Cannulated bone screws and intramedullary nails need straight, featureless bores drilled down the center of long titanium and stainless bars. Gundrills cut the 1–30 mm cannulas that guide wires slide through during minimally invasive surgery — at 20:1 to 200:1 L/D. ISO 13485 traceability, validated cleanliness, and biocompatible surface integrity define the discipline.
| Challenge | Why It Matters | Consequence |
|---|---|---|
| Guide-wire fit | Screw is placed over a Ø1.0–1.9 mm guide wire | Bore must pass the wire without binding or wobble |
| Work hardening | Ti-6Al-4V, 316L and CoCr harden at the cut | Rapid flank wear, built-up edge (BUE), drift |
| Long thin bars | Cannulas drilled in slender screw and nail blanks | Straightness governs wall thickness and burst strength |
| Cleanliness | Residual machining oil fails ISO 10993-5 tests | Cytotoxicity failure, rejected lot |
| Traceability | ISO 13485 device history record (DHR) | Every hole documented to the serialized part |
| Sterility | Debris and burrs trap bioburden and resist cleaning | Cleanability validation (ISO 19227) failure |
Cannulated (hollow) bone screws are passed over a guide wire to maintain trajectory in minimally invasive fracture fixation of the wrist, ankle, pelvis and scaphoid. Instrument systems pair guidewires (Ø1.0–1.9 mm) with graduated cannulated drills (Ø1.8, 2.0, 2.3, 2.9 and 3.6 mm), drill sleeves to protect soft tissue, depth gauges for screw length, and cannulated taps and drivers. The cannula in the screw itself is the deep hole — gundrilled into the blank before thread rolling.
Implant machining is restricted to specific ASTM/ISO grades with biocompatibility documentation (ISO 10993) and certificates of conformance. Every material lot is traceable to the finished part serial number.
| Material | Standard | Machinability | Typical Use | Cutting Notes |
|---|---|---|---|---|
| Ti-6Al-4V ELI (Grade 23) | ASTM F136 / ISO 5832-3 | Moderate–difficult | Cannulated screws, IM nails | Low speed, high-pressure coolant, razor edge |
| Ti-6Al-4V (Grade 5) | ASTM F1472 | Moderate–difficult | Instrument shafts, screws | Low speed, never underfeed (work hardening) |
| 316L / 316LVM | ASTM F138 | Difficult (~35%) | Instruments, short-term implants | Constant feed, no dwelling, passivation after |
| Co-28Cr-6Mo | ASTM F75 / F1537 | Very difficult | Joint and wear components | High-EP oil, low speed, tool monitoring |
| MP35N | ASTM F562 | Very difficult | High-strength bone screws | High-EP oil, conservative feeds, monitor torque |
| 17-4PH / 420B | — | Moderate | Surgical drivers, taps, housings | Oil or high-perf emulsion |
| Nitinol (NiTi) | ASTM F2063 | Very difficult | Cannulated tubing for delivery systems | Sharp carbide, frequent resharpening, straightness is critical |
The cannula is sized to the guide-wire diameter plus a running clearance that allows the wire to slide freely without slop, so trajectory control is preserved. Wall thickness around the cannula is a critical-to-quality dimension — it governs the screw’s burst strength and must stay concentric along the whole length.
Small screw cannulas, nitinol starting bores; high-RPM spindles, 5 μm coolant filtration
IM nail cannulas, instrument channels; single-lip, one-pass, guided entry
Non-round or pre-hardened bores; adds a recast layer that needs post-treatment
After gundrilling when Ra 0.2–0.4 μm and burr-free edges are specified
Micro gun-drilling machines built for implant production drill Ø1–6 mm (down to ~Ø0.3 mm in specialist machines) to depths up to ~300 mm. They run high-RPM motorized spindles (roughly 1,000–25,000 RPM), high-pressure coolant systems with fine filtration at ~5 μm purity, and tool monitoring with automated retract.
Implant cannula gundrilling runs conservative and coolant-rich. These are starting points — qualify every hole with a process study and tool manufacturer data before production.
| Operation | Material | Vc (m/min) | Feed (mm/rev) | Notes |
|---|---|---|---|---|
| Gundrill, Ø6–10 mm carbide | Ti-6Al-4V ELI | 15–30 | 0.02–0.09 | High-pressure coolant; razor edge; resharpen early |
| Micro gundrill, Ø1–3 mm | Ti-6Al-4V | 10–20 | 0.005–0.02 | 1,000–25,000 RPM spindle; peck for chip control |
| Gundrill, carbide | 316L | 30–50 | 0.03–0.10 | Constant feed, TSC; no dwelling (work hardening) |
| Gundrill, carbide | Co-28Cr-6Mo | 12–20 | 0.02–0.06 | High-EP oil; low speed; monitor torque |
| Gundrill, carbide | MP35N | 8–15 | 0.01–0.04 | Tool monitoring mandatory; conservative feeds |
| Gundrill, carbide | Nitinol | 5–10 | 0.005–0.015 | Frequent resharpening; straightness critical for drawing |
For implants, surface finish is a clinical and regulatory requirement, not a cosmetic one. It determines cleanability, osseointegration, corrosion resistance and sterilization compatibility.
| Surface | Ra Target | Method |
|---|---|---|
| Surgical instruments | ≤0.8 μm | As-machined or electropolished |
| Implant contact surfaces | ≤0.4 μm | Gundrill + finish pass |
| Articulating / bearing surfaces | 0.1–0.2 μm | Electropolishing (ECP) |
| CoCr articulating surfaces | ≤0.025 μm | Electrochemical polish, wear-particle control |
| Cannula bore | 0.2–0.8 μm | Gundrill, burr-free both ends |
Implant-grade mill cert with ASTM/ISO spec, chemical analysis, and ISO 10993 biocompatibility documentation for the lot.
Verify spindle alignment, coolant filtration (~5 μm) and cleanliness. Assembly/cleaning areas per ISO 14644-1 Class 7.
Inspect gundrill geometry and coating; serialize each tool with regrind history and life count.
Installation Qualification, Operational Qualification at feed extremes, Performance Qualification over a production batch with Cpk ≥1.33.
Multi-stage ultrasonic wash, cascade/DI rinse, conductivity monitoring; validate removal of oils, particles and bioburden.
Log every hole (parameters, inspection, tool serial); build the Device History Record for regulatory submission.
Reference class: micro gun-drilling machines purpose-built for implant production, e.g., Precihole tabletop units distributed by Absolute Machine Tools.
| Capability | Specification |
|---|---|
| Diameter range | Ø1–6 mm standard (Ø0.3 mm specialist); to ~Ø30 mm for IM nail cannulas |
| Spindle speed | 1,000–25,000 RPM motorized spindle |
| Drill depth | Up to ~300 mm (12 in) in one pass |
| Coolant | High-pressure TSC, filtered to ~5 μm to prevent particle embedding |
| Tool monitoring | Spindle torque / coolant pressure sensing with automated retract on threshold |
| Guidance | Entry guide bushings and whip guides for slender bars; optional counter-rotation for >100:1 straightness |
| Control | Peck cycles for micro-chip evacuation; SPC logging per hole |
| Parameter | Typical Tolerance | Measurement Method |
|---|---|---|
| Cannula diameter | ±0.02 mm | Air gauge, pin gauge, bore scope |
| Straightness | 0.03–0.10 mm/m | Laser bore alignment, straightness mandrel |
| Surface roughness | Ra 0.2–0.8 μm | Profilometer, surface comparator |
| Wall thickness / concentricity | ±0.03 mm | Ultrasonic, CMM, gauge pin |
| Burr / edge condition | Burr-free both ends | 10–40× bore scope, feeler check |
| Cleanliness | Residue-free per validated wash | Rinsate particle count, cleanliness cert |
Medical device quality management system
Biocompatibility evaluation
US QSR and European device regulation
Risk management
Implant cleanability validation
ASTM F136 / F138 / F75 / F562 / F2063
| Symptom | Cause | Fix |
|---|---|---|
| Bore wander / drift | Off-square entry, worn bushing | Face the bar end, replace bushing, use spot-faced entry |
| Tool breakage in bore | Chip packing, dull edge, feed too high | Reduce feed, resharpen early, torque monitoring with retract |
| Exit burr on cannula | Dwell at exit, dull edge, wrong feed | Correct speed/feed, controlled exit pass, deburr or electropolish |
| Poor finish / chatter | Runout, insufficient coolant | Verify holder ≤0.005 mm runout, raise pressure, add whip guide |
| Oversize hole entrance | Tool runout, off-center start | Check spindle/bushing runout, align guide bushing |
| Work-hardened hole floor | Dwelling or underfeeding | Hold constant feed, never stop mid-hole, re-sharpen edge |
| Cytotoxicity failure | Residual oil in the bore | Validated wash, low-residue cutting fluid, cleanroom handling |