🌀 DOUBLE TUBE SYSTEM · VENTURI EFFECT

Ejector Drilling (DTS)

Double Tube System — uses a concentric double-tube structure and Venturi effect for chip evacuation. No pressure head, no workpiece face seal required. Ideal for retrofitting existing lathes and machining centers. Effective from L/D > 3.

18–250mmDiameterOptimal 20–150mm

~100:1L/DDepth capability

10–50 barCoolantModerate pressure

No SealWorkpieceKey advantage vs BTA

PRINCIPLE

How Ejector Drilling Works

💡 Venturi effect: 60–70% of coolant flows through the annular space between outer and inner tubes to the drill head for cooling and lubrication. The remaining 30–40% passes through Venturi nozzles (ejector slots) that create a pressure drop, generating suction that draws chips and spent coolant back through the inner tube. No pressure head required — unlike BTA, there is no need to seal against the workpiece face.

Component	Function
Outer tube	Delivers coolant to the drill head region (~65% of flow)
Inner tube	Evacuates chips and coolant under Venturi suction
Venturi ring	Slotted nozzle ring that generates the suction force — critical design element
Drill head	Cutting edges and guide pads, fed by annular coolant flow

SPECIFICATIONS

Technical Data

18–250

Diameter range

~100:1

L/D

Max depth ratio

10–50

bar

Coolant pressure

25–200+

L/min

Coolant flow rate

Ra 0.8–3.2

μm

Surface finish

IT9–IT11

Tolerance

Hole quality

⚠️ Practical depth cap: the Venturi chip-return tube limits ejector depth to roughly 1,000–2,000 mm in practice (the top end needs high-pressure DTC-R tube connectors). The ~100:1 L/D figure only holds at small diameters — a Ø200 mm ejector bore is not going to go 20 m.

CUTTING DATA

Cutting Parameters

Material	Vc (m/min)	Feed D=25mm	Feed D=50mm	Pressure	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

⚠️ Critical: Ejector drilling relies on Venturi effect — below ~8 bar inlet pressure, the suction effect collapses. Monitor coolant pressure continuously. A sudden drop = blocked inner tube or failing pump. A sudden increase = blockage in chip evacuation channel. Unlike other methods, ejector drilling needs high flow at moderate pressure, not high pressure.

PROS & CONS

Strengths & Limitations

✅ Advantages

No pressure head — workpiece face does not need sealing
Retrofits onto existing lathes, boring mills, machining centers
Low coolant pressure (10–50 bar) vs gundrilling's 50–150+ bar
Effective from relatively shallow holes (L/D > 3)
Suitable for irregular workpiece faces and non-round parts
Lower entry cost ($30k–$100k retrofit vs $200k+ BTA machine)

⚠ Limitations

L/D limited to ~100:1 (vs 300:1 gundrilling, 200:1 BTA)
Min diameter ~18mm (insufficient chip clearance below this)
Requires higher flow rates than gundrilling — pump upgrades may be needed
Tool design more complex due to Venturi nozzle geometry
Productivity between gundrilling and BTA
Venturi slots can wear or clog over time

COOLANT FLOW

Flow Requirements & Research

Recent 2025 research (Production Engineering, Springer) used Smoothed Particle Hydrodynamics (SPH) simulation to analyze flow in ejector drill heads. Key flow rate data for a 30mm drill in 42CrMo4+QT steel at Vc 60 m/min, feed 0.2 mm/rev:

Drill Head Type	At 60 m/min	At 80 m/min
Standard head (reference)	~29.5 L/min min	~51.5 L/min min
Optimized head (20° outlets)	~24.7 L/min min	~29.5 L/min min
Manufacturer recommended starting flow	55 L/min (for 30mm drill)

💡 Industry practice vs research: Standard practice over-feeds coolant by 10–80% above the true minimum. Optimized drill head designs can reduce minimum required flow by up to 43% while maintaining process stability. Optimized heads produce directed, nearly laminar flow even at very low rates (as low as 18 L/min). Vortex formation near the outer cutting edge — which can trap chips — is significantly reduced with optimized geometry. Normal feed force range: 2.5–4 kN; >6 kN = chip blockage.

APPLICATIONS

Where It's Used

🏯 Mold Cooling ChannelsDeep straight holes in mold plates, uniform temperature control. L/D 20:1–50:1

🔧 Hydraulic CylindersMedium-diameter bores retrofitted on existing lathes. Cost-effective vs BTA

⚙ Valve BodiesStepped and straight holes in hydraulic and pneumatic manifolds

🔉 Shaft PartsAxial bores in transmission and motor shafts on existing turning centers

📝 Oil & GasDownhole tool components where workpiece face is not perfectly square

🏭 Heavy MachineryStructural parts where dedicated deep hole machines are not available

RETROFIT GUIDE

Converting an Existing Machine

Check spindle

Verify through-spindle coolant capability. Min 50 bar recommended. Add rotary union + high-pressure pump if needed.

Check spindle bore

Existing lathe spindle bore must accommodate the double-tube diameter — typically 1.5–2× the drill diameter for the outer tube.

Install guide bushing

Position at workpiece entry. Outer tube must extend >5mm into bushing. Gap between workpiece and bushing <1mm.

Upgrade coolant system

Install pump delivering high flow at moderate pressure — NOT a high-pressure/low-flow gun drilling pump. Filtration to 50μm is adequate.

Return system

Provide coolant return system to handle the significant fluid volume returning from the bore. Whip guide supports every 40–60×D for deep holes.

Pilot hole & start

For L/D >12:1, pre-drill pilot hole. Use conservative parameters, verify C-shaped chips, optimize upward.

COMPARISON

Ejector vs. BTA vs. Gundrill

Criteria	Ejector	BTA	Gundrill
Diameter	18–250mm	6–2000mm	0.5–50mm
L/D max	~100:1	~200:1	~300:1
Coolant pressure	10–50 bar	15–100 bar	50–150+ bar
Face seal needed	No	Yes	Yes
Retrofit ready	Yes	Limited	Limited
Machine cost	$30k–$100k	$200k–$800k+	$50k–$200k
Productivity	3–5×	5–10×	1×

🎯

D < 15mm→ Gundrill

⚖

D 15–30mm→ Compare

⚡

D 30–150mm→ Ejector sweet spot

🏭

D > 150mm→ BTA or trepan

SAFETY

Key Safety Points

⚠️ Coolant pressure: Although lower than gundrilling, never disconnect fittings while pressurized. Relieve at pump before maintenance. Use whip-checks on all high-pressure hoses.

⚠️ Venturi nozzle inspection: The Venturi slots in the inner tube are critical for chip evacuation. Inspect for wear or clogging regularly — a blocked nozzle causes immediate chip evacuation failure. Feed force >6 kN = reliable chip blockage detection threshold.

Ejector Drilling (DTS)

How Ejector Drilling Works

Technical Data

Cutting Parameters

Strengths & Limitations

✅ Advantages

⚠ Limitations

Flow Requirements & Research

Where It's Used

Converting an Existing Machine

Ejector vs. BTA vs. Gundrill

Key Safety Points

Keep Reading