Appearance
A German hydraulic cylinder manufacturer operating two dedicated BTA deep hole drilling machines for 80 mm diameter × 2,000 mm deep bores in 1026 steel found itself capacity-constrained when a major customer increased annual order volume by 60%. The lead time for a new BTA machine was 14 months — too long to meet the demand increase — and the capital investment of approximately €380,000 for a machine with the required stroke length was difficult to justify for a single product line. The manufacturer's application engineers evaluated ejector drilling as an alternative: by converting a standard CNC lathe (existing asset, fully depreciated) to ejector drilling, the total investment was €57,000 — a double-tube boring bar assembly, coolant system upgrade to 30 bar, and tooling — with a conversion time of 6 weeks. The ejector drilling performance was impressive for a converted conventional lathe: cycle time of 18 minutes per bore (versus 14 minutes on the dedicated BTA machine), bore straightness of 0.08 mm over 2,000 mm, and surface finish of Ra 1.2 µm. The coolant pressure requirement of only 30 bar (versus 80 bar for BTA) was handled by a standard coolant pump with a booster, and the absence of a pressure head seal — the double tube system contains the coolant and chip flow entirely within the boring bar assembly — simplified setup and changeover. The 28% longer cycle time was acceptable because the machine was running two shifts and the additional time per bore was offset by the lower capital cost. The ejector drilling conversion allowed the manufacturer to meet the increased demand without a major capital investment, and the CNC lathe retained its ability to perform conventional turning operations when deep hole drilling capacity was not required. This case illustrates the strategic value of ejector drilling as a flexible, low-capital alternative to dedicated BTA machines for medium-diameter, medium-precision deep hole drilling applications.
Principles of Ejector Drilling
The Double Tube System
Ejector drilling operates on a concentric double-tube boring bar principle. The outer tube — typically 10–20 mm smaller in diameter than the bore — connects to the drill head at the cutting end and to a coolant connector at the machine end. The inner tube — approximately 50–60% of the outer tube diameter — runs concentrically inside the outer tube and provides the chip evacuation passage. The annular space between the inner and outer tubes is the coolant delivery passage.
The drill head for ejector drilling is similar to a BTA drill head in its cutting geometry — it uses indexible carbide inserts for cutting and carbide guide pads for bore wall support — but differs in an essential detail: the ejector drill head incorporates angled nozzles or slots (the ejector nozzles) that divert a portion of the incoming coolant from the annular delivery passage into the inner tube at high velocity. These nozzles are positioned behind the cutting zone, typically 20–50 mm from the cutting edge, and are angled toward the machine end of the inner tube at 15–30° relative to the tube axis.
The operating principle is a controlled coolant pressure and flow balance. Coolant enters the coolant connector at the machine end at 20–60 bar and flows through the annular space toward the drill head. At the drill head, the coolant flow splits: approximately 40–60% of the coolant flows through the ejector nozzles into the inner tube, creating a high-velocity jet that induces suction (the Venturi effect) in the inner tube. The remaining 40–60% of the coolant flows to the cutting zone, where it cools and lubricates the cutting inserts and guide pads before being drawn — together with the chips — through the chip mouth in the drill head and into the inner tube. The combined coolant-and-chip flow is then transported through the inner tube by the suction generated by the ejector nozzles and returned to the coolant system for filtration and recirculation.
Coolant Flow Dynamics
The critical design parameter in ejector drilling is the coolant flow balance between the ejector nozzles and the cutting zone. If too much coolant goes through the ejector nozzles, the cutting zone is under-lubricated and the tool overheats. If too little coolant goes through the ejector nozzles, the suction is insufficient for chip evacuation and chips accumulate in the inner tube. The flow balance is determined by the geometry and cross-sectional area of the ejector nozzles relative to the cutting zone coolant passages, and is fixed by the drill head design — it cannot be adjusted by the operator.
The suction generated by the ejector nozzles is proportional to the coolant pressure drop across the nozzles and the nozzle velocity. For a typical ejector drill head operating at 30 bar inlet pressure, the pressure drop across the ejector nozzles is 15–25 bar, producing a nozzle exit velocity of 50–80 m/s. The resulting suction at the chip mouth is equivalent to a pressure of 0.2–0.5 bar below atmospheric pressure — sufficient to transport chips through the inner tube at velocities of 5–15 m/s, depending on chip size and density.
The coolant pressure requirement for ejector drilling is substantially lower than for BTA drilling of the same bore diameter. For a 60 mm diameter bore at 2,000 mm depth, BTA drilling requires 80–120 bar coolant pressure, while ejector drilling requires 20–40 bar for the same chip evacuation performance. This lower pressure requirement is the key operational advantage of ejector drilling — it allows the use of standard machine tool coolant systems (typically rated for 20–70 bar) rather than the dedicated high-pressure systems required for BTA drilling.
Pressure and Flow Characteristics by Diameter
The table below summarizes typical coolant pressure and flow requirements for ejector drilling by bore diameter range, compared to BTA drilling of the same diameter.
| Bore Diameter (mm) | Ejector Coolant Pressure (bar) | Ejector Coolant Flow (L/min) | BTA Coolant Pressure (bar) | BTA Coolant Flow (L/min) | Ejector Max Depth (mm) | Ejector Typical MRR (cm³/min) |
|---|---|---|---|---|---|---|
| 18–25 | 30–50 | 40–80 | 80–150 | 50–100 | 1,500 | 15–40 |
| 25–40 | 25–45 | 60–120 | 70–120 | 80–150 | 2,500 | 40–100 |
| 40–65 | 20–40 | 100–200 | 60–100 | 120–250 | 4,000 | 100–250 |
| 65–100 | 18–35 | 150–300 | 50–90 | 200–400 | 6,000 | 250–600 |
| 100–150 | 15–30 | 200–400 | 40–80 | 300–600 | 8,000 | 400–1,000 |
| 150–200 | 12–25 | 300–500 | 30–60 | 400–800 | 10,000 | 600–1,500 |
Technical Comparison with BTA and Gun Drilling
Method Selection Criteria
The choice between ejector drilling, BTA drilling, and gun drilling depends on bore diameter, depth, precision requirements, machine tool availability, and production volume. The table below provides a comprehensive technical comparison.
| Parameter | Ejector Drilling (DTS) | BTA Drilling (STS) | Gun Drilling |
|---|---|---|---|
| Diameter Range | 18–200 mm (up to 700 mm with special tooling) | 6–300 mm | 0.8–40 mm |
| Depth-to-Diameter Ratio | Up to 250:1 | Up to 200:1 | Up to 300:1 |
| Typical Surface Finish Ra | 1.0–2.5 µm | 0.8–2.0 µm | 0.4–1.5 µm |
| Typical Diameter Tolerance | IT9–IT11 | IT8–IT10 | IT7–IT9 |
| Metal Removal Rate (relative) | High | Very High | Low to Moderate |
| Coolant Pressure Required | 15–50 bar | 30–150 bar | 50–200 bar |
| Machine Requirement | Conventional lathe or machining center with coolant booster | Dedicated BTA machine with pressure head seal | Dedicated gun drilling machine or gun drilling attachment |
| Pressure Head Seal Required | No — coolant contained within double tube | Yes — must seal against workpiece bore entry | No — coolant enters through drill tube |
| Tooling Cost (relative) | Moderate | Moderate | Lower (solid carbide or carbide-tipped) |
| Setup Complexity | Low — standard tool post or spindle mount | High — pressure head and seal setup | Moderate — guide bush and collet setup |
| Chip Evacuation | Venturi suction through inner tube | Coolant pressure through single tube | Coolant pressure through single tube (external chip evacuation in flute) |
| Bore Straightness (typical) | 0.05–0.15 mm / 1,000 mm | 0.03–0.10 mm / 1,000 mm | 0.02–0.08 mm / 1,000 mm |
When to Choose Ejector Drilling
Ejector drilling is the optimal choice — among the three deep hole drilling methods — in the following situations:
Converting an existing conventional machine tool — When a BTA or gun drilling machine is not available and the capital investment for a dedicated machine is not justified, ejector drilling converts a standard lathe or machining center to deep hole drilling capability with relatively low investment. The conversion requires: a double-tube boring bar assembly, a coolant system upgrade to 20–50 bar, a coolant connector mounted on the tool post or spindle, and the drill head and guide bush tooling. The total conversion cost is typically 10–25% of the cost of a dedicated deep hole drilling machine.
Medium-diameter bores (20–100 mm) — Ejector drilling is most efficient in the medium-diameter range. Below 18 mm diameter, the double-tube assembly becomes mechanically impractical — the inner tube diameter is too small for adequate chip evacuation. Above 100 mm diameter, the weight and cost of the double-tube assembly increase substantially, and BTA drilling's higher coolant flow capacity provides higher metal removal rates that offset the higher capital cost.
Moderate precision requirements (IT9–IT11) — Ejector drilling produces bore quality that is adequate for hydraulic cylinders, structural components, and general mechanical applications. For applications requiring tighter tolerances (IT7–IT8) or very fine surface finishes (Ra < 0.8 µm), gun drilling or BTA drilling with precision tooling is preferred.
Low to moderate production volumes — Ejector drilling's lower capital investment and shorter setup time make it cost-effective for job shop and low-volume production where the machine can be used for conventional turning operations when deep hole drilling is not required.
When BTA Drilling Is Preferable
BTA drilling is the preferred method when: the highest metal removal rate is required (BTA achieves 30–50% higher MRR than ejector drilling at equivalent bore diameters), the bore diameter is below 18 mm (where ejector drilling is not feasible), the available machine is already configured for BTA with a pressure head system, or the bore quality requirements demand IT8 tolerance or better. BTA drilling also achieves better bore straightness than ejector drilling (0.03–0.10 mm/1,000 mm versus 0.05–0.15 mm/1,000 mm) due to the higher coolant pressure providing more rigid hydraulic support for the drill head.
When Gun Drilling Is Preferable
Gun drilling is the preferred method for: bore diameters below 18 mm (particularly below 6 mm where neither BTA nor ejector can operate), applications requiring the highest bore quality (IT7–IT8 tolerance, Ra < 0.8 µm finish), very high depth-to-diameter ratios (exceeding 100:1), and through-bore applications where the chip evacuation through the external flute is acceptable. Gun drilling is also preferred for small-diameter precision components such as fuel injectors, medical devices, and hydraulic valve components.
Cutting Parameters and Tooling
Recommended Cutting Parameters
The following table provides recommended starting parameters for ejector drilling of common workpiece materials. Parameters should be adjusted based on specific machine rigidity, coolant system capability, and bore geometry.
| Workpiece Material | Cutting Speed (m/min) | Feed Rate (mm/rev) | Insert Grade | Guide Pad Material | Coolant Pressure (bar) | Coolant Type |
|---|---|---|---|---|---|---|
| Carbon Steel (150–250 HB) | 80–120 | 0.08–0.20 | P20–P30 carbide (TiN/TiAlN) | C2 grade carbide | 20–35 | Oil or emulsion 8–10% |
| Alloy Steel (250–350 HB) | 60–90 | 0.06–0.15 | P25–P35 carbide (TiAlN) | C2 grade carbide | 25–40 | Oil-based EP |
| Cast Iron (180–280 HB) | 80–130 | 0.10–0.30 | K10–K20 carbide (uncoated or TiN) | C3 grade carbide | 15–30 | Emulsion 5–8% |
| Stainless Steel (austenitic) | 50–80 | 0.05–0.12 | M10–M20 carbide (AlCrN) | C2 grade carbide | 30–50 | Oil-based EP |
| Aluminum (wrought, 6061) | 150–300 | 0.10–0.30 | K10–K20 carbide (polished) | C3 grade carbide or PCD | 15–25 | Emulsion 5–8% or MQL |
| Brass / Bronze | 120–200 | 0.08–0.20 | K10–K20 carbide (polished) | C3 grade carbide | 15–25 | Emulsion 5–8% |
| Titanium Alloys (Ti-6Al-4V) | 25–40 | 0.04–0.10 | S10–S20 carbide (AlTiN) | C2 grade carbide | 35–55 | Oil-based EP |
Drill Head and Tooling Design
Ejector drill heads are available in two primary configurations: indexible insert heads (for diameters above 20 mm) and brazed carbide tip heads (for diameters 18–25 mm). Indexible insert heads use standard or specialized indexible carbide inserts mounted in pockets on the drill head body, with guide pads (typically two or three) positioned on the drill head periphery to support and stabilize the head in the bore.
The indexible insert configuration for ejector drilling is distinguished from BTA inserts by the chip mouth design. In ejector drilling, the chip mouth opens into the inner tube through the drill head body, and the chip passage is designed to guide chips smoothly into the suction zone of the ejector nozzles. The chip mouth should have a smoothly radiused entry (minimum 2 mm radius) to prevent chip hang-up at the entrance, and the chip passage should have a surface finish of Ra < 0.4 µm to minimize chip friction.
The guide pads on ejector drill heads use the same materials and geometries as BTA drill head pads: C2 grade carbide for steel and cast iron, C3 grade for non-ferrous materials, and PCD for aluminum. The pad land width should be matched to the workpiece material — 0.8–1.5 mm for carbon steel, 0.5–1.0 mm for cast iron, and 1.0–2.0 mm for aluminum.
Double Tube Boring Bar Assembly
The double-tube boring bar assembly consists of: the outer tube (connects to drill head at one end and coolant connector at the other), the inner tube (inserted concentrically within the outer tube, extending from the chip mouth area to the coolant return), and the coolant connector (mounts to the machine tool post or spindle and provides connections for coolant supply and return).
The outer tube diameter is typically 10–20 mm smaller than the bore diameter to provide clearance for the guide bush. The inner tube diameter is 50–60% of the outer tube diameter — large enough for chip evacuation but small enough to maintain adequate annular flow area for coolant delivery. The tube lengths are matched to the maximum bore depth, with a safety margin of 200–500 mm.
The tube material is typically high-strength alloy steel (AISI 4140 or 4340, heat-treated to 280–350 HB) for the outer tube and carbon steel for the inner tube. The tube straightness should be within 0.10 mm per meter of length, and the inner tube should be concentric with the outer tube within 0.10 mm TIR.
Application Limitations and Considerations
Diameter and Depth Limitations
The minimum practical diameter for ejector drilling is 18–20 mm. Below this diameter, the double-tube assembly cannot provide sufficient annular flow area for coolant delivery while maintaining adequate inner tube diameter for chip evacuation. A 15 mm bore requires an outer tube of approximately 8–10 mm diameter — the inner tube would be only 4–5 mm diameter, and the annular flow area would be approximately 25–40 mm², which is insufficient for the coolant flow required to operate the ejector effect and provide cutting zone lubrication.
The maximum depth for ejector drilling is limited by the pressure drop in the coolant delivery and return passages. As the bore depth increases, the friction losses in the annular delivery passage and the inner tube reduce the pressure available at the ejector nozzles and the suction available for chip evacuation. For diameters above 40 mm, depths of 5,000–8,000 mm are achievable. For diameters below 25 mm, the practical depth limit is approximately 1,500–2,500 mm.
Coolant Filtration Requirements
Ejector drilling requires coolant filtration to 20–30 µm or better. The ejector nozzles have small-diameter orifices (typically 1–3 mm diameter) that can become blocked by chips or debris larger than the nozzle diameter. A blocked nozzle disrupts the flow balance between the cutting zone and the ejector, reducing suction and causing chip evacuation failure. The coolant system should include a filter with automatic cleaning (self-cleaning paper filter or centrifugal separator) rated for the coolant flow rate of the ejector drilling system.
Machine Rigidity Considerations
Ejector drilling on conventional lathes requires adequate machine rigidity to handle the cutting forces. The radial cutting forces in ejector drilling are comparable to those in BTA drilling of the same diameter — typically 500–3,000 N for 40–100 mm diameter bores. The machine spindle bearings, bed, and tool post must be capable of handling these forces without excessive deflection or chatter. A CNC lathe with a 50–100 mm bar capacity and 15–30 kW spindle power is adequate for ejector drilling of bores up to 80 mm diameter. Larger bores (80–200 mm) require machines with 30–75 kW spindle power and reinforced bed construction.
FAQ
What is the difference between ejector drilling and BTA drilling?
The fundamental difference is the coolant and chip evacuation path and the requirement for a pressure head seal. In BTA drilling (single tube system), coolant is delivered through the annular space between the outer drill tube and the bore wall, and a pressure head seal must be pressed against the workpiece entry face to contain the high-pressure coolant. In ejector drilling (double tube system), coolant is delivered through the annular space between two concentric tubes and contained entirely within the boring bar assembly — no seal against the workpiece is required. Chip evacuation in BTA uses the coolant pressure to push chips through the center of the single tube, while ejector drilling uses a Venturi suction effect (created by coolant passing through angled ejector nozzles) to draw chips through the inner tube. Ejector drilling operates at lower coolant pressure (15–50 bar vs. 30–150 bar for BTA) and can be used on conventional machine tools without dedicated deep hole drilling equipment.
What bore diameters can ejector drilling achieve?
Ejector drilling is practical for bore diameters from 18 mm to 200 mm in standard applications, with special tooling available for diameters up to 700 mm. The minimum diameter is constrained by the double-tube assembly geometry — the outer tube must fit within the bore, the inner tube must be large enough for chip evacuation, and the annular gap between the tubes must be sufficient for coolant flow. Below 18 mm diameter, the inner tube becomes too small for effective chip transport, and the annular coolant passage is too restrictive for the required flow. The maximum diameter is constrained more by practical and economic factors than technical limitations — above 200 mm diameter, the weight and cost of the double-tube assembly increase significantly, and BTA drilling typically provides better economics and metal removal rates at these diameters.
Can ejector drilling be used on a standard CNC lathe?
Yes — this is the primary advantage of ejector drilling. A standard CNC lathe can be converted to ejector drilling by installing: a double-tube boring bar assembly with the coolant connector mounted on the tool post or turret; a coolant system upgrade to achieve 20–50 bar pressure and appropriate flow rate (typically 100–300 L/min for medium diameters); a guide bush mounted in the spindle or on a steady rest; and appropriate chip handling and filtration for the coolant return. The conversion does not require any permanent modification to the lathe — the coolant connector mounts to the tool post, and the guide bush can be clamped to the lathe bed or mounted in a spindle adapter. The lathe retains its full turning capability when the ejector drilling tooling is removed. Most ejector drilling conversions on CNC lathes use the lathe's existing spindle drive (C-axis) for rotation and the Z-axis for feed, with the boring bar stationary on the tool post.
How does coolant pressure in ejector drilling compare to BTA?
Ejector drilling typically requires 30–60% lower coolant pressure than BTA drilling for the same bore diameter and depth. For a 60 mm diameter bore at 2,000 mm depth in carbon steel: ejector drilling requires 20–35 bar, while BTA drilling requires 60–100 bar. For a 100 mm diameter bore at 3,000 mm depth: ejector drilling requires 15–30 bar, while BTA drilling requires 40–80 bar. The lower pressure requirement is a consequence of the Venturi suction principle — the ejector nozzles create a localized low-pressure zone at the chip mouth that draws chips and coolant through the inner tube at lower pump pressure than the positive displacement approach used in BTA. The lower coolant pressure also reduces the machine modification requirements — a standard coolant pump with a pressure booster can typically achieve 30–50 bar, while BTA often requires a dedicated high-pressure coolant system.
What are the main limitations of ejector drilling?
The main limitations of ejector drilling are: minimum bore diameter of approximately 18 mm (compared to 6 mm for BTA and 0.8 mm for gun drilling); lower metal removal rate than BTA (typically 20–30% lower at equivalent bore diameters due to lower coolant flow capacity); lower bore quality than gun drilling (typical IT9–IT11 tolerance and Ra 1.0–2.5 µm finish, compared to IT7–IT9 and Ra 0.4–1.5 µm for gun drilling); sensitivity to coolant nozzle blockage (the ejector nozzles can clog if coolant filtration is inadequate); and higher tooling cost than gun drilling for small diameters. Ejector drilling is also not suitable for applications requiring very high straightness — the typical straightness of 0.05–0.15 mm per 1,000 mm is adequate for hydraulic cylinders and structural components but does not match the 0.02–0.08 mm per 1,000 mm achievable with precision gun drilling.
Disclaimer: The process parameters, performance data, and comparison information presented in this article are based on published technical literature, tooling manufacturer specifications, and industry-reported experience with ejector drilling. Actual results depend on machine tool condition, coolant system capability, workpiece material, bore geometry, and tooling selection. The cutting parameters provided should be used as starting recommendations and verified through process development trials. The conversion of conventional machine tools to ejector drilling should be reviewed by a qualified machine tool engineer to ensure the machine has adequate rigidity, power, and safety provisions for deep hole drilling operations. No guarantee of specific performance, bore quality, or tool life is expressed or implied. All data is provided for informational purposes and reflects industry practices as of 2026.