Holes under about 3 mm with depth-to-diameter ratios a twist drill can never reach. Single-lip micro gundrills hold straightness to 400:1; micro-EDM and femtosecond laser step in where cutters break — diesel injector spray holes, turbine blade cooling holes, cannulated implants, and watch jewel bearings.
A micro hole is conventionally under 1.0 mm diameter; microdrilling as a practical discipline spans roughly 0.1–3 mm. A deep hole is one with depth at least five times the diameter (L/D ≥ 5:1). Put the two together and you reach the hardest region in holemaking: a 0.3 mm hole drilled 30 mm deep is L/D 100:1 — the tool is a hundred diameters long, chips must escape through a channel a hair wide, and every angular error is amplified over depth.
| L/D Ratio | Classification | Typical Micro Process |
|---|---|---|
| ≤ 5:1 | Standard hole | Conventional twist drill |
| 5:1–10:1 | Semi-deep | Peck drilling, through-coolant drill |
| 10:1–20:1 | Deep hole | Micro gundrill or micro-EDM |
| 20:1–50:1 | Very deep | Micro gundrill with counter-rotation |
| 50:1–100:1 | Ultra-deep | Specialized micro gundrill or EDM |
| > 100:1 | Extreme | Micro gundrill with whip guides |
The single-lip gun drill is the default for mechanical micro holes. One cutting edge, a V-shaped chip flute along the outside, and an internal coolant hole down the center: coolant blasts to the tip and chips flush back along the flute. Gundrills run from about Ø0.5 mm up to 30 mm and hold depth-to-diameter ratios to 400:1 — a 1 mm hole drilled 400 mm deep.
| Material | Spindle Speed (rpm) | Feed (mm/rev) |
|---|---|---|
| Low / unalloyed steel (<1000 N/mm²) | 29,000–18,000 | 0.015–0.020 |
| High alloyed steel (700–1500 N/mm²) | 17,000–15,000 | 0.015–0.020 |
| Stainless (ferritic / martensitic / austenitic) | 15,700–14,000 | 0.005–0.008 |
| Cast iron | 29,000–14,000 | 0.015–0.020 |
| Titanium alloys | 7,000–6,500 | 0.005–0.008 |
| Fe / Ni / Co superalloys | 6,500–6,000 | 0.005–0.008 |
Micro-EDM erodes material with controlled sparks, so there is no cutting force, no torque, and no drill to break. It is the method of record for hardened steels, superalloys, and tiny spray orifices. Tube electrodes of Ø0.1–0.3 mm are standard; holes down to 0.1 mm are routine, and diesel injector spray holes hold diameter accuracy around ±2 µm.
| Parameter | Typical Range |
|---|---|
| Electrode diameter | Ø0.1–0.3 mm brass or tungsten tube |
| Peak current (Ip) | 1–10 A |
| Pulse on-time (Ton) | 5–50 µs |
| Electrode rotation | Up to 300 rpm (wear compensation + flushing) |
| Dielectric | Deionized water at 50–100 bar |
| Aspect ratio | 50:1 as a controlled spec; up to 100:1 with adaptive control |
When the hole is too small or the material too hard for a mechanical cutter, non-contact methods take over. Their aspect-ratio ceilings differ sharply, which is why process choice matters.
| Process | Practical Max L/D | Note |
|---|---|---|
| EDM drilling | ~10:1 (30:1 specialized) | Up to 100:1 with adaptive control |
| ECM (electrochemical) | ~8:1 | Burr-free, no thermal damage |
| Laser (LBM) | ~16:1 | Percussion 100 µm–1 mm; quality falls past ~20:1 |
| Electron beam (EBM) | ~6:1 | Vacuum only, shallow micro holes |
| Ultrasonic (USM) | ~2.5:1 | Mainly an assist, not a deep-hole method |
| Shaped-tube (STEM) | ~16:1 | Acid drilling, mostly EDM families |
Micro deep hole drilling is an equipment discipline: the machine, not the tool, decides whether a 0.3 mm hole reaches depth. Rigidity, runout, and coolant are non-negotiable.
| Machine / Class | Diameter | Depth | Spindle | Notes |
|---|---|---|---|---|
| Precihole micro gundrill | 1–6 mm | 300 mm | 1,000–25,000 rpm (2 × 1.1 kW) | Counter-rotation, tool monitoring, steadies |
| Widma MG series | 1–6 mm | — | Up to 18,000 rpm | Automotive & medical micro work |
| Sugino Gun Feeder | Ø0.9–5.0 mm | 300 mm | 2,000–12,000 min⁻¹ | 11 MPa coolant, 20 µm filtration |
| High-frequency spindle (e.g. FISCHER MFW 860 D) | Micro band | — | 80,000 rpm | Liquid-cooled micron machining |
Confirm it holds a stable speed at target RPM; below 0.5 mm, pick the most stable speed band.
TIR under 3 µm at the cutting edge; reseat the holder or re-chuck if not.
Spot-face a burr-free entry; an off-square face is the #1 cause of drift at depth.
Peck depth (0.2–0.3)×D for 0.30–0.50 mm drills to clear chips.
Pressure and flow at the tip, filtration ≤20 µm, chip shape observed on first part.
Torque/thrust or coolant pressure with auto-retract on threshold.
Everything in micro deep hole drilling depends on getting chips out of a channel measured in tenths of a millimeter. Coolant does double duty — it cools the cutting edge and it flushes chips. Lose the flush and the tool re-cuts its own chips, packs the flute, and breaks.
| Method | Coolant | Pressure | Filtration |
|---|---|---|---|
| Micro gundrill | High-EP cutting oil | 50–150+ bar (up to ~2,500 PSI on micro machines) | ≤20 µm (≤10 µm for superalloys) |
| Sugino Gun Feeder class | Cutting oil | 11 MPa (~1,595 PSI) | 20 µm |
| Micro EDM | Deionized water | 50–100 bar | Deionized, particulate-controlled |
| Conventional micro drilling | Oil or MQL | Through-tool where possible | Fine filter |
| Parameter | Typical Capability | Measurement Method |
|---|---|---|
| Diameter tolerance | IT6–IT9 (injector work IT6–8) | Air gauge, optical, CMM micro-probe |
| Surface finish (Ra) | 0.2–0.8 µm gundrill; 0.08–1.25 µm injector; <0.8 µm EDM | Profilometer, surface comparator |
| Straightness | Best with counter-rotation | Straightness mandrel, laser bore alignment |
| Roundness / circularity | (max–min diameter) / 2 | CCD microscope, air-gauge cross-sections |
| Drill deflection (injector) | 0.01–0.03 mm / 100 mm | — |
| Recast layer (EDM) | <0.02 mm optimized | Metallurgical section |
| Symptom | Likely Cause | Fix |
|---|---|---|
| Tool breaks / snaps | Chip packing, off-square entry, too-low RPM, no pecking | Peck (0.2–0.3)×D, spot-face the start, hold stable RPM, monitor torque/thrust |
| Hole drifts or bends | Off-square start, deflection, no counter-rotation | Square the start face, counter-rotate the part, reduce feed, add steadies |
| Poor surface finish | Under-feed rubbing, worn tip, contaminated coolant | Hold chip load, resharpen on schedule, filter to ≤20 µm |
| Chip clogging | Flute packed, coolant pressure drop | Raise pressure, peck more, verify flush at the tip |
| Short tool life | Too high speed, poor coolant, runout | Cut speed, check TIR < 3 µm, use high-EP oil |
| Burrs at exit | Mechanical cutting, no back support | Vibration assist, EDM for the smallest, deburr pass |
| EDM recast / white layer | High pulse energy, poor flushing | Lower pulse energy, adaptive servo, post-process |
Research shows all test drills broke on the first hole when pecking was omitted.
Auto-retract on threshold before the tool fractures and jams the hole.
A wobbly tip is a broken drill waiting to happen.
C-shaped chips mean healthy evacuation; strings or dust mean stop and inspect.
Log tool life per material and replace before failure.