The Limits of High-Speed CNC Machining
Mapping thermal distortion vs. feed rates in Inconel 718 under heavy roughing conditions.
CAM software will allow you to program aggressive feed rates in superalloys. Tooling manufacturers will claim their coatings can withstand the heat. But when you are running a 5-axis trunnion cutting Inconel 718, the physical reality of thermal expansion dictates the true limit.
We instrumented an ISO 10791 test block and ran 140 hours of roughing cycles to measure exactly when dynamic tool deflection and thermal distortion caused parts to fail a ±.0005" tolerance threshold.
Test Parameters
- Material: Inconel 718 (AMS 5662)
- Machine: 5-Axis VMC, HSK-63A spindle, 20,000 RPM max
- Tooling: 1/2" 5-Flute TiAlN coated solid carbide end mill
- Coolant: High-pressure through-spindle (1,000 PSI), 10% concentration
Key Findings: The Thermal Knee
There is a non-linear relationship between Surface Footage (SFM) and thermal transfer into the part. Below 120 SFM, the heat is carried away by the chip. At 145 SFM, we observed a rapid inflection point.
| Cutting Speed (SFM) | Tool Life (Minutes) | Part Temp (Delta T °F) | Observed Deflection (in) |
|---|---|---|---|
| 90 | 42 | +12°F | .0002" |
| 120 | 28 | +18°F | .0004" |
| 145 (The Knee) | 11 | +45°F | .0011" (Failed Tol) |
Conclusion: When roughing superalloys, pushing feed rates past the thermal knee destroys not just the tool, but the dimensional integrity of the part. If your scrap rate is mysteriously high on tight-tolerance aerospace parts, drop your SFM by 15% and measure the thermal delta.
Quantify Your Scrap
Are aggressive feeds actually saving you money if they increase your scrap rate by 5%? Run the math.