Appearance
Ejector drilling brings deep hole drilling capability to conventional machine tools by using a venturi effect instead of a pressure head seal — sacrificing some chip evacuation efficiency in exchange for machine compatibility.
Overview
Ejector drilling, also known as the Double Tube System (DTS), is a variant of the BTA deep hole drilling process. It was developed to address a key limitation of standard BTA drilling: the requirement for a pressure head (sealing unit) that presses against the workpiece to contain high-pressure coolant.
By using two concentric tubes instead of one, ejector drilling creates a self-contained coolant circuit that needs no external seal. This allows the process to run on standard CNC lathes and machining centers without dedicated deep hole drilling equipment, making it a popular choice for shops that need deep hole capability without investing in specialized machinery.
The trade-off is reduced chip evacuation efficiency and a more restricted diameter range compared to standard BTA drilling.
How the Ejector (Venturi) Effect Works
The ejector effect is the defining innovation of this drilling method.
The Two-Tube Construction
The tool assembly consists of:
- Outer tube — connects to the cutting head and transmits torque
- Inner tube — sits concentrically inside the outer tube, creating an annular gap
- Cutting head — structurally similar to a BTA head, with additional coolant outlets on its circumference
Coolant and Chip Flow Path
- High-pressure coolant is introduced through a connector at the machine spindle
- Coolant flows down the annular gap between the outer and inner tubes
- At the cutting head, the flow splits:
- Primary flow exits through the cutting face to cool and lubricate the cutting edges and guide pads
- Secondary flow is diverted through a ring nozzle into the inner tube
- The secondary flow passing through the ring nozzle creates a venturi effect — a localized low-pressure zone that draws coolant and chips from the cutting face into the inner tube
- Chips are evacuated through the hollow center of the inner tube and out the back of the machine
The venturi principle
The ejector effect is the same physics principle used in vacuum ejectors and aspirators: a fluid flowing through a constriction increases in velocity and drops in pressure, creating suction that pulls additional fluid along with it.
No Pressure Head Required
Because coolant flows entirely within the tube assembly (rather than through the annular gap between the tube and the bore wall), there is no pressurized fluid at the entry point that needs to be sealed. A simple guide bush replaces the BTA pressure head, dramatically simplifying the machine interface.
Ejector Drilling vs. BTA Drilling
| Parameter | BTA Drilling (STS) | Ejector Drilling (DTS) |
|---|---|---|
| Tube configuration | Single tube | Double tube (inner + outer) |
| Coolant path | Annular gap between tube and bore wall | Annular gap between outer and inner tube |
| Chip evacuation | Through hollow single tube | Through inner tube (smaller cross-section) |
| Pressure head required | Yes (seals against workpiece) | No (only guide bush needed) |
| Machine compatibility | Dedicated deep hole machines | Conventional lathes and machining centers |
| Diameter range | 8 – 200 mm (standard) | 18 – 250 mm |
| Tolerance (IT grade) | IT7 – IT9 | IT9 – IT11 |
| Chip evacuation efficiency | Higher (larger passage) | Lower (restricted by inner tube) |
| Torsional rigidity | High | Higher (larger outer tube diameter) |
| Cutting capacity | Higher | Lower |
| Coolant pressure required | 30 – 80 bar | 15 – 40 bar |
Key Takeaway
BTA drilling wins on chip evacuation, accuracy, and diameter range — it is the higher-performance system. Ejector drilling wins on machine compatibility and simplicity of setup — it brings deep hole capability to machines that cannot accommodate a BTA pressure head.
Ejector Drilling vs. Gun Drilling
| Parameter | Gun Drilling | Ejector Drilling (DTS) |
|---|---|---|
| Diameter range | 0.5 – 50 mm | 18 – 250 mm |
| Tube cross-section | V-flute (non-circular) | Circular tube (higher torsional rigidity) |
| Chip evacuation | External V-flute (contacts bore wall) | Internal (no contact with bore wall) |
| Surface finish (Ra) | 0.4 – 1.6 µm | 0.8 – 3.2 µm |
| Coolant pressure | 30 – 150 bar | 15 – 40 bar |
| Cutting performance | Lower | Higher |
| Machine compatibility | Machining centers or dedicated | Conventional lathes and machining centers |
Key Takeaway
Gun drilling is the choice for small diameters and high surface finish. Ejector drilling takes over where diameters exceed 18 mm and productivity matters more than surface finish — similar to the BTA vs. gun drilling divide, but with the convenience of conventional machine compatibility.
Applications
Ejector drilling is most commonly used in:
- Retrofitting conventional machine tools for deep hole capability — the primary market for ejector systems
- Hydraulic cylinder manufacturing — medium-diameter bores with moderate depth requirements
- Oilfield components — drill collars, tool joints, and other components where conventional machines are used
- General engineering — shafts, spindles, and structural components requiring bores 20–100 mm diameter
- Automotive — transmission shafts, axle components, and suspension parts
When to Choose Ejector Drilling
| Condition | Reason |
|---|---|
| Existing conventional machine tools | No dedicated deep hole machine needed |
| Limited capital investment | Lower equipment cost than BTA |
| Diameter 20 – 100 mm | Sweet spot for ejector system efficiency |
| Moderate production volume | Not optimized for high-volume production |
| Retrofitting existing生产线 | Simple integration with guide bush only |
Advantages and Limitations
Advantages
- No pressure head required — simplifies machine setup and reduces cost
- Compatible with conventional machines — can be retrofitted to standard CNC lathes and machining centers
- Higher torsional rigidity — the double tube assembly has a larger outer diameter than a comparable BTA tube, providing greater resistance to twisting
- Integrated chip evacuation — chips exit internally, avoiding bore wall scoring
- Lower coolant pressure requirement — 15–40 bar vs. 30–80 bar for BTA
- Can use machine's internal coolant supply in some configurations
Limitations
- Restricted chip evacuation — the inner tube's smaller cross-section limits chip size and can cause blockages, especially in materials that produce large or stringy chips
- Poorer concentricity (IT9–IT11) — reduced rigidity from the double tube arrangement affects hole accuracy compared to BTA (IT7–IT9)
- Limited small-diameter capability — impractical below 18 mm due to the space taken by the dual tube construction
- Lower cutting capacity — the restricted chip passage limits achievable feed rates in difficult materials
- More complex tool assembly — two concentric tubes require precise alignment
Parameter Selection
Parameter selection for ejector drilling follows similar principles to BTA drilling, with adjustments for the restricted chip evacuation path.
Cutting Speed by Material
| Material | Cutting Speed (m/min) |
|---|---|
| Aluminum alloys | 80 – 160 |
| Low-carbon steel | 80 – 120 |
| Medium-carbon/alloy steel | 60 – 100 |
| Stainless steel (austenitic) | 50 – 100 |
| Gray cast iron | 80 – 120 |
| Titanium alloys | 30 – 50 |
| Superalloys (Inconel) | 15 – 30 |
Feed Rate by Diameter
| Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) | Aluminum (mm/rev) |
|---|---|---|---|
| 18 – 25 | 0.08 – 0.18 | 0.10 – 0.20 | 0.12 – 0.25 |
| 25 – 50 | 0.10 – 0.23 | 0.12 – 0.25 | 0.15 – 0.30 |
| 50 – 100 | 0.15 – 0.30 | 0.18 – 0.35 | 0.20 – 0.40 |
Coolant Parameters
| Diameter (mm) | Pressure (bar) | Flow Rate (L/min) |
|---|---|---|
| 18 – 25 | 20 – 40 | 80 – 130 |
| 25 – 50 | 15 – 30 | 130 – 250 |
| 50 – 100 | 10 – 25 | 250 – 450 |
Coolant filtration is critical
Ejector drilling requires filtration to ≤ 20 µm, and ideally ≤ 10 µm. The restricted chip passage makes the system particularly sensitive to contamination and oversized particles that can lodge in the evacuation path.
Startup Procedure
- Drill pilot hole — 1–2× diameter deep, slightly larger than ejector drill diameter
- Insert tool at low speed — 50–100 rpm with minimal feed
- Activate coolant — wait for flow to stabilize (2–3 seconds)
- Ramp to full speed — over 2–3 seconds
- Apply target feed — ramp over the first 2–3× diameter of depth
- Monitor pressure gauge — stable reading confirms normal operation
- Reduce speed before exit — drop to 50–100 rpm before retracting
Summary
Ejector drilling fills a specific niche in the deep hole drilling landscape: it offers BTA-like capability on conventional machine tools by trading some performance for compatibility.
| Aspect | Assessment |
|---|---|
| Best for | Retrofitting conventional machines for deep hole drilling |
| Diameter range | 18 – 250 mm |
| Key advantage | No pressure head required |
| Key limitation | Reduced chip evacuation efficiency |
| Accuracy | IT9 – IT11 |
| Machine types | Standard CNC lathes and machining centers |
FAQ
Is ejector drilling the same as BTA drilling?
No — ejector drilling is a variant of BTA drilling. Both use multi-edge cutting heads and internal chip evacuation, but ejector drilling uses a double tube system with a venturi effect for chip removal, while standard BTA uses a single tube with coolant flowing through the annular gap between tube and bore wall. The practical difference is that ejector drilling needs no pressure head seal.
What is the ejector effect in drilling?
The ejector effect (also called the venturi effect) occurs when a portion of the coolant flow is redirected through a ring nozzle into the inner tube. This creates a localized low-pressure zone that draws coolant and chips from the cutting face into the inner tube, assisting evacuation. It is the same principle used in vacuum ejectors and fluid aspirators.
Can ejector drilling be used on any CNC machine?
Ejector drilling can be retrofitted to most CNC lathes and machining centers that have through-spindle coolant capability or can accommodate an external high-pressure coolant pump. The key requirement is a suitable connector to introduce coolant into the annular gap between the tubes. No pressure head or specialized deep hole drilling machine is needed.
What diameter range is practical for ejector drilling?
The practical range is approximately 18–250 mm. Below 18 mm, the dual-tube construction occupies too much space, leaving insufficient cross-section for chip evacuation. Above 250 mm, the size and weight of the double tube assembly become cumbersome, and standard BTA drilling is generally preferred.
Why is ejector drilling accuracy lower than BTA?
The double tube system has lower structural rigidity than a solid single tube of equivalent diameter. The inner tube, through which chips are evacuated, also constrains chip flow — any chip congestion can push against the tool and affect concentricity. Typical tolerances are IT9–IT11, compared to IT7–IT9 for standard BTA drilling.
How does coolant pressure compare between ejector and BTA drilling?
Ejector drilling typically requires lower coolant pressure (15–40 bar) than BTA drilling (30–80 bar) because the coolant flows through the internal annular gap between tubes rather than the narrow annular gap between tube and bore wall. However, the flow rate requirement remains substantial — comparable to BTA at equivalent diameters.
Parameters are starting recommendations. Actual values depend on machine condition, coolant system capacity, workpiece material, and specific tooling geometry. Consult your tool supplier for application-specific data. This article reflects industry knowledge as of 2026.