Appearance
A medium-sized machine shop produces hydraulic cylinder barrels with bores of Ø50 mm at 3,000 mm depth. The shop has a standard CNC lathe with through-coolant capability but no dedicated deep hole drilling machine. They need to drill deep holes but cannot justify the investment in a full BTA machine with a BOZA pressure head. The solution is the ejector drilling system — a double-tube arrangement where coolant flows between the inner and outer tubes, and a venturi effect creates suction that pulls chips back through the inner tube. No pressure head seal is required because the coolant path is entirely contained within the tool assembly. The shop retrofits their lathe with an ejector system capable of drilling Ø18–200 mm holes at depth-to-diameter ratios exceeding 100:1. The resulting bores achieve IT10 tolerance and Ra 3.2 µm finish — suitable for hydraulic cylinder service.
How the Ejector System Works
The ejector system (also called the double tube system or DTS) is a variant of BTA deep hole drilling that eliminates the need for a pressure head seal between the tool and the workpiece.
Coolant Flow Path
The key difference from single-tube BTA is that coolant flows entirely within the tool assembly:
- Coolant enters through the machine connector at the tailstock end
- It travels forward through the annular space between the outer tube and the inner tube
- At the drill head, coolant exits through side apertures and flows around the cutting edges for cooling and lubrication
- The chip-laden coolant returns through the inner tube and exits at the machine end
The entire coolant path is self-contained — no coolant flows between the outer tube and the bore wall in the cutting zone.
The Ejector Effect
The defining feature of the system is the ejector (venturi) effect:
- Notches or slots in the inner tube near the machine connector divert a portion of the coolant into the inner tube
- These venturi slots convert pressure energy into velocity energy
- The resulting low-pressure zone creates suction that draws coolant and chips from the cutting zone back through the inner tube
- The suction assists chip evacuation without requiring high pump pressure
This is the same principle as an ejector pump — the coolant flowing at high velocity through the venturi creates a vacuum that pulls the chip-laden fluid behind it.
Coolant path (forward): Pump → Connector → Annular gap → Drill head → Cutting zone
Coolant path (return): Cutting zone → Chip mouth → Inner tube → Venturi slots → ExitEjector vs BTA Single Tube System
| Feature | BTA (Single Tube / STS) | Ejector (Double Tube / DTS) |
|---|---|---|
| Tube configuration | Single tube | Inner tube + outer tube |
| Coolant supply path | Between tube and bore wall | Between inner and outer tubes |
| Pressure head seal required | Yes (BOZA) | No |
| Coolant pressure | High (30–120 bar) | Lower (10–40 bar) |
| Chip evacuation mechanism | Pressure-driven | Pressure + venturi suction |
| Chip evacuation cross-section | Full tube bore | Reduced by inner tube |
| Machine requirement | Specialised BTA machine | Conventional machine + retrofit |
| Diameter range | 6–2,000+ mm | 18–200 mm |
| Achievable tolerance | IT7–IT9 | IT9–IT11 |
| Surface finish | Ra 1.6–3.2 µm | Ra 3.2–6.3 µm |
| Depth capability | Up to 100× diameter | Up to 100× diameter+ |
| Torsional rigidity | Moderate | Higher (larger outer tube) |
| Chip evacuation in difficult materials | More reliable | Less reliable |
When to Choose Ejector
- Retrofitting existing conventional machines — the ejector system does not require a dedicated BTA machine
- No pressure head possible — when the workpiece geometry does not allow sealing
- Limited coolant pressure — the venturi effect reduces pump pressure requirements
- Non-production drilling — job shop or maintenance work where a dedicated machine is not justified
- Moderate tolerance requirements — IT9–IT11 is acceptable for many hydraulic and mechanical applications
Tip: The ejector system can be retrofitted to most CNC lathes with through-coolant capability. The minimum requirements are: a coolant pump capable of 20–40 bar, a tailstock with sufficient Z-axis thrust, and a machine connector to mount the outer tube and coolant inducer.
System Components
Drill Head
The drill head is identical or similar to a BTA drill head in cutting geometry — it uses indexable carbide inserts and carbide guide pads. The head is threaded onto the outer tube.
| Component | Function |
|---|---|
| Cutting inserts | Remove material (centre, intermediate, peripheral positions) |
| Guide pads | Support and guide the head, burnish the bore wall |
| Coolant apertures | Direct coolant to cutting edges |
| Chip mouth | Opening through which chips enter the inner tube |
Outer Tube (Boring Bar)
The outer tube connects the drill head to the machine connector. It must be:
- Rigid enough to resist bending and torsion at depth
- Concentric with the inner tube to maintain uniform coolant flow
- Threaded at both ends for connection to the head and connector
Inner Tube
The inner tube is positioned concentrically inside the outer tube. It:
- Forms the coolant delivery annulus together with the outer tube
- Provides the chip evacuation passage
- Contains the venturi slots that create the ejector effect
- Is removable for cleaning and replacement
Coolant Inducer / Machine Connector
This component mounts between the machine tailstock and the outer tube:
- Seals the coolant between the stationary connector and rotating tool
- Delivers coolant into the annular gap
- Houses the venturi slot region
- Provides the chip outlet connection
Ejector Drill Head Design
Insert Arrangement
The cutting geometry follows the same principles as BTA drilling heads:
| Insert Position | Function |
|---|---|
| Centre insert | Cuts the centre zone of the bore face |
| Intermediate insert | Cuts the mid-annular zone |
| Peripheral insert | Cuts the outer diameter, establishes bore size |
The number of inserts depends on diameter: two inserts for small diameters (18–30 mm), three to four for medium diameters (30–80 mm), and four to six for larger diameters (80–200 mm).
Guide Pad Configuration
| Parameter | Recommendation |
|---|---|
| Number of pads | 2 (primary and secondary) |
| Primary pad position | 45–65° from the cutting edge corner |
| Secondary pad position | 180° from the cutting edge |
| Pad material | Carbide (K10–K20) or DLC-coated |
The guide pad angle — the angle between the cutting edge corner and the first guide pad — is a critical design parameter. Patents recommend an angle of 30–60° (preferably 45–55°), which is lower than traditional designs that used 85–90°. The reduced angle decreases the tilting moment on the head and improves stability.
Warning: The guide pad angle has a direct effect on bore quality and tool life. A pad angle above 70° increases the tilting moment on the drill head, causing uneven pad wear and reduced straightness. If you are repurposing a BTA drill head for ejector drilling, verify that the pad angle is within the recommended range.
Cutting Parameters
Speed and Feed by Material
| Workpiece Material | Cutting Speed (m/min) | Feed (mm/rev) |
|---|---|---|
| Low-carbon steel | 60–90 | 0.08–0.20 |
| Alloy steel (4140) | 50–70 | 0.06–0.15 |
| Stainless steel (304) | 50–70 | 0.06–0.12 |
| Cast iron | 60–90 | 0.10–0.25 |
| Aluminum | 80–160 | 0.12–0.30 |
| Brass/bronze | 80–150 | 0.10–0.25 |
Note: These values are starting recommendations. Adjust based on chip form — the ideal chip shape for ejector drilling is small "C" or comma-shaped chips that flow freely through the inner tube.
Coolant Parameters
| Diameter | Pressure | Flow Rate (approximate) |
|---|---|---|
| 18–30 mm | 20–40 bar | Q = 4.5 × D (L/min) |
| 30–60 mm | 15–30 bar | Q = 4.5 × D (L/min) |
| 60–100 mm | 10–25 bar | Q = 4.5 × D (L/min) |
| 100–200 mm | 8–20 bar | Q = 4.5 × D (L/min) |
Example: For Ø50 mm, target flow ≈ 225 L/min.
Effect of Ejector Suction Distance
The venturi effect has a limited effective suction distance — typically up to 5 metres. Beyond this depth, a displacement-based chip evacuation mechanism takes over, where incoming coolant physically pushes the chip-laden fluid through the inner tube. This transition is automatic and does not require parameter changes.
Setup Requirements
Machine Requirements
| Requirement | Specification |
|---|---|
| Spindle power | Sufficient for BTA drilling at the target diameter (typically 10–30 kW for Ø50 mm) |
| Coolant pump | 20–40 bar, flow per the diameter-based formula above |
| Tailstock | Removable or with through-bore capability for the inner tube |
| Z-axis travel | 1.2× required drilling depth |
| Coolant filtration | 20–50 µm minimum — essential for ejector system reliability |
Retrofitting a Conventional CNC Lathe
The most common application of the ejector system is retrofitting onto a standard CNC lathe:
- Remove or modify tailstock — the centre of the tailstock must accommodate the inner tube exit
- Mount the coolant inducer — fixed to the lathe bed or tailstock, connecting the coolant supply to the rotating tool
- Install the outer tube — supported by steady rests along its length
- Connect coolant supply — from the pump through the inducer
- Install chip collection — at the inner tube exit
Workholding
Unlike single-tube BTA, ejector drilling does not require sealing at the workpiece entry. A standard chuck or faceplate is sufficient. However, the workpiece must still be rigidly clamped to resist the torque and thrust forces.
Guide Bush
A guide bush at the workpiece entry is recommended even though no pressure seal is required. The bush:
- Supports the drill at entry
- Maintains concentricity during the first few diameters of cut
- Prevents bell-mouthing at the bore entry
Troubleshooting
Chip Evacuation Problems
| Problem | Likely Cause | Correction |
|---|---|---|
| Chips not exiting inner tube | Insufficient coolant flow | Increase flow, check venturi slots for blockage |
| Long stringy chips blocking tube | Feed too low | Increase feed, check chip breaker |
| Chips exiting from drill entry | Coolant bypassing the cutting zone | Check head-to-tube seal, replace worn components |
| Intermittent chip flow | Pump pressure fluctuation | Check pump, filters, and pressure gauge |
| Chip build-up at venturi slots | High-viscosity coolant or debris | Clean inner tube, check coolant filtration |
Surface Finish Problems
| Problem | Likely Cause | Correction |
|---|---|---|
| Rough bore surface | Feed too high, worn inserts | Reduce feed, replace inserts |
| Scored bore surface | Chips contacting bore wall | Improve chip evacuation, increase coolant |
| Vibration marks | Speed too high, insufficient rigidity | Reduce speed, check steady rests |
| Bell-mouth at entry | Worn or missing guide bush | Replace guide bush |
Tool Life Problems
| Problem | Likely Cause | Correction |
|---|---|---|
| Rapid insert wear | Speed too high | Reduce speed, check coolant supply |
| Chipped inserts | Feed too high or interrupted cut | Reduce feed, check for hard spots |
| Uneven guide pad wear | Misalignment or excessive pad angle | Check alignment, verify pad angle |
| Built-up edge on inserts | Insufficient coolant or low speed | Increase coolant, adjust speed |
Setup Problems
| Problem | Likely Cause | Correction |
|---|---|---|
| Coolant leaking at inducer | Worn seals, incorrect pressure | Replace seals, check pump pressure |
| Tube vibration at depth | Insufficient steady rest support | Add steady rests, reduce overhang |
| High thrust force | Dull inserts or incorrect feed | Replace inserts, verify parameters |
| Bore oversize at exit | Worn guide pads or misalignment | Replace pads, check machine alignment |
FAQ
What is the ejector drilling system?
The ejector system is a double-tube deep hole drilling method where coolant flows down between the inner and outer tubes and returns through the inner tube. A venturi effect at the machine end creates suction that assists chip evacuation.
How is ejector drilling different from BTA drilling?
In BTA (single tube system), coolant flows between the tube and the bore wall, requiring a pressure head seal at the workpiece. In ejector drilling, coolant is contained within the tool assembly, eliminating the need for a pressure head seal.
What diameter range can ejector drilling handle?
Typically 18–200 mm. Below 18 mm, the double-tube arrangement leaves insufficient space for chip evacuation. Above 200 mm, single-tube BTA is more efficient.
Can I retrofit my existing CNC lathe for ejector drilling?
Yes — the ejector system was designed for this purpose. You need a coolant pump capable of 20–40 bar, a modified tailstock, a coolant inducer, and the double-tube tool assembly.
What accuracy does ejector drilling achieve?
IT9–IT11 tolerance and Ra 3.2–6.3 µm surface finish are typical. This is adequate for hydraulic cylinders, mechanical bores, and most general engineering applications.
What coolant pressure does the ejector system need?
10–40 bar depending on diameter. This is significantly lower than single-tube BTA, which may require 50–120 bar. The venturi effect reduces the pressure requirement.
What chip shape is ideal for ejector drilling?
Small "C" or comma-shaped chips that flow freely through the inner tube. Long stringy chips will jam the tube and stop chip evacuation.
Why is there no pressure head required in ejector drilling?
Because the coolant path is fully contained within the tool assembly — between the inner and outer tubes. No coolant flows between the tool and the bore wall, so no external seal is needed.
What is the ejector effect?
The ejector effect is a venturi phenomenon: slots in the inner tube divert coolant flow into the tube, creating a low-pressure zone that draws coolant and chips from the cutting zone, assisting evacuation.
Is ejector drilling replacing BTA drilling?
No — the two methods serve different applications. BTA remains preferred for high-volume production, tighter tolerances, and difficult materials. Ejector drilling is chosen for retrofitting conventional machines and applications where a pressure head cannot be used.
Summary
The ejector double tube drilling system is a practical alternative to single-tube BTA when a dedicated deep hole drilling machine is not available:
- How it works — coolant flows forward through the annular gap between inner and outer tubes and returns through the inner tube, assisted by a venturi suction effect
- No pressure head required — the coolant path is contained within the tool, enabling retrofitting to conventional CNC lathes and machining centres
- Diameter range — 18–200 mm with depth-to-diameter ratios exceeding 100:1
- Accuracy — IT9–IT11 with Ra 3.2–6.3 µm finish, suitable for hydraulic and general engineering bores
- Coolant pressure — 10–40 bar, significantly lower than single-tube BTA
- Chip control — small "C" shaped chips are essential; long chips will block the inner tube
The hydraulic cylinder shop in the opening scenario achieved production-ready bores on their existing CNC lathe with the ejector retrofit, avoiding the capital investment in a dedicated BTA machine while meeting their quality requirements.