30-Second Summary: Understanding the true cost per hole in deep hole drilling requires looking beyond tool price to consider tool life, cycle time, machine rate, labor, and scrap. The L/D ratio is the dominant cost driver, scaling non-linearly — a hole at L/D 100 can cost 10x more than one at L/D 10. This guide breaks down the Cost Per Hole (CPH) formula, key cost factors by material and process, and provides a worked example plus strategies for reducing per-hole costs.
The Cost Per Hole Formula
Total Cost Per Hole (CPH) is calculated as:
CPH = (Tool Cost ÷ Holes per Tool) + (Cycle Time × (Machine Rate + Labor Rate)) + (Scrap Rate × Part Cost)
Each term represents a distinct cost component that must be tracked independently to identify where savings are possible. The tooling term is often the smallest but most visible; the machine time term is typically the largest.
Key Cost Drivers
L/D Ratio — The Dominant Factor
Cost scales non-linearly with L/D ratio. This table provides estimated multipliers:
| L/D Ratio | Cost Multiplier vs. L/D 10 | Primary Cost Driver |
|---|---|---|
| ≤ 5:1 | 0.5x - 0.8x | Standard drilling, no special equipment needed |
| 10:1 (baseline) | 1.0x | Gundrill setup, moderate cycle time |
| 30:1 | 1.5x - 2.5x | Reduced feed rate, increased tool wear |
| 50:1 | 3x - 5x | Whip guides required, lower Vc |
| 100:1 | 5x - 10x | Specialized supports, frequent tool changes, high scrap risk |
Reducing L/D by even 20% often yields proportionally larger savings because the nonlinear relationship amplifies any reduction.
Material Cost Factors
| Material Group | Cost Factor vs. Carbon Steel | Key Challenges | Typical Tool Life (holes per regrind, D=10mm) |
|---|---|---|---|
| Carbon steel (baseline) | 1.0x | Standard machinability | 200-500 |
| Stainless steel (304/316) | 1.2x - 1.5x | Work hardening, chip adhesion | 80-200 |
| Hardened alloys (35-45 HRC) | 1.5x - 2.0x | Abrasive wear, heat generation | 50-150 |
| Titanium (Ti-6Al-4V) | 2.0x - 3.0x | Low thermal conductivity, high strength at temperature | 30-80 |
| Superalloys (Inconel 718) | 3.0x - 5.0x | Extreme hot hardness, notch wear | 10-40 |
Tooling Cost
The per-hole tool cost is simply tool price divided by tool life in holes. Examples for a 10 mm diameter gundrill:
| Scenario | Tool Price | Tool Life (holes) | Cost Per Hole (tooling only) |
|---|---|---|---|
| Carbon steel, optimized parameters | $120 | 400 | $0.30 |
| Stainless steel, moderate tooling | $120 | 150 | $0.80 |
| Inconel 718, premium coating | $200 | 25 | $8.00 |
| Inconel 718, PCBN tooling | $600 | 150 | $4.00 |
Note how a more expensive PCBN tool can produce a lower per-hole cost than a cheaper carbide tool because of longer tool life. This is the core argument for evaluating on CPH rather than initial tool price.
Machine Time
Machine rates for deep hole drilling operations typically range from $40-120 per hour depending on machine complexity, size, and features. BTA drilling machines at the high end; modified conventional machines at the low end. This rate includes machine depreciation, maintenance, floor space, and energy. For a 50 mm × 2000 mm hydraulic cylinder barrel (L/D=40:1), a 15-minute cycle yields a machine-time cost of $10-30 per hole depending on the machine rate.
Scrap and Rework
Without proper process controls, scrap rates in deep hole drilling can reach 18%. Each scrap part represents the full accumulated cost of material plus all prior operations. Rework — such as welding and re-drilling — typically costs 30-60% of the original operation but risks introducing new defects. Investing in process monitoring (coolant pressure, torque, feed force) can reduce scrap to below 5% in most applications.
Hidden Costs
- Coolant management: Filtration system maintenance, coolant replacement, and disposal. Budget $1-5 per hole for complex BTA operations.
- Tool regrinding: Regrinding a gundrill costs 20-30% of a new tool. Factor this into the tool cost calculation.
- Setup and changeover: Each tool change costs 5-15 minutes of machine time plus setup labor.
Process Cost Comparison by Batch Size
| Process | Low Volume (1-50 pcs) | Medium Volume (50-500 pcs) | High Volume (>500 pcs) | Best For |
|---|---|---|---|---|
| Gundrilling | $$ (standard tooling) | $$ (good tool life) | $ (optimized) | Small diameters (1-50 mm), tight tolerances |
| BTA drilling | $$$ (long setup, tool cost) | $$ (setup amortized) | $ (lowest per-hole cost) | Large diameters (12-250+ mm), high volume |
| Ejector drilling | $$ (moderate setup) | $$ (good for medium batches) | $$ (less efficient than BTA) | Medium diameters (18-65 mm), moderate volume |
Example Calculation: Hydraulic Cylinder Barrel
Part: Hydraulic cylinder barrel
Specification: 50 mm diameter × 2000 mm length (L/D = 40:1)
Material: 4140 steel (28-32 HRC)
Batch: 200 pieces per month
| Cost Component | Calculation | Per Hole Cost |
|---|---|---|
| Tool cost | $180 drill ÷ 250 holes | $0.72 |
| Machine time | 15 min cycle × ($60/hr machine + $40/hr labor) × 0.25 hr | $25.00 |
| Scrap (estimated 8%) | 0.08 × $45 part cost | $3.60 |
| Coolant / consumables | Estimated (filtration, coolant replacement, regrinding amortization) | $1.50 |
| Total CPH | $30.82 |
To reduce CPH for this part, the biggest lever is reducing cycle time (machine time is 81% of the total). A 20% cycle time reduction saves $5.00/hole — far more than any tool cost optimization.
Cost Reduction Strategies
- Upgrade tooling: Longer-life coatings (TiAlN, AlCrN) can increase tool life 2-5x, reducing the tool-cost-per-hole component. For difficult materials, PCBN tooling, despite higher upfront cost, often delivers lower cost per hole.
- Optimize parameters: Running at the optimal speed/feed combination for the specific material can reduce cycle time while maintaining or improving tool life. Use manufacturer cutting data as a starting point, then optimize based on actual chip shape and tool wear.
- Reduce tool changes: Minimize tool changes through better tool life management and batch scheduling. Each tool change costs both time and the potential for setup errors.
- Minimize scrap: Implement real-time monitoring (coolant pressure, torque, feed force) to detect problems before they produce scrap. A 5% scrap reduction can yield savings exceeding the CPH reduction target.
- Design adjustments: Work with design engineering to reduce L/D ratio where possible, convert blind holes to through holes, and standardize diameters across part families.