Appearance
A manufacturer of hydraulic cylinders switched from gun drilling to BTA drilling for a 50 mm diameter bore in a 2,000 mm long cylinder. The tooling cost increased by 40% per station. The cycle time dropped from 45 minutes to 12 minutes. The per-part cost fell by 55%. A manufacturer of fuel injector bodies switched from twist drilling to gun drilling for a 4 mm diameter × 200 mm long oil return bore. The tooling cost increased by 300% per hole. The reject rate dropped from 18% to 0.5%. The per-part cost fell by 35%. Both decisions were correct. Both decisions required understanding not just the cost of the tool and the machine time, but the total cost of producing a conforming hole — including scrap, rework, inspection, and the hidden cost of unreliable chip evacuation at depth. Deep hole drilling economics is not about choosing the cheapest tool or the fastest cycle. It is about matching the process to the geometry, material, tolerance, and volume of the hole to minimise the total cost per good part.
Cost Structure of Deep Hole Drilling
The total cost of a deep drilled hole consists of five components:
Cost Components
| Component | Typical Share | Notes |
|---|---|---|
| Machine time (labour + depreciation) | 40–60% | Dominant for long cycle times |
| Tooling (drill + inserts per hole) | 15–25% | Highly variable by material and diameter |
| Coolant and filtration | 5–10% | High-pressure systems are expensive to operate |
| Setup and alignment | 5–15% | Amortised over batch size |
| Scrap, rework, and inspection | 5–20% | Often underestimated |
The Non-Linear Depth Cost
The most important concept in deep hole drilling economics: cost does not scale linearly with depth.
| L/D Ratio | Relative Cost per mm of Depth | Reason |
|---|---|---|
| ≤ 10:1 | 1.0× (baseline) | Conventional twist drilling possible |
| 10:1–30:1 | 1.5–2.0× | Specialised tooling, coolant-through required |
| 30:1–50:1 | 2.0–4.0× | Chip evacuation becomes unreliable |
| 50:1–80:1 | 4.0–8.0× | Step drilling or peck cycles required |
| 80:1–150:1 | 8.0–20× | Specialised process, high scrap risk |
The non-linearity arises from three factors:
- Chip evacuation — at L/D > 30:1, chip friction against the bore wall increases exponentially, requiring higher coolant pressure and more frequent peck cycles
- Tool deflection — the drill shank torsional wind-up at depth reduces the effective feed at the cutting edge, requiring speed/feed adjustments
- Failure consequence — a tool breakage at 80:1 depth scraps the component; at 10:1 depth it is recoverable
Comparative Economics by Drilling Method
Cost Comparison: Gun Drilling vs. BTA Drilling vs. Trepanning
| Factor | Gun Drilling | BTA Drilling | Trepanning |
|---|---|---|---|
| Diameter range | 1–50 mm | 18–300 mm | 50–1,000 mm |
| Relative tool cost (per hole) | Medium | High | Very high |
| Material removal rate | Low | High (3× gun drilling) | Medium |
| Surface finish (as-drilled) | Ra 0.4–0.8 µm | Ra 0.8–3.2 µm | Ra 1.6–6.3 µm |
| Typical tolerance | H8–H9 | H9–H11 | H11–H12 |
| Scrap risk (at L/D > 50:1) | Moderate | Low | Low |
| Setup cost | Low-moderate | Moderate | High |
| Break-even batch size | 1–500 | 50–10,000 | 1–100 |
| Secondary finishing | Often not needed | Often required (hone/skive) | Always required |
| Machine cost | €100k–€500k | €300k–€1,000k | €500k–€2,000k |
Cost per Hole by Diameter (Estimates for Steel, L/D = 20:1)
| Diameter | Gun Drilling | BTA Drilling | Trepanning |
|---|---|---|---|
| 10 mm × 200 mm | €3–€8 | N/A | N/A |
| 25 mm × 500 mm | €12–€25 | €10–€20 | N/A |
| 50 mm × 1,000 mm | €30–€60 | €15–€30 | €25–€50 |
| 100 mm × 2,000 mm | N/A | €40–€80 | €50–€100 |
| 200 mm × 3,000 mm | N/A | €100–€200 | €120–€250 |
These are order-of-magnitude estimates. Actual costs depend on material, tolerance, surface finish requirements, and batch size.
Process Selection Guide
Decision Matrix
| Primary Criteria | Gun Drilling | BTA / STS | Trepanning | Ejector (Double-Tube) |
|---|---|---|---|---|
| Diameter ≤ 20 mm | ✔ Best choice | Not suitable | Not suitable | Not suitable |
| Diameter 20–50 mm | ✔ Good | ✔ Good | Not suitable | ✔ Good |
| Diameter 50–150 mm | Not suitable | ✔ Best choice | ✔ Good | ✔ Best choice |
| Diameter > 150 mm | Not suitable | ✔ Good | ✔ Best choice | ✔ Good |
| L/D ≤ 10:1 | Overkill | Overkill | Overkill | ✔ Good |
| L/D 10:1–50:1 | ✔ Good | ✔ Good | ✔ Good | ✔ Best choice |
| L/D 50:1–100:1 | ✔ Best choice | ✔ Good | Fair | ✔ Good |
| L/D > 100:1 | ✔ Best choice | Fair | Not suitable | Fair |
| Tolerance H7–H8 | ✔ Best choice | Moderate | Not suitable | Moderate |
| Tolerance H9–H11 | ✔ Good | ✔ Good | Fair | ✔ Good |
| Batch < 100 | ✔ Best choice | Fair | Fair | Fair |
| Batch 100–1,000 | ✔ Good | ✔ Best choice | Fair | ✔ Good |
| Batch > 1,000 | Moderate | ✔ Best choice | Moderate | ✔ Best choice |
Material Cost Multipliers
Different materials require different cutting speeds, feed rates, and tool grades — all of which affect cost:
| Material | Relative Machinability | Speed vs. Steel | Tool Life vs. Steel | Cost Multiplier vs. Mild Steel |
|---|---|---|---|---|
| Mild steel (C45, 1020) | Excellent | 1.0× | 1.0× | 1.0× (baseline) |
| Alloy steel (4140, 42CrMo4, 300 HB) | Good | 0.7–0.8× | 0.5–0.7× | 1.2–1.5× |
| Tool steel (D2, H13, 250 HB) | Fair | 0.5–0.6× | 0.3–0.5× | 1.5–2.0× |
| Stainless steel (304, 316) | Fair | 0.5–0.7× | 0.3–0.5× | 1.5–2.0× |
| Titanium (Ti-6Al-4V) | Fair-poor | 0.3–0.5× | 0.2–0.4× | 2.0–3.0× |
| Inconel 718 (annealed) | Poor | 0.2–0.3× | 0.1–0.2× | 3.0–5.0× |
| Inconel 718 (40 HRC) | Very poor | 0.15–0.25× | 0.02–0.10× | 5.0–10× |
Cost Optimisation Strategies
Speed-Feed Optimisation
The relationship between cutting speed, feed, tool life, and cost is not linear — there is an economic optimum that balances tooling cost against machine time:
| Parameter | Effect on Tool Life | Effect on Cycle Time | Effect on Cost |
|---|---|---|---|
| Increase cutting speed 20% | Decrease tool life 40–60% | Decrease cycle time 17% | Variable — often increases total cost |
| Increase feed 20% | Decrease tool life 15–25% | Decrease cycle time 17% | Often reduces total cost |
| Both increased 10% each | Decrease tool life 30–40% | Decrease cycle time 17% | Usually increases cost |
| Optimise speed for max tool life | Increase 2–5× | Increase 20–50% | Lower cost at small batch sizes |
Rule of thumb: For production deep hole drilling, the economic optimum is typically at a cutting speed 20–30% below the maximum recommended speed and a feed 10–20% above the minimum recommended feed.
Tool Life Management
| Strategy | Effect on Per-Hole Cost | Best For |
|---|---|---|
| Replace tool at fixed interval (before end of life) | +5–15% tool cost, −50–80% scrap | Critical components, expensive materials |
| Run to failure (minimum tool changes) | Lowest tool cost, highest scrap | Low-volume, cheap materials |
| Regrind vs. replace | −30–50% per regrind (3–5 regrinds possible) | Gun drills > 6 mm diameter |
| Indexable inserts (BTA) | +10–20% per edge, −50% change time | High-volume BTA drilling |
Coolant Cost Management
| Strategy | Saving | Trade-off |
|---|---|---|
| Optimise pressure to minimum required | −10–30% pump energy | Risk of chip packing at marginal pressure |
| Fine filtration (≤ 20 µm) | −20–40% tool wear | Higher filter element cost |
| Temperature control (20–30°C) | −10–20% tool wear variation | Chiller capital cost |
| Coolant life extension (centrifuge) | −30–50% coolant purchases | Centrifuge capital cost |
Investment Decision Framework
When to Upgrade from Gun Drilling to BTA
| Indicator | Threshold |
|---|---|
| Bore diameter | > 25 mm |
| Annual volume | > 500 parts |
| Current cycle time vs. target | Current > 2× target |
| Scrap rate | > 5% from gun drilling issues |
| Available machine utilisation | > 70% |
Payback Calculation Example
Converting a hydraulic cylinder bore from gun drilling to BTA drilling:
| Item | Gun Drilling | BTA Drilling |
|---|---|---|
| Bore diameter | 50 mm | 50 mm |
| Bore length | 1,500 mm | 1,500 mm |
| Cycle time (including handling) | 35 minutes | 10 minutes |
| Tool cost per hole | €4 | €6 |
| Scrap rate | 4% | 1% |
| Machine rate | €80/hour | €80/hour |
| Cost per good part | €52.30 | €16.50 |
Investment: BTA machine — €450,000 (or €50,000 BTA conversion kit) Savings per part: €35.80 Annual volume: 3,000 parts Annual savings: €107,400 Payback period: 4.2 years (full machine) / 5.6 months (conversion kit)
When to Choose Trepanning
Trepanning is economically justified when:
- Bore diameter exceeds 50 mm — the core removed is valuable material
- Core material can be reused — a 150 mm core from a 300 mm bore × 2,000 mm length weighs approximately 110 kg in steel — worth €100–€300 in material alone
- Cutting forces must be minimised — trepanning removes only 30–40% of the cross-sectional area vs. 100% for solid drilling
- Available machine power is limited — lower cutting forces allow smaller machines
Case Studies
Case 1: Hydraulic Cylinder — Gun Drilling to BTA
| Before | After | Improvement |
|---|---|---|
| Gun drilling, 50 mm × 1,500 mm | BTA drilling | −55% cost per hole |
| 35 min cycle | 10 min cycle | −71% cycle time |
| 4% scrap | 1% scrap | −75% scrap |
| 200 holes per regrind | 500 holes per insert edge | +150% tool life |
Key insight: The BTA method's internal chip evacuation eliminated the chip packing that caused most of the gun drilling scrap.
Case 2: Fuel Injector Body — Twist Drill to Gun Drill
| Before | After | Improvement |
|---|---|---|
| Twist drill, 4 mm × 200 mm | Gun drilling | −35% cost per hole |
| 18% reject rate | 0.5% reject rate | −97% rejects |
| 3 min cycle | 4 min cycle | +33% cycle time |
| No straightness control | ±0.01 mm straightness | Predictable quality |
Key insight: The higher cycle time was more than offset by the elimination of scrap and rework.
Case 3: Military Component — Gun Drilling to Double-Tube BTA
Reported in Cutting Tool Engineering: a military contractor switched from gun drilling to a double-tube BTA system.
| Item | Before | After |
|---|---|---|
| Per-part cost | $33 | $15 |
| Investment | — | $20,000 (conversion) |
| Payback period | — | 21 days |
| Annual savings | — | $75,000 |
Case 4: Nuclear Steam Generator Tube Sheet — Single-Spindle to Multi-Spindle
| Before | After | Improvement |
|---|---|---|
| Single-spindle BTA | 4-spindle BTA | −60% total drilling time |
| 8,000 holes, 45 s each | 8,000 holes, 45 s each (×4) | 90 hours → 22.5 hours |
| 4 machines required | 1 machine required | −75% capital |
Key insight: Multi-spindle drilling requires higher initial investment but delivers the lowest per-hole cost for very large hole quantities (> 5,000 holes per tube sheet).
FAQ
Q: What percentage of deep hole drilling cost is machine time? 40–60% in most production environments. Tooling accounts for 15–25%, coolant 5–10%, setup 5–15%, and scrap/rework 5–20%. Machine time dominates because deep hole drilling is inherently slow — cutting speeds are low and feed rates are constrained by chip evacuation.
Q: How does L/D ratio affect cost? Non-linearly. A hole with L/D = 50 costs 2–4× more per mm of depth than L/D = 10. At L/D > 80:1, the cost per mm can be 8–20× higher because of chip evacuation problems, tool deflection, and the catastrophic cost of a tool breakage.
Q: When should I choose gun drilling over BTA drilling? Gun drilling is the best choice for: diameters below 20 mm, L/D ratios above 80:1, tight tolerances (H8 or better), small batch sizes (< 100), and materials that are difficult to machine with multi-edge tools.
Q: When is BTA drilling more economical? BTA drilling is more economical for: diameters above 25 mm, medium to high volumes (> 500 parts/year), when internal chip evacuation improves process reliability, and when the higher material removal rate (3× gun drilling) can reduce cycle time.
Q: What is trepanning and when is it economical? Trepanning cuts a ring-shaped groove around a solid core, leaving the core intact for reuse. It is economical for bores > 50 mm diameter where the core material is valuable, when cutting forces must be minimised, or when available machine power is limited.
Q: What is the most effective cost reduction strategy in deep hole drilling? Eliminating scrap caused by chip evacuation problems. A 5% scrap rate adds 5.3% to the cost of every good part. A 15% scrap rate (common in difficult materials) adds 17.6%. Improving chip control through method selection, coolant optimisation, and chip breaker geometry typically delivers the largest single cost reduction.
Q: How does material selection affect deep hole drilling cost? Material is the second-largest cost driver after L/D ratio. Inconel 718 at 40 HRC costs 5–10× more per hole than mild steel. Titanium costs 2–3× more. The cost multiplier comes from lower cutting speeds, shorter tool life, higher coolant pressure requirements, and higher scrap risk.
Q: What batch size justifies a dedicated deep hole drilling machine? A dedicated gun drilling machine (€100k–€300k) is typically justified at 500–2,000 parts/year for small-diameter work. A BTA machine (€300k–€1,000k) is justified at 1,000–5,000 parts/year for larger diameters. Contract drilling services are more economical below these volumes.
Q: Can a multi-spindle machine reduce per-hole cost? Yes, for large quantities of identical holes. A 4-spindle BTA machine can reduce drilling time by 75% compared to a single-spindle machine. The breakeven is typically at 5,000+ holes per tube sheet or 10,000+ parts per year.
Q: What is the most common economic mistake in deep hole drilling? Choosing a process based on cycle time alone without considering scrap rate and tooling cost. A fast process with 10% scrap is usually more expensive than a slower process with 0.5% scrap. The total cost per good part — not the cycle time — is the correct economic metric.