Quick Q: Can MIM and CNC be combined on the same part?
Yes — this is a common and highly effective strategy. MIM produces the near-net shape (complex geometry, all features molded), then CNC machining adds precision features that MIM cannot hold (threads, tight bores, sealing surfaces, flatness-critical faces). The key is designing the MIM part with 0.2-0.5 mm stock on surfaces that will be CNC-machined.
When to combine MIM and CNC:
| Feature Type |
MIM Alone |
MIM + CNC |
Best Approach |
| General geometry |
✓ |
✓ |
MIM alone — no CNC needed |
| Threaded holes |
✗ (must post-tap) |
✓ |
MIM cored hole → CNC tap |
Tight bore (
| ✗ (CpK < 1.33) |
✓ |
MIM with 0.3 mm stock → CNC bore |
|
| Sealing surface (flatness < 0.02 mm) |
✗ |
✓ |
MIM with 0.3 mm stock → CNC face |
| Surface grinding (Ra < 0.4 µm) |
✗ |
✓ |
MIM → CNC grinding |
| High-precision alignment hole |
✗ |
✓ |
MIM → CNC ream or bore |
Stock allowance design rules:
| CNC Operation |
Stock Per Side (mm) |
Notes |
| Drilling (from cored hole) |
0.1-0.2 |
Cored hole guides the drill |
| Tapping (from cored hole) |
0.05-0.15 |
Per thread size table |
| Boring |
0.3-0.5 |
Must clear sintering decarb layer |
| Turning (OD) |
0.3-0.5 |
Must clear sintering surface |
| Facing / milling |
0.2-0.4 |
Minimize stock to reduce machining time |
| Surface grinding |
0.1-0.3 |
Minimum stock for flatness correction |
Cost comparison: MIM alone vs MIM + CNC:
| Scenario |
MIM Alone |
MIM + CNC (1 bore + 1 face) |
All-CNC from Bar |
| Per-part cost (10 g, 50k/yr) |
$0.80 |
$1.15 |
$6.50 |
| Additional operations |
None |
CNC: bore Ø5 H7 + face seal surface |
All features CNC |
| Tolerance on seal surface |
±0.10 mm (as-sintered) |
±0.01 mm (CNC) |
±0.01 mm |
| Tolerance on bore |
±0.05 mm |
±0.008 mm (H7) |
±0.008 mm |
| Total cost (3 yr, 150k parts) |
$120,000 |
$172,500 |
$975,000 |
Best value: MIM + minimal CNC = 82% savings vs all-CNC, 44% more than MIM alone but with precision that MIM alone cannot achieve.