English: Drawing Requirements and Process Risk
This CNC case study considers an Inconel 718 turbine seal ring with a 420 mm outside diameter, a 390 mm bore and a finished wall close to 4.5 mm. The drawing calls for 0.03 mm bore roundness, 0.04 mm face-to-bore runout, two narrow sealing grooves and a 1.6 micrometre Ra finish on the functional diameter. These values describe a realistic process-planning example rather than a universal production specification.
Three problems dominate the plan. Inconel 718 retains strength at cutting temperature and tends to work-harden when the tool rubs. The thin ring releases residual stress as stock is removed. Finally, chuck pressure can make a round part appear acceptable in the machine but spring out of tolerance after unclamping. The process therefore needs balanced stock removal, low-friction cutting edges and inspection in a relaxed condition.
Datum and Workholding Plan
Machine a sacrificial locating face and pilot bore while the blank is still rigid. For the second setup, use a segmented expanding mandrel that supports most of the bore circumference. Expansion force should be only high enough to resist the predicted tangential cutting load. A face stop establishes axial location, while three equally spaced confirmation points help detect chips or burrs before clamping.
Rough both faces and diameters in alternating steps, leaving 0.7 to 1.0 mm radial stock and about 0.4 mm per face. Remove the ring, allow it to stabilize, and check free-state roundness before semi-finishing. This intermediate measurement shows whether the process needs a stress-relief allowance adjustment before expensive finishing operations.
Tool Selection
| Operation | Suggested tool | Purpose |
|---|---|---|
| OD and face roughing | SiAlON ceramic round insert | Stable high-temperature material removal on an uninterrupted cut |
| Bore semi-finishing | Positive PVD-coated carbide insert | Lower radial force and reduced wall deflection |
| Finish turning | Sharp ground carbide, 0.4 mm nose radius | Control size and surface without burnishing |
| Seal grooves | Rigid precision grooving blade with through coolant | Control groove width, root radius and chip evacuation |
| Edge break | Small carbide profiling tool | Create consistent drawing-defined chamfers |
Starting Cutting Parameters
For a stable ceramic roughing cut, a starting range of 180-240 m/min, 0.15-0.25 mm/rev and 0.8-1.5 mm depth of cut can be evaluated. Ceramic tools should not dwell, and coolant use must follow the insert supplier’s thermal-shock guidance. For positive-carbide semi-finishing, begin near 35-50 m/min and 0.08-0.14 mm/rev. Finish turning may start around 30-45 m/min, 0.05-0.10 mm/rev and 0.15-0.30 mm depth of cut.
These figures are commissioning ranges, not guaranteed production values. Adjust them for machine rigidity, insert grade, heat-treatment condition, engagement and coolant delivery. A stable chip and predictable flank wear matter more than chasing maximum speed on the first part.
Operation Sequence and Quality Gates
- Verify blank condition, heat-treatment record and stock distribution.
- Rough the first face, pilot bore and outside diameter.
- Reverse on soft jaws; balance the opposite face and diameter.
- Stabilize unclamped and record free-state roundness.
- Mount on the expanding mandrel and semi-finish bore, OD and faces.
- Finish the datum bore and functional diameter without an intervening unclamp.
- Cut seal grooves last with controlled infeed and uninterrupted coolant.
- Deburr under magnification, clean and inspect after release from the fixture.
Use an air gauge or bore gauge for diameter, a roundness tester for harmonic error, and a height or form system for groove location. Record measurements both on the mandrel and after release. The difference is valuable process data: it separates true cutting error from elastic clamping distortion.
For alloy-first comparison across pockets, holes, seats, grooves and thin sections, use the nickel-alloy CNC tooling decision hub.
