Reverse Flow vs Forward Flow Coolant Systems in BTA Drilling

The difference between forward flow and reverse flow coolant in BTA drilling is not merely a matter of plumbing. It determines whether you need a dedicated machine with a pressure head or can run on a standard lathe, whether you achieve IT7 or IT11 tolerances, and whether chip evacuation remains reliable at the far end of a 3-meter bore.

Overview ​

BTA deep hole drilling uses two distinct coolant delivery architectures. The forward flow (STS/Single Tube System) is the original BTA configuration, delivering coolant externally through the annular gap between the drill tube and the bore wall. The reverse flow (ejector/DTS/Double Tube System) is a later development that delivers coolant through concentric tubes and uses a venturi to create suction for chip evacuation.

Forward Flow: STS/BTA System ​

Operating Principle ​

In the forward flow system, coolant is supplied through a pressure head unit called a BOZA (Bohrölzuführungsarmatur — drilling oil supply fitting) that seals against the workpiece face. The coolant travels forward through the annular space between the outer diameter of the drill tube and the wall of the drilled hole.

Coolant path:  Pump → BOZA → Annular gap (tube OD to bore wall) → Cutting face
Chip path:     Cutting face → Chip mouth → Inner tube → Machine spindle → Filtration

Components ​

Coolant Flow Path Detail ​

Forward flow coolant follows this path:

1. Pump discharge — High-pressure coolant pump delivers oil at 3–10 MPa
2. Rotary union — Transfers coolant from stationary supply to rotating machine spindle
3. BOZA entry — Coolant enters the pressure head, which seals against the workpiece face
4. Annular gap — Coolant flows forward through the gap between the drill tube OD and the bore wall
5. Cutting zone — Coolant reaches the cutting inserts, providing lubrication, cooling, and hydraulic chip transport
6. Chip mouth entry — Chips are forced by coolant pressure into the chip mouth openings on the drill head
7. Inner tube return — The chip/coolant mixture travels back through the hollow interior of the drill tube
8. Spindle exit — Mixture exits through the machine spindle to the filtration system

Sealing Requirements ​

The BOZA must maintain a pressure-tight seal at the workpiece face. This requires:

Advantages of Forward Flow ​

Reverse Flow: Ejector/DTS System ​

Operating Principle ​

In the reverse flow (ejector) system, coolant is delivered through a double tube — an inner tube inside an outer tube. The coolant flows forward through the annular gap between the two tubes, and chips return through the inner tube. A venturi nozzle creates suction at the chip mouth to assist chip evacuation.

Coolant path:  Pump → Spindle connector → Annular gap (inner tube OD to outer tube ID) → 
               Drill head → Partial flow through lateral outlets → Cutting face
               Partial flow through ring nozzle → Ejector (suction generation)
Chip path:     Cutting face → Chip mouth → Inner tube (vacuum-assisted) → Ejector → Filtration

Components ​

The Ejector Effect ​

The key innovation of the reverse flow system is the ejector effect, which creates suction at the chip mouth without moving parts:

1. Approximately 2/3 of the coolant flows through the lateral outlets on the drill head, flushing chips from the cutting zone
2. The remaining 1/3 of the coolant is directed through a ring nozzle at the rear of the inner tube
3. The ring nozzle creates a venturi: high-velocity coolant exiting the nozzle reduces pressure in the inner tube
4. The resulting vacuum (typically 0.1–0.3 MPa below atmospheric) pulls chips and coolant back through the inner tube
5. The combined flow exits the system at the spindle end

No Sealing Required ​

The ejector system is self-contained — coolant circulates entirely within the double-tube assembly. No BOZA or workpiece seal is needed. This is the defining advantage of the reverse flow system:

Limitations of Reverse Flow ​

Coolant Flow Path Comparison ​

Path Schematic ​

Hydraulic Characteristics ​

Chip Evacuation Mechanism ​

Forward Flow Chip Transport ​

In the forward flow system, chips are evacuated by positive pressure:

The chip mouth openings on the drill head are sized to admit broken chips but block oversized fragments. Typical chip mouth dimensions:

Reverse Flow Chip Transport ​

In the ejector system, chip evacuation relies on a combination of pressure and vacuum:

Warning — The ejector effect weakens at depths beyond approximately 80× diameter. For very deep holes (L/D > 100:1), the forward flow system provides more reliable chip evacuation because the pressure-driven transport does not depend on suction that must propagate the full length of the tube.

Pressure Requirements and Hydraulics ​

Forward Flow Pressure Profile ​

The pressure in a forward flow system drops along the flow path:

The pressure loss in the annular gap is a function of:

• Gap size (bore diameter minus tube OD) — smaller gaps create higher pressure loss
• Tube length — longer tubes increase friction loss
• Coolant viscosity — higher viscosity increases loss
• Flow rate — loss increases with the square of flow rate

Reverse Flow Pressure Profile ​

Pump Selection ​

Machine Compatibility and Integration ​

Forward Flow Machine Requirements ​

Suitable applications: high-volume production, difficult materials, deep holes (L/D > 80:1), tight tolerance requirements.

Reverse Flow Machine Requirements ​

Suitable applications: retrofit applications, moderate depths (L/D < 80:1), multi-operation parts, lower volume production.

Retrofit Considerations ​

The reverse flow (ejector) system was developed specifically to enable deep hole drilling on conventional machine tools. Key retrofit considerations:

Application-Specific Selection ​

Select Forward Flow (STS/BTA) When ​

Select Reverse Flow (Ejector/DTS) When ​

Diameter Range Overlap ​

For diameters in the overlap range (18–250 mm) where both systems can be used:

Summary ​

FAQ ​

What is the difference between forward flow and reverse flow coolant in BTA drilling? ​

Forward flow (STS/BTA) delivers coolant through the annular gap between the drill tube outer diameter and the bore wall. Coolant is pumped forward under high pressure (3–10 MPa) and chips return through the inner tube. Reverse flow (ejector/DTS) delivers coolant through the annular gap between an inner and outer concentric tube. It uses a venturi nozzle to create suction that pulls chips back through the inner tube at lower pressure (1–4 MPa), requiring no seal against the workpiece.

Which system provides better chip evacuation? ​

Forward flow (STS/BTA) provides more reliable chip evacuation, especially at high L/D ratios. The pressure-driven flow maintains chip transport velocity regardless of depth. The full inner tube cross-section is available for chip passage. Reverse flow (ejector/DTS) relies on suction from the venturi effect, which weakens with depth and has a reduced chip passage cross-section due to the double-tube design. For L/D ratios beyond 80:1, forward flow is the preferred choice.

Can the ejector system be used on any CNC lathe? ​

The ejector system can be retrofitted to most CNC lathes and machining centers with through-spindle or through-turret coolant capability. Requirements include: minimum coolant pressure of 1 MPa at the required flow rate, a spindle connector for the double-tube assembly, a guide bushing holder that fits the turret or spindle adapter, and coolant filtration to 20–30 µm. Most modern CNC machines with high-pressure coolant options meet these requirements.

What diameter range does each system cover? ​

Forward flow (BTA/STS) covers from approximately 7.76 mm to 2,000 mm, with typical industrial applications starting at 16 mm. Reverse flow (ejector/DTS) covers approximately 18–250 mm. Below 18 mm, the double-tube configuration cannot accommodate both the inner and outer tubes with sufficient cross-section for chip flow. Above 250 mm, the ejector effect becomes less effective and the stiffness advantage of the single-tube forward flow system dominates.

Why does the ejector system not require a pressure head? ​

The ejector system is self-contained — coolant circulates entirely within the double-tube assembly. The outer tube contains the coolant flow, and the inner tube conducts chips. No sealing against the workpiece is needed because coolant never contacts the bore wall at high pressure. In contrast, the forward flow system delivers coolant through the annular gap between the tube and the bore wall, which requires a seal (BOZA) at the workpiece entry point to prevent coolant from escaping.

Which system achieves tighter tolerances? ​

Forward flow (STS/BTA) achieves tighter tolerances: IT7–IT9 compared to IT9–IT11 for the reverse flow (ejector) system. The difference is due to the higher torsional rigidity of the single tube (no inner tube reducing the effective cross-section) and the more stable cutting conditions provided by higher coolant pressure and more effective chip evacuation. The ejector system's reduced rigidity allows more vibration, which degrades concentricity.

Is the ejector system suitable for high-volume production? ​

The ejector system is suitable for moderate-volume production but is generally outperformed by the forward flow system for high-volume applications. The forward flow system offers higher cutting performance (higher metal removal rate), tighter tolerances (reducing or eliminating secondary operations), and more reliable chip evacuation (reducing downtime for chip clearing). The ejector system's main advantage is machine compatibility, not production speed.

What are the pressure requirements for each system? ​

Forward flow (STS/BTA) requires 3–10 MPa (435–1,450 psi) at the pump, depending on bore diameter, depth, and material. Reverse flow (ejector/DTS) requires 1–4 MPa (145–580 psi). The ejector system operates at lower pressure because the coolant path is entirely within the tool assembly (shorter distance, no annular gap friction loss against the bore wall). The forward flow system must overcome friction loss through the long annular gap between the tube and bore wall.

Can both systems use the same drill heads? ​

Drill heads for forward flow and reverse flow are structurally similar but have different coolant outlet configurations. Forward flow drill heads have coolant passages that direct flow from the annular gap to the cutting edges and chip mouth. Reverse flow drill heads have additional lateral coolant outlets on the circumference that direct flow from the inter-tube annular space to the cutting zone and chip mouth. The cutting insert geometry and guide pad layout are typically similar between the two systems.

Which system should I choose for retrofitting an existing machine? ​

Choose the reverse flow (ejector/DTS) system for retrofitting an existing conventional machine tool. The ejector system requires only a spindle connector, guide bushing holder, and adequate coolant supply — no pressure head or machine structure modifications. This allows deep hole drilling capability to be added to a standard CNC lathe or machining center at a fraction of the cost of a dedicated BTA machine. The trade-off is reduced cutting performance and looser tolerances compared to a dedicated forward flow system.

Coolant system selection depends on hole geometry, material, production volume, and available machine tools. The specifications in this article represent typical ranges for production BTA drilling as of 2026. Consult machine and tooling suppliers for application-specific recommendations.