About 100 to 500 times the hole diameter. A gundrill on a dedicated machine routinely reaches 100:1 depth-to-diameter, high-performance machines push to 200:1, and special-geometry gundrills reach 300:1 to 400:1 — but only with the right machine, whip support, and coolant pressure. This guide breaks down the real limits by method, by diameter, and by depth.
Different deep hole methods trade depth ratio against metal-removal rate and diameter range. The table below gathers typical and maximum figures from Wikipedia, UNISIG, Botek, NTS Amega, and CTE Magazine.
| Method | Typical L/D | Practical Maximum | Diameter Range |
|---|---|---|---|
| Twist drill (standard) | ≤5:1 | ~5:1 | Any |
| Twist drill with through-tool coolant | ~10:1 | ~20:1 (special deep-hole drills) | 3–40 mm |
| Gundrill on dedicated machine | 100:1 | 200:1 on high-performance machines | 0.5–50 mm (75 mm possible) |
| Gundrill, small diameters / special geometry | 300:1 | 400:1 (500:1 claimed) | Ø1 mm × 400 mm demonstrated in Ti-6Al-4V |
| BTA drilling | 30:1 | ~100:1; up to 12 m absolute depth | 20–400+ mm (to 2,000 mm special) |
| Ejector drilling | 30:1 | ~100:1 (limited to ~100 diameters) | 18–250 mm |
Absolute depth capability grows with diameter because bigger drills carry longer flutes. Below about Ø3 mm the practical limit is roughly 75–80 diameters; from Ø16 mm up, tool series are usually rated 40–50 diameters with much larger absolute flute lengths.
| Diameter Range | Practical Depth Rating | Notes |
|---|---|---|
| 0.5–3 mm | 75–80xD | Specialized micro machines required; a Ø1 mm drill has a flute only ~80–90 mm long |
| 3–16 mm | 40–80xD | Standard stock: Gühring EB 100 = 80xD; HARTNER E 100 = 50xD |
| 16–40 mm | 40xD per pass; 80–120xD stepped | Brazed and indexable gundrills; blank lengths to 2,200 mm |
| 40–50 mm | To 3,000 mm total length | Gühring EB 80 specials to 3,000 mm overall |
| 50–75 mm | Possible, less efficient | BTA deep hole drilling becomes the better fit |
| Tool Series | Diameter | Depth Rating | Overall Length |
|---|---|---|---|
| Gühring EB 100 M (solid carbide) | 1.0 mm | 75xD | 115 mm |
| Gühring EB 100 M | 6.0 mm | 75xD | 525 mm |
| Gühring EB 100 M | 7.14 mm | 75xD | 625 mm |
| HARTNER E 100 | 16.0 mm | 50xD | 565 mm |
| HARTNER E 80 | 25.4 mm | 40xD | 910 mm |
Three independent constraints decide how deep a gundrill can actually go. All three grow worse as the hole gets deeper, so pushing any one of them hard is what separates a 100:1 production hole from a broken tool.
The gundrill is a long, thin tube with a single cutting lip. Unsupported, the tool body begins to whirl (whip) and vibrate once the unsupported length exceeds roughly 40:1 — Star Cutter quotes 40:1 to control tool-body whipping without predrilled holes or machine guide supports. Beyond that, the tool is self-piloting only after a proper start through a bushing or pilot hole.
Coolant pressure loss is dominated not by pipe friction but by flow deflection at the bottom of the hole, in the small clearance between the gundrill flank and the hole bottom (Astakhov, Frazao & Osman, 1994). If pressure at the cutting edge collapses, chips pack in the V-shaped flute, form a plug, and over-torque the tool until the tip separates.
Long, flexible gundrill shafts deviate more as depth increases, especially in difficult-to-cut materials such as Inconel 718. Straightness deviation is not linear — a small entry error or pressure deficiency compounds over hundreds of millimetres (published Inconel 718 studies).
| Limiter | Symptom | Countermeasure |
|---|---|---|
| Whip | Chatter marks, ovality, drift | Whip guides / steady rests, raise critical RPM |
| Coolant pressure drop | Chip packing, flute plug, tool fracture | Higher pressure, optimized coolant passage |
| Straightness drift | Deviation grows with depth | Guide bushing, counter-rotation, high pressure |
Dedicated gundrilling machines mount a steady rest (whip guide) between the spindle and the workpiece to support the shank. A common design uses flexible plastic bushings (such as International Drill Guide’s Snapguide®) held in a bearing with a slight compression fit, which damps vibration and tool whipping.
Coolant pressure is the hidden lever on depth. Higher pressure simultaneously clears chips and stiffens the gundrill shaft: pressurised coolant inside the bore raises the shaft’s first critical RPM, and straighter holes result (rotating Euler–Bernoulli beam model). Published Inconel 718 work shows straightness deviation falling from 1.62 mm to 0.37 mm as coolant pressure rose to 137.89 bar at 1,600 rpm.
Straightness is usually specified per metre (for example 1 mm/m or, with counter-rotation, 0.05 mm/m). Deviation grows non-linearly with depth, which is why a hole can be straight for the first 200 mm and visibly wander by the 500 mm mark.
| Depth | Low-pressure / unoptimized | High-pressure + support |
|---|---|---|
| 100 mm | ~0.5 mm deviation | ~0.05 mm deviation |
| 350 mm (Inconel 718) | 1.62 mm deviation (low coolant pressure) | 0.19 mm deviation (EDMG + 137.89 bar), 48.65% better |
| 500 mm (Inconel 718) | Fails a 1 mm/m spec | 0.37 mm deviation at 137.89 bar / 1,600 rpm |
When people ask “how deep can a gundrill go,” these are the documented extremes they mean — from mass-produced rifle barrels to single-purpose research machines.
| Case | Dimensions | L/D | How It Was Done |
|---|---|---|---|
| M16 rifle bore | .223 caliber, 20 in (508 mm) long | ~90:1 | Production gundrilling of rifle barrels |
| Titanium micro-hole research | Ø1.0 mm × 400 mm in Ti-6Al-4V | 400:1 | Custom machine, servo spindle control, high-pressure coolant (PatSnap patent landscape) |
| BTA absolute depth | Up to 12 m in Ø20–400+ mm | to 100:1 | Contract deep hole drilling (Breitenbach) |
| Landing gear bores | Ø230 mm × 2.1 m in 300M | ~9:1 but huge absolute | Counter-rotated BTA on a UNISIG B700 |
| Long-stroke gundrill machines | 3,000–6,000 mm depth | — | XL-3000 / XL-6000 machine series |
Holes at 20:1 or greater generally require dedicated equipment; the practical depth ceiling of a standard machining centre is far lower. Here is what a deep-hole-capable machine must bring to the table.
| Requirement | Why | Typical Specification |
|---|---|---|
| Dedicated gundrill machine | Long stroke + steady rest + guide-bushing arrangement | 20:1+ only practical on dedicated machines |
| Deep Z stroke | Absolute depth must exceed target hole length | 3,000 mm common; 6,000 mm available |
| High-pressure coolant | Chip evacuation + shaft stiffening + straightness | 50–150+ bar oil system, temperature controlled |
| Whip guide / steady rest | Damp whip beyond ~40:1 unsupported length | Bearing + flexible plastic bushings |
| Starting bushing / pilot | Near-zero-clearance entry, prevents bell-mouth | Gundrill-specific starting bushing |
| High spindle speed | Small diameters need high surface speeds | 5,000–8,000 rpm on micro gundrill machines |
| Filtration | Clean oil protects the single cutting edge | To 20 μm standard; 10 μm for superalloys |
Match the method and machine to the target L/D, then engineer the supporting systems.
One rigid tool at full depth is always better than several short ones — until the target L/D exceeds the tool’s rated depth, whip becomes unmanageable, or the material is too difficult to cut in one pass. Then step-drill.
If target depth exceeds the tool series rating (40–80xD), plan multiple diameters or an EDM guide hole.
Spot-face square, then cut the first step with a rigid, short tool through a starting bushing.
Start large, step down to final size; each new tool re-engages rigidly and re-centres.
For extreme L/D in hard materials, an EDM-predrilled guide hole + gundrill (EDMG) cut deviation by 48.65% in Inconel 718 studies.
Check straightness and runout before committing the next, longer tool.
| Single Pass | Step-Drill Instead |
|---|---|
| L/D within the tool’s rated depth | Target L/D exceeds rated depth |
| Tool supported (bushing + whip guide) | Whip-limited or unsupported length |
| Easy-to-cut material, clean chip | Hard / work-hardening material (Inconel, titanium) |
| Good entry control and alignment | Entry drift or off-square start suspected |