Ejector Drilling vs BTA vs Gun Drilling
Three deep hole drilling methods compete for holes in the 18–50 mm diameter range — the range where all three can potentially work. Ejector drilling (DTS) offers a unique combination of BTA-like productivity with gun-drilling-like machine flexibility.
This guide compares all three methods and provides a selection framework for choosing the right method for your application.
How Each Method Works
Ejector drilling (DTS) uses a double-tube boring bar. Coolant flows between the inner and outer tubes, and a Venturi effect at the drill head creates suction that evacuates chips through the inner tube. No workpiece seal is needed.
BTA drilling (STS) uses a single thick-walled tube. Coolant is pumped between the tube and the bore wall, and chips exit through the hollow center of the tube. A pressure head seal is required at the workpiece entry.
Gun drilling uses a single-lip carbide tool with an internal coolant hole. Coolant flows through the tool and chips exit along an external V-shaped flute.
| Feature | Ejector (DTS) | BTA (STS) | Gun Drilling |
|---|---|---|---|
| Tube system | Double tube | Single tube | Solid shaft + V-flute |
| Coolant path | Between inner/outer tubes | Annulus (tube-to-bore) | Through tool center |
| Chip exit | Venturi suction, inner tube | Pressure, through tube center | External V-groove |
| Seal required? | No | Yes (pressure head) | No (simple bushing) |
| Retrofit CNC lathe? | Yes | No | Limited (40:1) |
Diameter Range
| Method | Minimum | Maximum | Optimal |
|---|---|---|---|
| Gun drilling | 0.5 mm | 50 mm | 1–25 mm |
| BTA drilling | 18 mm | 250 mm (500+ special) | 25–150 mm |
| Ejector drilling | 18 mm | 200 mm | 20–65 mm |
Key insight: Below 18 mm, the choice is simple — gun drilling is the only option. In the 20–65 mm range, all three methods can work, and the decision depends on your machine, production volume, and workpiece geometry.
Penetration Rate
| Method | Relative Rate | Typical Feed (mm/rev, Ø40 mm steel) |
|---|---|---|
| Gun drilling | Baseline (1×) | 0.04–0.07 |
| Ejector drilling | 4–6× gun drilling | 0.12–0.30 |
| BTA drilling | 5–7× gun drilling | 0.15–0.40 |
Ejector drilling is approximately 10–20% slower than BTA due to the less efficient Venturi chip evacuation. In practice, the difference is often offset by faster setup times (no pressure head alignment).
Precision and Surface Finish
| Metric | Gun Drilling | BTA Drilling | Ejector Drilling |
|---|---|---|---|
| Diameter tolerance | ±0.025 mm (±0.001") | ±0.05 mm (±0.002") | ±0.04 mm (±0.0015") |
| Straightness | 0.08 mm per 300 mm | 0.10 mm per 300 mm | 0.10 mm per 300 mm |
| Surface finish (Ra) | 0.4–0.8 µm | 0.8–3.2 µm | 0.8–3.2 µm |
Gun drilling still wins on precision. BTA and ejector are comparable, with BTA having a slight edge on very deep holes.
Machine Requirements
| Factor | Gun Drilling | BTA Drilling | Ejector Drilling |
|---|---|---|---|
| Machine type | Dedicated or CNC retrofit | Dedicated BTA machine | Standard CNC lathe/MC + coolant upgrade |
| Coolant pressure | Up to 2,000 PSI | 300–870 PSI | 290–580 PSI |
| Workpiece seal | Bushing only | Pressure head seal | None |
| Setup time | Short | Long (seal alignment) | Short |
| Capital investment | $50K–$500K (retrofit to dedicated) | $200K–$3M | $30K–$100K (coolant upgrade) |
Ejector drilling requires the lowest capital investment because it can use existing machine tools.
Selection Matrix
| Your Situation | Best Method | Why |
|---|---|---|
| Hole under 18 mm | Gun drilling | Only option |
| Existing CNC lathe, adding deep hole capability | Ejector drilling | Lowest cost entry, no dedicated machine |
| Irregular workpiece entry face | Ejector drilling | No seal needed |
| Maximum production rate | BTA drilling | Fastest penetration |
| Highest precision required | Gun drilling | Best tolerance and surface finish |
| Extreme depth ratio (> 100:1) | Gun drilling | Only option |
| Large diameter (> 65 mm) | BTA drilling | Ejector tooling limited above 65 mm |
| Dedicated machine available | BTA drilling | Faster than ejector at same diameter |
| Low production volume | Ejector or gun drilling | Lower capital investment |
Cost Comparison
For a hole Ø40 mm × 1,000 mm deep in steel, batch of 1,000:
| Cost Factor | Gun Drilling | BTA Drilling | Ejector Drilling |
|---|---|---|---|
| Machine rate per hour | $100 (retrofit) | $150 (dedicated) | $80 (CNC lathe retrofit) |
| Cycle time | ~7 min | ~5 min | ~6 min |
| Machine cost per hole | $11.67 | $12.50 | $8.00 |
| Tool cost per hole | $0.05 | $0.15 | $0.20 |
| Setup per hole (batch 1K) | $0.20 | $0.50 | $0.20 |
| Total per hole | ~$12.00 | ~$13.15 | ~$8.40 |
Note: Ejector drilling’s lower machine rate (using existing equipment) often offsets its slightly longer cycle time versus BTA. For lower volumes, the difference in capital cost makes ejector drilling even more attractive.
Practical Examples
Example 1: Job shop adding deep hole capability, Ø25 mm holes
- Current equipment: Standard CNC lathes
- Volume: 200 holes/year across various parts
- Best choice: Ejector drilling — add coolant swivel and pump for under $50K
- Alternative: Contract BTA service provider
Example 2: High-volume production, Ø50 mm × 600 mm, 10,000/year
- Best choice: BTA drilling — dedicated multi-spindle machine
- Ejector drilling possible but slower per hole
- Gun drilling too slow for this volume
Example 3: Casting with rough entry face, Ø30 mm × 900 mm
- Best choice: Ejector drilling — no seal needed on rough surface
- BTA would require machining a sealing face
- Gun drilling possible if diameter allows single-lip tool
Example 4: Precision valve bore, Ø12 mm × 300 mm
- Best choice: Gun drilling — only option below 18 mm, best precision
- Ejector and BTA cannot reach this diameter
Summary
| When you need… | Choose… |
|---|---|
| Lowest capital investment for deep hole capability | Ejector drilling (on existing CNC lathe) |
| No workpiece seal possible | Ejector drilling |
| Highest production rates | BTA drilling |
| Smallest holes (< 18 mm) or extreme precision | Gun drilling |
| Large diameters (> 65 mm) | BTA drilling |
| Quick setup, frequent changeovers | Ejector drilling |
Ejector drilling fills a specific niche — it is the most practical deep hole drilling method for shops that want to add deep hole capability to existing CNC machine tools, particularly for medium-diameter holes (20–65 mm) at low to moderate production volumes. For high-volume production at larger diameters, BTA drilling is more productive. For small diameters and extreme precision, gun drilling remains the standard.
For ejector drilling fundamentals, see what is ejector drilling. For CNC setup guidance, see ejector drilling CNC setup. For a complete overview, visit the ejector drilling guide.