Gun Drilling Cost Guide
Gun drilling is a specialized process, and its cost structure differs significantly from conventional drilling. The combination of specialized tooling, high-pressure coolant systems, and slow-but-steady feed rates creates a unique cost profile that scales non-linearly with hole depth.
This guide breaks down the factors that drive gun drilling costs, provides estimation methods, and offers practical benchmarks to help you budget and compare options.
Key Cost Drivers
1. Depth-to-Diameter Ratio (L/D) — The Dominant Factor
Depth-to-diameter ratio is the single most influential cost driver in gun drilling. Unlike conventional drilling where cost scales roughly linearly with depth, gun drilling cost scales non-linearly.
| L/D Ratio | Relative Cost vs. L/D 10 | Why |
|---|---|---|
| 10:1 | 1.0× (baseline) | Short hole, fast cycle, easy chip evacuation |
| 25:1 | 1.5–2.0× | Moderate depth, may need whip guide |
| 50:1 | 2.5–4.0× | Long cycle time, higher coolant pressure, tool wear |
| 100:1 | 5.0–8.0× | Very long cycle, whip guides required, multiple passes may be needed |
| 200:1+ | 10–20× or more | Extreme depth, specialized machine required, high risk |
The cost curve steepens dramatically past 50:1. A 200:1 ratio hole can cost 10–20 times more per mm of depth than a 10:1 hole—not 20 times the hole, but 10–20 times the per-unit-depth cost.
2. Material
Workpiece material affects both cutting speed and tool life, which directly impacts cost.
| Material Group | Relative Machining Cost vs. Low-Carbon Steel | Notes |
|---|---|---|
| Low-carbon steel | 1.0× (baseline) | Standard parameters, good tool life |
| Medium-carbon steel | 1.1–1.3× | Slightly slower speeds |
| Alloy steel (annealed) | 1.3–1.6× | Lower cutting speeds, more tool wear |
| Stainless steel (304/316) | 1.5–2.0× | Work hardening, chip breaking challenges |
| Titanium (Ti-6Al-4V) | 3.0–5.0× | Very slow speeds, high tool wear |
| Nickel alloys (Inconel 718) | 4.0–8.0× | Extremely slow, short tool life |
| Hardened steel (HRC 40+) | 2.5–4.0× | Slow speeds, frequent regrinds |
| Aluminum (6061) | 0.7–0.9× | Fast speeds, good tool life |
| Brass | 0.6–0.8× | Fast, easy cutting |
3. Diameter
Smaller diameter drills are more expensive to produce per mm of tool length, run at higher spindle speeds (more wear), and require higher coolant pressure—all of which increase per-hole cost.
| Diameter | Relative Tool Cost per mm Length | Relative Coolant Pressure Required |
|---|---|---|
| 3 mm (0.125") | High | 1,500 PSI |
| 6 mm (0.250") | Moderate | 925 PSI |
| 12 mm (0.500") | Moderate | 525 PSI |
| 25 mm (1.000") | Low | 300 PSI |
- A 3 mm gun drill may cost 20–40% more than a 6 mm drill of the same length
- Coolant pump requirements are more demanding for small diameters (higher pressure needed)
- Small diameter tools are more fragile and prone to breakage, adding risk cost
4. Tolerance and Surface Finish Requirements
Tighter tolerances increase cost because they require:
- More frequent tool regrinding (dull tools produce oversized holes)
- Slower feed rates to maintain finish
- More inspection time
- Higher scrap rates if tolerance is missed
| Tolerance Class | Typical Cost Multiplier vs. Standard (±0.05 mm) |
|---|---|
| Standard (±0.05 mm / ±0.002") | 1.0× |
| Precision (±0.025 mm / ±0.001") | 1.3–1.6× |
| Ultra-precision (±0.013 mm / ±0.0005") | 2.0–3.0× |
5. Production Volume
Volume affects cost primarily through setup amortization and tooling investment.
| Volume | Setup Cost per Hole | Tooling Cost per Hole | Total Relative Cost |
|---|---|---|---|
| 1–10 pieces (prototype) | High | High | 5–10× baseline |
| 100–1,000 pieces (low production) | Moderate | Moderate | 1.5–3× baseline |
| 10,000+ pieces (mass production) | Low | Low | 1.0× (baseline) |
Setup costs include machine programming, fixturing, guide bushing selection, and pilot hole preparation. For small batches, these fixed costs dominate per-hole pricing.
Cost Structure Breakdown
For a typical production run, gun drilling cost breaks down as follows:
| Cost Component | Typical Share of Total Cost | Description |
|---|---|---|
| Machine time | 40–55% | Cycle time × machine rate (depreciation, labor, overhead) |
| Tool cost | 15–25% | Gun drill purchase price amortized over regrind life |
| Coolant and consumables | 5–10% | Cutting oil/filter replacement, guide bushings, seals |
| Setup and fixturing | 10–15% | Engineering, programming, fixturing per batch |
| Inspection | 5–10% | Dimensional checks, surface finish measurement, CMM time |
| Scrap and rework | 5–10% | Rejected parts, rework labor |
Cost Estimation Formula
A practical cost-per-hole estimate can be built from four components:
Cost per hole = Machine time cost + Tool cost per hole + Setup cost per hole + Consumables
Machine Time Cost
Machine time (minutes) = Hole depth (mm) ÷ Feed rate (mm/min)
Machine cost = Machine time × Shop hourly rate ÷ 60
Typical machine rates for gun drilling equipment:
- Dedicated gun drilling machine: $75–150/hour
- CNC lathe with gun drilling attachment: $60–100/hour
- Multi-spindle gun drilling machine: $100–200/hour
Tool Cost per Hole
Tool cost per hole = Tool purchase price ÷ (Total regrinds + 1) ÷ Holes per regrind
Example: A $120 gun drill that can be reground 8 times, with each regrind lasting for 500 holes:
- Tool cost per hole = $120 ÷ (8 + 1) ÷ 500 = $0.027/hole
Separately, regrinding cost (typically $15–30 per regrind) must be added:
- Regrind cost per hole = $20 ÷ 500 = $0.04/hole
Total tool cost per hole = $0.067
Setup Cost per Hole
Setup cost per hole = Total setup hours × Hourly rate ÷ Batch size
For a 2-hour setup at $100/hour on a batch of 500 parts: $200 ÷ 500 = $0.40/part
Cost Examples by Application
Example 1: Ø6 mm × 400 mm deep in 4140 alloy steel
| Parameter | Value |
|---|---|
| Material | 4140 alloy steel (annealed) |
| Hole dimensions | Ø6 mm × 400 mm deep (67:1 L/D) |
| Feed rate | 0.015 mm/rev → 48 mm/min (at 3,180 RPM) |
| Cycle time | 400 ÷ 48 = 8.33 minutes |
| Machine rate | $100/hour |
| Machine cost | $13.89 |
| Tool cost per hole | ~$0.07 |
| Setup per hole (batch 200) | ~$0.50 |
| Consumables | ~$0.50 |
| Total estimated cost per hole | ~$15.00 |
Example 2: Ø12 mm × 300 mm deep in low-carbon steel
| Parameter | Value |
|---|---|
| Material | 1018 low-carbon steel |
| Hole dimensions | Ø12 mm × 300 mm deep (25:1 L/D) |
| Feed rate | 0.045 mm/rev → 143 mm/min |
| Cycle time | 300 ÷ 143 = 2.10 minutes |
| Machine rate | $90/hour |
| Machine cost | $3.15 |
| Tool cost per hole | ~$0.04 |
| Setup per hole (batch 1,000) | ~$0.20 |
| Consumables | ~$0.25 |
| Total estimated cost per hole | ~$3.65 |
Example 3: Ø3 mm × 200 mm deep in stainless steel 304
| Parameter | Value |
|---|---|
| Material | 304 stainless steel |
| Hole dimensions | Ø3 mm × 200 mm deep (67:1 L/D) |
| Feed rate | 0.008 mm/rev → 27 mm/min |
| Cycle time | 200 ÷ 27 = 7.41 minutes |
| Machine rate | $110/hour |
| Machine cost | $13.58 |
| Tool cost per hole | ~$0.15 |
| Setup per hole (batch 100) | ~$1.00 |
| Consumables | ~$0.75 |
| Total estimated cost per hole | ~$15.50 |
Cost Comparison vs. Other Deep Hole Drilling Methods
For diameters where multiple methods are feasible (20–50 mm):
| Factor | Gun Drilling | BTA Drilling | Ejector Drilling |
|---|---|---|---|
| Cycle time (Ø25 mm × 500 mm) | ~7 min | ~1.5 min | ~2 min |
| Machine cost per hour | $100 | $130 | $80 (on retrofitted machine) |
| Machine cost per hole | ~$11.67 | ~$3.25 | ~$2.67 |
| Tool cost per hole | $0.05 | $0.40 | $0.35 |
| Secondary ops needed | Rare | Sometimes | Sometimes |
At larger diameters where BTA and ejector can run, gun drilling’s slower cycle time makes it the most expensive per hole despite lower tool cost. However, when secondary operations are eliminated, the total cost gap narrows.
See our gun drilling vs BTA vs ejector comparison for a detailed method comparison.
How to Reduce Gun Drilling Costs
Optimize Parameters
- Increase feed rate until chips become long and stringy, then back off slightly. Higher feed = shorter cycle time.
- Match nose grind to material for optimal tool life.
- Monitor chip shape continuously to catch issues before they cause tool damage or scrapped parts.
Extend Tool Life
- Use proper coolant filtration (10–20 micron) to reduce abrasive wear.
- Regrind at the first sign of wear (0.25 mm wear land). Waiting too long degrades hole quality and shortens total tool life.
- Use coated gun drills for abrasive materials (AlTiN, TiAlN coatings).
Design for Gun Drilling
- Specify the largest diameter that meets the design requirement. Larger drills run faster and cost less per mm of depth.
- Avoid unnecessarily tight tolerances—each class of precision adds 30–100% to cost.
- Consider through-holes instead of blind holes where possible (chip evacuation is easier).
Estimating Scrap Risk Cost
Gun drilling has a higher scrap risk than conventional drilling, especially at extreme depth ratios. This risk should be built into cost estimates.
| L/D Ratio | Typical Scrap Rate | Risk Cost per Good Part |
|---|---|---|
| < 20:1 | 0.5–1% | Negligible |
| 20:1–50:1 | 1–3% | Low |
| 50:1–100:1 | 3–8% | Moderate |
| > 100:1 | 5–15% | Significant |
Quick Estimation Table
Use this table for ballpark cost estimates based on hole volume.
| Hole Volume (mm³) | Typical Cost Range (per hole, batch of 500, steel) |
|---|---|
| < 1,000 | $2–5 |
| 1,000–10,000 | $5–15 |
| 10,000–50,000 | $10–30 |
| 50,000–200,000 | $25–60 |
| > 200,000 | $50–150+ |
Note: These are rough estimates for standard tolerance work. Tight tolerances, difficult materials, or extreme depth ratios can multiply these ranges by 2–5×.
Summary
Gun drilling cost is driven primarily by depth ratio, material, diameter, and tolerance requirements—in that order. The process is most economical for holes between 20:1 and 50:1 depth ratio in standard steels at moderate production volumes. At extreme depth ratios or in difficult materials, costs escalate rapidly and alternative methods should be considered when feasible.
For accurate pricing, the most reliable approach is to provide a detailed part drawing to 2–3 gun drilling service providers and request quotes. See our guide on how to choose a gun drilling service provider for what to include in your RFQ.
For technical background, see what is gun drilling and gun drilling parameters guide. For a complete overview, visit the gun drilling guide.
For the method-agnostic buyer’s guide — what to specify in an RFQ and how to compare quotes fairly — see deep hole drilling cost factors and RFQ.