Gun Drilling Speeds and Feeds
Selecting the correct cutting parameters is critical to successful gun drilling. Unlike conventional drilling, gun drilling parameters must balance chip evacuation, tool life, hole quality, and productivity—and the relationships between speed, feed, and coolant pressure are tightly coupled.
This guide provides practical parameter tables and selection rules for gun drilling common engineering materials.
How to Use This Guide
Gun drilling parameters depend on three primary variables:
Material. The workpiece material determines the cutting speed range and influences feed rate selection. Harder materials require lower speeds and feeds.
Drill diameter. Larger diameter drills can run at higher surface speeds and feed rates, but require careful coolant pressure management.
Depth ratio. As depth-to-diameter ratio increases, reduce both speed and feed. Deeper holes generate more heat, require more coolant pressure, and put more stress on the tool.
Cutting Speed by Material
Cutting speed (surface speed of the drill at the cutting edge) is the first parameter to select. Use these starting values for carbide-tipped gun drills.
| Material Group | Cutting Speed (m/min) | Cutting Speed (SFM) |
|---|---|---|
| Low carbon steel (< 0.25% C) | 120–180 | 400–600 |
| Medium carbon steel (0.25–0.55% C) | 100–140 | 330–460 |
| Alloy steel (low alloy, annealed) | 80–130 | 260–430 |
| Tool steel / high alloy | 60–100 | 200–330 |
| Stainless steel (austenitic 304/316) | 50–80 | 160–260 |
| Stainless steel (martensitic/ferritic) | 60–100 | 200–330 |
| Gray cast iron | 70–100 | 230–330 |
| Ductile iron | 60–90 | 200–300 |
| Aluminum (wrought, 6061) | 80–160 | 260–525 |
| Aluminum (cast) | 60–120 | 200–400 |
| Brass (free machining) | 80–150 | 260–490 |
| Copper (pure) | 40–70 | 130–230 |
| Titanium (Ti-6Al-4V) | 15–25 | 50–80 |
| Nickel alloys (Inconel 718) | 10–20 | 33–65 |
| Hardened steel (HRC 40+) | 15–30 | 50–100 |
Starting recommendation: Begin at the lower end of the speed range for the material. Increase speed only after confirming stable chip evacuation and acceptable tool wear.
Feed Rate by Drill Diameter
Feed rate is the most influential parameter on chip evacuation. Higher feed rates produce thicker, shorter chips that evacuate more easily—but excessive feed overloads the tool.
| Drill Diameter (mm) | Steel (mm/rev) | Cast Iron (mm/rev) | Aluminum (mm/rev) | Brass (mm/rev) |
|---|---|---|---|---|
| 3.0–4.0 | 0.007–0.013 | 0.009–0.040 | 0.006–0.040 | 0.006–0.030 |
| 4.0–6.0 | 0.012–0.022 | 0.015–0.060 | 0.010–0.080 | 0.010–0.060 |
| 6.0–8.0 | 0.018–0.030 | 0.025–0.080 | 0.012–0.125 | 0.012–0.080 |
| 8.0–12.0 | 0.025–0.050 | 0.050–0.120 | 0.025–0.175 | 0.020–0.140 |
| 12.0–16.0 | 0.040–0.070 | 0.060–0.150 | 0.040–0.200 | 0.030–0.180 |
| 16.0–20.0 | 0.050–0.085 | 0.080–0.170 | 0.050–0.210 | 0.035–0.200 |
| 20.0–25.0 | 0.060–0.110 | 0.100–0.210 | 0.060–0.255 | 0.040–0.250 |
Starting recommendation: Use the middle of the feed range. If chips are long and stringy, increase feed. If the tool chatters or overloads, decrease feed.
Coolant Pressure by Drill Diameter
Coolant pressure is the most critical parameter for reliable gun drilling. Insufficient coolant pressure is the leading cause of chip packing and tool breakage.
| Drill Diameter | Ideal Pressure | Minimum Pressure |
|---|---|---|
| 3.0 mm (0.125") | 10,000 kPa (1,500 PSI) | 3,500 kPa (500 PSI) |
| 6.0 mm (0.250") | 6,400 kPa (925 PSI) | 2,400 kPa (350 PSI) |
| 12.0 mm (0.500") | 3,600 kPa (525 PSI) | 1,700 kPa (250 PSI) |
| 19.0 mm (0.750") | 2,800 kPa (400 PSI) | 1,200 kPa (175 PSI) |
| 25.0 mm (1.000") | 2,100 kPa (300 PSI) | 1,000 kPa (150 PSI) |
Coolant flow rule of thumb: Supply enough coolant volume to fill the volume of the drilled hole once per revolution of the drill.
Coolant type: For dedicated gun drilling machines, use neat cutting oil (viscosity 7–20 mm²/s at 40°C). For CNC machines, use water-miscible emulsion at 8–12% oil content.
Depth Ratio Adjustments
As the hole gets deeper relative to its diameter, parameters must be reduced.
| Depth Ratio | Speed Adjustment | Feed Adjustment | Coolant Pressure |
|---|---|---|---|
| Up to 20:1 | 100% | 100% | Standard |
| 20:1 to 50:1 | 90% | 90% | Increase 15% |
| 50:1 to 100:1 | 80% | 80% | Increase 25% |
| Over 100:1 requires a dedicated machine with whip guide support. See our gun drilling machines guide for machine selection criteria based on depth ratio. |
Parameter Selection by Hardness
For alloy and tool steels, hardness significantly affects recommended parameters.
| Condition | Material Example | Hardness | Speed (RPM, Ø12mm) | Feed (mm/min, Ø12mm) | Coolant (bar) |
|---|---|---|---|---|---|
| Soft | 1.1730 (C45) | Low | 1,860 | 61–65 | 12 |
| Medium | 1.2311 (P20) | Medium | 1,670 | 43–44 | 12 |
| Hard | 1.2711 | High (HRC 45+) | 1,460 | 22 | 15 |
| Very Hard | 1.2714 | Very High | 1,380 | 21 | 15 |
Nose Grind Selection
The nose grind geometry of the carbide tip should be matched to the workpiece material.
| Nose Grind | Best For |
|---|---|
| N-8, R1 relief | Standard for steel, stainless steel, Inconel |
| N-4, R4 relief | Aluminum, brass, soft non-ferrous materials |
| N-73 | Cast iron, brittle materials |
| Facet grind | General purpose, good for most steels |
Practical Examples
Example 1: Gun drilling 304 stainless steel, Ø6 mm × 400 mm deep
- Cutting speed: 60 m/min → spindle speed = 60 ÷ (0.006 × π) = 3,180 RPM
- Feed rate: 0.015 mm/rev → 48 mm/min
- Coolant pressure: ~1,000 PSI (minimum)
- Depth ratio: 67:1 → reduce speed to 90% = 2,860 RPM, feed to 90% = 43 mm/min
Example 2: Gun drilling medium carbon steel, Ø12 mm × 300 mm deep
- Cutting speed: 120 m/min → spindle speed = 120 ÷ (0.012 × π) = 3,180 RPM
- Feed rate: 0.045 mm/rev → 143 mm/min
- Coolant pressure: 500 PSI
- Depth ratio: 25:1 → no reduction needed
Example 3: Gun drilling aluminum 6061, Ø8 mm × 600 mm deep
- Cutting speed: 140 m/min → spindle speed = 140 ÷ (0.008 × π) = 5,570 RPM
- Feed rate: 0.080 mm/rev → 446 mm/min
- Coolant pressure: 800 PSI
- Depth ratio: 75:1 → reduce speed to 80% = 4,456 RPM, feed to 80% = 357 mm/min
Chip Monitoring
Monitor chip appearance during gun drilling—it provides real-time feedback on parameter correctness.
| Chip Appearance | Indication | Action |
|---|---|---|
| Silver or light straw colored | Good parameters | Maintain |
| Blue or dark blue | Excessive heat | Reduce speed, increase coolant |
| Long, stringy chips | Feed too low | Increase feed 10–15% |
| Powdered or dusty chips | Feed too high or tool dull | Reduce feed, check tool |
| Variable chip shape | Inconsistent material or cutting | Check material hardness, coolant pressure |
Frequently Asked Questions
What happens if coolant pressure is too low? Chips pack in the V-flute, causing heat buildup, poor surface finish, and eventually tool breakage. Low coolant pressure is the most common cause of gun drilling failures.
Should I peck with a gun drill? No. Gun drilling is a continuous process. Pecking interrupts coolant flow and chip evacuation, leading to chip packing. The tool should feed continuously to full depth.
How do I know if my feed rate is correct? Monitor chip shape. Short C-shaped chips indicate good feed. Long stringy chips mean feed is too low. Powdered chips mean feed is too high or the tool is worn.
Summary
Selecting gun drilling parameters requires balancing speed, feed, and coolant pressure for the specific material and depth ratio. Start with the recommended values in the tables above, then fine-tune based on chip appearance and tool wear. Proper coolant pressure is the most critical factor for reliable operation.
For material-specific parameter recommendations, see our gun drilling by material guide. For troubleshooting advice, see common gun drilling problems and solutions. For a complete overview, visit the gun drilling guide.