How to Choose the Right Deep Hole Drilling Method
With multiple deep hole drilling methods available — each optimized for a different combination of diameter, depth, material, and production volume — choosing the wrong method can lead to excessive cost, poor quality, or missed delivery dates.
This guide provides a structured decision framework that narrows the options step by step, based on your hole requirements and available resources.
Step 1: Determine Hole Diameter
Diameter is the most restrictive factor — it immediately eliminates methods that cannot physically fit.
| Diameter Range | Available Methods |
|---|---|
| < 18 mm | Gun drilling only |
| 18–50 mm | Gun drilling, BTA, ejector drilling |
| 50–200 mm | BTA, ejector drilling, trepanning |
| > 200 mm | BTA, trepanning |
| < 1 mm | EDM, laser, ECM |
Decision: If diameter is under 18 mm, the choice is already made — gun drilling is the only practical mechanical method. For diameters under 1 mm, unconventional methods (EDM, laser) are required.
Step 2: Evaluate Depth Ratio
Depth ratio (L/D) determines whether a method can physically reach the required depth.
| Depth Ratio | Available Methods |
|---|---|
| < 20:1 | All methods feasible (including conventional twist drilling) |
| 20:1 to 40:1 | Gun drilling, BTA, ejector (CNC retrofit ok for gun/ejector) |
| 40:1 to 100:1 | Gun drilling, BTA, ejector (dedicated gun drill for best results) |
| 100:1 to 300:1 | Gun drilling only |
| > 300:1 | Specialized gun drilling (extreme capability) |
Decision: For depth ratios exceeding 100:1, gun drilling is the only option. For 40:1 to 100:1, all three mechanical methods can work, but ejector and gun drilling on a CNC retrofit are marginal at the upper end.
Step 3: Consider Workpiece Material
Most mechanical deep hole drilling methods work on machinable metals. Unconventional methods extend the range to difficult materials.
| Material Condition | Recommended Method |
|---|---|
| Standard steels and alloys | Gun drilling, BTA, ejector (all work well) |
| Hardened steels (HRC 45+) | Gun drilling (carbide tooling), EDM |
| Titanium and superalloys | Gun drilling, BTA (slower speeds, specialized tooling) |
| Non-conductive materials (ceramics, composites) | Laser, abrasive water jet |
| Very hard materials (carbide, hardened tool steel) | EDM, laser |
| Heat-sensitive materials | ECM (no heat-affected zone), gun drilling (low heat) |
Step 4: Assess Production Volume
Volume drives the economic case for dedicated equipment.
| Annual Volume | Recommended Approach |
|---|---|
| 1–100 holes | Contract service provider (outsource) |
| 100–500 holes | Ejector drilling on existing CNC lathe, or contract BTA |
| 500–5,000 holes | Dedicated gun drilling or ejector drilling |
| 5,000–50,000 holes | BTA drilling (fastest penetration, justifies dedicated machine) |
| > 50,000 holes | Multi-spindle BTA or gun drilling |
Step 5: Evaluate Precision Requirements
| Tolerance Required | Recommended Method |
|---|---|
| ±0.050 mm or looser | Any method suitable |
| ±0.025 mm (±0.001") | Gun drilling (best), BTA (good), ejector (good) |
| ±0.013 mm or tighter | Gun drilling (only viable mechanical method) |
| ±0.005 mm | EDM (for small holes), gun drilling with optimized parameters |
Step 6: Consider Available Equipment
| Equipment Available | Best Method |
|---|---|
| Standard CNC lathe | Ejector drilling (DTS retrofit) or gun drilling (limited to 40:1) |
| Dedicated gun drilling machine | Gun drilling |
| Dedicated BTA machine | BTA drilling |
| No deep hole equipment | Contract service provider or DTS retrofit on existing CNC |
| EDM machine | EDM for small holes in hard materials |
Decision Matrix
| Hole Ø < 18 mm | Hole Ø 18–50 mm | Hole Ø 50–200 mm | Hole Ø > 200 mm |
|---|---|---|---|
| Gun drilling | Check depth ratio: | Check depth ratio: | Check depth ratio: |
| (EDM if < 1 mm) | ≤ 40:1 → all three | ≤ 40:1 → BTA or trepan | BTA or trepan |
| (Laser if < 0.5 mm) | ≤ 100:1 → gun or BTA | ≤ 100:1 → BTA | ≥ 40:1 → BTA required |
| > 100:1 → gun only | > 100:1 → gun only |
Decision Tree: Step by Step
1. What is the hole diameter?
├── < 1 mm → EDM / Laser / ECM
├── 1–18 mm → Gun drilling
└── > 18 mm → Go to step 2
2. What is the depth ratio?
├── > 100:1 → Gun drilling
└── ≤ 100:1 → Go to step 3
3. What is the production volume?
├── < 500/year → Ejector drilling (CNC retrofit) or contract service
├── 500–5,000/year → Gun drilling or ejector on dedicated machine
└── > 5,000/year → BTA drilling (highest productivity)
4. What is the available machine?
├── CNC lathe only → Ejector drilling (DTS)
├── Gun drill machine → Gun drilling
└── BTA machine → BTA drilling
5. Is the workpiece entry face irregular?
├── Yes → Ejector drilling (no seal needed)
└── No → BTA or gun drilling
Cost Ranking (per hole at same diameter)
From lowest to highest cost per hole for a given diameter:
| Rank | Method | Why |
|---|---|---|
| 1 | BTA drilling | Fastest penetration, lowest cycle time (highest machine cost offset by speed) |
| 2 | Ejector drilling | Slightly slower than BTA but lower machine rate on existing equipment |
| 3 | Gun drilling | Slowest penetration, but lowest tooling cost per hole |
| 4 | Trepanning | Higher tooling cost, but material savings on expensive alloys |
| 5 | EDM | Very slow removal rate, high operating cost |
| 6 | Laser / ECM | High equipment cost, slow for deep holes |
Common Mistakes in Method Selection
| Mistake | Why It Fails |
|---|---|
| Choosing gun drilling for a large-diameter hole | Gun drills are limited to ~50 mm max diameter |
| Choosing BTA for a small batch on a CNC lathe | BTA requires a dedicated machine — minimum $200K investment |
| Choosing ejector drilling for a hole under 18 mm | DTS tooling cannot physically fit below 18 mm |
| Choosing BTA for a rough casting surface | BTA’s pressure head seal requires a flat, square entry face |
| Choosing EDM for a standard steel hole | EDM is 10–50× slower than mechanical drilling |
| Choosing gun drilling for high-volume production | Gun drilling’s slow penetration rate drives up per-hole cost |
Summary
Choosing the right deep hole drilling method requires a structured evaluation of diameter, depth ratio, material, production volume, precision requirements, and available equipment. Start with diameter (the most restrictive constraint), then depth ratio, then volume. The most common mistake is selecting a method optimized for one dimension (e.g., BTA for speed) without considering whether the constraints in other dimensions (e.g., workpiece entry face condition) make it impractical.
For a complete guide to each method, see deep hole drilling methods overview. For a head-to-head comparison table, see deep hole drilling method comparison. For a complete overview, visit the drilling methods guide.