30-Second Summary: Ejector drilling (Double Tube System — DTS) uses a concentric double-tube structure, employing the Venturi effect to create negative pressure that assists chip evacuation. Its biggest advantage is that it requires no workpiece face seal, allowing it to be used on conventional lathes, boring mills, and machining centers without a dedicated deep hole drilling machine. It can be used efficiently from L/D > 3, making it suitable for relatively shallow holes as well.
How It Works
Approximately 60–70% of the coolant is delivered to the drill head through the annular space between the outer and inner tubes for cooling and lubrication of the cutting edges and guide pads. The remaining 30–40% passes through Venturi nozzles (ejector slots) machined into the inner tube near the drill head connection. These nozzles create a pressure drop that generates suction, drawing chips and spent coolant back through the inner tube for evacuation.
- Outer tube: Delivers coolant to the drill head region
- Inner tube: Evacuates chips and coolant under suction
- Venturi ring: Slotted nozzle ring on the inner tube that generates the suction force — this is the critical design element
- No pressure head required: Unlike BTA, there is no need to seal against the workpiece face, enabling use on standard machine tools
Coolant Flow Requirements
Unlike other deep hole methods, ejector drilling requires high volume flow at moderate pressure. The Venturi effect is driven by flow rate, not static pressure. Minimum flow rates from recent research (2025, 30 mm drill in 42CrMo4+QT steel at vc = 60 m/min, f = 0.2 mm/rev):
| Drill Head Type | At vc 60 m/min | At vc 80 m/min |
|---|---|---|
| Standard (reference) head | ~29.5 L/min minimum | ~51.5 L/min minimum |
| Optimized head (angled outlets 20°) | ~24.7 L/min minimum | ~29.5 L/min minimum |
| Manufacturer's recommended starting flow | 55 L/min (for 30 mm drill) | |
Industry practice often sets flow rates significantly higher than the actual minimum — the 2025 study found that standard practice over-feeds coolant by 10–80% above the true minimum required for stability. Optimized drill head designs can reduce minimum required flow by up to 43% while maintaining process stability.
Venturi Effect Principles
- The Venturi nozzles convert pressure energy into velocity energy, creating a localized low-pressure zone
- The pressure drop at the chip collection opening creates suction that draws chips into the inner tube
- The effect is critically dependent on maintaining minimum flow velocity — below ~8 bar inlet pressure, the suction effect collapses
- Flow instability (Taylor macrovortices) can occur in the annular channels under certain combinations of boring bar rotational velocity and axial flow velocity, affecting pressure distribution and hydraulic resistance
- Reynolds number in the annular channels influences flow stability — channel clearance variation and eccentricity affect coolant pressure distribution
Advantages
- No pressure head required — workpiece face does not need to be flat or sealed
- Can be installed on conventional machine tools (retrofit solution for existing lathes and machining centers)
- Lower coolant pressure requirements (10–50 bar) compared to gundrilling (50–150+ bar)
- Suitable for irregular workpiece faces and non-round workpieces
- Effective from relatively shallow holes (L/D > 3)
Limitations
- Limited depth-to-diameter ratio (approx. 100:1 maximum in production)
- Minimum diameter typically 18 mm — insufficient chip evacuation space for smaller diameters due to the double-tube construction
- Productivity between gundrilling and BTA (typically 3–5× gundrilling)
- Requires higher volume flow rates than gundrilling — may require pump upgrades on retrofit installations
- Tool design is more complex due to the Venturi nozzle geometry
Cutting Parameter Reference
| Material | Vc (m/min) | Feed (mm/rev) D=25mm | Feed (mm/rev) D=50mm | Coolant Pressure | Typical Flow (L/min) |
|---|---|---|---|---|---|
| Low-carbon steel | 60–100 | 0.06–0.14 | 0.12–0.28 | 10–35 bar | 30–80 |
| Alloy steel (annealed) | 40–70 | 0.05–0.12 | 0.10–0.25 | 15–40 bar | 40–100 |
| Stainless steel | 25–50 | 0.04–0.10 | 0.08–0.20 | 20–50 bar | 50–120 |
| Aluminum | 60–120 | 0.10–0.22 | 0.20–0.45 | 10–30 bar | 30–80 |
| Cast iron | 50–80 | 0.08–0.18 | 0.15–0.35 | 10–30 bar | 30–80 |
| Copper alloys | 50–100 | 0.08–0.20 | 0.15–0.40 | 10–30 bar | 30–80 |
Recent Research: Optimized Drill Head Designs
A 2025 study (Production Engineering, Springer) used Smoothed Particle Hydrodynamics (SPH) simulation to analyze and optimize fluid flow in ejector drill heads. Key findings:
- Extended chip mouth: Lengthening the chip intake opening reduces flow restriction and improves chip entry
- Angled coolant outlets (20°): Directing coolant jets at 20° toward the cutting zone improves lubrication and cooling while reducing overall flow requirement
- Targeted near-laminar flow: Optimized designs produce directed, nearly laminar flow even at very low flow rates (as low as 18 L/min), improving chip evacuation reliability
- Vortex reduction: Vortex formation near the outer cutting edge (which can trap chips and delay removal) is significantly reduced with optimized geometry
- Chip blockage detection: Feed force exceeding 6 kN is a reliable termination criterion — normal operating feed forces in ejector drilling range from 2.5–4 kN
Installation Considerations for Retrofits
- Existing lathe spindle bore must accommodate the double-tube diameter (typically 1.5–2× the drill diameter for the outer tube)
- Coolant pump must deliver sufficient flow rate at moderate pressure — a common mistake is installing a high-pressure/low-flow pump suitable for gundrilling
- Coolant filtration to 50 μm is adequate (less stringent than gundrilling due to larger coolant passages)
- A coolant return system must be provided to handle the significant fluid volume returning from the bore
- Whip guide supports are still needed for deep holes — support the outer tube every 40–60× D
⚠️ Important: Ejector drilling relies on the Venturi effect for chip evacuation. If coolant pressure drops below ~8 bar, the suction effect is lost and chips will accumulate in the bore. Monitor coolant pressure continuously — a sudden drop indicates a blocked inner tube or failing pump. A sudden increase in pressure typically signals a blockage in the chip evacuation channel.
Typical Applications
Mold cooling holes, hydraulic cylinders, valve body bores, pump housings, and deep hole machining of shaft parts. Particularly well-suited for retrofitting and upgrading existing equipment where the cost of a dedicated BTA machine cannot be justified. Also ideal for workpieces with irregular or non-flat faces where BTA pressure head sealing would be problematic.