I use single lip drills for most work and double lip for high production. The choice between these two gun drill types depends on the application requirements, the material being drilled, and the production volume. Here is a detailed comparison of single lip vs double lip gun drills based on my experience.
Single Lip Gun Drills
Single lip gun drills have one cutting edge and one chip flute. Coolant is delivered through a single hole in the tip. The single cutting edge is supported by guide pads that bear against the bore wall and keep the drill tracking straight.
Advantages:
- Simpler design, lower cost
- Better for small diameters under 10 mm
- Produce excellent surface finish down to Ra 0.1-0.8 micrometers
- Easier to regrind because there is only one edge to set up
- Superior self-piloting action from the single cutting edge
- Available in very small diameters, from 2 mm and up
- Achieve tighter hole tolerances (IT7-IT9)
- Better straightness for depth-to-diameter ratios above 50:1
Disadvantages:
- Lower feed rates than double lip
- Higher cutting forces per edge
- Single flute limits chip evacuation capacity
Single lip drills are the standard for most gun drilling work. I reach for a single lip drill first on almost every new job. The versatility and reliability make them the default choice.
Double Lip Gun Drills
Double lip gun drills have two cutting edges and two chip flutes. Coolant is delivered through two holes in the tip. Dual cutting edges distribute the cutting load and allow faster feed rates.
Advantages:
- Higher feed rates up to 2 times single lip
- Better balance in the hole due to opposing cutting forces
- Lower cutting forces per edge, longer tool life per edge
- Two chip flutes improve chip evacuation capacity
- Good for non-ferrous materials where chip formation is favorable
Disadvantages:
- Higher cost, typically 30-50 percent more than single lip
- More complex to regrind because both edges must be identical
- Not available in very small diameters — typically 6 mm minimum
- Reduced self-piloting action compared to single lip
- Require 20-30 percent more coolant pressure and volume
- Not recommended for hardened steels or high-strength alloys
Comprehensive Comparison Table
Here is a side-by-side comparison across all major factors:
| Factor | Single Lip | Double Lip |
|---|---|---|
| Number of cutting edges | 1 | 2 |
| Number of chip flutes | 1 | 2 |
| Coolant holes | 1 | 2 |
| Diameter range | 2-40 mm | 6-26 mm |
| Typical max depth | 2000+ mm | 1000 mm |
| Feed rate capability | Baseline | Up to 2x single lip |
| Surface finish (Ra) | 0.1-3.0 micrometers | 1.0-4.0 micrometers |
| Hole tolerance | IT7-IT9 | IT10 |
| Cost | Lower | 30-50% higher |
| Regrind difficulty | Easy | Requires skill and setup |
| Coolant pressure needed | Baseline | 20-30% higher |
| Best L/D ratio | Up to 200:1 | Up to 30:1 |
| Self-piloting | Excellent | Good |
The most significant difference I see in practice is the self-piloting behavior. Single lip drills have an inherent cutting force imbalance that pushes the tip against the bore wall, creating a burnishing action that keeps the hole straight. Double lip drills balance the forces better but lose some of that self-piloting action.
Application by Material
The choice between single lip and double lip depends heavily on the workpiece material. Here is what I use based on material type:
| Material | Recommended Type | Reason |
|---|---|---|
| Low carbon steel | Single or double lip | Both work well; choose by volume |
| Alloy steel (4140, 4340) | Single lip | Better straightness and finish in tough materials |
| Stainless steel (316, 304) | Single lip | Better chip control in stringy materials |
| Tool steel (H13, D2) | Single lip | Lower feed rates needed for hardness |
| Aluminum (6061, 7075) | Double lip | Higher feed rates possible, good chip formation |
| Cast iron | Double lip | Brittle chips evacuate easily through two flutes |
| Brass and bronze | Double lip | High feed rates without finish concerns |
| Titanium (Ti-6Al-4V) | Single lip | Better for difficult materials |
| Inconel and superalloys | Single lip | Requires lowest feed rates for tool life |
| Copper alloys | Double lip | Soft material allows high feed rates |
Cost Analysis
The cost difference between single lip and double lip gun drills affects the decision for high-volume production. Here is a rough cost comparison:
| Drill Type | Cost (10 mm diameter) | Tool Life per Regrind | Regrinds Possible | Cost per Hole |
|---|---|---|---|---|
| Single lip, carbide | $80-120 | 80-120 holes in steel | 5-8 | $0.15-0.30 |
| Double lip, carbide | $120-180 | 60-90 holes in steel | 5-8 | $0.25-0.45 |
| Single lip, HSS | $40-60 | 40-60 holes in steel | 3-5 | $0.20-0.40 |
| Double lip, HSS | $60-90 | 30-50 holes in steel | 3-5 | $0.35-0.60 |
The cost per hole for double lip drills is higher, but the faster feed rate can reduce cycle time enough to offset the tooling cost in high-volume production. I switch to double lip when the cycle time savings exceed the additional tooling cost.
When to Use Each Type
I use single lip gun drills for the majority of my work. The superior surface finish, better straightness, and wider range of applicable sizes make them the versatile choice. For job shop work where materials change frequently, single lip drills cover almost every situation.
I switch to double lip gun drills in specific situations:
- High-production runs of 500+ identical parts
- Aluminum and cast iron components where feed rate is the bottleneck
- Parts where surface finish requirements are Ra 1.6 or looser
- When the machine has excess coolant capacity to handle the higher volume
For more on tool selection and holder setup, see my comparison of hydraulic vs mechanical tool holders. The tool holder choice affects both single lip and double lip drill performance.
Key Takeaways
- For most shops, single lip gun drills are the right choice — they are versatile, produce excellent surface finish, and are easy to maintain
- Double lip gun drills offer up to 2x higher feed rates but require more coolant pressure and are limited to specific applications
- Single lip drills are better for small diameters, hard materials, and deep holes with high L/D ratios
- Double lip drills work best for non-ferrous materials, high-volume production, and moderate depth-to-diameter ratios
- Single lip drills achieve IT7-IT9 tolerances with Ra 0.1-0.8 micrometer surface finish compared to IT10 for double lip
- Tooling cost per hole is higher for double lip drills, but the faster cycle time can offset the difference in high-volume runs
- The self-piloting action of single lip drills provides better straightness in deep holes
- Double lip drills are worth considering for high-production jobs where feed rate is the limiting factor