | 18–200 mm (typical 20–65 mm) |
| Max L/D ratio | 100:1 (up to 200:1 with special tools) | 400:1 | 100:1 |
| Relative penetration rate | 1× (baseline) | 5–10× | 3–5× |
| Typical penetration (mm/min at Ø20 mm) | 30–80 | 150–400 | 80–250 |
| Coolant delivery | Internal through drill | External annulus between tube and bore wall | Between inner and outer tubes |
| Chip evacuation | External V-flute (22–26% of hole area) | Internal through hollow tube (>60% of hole area) | Internal through inner tube (Venturi effect) |
| Coolant pressure | 50–200 bar | 30–150 bar | 15–50 bar |
| Coolant flow (at Ø20 mm) | 40–80 L/min | 80–200 L/min | 100–250 L/min |
| Pressure head/seal required | No (rotating or stationary bush) | Yes (pressure head seals against workpiece face) | No (self-sealing double tube system) |
| Surface finish Ra | 0.2–0.8 µm | 0.8–3.2 µm | 0.8–2.5 µm |
| Bore straightness (mm/m) | 0.05–0.15 | 0.1–0.3 | 0.15–0.4 |
| Typical tolerance (IT grade) | IT7–IT9 | IT9–IT11 | IT9–IT11 |
| Machine capital cost | Lower ($80k–$250k per spindle) | Higher ($200k–$600k per spindle) | Similar to BTA |
| Tool cost per bore | $2–$8 (solid carbide) | $1–$4 (indexable inserts) | $1–$5 |
| Setup time per job | 10–20 min | 20–45 min | 15–30 min |
| Best for small diameters (< 12 mm) | Yes — only practical option | No — minimum diameter ~18 mm | No — minimum diameter ~18 mm |
| Best for high production volume | Low to moderate | High | Moderate to high |
| Best for retrofitting existing machines | Moderate | Low (needs pressure head) | High (no pressure head, works on lathes) |
| Suitable for interrupted surfaces | No (sensitive to starting surface condition) | No (seal requires smooth face) | Yes (no face seal required) |
Chip Evacuation Mechanism Details
The chip evacuation area is the defining characteristic that determines each method's material removal rate capability:
Gun drilling uses a single-flute tool with a V-shaped groove that occupies 22–26% of the hole cross-sectional area. Coolant is delivered at high pressure (50–200 bar) through an internal coolant channel in the drill tube, and chips are flushed back through the V-groove external to the tube. The limited chip clearance area restricts the feed rate — if too high, chips pack in the groove and cause tool jamming or breakage. The maximum chip cross-sectional area for a gun drill is approximately (π × D²/4) × 0.22 × (chip compression ratio), which limits the product of feed × cutting speed.
BTA drilling delivers coolant through the annular space between the drill tube and the bore wall, and chips evacuate through the hollow interior of the tube. The chip evacuation area is the internal tube diameter — typically >60% of the hole cross-sectional area. This large chip clearance allows 5–10× higher feed rates than gun drilling. The annular coolant passage also provides more effective cooling of the cutting zone because coolant flows directly over the cutting edge before reversing direction and carrying chips through the tube interior.
Ejector drilling uses a double tube system: coolant flows between the inner and outer tubes to the cutting head. Approximately two-thirds of the coolant is ejected through the cutting head and returns through the inner tube, while one-third is diverted through Venturi nozzles in the drill head that create a suction effect (ejector principle) to assist chip evacuation. This Venturi effect means ejector drilling does not require a pressure head sealed against the workpiece face — the suction draws chips through the inner tube even when the starting face is irregular or interrupted.
Method Selection Framework
Selection by Diameter
Diameter is the primary constraint in method selection because each method has a practical minimum and optimal range:
| Diameter Range | Recommended Method | Rationale |
|---|---|---|
| 0.5–12 mm | Gun drilling only | BTA and ejector minimum diameter ~18 mm due to chip passage constraints |
| 12–18 mm | Gun drilling (preferred) or ejector | Gun drilling provides better tolerance and surface finish at these diameters |
| 18–50 mm | Gun drilling (low volume/precision) or BTA (high volume) | BTA becomes economical above approximately 1,000 bores/year at Ø20 mm |
| 50–200 mm | BTA drilling (preferred) or ejector drilling | BTA provides highest penetration rate; ejector useful for retrofitting |
| 200–500 mm | BTA drilling only | Gun and ejector are not practical at these diameters |
| >500 mm | BTA drilling (special large-diameter heads) | Special BTA trepanning or counter-boring heads |
Selection by Production Volume
Production volume determines whether the higher capital cost of BTA drilling is justified by the lower per-bore cost:
| Annual Volume | Ø12 mm | Ø25 mm | Ø50 mm | Ø100 mm |
|---|---|---|---|---|
| <500 bores | Gun (any diameter within range) | Gun | Gun | BTA (only option above 50 mm) |
| 500–2,000 | Gun | Gun | BTA if volume >1,000 | BTA |
| 2,000–10,000 | Gun | BTA (evaluate at >5,000) | BTA | BTA |
| >10,000 | BTA (if diameter permits) or multi-spindle gun | BTA | BTA (consider multi-spindle) | BTA (dedicated machine) |
Selection by Surface Finish Requirement
Surface finish requirements can override other selection criteria. When bore surface finish of Ra < 0.8 µm is required, gun drilling is preferred for diameters within its range. When larger diameters (above 18–25 mm) require Ra < 0.8 µm, a two-step process is typically used: BTA drilling for roughing followed by skiving and burnishing or honing for finishing.
| Surface Finish Requirement | Diameter < 18 mm | Diameter 18–50 mm | Diameter > 50 mm |
|---|---|---|---|
| Ra < 0.4 µm | Gun drilling + burnishing | BTA + skiving/burnishing | BTA + skiving/burnishing |
| Ra 0.4–1.0 µm | Gun drilling | Gun drilling or BTA + finishing | BTA + finish boring |
| Ra 1.0–2.0 µm | Gun drilling | BTA drilling | BTA drilling |
| Ra > 2.0 µm | Gun drilling | BTA drilling | BTA drilling |
Selection by Existing Machine Infrastructure
When retrofitting an existing conventional machine tool (CNC lathe, milling machine, or machining center), ejector drilling offers the fastest implementation path because: no pressure head is required (the double tube system is self-sealing), the tool can be presented to a rotating or stationary workpiece, coolant pressure requirements are lower (15–50 bar, achievable with a moderate coolant pump upgrade), and the tool system can be mounted on the machine turret or tool changer. BTA retrofitting requires a pressure head assembly and higher coolant pressure (30–150 bar), which typically requires a significant machine modification. Gun drilling retrofitting is feasible for smaller diameters but requires high-pressure coolant (100–200 bar) and a guide bush support system.
FAQ
What is the most important factor in selecting a deep hole drilling method?
The diameter of the hole is the primary constraint because each method has a minimum practical diameter determined by its chip evacuation mechanism. Gun drilling is the only method available for diameters below 12 mm. For diameters between 18 and 50 mm, both gun drilling and BTA are technically feasible, and the selection is driven by production volume, surface finish requirements, and existing equipment. Above 50 mm, BTA or ejector drilling is required. The second most important factor is production volume — BTA drilling requires higher capital investment but delivers 5–10× higher penetration rates, making it the preferred method for production volumes above approximately 1,000–5,000 bores per year (depending on diameter).
Can BTA drilling achieve the same surface finish as gun drilling?
BTA drilling typically produces Ra 0.8–3.2 µm versus Ra 0.2–0.8 µm for gun drilling. The rougher surface is a result of the indexable insert cutting geometry and the chip formation mechanics of the BTA process — the cutting action of multiple inserts with corner radii of 0.4–1.2 mm produces a feed-mark pattern that is more pronounced than the single-point cutting action of a gun drill. However, for many applications (hydraulic cylinders, structural components, oilfield equipment), Ra 1.6–3.2 µm is acceptable. When finer finishes are required from BTA-drilled bores, a subsequent finishing operation — skiving and burnishing (Ra 0.05–0.40 µm), honing (Ra 0.1–0.8 µm), or finish boring — is applied. The economics of the combined process are often favorable because BTA drilling's higher penetration rate reduces the roughing cost enough to offset the additional finishing operation.
What are the advantages of ejector drilling over BTA drilling?
Ejector drilling offers three distinct advantages over conventional BTA drilling: no pressure head is required (the double tube system is self-sealing, so the workpiece face does not need to be smooth or perpendicular), it can be easily retrofitted to existing conventional machine tools (lathes, machining centers) without major machine modifications, and it operates at lower coolant pressure (15–50 bar versus 30–150 bar for BTA), reducing the coolant system cost and complexity. The disadvantages of ejector drilling compared to BTA are: lower penetration rate (typically 50–70% of BTA), limited diameter range (18–200 mm versus 18–1,250 mm for BTA), and lower maximum L/D ratio (100:1 versus 400:1). Ejector drilling is the preferred choice when retrofitting existing machines or when the workpiece has an irregular starting surface.
When is gun drilling the preferred choice despite lower penetration rates?
Gun drilling is the preferred choice in five situations: diameters below 12 mm (where BTA and ejector are not feasible), applications requiring bore surface finish below Ra 0.8 µm (gun drilling produces the best as-drilled finish of any deep hole drilling method), tight tolerance requirements (IT7–IT9 is achievable without secondary operations), low-to-moderate production volumes (gun drilling machine capital cost is lower, and the per-bore tooling cost is competitive at low volumes), and small lot sizes with frequent changeovers (gun drilling setup time is 10–20 minutes versus 20–45 minutes for BTA). Many job shops and tool-and-die operations use gun drilling exclusively because of its flexibility and the wide range of diameters achievable with a single machine platform.
Can both rotating workpiece and rotating tool configurations be used with all methods?
Not all methods are equally suited to both configurations. Gun drilling works well in both configurations: rotating workpiece (workpiece spins, tool feeds linearly, typical for shaft-type parts on gun drilling machines) and rotating tool (tool spins and feeds, typical for machining centers and retrofit applications). BTA drilling is most commonly configured as rotating workpiece with stationary tool, because the pressure head must maintain a sealed interface with the rotating workpiece face. BTA can be configured as rotating tool, but this requires a rotating coolant coupling (rotary union) capable of handling high pressure (30–150 bar) and a carefully designed pressure head that seals against a stationary workpiece face — this adds complexity and cost. Ejector drilling is the most flexible: it works well in both rotating workpiece and rotating tool configurations, and is the preferred choice for retrofit applications where a rotating tool configuration on a standard machining center is required.
Disclaimer: The process parameters, method selection recommendations, and performance data presented in this article are based on published technical literature, machine tool manufacturer specifications, and industry-reported experience with deep hole drilling method selection. Actual results depend on specific workpiece material and geometry, production volume requirements, existing machine tool infrastructure, coolant system capability, and quality requirements. The selection guidelines provided should be used as a starting framework and verified through process development trials for each specific application. BTA and ejector drilling require appropriate machine tool rigidity, coolant system capability, and operator training to realize their full potential. No guarantee of specific cost savings, cycle time reduction, or quality outcomes is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.