The geometry ground onto the tip of a deep hole drill determines everything — chip formation, cutting forces, surface finish, and tool life. For gun drills specifically, the nose grind is the single most important variable for matching the tool to the material.
This guide covers both gun drill nose grind types and general drill point geometry parameters, with material-specific recommendations.
1. Gun Drill Nose Grind Types
Gun drills use specialized nose grind configurations that balance cutting forces, control chip formation, and direct coolant flow. Five standard grinds cover the majority of applications.
N-8 — General Purpose (Steel, Inconel, Stainless)
| Parameter | Value |
|---|---|
| Inner angle | 20° |
| Outer angle | 30° |
| Dub-off | 25° |
| OD contour | R1 (standard radius) |
| Best for | Steel, Inconel, nickel alloys, stainless steel |
The N-8 is the default grind for most production gun drilling in ferrous materials. The 30° outer angle provides a good balance of cutting edge strength and chip formation.
N-4 — Aluminum, Brass, Soft Materials
| Parameter | Value |
|---|---|
| Inner angle | 20° |
| Outer angle | 15° (shallower than N-8) |
| Dub-off | 25° |
| OD contour | R4 (sharper radius) |
| Best for | Aluminum, brass, copper, plastics |
The shallower 15° outer angle reduces cutting forces in soft materials and produces the best surface finish. The R4 contour further improves finish quality.
N-73 — Stacked Parts, Angular Entries
| Parameter | Value |
|---|---|
| Point placement | Near center of drill |
| Key feature | Strongest grind geometry |
| Best for | Stacked plates, angled entry surfaces, interrupted cuts |
The N-73’s center-point placement makes it the most robust grind — it is significantly stronger than N-8 or N-4 and is specified when the drill must withstand impact loading at entry.
Facet Grind (F8) — European Standard
| Parameter | Value |
|---|---|
| Type | Facet (flat-ground) |
| Coolant clearance | Greater than cam ground |
| Best for | Production runs, European applications |
Facet grinds provide greater clearance for coolant to reach the cutting edge. They are ground as distinct flat facets rather than the blended curves of a sweep grind. This allows more coolant flow at the cutting edge, improving chip evacuation in deep holes.
Sweep Grind — Versatile, Blended
| Parameter | Value |
|---|---|
| Type | Cam ground, blended |
| Adjustability | Can be modified for specific materials |
| Best for | General-purpose, job shops |
Sweep grinds blend the geometric features using a cam fixture, creating smooth transitions between angles. This reduces stress concentration points and allows fine adjustments for specific material behaviors.
Sources: DME Tool nose geometry guide; industry standard grind definitions; UNISIG.
2. Material Selection Chart
| Material | Recommended Grind | Why |
|---|---|---|
| Low-carbon steel (1018, 1020) | N-8 | Standard general-purpose |
| Medium-carbon steel (1045) | N-8 | Good chip formation |
| Alloy steel (4140, 4340) | N-8 | Balance of strength and finish |
| Stainless 304/316 | N-8 | Work-hardening resistance |
| Inconel / superalloys | N-8 with modified relief | Reduced angles for edge strength |
| Aluminum (6061, 7075) | N-4 | Best finish, reduced cutting forces |
| Brass / bronze | N-4 | Prevents grabbing |
| Titanium (Ti-6Al-4V) | N-8 or modified | Sharp edge with proper relief |
| Cast iron | N-8 | Standard grind works well |
| Stacked / angled entries | N-73 | Strongest geometry for impact |
| High-production, European | Facet F8 | Better coolant flow at edge |
Source: DME Tool nose geometry guide; Hone-All Precision common mistakes guide.
3. Point Angle Selection (Twist Drills)
For twist drills and indexable insert drills used in deep hole applications, the point angle selection follows different rules:
| Material | Recommended Point Angle | Edge Strength |
|---|---|---|
| Aluminum, mild steel, plastics | 118° | Moderate |
| Alloy steel, stainless, titanium | 135° | High |
| Inconel, superalloys | 132–138° | Very high |
| Cast iron | 120–130° | High |
| Hardened steel (> 45 HRC) | 135–140° | Very high |
| Very hard materials | 80–90° | Highest |
Source: HNCarbide drill point angle guide; PMC split-point drill study.
118° vs 135°: The 118° point has sharper cutting edges and penetrates faster with lower thrust, but the cutting edge is weaker. The 135° point has stronger edges and better wear resistance but requires higher thrust. For deep hole drilling where tool life is critical, 135° is generally preferred for steel and stainless.
Helix Angle
| Material | Recommended Helix | Effect |
|---|---|---|
| Steel (general) | 25–30° | Standard chip evacuation |
| High-strength alloys | 33–37° | Higher torsional strength |
| Aluminum, soft materials | 35–40° | Better chip removal |
| Brass, bronze | 10–20° | Prevents grabbing |
Sources: Split-point twist drill patents; HNCarbide geometry guide.
4. Rake and Clearance Angles
Rake Angle
| Material Type | Rake Angle | Notes |
|---|---|---|
| General steel | ∼25° | Standard positive rake |
| High-strength alloys (> 500 MPa) | −15° (negative) | Edge strength priority |
| Aluminum, copper, plastics | ∼35° | Free-cutting |
| Composite + titanium sandwich | 10–14° positive | Balances dissimilar materials |
Lip Relief (Clearance) Angle
| Material | Lip Relief Angle |
|---|---|
| General steel | 4–6° |
| High-strength alloys, stainless | 10–14° |
| Aluminum | 6–8° |
| Bone / composites | 12–15° |
Higher relief angles reduce friction but weaken the cutting edge. For deep hole drilling in difficult materials, the higher range (10–14°) is recommended to prevent rubbing-induced work hardening.
5. Chisel Edge Modifications
The chisel edge consumes approximately one-third of the thrust force in drilling and generates significant heat because it extrudes rather than cuts.
| Modification | Effect | Application |
|---|---|---|
| Split point | Reduces chisel edge to 0.004–0.008 in | CNC drilling, all materials |
| Multi-facet (polygonal) | Reduces thrust by 40–50%, heat by 50–60% | High-performance, difficult materials |
| Web thinning | Reduces non-cutting chisel width | Deep hole twist drills |
For deep hole drilling, a split point is strongly recommended for all twist drills. The reduced thrust force improves hole straightness and reduces deflection at depth.
6. Practical Selection Workflow
Step 1: Identify workpiece material and hardness
Step 2: Select gun drill nose grind (N-8, N-4, N-73, or Facet)
Step 3: If using twist drill, select point angle (118° or 135°)
Step 4: Choose split point for all CNC applications
Step 5: Match coating to material
Step 6: Configure coolant pressure and flow
Step 7: Set cutting parameters from reference tablesFor parameter selection, see the Speeds and Feeds Reference. For coating selection, see the Tool Coatings Guide.
7. Signs of Incorrect Geometry
| Symptom | Likely Cause | Fix |
|---|---|---|
| Built-up edge on rake face | Rake angle too low (not enough positive) | Increase rake angle |
| Chipped cutting edge | Point angle too sharp for material | Increase to 135° |
| Oversized hole | Unequal lip heights | Regrind with equal heights |
| Poor surface finish | Wrong nose grind (gun drill) | Switch N-8 ↔ N-4 |
| High thrust force / machine overload | Point angle too large / chisel edge too long | Reduce point angle or split point |
| Chatter marks | Clearance angle too large (loss of edge support) | Reduce lip relief angle |
| Heat discoloration on drill | Rake angle too low, insufficient chip flow | Increase rake angle or helix |
Key Sources
- DME Tool, “Nose Geometries & Chip Breakers” — gun drill nose grind types and specifications
- Hone-All Precision / MTDCNC, “Common Mistakes to Avoid When Gundrilling” — grind selection guidance
- HNCarbide, “Choosing the Right Drill Point Angle” — point angle selection by material
- PMC / NIH, split-point drill geometry study — rake and clearance angle data
- UNISIG Technical Reference — gun drill geometry
- Patent US4556347, “Split-point twist drill” — helix and relief angle specifications