What is Ejector Drilling?
Ejector drilling, also called the Double Tube System (DTS), is a deep hole drilling method that uses two concentric tubes to deliver coolant and evacuate chips through a patented Venturi effect. Developed by Sandvik in the 1970s as an evolution of BTA drilling, ejector drilling eliminates the need for a high-pressure seal at the workpiece entry, making it the most flexible deep hole drilling method for medium-diameter holes.
The key innovation is the ejector (Venturi) effect: a portion of the coolant flow is diverted through narrow slots in the inner tube, creating a vacuum that actively sucks chips back through the system. This self-contained coolant circuit means no external pressure head is needed — the system can be retrofitted onto standard CNC lathes and machining centers.
Ejector drilling covers diameters from 18 mm to 200 mm with depth ratios up to 100:1, offering penetration rates approaching BTA drilling while requiring lower coolant pressure and no workpiece sealing.
How the Double Tube System Works
The ejector system is built around a two-tube design — an inner tube inside an outer tube — that creates a self-contained coolant and chip evacuation circuit.
The Flow Path
Coolant delivery: High-pressure coolant is pumped into the annular space between the outer tube and the inner tube. The coolant travels along the length of the boring bar toward the drill head.
Cutting action: The DTS drill head, equipped with carbide inserts and guide pads, removes material across the full radius of the hole — similar to a BTA head in cutting action.
Venturi effect: As the coolant reaches the drill head, a portion flows through specially designed Venturi slots (ejector nozzles) in the inner tube wall. These slots constrict the flow path, dramatically increasing velocity and creating a low-pressure zone.
Chip suction: This low-pressure zone creates a vacuum effect that draws the remaining coolant and chips from the cutting zone into the inner tube.
Chip evacuation: The combined coolant flow and suction force carries chips back through the inner tube, through the boring bar, and out for collection and filtration.
Why This Matters
The Venturi effect is the key differentiator. In BTA drilling, chip evacuation relies entirely on external coolant pressure pushing chips through the tube. In ejector drilling, the suction created by the Venturi effect actively pulls chips — making the system less dependent on high coolant pressure and eliminating the need for an external seal.
| Component | Function |
|---|---|
| Outer tube | Carries coolant forward to the drill head |
| Inner tube | Carries chips and coolant back from the drill head |
| Venturi slots | Create suction that evacuates chips |
| DTS drill head | Cutting edges + guide pads + Venturi passages |
| Coolant swivel | Transfers coolant from machine to rotating boring bar |
Diameter and Depth Capability
| Parameter | Ejector Drilling |
|---|---|
| Diameter range | 18–200 mm |
| Optimal sweet spot | 20–65 mm |
| Maximum depth ratio | 100:1 |
| Typical depth | Up to 5 m (16 ft) on standard machines |
| Diameter tolerance | IT7–IT10 |
| Surface finish | Ra 0.8–3.2 µm as-drilled |
Ejector Drilling vs BTA vs Gun Drilling
| Factor | Ejector Drilling | BTA Drilling | Gun Drilling |
|---|---|---|---|
| Tube system | Double tube (DTS) | Single tube (STS) | Solid shaft with V-flute |
| Coolant delivery | Between inner and outer tubes | External (tube-to-bore annulus) | Internal (through tool) |
| Chip evacuation | Venturi suction through inner tube | Pressure through tube center | External V-groove |
| Workpiece seal needed? | No — key advantage | Yes (pressure head) | No (simple bushing) |
| Can retrofit CNC lathe? | Yes — another key advantage | No (dedicated machine) | Limited (up to 40:1) |
| Diameter range | 18–200 mm | 18–250+ mm | 0.5–50 mm |
| Penetration rate | 4–6× gun drilling | 5–7× gun drilling | Baseline |
| Coolant pressure | Moderate (20–40 bar) | High (20–60 bar) | Very high (up to 140 bar) |
For a detailed comparison, see our ejector vs BTA vs gun drilling guide.
Advantages of Ejector Drilling
No workpiece seal required. The self-contained double-tube design means coolant never contacts the workpiece entry surface. This is ejector drilling’s most important advantage — it can drill holes in workpieces with irregular, rough, or non-square entry faces that would cause seal failure in BTA drilling.
Retrofittable to standard machine tools. Ejector drilling can be installed on existing CNC lathes and machining centers with a coolant system upgrade. There is no need for a purpose-built deep hole drilling machine. This dramatically reduces the capital investment required.
Lower coolant pressure requirements. The Venturi effect creates suction that assists chip evacuation, so ejector drilling requires lower coolant pressure than BTA drilling at the same diameter — typically 20–40 bar vs 30–60 bar for BTA.
Good penetration rate. Ejector drilling achieves 4–6× the penetration rate of gun drilling, approaching BTA performance while offering greater machine flexibility.
Clean chip evacuation. Chips exit through the inner tube and never contact the finished bore surface.
Limitations
Larger minimum diameter. The double-tube design requires space for both the inner and outer tubes, setting a minimum diameter of approximately 18 mm. Below this, the design cannot physically fit.
Slightly slower than BTA. The Venturi effect is less efficient at chip evacuation than BTA’s direct pressure method, making ejector drilling about 10–20% slower than BTA at the same diameter.
Limited depth ratio. Maximum depth ratio of 100:1, comparable to BTA but well below gun drilling’s 300:1.
Tooling complexity. The DTS drill head and dual-tube boring bar are more complex than equivalent BTA tooling, which can increase tool cost.
Applications
Ejector drilling is the preferred choice when:
- Retrofitting a standard CNC lathe for deep hole drilling capability
- The workpiece entry face is irregular — castings, forgings, or rough-machined surfaces where BTA’s pressure head seal would leak
- Production volumes are low to moderate — the lower capital investment makes sense for smaller production runs
- Diameters are in the 20–65 mm range — the sweet spot for ejector tooling availability
For a detailed review by industry, see ejector drilling applications.
History
Ejector drilling was developed by Sandvik Coromant in the 1970s as an alternative to BTA drilling for shops that did not have dedicated deep hole drilling machines. Sandvik’s CoroDrill 800 series became the industry standard for DTS tooling. Today, Sandvik has phased out the CoroDrill 800 line, and replacement tooling is available from manufacturers like Sunnen (DirectDex line) and BTA BORE UK, both of whom produce drop-in compatible DTS drill heads and boring bars.
Summary
Ejector drilling (DTS) is the most flexible deep hole drilling method for medium-diameter holes (18–200 mm). Its double-tube design and Venturi-effect chip evacuation eliminate the need for workpiece sealing, allowing it to be retrofitted onto standard CNC machine tools. While slightly slower than BTA drilling and not available below 18 mm diameter, ejector drilling offers the best combination of productivity and machine flexibility for shops adding deep hole drilling capability without investing in dedicated equipment.
For a step-by-step guide to the ejector drilling process, see how ejector drilling works. For parameter recommendations, see ejector drilling parameters guide. For a complete overview, visit the ejector drilling guide.