Appearance
An ejector drilling system operating at 25 bar coolant pressure can evacuate chips as effectively as a standard BTA system operating at 80 bar — because the venturi suction at the drill head creates a negative pressure differential that pulls chips through the inner tube regardless of the back pressure at the tube exit. The ejector system sacrifices approximately 50% of its coolant flow to drive the venturi effect, but gains the ability to operate on standard machine tools without high-pressure coolant seals, without external coolant sealing heads, and without modification to the workpiece entry face. This makes the ejector system the most practical deep hole drilling method for retrofitting onto existing CNC equipment.
Ejector System Design Principles
Venturi Nozzle Design Parameters
| Parameter | Typical Value | Effect on Performance | Measurement Method |
|---|---|---|---|
| Nozzle throat diameter | 1.5–4.0 mm (per nozzle) | Throat too small: choked flow — too large: insufficient velocity | Pin gauge — optical measurement |
| Nozzle exit diameter | 1.2–3.2 mm | Determines jet velocity and suction | Pin gauge |
| Nozzle angle (convergent) | 15–25° | Flow acceleration uniformity | Optical comparator |
| Nozzle angle (divergent) | 6–12° | Pressure recovery efficiency | Optical comparator |
| Number of nozzles | 3–8 (depending on diameter) | Even suction distribution | Visual count |
| Nozzle material | Carbide or hardened steel | Wear resistance — dimensional stability | Material specification |
| Venturi gap spacing | 0.5–2.0 mm | Suction rate — pressure recovery | Feeler gauge |
Coolant Split Ratio
| Parameter | Percentage of Total Flow | Purpose | Pressure Drop |
|---|---|---|---|
| Cutting edge coolant | 35–50% | Lubrication — cooling — chip breaking at cutting zone | High — through drill head coolant holes |
| Venturi nozzle flow | 40–55% | Generate suction — evacuate chips through inner tube | High — through restricted nozzle orifices |
| Guide pad lubrication | 5–15% | Reduce friction — cool guide pads | Moderate — through pad cooling passages |
Operating Parameters and Comparison
Ejector vs. BTA vs. DF Operating Parameters
| Parameter | Ejector (Jet-Suction) | Standard BTA | DF System |
|---|---|---|---|
| Coolant pressure at pump | 10–50 bar | 20–150 bar | 20–80 bar |
| Coolant pressure at drill head | 5–30 bar | 15–120 bar | 15–60 bar |
| Coolant flow rate (per mm diameter) | 2–5 L/min/mm | 1–4 L/min/mm | 1.5–4 L/min/mm |
| Suction vacuum at venturi | 0.2–0.8 bar | None | None |
| Maximum depth ratio | 100:1 | 200:1 | 100:1 |
| Hole diameter range | 18–120 mm | 15–300 mm | 15–80 mm |
| Surface finish (Ra) | 0.4–1.6 µm | 0.4–1.6 µm | 0.4–1.6 µm |
| Machine modification required | Coolant system only | Sealing head — high-pressure system | Coolant system — tube support |
Recommended Operating Parameters by Diameter
| Hole Diameter (mm) | Coolant Flow (L/min) | Coolant Pressure (bar) | Cutting Speed (m/min) — Steel | Feed Rate (mm/rev) — Steel | Suction Vacuum (bar) |
|---|---|---|---|---|---|
| 20 | 60–100 | 15–30 | 70–120 | 0.08–0.15 | 0.3–0.6 |
| 30 | 100–180 | 15–30 | 70–120 | 0.10–0.20 | 0.3–0.6 |
| 40 | 160–300 | 12–25 | 60–100 | 0.12–0.25 | 0.2–0.5 |
| 50 | 250–450 | 10–25 | 60–100 | 0.15–0.30 | 0.2–0.5 |
| 60 | 350–600 | 10–20 | 50–90 | 0.18–0.35 | 0.2–0.4 |
| 80 | 500–900 | 8–18 | 50–80 | 0.20–0.40 | 0.15–0.4 |
FAQ
How does the venturi effect create suction in the ejector system?
The venturi effect in the ejector system is created by diverting a portion of the high-pressure coolant through specially designed nozzles positioned near the drill head connection. As the coolant passes through the convergent section of each nozzle, its velocity increases and pressure decreases according to Bernoulli's principle. At the nozzle throat, the high-velocity jet creates a low-pressure zone that draws air and coolant from the chip evacuation tube. Downstream of the throat, the divergent section slows the jet and recovers pressure. The net effect is a continuous suction of approximately 0.2–0.8 bar at the venturi section, which pulls chips and coolant from the cutting zone through the inner tube and out the rear of the tool assembly. The venturi section is designed to maintain this suction regardless of the back pressure at the tube exit, making the system self-regulating.
What are the machine requirements for retrofitting an ejector system?
The machine requirements for retrofitting an ejector system are relatively modest compared to BTA. The spindle must have a through-bore large enough to pass the ejector tube assembly (typically 5–15 mm larger than the tube OD). The coolant system must deliver 10–50 bar pressure at the required flow rate — many standard CNC lathes with high-pressure coolant options already meet this requirement, though an additional booster pump may be needed. The coolant filtration system must be capable of 20–50 micron filtration to prevent nozzle blockage. The machine must have sufficient Z-axis travel for the required hole depth plus tool overhang. A guide bushing support must be mounted to maintain the tube position at the spindle exit. No external coolant sealing head is required, and the workpiece needs no special preparation at the entry face beyond a standard spot-facing or center-drilled pilot.
What causes suction loss in ejector drilling?
Suction loss in ejector drilling is caused by several factors. Venturi nozzle wear is the most common — nozzle erosion from abrasive coolant opens the throat diameter, reducing jet velocity and therefore suction. Nozzle blockage from coolant contamination or fine chip particles also reduces or eliminates suction on individual nozzles. Incorrect venturi gap spacing — the distance between the nozzle exit and the diffuser entry — is another frequent cause; if this gap is too large or too small, the pressure recovery is reduced and suction drops. Coolant temperature increase (above 50°C) reduces coolant viscosity and changes the flow characteristics through the nozzles, reducing suction efficiency. Suction loss is detected by monitoring the coolant return flow — reduced return volume through the inner tube indicates reduced suction. Regular nozzle inspection and replacement at scheduled intervals prevents suction-related production interruptions.
Can the ejector system achieve the same hole quality as standard BTA?
Yes — the ejector system can achieve equivalent hole quality to standard BTA in terms of diameter tolerance, surface finish, and hole straightness for applications within its operating range. The ejector system produces the same surface finish (Ra 0.4–1.6 µm) and diameter tolerances (IT7–IT10) as standard BTA because the cutting mechanics at the drill head are essentially identical — the difference is only in the coolant delivery and chip evacuation method. The ejector system's lower coolant pressure at the cutting edges is compensated by the venturi-assisted chip evacuation, which keeps the cutting zone clear of chips. The main quality limitation is depth-to-diameter ratio — beyond 100:1, the ejector system's chip evacuation may become less reliable than BTA, potentially affecting hole quality at extreme depths. For the vast majority of applications within 100:1 depth ratio, the ejector system produces fully equivalent hole quality.
What maintenance does the ejector system venturi section require?
The ejector system venturi section requires regular inspection and maintenance at intervals of 500–2000 holes depending on coolant quality and operating conditions. The venturi nozzles must be inspected for wear using pin gauges — if the nozzle throat diameter has increased by more than 0.05 mm from the original size, the nozzle should be replaced. The venturi gap (distance from nozzle exit to diffuser entry) should be measured using feeler gauges and adjusted to the manufacturer's specification if it has shifted. The coolant return passages should be inspected for erosion from abrasive chip flow — particularly at the tube entry to the venturi section where chip velocity is highest. Seals between the inner and outer tubes should be replaced at every venturi inspection. A suction test (measuring vacuum at the venturi with a manometer at standard coolant flow) should be performed after each maintenance procedure to verify the system is operating within specification.
Disclaimer: The ejector system design parameters and operating recommendations provided in this article are general guidelines based on industry-standard practices. Actual system configurations vary by manufacturer and specific application requirements. Ejector system selection should consider workpiece material, hole geometry, machine capabilities, and production volume. Venturi nozzle inspection and replacement should follow the manufacturer's specific procedures. The authors and publisher assume no liability for any damages or losses arising from the use of this information — always follow original equipment manufacturer guidelines for your specific equipment. Content is for informational purposes only and does not constitute professional engineering advice. Verify all parameters with qualified personnel before implementation as of 2026.