Appearance
A medical device manufacturer drills 4 mm diameter holes at L/D 20:1 in wrought CoCrMo alloy (ASTM F1537, 42 HRC) for femoral stem fixation channels. A solid carbide gun drill at 25 m/min and 0.012 mm/rev with 60 bar coolant delivers only 4 holes per edge — rapid flank wear and micro-chipping cause failure. Scrap from oversize bores is 12%. After switching to PVD AlTiN-coated carbide with 140° point angle, reducing speed to 18 m/min, raising coolant to 120 bar, and implementing a 1 mm peck cycle with 0.3 s dwell, tool life reaches 28 holes per edge (7× improvement) with 2% scrap.
Material Characteristics
Cobalt chrome (CoCr) alloys are among the most difficult materials to deep hole drill. Their combination of high hardness, low thermal conductivity, and high abrasiveness creates extreme conditions at the cutting edge.
Common Implant Grades
| Standard | Form | Hardness | Tensile Strength | Typical Use |
|---|---|---|---|---|
| ASTM F75 | Cast Co-28Cr-6Mo | 30–35 HRC | 700 MPa | Hip and knee bearing surfaces |
| ASTM F799 | Wrought Co-28Cr-6Mo | 35–40 HRC | 1,000 MPa | Hip stems, revision components |
| ASTM F1537 | Wrought Co-28Cr-6Mo (3 grades) | 35–45 HRC | 1,000–1,300 MPa | Femoral stems, spinal implants |
| ASTM F90 (L605) | Wrought Co-20Cr-15W-10Ni | 30–40 HRC | 1,000 MPa | Stents, surgical instruments |
| ISO 5832-12 | Wrought CoCrMo | 38–44 HRC | 1,100 MPa | Orthopaedic implants |
Machinability Compared to Other Materials
| Material | Relative Machinability (%) | Cutting Speed Range (m/min) |
|---|---|---|
| Low-carbon steel (1212) | 100 | 80–150 |
| 316L stainless steel | 45 | 40–80 |
| Ti-6Al-4V | 30 | 30–60 |
| CoCrMo (ASTM F75) | 10–19 | 15–35 |
| CoCrMo (ASTM F1537, 42 HRC) | 6–12 | 12–25 |
CoCrMo has one of the lowest machinability ratings of any common implant material.
Drilling Challenges
| Challenge | Cause | Effect |
|---|---|---|
| Extreme heat at cutting edge | Thermal conductivity 13–15 W/m·K (1/5 of steel) | Rapid crater wear, edge softening |
| Abrasive carbides | Chromium carbides in microstructure | Flank wear 3–5× faster than in titanium |
| Work hardening | Strain-induced martensite formation | Notch wear, difficult re-entry after peck |
| High cutting forces | 1,300 MPa tensile at 42 HRC | Tool deflection, hole straightness issues |
| Stringy chips | Ductile cobalt binder phase | Chip jamming in small-diameter flutes |
Gun Drilling Parameters
Speed and Feed
| Grade | Vc (m/min) | Feed (mm/rev) | Coolant Pressure | Expected Tool Life |
|---|---|---|---|---|
| ASTM F75 (cast) | 20–35 | 0.010–0.030 | 80–140 bar | 8–20 holes per edge |
| ASTM F799 (wrought) | 18–28 | 0.008–0.025 | 100–160 bar | 6–15 holes per edge |
| ASTM F1537 Grade 2 (38 HRC) | 16–25 | 0.008–0.022 | 100–160 bar | 6–15 holes per edge |
| ASTM F1537 Grade 3 (42 HRC) | 14–22 | 0.006–0.020 | 120–180 bar | 4–12 holes per edge |
Feed by Diameter (ASTM F1537, 38–42 HRC)
| Drill Diameter (mm) | Feed Range (mm/rev) | Speed at 18 m/min (RPM) |
|---|---|---|
| 2–3 | 0.004–0.012 | 1,910–2,860 |
| 4–5 | 0.006–0.018 | 1,150–1,430 |
| 6–8 | 0.008–0.022 | 720–960 |
| 10–12 | 0.010–0.025 | 480–570 |
Tip: In CoCr, the feed must be high enough to avoid work hardening but low enough to limit tool loading. The optimal feed range is narrower than for most materials — a change of 0.005 mm/rev can make the difference between acceptable tool life and rapid failure. Start at the middle of the feed range and adjust in 0.002 mm/rev increments.
BTA Drilling Parameters
BTA drilling is less common for CoCr medical implants because implant holes are typically small diameters (under 10 mm) where gun drilling is preferred. For larger CoCr components (e.g., trauma nails, spinal rods):
| Diameter (mm) | Vc (m/min) | Feed (mm/rev) | Coolant Pressure | Coolant Flow |
|---|---|---|---|---|
| 12–16 | 18–25 | 0.04–0.08 | 6–10 MPa | 60–90 L/min |
| 18–25 | 16–22 | 0.05–0.10 | 6–10 MPa | 90–140 L/min |
Tool Selection
Gun Drill Design
| Parameter | Recommendation | Reason |
|---|---|---|
| Carbide grade | Ultra-fine grain (0.2–0.5 µm) | Wear resistance at high temperature |
| Coating | PVD AlTiN or DLC | Thermal barrier, reduces adhesion |
| Point angle | 130–140° | Reduces thrust force, improves chip formation |
| Chip clearance | Larger than standard | Accommodates abrasive chips |
| Coolant hole | Standard single hole | Adequate for diameters above 2 mm |
Insert Grades (BTA)
| Requirement | Grade | Coating | Edge Prep |
|---|---|---|---|
| General CoCr | IC806, AH9130 | AlTiN PVD | T-land 0.05–0.08 mm |
| High hardness (40+ HRC) | CBN insert | None (PCBN) | Chamfer 0.10–0.15 mm |
| Maximum wear resistance | PCD-tipped | None (diamond) | Sharp |
Guide Pads
| Grade | Material | Clearance |
|---|---|---|
| Carbide pad | WC-Co with 6–10% Co | 0.005–0.008 mm per side |
| PCD pad | Polycrystalline diamond | 0.003–0.005 mm per side |
PCD-tipped guide pads are recommended for production CoCr drilling because carbide pads wear rapidly from the abrasive chip flow.
Coolant Requirements
Why High Pressure Is Essential
CoCr's low thermal conductivity means heat generated at the cutting edge cannot dissipate through the workpiece or chip — it must be carried away by the coolant. Insufficient coolant pressure leads to:
- Tool tip temperature exceeding 800 °C (softens the carbide edge)
- Rapid crater wear (diffusion mechanism activated above 700 °C)
- Chip welding to the cutting edge
- Catastrophic tool failure within 1–2 holes
Coolant Parameters
| Parameter | Recommendation |
|---|---|
| Coolant type | Neat oil with EP additives (preferred) or high-performance emulsion |
| Minimum pressure | 80 bar (1,160 psi) |
| Recommended pressure | 120–180 bar (1,740–2,610 psi) |
| Flow rate | 0.4–0.6 L/min per mm diameter |
| Filtration | 5 µm absolute, paper or cartridge filter |
| Temperature | Below 40 °C (monitor for heat buildup) |
| Oil viscosity | ISO VG 10–22 (low viscosity for chip evacuation) |
Warning: Standard flood coolant at 5–10 bar is completely inadequate for deep hole drilling of CoCr alloys. The heat generated cannot be removed, and the tool will fail within 1–3 holes. A minimum of 80 bar at the tool tip is required. For production applications, 120–180 bar is recommended. Verify pressure at the tool tip (not at the pump) using a pressure gauge in the tool holder.
Tool Life Expectations
Typical Tool Life
| Operation | Tool Life | Failure Mode |
|---|---|---|
| Gun drilling, ASTM F75 | 8–20 holes | Flank wear, micro-chipping |
| Gun drilling, ASTM F1537 Grade 2 | 6–15 holes | Flank wear, crater wear |
| Gun drilling, ASTM F1537 Grade 3 | 4–12 holes | Flank wear, edge chipping |
| BTA drilling, CoCr large diameters | 3–8 metres | Flank wear, notch wear |
| Reaming (after drilling) | 50–200 holes | Edge wear |
Wear Mechanisms
Research on CoCrMo machining (Tavares and Hassui, 2023) identified:
- Flank wear — the dominant mode, driven by abrasion from chromium carbides
- Crater wear — significant at cutting speeds above 30 m/min due to diffusion
- Adhesion — workpiece material builds up on the cutting edge, then breaks away, taking tool material with it
- Notch wear — at the depth-of-cut line from work hardening
- Micro-chipping — from mechanical fatigue at the cutting edge
Chip Breaking and Evacuation
CoCr produces hard, abrasive chips that are difficult to break:
| Factor | Effect | Strategy |
|---|---|---|
| Chip hardness | Similar to annealed steel (200–300 HV) | Must be flushed quickly, not allowed to recirculate |
| Chip shape | Curled but continuous at low feed | Use higher feed for chip breaking |
| Chip abrasiveness | Wears flutes and guide pads | Use coated tools, PCD pads |
Chip breaking strategy:
- Maintain feed above 0.008 mm/rev for chip breaking
- Use peck cycle with 0.5–1× diameter peck depth
- Apply 0.2–0.3 s dwell at bottom of each peck
- Ensure coolant flow is sufficient to clear chips on retract
Surface Quality Requirements
Medical implant drilling typically requires:
| Parameter | Typical Requirement | Achievable with Optimised Process |
|---|---|---|
| Surface roughness (Ra) | ≤0.8 µm | 0.4–0.6 µm |
| Diameter tolerance | IT8 (H8) | IT7–IT8 |
| Roundness | ≤0.01 mm | 0.005–0.010 mm |
| Burr height (entry/exit) | ≤0.05 mm | 0.02–0.05 mm |
Surface integrity is critical for medical implants — any surface defects (tearing, micro-cracks, white etching layer) can initiate fatigue failure in vivo.
Troubleshooting
| Problem | Likely Cause | Correction |
|---|---|---|
| Tool life under 5 holes | Speed too high or coolant pressure too low | Reduce speed 20%, increase coolant to 120 bar+ |
| Rapid flank wear | Abrasive carbides wearing edge | Switch to AlTiN or DLC coating, reduce speed |
| Edge chipping | Mechanical overload | Reduce feed, use tougher carbide grade |
| Oversize bore at entry | Tool deflection from high forces | Reduce feed, check guide bush condition |
| Rough surface finish | Built-up edge on tool | Increase speed 10%, check coolant flow |
| Chip jam in flute | Insufficient coolant pressure | Increase pressure, check coolant hole diameter |
| Tool breaks in hole | Chip packing causing torque spike | Reduce peck depth, increase dwell time |
| Burr at exit | Feed too high through breakthrough | Reduce feed in final 1 mm |
| Hole not straight | Workpiece not rigidly supported | Improve fixturing, reduce feed |
| Inconsistent tool life | Material batch variation | Verify hardness, adjust parameters per batch |
Process Recommendations
Starting Point for Process Development
- Select tool — PVD AlTiN-coated ultra-fine carbide gun drill, 140° point angle
- Set speed — 18 m/min for ASTM F1537 Grade 3; 22 m/min for F75
- Set feed — 0.012 mm/rev for 4 mm diameter; scale linearly with diameter
- Set coolant — 120 bar minimum, neat oil with EP additives
- Program peck cycle — 1–2 mm peck depth, 0.3 s bottom dwell, full retract every 5 mm
- Run trial — drill 3 holes, inspect tool wear and hole quality
- Adjust — if flank wear exceeds 0.10 mm after 3 holes, reduce speed or increase pressure
Incremental Optimisation
- Adjust speed in ±2 m/min steps
- Adjust feed in ±0.002 mm/rev steps
- Allow 3 holes per parameter set to confirm trend
- Document tool wear at each condition
FAQ
Why is cobalt chrome so difficult to deep hole drill?
CoCr combines high hardness (35–45 HRC), very low thermal conductivity (13–15 W/m·K), abrasive chromium carbides, and rapid work hardening — all factors that accelerate tool wear and challenge chip evacuation.
What cutting speed should I use for CoCr gun drilling?
Start at 14–22 m/min depending on the grade and hardness. ASTM F75 (cast): 20–35 m/min. ASTM F1537 Grade 3 (42 HRC): 14–22 m/min.
What feed rate is recommended for CoCr deep hole drilling?
0.006–0.025 mm/rev depending on diameter and grade. For a 4 mm hole in F1537: 0.008–0.018 mm/rev. The feed must be high enough to avoid work hardening but low enough to limit edge loading.
What coolant pressure is needed for CoCr deep hole drilling?
Minimum 80 bar, recommended 120–180 bar. Standard flood coolant at 5–10 bar will result in tool failure within 1–3 holes.
What tool coating works best for CoCr drilling?
PVD AlTiN (aluminium titanium nitride) provides the best thermal barrier and wear resistance. DLC (diamond-like carbon) coatings also perform well by reducing adhesion. Avoid TiN alone — it oxidises at the temperatures generated in CoCr drilling.
What point angle is best for CoCr gun drilling?
130–140°. Research on micro-drilling CoCrMo found that 140° produced the lowest forces and tool wear compared to 118° and 130°.
What tool life can I expect when gun drilling CoCr?
4–20 holes per edge depending on grade, hardness, and parameters. With optimised parameters (AlTiN coating, high coolant pressure, conservative speed), 15–30 holes per edge is achievable in wrought grades.
Can CoCr be BTA drilled?
Yes, but BTA drilling is practical only for diameters above 12 mm. Most medical implant holes are smaller (2–8 mm) and better suited to gun drilling.
How does CoCr compare to titanium for deep hole drilling?
CoCr is significantly more difficult. At equivalent hardness, tool life in CoCr is typically 20–30% of tool life in Ti-6Al-4V. Coolant pressure requirements are 2–3× higher.
What surface finish can I achieve in CoCr deep hole drilling?
With optimised parameters: Ra 0.4–0.6 µm. This meets the typical medical implant requirement of Ra ≤0.8 µm. Surface finish degrades rapidly as tool wear progresses — monitor hole quality every 2–3 holes.
Summary
Cobalt chrome alloy deep hole drilling is among the most challenging machining operations in medical device manufacturing:
- Material — CoCrMo (ASTM F75, F799, F1537) has 6–19% machinability relative to steel, with extreme abrasiveness and heat retention
- Gun drilling speeds — 14–35 m/min depending on grade and hardness; start conservatively
- Feed — 0.006–0.025 mm/rev; the optimal range is narrow and material-sensitive
- Coolant — 120–180 bar minimum; inadequate pressure causes immediate tool failure
- Tooling — PVD AlTiN or DLC-coated ultra-fine carbide, 130–140° point angle
- Tool life — 4–20 holes per edge in production; 7× improvement demonstrated through optimisation
- The medical device manufacturer in the opening scenario increased tool life from 4 to 28 holes per edge (7×) and reduced scrap from 12% to 2% by optimising coating, geometry, coolant pressure, and peck cycle parameters for ASTM F1537