Appearance
A precision machining company bids on a five-year contract to produce 12,000 deep-drilled hydraulic cylinder components annually. The sales team estimates the job using a simple per-kilogram rate, arriving at a competitive price that wins the contract. Six months into production, the job is losing money — the per-kilogram estimate did not account for tool wear at 3 metres depth, coolant filtration costs, or the 15% scrap rate on deep-hole features. The company rebuilds its quoting process around a per-hole cost model that factors in machine hourly rate, tool cost per edge, coolant and filtration overhead, cycle time, setup amortisation, quality yield, and post-drilling operations. The new model reveals the job needs a 22% higher price to achieve target margin. The company renegotiates with the customer, adjusts the process to reduce cycle time by optimising feed rates, and implements a tool regrind programme that cuts tool cost per hole by 35%.
Cost Per Hole Model Structure
| Cost Component | Symbol | Formula | Typical % of Total |
|---|---|---|---|
| Machine cost (labour + overhead) | Cm | Rm × tc / 60 | 40–55% |
| Tool cost per hole | Ct | Pt / (ne × nr) | 10–20% |
| Coolant and filtration cost | Cc | (Fc × Rc) / Nh | 3–8% |
| Setup and fixture amortisation | Cs | St / Nb | 2–5% |
| Quality and scrap cost | Cq | (Cm + Ct) × Rs / (1 − Rs) | 5–15% |
| Post-drilling operations | Cp | Σ(Pi × ti / 60) | 10–20% |
| Total cost per hole | Ch | Σ(all components) | 100% |
Machine Hourly Rate Calculation
| Cost Element | Description | Calculation Method | Example Value |
|---|---|---|---|
| Machine purchase price | Initial capital cost | Invoice price + installation | $250,000 |
| Depreciation period | Expected service life | 7–10 years (straight line) | 8 years |
| Annual depreciation | Yearly capital recovery | Purchase price / years | $31,250/yr |
| Floor space cost | Factory area occupied | m² × cost per m²/yr | $3,000/yr |
| Maintenance cost | Annual service and repairs | 3–5% of purchase price | $10,000/yr |
| Power consumption | Electrical energy | kW × hours × rate/kWh | $4,500/yr |
| Coolant system cost | Filtration + disposal | Annual coolant budget | $6,000/yr |
| Operating labour | Operator wage + burden | Hourly wage × 1.4 burden | $42/hr |
| Machine hourly rate | Rm | (Σ fixed costs) / hours + labour | $78/hr |
TIP
Machine hourly rate (Rm) is the single most important input to the cost per hole model. Use the VDMA 34100 standard framework which separates fixed costs (depreciation, interest, floor space) from variable costs (labour, power, consumables). The fixed cost portion is calculated as Rfixed = (Pm / Yd + Cfloor + Cmaint) / Ha, where Pm is machine purchase price, Yd is depreciation years, and Ha is annual operating hours. For deep hole drilling machines, actual utilisation is typically 70–85% due to setup time, tool changes, and maintenance — factor this into Ha to avoid under-estimating the effective hourly rate.
Cycle Time Calculation
| Operation Element | Symbol | Formula | Example (BTA, Ø20 × 500 mm) |
|---|---|---|---|
| Spindle speed | n | (vc × 1000) / (π × D) | 80 m/min → 1,273 rpm |
| Feed rate | vf | f × n | 0.08 mm/rev × 1,273 = 102 mm/min |
| Drilling time (through hole) | td | L / vf | 500 mm / 102 = 4.90 min |
| Drilling time (blind hole + approach) | td | (L + La) / vf | (500 + 5) / 102 = 4.95 min |
| Tool change time | ttc | Per tool change event | 3 min |
| Tool change frequency | ntc | td / T | 4.90 / 120 = 0.041 changes/hole |
| Cycle time per hole | tc | td + ttc × ntc | 4.90 + 3 × 0.041 = 5.02 min |
Where:
- vc = cutting speed (m/min), D = drill diameter (mm), f = feed (mm/rev), L = drilling depth (mm), La = approach length (mm), T = tool life (min)
Tool Cost Per Hole
| Tool Type | Purchase Price Pt | Regrinds nr | Edges ne | Cost Per Edge | Typical Tool Life T (min) | Tool Cost Per Hour |
|---|---|---|---|---|---|---|
| Gun drill (solid carbide, Ø10) | $85 | 8–12 | 1 | $7.08–8.50 | 45–90 | $4.72–11.33 |
| Gun drill (solid carbide, Ø20) | $150 | 8–12 | 1 | $11.54–15.00 | 60–120 | $5.77–15.00 |
| BTA head (brazed carbide, Ø20) | $95 | 0–2 (tip replace) | 3–5 (indexable) | $19.00–31.67 | 30–60 | $19.00–63.33 |
| BTA head (indexable insert, Ø30) | $45 (body) + $18 (insert) | N/A | 3–5 per insert side | $3.60–6.00 | 20–45 | $4.80–18.00 |
| STS drill head (Ø50) | $180 | 0–1 | 2–4 | $45.00–90.00 | 40–80 | $33.75–135.00 |
DANGER
Tool life T is not a fixed number — it varies significantly with cutting speed, feed rate, coolant pressure, material hardness, and hole depth-to-diameter ratio. Using manufacturer catalogue tool life values without adjustment for actual conditions leads to cost estimates that are 30–50% too low. Always run a tool life validation test under production conditions before finalising a quote. The Taylor tool life equation vc × Tn = C (where n ≈ 0.15–0.25 for carbide in deep hole drilling) must be calibrated with real test data — do not rely on handbook exponents for cost estimation.
Tool cost per hole: Ct = Pt / (ne × nr) where nr = 1 + regrinds (if regrindable). For indexable tools, Ct = Pbody/Ntotal + Pinsert/ne where Ntotal is total holes before body replacement.
Optimum Cutting Speed for Minimum Cost
| Parameter | Symbol | Formula | Example Value |
|---|---|---|---|
| Taylor tool life exponent | n | From tool life tests | 0.20 (carbide) |
| Machine + operator rate | Rm | From hourly rate calc | $78/hr = $1.30/min |
| Tool change time | ttc | Per change | 3 min |
| Tool cost per edge | Ce | Pt / ne | $11.54 |
| Optimum tool life | Topt | (1/n − 1) × (Ce / Rm + ttc) | (5 − 1) × (11.54/1.30 + 3) = 47.5 min |
| Taylor constant | C | vc × Tn (calibrated) | 80 × 1200.20 = 167 |
| Optimum cutting speed | vopt | C / Toptn | 167 / 47.50.20 = 87 m/min |
| Cost per hole at vopt | Ch,opt | Rm × tc + Ct + other costs | $8.42 |
TIP
The optimum cutting speed for minimum cost is always higher than the speed for maximum production rate. The economic tool life Topt = (1/n − 1) × (Ce/Rm + ttc) balances the cost of tool changes against the cost of running at a slower speed. When tool cost per edge Ce is low relative to machine rate Rm (e.g. indexable inserts vs solid gun drills), Topt decreases — meaning it is economical to run faster and change tools more frequently. For expensive solid carbide gun drills, Topt increases — run slower to maximise tool life per edge.
BTA vs Gun Drilling Cost Comparison
| Factor | BTA Drilling | Gun Drilling | Cost Impact |
|---|---|---|---|
| Typical diameter range | Ø12–500 mm | Ø1–40 mm | BTA covers larger holes, gun drilling for small |
| Feed rate (relative) | High (0.05–0.25 mm/rev) | Low (0.01–0.08 mm/rev) | BTA is 3–5× faster in material removal |
| Surface finish | Ra 1.6–6.3 µm | Ra 0.4–1.6 µm | Gun drilling may eliminate reaming operation |
| Tool cost per edge | $3.60–31.67 (indexable) | $7.08–15.00 (solid) | BTA indexable inserts are cheaper per edge |
| Coolant pressure | 10–80 bar | 60–180 bar | Gun drilling requires higher pump investment |
| Machine cost (new) | $150,000–500,000 | $80,000–250,000 | Gun drilling machines are typically lower cost |
| Cycle time (Ø20 × 500 mm) | ~5 min | ~12 min | BTA is ~60% faster for same hole |
| Cost per hole (same spec) | $8.50–12.00 | $14.00–20.00 | BTA is 30–40% lower cost per hole |
Coolant and Filtration Cost Allocation
| Cost Element | Annual Cost | Allocation Basis | Cost Per Hole (12,000 holes/yr) |
|---|---|---|---|
| Coolant oil purchase (neat oil) | $4,800 | Volume consumed | $0.40 |
| Filter media (paper band) | $3,200 | Operating hours | $0.27 |
| Filter media (cartridge) | $2,400 | Operating hours | $0.20 |
| Coolant disposal/waste treatment | $1,600 | Volume disposed | $0.13 |
| Hydrocyclone maintenance | $1,200 | Operating hours | $0.10 |
| Magnetic separator upkeep | $800 | Operating hours | $0.07 |
| Coolant analysis (lab fees) | $600 | Batch tests | $0.05 |
| Total coolant and filtration cost | $14,600 | $1.22/hole |
Setup and Fixture Cost Amortisation
| Item | Cost | Expected Life (holes) | Cost Per Hole |
|---|---|---|---|
| Dedicated fixture (cast iron) | $3,500 | 50,000 | $0.07 |
| Bushing plate (hardened steel) | $1,200 | 20,000 | $0.06 |
| Guide bushing set | $400 | 5,000 | $0.08 |
| Workholding jaws (custom) | $2,800 | 30,000 | $0.09 |
| Initial setup labour (first article) | $600 | 12,000 (batch) | $0.05 |
| Total fixture amortisation | $0.35/hole |
Quality Yield and Scrap Cost
| Scrap Scenario | Scrap Rate Rs | Impact on Cost Per Hole | Mitigation |
|---|---|---|---|
| Low (optimised process) | 1–2% | +1.0–2.0% | SPC monitoring, pre-production validation |
| Typical (stable production) | 3–5% | +3.1–5.3% | Regular tool change intervals, coolant monitoring |
| High (difficult material) | 8–12% | +8.7–13.6% | Process optimisation, tool geometry modification |
| Excessive (new material/process) | 15–20% | +17.6–25.0% | Full process review, alternative tooling strategy |
Scrap cost is calculated as: Cq = (Cm + Ct + Cc + Cs) × Rs / (1 − Rs)
WARNING
Scrap cost is often underestimated in drilling quotes because it only accounts for material loss and additional machining time. The true cost of a scrapped deep-hole component includes: (1) the full machining cost of all previous operations (turning, milling, heat treat); (2) inspection time for the scrap part; (3) disruption to production flow; (4) potential late-delivery penalties. For deep hole features that are the final operation before inspection, a 5% scrap rate may add 8–10% to the effective cost per good hole — include this in your margin calculation.
Cost Optimisation Levers
| Optimisation | Impact on Cost Per Hole | Implementation Effort | Risk Level |
|---|---|---|---|
| Increase feed rate (chip thinning) | −8 to −15% | Low (parameter change) | Low — check surface finish |
| Optimise cutting speed (economic Topt) | −5 to −12% | Low (parameter change) | Low — tool life changes predictably |
| Tool regrind programme | −15 to −35% on tool cost | Medium (vendor qualification) | Low — quality regrind extends life |
| Indexable vs solid tool conversion | −10 to −25% on tool cost | Medium (tool design change) | Medium — chip control may change |
| Coolant pressure optimisation | −3 to −8% (faster chip evacuation) | Medium (pump/plumbing) | Low — too low pressure causes chip jamming |
| Multi-spindle or multi-head | −30 to −50% (cycle time) | High (machine modification) | Medium — setup complexity increases |
| Automation (robot load/unload) | −15 to −25% (labour reduction) | High (capital investment) | Medium — reliability dependent |
| Pre-drilling (core hole) | −20 to −40% (material removal) | Medium (process change) | Medium — chip control changes |
Economic Analysis Example — Hydraulic Cylinder
| Parameter | Value | Unit |
|---|---|---|
| Material | 4140 steel, 280 HB | — |
| Hole diameter | Ø25 mm | — |
| Hole depth | 600 mm | — |
| Quantity | 12,000 holes/year | — |
| Machine hourly rate | $78/hr | — |
| Cutting speed (cost-optimised) | 85 m/min | — |
| Feed rate | 0.12 mm/rev | — |
| Calculated cycle time | 4.12 min/hole | — |
| Tool type | BTA indexable head | — |
| Tool cost per edge | $5.60 | — |
| Tool life | 55 min (13.3 holes) | — |
| Tool changes per hole | 0.075 changes | — |
| Tool change time | 3 min | — |
| Coolant and filtration cost | $1.22/hole | — |
| Fixture amortisation | $0.35/hole | — |
| Scrap rate | 4% | — |
| Cost per good hole | $9.87 | — |
| Target margin | 25% | — |
| Selling price per hole | $13.16 | — |
Bidding and Quoting Checklist
| Item | Details | Check |
|---|---|---|
| Machine hourly rate | Include depreciation, floor space, maintenance, power, labour, coolant system | ☐ |
| Tool cost per hole | Purchase price / (edges × regrinds) or indexable insert cost per edge | ☐ |
| Tool life validation | Run production-representative test, apply Taylor equation for speed adjustments | ☐ |
| Cycle time calculation | Include approach, retract, tool change frequency, inspection time | ☐ |
| Coolant and filtration | Annual coolant budget ÷ annual holes + filter media cost per hole | ☐ |
| Fixture and setup amortisation | Fixture cost ÷ expected total holes + setup hours ÷ batch size | ☐ |
| Scrap and rework cost | (machining cost + material) × scrap rate / yield | ☐ |
| Post-drilling operations | Deburring, honing, inspection, cleaning | ☐ |
| Overhead allocation | Factory overhead (15–25%), SG&A (5–10%), profit margin | ☐ |
| Volume adjustment | Lower unit cost at higher volumes (setup amortisation, tool purchase discounts) | ☐ |
FAQ
How is cost per hole calculated in deep hole drilling?
Cost per hole is calculated as the sum of all cost components: machine cost (machine hourly rate × cycle time), tool cost (tool price per edge divided by holes per edge), coolant and filtration cost, fixture amortisation, scrap cost, and post-drilling operations. The machine rate must account for depreciation, floor space, maintenance, power, labour, and coolant system overhead. The complete formula is Ch = Rm × tc / 60 + Pt / (ne × nr) + Cc + Cs + Cq + Cp.
What is a typical machine hourly rate for deep hole drilling?
A deep hole drilling machine hourly rate typically ranges from $65–120/hr depending on machine size, complexity, and geographic location. For a $250,000 BTA machine with 8-year depreciation, the fixed cost portion is approximately $36–45/hr, plus operator labour at $35–50/hr (including burden). High-pressure gun drilling machines ($80,000–250,000) tend toward the lower end of the range, while large STS or BTA machines ($300,000–500,000) with higher coolant system costs are at the upper end.
How does tool cost affect cost per hole?
Tool cost typically represents 10–20% of total cost per hole in deep hole drilling. The impact depends on tool type: solid carbide gun drills ($85–150 each with 8–12 regrinds) have a per-edge cost of $7–15; indexable BTA heads ($3.60–6.00 per cutting edge) are cheaper per edge but may have shorter tool life. The key optimisation is balancing cutting speed against tool life using the Taylor tool life equation — running 10% faster may reduce cycle time cost but increase tool cost by 30%, so the economic optimum must be calculated.
What is the Taylor tool life equation and how is it used for cost optimisation?
The Taylor tool life equation vc × Tn = C relates cutting speed vc to tool life T through exponent n (typically 0.15–0.25 for carbide in deep hole drilling) and constant C (calibrated from test data). For cost optimisation, the economic tool life Topt = (1/n − 1) × (Ce/Rm + ttc) gives the tool life that minimises total cost per hole. The corresponding optimum cutting speed is vopt = C / Toptn. Always calibrate n and C with production data — handbook values can be off by 40% for deep hole drilling conditions.
What are the biggest cost drivers in deep hole drilling?
The largest cost driver is cycle time-related machine cost (40–55% of total), which is driven by cutting speed, feed rate, and tool change frequency. The second largest is tool cost (10–20%), especially for solid carbide gun drills in small-diameter holes. Scrap cost is often the hidden driver — a 5% scrap rate can add 5–8% to effective cost per good hole. Coolant and filtration costs are typically only 3–8% but become significant at high volumes. Labour and setup amortisation decrease per hole as batch size increases.
How does BTA compare to gun drilling in terms of cost?
BTA drilling is typically 30–40% lower cost per hole than gun drilling for the same hole specification (Ø20 × 500 mm in steel). This is because BTA can use significantly higher feed rates (0.12–0.25 mm/rev vs 0.02–0.08 mm/rev for gun drilling) and indexable inserts are cheaper per cutting edge than solid carbide gun drills. However, BTA requires higher initial machine investment and is not suitable for holes under Ø12 mm or when extremely fine surface finish (Ra < 0.8 µm) is required.
What is the optimum cutting speed for minimum drilling cost?
The optimum cutting speed for minimum cost is determined by the economic tool life equation Topt = (1/n − 1) × (Ce/Rm + ttc). For a typical carbide deep hole drilling operation (n = 0.20, Rm = $78/hr, Ce = $11.54, ttc = 3 min), Topt ≈ 48 min. This is generally shorter than the maximum possible tool life, meaning the cost-optimum speed is higher than the speed for maximum tool life. Running at lower speeds to maximise tool life actually increases total cost because the longer cycle time outweighs the tool cost savings.
How should coolant and filtration costs be allocated in cost per hole?
Coolant and filtration costs should be calculated as the total annual coolant system operating cost divided by the annual number of holes produced. Include coolant oil purchases, filter media (paper band, cartridge, magnetic), coolant disposal fees, hydrocyclone and separator maintenance, and laboratory analysis costs. For a typical four-machine deep hole drilling cell producing 12,000 holes/year, total coolant and filtration cost is approximately $1.00–1.50 per hole. This is often overlooked in quoting but represents a real ongoing cost.
What scrap rate should be assumed when quoting deep hole drilling?
Assume 3–5% scrap rate for stable production in conventional materials (steel, cast iron) with established processes. For difficult materials (titanium, Inconel, hardened steel) or new processes, assume 8–12% until process capability is demonstrated. Include the scrap cost in the quote using the formula Cq = (machining cost + material) × Rs / (1 − Rs). Never quote at 0% scrap — it is unrealistic and erodes margin when the inevitable scrap occurs.
What are the most effective ways to reduce cost per hole in deep hole drilling?
The most effective cost reduction levers, in order of typical impact, are: (1) increase feed rate while maintaining surface finish (8–15% reduction); (2) implement a tool regrind programme for solid carbide tools (15–35% reduction in tool cost); (3) optimise cutting speed to the economic Topt (5–12% reduction); (4) convert to indexable tooling where applicable (10–25% tool cost reduction); (5) add pre-drilling or core hole operations to reduce material removal volume (20–40% reduction in drilling time). Each lever must be validated with production tests before committing to a quote.
Summary
Accurate cost estimation for deep hole drilling requires moving beyond simple per-kilogram or per-hour pricing to a structured per-hole cost model that accounts for all cost components: machine hourly rate, tool cost per edge, coolant and filtration overhead, fixture amortisation, scrap and rework, and post-drilling operations. The machine hourly rate — calculated using the VDMA 34100 framework separating fixed and variable costs — is the most significant input, representing 40–55% of total cost per hole. Tool cost optimisation using the Taylor tool life equation enables the selection of cutting parameters that minimise total cost rather than maximising tool life, typically reducing cost per hole by 5–12% compared to handbook parameter selection. BTA drilling offers 30–40% lower cost per hole than gun drilling for the same specification when diameter permits, driven by higher feed rates and lower-cost indexable tooling. Quality yield must be explicitly included in the model — a 5% scrap rate adds 5–8% to effective cost. The most impactful cost reduction measures — feed rate optimisation, tool regrind programmes, and pre-drilling operations — can collectively reduce cost per hole by 30–50% compared to unoptimised production. Implementing a structured cost model, calibrating it with production data, and reviewing it periodically ensures that deep hole drilling quotes are both competitive and profitable.