Gun Drilling Cost Analysis: Economics Per Hole

Gun Drilling Cost Analysis: Economics Per Hole

Gun drilling is often chosen for its quality — single-pass precision IT6-IT9, surface finish Ra 0.4-0.8 µm, and depth ratios up to 300:1. But these capabilities come at a cost. Understanding the economics per hole is essential for process planning, equipment justification, and cost comparison with alternative methods.

This guide provides a structured framework for calculating gun drilling cost per hole, with worked examples and comparison data.

Cost Per Hole Formula

The total cost per gun-drilled hole is the sum of four components:

Cost per hole = Tool cost + Machine cost + Coolant cost + Labor/overhead

1. Tool Cost Per Hole

Tool cost per hole = (Tool purchase price + Total regrind costs) ÷ Total holes over tool life

For a new tool with regrinds:
  Total tooling cost = Purchase price + (Number of regrinds × Regrind cost per cycle)
  Total holes = (Holes per regrind cycle) × (1 + Number of regrinds)
  Tool cost per hole = Total tooling cost ÷ Total holes

Example: Ø8 mm brazed carbide gun drill

2. Machine Cost Per Hole

Machine cost per hole = Machine hourly rate × Cycle time per hole (hours)

Machine hourly rate = (Machine purchase price × Depreciation factor + Annual maintenance) ÷ Annual operating hours

Example: Dedicated gun drilling machine

Cycle time calculation:

Cycle time (min) = (Hole depth ÷ Feed rate (mm/min)) + Retract + Indexing

For a 200 mm deep hole at 0.02 mm/rev, 4,000 RPM:
  Feed rate = 0.02 × 4,000 = 80 mm/min
  Drilling time = 200 ÷ 80 = 2.5 min
  Retract + indexing = 0.5 min
  Total cycle = 3.0 min

Machine cost per hole = $11.25/hr × (3.0 ÷ 60) hr = $0.56

3. Coolant Cost Per Hole

Coolant cost per hole = (Coolant consumption per hole × Coolant unit cost) + (Filtration amortization per hole)

4. Labor and Overhead

Total Cost Per Hole

Cost Sensitivity Analysis

Tool Diameter vs Cost Per Hole

L/D Ratio Effect on Cost

As L/D ratio increases:
  - Cycle time increases (linearly)
  - Tool wear rate increases (nonlinearly) 
  - Scrap/breakage risk increases
  - Regrind yield decreases

Regrind vs Replace: Break-Even Analysis

Break-even: Regrinding saves money as long as the first regrind yields ≥ 150 holes (47% of new edge performance).

Cost Comparison: Gun Drilling vs BTA vs Twist Drilling

Ø10 mm × 300 mm (30:1)

Ø25 mm × 500 mm (20:1)

When Twist Drilling Is Cheaper

For short holes (< 10:1 L/D), twist drilling (with pecking for moderate depths) is usually cheaper:

Machine Investment Payback Analysis

Cost Reduction Levers

Highest Impact First

Summary

Gun drilling cost per hole is driven primarily by cycle time (machine cost + labor) at small diameters and tool cost at larger diameters. For a typical Ø8 mm × 200 mm hole, total cost is approximately $3.08 per hole, with labor and overhead representing the largest component (65%). Regrinding reduces tool cost by 63% over the life of the tool compared to using new tools exclusively. At diameters below 10:1 L/D, twist drilling is more economical; above 20:1 L/D, gun drilling is the most cost-effective method. For a detailed cost comparison across all deep hole drilling methods, see deep hole drilling method cost comparison. For tooling cost optimization through regrinding, see gun drill regrinding best practices.