Deep Hole Drilling Method Cost Comparison: Comprehensive Per-Hole Analysis

Deep Hole Drilling Method Cost Comparison

Selecting the most economical deep hole drilling method requires comparing costs across all available options at the required diameter, depth, material, and production volume. Each method has a different cost structure — some favor low investment and higher per-hole cost (gun drilling), others favor higher investment and lower per-hole cost (BTA, multi-spindle).

This guide provides a consistent cost comparison framework across all methods, with practical examples and decision guidelines.

Cost Modeling Framework

Cost Components

Total cost per hole = C_tool + C_machine + C_coolant + C_labor + C_overhead

Cost Component	Typical Share of Total	Most Influenced By
Tool cost	10–30%	Tool life, regrind frequency, number of cutting edges
Machine cost	20–40%	Cycle time, machine investment, depreciation
Coolant cost	5–15%	Coolant volume, filtration requirements, replacement interval
Labor cost	20–40%	Cycle time, operator skill level, machine automation level
Overhead	10–20%	Facility, management, quality inspection, utilities

Assumptions Used in This Comparison

Parameter	Value
Machine operation	2 shifts/day, 4,000 hours/year
Labor rate	$35/hr (including benefits)
Coolant type	Neat oil
Material	Medium-carbon steel (0.35% C)
Depth	200 mm except where noted
Depreciation	10-year straight line

Cost Per Hole by Method and Diameter

10 mm Diameter × 200 mm Deep

Method	Cycle Time	Tool Cost/Hole	Machine + Labor	Total per Hole
Gun drilling (single-spindle)	3.0 min	$0.25	$2.25	$3.00
Gun drilling (multi-spindle, 4×)	1.0 min	$0.35	$2.50	$3.35
Solid carbide extended drill (20×D)	1.5 min	$0.18	$1.80	$2.30
Twist drill + ream (peck cycle)	6.0 min	$0.12	$4.50	$4.80

Winner: Solid carbide extended drill (cheapest + fastest) at 10 mm × 20×D.

30 mm Diameter × 200 mm Deep

Method	Cycle Time	Tool Cost/Hole	Machine + Labor	Total per Hole
Gun drilling	8.0 min	$0.52	$5.60	$6.12
BTA drilling	2.5 min	$0.35	$2.80	$3.65
Ejector drilling (CNC lathe)	3.5 min	$0.30	$2.60	$3.20

Winner: Ejector drilling (lowest total cost + no dedicated machine required).

60 mm Diameter × 300 mm Deep

Method	Cycle Time	Tool Cost/Hole	Machine + Labor	Total per Hole
BTA drilling	4.0 min	$0.60	$4.00	$4.60
Ejector drilling (CNC lathe)	5.5 min	$0.50	$3.60	$4.80
Trepanning	6.0 min	$0.80	$4.20	$5.50

Winner: BTA drilling (highest productivity at larger diameters).

Crossover Diameter Analysis

The crossover diameter — where one method becomes cheaper than another — depends on material, depth, and volume.

Gun Drilling vs Solid Carbide Extended Drill

Diameter	20:1 Depth	50:1 Depth	100:1 Depth
3 mm	Solid carbide	Solid carbide	Gun drill
6 mm	Solid carbide	Gun drill	Gun drill
10 mm	Solid carbide	Gun drill	Gun drill
15 mm	Gun drill	Gun drill	Gun drill

Rule: Below 10 mm and moderate depth (< 30:1), solid carbide extended drills are cheaper.

Gun Drilling vs BTA

Diameter	50:1 Depth	100:1 Depth	200:1 Depth
15 mm	Gun drill	Gun drill	Gun drill
20 mm	Compare	Gun drill	Gun drill
30 mm	BTA	BTA	Compare
50 mm	BTA	BTA	BTA

Rule: Above 25–30 mm diameter, BTA is cheaper per hole at any depth.

Volume-Based Selection

Low Volume (< 5,000 holes/year)

Diameter	Recommended Method	Rationale
< 10 mm	Solid carbide drill or gun drilling	No BTA available at this diameter
10–25 mm	Gun drilling	Lower machine investment
25–200 mm	Ejector (retrofit) or gun drilling	Ejector: low retrofit cost vs BTA machine

Medium Volume (5,000–50,000 holes/year)

Diameter	Recommended Method	Rationale
< 10 mm	Gun drilling (multi-spindle)	Higher throughput without second ops
10–30 mm	Gun drilling or BTA	Compare costs at exact specs
30–200 mm	BTA	Lower per-hole cost at this volume

High Volume (> 50,000 holes/year)

Diameter	Recommended Method	Rationale
< 10 mm	Multi-spindle gun drilling	Highest throughput
10–30 mm	BTA (multi-spindle)	Lowest per-hole cost at volume
30–200 mm	BTA (multi-spindle)	BTA clearly dominant

Total Cost of Ownership Comparison

30 mm Diameter, 50,000 holes/year, 5-year analysis

Cost Element	Gun Drilling	BTA	Ejector (Retrofit)
Machine investment	$250,000	$550,000	$30,000 (retrofit)
5-year depreciation	$125,000	$275,000	$15,000
Tooling (5 years)	$425,000	$175,000	$225,000
Labor (5 years)	$875,000	$350,000	$437,500
Coolant/maintenance (5 years)	$125,000	$175,000	$100,000
Total 5-year cost	$1,550,000	$975,000	$777,500
Cost per hole	$6.20	$3.90	$3.11

Conclusion: Ejector drilling on a retrofitted CNC lathe has the lowest 5-year total cost of ownership at this diameter and volume.

Cost Reduction Recommendations

Strategy	Savings Potential	Best Method for Strategy
Increase cutting speed 15%	10–15% cycle time reduction	Any — verify stability
Extend regrind intervals	20–40% tool cost reduction	Gun drilling
Switch from brazed to indexable	15–25% per-edge cost reduction	BTA, ejector, some gun drilling
Add multi-spindle capability	40–60% labor cost reduction per hole	All at high volume
Automate part loading/unloading	30–50% labor reduction	All at high volume
Use contract deep hole drilling	100% capital avoidance	Low volume (< 1,000/year)

Summary

The most economical deep hole drilling method depends on diameter, depth, material, and volume. At small diameters (< 10 mm), solid carbide extended drills or gun drilling are the only options. In the 10–30 mm overlap zone, ejector drilling (CNC lathe retrofit) often provides the lowest total cost at moderate volumes, while BTA dominates above 30 mm diameter. Multi-spindle configurations reduce per-hole cost by 40–60% for high-volume production. For a detailed gun drilling cost model, see gun drilling cost per hole analysis. For BTA vs gun drilling comparison, see BTA vs gun drilling economics.