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In deep hole drilling, the cost of the tool is not the tool price divided by the number of holes — it is the tool price, plus the cost of every hole it failed to make, every regrind it required, and every minute of downtime it caused, all divided by the number of good holes it actually produced.
Overview
Deep hole drilling economics differ from conventional machining economics in two important ways: the tooling cost per hole is higher (specialized drills cost more), but the cost of failure is also higher — a broken tool at 100× diameter depth can scrap a workpiece worth hundreds or thousands of dollars.
| Cost Component | Typical Share of Total Hole Cost | Notes |
|---|---|---|
| Machine cost (depreciation + operating) | 35 – 50% | Dominant for dedicated machines |
| Tooling cost (drill + regrinds) | 15 – 30% | Highly variable with tool life |
| Labor cost | 10 – 20% | Lower for automated systems |
| Coolant (pump power + filtration + disposal) | 5 – 10% | Higher for BTA systems |
| Scrap and rework | 5 – 15% | Often underestimated |
Cost per Hole Calculation
Basic Formula
Cost per hole = (Machine cost per hour × Cycle time in hours) + Tooling cost per hole + Consumables per hole
Where:
Machine cost per hour = (Machine purchase price × Depreciation rate + Annual maintenance + Floor space cost) / Annual operating hours
Tooling cost per hole = (Tool purchase price / Holes per tool life) + (Regrind cost / Holes per regrind interval)
Worked Example: Gun Drilling a 10 mm Hole in 4140 Steel
| Parameter | Value |
|---|---|
| Machine cost (CNC lathe with TSC) | $80,000 |
| Machine life | 10 years |
| Annual operating hours | 2,000 |
| Machine cost per hour | $4.00 + $2.00 (labor) = $6.00/hr |
| Cycle time per hole (100 mm deep) | 2.5 minutes (0.042 hr) |
| Machine cost per hole | $6.00 × 0.042 = $0.25 |
| Gun drill cost | $85.00 |
| Holes per regrind | 500 |
| Regrinds per tool | 8 |
| Total holes per tool | 4,500 |
| Tooling cost per hole | $85.00 / 4,500 = $0.019 |
| Regrind cost per hole | $15.00 / 500 = $0.030 |
| Coolant and consumables per hole | $0.02 |
| Total cost per hole | $0.32 |
Worked Example: BTA Drilling a 50 mm Hole in 4140 Steel
| Parameter | Value |
|---|---|
| Machine cost (BTA machine) | $250,000 |
| Machine life | 10 years |
| Annual operating hours | 4,000 (2 shifts) |
| Machine cost per hour | $6.25 + $3.00 (labor) = $9.25/hr |
| Cycle time per hole (500 mm deep) | 8 minutes (0.133 hr) |
| Machine cost per hole | $9.25 × 0.133 = $1.23 |
| BTA drill head cost (indexable) | $120.00 |
| Inserts per head | 3 inserts, 4 edges each = 12 edges |
| Holes per edge | 200 |
| Total holes per head | 2,400 |
| Tooling cost per hole | $120.00 / 2,400 = $0.050 |
| Guide pads per hole | $0.03 |
| Coolant and consumables per hole | $0.15 |
| Total cost per hole | $1.46 |
Tooling Cost by Method
Gun Drilling Tooling
| Drill Diameter (mm) | Typical Drill Cost | Holes per Regrind | Regrinds | Total Holes | $ per Hole |
|---|---|---|---|---|---|
| 3 | $45 – $65 | 200 – 800 | 5 – 10 | 1,000 – 8,000 | $0.01 – $0.07 |
| 6 | $55 – $85 | 300 – 1,200 | 5 – 10 | 1,500 – 12,000 | $0.01 – $0.06 |
| 10 | $70 – $120 | 500 – 2,000 | 5 – 10 | 2,500 – 20,000 | $0.01 – $0.05 |
| 16 | $90 – $160 | 500 – 2,000 | 5 – 8 | 2,500 – 16,000 | $0.01 – $0.06 |
| 25 | $120 – $220 | 400 – 1,500 | 5 – 8 | 2,000 – 12,000 | $0.01 – $0.11 |
BTA Drilling Tooling
| Drill Diameter (mm) | Head Cost | Inserts per Head | Edges per Insert | Holes per Edge | $ per Hole |
|---|---|---|---|---|---|
| 25 | $80 – $150 | 2 – 3 | 3 – 4 | 150 – 400 | $0.03 – $0.17 |
| 40 | $120 – $250 | 3 – 4 | 3 – 4 | 150 – 400 | $0.03 – $0.19 |
| 65 | $200 – $400 | 4 – 5 | 3 – 4 | 150 – 400 | $0.04 – $0.22 |
| 100 | $350 – $600 | 4 – 6 | 3 – 4 | 100 – 300 | $0.05 – $0.33 |
Indexable tools reduce tooling cost variance
Indexable BTA and gun drill systems eliminate the cost of regrinding and the risk of poor regrind quality. While the per-edge cost is slightly higher than a reground brazed tool, the total cost of ownership is often lower because indexable systems eliminate regrind inventory management and provide predictable edge life.
Machine Investment ROI
Retrofit vs Dedicated Machine
| Approach | Investment | Typical Payback | Best For |
|---|---|---|---|
| CNC lathe gun drilling retrofit | $7,000 – $20,000 | 3 – 6 months | Low-medium volume, < 20 mm Ø |
| TSC retrofit on VMC/lathe | $10,000 – $30,000 | 3 – 6 months | Shallow deep holes, < 20:1 L/D |
| Dedicated micro gun drilling machine | $80,000 – $200,000 | 12 – 24 months | Medical, precision, < 6 mm Ø |
| Dedicated gun drilling machine | $100,000 – $300,000 | 18 – 36 months | Production gun drilling |
| Dedicated BTA machine | $200,000 – $600,000 | 24 – 48 months | High-volume, > 20 mm Ø |
ROI Calculation Example
A $20,000 gun drilling retrofit on a CNC lathe, used to produce one hole per part at 500 parts per month:
Before retrofit (outsourced deep hole drilling):
- Outside cost per hole: $5.00
- Monthly cost: 500 × $5.00 = $2,500
After retrofit (in-house):
- Machine cost per hour: $6.00
- Cycle time per hole: 3 minutes
- Machine cost per hole: $0.30
- Tooling cost per hole: $0.03
- Total in-house cost per hole: $0.33
- Monthly cost: 500 × $0.33 = $165
Monthly savings: $2,500 – $165 = $2,335
Payback period: $20,000 / $2,335 = 8.6 months
If the alternative is not outsourcing but instead using a conventional twist drill with peck cycles that takes 15 minutes per hole (instead of 3 minutes with gun drilling), the savings come from cycle time reduction:
- Old cycle time: 15 min/hole = $1.50/hole machine cost
- New cycle time: 3 min/hole = $0.30/hole machine cost
- Savings: $1.20/hole × 500 = $600/month
- Payback: $20,000 / $600 = 33 months (less attractive)
ROI depends on what you are comparing against
The ROI of a deep hole drilling investment depends entirely on the baseline. Comparing against outsourcing (high per-hole cost) gives fast payback. Comparing against a slow existing process gives moderate payback. Comparing against doing nothing (no requirement for deep holes) gives no payback. Always calculate against your actual alternative, not an idealized baseline.
Method Economics by Volume
Low Volume (1 – 100 holes per year)
| Method | Setup Cost per Hole | Tooling Cost per Hole | Total Cost per Hole |
|---|---|---|---|
| Conventional (twist drill, peck) | $0 – $5 | $0.50 – $2.00 | $0.50 – $7.00 |
| Outsourced gun drilling | $0 (vendor) | $5.00 – $50.00 | $5.00 – $50.00 |
| In-house gun drilling (retrofit) | $5 – $20 | $0.50 – $2.00 | $5.50 – $22.00 |
At very low volumes, outsourcing is often cheaper than in-house investment.
Medium Volume (100 – 5,000 holes per year)
| Method | Machine Cost per Hole | Tooling Cost per Hole | Total Cost per Hole |
|---|---|---|---|
| Conventional drilling | $1.00 – $5.00 | $0.20 – $1.00 | $1.20 – $6.00 |
| Gun drilling (retrofit) | $0.20 – $1.00 | $0.02 – $0.10 | $0.22 – $1.10 |
| Gun drilling (dedicated) | $0.50 – $2.00 | $0.02 – $0.10 | $0.52 – $2.10 |
| BTA drilling | $0.50 – $2.00 | $0.05 – $0.20 | $0.55 – $2.20 |
At medium volumes, in-house gun drilling with a retrofit offers the lowest per-hole cost.
High Volume (> 5,000 holes per year)
| Method | Machine Cost per Hole | Tooling Cost per Hole | Total Cost per Hole |
|---|---|---|---|
| Gun drilling (multi-spindle) | $0.05 – $0.20 | $0.01 – $0.05 | $0.06 – $0.25 |
| BTA drilling (dedicated) | $0.10 – $0.40 | $0.03 – $0.10 | $0.13 – $0.50 |
| Indexable gun drilling | $0.05 – $0.20 | $0.02 – $0.06 | $0.07 – $0.26 |
At high volumes, dedicated multi-spindle gun drilling or BTA achieves the lowest per-hole cost.
Hidden Costs
Scrap and Rework
The cost of a scrapped workpiece in deep hole drilling can be many times the cost of drilling the hole:
| Workpiece Value | Scrap Rate | Effective Cost per Good Hole |
|---|---|---|
| $50 | 2% | $1.02 × drilling cost |
| $50 | 10% | $1.11 × drilling cost |
| $500 | 2% | $10.20 + drilling cost |
| $500 | 10% | $55.56 + drilling cost |
A 2% scrap rate on a $500 workpiece adds $10.20 to the effective cost of every hole. Reducing scrap through better tool life management, process monitoring, and operator training is often the highest-ROI activity in deep hole drilling.
Coolant System Operating Cost
| System Type | Pump Power | Annual Power Cost (2,000 hrs) | Filter Element Cost | Total Annual |
|---|---|---|---|---|
| Flood coolant | 3 HP | $450 | $200 | $650 |
| TSC retrofit (1,000 psi) | 10 HP | $1,500 | $1,200 | $2,700 |
| Gun drilling machine | 20 HP | $3,000 | $2,400 | $5,400 |
| BTA machine (large) | 50 HP | $7,500 | $6,000 | $13,500 |
Tooling Management Cost
Regrind management adds overhead that is often overlooked:
| Activity | Cost per Regrind |
|---|---|
| Tool removal and replacement | $2 – $5 |
| Shipping to regrind service | $5 – $15 |
| Regrind service fee | $10 – $30 |
| Inspection after regrind | $3 – $8 |
| Inventory carrying cost | $1 – $3 |
| Total per regrind cycle | $21 – $61 |
Cost Reduction Strategies
| Strategy | Impact | Effort | Time to Result |
|---|---|---|---|
| Optimize cutting speed for tool life | Reduces tooling cost 20–50% | Low | Immediate |
| Implement coolant pressure monitoring | Reduces scrap from chip packing | Medium | 1 – 4 weeks |
| Switch from brazed to indexable tooling | Eliminates regrind management | Medium | 1 – 2 months |
| Add coolant chiller | Extends tool life 15–30% | Medium | 1 – 2 months |
| Upgrade filtration to ≤ 10 µm | Extends tool and pad life 20–40% | Medium | 1 – 2 months |
| Retrofit existing machine | Avoids dedicated machine purchase | High | 2 – 8 weeks |
| Multi-spindle machine for high volume | 2–4× throughput at 1.5× cost | High | 4 – 12 months |
| Automate tool change based on hole count | Prevents catastrophic failure | Medium | 2 – 6 weeks |
Summary
| Scenario | Most Economical Method | Typical Cost per Hole | Key Cost Driver |
|---|---|---|---|
| < 5 mm Ø, any volume | Gun drilling | $0.10 – $2.00 | Tool life (small drills break easily) |
| 5 – 20 mm Ø, low volume | Gun drilling (retrofit) | $0.20 – $1.00 | Machine utilization |
| 5 – 20 mm Ø, high volume | Multi-spindle gun drilling | $0.06 – $0.25 | Cycle time |
| 20 – 65 mm Ø, medium volume | BTA drilling | $0.30 – $1.50 | Insert cost + cycle time |
| 20 – 65 mm Ø, high volume | BTA drilling | $0.13 – $0.50 | Coolant power |
| > 65 mm Ø, any volume | BTA or trepanning | $0.50 – $5.00 | Material removal volume |
FAQ
What is the typical cost per hole for gun drilling?
For gun drilling in steel with diameters between 6–20 mm, the typical cost per hole ranges from $0.20 to $1.00 including machine, tooling, and consumable costs. The tooling component is usually $0.01–$0.06 per hole. The machine cost (depreciation + operating) accounts for 40–60% of the total. Small diameters under 6 mm have higher per-hole costs due to shorter tool life.
How long does it take to pay back a gun drilling machine?
A CNC lathe gun drilling retrofit ($7,000–$20,000) typically pays back in 3–6 months when replacing outsourced deep hole drilling. A dedicated gun drilling machine ($100,000–$300,000) typically requires 18–36 months payback depending on utilization and part volume. The fastest payback comes from converting outsourced work to in-house production.
Is BTA drilling cheaper than gun drilling?
At diameters below 20 mm, gun drilling is cheaper. At diameters above 20 mm with medium-to-high volume, BTA drilling is cheaper per hole because its 4× higher metal removal rate reduces cycle time enough to offset higher tooling and coolant costs. The crossover point depends on material, depth, and volume — but 20 mm diameter is a reasonable rule of thumb.
What is the biggest hidden cost in deep hole drilling?
Scrap and rework are the biggest hidden costs. A single scrapped high-value workpiece ($200–$2,000) can eliminate the profit from hundreds of good holes. Scrap rates of 5–10% are common in shops that do not monitor tool wear or coolant pressure. Investing in process monitoring (coolant pressure, feed force, spindle power) often pays for itself within weeks through scrap reduction alone.
How many regrinds should I expect from a gun drill?
Typical gun drills can be reground 5–15 times depending on the original carbide tip size and the regrind allowance per cycle (0.1–0.3 mm of carbide removed). Small-diameter drills (< 6 mm) average 5–10 regrinds, while larger drills (> 12 mm) average 8–15 regrinds. The regrind cost ($10–$30 per cycle) should be factored into the cost-per-hole calculation.
Does coolant filtration affect cost per hole?
Yes. Inadequate filtration (above 20 µm) accelerates guide pad wear and can reduce tool life by 30–50%. This directly increases cost per hole. Upgrading from 20 µm to 10 µm filtration typically costs $1,000–$3,000 for a filter system upgrade but reduces tooling cost by 20–40%, yielding ROI in 2–6 months for production operations.
What is the cheapest way to start deep hole drilling?
The cheapest entry point is a gun drilling retrofit on an existing CNC lathe — typically $7,000–$20,000 for a high-pressure coolant pump, rotary union, guide bush holder, and tooling. This works for diameters up to 20 mm at L/D ratios up to 40:1. For larger diameters, a BTA conversion of a lathe costs approximately $20,000 and can achieve 5× the material removal rate of gun drilling.
How does hole depth affect cost per hole?
Cost per hole increases with depth for two reasons: longer cycle time (more machine cost) and reduced tool life at high L/D ratios (higher tooling cost). At L/D > 50:1, expect a 20–30% increase in cost per hole compared to L/D < 20:1 due to speed and feed reductions required for deep holes. At L/D > 100:1, the cost increase can exceed 50%.
Costs depend on machine type, location, labor rates, material, production volume, and specific process parameters. The values in this article are typical ranges for North American and European production environments. Consult equipment suppliers and conduct your own cost analysis for application-specific economics. This article reflects industry knowledge as of 2026.