Appearance
A manufacturer of oilfield valve bodies was drilling Ø50 mm × 1,200 mm bores (L/D 24:1) in 4130 steel (22 HRC) for Christmas tree valve blocks. The existing process used a single-point boring bar on a horizontal boring mill, requiring three passes with a total cycle time of 8.5 hours per valve body. The company compared BTA drilling with a pressure head (requiring a flat, sealed entry surface and a dedicated machine at €450,000) versus ejector drilling retrofitted onto an existing CNC lathe (€95,000: €55,000 for tooling, €25,000 for coolant pump upgrade to 50 bar, €15,000 for chip conveyor and filtration). The ejector process achieved Ø50 mm × 1,200 mm in a single pass at Vc = 120 m/min, f = 0.35 mm/rev, 267 mm/min penetration, with drilling time of 4.5 minutes per bore. Bore straightness was 0.06 mm/m, diameter IT9, surface finish Ra 3.2 µm. The single-point boring process was eliminated, reducing total machining time from 8.5 hours to 45 minutes per valve body.
Ejector Drilling Principles
How the Ejector System Works
The ejector drilling system uses a double-concentric tube assembly: an outer tube (typically 1.5–2.0 mm wall thickness) and an inner tube (typically 1.0–1.5 mm wall thickness), with the drill head attached to the end of the outer tube. Coolant is pumped into the annulus between the outer and inner tubes at moderate pressure (10–50 bar). At the drill head end, approximately two-thirds of the coolant flows through the drill head to the cutting zone (providing cooling and lubrication), while the remaining one-third passes through specially designed ejector nozzles or slots near the drill head. These ejector nozzles create a Venturi effect — the high-velocity coolant passing through the nozzles creates a low-pressure region that draws the chip-laden coolant from the cutting zone back through the inner tube. The chip-laden coolant then flows through the inner tube and out of the system to the filtration and cooling system.
The key advantages of this design are: no high-pressure seal at the workpiece is needed — the chip evacuation is driven by the Venturi effect rather than by external pressure; the coolant pressure requirement is lower (10–50 bar versus 10–100 bar for BTA STS and 50–200 bar for gun drilling); and the system can be used on standard machine tools without a dedicated pressure head feed unit.
Ejector Drilling vs BTA Single Tube System
| Parameter | Ejector Drilling (Double Tube) | BTA Drilling (Single Tube) |
|---|---|---|
| Tube configuration | Double concentric tube (coolant in annulus, chips through inner tube) | Single tube (coolant in annulus between tube OD and bore ID, chips through tube ID) |
| Coolant pressure | 10–50 bar (moderate) | 10–100 bar (low to high) |
| Coolant flow | Higher (ejector flow + cutting zone flow) | Lower (all coolant through cutting zone) |
| Pressure head seal | Not required | Required at workpiece entry |
| Workpiece entry surface | No special requirement; irregular surfaces acceptable | Requires flat, sealed surface perpendicular to bore axis |
| Retrofit on standard machines | Excellent — can be fitted to standard lathes and machining centers | Limited — requires pressure head feed unit |
| Diameter range | 18–180 mm (typical), up to 250 mm (special) | 18–300 mm (standard), up to 1,250 mm (special) |
| L/D ratio capability | Up to 80:1 | Up to 100:1 |
| Penetration rate (steel) | 80–250 mm/min | 100–400 mm/min |
| Surface finish Ra (steel) | 2.0–5.0 µm | 1.5–4.0 µm |
| Diameter tolerance | IT8–IT10 | IT8–IT10 |
| Tool cost | Moderate (BTA-type head with indexable inserts) | Moderate (BTA head with indexable inserts) |
| System cost (retrofit) | €80,000–150,000 | €100,000–250,000 (including pressure head) |
| Machine cost (new) | €250,000–500,000 | €250,000–600,000 |
Recommended Cutting Parameters for Ejector Drilling
| Material | Vc (m/min) | f (mm/rev) | Coolant Pressure (bar) | Penetration Rate (mm/min) | Expected Ra (µm) | Expected Tool Life (m) |
|---|---|---|---|---|---|---|
| Low-carbon steel (1018, 1026) | 120–180 | 0.25–0.50 | 15–30 | 200–450 | 2.0–4.0 | 200–500 |
| Alloy steel (4140, 4340) annealed | 80–130 | 0.20–0.40 | 20–40 | 120–320 | 2.0–4.5 | 150–400 |
| Alloy steel (4140, 4340) Q&T (28–35 HRC) | 60–100 | 0.15–0.30 | 25–50 | 90–240 | 2.5–5.0 | 100–300 |
| Stainless steel (304, 316) | 60–90 | 0.12–0.25 | 30–50 | 72–180 | 3.0–5.5 | 60–200 |
| Cast iron (gray) | 80–140 | 0.30–0.60 | 10–25 | 240–600 | 2.5–5.0 | 200–600 |
| Cast iron (CGI) | 50–70 | 0.15–0.30 | 20–40 | 75–150 | 3.0–5.5 | 40–120 |
| Aluminum (6061, 7075) | 200–400 | 0.30–0.60 | 10–20 | 600–1,200 | 1.5–3.5 | 500–2,000 |
| Copper alloys | 100–250 | 0.20–0.45 | 10–25 | 200–500 | 2.0–4.0 | 300–1,000 |
Tooling System Components
Ejector Drill Head Design
| Component | Function | Typical Design | Material | Replacement Interval |
|---|---|---|---|---|
| Cutting inserts (indexable, typically 3–5 per head) | Remove material; form the bore | ISO standard trigon or square inserts with chip breaker; coated carbide (TiAlN, AlCrN) | Carbide P20–P40 grade | Every edge: 50–300 m (steel); index 3–6 times per insert |
| Guide pads (2–4 per head) | Guide the head in the bore; burnish the bore surface | Rectangular or wedge-shaped pads brazed or clamped into head body | Carbide K10–K20; PCD for high-wear materials | Every 20–100 bores depending on material abrasiveness |
| Head body | Holds inserts and guide pads; connects to outer tube | Precision-machined from alloy steel or heat-treated stainless | 4140 HT (35–42 HRC) or 17-4 PH | 500–2,000 bores (body life) |
| Ejector nozzle ring | Creates Venturi effect; directs ejector flow | Annular ring with angled slots or holes positioned behind the drill head | Hardened steel or carbide | 200–1,000 bores (wear from coolant flow) |
| Outer tube connection | Threaded connection between head body and outer tube | API-type or custom thread | Alloy steel HT | 200–500 cycles (thread life) |
Double Tube Assembly
| Component | Function | Material | Typical Dimensions | Maintenance |
|---|---|---|---|---|
| Outer tube | Carries coolant to the drill head; provides torsional drive | Carbon steel (seamless, cold drawn) or alloy steel | OD: drill diameter − 3–6 mm; Wall: 1.5–2.5 mm; Length: bore depth + 300 mm | Inspect for straightness (bend < 0.2 mm/m); replace if worn OD below minimum |
| Inner tube | Carries chips back to the machine exit; provides the return flow path | Carbon steel or stainless steel | OD: outer tube ID − 2–4 mm (annulus gap); Wall: 1.0–2.0 mm | Inspect for wear (inner tube ID increases from chip abrasion); replace if wall < 0.8 mm |
| Tube connector / coupling | Connects tube sections (for deep bores requiring multiple tube lengths) | Alloy steel HT | Threaded or flanged | Inspect threads each use; replace if thread wear exceeds 50% of thread height |
| Coolant inlet / rotary union | Transfers coolant from stationary supply to rotating outer tube | Multi-seal assembly | Custom to machine spindle | Seal replacement: 1,000–3,000 hours; bearing replacement: 5,000–8,000 hours |
Retrofit Considerations
Machine Tool Requirements for Ejector Drilling Retrofit
| Machine Requirement | Minimum Specification | Preferred Specification | Retrofit Cost if Not Existing |
|---|---|---|---|
| Spindle power | 30 kW (for Ø50 mm in steel) | 50+ kW | €20,000–60,000 (spindle motor upgrade) |
| Spindle speed range | 100–3,000 RPM | 50–4,000 RPM | Typically sufficient on most CNC lathes |
| Feed force (Z-axis) | 20 kN | 40+ kN | Usually sufficient on standard CNC lathes |
| Coolant pump | 20 bar, 200 L/min | 50 bar, 500 L/min | €15,000–35,000 (pump + plumbing upgrade) |
| Coolant filtration | 50 µm paper band | 30 µm paper band + magnetic separator | €15,000–30,000 |
| Chip conveyor | Hinged belt or drag chain | Heavy-duty drag chain | €10,000–25,000 |
| Coolant tank | 1,000 L | 3,000–5,000 L | €5,000–15,000 |
| CNC control | Standard | Standard with M-codes for coolant control | No change typically required |
| Rotary union (through-spindle coolant) | 50 bar rated | 70 bar rated | €8,000–20,000 |
| Tool holding | Hydraulic or mechanical chuck for outer tube | Custom collet system for tube diameter | €3,000–8,000 |
FAQ
What is the main advantage of ejector drilling over BTA single tube drilling?
The main advantage of ejector drilling is that it does not require a high-pressure seal between the drill head and the workpiece. In BTA single tube (STS) drilling, coolant is pumped into the annulus between the drill tube OD and the bore ID, and a pressure head seal at the workpiece entry is required to contain the coolant pressure and direct it into the annulus. This seal requires: a flat, machined entry surface on the workpiece perpendicular to the bore axis; a pressure head (a specialized clamping and sealing device) mounted on the machine feed unit; and the workpiece to be positioned such that the pressure head can seal against the entry surface. These requirements add significant cost and complexity and prevent BTA STS drilling of parts with irregular or non-flat entry surfaces. Ejector drilling eliminates the pressure head entirely because the chip evacuation is driven by the Venturi effect within the tool itself — the coolant pressure difference that drives chip flow is generated at the drill head, not at the workpiece entry. This means: the workpiece entry surface can be irregular (casting surface, forged surface, angled face); no pressure head or specialized feed unit is required (the tool can be mounted in a standard tool holder); the system can be easily retrofitted onto standard lathes and machining centers; and the setup time is shorter because there is no pressure head alignment.
What is the Venturi effect in ejector drilling?
The Venturi effect in ejector drilling works as follows: coolant is pumped into the annulus between the outer and inner tubes at 10–50 bar. At the drill head end, a portion of the coolant (typically 30–40%) is diverted through specially designed ejector nozzles or slots positioned in the head body or a nozzle ring. These nozzles are designed with a converging-diverging cross-section that accelerates the coolant to high velocity, creating a low-pressure region at the nozzle exit (the Venturi effect — the same principle used in a carburetor or a vacuum ejector). This low-pressure region creates a suction effect that draws the chip-laden coolant from the cutting zone through the inner tube. The remaining 60–70% of the coolant flows through the drill head to the cutting zone, providing cooling and lubrication. The ejector action is self-regulating — as the drilling resistance changes (e.g., tool wear, material hardness variation), the back pressure in the cutting zone changes, which automatically adjusts the pressure differential across the ejector nozzles and maintains approximately constant chip evacuation flow. The magnitude of the Venturi effect depends on: the ratio of ejector flow to total flow (30–40% is optimal); the nozzle geometry (converging angle 15–20°, diverging angle 8–12°); the nozzle cross-sectional area relative to the annulus area; and the coolant supply pressure.
What are the limitations of ejector drilling?
Ejector drilling has four main limitations compared to BTA STS: lower maximum L/D ratio — ejector drilling is typically limited to L/D 80:1 versus 100:1 for BTA STS. The Venturi effect becomes less effective at very long bore depths because the flow resistance in the inner tube increases with length, reducing the pressure differential available for chip evacuation. Lower pressure capability — the double tube design limits the maximum coolant pressure to approximately 50 bar (higher pressures risk collapsing the inner tube or causing excessive deflection of the outer tube). This makes ejector drilling less suitable for materials requiring high coolant pressure (stainless steel > 150 bar, titanium > 180 bar). Lower penetration rate — typically 20–40% lower than BTA STS for the same material and diameter, because the ejector flow (30–40% of total coolant bypassing the cutting zone) reduces the coolant available for chip evacuation from the cutting zone. Higher coolant flow requirement — the total coolant flow is higher for ejector drilling than for BTA STS because 30–40% of the flow is used for the ejector effect rather than for cutting zone cooling. This requires a larger coolant pump and filtration system for the same bore diameter. The practical implication is that ejector drilling is best suited for moderate L/D ratios (up to 80:1), moderate materials (carbon steel, alloy steel, cast iron, aluminum), and applications where the BTA pressure head requirement is a barrier.
Can ejector drilling be used on a standard CNC lathe?
Yes, ejector drilling can be retrofitted onto a standard CNC lathe with relatively modest modifications. The requirements are: spindle power sufficient for the drilling operation (30–50 kW for Ø40–80 mm in steel); through-spindle coolant capability (a rotary union rated for 50 bar is typically required); a coolant pump upgrade (from the standard 5–10 bar to 20–50 bar, with flow capacity of 200–500 L/min); a coolant filtration system upgrade (paper band filter at 30–50 µm, or magnetic separator + paper band for cast iron); a chip conveyor (standard hinged belt is usually adequate); a coolant tank of sufficient capacity (3,000–5,000 L for a typical retrofitted system); and a tool holding system for the ejector drill outer tube (a hydraulic chuck with a bushing to match the tube diameter, or a custom collet). The total retrofit cost is typically €80,000–150,000 including tooling, which is substantially less than the €250,000–600,000 cost of a new dedicated BTA drilling machine. The retrofit can typically be completed in 2–4 weeks including installation and commissioning (plus 2–4 weeks for tooling delivery). The lathe retains its full functionality for conventional turning operations after the retrofit, making ejector drilling a cost-effective entry point for companies adding deep hole drilling capability.
What is the difference between ejector drilling and BTA STS tooling?
The tooling differences between ejector drilling and BTA STS are: drill head — the ejector drill head includes an integral ejector nozzle ring (or nozzles in the head body) that creates the Venturi effect. The BTA STS head has a simpler construction without ejector nozzles. Tube assembly — ejector drilling uses a double concentric tube (outer tube for coolant supply, inner tube for chip return). BTA STS uses a single tube (the annulus between the tube OD and the bore ID is the coolant supply path, and the tube ID is the chip return path). Coolant connection — ejector drilling requires a coolant inlet at the machine end that connects to the annulus between the tubes. BTA STS requires a rotary union at the spindle connection and a pressure head at the workpiece. The drill heads look similar externally — both use indexable carbide inserts and guide pads — but the ejector head has internal coolant passages that divide the flow between the ejector nozzles and the cutting zone. Interchangeability is limited — an ejector drill head cannot be used on a BTA STS system and vice versa. The tube assemblies are also not interchangeable because the coolant flow path is fundamentally different.
Disclaimer: The ejector drilling parameters, design guidelines, and retrofit cost estimates presented in this article are based on published technical literature, tool manufacturer data, and industry-reported experience. Actual results depend on specific machine tool condition, workpiece material, coolant system capability, and operating parameters. Retrofit feasibility and cost should be evaluated by qualified machine tool integrators for each specific application. No guarantee of specific penetration rate, tool life, or retrofit performance is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.