Correctly specifying tolerances on a MIM part drawing is one of the most effective ways to control cost and quality. Over-tolerancing drives up scrap and inspection cost; under-tolerancing risks functional failure.
General MIM tolerance capabilities by feature type:| Feature Type | As-Sintered Tolerance (mm) | With Coining (mm) | Drawing Note Example |
|---|---|---|---|
| Linear dimension (parting line direction) | ±0.05-0.15 | ±0.03-0.08 | 20.00 ±0.10 |
| Linear dimension (mold-closing direction) | ±0.08-0.20 | ±0.05-0.10 | 10.00 ±0.15 |
| Hole diameter (molded core pin) | ±0.05-0.12 | ±0.03-0.06 | Ø3.00 ±0.08 |
| Center-to-center pitch | ±0.08-0.20 | ±0.05-0.10 | 15.00 ±0.15 |
| Wall thickness | ±0.05-0.10 | ±0.03-0.06 | 1.00 ±0.08 |
| Flatness (per 25 mm) | ±0.08-0.15 | ±0.03-0.08 | Flatness 0.10 |
| Roundness | ±0.05-0.12 | ±0.03-0.06 | Circularity 0.08 |
| Angularity | ±0.5-1.0° | ±0.3-0.5° | 90° ±0.5° |
Add these notes to every MIM part drawing:
| Note | Purpose |
|---|---|
| 1. "All tolerances ±0.3 mm unless otherwise specified" | Sets the default tolerance for all dimensions not individually toleranced |
| 2. "Gate vestige not to exceed 0.15 mm above surrounding surface" | Specifies acceptable gate mark height |
| 3. "Parting line flash not to exceed 0.10 mm" | Specifies maximum flash at parting line |
| 4. "Draft angles: 0.5° minimum on cavity surfaces, 1.0° on core surfaces" | Ensures mold release |
| 5. "Critical characteristics (CC) identified by symbol — CpK ≥ 1.33" | Identifies which dimensions need capability study |
| 6. "Material: MIM-316L per MPIF Standard 35" | Specifies the material standard |
Assign each dimension to one of three categories on the drawing:
| Category | Symbol | CpK Required | Inspection Frequency | Typical Examples |
|---|---|---|---|---|
| Critical (CC) | ⚠️ | ≥ 1.67 | 100% or CpK-based sampling | Press-fit diameters, sealing surfaces, mating alignment |
| Significant (SC) | ▲ | ≥ 1.33 | Regular sampling | Mounting hole pitch, clearance fits, wall thickness |
| General | — | ≥ 1.00 | Audit only | Overall length, non-functional radii, draft angles |
Use GD&T per ASME Y14.5. Add default notes for gate vestige, flash, and draft angle limits. Classify dimensions as critical (CpK ≥ 1.67), significant (CpK ≥ 1.33), or general. Limit critical dimensions to 1-3 features to control inspection cost — assigning every dimension as critical unnecessarily increases per-part cost by 30-50%.