Understanding the true cost per hole in deep hole drilling requires looking beyond tool price to tool life, cycle time, machine rate, labor, and scrap. The L/D ratio is the dominant cost driver — a hole at L/D 100 can cost 10× more than one at L/D 10. This guide breaks down the Cost Per Hole (CPH) formula, the key cost factors by material and process, a fully worked numeric example, and the levers that actually move the number.
Total Cost Per Hole (CPH) is the sum of three independent terms. Each must be tracked separately — a shop that only watches tool price will miss the fact that machine time is usually the biggest line on the bill.
| Component | What It Captures | Typical Share | Why It Matters |
|---|---|---|---|
| Tool cost | Tool price ÷ holes per tool, plus regrinding amortized over life | 5–20% (often smallest) | Easiest to see, easiest to misjudge — a pricier tool can still win on CPH |
| Machine time | Cycle time × (machine rate + labor rate) | 50–80% (largest) | Cycle time is the single biggest lever in almost every deep hole job |
| Scrap & rework | Scrap rate × accumulated part cost, plus rework cost | 5–15% | Can reach 18% without process monitoring; each scrap carries all prior operation cost |
| Overhead & consumables | Coolant, filtration, energy, inspection, setup amortization | 5–15% | Budget $1–5 per hole for complex BTA coolant operations |
Cost scales non-linearly with the length-to-diameter (L/D) ratio. Doubling hole depth can more than double machining time, because feed must drop, tool wear climbs, and chip-evacuation pressure rises. The table below gives practical multipliers against a D=10mm gundrill baseline.
| L/D Ratio | Cost Multiplier vs. L/D 10 | Primary Cost Driver |
|---|---|---|
| ≤ 5:1 | 0.5× – 0.8× | Standard drilling, no special equipment needed |
| 10:1 (baseline) | 1.0× | Gundrill setup, moderate cycle time |
| 30:1 | 1.5× – 2.5× | Reduced feed rate, increased tool wear |
| 50:1 | 3× – 5× | Whip guides required, lower Vc |
| 100:1 | 5× – 10× | Specialized supports, frequent tool changes, high scrap risk |
Material machinability drives tool life, cutting speed, and scrap risk all at once. Published cost guides put stainless at roughly 20–50% above carbon steel, hardened alloys at 50–100% above — and exotic aerospace alloys far higher still.
| Material Group | Cost Factor vs. Carbon Steel | Key Challenges | Tool Life (holes per regrind, D=10mm) |
|---|---|---|---|
| Carbon steel (baseline) | 1.0× | Standard machinability | 200–500 |
| Stainless steel (304/316) | 1.2× – 1.5× | Work hardening, chip adhesion | 80–200 |
| Hardened alloys (35–45 HRC) | 1.5× – 2.0× | Abrasive wear, heat generation | 50–150 |
| Titanium (Ti-6Al-4V) | 2.0× – 3.0× | Low thermal conductivity, high strength at temperature | 30–80 |
| Superalloys (Inconel 718) | 3.0× – 5.0× | Extreme hot hardness, notch wear | 10–40 |
Per-hole tool cost is simply tool price divided by tool life in holes — plus regrinding amortization. For a 10 mm diameter gundrill the spread is dramatic:
| Scenario | Tool Price | Tool Life (holes) | Cost Per Hole (tooling only) |
|---|---|---|---|
| Carbon steel, optimized parameters | $120 | 400 | $0.30 |
| Stainless steel, moderate tooling | $120 | 150 | $0.80 |
| Inconel 718, premium coating | $200 | 25 | $8.00 |
| Inconel 718, PCBN tooling | $600 | 150 | $4.00 |
Machine time plus tooling accounts for 70–85% of total deep hole drilling cost, and machine time usually outweighs tooling. The hourly rate depends on machine class, location, and overhead load.
| Machine Class | Typical Hourly Rate | Notes |
|---|---|---|
| 3-axis CNC mill | $40–80 | Lowest-cost base for retrofits |
| CNC turning center | $60–120 | Common ejector / gundrill host |
| Dedicated deep hole / BTA machine | $40–150 | BTA class sits at the high end |
| 5-axis / multi-axis center | $120–200 | High overhead; rarely dedicated to drilling |
| Regional reference | CNC China ~$10–21; UK ~£30 | Location swings the rate 3–5× |
Feed rate differences between methods dominate cycle time. BTA (single-tube system) drills at 100–300 mm/min in steel versus 15–60 mm/min for gundrilling — a 5–7× advantage — because internal chip removal eliminates pecking and feed interruptions. Gundrilling is in turn up to ~6× faster than conventional twist drilling on deep holes.
| Factor | Gundrilling | BTA (STS) | Ejector (DTS) |
|---|---|---|---|
| Machine investment | $50k–$200k | $200k–$800k+ | $30k–$100k (retrofit kit) |
| Feed rate (relative) | 1× baseline (15–60 mm/min) | 5–7× (100–300 mm/min) | Between gundrill and BTA |
| Tool cost per edge | Low–Moderate (regrindable) | Higher (indexable) | Moderate |
| Setup time per job | Short | Longer (pressure head) | Moderate |
| Cost per hole (low volume) | Lowest | Highest | Moderate |
| Cost per hole (high volume) | Highest | Lowest | Moderate |
| Break-even batch | Baseline | >500–1000/mo | >200/mo |
| Sweet-spot diameter | 1–50 mm | 12–250+ mm | 18–65 mm |
| Head Type | Size Range | Pros | Cost Trap |
|---|---|---|---|
| Brazed BTA head | 12–20 mm | Lower initial cost, ±0.01 mm tolerance, Ra 0.8–1.6 µm | Whole head discarded when dull — cost per hole becomes prohibitive above ~20 mm |
| Indexable BTA head | ≥20 mm (now from 8 mm) | Inserts indexed 3–6 times; reusable body; lower CPH in high volume | Slightly looser ±0.02–0.05 mm and Ra 1.6–3.2 µm |
| Hidden Cost | Typical Impact | How to Control |
|---|---|---|
| Coolant management | $1–5 per hole for complex BTA ops (filtration, replacement, disposal) | Track coolant cost per hole; high-pressure filtration is mandatory |
| Tool regrinding | 20–30% of a new tool per regrind | Factor into CPH; log regrinds per tool |
| Setup & changeover | 5–15 min machine time per change, plus setup labor | Batch scheduling and tool-life management cut change frequency |
| Drill wander | Unforeseen deviations → rework or scrap; standard quoting misses it | Guide bushings, pilot holes, counter-rotation for long bores |
| Compliance & inspection | EN 9100 / AS9100 paperwork adds admin and inspection cost | Build into the quote; scope documentation requirements up front |
| Poor drawings | Incomplete or inaccurate drawings add cost and time at every stage | Verify drawing completeness before quoting or scheduling |
Without proper process controls, scrap rates in deep hole drilling can reach 18%. Every scrapped part carries the full accumulated cost of material plus all prior operations — not just the drilling step. Rework — such as welding and re-drilling — typically costs 30–60% of the original operation but risks introducing new defects.
Real-time coolant pressure, torque, and feed-force monitoring detects problems before they produce scrap. Most applications can be brought from 18% down to below 5% scrap.
On a $45 part with a 5% scrap improvement, the saving is $2.25 per hole before counting the machine time that was also saved. Scrap is the cheapest line to improve because it recovers both material and cycle time.
$180 drill ÷ 250 holes = $0.72/hole — the visible line item, and the smallest.
15 min cycle × ($60/hr machine + $40/hr labor) × 0.25 hr = $25.00/hole — 81% of the total.
Estimated 8% scrap × $45 part cost = $3.60/hole.
Filtration, coolant replacement, regrinding amortization = $1.50/hole.
$0.72 + $25.00 + $3.60 + $1.50 = $30.82 per hole.
TiAlN and AlCrN coatings extend tool life 2–5×. For difficult materials PCBN often delivers lower CPH despite the upfront cost.
Run the correct speed/feed for the material; validate on chip shape and wear, not manufacturer data alone.
Better tool-life management and batch scheduling cut change frequency — each change costs 5–15 min of machine time.
Real-time torque, coolant, and feed-force monitoring can take scrap from 18% to below 5%.
Reduce L/D where possible, convert blind to through holes, standardize diameters across part families.
Evaluate every tooling investment on CPH, not purchase price — a $300 tool at $0.50/hole beats a $100 tool at $2.00/hole.
Capital cost spans two orders of magnitude depending on method. Payback = machine cost ÷ (saving per hole × holes per month), and the sourced paybacks below show how quickly the economics can work.
| Investment | Method | Typical Payback | When It Works |
|---|---|---|---|
| $30k–$100k retrofit kit | Ejector (DTS) on existing machine | 6–12 months | Medium diameters, existing spindles, >200 holes/month |
| $50k–$200k machine | Gundrilling | Fine for ~50 parts/month | Small diameters, precision holes, modest volume |
| $200k–$800k+ machine | BTA (STS) | Economical at >500–1000 holes/month | Large diameters, high volume, 7–10× faster |
| U-Drill class | Large-diameter drilling | 6–12 months ROI | 40–60% manpower cut, dual-part output, less rework |
Reputable deep hole shops issue bespoke quotes (often within 1–2 working days) because no standard per-part price exists. For the same part, five suppliers can quote wildly different numbers.
| Reference Rate Model | Example Figure | Meaning |
|---|---|---|
| Per-mm of depth (India market) | ₹1–1.5 / mm depth; ₹360–500 / piece | Roughly ₹1,000/meter — a floor, not a standard |
| D/d ≤ 2.5, D ≤ 25 mm | charge = bit dia × 0.05 (RMB) | Shallow small holes price by diameter |
| D 25–60 mm | charge = bit dia × 0.12 (RMB) | Larger diameter → higher base charge |
| D/d > 2.5 | base × 0.4 × D/d ratio (RMB) | Depth ratio multiplies the charge directly |
| Precision < 0.1 mm | base price × 5 | Tolerance multiplies cost fivefold |
| Batch discount | base × 0.2–0.8 | Volume drives the biggest discount |