How Ejector Drilling Works
Ejector drilling uses a unique double-tube design that creates a self-contained coolant circuit, eliminating the need for external sealing at the workpiece. This makes it the most adaptable deep hole drilling method for standard machine tools.
This guide walks through the ejector drilling process step by step, explains the Venturi effect that makes it work, and covers setup considerations for CNC lathe installations.
The Double Tube System (DTS)
The heart of the ejector drilling system is the boring bar — a two-tube assembly that handles coolant delivery, chip evacuation, and torque transmission simultaneously.
Boring Bar Design
The boring bar consists of:
- Outer tube — A heavy-walled steel tube that carries coolant forward to the drill head. The outer diameter of this tube fits inside the drilled hole with annular clearance.
- Inner tube — A smaller-diameter tube inside the outer tube. The annular space between the outer and inner tubes is the coolant supply path; the hollow interior of the inner tube is the chip evacuation path.
- Venturi section — Located at the drill head end of the inner tube, this section contains precisely machined slots (ejector nozzles) that create the Venturi effect.
The DTS Drill Head
The drill head threads onto the front of the boring bar and contains:
- Carbide cutting inserts — 2–4 indexable inserts positioned to cut specific zones of the hole cross-section
- Guide pads — Carbide pads that provide self-piloting and bore burnishing
- Coolant passages — Internal channels that direct coolant to the cutting edges
- Venturi slots — Openings that connect the coolant annulus to the inner tube
The Venturi Effect Explained
The Venturi effect is the physical principle that makes ejector drilling possible without external sealing.
How It Works
- Coolant enters the boring bar at moderate pressure (20–40 bar) through the coolant swivel connection
- Coolant flows forward through the annular space between the outer and inner tubes
- At the drill head, the coolant path splits:
- Approximately 60–70% flows through the Venturi slots in the inner tube wall
- Approximately 30–40% flows through the cutting insert coolant passages
- The Venturi slots constrict the flow, causing the coolant velocity to increase and pressure to drop (Bernoulli’s principle)
- This low-pressure zone creates suction inside the inner tube — the “ejector” effect
- Chips and remaining coolant are drawn from the cutting zone into the inner tube by this suction
- The combined flow of chips and coolant exits through the inner tube
Why the Venturi Effect Matters
The Venturi effect replaces the external pressure head seal used in BTA drilling. Since the suction is generated inside the boring bar itself, there is no need to seal the coolant annulus at the workpiece entry. This is what allows ejector drilling to work on standard machine tools and with irregular workpiece surfaces.
Step-by-Step Process
Step 1: Machine Setup
Ejector drilling on a CNC lathe requires:
- Coolant system upgrade — A high-pressure coolant pump (20–40 bar, 80–200 L/min depending on diameter) with appropriate filtration (10–20 micron)
- Coolant swivel — A rotating coolant union mounted on the lathe turret or tailstock that transfers coolant to the rotating boring bar
- Boring bar support — Steady rests or guide bushings to support the boring bar between the turret and workpiece
- Workpiece clamping — Standard lathe chuck or faceplate; no special seal is needed
Step 2: Workpiece Preparation
- Face the workpiece entry — While no seal is required, a faced entry improves hole start accuracy
- Drill a pilot hole — A short pilot hole (1.5–2× diameter deep) guides the DTS head at entry
- Spot face for irregular surfaces — If the entry face is extremely irregular, a light spot face ensures the pilot hole starts on-axis
Step 3: Coolant Flow Initiation
- Start the coolant pump and verify pressure at the coolant swivel
- Confirm coolant return flow through the boring bar — visible at the chip collection point
- Check for leaks at the coolant swivel connection
Step 4: Tool Entry
- Advance the DTS head toward the workpiece at reduced feed (50% of normal)
- The guide pads engage the pilot hole bore as the head enters
- Once the cutting inserts contact the full cross-section, increase to full feed
Step 5: Continuous Cutting
The cutting action is similar to BTA drilling:
- Multiple cutting edges remove material simultaneously
- Guide pads provide self-piloting and burnish the bore wall
- The Venturi effect continuously evacuates chips through the inner tube
Chip monitoring is the primary indicator of process health:
- Short C-shaped chips — Ideal; good chip breaking and evacuation
- Long stringy chips — Feed too low; can block the inner tube
- Powdered chips — Feed too high or tool worn
- Burned/blue chips — Excessive heat; reduce speed or increase coolant
Step 6: Depth Monitoring
Monitor these parameters during the cut:
| Parameter | Normal Range | Warning Sign |
|---|---|---|
| Coolant pressure | 20–40 bar | Gradual drop indicates pump issue or blockage |
| Spindle load | Stable | Gradual increase = tool wear |
| Chip flow | Continuous | Intermittent = Venturi blockage |
| Coolant return temperature | 30–40°C | Above 45°C = cooling insufficient |
Step 7: Breakthrough and Withdrawal
- For through-holes, reduce feed to 50% for the last 5–10 mm
- Stop spindle rotation before retracting
- Continue coolant flow for 5–10 seconds after feed stop to flush remaining chips
- Withdraw the head at rapid traverse
Boring Bar Support Configurations
On a CNC Lathe
| Support Method | Configuration | Max Depth Ratio |
|---|---|---|
| Steady rest on boring bar | Steady rest mounted on lathe bed, supporting the boring bar near the workpiece | 40:1 |
| Guide bushing in turret | Bushing mounted in a turret station, supporting the bar close to the head | 60:1 |
| Multiple steady rests | 2+ steady rests along the boring bar length | 80:1 |
| Tailstock support | Boring bar supported between centers with a rotating center | 30:1 (limited by tailstock interference) |
On a Machining Center
On a machining center, the boring bar is held in a tool holder, and the workpiece is stationary. Coolant is delivered through the machine spindle (through-tool coolant). This arrangement is limited to lower depth ratios (approximately 30:1) due to tool overhang.
Coolant System Requirements
| Drill Diameter | Pressure (bar) | Flow Rate (L/min) | Minimum Filtration |
|---|---|---|---|
| 20 mm | 25–40 | 80–120 | 20 micron |
| 40 mm | 25–35 | 120–180 | 20 micron |
| 60 mm | 20–30 | 150–250 | 20 micron |
| 80 mm | 20–30 | 200–300 | 20 micron |
| 100 mm | 15–25 | 250–350 | 20 micron |
Coolant type: Neat cutting oil with EP additives (sulfur, chlorine) for dedicated systems. High-EP emulsion (8–12%) for CNC machine retrofits where the same coolant is used for other machining operations.
Comparison: Ejector vs BTA Process
| Process Step | Ejector Drilling | BTA Drilling |
|---|---|---|
| Machine required | Standard CNC lathe or MC (with coolant upgrade) | Dedicated BTA machine |
| Workpiece preparation | Simple pilot hole | Pilot hole + flat sealing face |
| Seal required? | No | Yes (pressure head / BOZA) |
| Coolant pressure | Lower (20–40 bar) | Higher (30–60 bar) |
| Chip evacuation | Venturi suction | Pressure-driven |
| Setup time | Shorter (no seal alignment) | Longer (seal alignment critical) |
Summary
Ejector drilling works by using a double-tube boring bar and the Venturi effect to create a self-contained coolant and chip evacuation circuit. The key advantage is that no external seal is needed at the workpiece, making ejector drilling the most practical deep hole drilling method for standard CNC machine tools. The process sequence — setup, coolant initiation, continuous cutting, and withdrawal — follows the same pattern as BTA drilling, but with simpler workpiece preparation and lower coolant pressure requirements.
For foundational knowledge, see what is ejector drilling. For parameter selection, see ejector drilling parameters guide. For setup on a CNC lathe, see ejector drilling CNC setup. For a complete overview, visit the ejector drilling guide.