BTA STS vs DTS: Single-Tube & Double-Tube System Comparison

The BTA system exists in two configurations that share the same drill head and cutting principle but differ fundamentally in coolant delivery. The Single-Tube System (STS) delivers coolant through the annular gap between the drill tube and the hole wall, requiring a pressure head sealed against the workpiece. The Double-Tube System (DTS) delivers coolant through the space between two concentric tubes, eliminating the pressure head entirely. This distinction determines the machine requirements, depth capability, and application suitability of each system.

Overview ​

The BTA deep hole drilling system was developed by the Boring and Trepanning Association in the 1940s as an alternative to gun drilling for larger diameter holes. BTA drilling uses multi-edge drill heads (typically 2–4 cutting edges) with indexable carbide inserts, delivering high feed rates and excellent hole quality.

Two principal configurations evolved:

Both systems evacuate chips through the internal bore of the drill tube, keeping the finished bore wall undisturbed by chip flow.

Single-Tube System (STS) ​

Operating Principle ​

The STS drill consists of a single thick-walled tube with a drill head attached at the cutting end and a pressure head assembly at the machine spindle end.

Coolant flow path:

1. High-pressure coolant is delivered to the pressure head
2. The pressure head seals against the workpiece face, directing coolant into the annular gap between the drill tube OD and the machined bore ID
3. Coolant flows forward to the drill head, exiting through strategically positioned nozzles directed at each cutting edge
4. Coolant, now carrying chips, enters chip openings in the drill head and flows back through the internal bore of the drill tube
5. The chip-laden coolant exits through the machine spindle and returns to filtration

Chip evacuation: Chips are evacuated entirely through the internal bore of the drill tube. The chip passage cross-section is the full internal diameter of the tube — the largest possible chip evacuation path in any deep hole drilling system.

Pressure Head Requirements ​

The pressure head is the defining hardware requirement of STS drilling. It consists of:

• A sealing ring that contacts the workpiece face
• A drill bush that guides the drill at hole entry
• Coolant inlet ports
• Seals around the rotating drill tube

The pressure head must maintain a leak-tight seal against the workpiece face at coolant pressures of 10–50 bar. This requires:

• A flat, perpendicular workpiece face at the entry point
• Sufficient clamping force to resist the coolant pressure force (pressure × area can be several tonnes)
• A machine with the structural capacity to react this force

Performance Characteristics ​

Advantages of STS ​

• Largest chip evacuation passage — the full internal bore of the tube, reducing chip packing risk in difficult materials
• Highest feed rates — 5–7× higher than gun drilling at equivalent diameter due to balanced multi-edge cutting
• Superior surface finish — guide pads burnish the bore as the drill advances
• Continuous drilling — no peck cycles needed; coolant continuously clears chips
• Deepest holes — up to 400:1 L/D in suitable conditions
• Best chip evacuation for difficult materials — the large internal passage handles long chips from low-carbon steel and stainless steel reliably

Disadvantages of STS ​

• Pressure head required — adds cost, setup time, and machine requirements
• Workpiece sealing — the workpiece face must be flat and perpendicular to the drill axis
• Machine requirements — typically requires a dedicated BTA drilling machine or substantial machine modification
• Higher coolant volume — the annular gap has a large cross-section, requiring higher pump flow capacity
• Chip return through spindle — the machine spindle must accommodate chip-laden coolant flow

Double-Tube System (DTS / Ejector) ​

Operating Principle ​

The DTS drill consists of two concentric tubes: an outer tube (boring bar) that carries the drill head and an inner tube through which chips are evacuated.

Coolant flow path:

1. Coolant is introduced through a rotating union or spindle connector at the machine side
2. Coolant flows forward through the annular space between the outer tube and the inner tube
3. At the drill head, coolant exits through nozzles directed at the cutting edges
4. A portion of the coolant is diverted through ejector nozzles that create a vacuum (ejector effect) in the inner tube
5. Chips and coolant are drawn through the inner tube by the combined force of the incoming coolant pressure and the ejector vacuum
6. The chip-laden coolant exits through the rear of the system

Ejector effect: The ejector nozzles create a pressure differential that actively draws the chip-coolant mixture into the inner tube. This vacuum assist is what enables the system to operate without a pressure head. The ejector effect is most effective when the inner tube cross-section is properly sized relative to the coolant flow rate.

Elimination of the Pressure Head ​

By delivering coolant entirely within the double-tube assembly, the DTS system bypasses the need for a seal against the workpiece. This has two important consequences:

1. Standard machine tools can be used — lathes and machining centres without pressure head capability
2. Non-rotating workpieces can be drilled — the workpiece does not need to be clamped against a seal

Performance Characteristics ​

Advantages of DTS ​

• No pressure head required — simpler setup, lower capital cost
• Standard machine tool compatibility — can be used on CNC lathes and machining centres
• No workpiece sealing — non-flat or non-perpendicular faces are acceptable
• Lower coolant pressure — 10–30 bar vs 10–50 bar for STS
• Easy retrofitting — existing conventional machines can be adapted
• Lower coolant volume — the annular space between tubes has a smaller cross-section than the tube-to-bore annulus in STS

Disadvantages of DTS ​

• Smaller chip evacuation passage — the inner tube diameter is smaller than the STS internal bore, limiting chip size
• Shallower maximum depth — typically limited to 1,000 mm (2,000 mm with special connectors)
• Lower feed rates — approximately 20–30% lower than STS for equivalent conditions
• Less tolerance capability — IT 9–11 vs IT 7–9 for STS
• Ejector efficiency decreases with depth — the vacuum assist becomes less effective as the chip column length increases

Head-to-Head Technical Comparison ​

Application Selection Guide ​

Choose STS When: ​

1. Maximum depth is required — L/D > 100:1, up to 400:1
2. Largest diameters — above 250 mm
3. Difficult chip-breaking materials — low-carbon steel, stainless steel, titanium
4. Highest production volumes — STS feed rates are 20–30% higher than DTS
5. Tightest tolerances required — IT 7–8
6. Dedicated BTA machine available — the capital investment is justified by production volume
7. Continuous production of similar parts — the setup time for the pressure head is amortised over long runs

Choose DTS / Ejector When: ​

1. Existing standard machine tools — no dedicated BTA machine available
2. Shallow to moderate depths — L/D < 50:1 (up to 100:1 with optimised hardware)
3. Diameters 18–250 mm — the standard range for ejector tooling
4. Lower production volumes — the simpler setup compensates for lower feed rates
5. Frequent part changeovers — DTS tooling can be changed more quickly
6. Non-flat workpiece entry faces — no sealing surface is required
7. Budget constraint — lower capital investment

Decision Matrix ​

Tube Design and Rigidity Comparison ​

The tube construction difference between STS and DTS affects torsional rigidity and bending stiffness — important factors for hole straightness and vibration resistance.

The STS solid tube provides the highest torsional rigidity, which translates to better resistance to torsional vibration (chatter) at high feed rates. The DTS double-tube construction is stiffer than a single tube of the same outer diameter due to the inner tube adding structural support, but the chip evacuation passage is necessarily smaller.

Process Economics ​

Cost per Hole Comparison ​

For equivalent hole specifications (same diameter and depth), the cost per hole comparison depends on production volume:

Breakeven analysis: For a given hole specification, STS becomes more economical when the production volume exceeds approximately 500–2,000 holes per year, depending on the capital cost of the dedicated machine and the value of cycle time reduction.

Summary ​

FAQ ​

What does STS stand for in deep hole drilling? ​

STS stands for Single-Tube System. It is the original BTA drilling configuration, using a single thick-walled tube. Coolant is delivered through the annular gap between the tube OD and the hole wall, and chips are evacuated through the internal bore of the tube. A pressure head seals against the workpiece.

What does DTS stand for in deep hole drilling? ​

DTS stands for Double-Tube System, also called the ejector system. It uses two concentric tubes — an outer tube that carries the drill head and an inner tube for chip evacuation. Coolant flows forward between the two tubes, and an ejector effect (vacuum) assists chip evacuation. No pressure head is required.

What is the main advantage of the ejector system over BTA STS? ​

The main advantage is that the ejector system does not require a pressure head sealed against the workpiece. This allows it to be used on standard machine tools (lathes, machining centres) without modification. It also simplifies setup, reduces changeover time, and allows drilling of parts with non-flat entry faces.

Which system achieves deeper holes, STS or DTS? ​

STS achieves significantly deeper holes. The maximum L/D ratio for STS is 400:1, while DTS is typically limited to approximately 50:1 (up to 100:1 with optimised hardware). The limitation for DTS is the smaller chip evacuation cross-section and the decreasing efficiency of the ejector effect at greater depths.

Can I use DTS tooling on a standard CNC lathe? ​

Yes. The ejector system is designed specifically for use on standard machine tools. The tool assembly (outer tube, inner tube, and drill head) mounts in the turret or tool holder, and coolant is supplied through a rotating union at the rear of the assembly. No machine modification is required beyond the coolant supply connection.

What coolant pressure is needed for STS drilling? ​

STS drilling typically requires 10–50 bar coolant pressure, depending on the diameter, depth, and material. The coolant volume requirement is higher than DTS because the annular gap between the tube and the bore wall has a larger cross-section than the annular space between the two tubes in ejector drilling.

Which system gives better surface finish? ​

STS generally achieves better surface finish (IT 7–9, Ra 0.8–1.6 µm) compared to DTS (IT 9–11, Ra 1.6–3.2 µm). The superior finish is due to the higher rigidity of the single tube and more consistent chip evacuation through the larger internal bore.

What diameter range does each system cover? ​

STS covers 18–2,000+ mm for solid drilling and up to 2,000+ mm for trepanning and boring. DTS covers 18–250 mm for solid drilling, with standard tooling. Above 250 mm, STS is the only BTA option.

Is the drill head the same for STS and DTS? ​

Drill heads are similar in principle (multi-edge with indexable inserts and guide pads) but are designed for the specific system. STS drill heads have coolant openings positioned to receive coolant from the annular gap between the tube and the bore wall. DTS drill heads have coolant passages that align with the annular space between the two tubes and include ejector nozzle passages.

Which system is better for a job shop with varied work? ​

DTS (ejector) is generally better for a job shop because it can be used on multiple standard machine tools, requires minimal setup time, handles frequent changeovers, and does not require a dedicated deep hole drilling machine. The trade-off is slightly lower feed rates and a shallower maximum depth capability.