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A deep hole drilling machine is a 20-year capital investment. Choosing the wrong configuration — undersized coolant system, incorrect guide bushing arrangement, or mismatched spindle power — locks in process limitations that cannot be retrofitted economically.
1. Hole Diameter Range
Diameter determines the drilling method and therefore the machine type.
| Diameter Range | Recommended Method | Machine Configuration |
|---|---|---|
| 0.5 – 6 mm | Gun drilling | High-speed spindle (10,000–28,000 RPM), high coolant pressure |
| 6 – 40 mm | Gun drilling | Standard gun drilling machine, 35–70 bar coolant |
| 8 – 65 mm | Gun drilling or BTA | Combination machine preferred for flexibility |
| 20 – 400 mm | BTA / STS | High-torque spindle, high-volume coolant system |
| > 400 mm | BTA / Trepanning | Heavy-duty machine, workhead spindle required |
Select a machine rated for the full range of diameters you expect to drill, not just the current job. A machine that can handle both gun drilling and BTA (e.g., UNISIG UNI-50BTA with 10-minute changeover) provides maximum flexibility.
2. Drilling Depth and L/D Ratio
Depth capability is determined by machine bed length, guide bushing configuration, and intermediate support arrangement.
- Standard gun drilling machines: Up to 3,000 mm depth with fixed whip guide
- Extended bed machines: Up to 12,000 mm with moving whip guides
- Multi-support configurations: For L/D > 100:1, intermediate supports at 1,000–2,000 mm spacing prevent drill whipping
The machine bed must be at least 500 mm longer than the maximum work piece length to accommodate drill approach and retraction.
3. Spindle Power and Speed
| Method | Spindle Power | Speed Range |
|---|---|---|
| Gun drilling (small diameter) | 5 – 22 kW | 1,500 – 28,000 RPM |
| Gun drilling (medium diameter) | 15 – 30 kW | 500 – 8,000 RPM |
| BTA drilling (small-medium) | 22 – 51 kW | 40 – 1,200 RPM |
| BTA drilling (large diameter) | 51 – 115 kW | 40 – 800 RPM |
For dedicated gun drilling machines, high spindle speed is the priority. For BTA machines, spindle torque is the priority. Combination machines must balance both requirements.
4. Coolant System
The coolant system specification is the most frequently underestimated parameter in machine selection.
| Parameter | Gun Drilling | BTA Drilling |
|---|---|---|
| Pressure | 35 – 200 bar | 20 – 100 bar |
| Flow rate | 1 – 20 L/min | 80 – 380 L/min |
| Filtration | 10 – 20 µm | 10 – 20 µm |
| Tank capacity | 5× pump flow per minute | 5–10× pump flow per minute |
| Temperature control | Chiller recommended | Chiller recommended |
The coolant pump power must be sized for the largest diameter and deepest hole expected, not the current production average. Undersized coolant systems are the most common cause of machine performance limitations.
5. Guide Bushing and Whip Guide Configuration
Guide bushings and whip guides maintain tool alignment and prevent whipping.
- Entry guide bushing: Hardened steel or carbide, clearance +0.003 to +0.008 mm on drill diameter. Replaceable bushing inserts are preferred.
- Fixed whip guide: Standard for depths up to 100× diameter. Supports the drill tube at a fixed position near the workpiece.
- Moving whip guide: Rides on the drill tube and moves with the cutting head. Required for L/D > 100:1.
- Pressure head (BTA): Seals coolant at the workpiece entry and guides the tool. Must accommodate the full workpiece OD range.
6. CNC Control
CNC controls for deep hole drilling require specific cycles not found on standard machining centre controls.
| Control System | Typical Application | Deep Hole Cycles |
|---|---|---|
| Siemens 828D | Medium complexity | Pecking, constant feed, torque monitoring |
| Siemens 840D | High complexity, multi-axis | Full deep hole package, synchronous spindles |
| Fanuc Oi-TC / MF | Standard gun drilling | G73/G83 pecking, custom macros |
| Fanuc 31i / 35i | High-speed, multi-spindle | High-speed pecking, in-process monitoring |
The control should provide real-time monitoring of coolant pressure, feed force, and spindle torque with configurable thresholds for automatic process interruption.
7. Spindle Configuration
| Configuration | Best For | Typical Manufacturers |
|---|---|---|
| Single spindle | Low-medium volume, wide diameter range | All manufacturers |
| Dual spindle | Medium volume, paired operations | UNISIG (UNE6-2i), TBT (DM 50) |
| Multi-spindle (4–6) | High volume, single diameter | TBT (ML series) |
| Multi-spindle knee-type | Off-centre drilling | Mollart (Omnisprint) |
Multi-spindle machines reduce cost per hole but limit diameter flexibility. Single-spindle machines maximise flexibility at higher cost per hole.
8. Workpiece Handling
- Workhead spindle: Required for BTA (workpiece rotation) and counter-rotation. Power typically 15–50% of the main spindle.
- Steady rests: 2-point or 3-point supports for slender workpieces. Required for L/D > 30:1 on the workpiece.
- Automatic loading: Gantry robots, pick-and-place systems, or automated guided vehicles for high-volume production.
- Pallet systems: Enable part changeover while the machine is running.
9. Precision and Tolerance Requirements
Verify that the machine's rated precision matches your requirements:
| Requirement | Gun Drilling Machine | BTA Machine |
|---|---|---|
| Diameter tolerance | IT7 – IT9 | IT8 – IT11 |
| Surface finish Ra | 0.4 – 1.6 µm | 1.2 – 6.3 µm |
| Straightness | 0.1 mm / 100 mm | 0.1 mm / 100 mm |
Machine acceptance tests per VDI 3212 should be specified in the purchase contract. Run test holes in the actual workpiece material before acceptance.
10. Manufacturer Support
| Manufacturer | Country | Diameter Range | Max Depth | Key Strength |
|---|---|---|---|---|
| UNISIG | USA | 1 – 500 mm | 20,000 mm | BTA/gundrill changeover, broad range |
| TBT | Germany | 0.8 – 400 mm | 15,000 mm | Multi-spindle, extreme depth |
| Mollart | UK | 0.5 – 400 mm | 4,000 mm | Custom machines, off-centre drilling |
| Precihole | India | 1 – 400 mm | 12,000 mm | Cost-effective BTA solutions |
Evaluate spare parts availability, service response time, training programs, and installation support. A machine from a manufacturer with local representation will have lower downtime than one from a manufacturer that requires international travel for service.
FAQ
What spindle power do I need for gun drilling?
5–22 kW for small to medium diameters. Medical micro-drilling machines may use as little as 5 kW. Production gun drilling typically uses 15–30 kW.
What coolant pressure is needed for BTA drilling?
20–100 bar depending on diameter. Smaller BTA diameters require higher pressure. Flow rate (80–380 L/min) is equally important.
Can one machine do both gun drilling and BTA?
Yes. Combination machines (e.g., UNISIG UNI-50BTA) can switch between methods in approximately 10 minutes by changing the toolhead and coolant configuration.
How many spindles should I choose?
Single spindle for flexibility, multi-spindle (4–6) for high volume. Multi-spindle machines are economical when drilling the same hole pattern in large quantities.
What CNC control is best for deep hole drilling?
Siemens 840D for complex multi-axis machines, Siemens 828D for medium complexity, Fanuc Oi-TC for standard gun drilling. All provide deep hole pecking cycles and process monitoring.
How deep can a standard deep hole drilling machine drill?
Standard gun drilling machines reach 3,000 mm. Extended-bed machines with moving whip guides reach 12,000–20,000 mm. TBT B-series drills to 15,000 mm.
What is a whip guide and when do I need one?
A whip guide supports the drill tube mid-length to prevent whipping (centrifugal deflection). Fixed whip guides are standard. Moving whip guides are required for L/D > 100:1.
Is counter-rotation worth the investment?
Yes for thin-wall workpieces and tight straightness requirements. Counter-rotation can halve straightness deviation. It requires a workhead spindle, adding 15–30% to machine cost.
What is the typical lead time for a deep hole drilling machine?
Custom-configured machines: 6–12 months. Standard models: 3–6 months. Used machines are available from specialty dealers with shorter lead times.
Which deep hole drilling machine manufacturer is most cost-effective?
Precihole (India) offers the most competitive pricing for BTA machines at 30–50% below European equivalents. Mollart's Matrix range starts at approximately £130,000 for a basic gundrilling machine.
Conclusion
The 10-point checklist for deep hole drilling machine selection prioritises: diameter range (determines method), drilling depth (determines bed length and guide configuration), spindle power (torque vs speed trade-off), coolant system (the most commonly underestimated parameter), guide bushing arrangement, CNC control with deep hole cycles, spindle count, workpiece handling, precision requirements, and manufacturer support. Evaluating all 10 criteria before issuing a specification prevents costly configuration errors that would otherwise be discovered during commissioning.