Deep Hole Drilling Cost Calculation: Complete Guide to Per-Hole Estimation and Optimization

The total cost of a deep hole drilled component is not the sum of obvious expenses — it is the result of interactions between cutting parameters, tool wear characteristics, machine rate, and batch size. A feed rate that is 20% too low may double the tool cost per hole, while a speed that is 10% too high may halve the tool life, increasing machine time per hole by more than the speed gain saved.

In deep hole drilling, the cost structure differs from conventional machining because the cutting edge cannot be observed mid-cycle, tool wear directly affects hole geometry in ways that can scrap the part, and the regrinding economics of brazed carbide tools follow different rules than indexable inserts.

This guide provides the complete cost framework for deep hole drilling operations, with formulas, worked examples, and optimization guidance based on published research and industry practice.

Core Cost Per Hole Formula

The total cost to produce one deep hole is:

C_total = C_machine + C_tool + C_setup + C_coolant

Where:

Machine Hour Rate

The machine hour rate is the cost of operating the deep hole drilling machine for one hour. It must cover all fixed and variable costs.

H_rate = C_labor + C_overhead + C_depreciation

Where:

• C_labor = operator wage + burden (typically $25–45/hr depending on region)
• C_overhead = facility cost, management, quality, maintenance allocated per machine hour
• C_depreciation = machine purchase price ÷ service life in hours

Typical Rates by Machine Type

These rates assume 70–80% machine utilization. Below 60% utilization, the effective rate increases significantly because fixed costs must be spread over fewer productive hours.

Machine Capital Cost Reference

Used machine prices range from €20,000 for basic models to €250,000 for advanced high-capacity units.

Machine Hour Rate Calculation Example

Machine: Mid-size BTA drilling machine, purchase price $600,000 Service life: 20 years, 4,000 hours/year → 80,000 total hours Depreciation: $600,000 ÷ 80,000 = $7.50/hr

Cycle Time Calculation

The cycle time for a deep hole drilling operation is:

T_cycle = T_entry + T_drill + T_retract + T_index

Where:

• T_entry = approach and pilot hole engagement time (seconds)
• T_drill = actual drilling time = Hole length ÷ Penetration rate
• T_retract = tool withdrawal time (typically 2–5 seconds)
• T_index = tool change and positioning time (if multi-hole)

Penetration Rate

The penetration rate (feed rate in distance per unit time) is:

P_rate = n × f

Where:

• n = spindle speed (RPM) = (Vc × 1,000) ÷ (π × D) — metric
• n = (Vc × 3.82) ÷ D — imperial (Vc in SFM, D in inches)
• f = feed per revolution (mm/rev or IPR)
• P_rate = penetration rate (mm/min or IPM)

Drilling Time

T_drill = (Hole depth + Pilot depth + Over-travel) ÷ P_rate

For blind holes: T_drill = Hole depth ÷ P_rate For through holes: T_drill = (Hole depth + 0.3 × D) ÷ P_rate (the 0.3×D accounts for breakthrough)

Material Removal Rate

MRR = (π × D² ÷ 4) × P_rate

In metric: MRR (cm³/min) = (D × f × Vc) ÷ 4 (D in cm) In imperial: MRR (in³/min) = (D × f × Vc) ÷ 4 (D in inches, Vc in SFM, f in IPR)

MRR is the direct measure of productivity. For a given hole diameter, increasing penetration rate proportionally increases MRR. BTA drilling achieves 5–7× higher MRR than gun drilling at the same diameter due to its higher feed rate capability.

Cycle Time Example — Gun Drilling

Hole: 12 mm diameter, 200 mm deep, 1045 steel From manufacturer data: Vc = 70 m/min, f = 0.035 mm/rev Pilot depth: 15 mm, breakthrough over-travel: 4 mm

Cycle Time Example — BTA Drilling (Same Hole)

Hole: 12 mm diameter, 200 mm deep, 1045 steel BTA parameters: Vc = 80 m/min, f = 0.15 mm/rev

The BTA cycle is 4.5× faster than gun drilling for this hole. This ratio is consistent with the published 5–7× feed rate advantage of BTA over gun drilling.

Tool Cost Per Edge

Brazed Carbide Tools (Reground)

For gun drills and brazed BTA heads that are reground through their service life:

C_edge = (C_tool + N_regrinds × C_regrind) ÷ (1 + N_regrinds)

Research on deep hole drilling economics provides a more detailed model that accounts for the fact that regrind frequency and quantity depend on cutting parameters. The number of possible regrinds is not fixed but is determined by:

N_regrinds = Int[(L_pmax − L_pmin) ÷ L_r]

Where:

• L_pmax = new pad length
• L_pmin = minimum pad length (below which tool cannot be reground)
• L_r = pad material removed per regrind = K_r × V^y × f^z

For a 22 mm BTA drill in cast iron, typical constants are:

• K_r = 0.0253, y = −0.6589, z = 0.2923
• L_pmax = 17 mm, L_pmin = 6 mm

Regrind time is also variable: t_r = t_rc + t_r1 × L_r Where t_rc = 15 min (fixed setup), t_r1 = 6.5 min/mm of material removed

Indexable Insert Tools

For BTA drill heads with replaceable inserts:

C_edge = (C_insert ÷ N_edges) × F_failure + (C_body ÷ N_body_life)

Tool Cost per Hole

C_tool_per_hole = C_edge ÷ Holes per edge

Gun drilling example: If a reground edge lasts 150 holes in 1045 steel: C_tool_per_hole = $35.00 ÷ 150 = $0.233/hole

BTA example: If an indexable edge lasts 300 holes in the same material: C_tool_per_hole = $5.52 ÷ 300 = $0.018/hole

At first glance, the BTA tool cost per hole is 13× lower than gun drilling. However, the BTA head body cost ($400) must be spread across a large number of edges to achieve this — which requires high production volume.

Total Cost Per Hole — Complete Example

Compare gun drilling vs. BTA drilling for a 12 mm × 200 mm hole in 1045 steel, batch of 10,000 parts, single hole per part.

Assumptions:

• Machine hour rate: $90/hr (both machines)
• Setup time: 1 hour per batch
• Annual coolant cost: $8,000 (gun drill), $12,000 (BTA)
• Annual holes: 30,000 (to account for capacity differences)

Gun Drilling

BTA Drilling

At 10,000 holes per year, the savings from BTA is $4.18 per hole × 10,000 = $41,800 annually — which must be weighed against the higher machine investment.

Annual Cost Comparison by Production Volume

The BTA machine premium (25–35% higher than gun drilling machine, or approximately $75,000–$200,000 additional investment) is recovered within 2–5 years at volumes above 5,000 holes per year.

Batch Cost Estimation

For small batches or prototype work, the setup cost dominates:

C_batch = H_rate × (T_setup + Batch_size × T_cycle) + Batch_size × C_tool_per_hole + C_coolant_per_hole × Batch_size

Example: Small Batch Gun Drilling

Batch of 50 parts, 12 mm × 200 mm hole:

The cost per hole at batch size 50 ($7.58) is 31% higher than at batch size 10,000 ($5.79) due to setup amortization.

Economic Tool Life Optimization

For deep hole drilling tools, the minimum cost condition and the maximum production rate condition do not coincide. Published research on deep hole drilling economics (Griffiths & Grieve, 1985 International Conference on Production Research) provides a method for determining the optimum.

Taylor Tool Life Equation

T = (K ÷ (V^m × F^n))

Where:

• T = tool life (min)
• V = cutting speed
• F = feed rate
• K, m, n = material-specific constants

For a 22 mm BTA drill in 0.2% Cr white cast iron:

• K = 0.0162, m = −1.875, n = −1.2105

Optimum Conditions

The two objectives cannot be achieved simultaneously. The operator must choose, based on whether capacity or cost is the constraint.

Regrind vs. Discard Decision

When the fractional part of the calculated number of possible regrinds (n_L) is less than 0.3, it is cheaper to discard the drill. When greater than 0.3, it is cheaper to regrind and use the remaining life.

L/D Ratio Cost Scaling

Cost scales non-linearly with the depth-to-diameter ratio. A hole with L/D = 10 is the baseline. As L/D increases:

The cost factor is not linear because the penetration rate must be reduced as L/D increases (see L/D correction factors in the Speeds and Feeds Reference), and the risk of tool failure increases, raising the expected scrap cost.

Material Cost Factors

Reducing Cost Per Hole

Increase Penetration Rate

Penetration rate is the dominant cost lever because machine time is the largest cost component. Increasing feed rate by 20% reduces machine time by 17%, reducing total cost per hole by approximately 12–15%.

Check: Can the tool edge survive the higher feed? If tool life drops in proportion to the feed increase, the tool cost per hole remains the same but the scrap risk from tool failure increases.

Extend Tool Life

Extending tool life by 50% reduces tool cost per hole by 33%, but has a smaller impact on total cost because tool cost is only 8–15% of the total.

Most effective method: Verify that the tool is being reground at the optimal interval (when flank wear reaches 0.15–0.20 mm, not after catastrophic failure). Running a tool past the optimal regrind point reduces tool life more than it saves on regrind costs.

Optimize Regrind Frequency

For reground tools, the cost per edge is minimized when the maximum number of regrinds is achieved without exceeding the minimum pad length. Keep regrind records to determine the actual number of regrinds possible for each tool diameter and material combination.

Match Method to Volume

For detailed guidance on selecting cutting parameters that affect cost, see the Speeds and Feeds Reference for Deep Hole Drilling. For tool regrinding economics, see the Tool Regrinding Guide. For machine selection cost factors, see the Total Cost of Ownership Guide.