I use the slowest speeds and highest coolant pressures for cobalt chrome drilling — and I still expect to change tools frequently. Cobalt chrome and other superalloys are among the most difficult materials to deep hole drill. They are hard, abrasive, and retain heat at the cutting edge because their thermal conductivity is a fraction of steel’s. I have broken more gun drills in cobalt chrome than in everything else combined, and every break taught me something about what this material demands.
The properties that make cobalt chrome ideal for medical implants and aerospace components — high wear resistance, corrosion resistance, and strength at temperature — are exactly what make it miserable to machine. Cobalt chrome sits at 40-50 HRC with abrasive carbide particles in its microstructure. The cutting edge wears by microchipping and abrasion, not the gradual flank wear I see in steel.
Cutting Parameters That Work
The cutting parameters for superalloys are extreme by normal standards. I run cutting speed at 15-25 m/min with a feed rate of 0.008-0.02 mm/rev. Coolant pressure needs to be 2000-3000 psi with AlTiN-coated carbide tools. The low speed is necessary because the material holds onto the heat generated at the cutting zone — 80-90% of the heat stays in the tool instead of going into the chip. That heat accumulation accelerates flank wear and edge breakdown.
Here is the parameter table I use when quoting new cobalt chrome and superalloy deep hole drilling jobs:
| Material | Hardness | Speed (m/min) | Feed (mm/rev) | Coolant Pressure | Tool Coating |
|---|---|---|---|---|---|
| Cobalt Chrome ASTM F75 | 40-50 HRC | 15-20 | 0.008-0.015 | 2500-3000 psi | AlTiN |
| Inconel 718 | 35-45 HRC | 12-18 | 0.01-0.02 | 2000-3000 psi | AlTiN or TiAlN |
| Hastelloy X | 35-40 HRC | 10-15 | 0.008-0.015 | 2000-2500 psi | AlTiN |
| Waspaloy | 38-44 HRC | 8-12 | 0.005-0.01 | 2500-3000 psi | AlTiN |
| Stellite 21 | 42-48 HRC | 12-18 | 0.008-0.015 | 2000-3000 psi | AlTiN |
| Rene 41 | 40-45 HRC | 10-15 | 0.005-0.012 | 2500-3000 psi | AlTiN or AlCrN |
| Haynes 25 | 42-50 HRC | 10-14 | 0.008-0.015 | 2500-3000 psi | AlTiN |
I set the spindle load monitoring to trigger an alarm at 120% of the baseline running load for a sharp tool. Superalloys wear tools fast, and the load spike from a worn tool is the best indicator I have found for when to change gun drill tips. The load increase is sharper in superalloys than in steel — a worn tool in cobalt chrome goes from normal to broken in about 30 seconds of cutting time.
Tool Coating Comparison for Superalloy Gun Drilling
The coating on the carbide tip makes the difference between 2 meters of drilling and 0.5 meters. Here is what I have found from testing coatings in production:
| Coating Type | Best For | Tool Life vs Uncoated | Max Temp Resistance | Cost Premium |
|---|---|---|---|---|
| AlTiN (Aluminum Titanium Nitride) | Cobalt chrome, Inconel, high-temp alloys | 2-3x | 900°C | Moderate |
| TiAlN (Titanium Aluminum Nitride) | Inconel, Hastelloy, general superalloys | 2-2.5x | 850°C | Moderate |
| AlCrN (Aluminum Chromium Nitride) | Stellite, abrasive superalloys | 2.5-3.5x | 1100°C | Higher |
| TiN (Titanium Nitride) | Mild steel only — not for superalloys | 1.2x | 600°C | Low |
| CVD Diamond | Not recommended for cobalt chrome (chemical reactivity) | — | — | High |
I standardize on AlTiN for my cobalt chrome gun drilling. The aluminum oxide layer that forms at high cutting temperatures acts as a thermal barrier, keeping the heat out of the carbide substrate. On Stellite 21, I switch to AlCrN because the higher aluminum content provides better oxidation resistance at the extreme temperatures generated in that material.
Tool Life Management
Tool life in cobalt chrome deep hole drilling is short but predictable. I get about 2-3 meters of drilling per edge before the gun drill tip needs replacement. I change tools on a set schedule rather than waiting for failure. The schedule is based on meters drilled, not parts completed, because the material removes the tool at a consistent rate per meter of cutting.
For a 300 mm deep hole in cobalt chrome, I change the drill after 8-10 holes. The last hole in that range shows visible edge wear but still produces a good bore with consistent surface finish and diameter. Hole 11 or 12 would start showing surface finish degradation and size drift outside tolerance. The cost of a replacement tip is far less than scrapping a $500 medical component.
I use a tool presetter to measure the drill OD before and after each run. A 0.03 mm reduction in OD means the tool is at end of life. I track the OD on a spreadsheet and plot the wear rate to predict when the next change is due. The wear rate in cobalt chrome is approximately linear — 0.003-0.005 mm per meter drilled — so I can forecast tool changes with reasonable accuracy.
Expected Tool Life by Superalloy
| Material | Meters per Edge | Holes per Tool (300mm depth) | Failure Mode |
|---|---|---|---|
| Cobalt Chrome ASTM F75 | 2-3 | 8-10 | Flank wear + microchipping |
| Inconel 718 | 3-5 | 10-16 | Notch wear at DOC line |
| Hastelloy X | 2-4 | 7-13 | Flank wear |
| Waspaloy | 1.5-2.5 | 5-8 | Edge chipping |
| Stellite 21 | 2-3 | 7-10 | Abrasive wear |
Chip Control Strategy
Chip control is the other major challenge in superalloy deep hole drilling. The chips are short and powdery at these low feeds. They need high coolant flow to clear the bore. I run the coolant at maximum flow and check the return line for blockages every 5 parts.
The chip shape in superalloys is different from steel. Instead of clean arcs, the chips look like crushed gravel — irregular fragments 0.5-2 mm across. These chips pack easily if the coolant velocity drops below 12 m/s in the return line. I have installed a flow meter on the return line to monitor the velocity and set an alarm at 10 m/s.
I also increase the peck frequency for superalloys. Instead of pecking every 20 mm, I peck every 10 mm. The more frequent pecks clear the chips before they have a chance to pack. The extra peck time adds about 15% to the cycle time but prevents drill breakage — a trade-off that pays for itself on the first hole that completes without issues. For more on chip control at these low feeds, see the chip management article.
Work Hardening Prevention
Cobalt chrome work-hardens aggressively if the tool dwells on the surface. I keep the tool advancing steadily with no pauses during the cut. Once the tool stops moving, the material at the cutting zone hardens by 20-30% in less than a second, and the next cut has to push through a harder layer that accelerates tool wear immediately.
I program a continuous feed motion with no G04 dwell commands in the deep hole drilling cycle. If the machine needs to pause for any reason, I retract the drill by at least 5 mm before stopping the feed. The retract pulls the tool out of the hardened zone so the re-entry is into fresh, unhardened material. This single practice cut my cobalt chrome tool breakage rate by half.
The same work hardening principle applies to spotting and pilot hole operations on these materials. A spot drill that dwells in cobalt chrome creates a hard spot that the gun drill must then penetrate. See the center drilling vs spot drilling guide for entry strategies that work on hard materials. Also check the cutting forces article for how the higher specific cutting force of superalloys affects thrust and torque requirements.
Key Takeaways
- Cutting speed of 15-25 m/min in cobalt chrome — anything above 30 m/min causes rapid edge failure within seconds.
- Coolant pressure above 2000 psi (138 bar) is mandatory for superalloy deep hole drilling — 80-90% of cutting heat stays in the tool.
- AlTiN coating is my standard for cobalt chrome gun drilling; AlCrN for Stellite and the most abrasive grades.
- Change tools on a set schedule based on meters drilled, not parts completed — 2-3 meters per edge in cobalt chrome.
- Peck every 10 mm instead of 20 mm in superalloys to prevent chip packing; the 15% cycle time penalty prevents breakage.
- Never dwell the tool in the cut — cobalt chrome work-hardens 20-30% in under a second on contact.
- Measure drill OD after every run and plot the wear rate; 0.03 mm OD reduction signals end of life.
- For Stellite grades, switch to AlCrN coating for better oxidation resistance at extreme cutting temperatures.