How to Select the Right Deep Hole Drilling Equipment
Choosing between gun drilling, BTA, ejector, or trepanning is one of the most consequential decisions you will make in a deep hole drilling operation. The wrong choice means either overspending on equipment or being unable to hold tolerance.
This guide provides a structured decision framework based on your part specifications, production volume, and budget. Use it as a checklist when evaluating new equipment.
Step 1: Categorize Your Workpiece
Start with five numbers that define which processes are feasible:
| Parameter | Your Value | Why It Matters |
|---|---|---|
| Hole diameter | ______ mm | Determines gun drilling vs BTA vs ejector |
| Hole depth | ______ mm | Calculates L/D ratio — determines method feasibility |
| L/D ratio | ______ :1 | Above 10:1 = deep hole; above 40:1 = dedicated machine needed |
| Tolerance (IT grade) | IT______ | Gun drilling holds IT7—IT9; BTA holds IT8—IT10 |
| Material | ______ | Affects cutting speed, tool grade, coolant type |
L/D ratio reference:
| L/D Range | Recommended Method | Equipment |
|---|---|---|
| < 5:1 | Conventional drilling | Standard CNC |
| 5:1 — 10:1 | Through-coolant twist drill | CNC with high-pressure coolant |
| 10:1 — 40:1 | Gun drilling or BTA | CNC or dedicated machine |
| 40:1 — 100:1 | Gun drilling (small Ø), BTA (large Ø) | Dedicated deep hole machine |
| 100:1+ | Gun drilling (specialist) | High-performance machine |
Step 2: Select the Right Method
Decision Tree
Hole diameter < 20 mm ?
├── Yes → L/D > 10:1 ?
│ ├── Yes → Gun drilling
│ └── No → Conventional or through-coolant twist drill
└── No → L/D > 100:1 ?
├── Yes → Gun drilling (specialist)
└── No → Production volume high ?
├── Yes → BTA drilling (5—7× faster feed)
└── No → Ejector drilling (retrofit on existing CNC)Method Comparison Table
| Factor | Gun Drilling | BTA Drilling | Ejector Drilling | Trepanning |
|---|---|---|---|---|
| Diameter range | 0.5 — 50 mm | 12 — 630 mm | 18 — 150 mm | 50 — 1000+ mm |
| Optimal L/D | Up to 100:1+ | Up to 100:1 | Up to 100:1 | Up to 7:1 |
| Tolerance | ±0.01 — 0.025 mm (IT7—IT9) | ±0.02 — 0.05 mm (IT8—IT10) | ±0.04 mm (IT9—IT11) | Lower — needs secondary ops |
| Surface finish Ra | 0.4 — 1.6 µm | 0.4 — 3.2 µm | Moderate | 3.2 — 12.5 µm |
| Feed rate (relative) | 1× (baseline) | 5 — 7× | 2 — 3× | 0.5 — 1× |
| Machine requirement | Lathe or dedicated machine | Dedicated BTA machine | Retrofittable on existing CNC | Large lathe or mill |
| Setup complexity | Moderate | High | Moderate | Low |
| Core recovery | No | No | No | Yes |
Selection summary:
- Gun drilling — for precision small-diameter deep holes
- BTA drilling — for high-productivity large-diameter holes
- Ejector drilling — for mid-range retrofits on existing equipment
- Trepanning — for very large diameters where material recovery matters
Step 3: Determine Machine Type
Option A: Dedicated Deep Hole Drilling Machine
| Configuration | Best For | Typical Cost Range |
|---|---|---|
| Single-spindle gun drill | Precision job shop, frequent changeover | $50,000 — $150,000 |
| Multi-spindle gun drill (2—8 spindles) | High-volume automotive production | $150,000 — $500,000+ |
| BTA machine (single tube) | Large-diameter production | $200,000 — $800,000+ |
| Combination gun drill + BTA | Mixed production | $250,000 — $1,000,000+ |
Leading manufacturers by tier:
| Tier | Brands | Origin | Positioning |
|---|---|---|---|
| Premium | TBT, IMSA, TIBO, Galbiati | Germany/Italy | Highest precision, automation integration |
| Mid-upper | UNISIG, Mollart, Kays Engineering | US/UK | Custom solutions, strong in automotive/aerospace |
| Mid-range | Precihole, Wim, GSM | India/Global | Cost-competitive, expanding reach |
| Value | Dezhou Jutai, Honge Precision | China | Lowest cost, competitive for mid-range specs |
Option B: CNC Machine Conversion
If your L/D ratio is below 40:1 and your production volume does not justify a dedicated machine, converting an existing CNC machining center is the most cost-effective approach.
What you need:
| Component | Specification | Estimated Cost |
|---|---|---|
| High-pressure coolant system | 70 — 140 bar, through-spindle | $15,000 — $40,000 |
| Coolant filtration | 10 — 15 µm paper band filter | $5,000 — $15,000 |
| Whip guide / bushing holder | Supports drill at entry | $2,000 — $8,000 |
| Tooling package | Gun drills, holders, bushings | $3,000 — $10,000 |
| Programming / setup | CAM post, peck cycles | $2,000 — $5,000 |
Total retrofit cost: $27,000 — $78,000
A retrofittable spray mist system from Mollart can convert any conventional or CNC machine for gun drilling of Ø5—25 mm holes up to 2,000 mm depth (Mollart, 2024). For higher production, a 140-bar high-pressure coolant system like the WBP-2230M with integrated drum filtration can be added to CNC milling and turning centers (Wuli Pump, 2024).
Limitations of CNC conversion:
- Practical L/D limit around 40:1
- No counter-rotation capability (reduces concentricity at high L/D)
- Limited to gun drilling and shallow BTA
- Machine spindle may lack the torque for large-diameter BTA
Option C: Used Machine
Used deep hole drilling machines from reputable manufacturers offer a lower entry cost but carry risks.
| Consideration | New Machine | Used Machine |
|---|---|---|
| Purchase price | $50,000 — $1M+ | $20,000 — $60,000 (entry level) |
| Warranty | Yes | Limited or none |
| Technology | Latest CNC, adaptive control | Potentially outdated controls |
| Lead time | 12 — 36 weeks | Immediate |
| Maintenance | Low first 5 years | Unknown history, higher risk |
Step 4: Evaluate Production Volume
The break-even point between gun drilling and BTA depends on production volume:
| Volume (holes/month) | Recommended Approach |
|---|---|
| < 100 | Job-shop: CNC conversion or single-spindle gun drill |
| 100 — 1,000 | Single-spindle dedicated machine |
| 1,000 — 10,000 | Multi-spindle gun drill or BTA (depending on diameter) |
| 10,000+ | Multi-spindle BTA with automation |
Case study: A six-spindle gun drilling machine drilling connecting rods (SAE 4140, 0.250“ diameter, 9“ depth) achieved 70 parts per hour with zero scrap (CTE Magazine, 2024). A UNISIG automated work cell producing rifle barrel receivers achieved over 100 parts per hour through multi-spindle configuration and pick-and-place automation (UNISIG/CTE, 2024).
Step 5: Calculate Total Cost of Ownership
TCO goes far beyond the purchase price. The largest cost driver in deep hole drilling is machine time, typically 70 — 85% of total cost per hole.
Cost Per Hole formula
Cost per hole = (Tool cost / holes per tool)
+ (Cycle time × machine hourly rate)
+ (Scrap rate × part value)
+ (Coolant cost per hole)
+ (Maintenance cost per hole)Cost comparison by method
| Cost Component | Gun Drilling | BTA Drilling |
|---|---|---|
| Machine time | Higher (slower feed) | Lower (5—7× faster feed) |
| Tool cost per hole | Low — $0.001 — 0.007 | Low — $0.002 — 0.010 |
| Coolant cost | Moderate (60 — 80 bar) | Higher (higher volume) |
| Scrap risk | Low (stable process) | Low (stable process) |
| Maintenance | Moderate | Higher (more complex system) |
Real-world TCO savings example: A Müller Hydraulik case study showed that optimizing the coolant system reduced annual drill costs from €10,272 to €4,800 (7× tool life increase), saved €24,276 in cycle time by eliminating chip clearing, and saved €2,633 in energy — a total rationalization of €42,686 per machine per year (Müller Hydraulik, 2024).
Another example: Switching from twist drilling to gun drilling for a hydraulic cylinder (L/D = 30) reduced scrap from 18% to below 4%, shortened cycle time 20%, and reduced overall cost 25 — 30%.
Step 6: Verify Infrastructure
Before purchasing, confirm your facility can support the equipment:
| Requirement | Gun Drilling (Small) | Gun Drilling (Production) | BTA Drilling |
|---|---|---|---|
| Floor space | 10 — 20 m² | 20 — 50 m² | 50 — 200 m² |
| Power | 15 — 30 kW | 30 — 60 kW | 60 — 150 kW |
| Coolant volume | 100 — 300 L | 300 — 1,000 L | 1,000 — 5,000 L |
| Coolant pressure | 60 — 120 bar | 60 — 120 bar | 30 — 100 bar |
| Filtration | 10 µm | 10 µm | 20 µm |
| Compressed air | 6 bar | 6 bar | 6 bar |
Selection Summary Checklist
Use this checklist when evaluating any deep hole drilling equipment:
- Hole diameter, depth, and L/D ratio calculated
- Tolerance and surface finish requirements defined
- Material identified (affects tool grade and coolant)
- Method selected (gun / BTA / ejector / trepanning)
- Production volume estimated (determines single vs multi-spindle)
- Cycle time target established
- New vs used vs conversion evaluated
- Machine manufacturer shortlisted (by tier and application fit)
- TCO calculated (not just purchase price)
- Coolant system specification confirmed (pressure, volume, filtration)
- Floor space, power, and utilities verified
- Tooling supply and regrind service available
- Maintenance and technical support accessible
References
- Precihole Machine Tools. How to Choose the Right Deep Hole Drilling Machine for Your Business. Technical Guide, 2024.
- Artizono. Comprehensive Guide to CNC Deep Hole Drilling Machines. Technical Resource, 2024.
- Artizono. CNC Deep Hole Boring Machine Price Comparison Guide. 2024.
- AGrade Carbide. Types of Deep Hole Drilling Machines — Ultimate Guide. 2024.
- CTE Magazine. Going Deep and Cellular — Multi-Spindle Automotive Drilling. Cutting Tool Engineering, 2024.
- UNISIG. Case Studies — Automated Deep Hole Drilling Work Cells. 2024.
- CNC Cookbook. Gun Drilling & BTA Drilling: Definitive Guide. 2024.
- Rapid-Protos. Deep Hole Drilling: Methods, Tolerance & Cost Guide. 2024.
- West Ohio Tool. How to Calculate Cost Per Hole for Aerospace. Technical Guide, 2024.
- Müller Hydraulik. Ratio Calculations for Deep Hole Drilling — TCO Case Study. 2024.
- Mollart. Retrofittable Spray Mist Coolant System. Technical Specification, 2024.
- Wuli Pump. WBP-2230M High Pressure Coolant System — 140 bar. Technical Data, 2024.
- HengCe Research. Global Deep Hole Drilling Market Report — Competitive Analysis. 2024.
- Market Publishers. Deep Hole Drilling Machinery Market, Global Outlook and Forecast. 2024.
- ProleanTech. Understanding CNC Drilling Costs — Step-by-Step Guide. 2024.