30-Second Summary: Three essential factors in tool selection — substrate hardness determines deformation resistance, geometry determines chip breaking effectiveness, and coating determines surface friction and thermal barrier. All three are indispensable. Coating choice must be matched to the workpiece material — the wrong coating can perform worse than uncoated carbide.
Drill Head Types
| Type | Application | Advantages |
|---|---|---|
| Brazed carbide gundrill | General gundrilling, D > 3 mm | Regrindable multiple times, moderate cost, proven geometry |
| Solid carbide gundrill | Small diameter (< 5 mm), high-precision | Highest rigidity, excellent wear resistance, long life between regrinds |
| Indexable insert gundrill | Production gundrilling, D 12–28 mm | No regrinding, quick insert indexing, up to 4× productivity vs. brazed |
| BTA indexable insert drill head | BTA drilling, D 16 mm and above | No regrinding, quick change, consistent geometry, multiple edge grades |
| BTA brazed drill head | High volume, special geometries | Custom geometry possible, maximum performance for specific applications |
| BTA spade drill type | D 10–114 mm | Replaceable blade, economical, good chip splitting |
| Replaceable head deep hole drill | Machining centers | Fast changeover, high consistency, no regrinding setup |
Guide Pads
Guide pads are the "secret weapon" of deep hole drilling tools — they not only provide guidance but also burnish the hole wall during machining, reducing surface roughness by up to 70%. They bear all radial cutting forces and are the most wear-sensitive component of any deep hole drilling system.
| Parameter | Specification |
|---|---|
| Standard material | Cemented carbide (ISO K10–K20 grade) |
| Premium material | PCD-coated carbide (reduces friction, prevents built-up edge); Cermet (for high-temp alloys) |
| Typical hardness | 1200–1800 HV (carbide); up to 6000 HV (PCD) |
| Number per drill head | 2 standard (gundrill, BTA); up to 4 for large diameter BTA |
| Wear limit | Replace when guide pad width wear exceeds 0.3 mm |
| Angular position (BTA) | Main pad at ~178°, secondary pad at ~276° |
| Angular position (gundrill) | Single pad opposite cutting edge at ~110–120° from lip |
| Surface finish requirement | Ra ≤ 0.2 μm (polished for reduced friction) |
Guide pad wear modes:
- Abrasive wear — most common; caused by hard particles in the workpiece material or insufficient coolant filtration
- Adhesive wear / galling — material transfer from workpiece to pad; indicates insufficient lubrication or incorrect pad material for the workpiece
- Edge chipping — mechanical shock at entry/exit or vibration during cutting
- Thermal cracking — intermittent coolant supply or excessive cutting speed
Cutting Tool Substrate Grades
| Grade Type | Hardness | Transverse Rupture Strength | Best For |
|---|---|---|---|
| Micrograin carbide (0.2–0.5 μm) | 1800–2200 HV | 3500–4500 N/mm² | Small gundrills, sharp edges, aluminum |
| Submicron carbide (0.5–0.8 μm) | 1600–2000 HV | 3000–4000 N/mm² | General purpose gundrills and BTA |
| Medium grain carbide (1–3 μm) | 1400–1700 HV | 2500–3200 N/mm² | Heavy-duty BTA, interrupted cuts |
| Coarse grain (3–5 μm) | 1200–1500 HV | 2000–2800 N/mm² | Cast iron, abrasive materials |
Coating Technology Comparison
| Coating | Hardness HV | Max Temp °C | Friction vs Steel | Applicable Materials |
|---|---|---|---|---|
| TiN (Titanium Nitride) | ~2300 | ~600 | 0.5–0.6 | General steel, low-carbon steel |
| TiCN (Titanium Carbonitride) | ~3000 | ~400 | 0.4–0.5 | Steel, cast iron, abrasive materials |
| TiAlN (Titanium Aluminum Nitride) | ~3300 | ~800 | 0.5–0.6 | Stainless steel, cast iron, alloy steel (general purpose) |
| AlTiN (Aluminum Titanium Nitride) | ~3600 | ~900 | 0.5–0.7 | Hardened steel >52 HRC, titanium, high-speed dry machining |
| AlTiCrN (AlTi Chromium Nitride) | ~3400 | ~850 | 0.5 | Steels, alloyed steels — but poor performance in drilling tests (edge breakage, adhesion) |
| TiSiN / TiAlSiN (Nanolaminate) | ~4000 | ~1100 | 0.3–0.4 | Superalloys, titanium, hardened materials (best drilling performance) |
| AlCrN (Aluminum Chromium Nitride) | ~3200 | ~900 | 0.5 | Cast iron, high-temperature alloys |
| Balinit Pertura (AlCrO-based) | ~3500 | ~1200 | 0.3–0.4 | Most difficult-to-machine materials |
| Uncoated (polished) | — | — | 0.6–0.8 | Aluminum alloys (prevents built-up edge), copper, plastics |
Coating selection guidance: For drilling, TiAlN-based coatings consistently outperform chromium-containing coatings (AlTiCrN, AlCrN) according to comparative studies — the chromium oxides formed during cutting have lower protective capability, leading to higher adhesion and abrasive wear. TiAlSiN nanolaminate coatings show the best overall drilling performance with lowest flank wear and best chip evacuation.
Recommended Edge Geometry by Material (Gundrills)
| Material | Point Offset | Rake Angle | Relief Angle | Point Angle | Chamfer Width |
|---|---|---|---|---|---|
| Low-carbon steel | 0.8–1.0 mm | 0° to +3° | 10°–12° | 30°–35° | 0.3–0.5 mm |
| Alloy steel (annealed) | 0.6–0.8 mm | 0° to +2° | 8°–10° | 28°–32° | 0.3–0.5 mm |
| Alloy steel (hardened) | 0.4–0.6 mm | -3° to 0° | 6°–8° | 25°–28° | 0.2–0.4 mm |
| Stainless steel (austenitic) | 0.6–0.8 mm | +3° to +6° | 10°–14° | 30°–35° | 0.2–0.4 mm |
| Stainless steel (ferritic) | 0.6–0.8 mm | 0° to +3° | 8°–12° | 28°–32° | 0.2–0.4 mm |
| Aluminum | 0.8–1.2 mm | +5° to +8° | 12°–16° | 30°–35° | 0.3–0.6 mm |
| Titanium (Ti-6Al-4V) | 0.5–0.7 mm | 0° to +2° | 8°–10° | 28°–32° | 0.1–0.3 mm |
| Cast iron (gray) | 0.5–0.7 mm | -3° to 0° | 6°–8° | 25°–28° | 0.2–0.4 mm |
| Nickel superalloy (Inconel) | 0.4–0.6 mm | +3° to +6° | 10°–14° | 28°–32° | 0.1–0.2 mm |
| Copper / Brass | 0.7–1.0 mm | +3° to +6° | 10°–14° | 30°–35° | 0.3–0.6 mm |
Values shown are for gundrills with D = 10–20 mm. Adjust proportionally for larger or smaller diameters. Negative rake angles improve edge strength for hard materials; positive rake angles reduce cutting forces for soft/gummy materials. Point offset determines the radial force balance — too little offset risks loss of self-piloting; too much increases torque and heat.
Edge Preparation
- Honed edge (R = 0.01–0.03 mm): Recommended for coated tools to prevent coating peeling at the cutting edge; improves edge strength for interrupted cuts
- Sharp edge: Best for aluminum and non-ferrous materials where built-up edge is a concern
- Chamfered edge (T-land): 0.05–0.15 mm at 20°–30° for hardened steels and cast iron to prevent micro-chipping
Major Tool Brands
- Botek: Specialist manufacturer of gundrills, BTA drill heads, and trepanning systems; extensive standard range and custom designs
- Allied Machine: GEN3SYS XT replaceable head system, designed for stainless steel/superalloys on machining centers
- Sandvik Coromant: CoroDrill 860, deep hole solutions for machining centers; extensive coating and substrate range
- Iscar: GD-DH / GD-DHL indexable insert gundrills, TRI-DEEP series for deep hole drilling on machining centers
- Guhring: Solid carbide deep hole drills, small diameter expertise (sub-1 mm), comprehensive technical documentation
- Walter: Titex X-treme series, DC150 and DC180 deep hole drills for machining centers
- Tungaloy: DeepTri-Drill indexable insert gundrills with triangular chip-splitting inserts