Appearance
Choosing between through-coolant and external coolant is not a technical preference — it is determined by the drilling method, and the drilling method is determined by the hole diameter. The coolant path defines the process.
Overview
Every deep hole drilling process must solve the same problem: deliver coolant to the cutting zone and evacuate chips from a hole that is too deep for flood coolant to reach. Two solutions have emerged:
- Through-coolant (gun drilling) — coolant flows through the center of the hollow drill shank to the cutting edge, and chips exit through an external V-shaped flute
- External coolant (BTA drilling) — coolant is delivered through the annular space between the drill tube and the bore wall, and chips exit through the center of the hollow drill tube
These are not interchangeable options for the same hole. The coolant path is built into the tool and machine design. Understanding the differences helps in selecting the right method for a given diameter range and production requirement.
How Coolant Delivery Works
Through-Coolant (Gun Drilling)
In gun drilling, the tool consists of a solid carbide head brazed to a hollow steel shank with a crimped V-shaped cross-section (kidney-shaped). Coolant flows from the machine's high-pressure pump, through the rotary feed unit, into the center of the hollow shank, and exits at the cutting face near the cutting edge.
After cooling the cutting edge and lubricating the guide pads, the coolant reverses direction and flows back along the outside of the drill through the V-shaped flute, carrying chips with it. The chip-laden coolant exits the hole at the tool entry point.
Path: Machine pump → rotary union → hollow shank interior → cutting face → external V-flute → hole entry → chip tray
External Coolant (BTA Drilling)
In BTA drilling, the tool head is mounted on a thick-walled, fully round tube. A pressure head seals against the workpiece at the hole entry. Coolant is pumped into the sealed annular space between the outside of the drill tube and the bore wall, traveling the full length of the hole to reach the cutting head.
After passing through the cutting zone, the coolant and chips are forced through internal openings in the drill head and into the center of the drill tube. They travel back through the tube interior and exit through the machine spindle.
Path: Machine pump → pressure head → tube-to-wall annulus → cutting head → drill tube interior → through spindle → chip collection
| Aspect | Through-Coolant (Gun Drilling) | External Coolant (BTA) |
|---|---|---|
| Delivery path | Through tool center | Through annulus |
| Chip return path | External V-flute | Through tube center |
| Pressure at tool | 30–170 bar | 20–80 bar |
| Flow requirement | 20–100 L/min | 100–800 L/min |
| Seal required | No | Yes (pressure head) |
Coolant path determines chip evacuation quality
The fundamental advantage of BTA's external coolant path is that chips never contact the finished bore surface — they are carried internally through the drill tube. Gun drilling chips travel along the V-flute in contact with the bore wall, creating the risk of chip-drag scoring. This difference affects surface finish quality independent of Ra values.
Through-Coolant Applications and Limits
Strengths
Through-coolant (gun drilling) excels in the small diameter range where BTA is impractical:
| Parameter | Through-Coolant (Gun Drilling) |
|---|---|
| Diameter range | 1 – 30 mm (primary range) |
| Depth capability | Up to 100× diameter |
| Surface finish | Ra 0.4 – 1.6 µm |
| Hole straightness | 0.1 – 0.5 mm per meter |
| Typical applications | Fuel injectors, medical devices, gun barrels, automotive shafts |
Limitations
- Tool rigidity — the V-shaped shank is less rigid than a full round tube, limiting feed rates
- Chip evacuation — chips travel along the bore surface, risking surface damage
- Diameter limit — becomes uneconomical above 30 mm due to material removal rate limitations
- Coolant pressure — requires higher pressure (up to 170 bar for small diameters) to overcome hydraulic resistance in the narrow shank
External Coolant Applications and Limits
Strengths
External coolant (BTA drilling) dominates the larger diameter range:
| Parameter | External Coolant (BTA) |
|---|---|
| Diameter range | 18 – 250 mm+ (up to 1,000 mm with trepanning) |
| Depth capability | Up to 100× diameter |
| Metal removal rate | 2–5× higher than gun drilling |
| Surface finish | Ra 0.8 – 3.2 µm (cleaner, no chip-drag) |
| Tool life | Longer (indexable inserts, rigid tube) |
Limitations
- Minimum diameter — BTA tools require a minimum diameter of approximately 18 mm to accommodate the internal chip passage
- Pressure head required — the sealing arrangement adds complexity and requires workpiece preparation
- Higher flow requirement — large volumes of coolant are needed to fill the annular gap
- Higher initial cost — the pressure head system and larger pump increase capital investment
Head-to-Head Comparison
| Attribute | Through-Coolant (Gun Drilling) | External Coolant (BTA) |
|---|---|---|
| Coolant delivery | Internal (through tool) | External (through annulus) |
| Chip evacuation | External (along bore) | Internal (through tube) |
| Pressure range | 30–170 bar | 20–80 bar |
| Flow range | 20–100 L/min | 100–800 L/min |
| Filtration required | ≤ 10 µm | ≤ 20 µm |
| Coolant temperature control | ±2°C recommended | ±5°C typical |
| Surface finish | Excellent (low Ra) | Good (cleaner, less scoring) |
| Metal removal rate | Moderate | High |
| Tool rigidity | Lower (V-shaped shank) | Higher (round tube) |
| Tool cost per hole | Moderate | Moderate to low (indexable) |
| Machine complexity | Lower (no pressure head) | Higher (pressure head system) |
| Automation integration | Straightforward | More complex |
| Retrofittable to standard machines | Limited | Yes (ejector variant) |
The Transition Point
Industry practice places the transition between gun drilling and BTA at approximately 20–30 mm diameter:
| Diameter Range | Recommended Method | Coolant Delivery |
|---|---|---|
| 1 – 18 mm | Gun drilling only | Through-coolant |
| 18 – 30 mm | Either (application-dependent) | Both possible |
| 30 – 250 mm+ | BTA drilling | External coolant |
Below 18 mm, BTA is generally not feasible because the internal chip passage through the drill tube becomes too small for reliable evacuation. Above 30 mm, gun drilling becomes uneconomical due to low metal removal rates and the high cost of large-diameter solid carbide heads.
The crossover is not a hard boundary
A gun drilling machine can produce holes up to 50 mm, and BTA machines exist for diameters as small as 12 mm. The transition point at 20–30 mm is an economic guideline based on productivity and tooling cost, not a physical limit. Evaluate both methods at the crossover range based on production volume, quality requirements, and existing equipment.
Cost and Complexity
Through-Coolant System Costs
| Component | Cost Level | Notes |
|---|---|---|
| Machine base | Moderate | Gun drilling machines are less complex than BTA |
| Coolant pump | Moderate to high | Positive displacement pump, 30–170 bar |
| Filtration | Moderate | 10 µm, paper or media filter |
| Rotary union | Moderate | Wears over time, requires maintenance |
| Tooling per hole | Moderate | Solid carbide gun drills |
External Coolant (BTA) System Costs
| Component | Cost Level | Notes |
|---|---|---|
| Machine base | Higher | Requires pressure head and larger coolant system |
| Coolant pump | Higher | Centrifugal with VFD, high flow capacity |
| Filtration | Higher | Larger filters, automatic backwash recommended |
| Pressure head | Additional | Custom-sized to workpiece diameter |
| Tooling per hole | Moderate to low | Indexable inserts reduce per-hole cost at volume |
Total Cost of Ownership Factors
- Through-coolant (gun drilling) — lower machine cost, moderate tooling cost, higher per-hole tool cost at larger diameters
- External coolant (BTA) — higher machine cost, lower per-hole tooling cost at volume, higher coolant system operating cost
Retrofitting and Conversion
Converting Standard Machines to Through-Coolant
Standard CNC machines can be equipped with through-spindle coolant capability:
- Rotary feed unit (rotary union) — enables coolant passage through the spindle
- High-pressure pump — typically 20–70 bar for general deep hole work
- Filtration upgrade — required to protect the rotary union seals
- Coolant-through tooling — drills, holders, and pull studs with internal passages
Retrofit systems such as REGO-FIX reCool can convert external-flood machines at approximately half the cost of an OEM through-spindle system, handling pressures up to 140 bar and speeds up to 12,000 rpm.
Converting to External Coolant (BTA)
Converting a standard machine to BTA is more involved:
- Pressure head system — must be designed for the workpiece diameter range
- Coolant pump upgrade — high-flow centrifugal pump with VFD
- Spindle modification — must allow chip passage through the spindle center
- Chip collection system — modified for high-volume chip transport
The ejector drilling (DTS) variant eliminates the pressure head by using a double-tube system, making BTA-style drilling more practical for retrofit applications.
Summary
| Aspect | Through-Coolant (Gun Drilling) | External Coolant (BTA) |
|---|---|---|
| Best diameter range | 1 – 30 mm | 18 – 250 mm+ |
| Coolant delivery | Through tool shank | Through tube-to-wall annulus |
| Chip evacuation | External V-flute | Internal through tube |
| Pressure range | 30 – 170 bar | 20 – 80 bar |
| Flow range | 20 – 100 L/min | 100 – 800 L/min |
| Surface finish quality | Ra 0.4 – 1.6 µm | Ra 0.8 – 3.2 µm |
| Metal removal rate | Moderate | High (2–5× gun drilling) |
| Machine complexity | Lower | Higher |
| Retrofittable | Yes (rotary union + pump) | Yes (ejector variant) |
FAQ
What is the difference between through-coolant and external coolant in deep hole drilling?
Through-coolant delivers fluid through the center of the drill shank to the cutting edge, with chips exiting along an external V-flute — this is the gun drilling method. External coolant delivers fluid through the annular space between the drill tube and the bore wall, with chips exiting through the center of the tube — this is the BTA method. The coolant path is determined by the drilling method, not chosen independently.
Which is better: through-coolant or external coolant?
Neither is universally better. Through-coolant (gun drilling) is better for small diameters (1–30 mm) where precision and surface finish are priorities. External coolant (BTA) is better for larger diameters (18–250 mm+) where metal removal rate and productivity matter. At the crossover range of 18–30 mm, the choice depends on production volume, quality requirements, and existing equipment.
Can a standard CNC machine be used for through-coolant drilling?
Yes, standard CNC machines can be retrofitted with through-spindle coolant capability by adding a rotary feed unit (rotary union), a high-pressure pump (20–70 bar), and upgraded filtration. Retrofit systems from suppliers such as REGO-FIX reCool can convert external-flood machines at approximately half the cost of an OEM through-spindle system. This makes gun drilling accessible on existing machine tools for many applications.
What coolant pressure is needed for through-coolant drilling?
Through-coolant (gun drilling) requires 30–170 bar depending on hole diameter. Smaller diameters need higher pressure: 100–170 bar for diameters under 3 mm, 50–120 bar for 3–6 mm, and 20–80 bar for 6–30 mm. The pressure must overcome the hydraulic resistance of the narrow coolant passage through the shank and maintain sufficient velocity for chip evacuation through the V-flute.
Does BTA drilling always require a pressure head?
Standard BTA drilling requires a pressure head that seals against the workpiece to deliver coolant into the annular space. However, the ejector drilling variant (DTS, double-tube system) eliminates the pressure head by using a concentric double tube — coolant flows down through the outer annulus and returns with chips through the inner tube. Ejector drilling is easier to retrofit onto conventional machines but requires a minimum diameter of approximately 18 mm.
How does coolant delivery affect surface finish?
Coolant delivery directly affects surface finish through chip evacuation. In through-coolant (gun drilling), chips travel along the external V-flute in contact with the bore wall, creating a risk of chip-drag scoring. In external coolant (BTA), chips never contact the finished surface — they are carried internally through the drill tube. This means BTA produces a cleaner surface free from chip-drag damage, even though gun drilling typically achieves lower Ra values through its burnishing action.
Coolant delivery method selection depends on hole diameter, depth, material, production volume, and quality requirements. The guidelines in this article represent industry practice for production deep hole drilling. Consult machine builders and tool suppliers for application-specific recommendations. This article reflects industry knowledge as of 2026.