Substrate hardness, cutting-edge geometry, and coating chemistry decide whether a deep hole comes out straight, round, and profitable — or whether chips jam, the axis drifts, and the tool dies at 10× diameter. This reference covers drill head types, guide pads, the full coating family (TiN to diamond), carbide grades, gundrill point geometry, and BTA/ejector insert design.
Three essential factors in tool selection — and all three are indispensable.
| 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 |
| Replaceable head deep hole drill | Machining centers | Fast changeover, high consistency, no regrinding setup |
Guide pads are what make a deep hole drill self-piloting. They are the only radial support for a drill that can be 100× its diameter long — and they do double duty, burnishing the hole wall during machining and reducing surface roughness by up to 70%.
Hardness is only half the story — the oxidation temperature limit decides whether the coating survives the cutting zone. AlTiN beats TiN by roughly 40–50% in hardness and survives ~200–300°C hotter.
| Coating | Hardness (HV) | Max Temp | Best Use / Notes |
|---|---|---|---|
| TiN (Titanium Nitride) | ~2300 | ~600°C | General steel, low-alloy steel; gold color doubles as a wear-monitor indicator |
| TiCN (Titanium Carbonitride) | ~3000 | ~400°C | Steel, cast iron, abrasive materials; high sliding-wear resistance, lower friction |
| TiAlN (Titanium Aluminum Nitride) | ~3300 | ~800°C | Stainless steel, cast iron, alloy steel; dry/high-speed cutting; forms protective Al²O³ layer |
| AlTiN (Aluminum Titanium Nitride) | ~3600 | ~900°C | Hardened steel >52 HRC, titanium, high-speed finishing; higher Al content |
| AlTiCrN / TiSiN | ~3500 | ~1000°C | Superalloys, titanium (TiSiN widely recommended for drilling titanium) |
| CrN (Chromium Nitride) | ~1500–2200 | ~800°C | Copper, aluminum, brass — anti-stick for galling-prone non-ferrous work |
| DLC (Diamond-Like Carbon) | 2000–3500 | ~450°C | Aluminum, composites, low-friction; resists built-up edge; limited heat range |
| Diamond (CVD) | Hardest known | Ferrous-unstable | Graphite, CFRP, ceramics, non-ferrous; iron catalyzes graphite so steel tools need a CrN interlayer |
| Balinit Pertura | ~3500 | ~1200°C | Most difficult-to-machine materials; the highest oxidation ceiling in this table |
| Uncoated (polished) | — | — | Aluminum, copper, plastics; razor-sharp edge prevents built-up edge |
A coating does four jobs: it is a friction reducer, a thermal barrier, a diffusion shield (stopping workpiece atoms migrating into the tool), and — in the case of TiN and its golden color — a visible wear indicator. The measurable gains published across coating guides and tool-maker data are substantial:
| Coating / System | Reported Gain | Context |
|---|---|---|
| TiN | 3–4× tool life | vs. uncoated on general steel |
| TiAlN | Up to 10× tool life; >2× TiN on drills | vs. uncoated; high-speed & stainless work |
| TiAlN cutting speed | 100–200 m/min | vs. ~60 m/min for TiN — higher productivity window |
| DLC | +30–50% tool life | Aerospace titanium (Ti-6Al-4V), semi-dry, high precision |
| Coated BTA inserts | +200–400% insert life | Multi-layer PVD/CVD vs. uncoated alternatives |
| Submicron carbide solid gundrill | Up to +100% feeds & speeds | Iscar IC08 class vs. brazed carbide gundrills |
| Replaceable-head BTA tooling | −60–80% tooling cost | Indexable inserts vs. brazed heads (no regrind setup) |
| Workpiece Material | Recommended Coating | Why |
|---|---|---|
| Mild / low-alloy steel | TiN or TiAlN | TiN is cheap and general; TiAlN opens up dry / high-speed cutting |
| Stainless steel (304/316) | TiAlN / AlTiN | Heat resistance to ~800–900°C beats work-hardening built-up edge |
| Hardened stainless (420/440C) | AlTiN | Higher Al ratio, alumina layer, suited to finishing at speed |
| Hardened steel (>52 HRC) | AlTiN | Highest hardness tier plus oxidation stability |
| Cast iron | TiCN / TiAlN / CVD Al²O³ | Abrasive graphite flecks; thick Al²O³ top layer for high speed |
| Titanium alloys (Ti-6Al-4V) | TiSiN / TiAlN / DLC | Low thermal conductivity & BUE; TiSiN ~4000 HV, ~1000°C; DLC for low friction |
| Superalloys (Inconel, Waspaloy) | AlTiCrN / TiSiN / Balinit Pertura | Extreme heat; high-Al nano-multilayer PVD (e.g. Tungaloy AH8015) resists BUE |
| Aluminum / copper / brass | Uncoated polished, DLC, or CrN | Coated edges round & push; a sharp uncoated edge shears cleanly |
The substrate carries the edge; the coating protects it. Grain size sets the hardness–toughness balance, and the deposition process (CVD vs. PVD) changes how the coating behaves at the edge.
| Manufacturer | Grade | Coating / Substrate | Application |
|---|---|---|---|
| Tungaloy | AH725 | TiAlN on carbide | BTA / gundrill inserts for steel, stainless, cast iron, heat-resistant alloys |
| Tungaloy | AH8015 | High-Al nano-multilayer PVD | Superalloys & duplex stainless; resists BUE, longer predictable life |
| Iscar | IC08 | Submicron carbide, AlTiN nano-layer | Solid carbide gundrills; up to +100% feeds/speeds vs. brazed |
| Kennametal | KC7325 | Double coating + TiN top layer | Small drills — TiN top acts as a wear-monitor indicator |
| ESTool | YC1015–YC1335 | MT-CVD TiCN + Al²O³ + TiN | Steel, ISO P05–P35; finishing to light roughing |
| ESTool | YP3225 / YP3330 | AlCr + AlCrSiN on ultra-fine / rare-metal substrates | Stainless, hardened steel, superalloys |
| ESTool | YR3025 | TiAlN + TiN | High-temperature alloys & titanium, semi-finish / roughing |
A gundrill carries a single cutting edge eccentrically offset from the drill centerline, with one internal fluid passage and an external chip flute. It is self-piloting — the tip is ground as planar facets so the whole head can be redressed with a standard grinding wheel. Tip geometry controls chip shape, fluid flow, guidance, and straightness, so tolerances are tight.
| Feature | Value | Why It Matters |
|---|---|---|
| Point angle | Held ±1° | Consistency hole to hole; increasing the angle cuts tighter and straighter, decreasing it cuts larger and wanders |
| Outer edge approach angle φ1 | ~30° | Outer cutting edge geometry |
| Inner edge approach angle φ2 | ~20° | Inner cutting edge geometry toward center |
| Point centrality | ±0.001 in (0.025 mm) | Off-center points break tools at cut start or produce oversized holes |
| Cutting lip heights | 0.0005 in (0.013 mm) | Unequal lips cause "fly cutting" — oversized holes, premature wear |
| Primary relief (outer edge) | 8–12° | Clearance behind the edge; lets coolant reach the cutting point |
| Secondary flank | ~20° | Opens space for drilling fluid to reach the edge |
| Inner edge flank | 8–12° | Clearance on the inner edge |
| Auxiliary flank | 25–35° | Prevents rib interference between flanks |
| Shoulder dub-off | ~30° | Free penetration at the periphery corner |
| Web thickness | 10–13% of D | Balance of rigidity and chip space; developed after notch grinding |
| Secondary margin trail | ~15% of D (max 0.125 in) | Supports size and straightness control |
| Notch / gash | Breaks into coolant hole ¼–½ its diameter | Gets chips into the flute; coolant forces chips out |
| Fluid exit angle β | >66° (prefer 75–80°) | Steeper exit angle minimizes fluid stagnation at the hole bottom |
Gundrill rake, relief, point offset, and point angle shift with the workpiece. Harder materials take negative rake and smaller relief; gummy materials take positive rake and open relief.
| Material | Point Offset | Rake Angle | Relief Angle | Point Angle |
|---|---|---|---|---|
| Low-carbon steel | 0.8–1.0 mm | 0° to +3° | 10°–12° | 30°–35° |
| Alloy steel (annealed) | 0.6–0.8 mm | 0° to +2° | 8°–10° | 28°–32° |
| Alloy steel (hardened) | 0.4–0.6 mm | −3° to 0° | 6°–8° | 25°–28° |
| Stainless steel | 0.6–0.8 mm | +3° to +6° | 10°–14° | 30°–35° |
| Aluminum | 0.8–1.2 mm | +5° to +8° | 12°–16° | 30°–35° |
| Titanium | 0.5–0.7 mm | 0° to +2° | 8°–10° | 28°–32° |
| Cast iron | 0.5–0.7 mm | −3° to 0° | 6°–8° | 25°–28° |
BTA and ejector (DTS) heads mount multiple indexable inserts on a ring of guide pads. Geometry and coating are engineered per insert position, because the center, intermediate, and peripheral edges see completely different cutting conditions.
Large BTA and ejector heads use more pads — up to 4 on big diameters — positioned to balance the radial cutting force. For BTA the main pad sits at 178° and the secondary at 276°, with a 0.3 mm width-wear replacement limit and carbide, PCBN, or PCD pad materials depending on the work material.
Steel → TiN/TiAlN; stainless & hardened → AlTiN; titanium & superalloys → TiSiN/AlTiCrN/pertura class; aluminum & brass → uncoated polished, DLC, or CrN.
Check point centrality (±0.025 mm), lip-height equality (0.013 mm), and point angle (±1°) before the tool goes back in the spindle.
Look for uniform color, edge chipping, and localized discoloration — patchy or burned coating means the edge already overheated.
Replace at 0.3 mm width wear; keep coolant filtered (≤30 μm, ≤10 μm for superalloys) so hard particles cannot embed in pads.
Small C-shaped chips = healthy cut. Long ribbons or powder = wrong geometry or coating; stop and adjust before the hole drifts.
Serialize tools and log regrinds and holes per edge. Deep hole drilling rewards data — the regrind-interval trend predicts failure before it happens.