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A job shop specialising in gun drilling (3 machines, 2000 drilling hours/year each) was quoting based on a simple $95/hour machine rate + tooling at 1.5× purchase price. This consistently underquoted stainless steel and titanium jobs (3–5× more tool changes) and overquoted aluminium jobs (10× less tooling). Switching to the Waller-Rowsell per-metre drilling cost model: C_per_m = R·T_m/L + (B + R·H·k_A)/(P·E·L) — where R = machine rate, T_m = machining time, L = tool life in metres, B = tool purchase cost, H = resharpenings, k_A = resharpening cost factor, P = production quantity, E = efficiency — enabled material-specific costing. True cost per metre for Ø12 mm × 300 mm bores: $4.20/m (1018 steel), $8.50/m (316L), $14.80/m (Ti-6Al-4V), $2.10/m (6061-T6). The shop increased margins on stainless/titanium from 8% to 22% while remaining competitive on steel and aluminium.
Deep Hole Drilling Cost Models
Comparison of Cost Models for Deep Hole Drilling
| Cost Model | Formula | Input Parameters Required | Output | Best Suited For | Accuracy | Complexity | Data Requirements |
|---|---|---|---|---|---|---|---|
| Simple machine hour rate | C = R · t_m + B / (P · L) | R = machine rate ($/h), t_m = machining time per part (h), B = tool purchase cost ($), P = bores per tool life, L = tool life (bores) | Cost per part ($) | Quick estimates; standard materials (steel, aluminium); low-cost tooling | ±30–50% (large error margin for different materials and tool types) | Very low | Machine rate, tool purchase cost, estimated tool life from experience |
| Ben-Arieh per-hole cost (Computers in Industry, 1994) | C_tot = C_setup + C_fixture + C_machining + C_tool_change + C_tool_regrind | C_setup = S / B_s (setup cost per part, S = setup cost, B_s = batch size); C_fixture = F / (B_f · Y_f) (fixture amortisation); C_machining = R_m · t_m; C_tool_change = R_m · t_c · (t_m / T); C_tool_regrind = (B_p / (n_r + 1) + n_r · R_g · t_g / (n_r + 1)) / (T / t_m) where n_r = number of resharpenings per tool, R_g = resharpening labour rate | Total cost per hole ($), broken down into setup, fixture, machining, tool change, and tool regrind components | Production cost analysis; comparison of tool life and regrind strategies; identification of dominant cost drivers | ±10–20% (good accuracy when tool life is known from production data) | Moderate | Setup cost, fixture cost, batch size, tool purchase cost, tool regrind cost, tool life (from published data or experience) |
| Waller-Rowsell per-metre cost (International Journal of Machine Tools & Manufacture) | C_per_m = R · T_m / L + (B + R · H · k_A) / (P · E · L) | R = machine + operator rate ($/h), T_m = machining time per metre (h/m), L = tool life per regrind interval (m), B = tool purchase cost ($), H = number of resharpenings, k_A = resharpening cost factor ($/sharpen), P = number of tools in circulation, E = efficiency factor | Cost per metre of drilled bore ($/m) | Cost comparison across materials; optimisation of feed and speed for minimum cost per metre; job shop quoting for diverse materials | ±10–20% (similar to Ben-Arieh but expressed per metre for easier comparison across different bore lengths) | Moderate | Machine rate, tool life (per metre), tool purchase cost, resharpening cost, efficiency factor |
| Kronenberg tool life model (Taylor extended for drilling) | T = (K · D · N) / (V^a · f^b · r^c) where T = tool life in minutes cutting time, K = constant dependent on material + tool combination, D = drill diameter, N = spindle speed, V = cutting speed, f = feed, r = edge radius, a, b, c = exponents | K, a, b, c from orthogonal cutting tests or published data for the specific material-tool combination; D, V, f, r from cutting parameters | Tool life (minutes or metres) for a given set of parameters | Optimisation of cutting parameters for maximum tool life at a given material removal rate; identification of the feed-speed combination that minimises cost per metre | ±20–40% (tool life prediction has inherent variability from material and tool geometry variation) | High (requires calibration or literature data for exponents) | Material-specific exponent data; tool geometry data; cutting parameter data |
Cost Model Application Example: Per-Metre Cost Comparison by Material
| Material | Cutting Speed Vc (m/min) | Feed f (mm/rev) | Tool Life per Regrind L (m) | Tool Purchase Cost B ($) | Resharpening Cost R_sh ($) | Number of Resharpenings H | Machining Time per Metre T_m (h/m) | Machine Rate R ($/h) | Cost per Metre C_per_m ($/m) | Cost per Bore (300 mm) ($) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1018 steel | 120 | 0.08 | 80 | 180 (carbide, uncoated) | 18 | 10 | 0.022 (1.3 min/m) | 85 | 4.20 | 1.26 |
| 4140 steel (32 HRC) | 90 | 0.06 | 60 | 180 (carbide, uncoated) | 18 | 10 | 0.025 (1.5 min/m) | 85 | 5.80 | 1.74 |
| 4140 steel (45 HRC) | 55 | 0.04 | 25 | 220 (carbide, TiAlN) | 22 | 8 | 0.036 (2.2 min/m) | 85 | 11.20 | 3.36 |
| 316L stainless | 60 | 0.03 | 30 | 250 (carbide, TiAlN) | 22 | 8 | 0.037 (2.2 min/m) | 85 | 8.50 | 2.55 |
| 17-4 PH H900 | 50 | 0.04 | 20 | 250 (carbide, TiAlN) | 22 | 8 | 0.035 (2.1 min/m) | 85 | 11.80 | 3.54 |
| Ti-6Al-4V (annealed) | 35 | 0.03 | 12 | 600 (PCD gun drill) | 35 | 8 | 0.048 (2.9 min/m) | 95 | 14.80 | 4.44 |
| Ti-6Al-4V (STA 40 HRC) | 22 | 0.02 | 6 | 600 (PCD) | 35 | 6 | 0.068 (4.1 min/m) | 95 | 26.50 | 7.95 |
| Inconel 718 (annealed) | 18 | 0.02 | 5 | 600 (PCD) | 35 | 6 | 0.083 (5.0 min/m) | 100 | 32.00 | 9.60 |
| 6061-T6 aluminium | 240 | 0.12 | 200 | 120 (carbide, uncoated) | 15 | 12 | 0.007 (0.4 min/m) | 85 | 2.10 | 0.63 |
| C110 copper | 100 | 0.05 | 30 | 400 (PCD) | 30 | 8 | 0.014 (0.85 min/m) | 85 | 8.60 | 2.58 |
Machine Hour Rate Calculation
Machine Hour Rate Breakdown for Deep Hole Drilling
| Cost Category | Cost Component | Calculation Method | Cost per Hour ($/h) — Gun Drilling Machine (single-spindle, 15 kW, with high-pressure coolant) | Cost per Hour ($/h) — BTA Drilling Machine (single-spindle, 50 kW, with central coolant system) | Cost per Hour ($/h) — Micro Gun Drilling Machine (single-spindle, 5 kW, 30 000 rpm) | Percentage of Total (typical range) |
|---|---|---|---|---|---|---|
| Ownership — depreciation | Machine purchase price divided by expected life in hours (7–10 years × 2000–4000 h/year) | Price = $300 000; life = 8 yr × 3000 h/yr = 24 000 h; dep = $300 000 / 24 000 = $12.50/h | Price = $600 000; life = 8 yr × 3000 h/yr = 24 000 h; dep = $600 000 / 24 000 = $25.00/h | Price = $200 000; life = 8 yr × 2000 h/yr = 16 000 h; dep = $200 000 / 16 000 = $12.50/h | 15–30% | |
| Ownership — interest / cost of capital | (Purchase price × interest rate) / operating hours per year; interest rate = 5–8% | Interest = $300 000 × 0.07 / 3000 h = $7.00/h | Interest = $600 000 × 0.07 / 3000 h = $14.00/h | Interest = $200 000 × 0.07 / 2000 h = $7.00/h | 8–15% | |
| Ownership — insurance and taxes | 1–3% of purchase price per year; insurance + property tax | Insurance = $300 000 × 0.015 / 3000 h = $1.50/h | Insurance = $600 000 × 0.015 / 3000 h = $3.00/h | Insurance = $200 000 × 0.015 / 2000 h = $1.50/h | 2–5% | |
| Operating — power (electricity) | Motor power × load factor × cost per kWh; load factor = 0.5–0.8 (average load during cutting) | Power = 15 kW × 0.6 × $0.12/kWh = $1.08/h | Power = 50 kW × 0.6 × $0.12/kWh = $3.60/h | Power = 5 kW × 0.5 × $0.12/kWh = $0.30/h | 1–5% | |
| Operating — coolant (consumables) | Coolant purchase + disposal cost per hour; depends on coolant type (oil, water-miscible, cryogenic) | Oil coolant: $3.00/h (purchase $1.50 + disposal $0.80 + filtration $0.70) | Oil or water-miscible: $5.00/h (higher flow rate, larger system) | MQL + cryogenic: $8.00/h (cryogen $6 + MQL oil $1 + air $1) | 3–12% | |
| Operating — coolant system maintenance | Filter element replacement, coolant analysis, tank cleaning, biocide dosing | $2.00/h (filter $0.50, analysis $0.50, cleaning $1.00) | $3.50/h (larger system, more filters, biocide dosing) | $1.00/h (smaller system, minimal coolant management) | 2–5% | |
| Operating — machine maintenance | Spindle bearing replacement, guideway lubrication, hydraulic system maintenance, PM labour | $3.00/h (lubricants $0.50, PM parts $1.00, PM labour $1.50) | $5.00/h (more complex spindle, larger hydraulics, bearing replacement) | $2.00/h (high-speed spindle bearing replacement more frequent) | 3–8% | |
| Operating — operator labour | Operator wage + burden (health insurance, retirement, training) | $25.00/h (wage $18 + burden $7) | $25.00/h (same operator rate) | $25.00/h (same operator rate) | 20–40% | |
| Operating — floor space (rent / allocated overhead) | Machine footprint × floor space cost per m² per year; $200–500/m²/year | 20 m² × $300/m²/year / 3000 h = $2.00/h | 40 m² × $300/m²/year / 3000 h = $4.00/h | 15 m² × $300/m²/year / 2000 h = $2.25/h | 2–5% | |
| TOTAL MACHINE HOUR RATE | Sum of all components | $57.08/h | $88.10/h | $59.55/h | 100% |
FAQ
What is the most accurate cost model for deep hole drilling quoting, and how should it be applied across different materials?
The most accurate cost model for deep hole drilling quoting is the Waller-Rowsell per-metre drilling cost model, because it expresses cost in units that are directly comparable across different bore diameters, depths, and materials — cost per metre of drilled bore. The model accounts for all significant cost components: machine rate (including ownership and operating costs), tooling cost (purchase price amortised over the tool life), resharpening cost (the cost of each regrind plus the number of resharpenings per tool), and efficiency (the fraction of machine time spent cutting versus non-cutting activities such as part loading, tool change, and inspection). The formula is:
C_per_m = R · T_m / L + (B + R_sh · H) / (P · L)
where:
- C_per_m = cost per metre of drilled bore ($/m)
- R = machine + operator rate ($/h)
- T_m = machining time per metre (h/m) = 1 / (f · Vc · 1000 / (π · D)) where f = feed per rev (m/rev), Vc = cutting speed (m/min), D = bore diameter (m)
- L = tool life per regrind interval (m) — the cumulative cutting length between one regrind and the next
- B = tool purchase cost ($)
- R_sh = resharpening cost per regrind ($)
- H = number of resharpenings per tool before end of life
- P = effective number of lives per tool (1 new + H × life factor after each regrind, typically 1 + 0.85·H)
- E = efficiency factor (typically 0.70–0.85 for job shops, 0.85–0.95 for production shops)
For job shop quoting across multiple materials, the Waller-Rowsell model should be applied with material-specific tool life input. Tool life L for a given material can be estimated from the Kronenberg tool life equation: L = (K · D · N) / (V^a · f^b), where K is a constant specific to the material-tool combination and a and b are exponents. For practical quoting without Kronenberg exponents, tool life can be estimated from published machinability ratings: L = L_reference · (V_reference / V)^a, where a = 3–5 for carbide tools (typical for deep hole drilling). For steel grades, a reference tool life of 60 m at Vc = 90 m/min for 4140 steel (32 HRC) with carbide tooling can be scaled: for a harder steel (45 HRC), reduce speed to Vc = 55 m/min and the tool life becomes L = 60 · (90/55)^4 ≈ 60 · (1.64)^4 ≈ 60 · 7.2 ≈ 432 m? That doesn't seem right. The Kronenberg exponent a = 4-5 implies very strong speed sensitivity. Let me recalculate: L = L_ref · (V_ref / V)^a = 60 · (90/55)^4 = 60 · (1.64)^4 = 60 · 7.2 = 432 m. But in practice, 45 HRC 4140 tool life at 55 m/min is not 432 m — it's about 25 m. This illustrates the danger of using generic Kronenberg exponents without material-specific calibration. The exponents a and b are not generic constants — they depend on the specific material-tool-coating combination. For accurate quoting, the shop should establish its own tool life database from production records: track tool consumption, cumulative cutting metres, and material for each job, then calculate the average tool life per material-tool combination after 10–20 jobs. The Waller-Rowsell model with a shop-specific tool life database provides quoting accuracy of ±10–20%, compared to ±30–50% for a simple machine rate model.
How do tool life and resharpening strategy affect the cost per hole in deep hole drilling?
Tool life and resharpening strategy are the dominant cost drivers for deep hole drilling — together they account for 30–50% of the total cost per hole, depending on the tool cost and the material's abrasiveness. The key relationship is that increasing the number of resharpenings per tool reduces the tool purchase cost per hole but increases the resharpening cost per hole, and there is an optimum number of resharpenings that minimises the total tooling cost per hole. The total tooling cost per metre of drilling is: C_tooling = (B + n · R_sh) / (L_1 + L_2 + ... + L_n), where B = purchase cost, n = number of resharpenings, R_sh = resharpening cost per regrind, and L_i = tool life in metres after regrind i. After each regrind, the tool life decreases by 5–15% relative to the previous regrind (the regrind removes carbide material from the tip, reducing the tool's thermal mass and changing the geometry slightly). The cumulative tool life over the tool's life is L_total = L_0 · (1 + r + r² + ... + r^n), where r = 0.85–0.95 (the life retention factor per regrind) and L_0 is the new-tool life. For a carbide gun drill with L_0 = 60 m in 4140 steel, r = 0.90, and n = 10 resharpenings: L_total = 60 · (1 + 0.9 + 0.81 + ... + 0.9^10) = 60 · 6.74 = 404 m. The tool purchase cost B = $200, and the resharpening cost per regrind R_sh = $20 (labour + wheel wear + coolant for the grinding machine). The total tooling cost per metre = ($200 + 10 × $20) / 404 m = $400 / 404 m = $0.99/m. For a 300 mm deep bore, the tooling cost per bore = $0.99 × 0.3 = $0.30.
The optimal number of resharpenings is found by plotting the tooling cost per metre as a function of n and selecting the n that minimises C_tooling. For the example above, the cost per metre decreases from $1.11/m at n = 5 to $0.99/m at n = 10 and begins to increase after n = 12 (because the tool life after each regrind becomes very short, and the resharpening cost per metre increases). The optimum is typically 8–12 resharpenings for carbide gun drills and 10–15 for PCD-tipped gun drills (which have a thicker diamond layer that can withstand more regrinds). The practical rule of thumb is: resharpen until the tool life after a regrind drops below 50% of the new tool life, then discard. For the case above, tool life after 12 resharpenings would be L_12 = L_0 × 0.9^12 = 60 × 0.28 = 17 m — below 50% of 60 m = 30 m, so the tool should be discarded after 10–11 resharpenings. The economic impact of extending resharpenings from 5 to 10 reduces tooling cost per bore by $0.30 to $0.60 per bore in the example — a significant saving for a shop drilling 10 000 bores per year ($3000–6000/year). The tooling cost per bore for different materials (from the cost model table above) ranges from $0.02/bore for aluminium (long tool life + low tool cost) to $3.00–6.00/bore for Inconel 718 (short tool life + PCD tooling cost). The cost of tooling per bore in difficult materials is high enough that PCD tooling (with 10× the purchase cost but 20× the tool life) becomes the economic choice, as the PCD cost per bore is $1.00–2.00 versus $3.00–5.00 for carbide in Inconel 718.
What is the correct machine hour rate for deep hole drilling, and why should it include both ownership and operating costs?
The correct machine hour rate for deep hole drilling is the sum of the ownership costs (depreciation, interest, insurance, and taxes — costs that accrue regardless of whether the machine is running) and the operating costs (power, coolant, maintenance, operator labour, and floor space — costs that accrue only when the machine is operating). The machine hour rate is the centrepiece of any drilling cost estimate, and an incorrect rate — whether too low (underquoting, losing profit) or too high (overquoting, losing jobs) — directly affects the shop's competitiveness. The ownership costs are fixed: depreciation ($12.50/h for a $300 000 gun drilling machine amortised over 8 years at 3000 h/year), interest ($7.00/h at 7% interest), insurance and taxes ($1.50/h at 1.5% of purchase price per year). The ownership costs total $21.00/h and do not change if the machine is cutting chips or idle — they represent the cost of having the machine available. The operating costs are variable: operator labour ($25.00/h for an experienced deep hole drilling operator), power ($1.08/h for a 15 kW spindle at 60% load), coolant ($3.00/h for oil-based system), coolant system maintenance ($2.00/h), machine maintenance ($3.00/h), and floor space ($2.00/h). The operating costs total $36.08/h and accrue only when the machine is running. The total machine hour rate is $21.00 + $36.08 = $57.08/h for a single-spindle gun drilling machine.
The machine hour rate must be applied to the effective cutting time (the time the spindle is cutting), not the total job time (which includes loading, unloading, inspection, and downtime). If the shop uses a utilisation factor (typically 0.70–0.85 for job shops), the effective machine rate applied to quoted jobs should be the total rate divided by the utilisation factor: $57.08 / 0.80 = $71.35/h. This means the shop must charge $71.35 per hour of spindle cutting time to cover both the cutting time overhead and the idle time overhead. For BTA drilling machines (higher capital cost, larger coolant system, higher power consumption), the machine hour rate is higher ($88.10/h), and for micro gun drilling machines (lower capital cost but higher spindle maintenance), the rate is similar to standard gun drilling ($59.55/h). The machine hour rate should be recalculated annually based on the actual operating hours, maintenance costs, and tooling consumption from the previous year. A common mistake is to use the same machine rate for all machines in the shop — a BTA machine with a $600 000 purchase price has a machine rate that is 54% higher than a $300 000 gun drilling machine, and quoting BTA jobs at the gun drilling rate loses money on every BTA job. The machine hour rate should be machine-specific, not averaged across the shop.
How do I calculate the setup, fixture, and programming costs for a deep hole drilling job, and how are they amortised across the batch size?
Setup, fixture, and programming costs for a deep hole drilling job differ from the per-hole drilling cost in that they are fixed costs — they do not change with the number of bores drilled — and they must be amortised across the batch size to determine the total cost per part. The setup cost includes: the time required for the operator to set up the workpiece(fixture, clamping, alignment), to load the CNC programme(if any), to verify the first part (first-article inspection), and to adjust the parameters if the first article is out of tolerance. For a typical deep hole drilling job on a gun drilling or BTA machine, the setup time is 30 minutes to 4 hours, depending on the complexity(how many bores, whether the workpiece must be repositioned between bores, whether coolant pressure or tooling must be changed between bores). At a machine rate of $85/h, the setup cost is $42–340 per job. The fixture cost is the cost of designing and manufacturing the workpiece fixture, amortised over the expected life of the fixture (typically 500–5000 parts for a dedicated fixture, or 10–100 parts for a modular fixture system). For a deep hole drilling fixture with locating pads, hydraulic clamping, and a drill bushing plate, the fixture cost is $2000–10 000. The programming cost(if the deep hole drilling machine is CNC-controlled) includes the time to write, simulate, and verify the CNC programme for the hole pattern. For a typical multi-bore part, the programming cost is $200–1000. For a shop quoting on a batch of 100 parts (4 bores per part, requiring a dedicated fixture at $5000, setup 3 h at $85/h, programming $600), the fixed cost per part is ($5000/100) + (3 × $85)/100 + $600/100 = $50 + $2.55 + $6 = $58.55 per part in fixed costs. The variable cost per part (drilling cost per bore × 4 bores) for 4140 steel at 32 HRC, 300 mm deep bores, is $1.74 × 4 = $6.96 per part. The total cost per part is $58.55 + $6.96 = $65.51 per part.
The quoting strategy for different batch sizes should apply the fixed costs differently. For small batches (1–10 parts), the setup and programming costs dominate — the shop should quote a fixed setup charge + per-part cost, with the setup charge covering the setup and programming time regardless of the actual number of parts produced. For large batches (> 100 parts), the fixture amortisation dominates the fixed cost, and the shop should amortise the fixture over the expected batch size plus a spare fixture for future orders. The quoting equation is: Quote price = (S + P + F/N_batch) / N_batch + C_per_part × N_bore_per_part + margin, where S = setup cost, P = programming cost, F = fixture cost, N_batch = batch size, C_per_part = variable drilling cost per bore, and margin = 15–30% depending on the market and utilisation. For a batch of 100 parts with the costs above: Quote = ($255 + $600 + $5000/100) / 100 + $6.96 + 20% margin = ($855 + $50) / 100 + $6.96 × 1.20 = $9.05 + $8.35 = $17.40 per part. The shop can verify this quote by comparing it to the estimate from the Waller-Rowsell model, which should give a similar total cost (±10%) when the fixed costs (setup, fixture, programming) are added to the variable drilling cost.
The information provided in this article is for general informational purposes only and does not constitute professional financial or business advice. Always consult qualified cost estimators, accountants, and industry specialists for specific quoting and pricing decisions. Data and recommendations are based on published research and industry experience as of 2026.