ZAMAK 3 Pearl Chrome Die Casting Case Study: Assembly Step Dimension Analysis and Tolerance Review
ZAMAK 3 Pearl Chrome Die Casting Case Study: Assembly Step Dimension Analysis and Tolerance Review: material behavior, mold control, plating risk, dimensional verification,.
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ZAMAK 3 Pearl Chrome Die Casting Case Study: Assembly Step Dimension Analysis and Tolerance Review: material behavior, mold control, plating risk, dimensional verification,.
Case Background
| Item | Engineering Record |
|---|---|
| Material and process | ZAMAK 3 zinc alloy die casting with hexavalent pearl chrome plating. |
| Production route | Die casting, degating, deburring, drilling and tapping, grinding and polishing, pearl chrome plating, threaded insert assembly, inspection and packing. |
| Quality concern | Dimensional concern suspected to cause assembly appearance issues, especially step protrusion after fitting with the mating plastic part. |
| In-process lot | 3500 pcs were checked by fitting inspection; after fitting, the assembly step protrusion was confirmed acceptable. |
| Dimensional sample | 200 pcs were randomly selected from this batch for measurement of two key dimensions. |
Inspection Logic
Measurement Summary
| Dimension | Current specification | Current lower limit | Current upper limit | Sample minimum | Sample maximum | Result |
|---|---|---|---|---|---|---|
| 14.3±0.1 | 14.20 to 14.40 | 14.20 | 14.40 | 14.26 | 14.75 | Exceeds upper specification limit. |
| 37.34+0.15/-0.1 | 37.24 to 37.49 | 37.24 | 37.49 | 37.35 | 37.65 | Exceeds upper specification limit. |
Assembly Result
Although both measured dimensions exceed the current drawing upper limits under the present manufacturing route, the fitting inspection showed that the assembly step protrusion was acceptable after adaptation. Based on this batch evidence, the step appearance result did not show a significant correlation with these two measured dimensions alone. This finding applies to step protrusion appearance in that production batch and should not be generalized to the separate loose-fit response evaluated in the follow-up DOE.
Engineering Analysis
- The two metal-part dimensions are out of the current drawing limits, so the drawing and process capability cannot be judged only by nominal conformance.
- The accepted assembly step result indicates that the actual appearance risk is likely controlled by the full assembly stack-up, not by either dimension alone.
- Based on the dimensional distribution and the fitting result, the mating plastic part should be checked for shrinkage, because plastic shrinkage may shift the actual assembly relationship.
- A metal-part-only rejection rule could create unnecessary sorting if the final assembly function and appearance are still acceptable.
Follow-up DOE: 37.50, 37.55 and 37.60
A follow-up full inspection compared three nominal dimension levels against loose-fit results. Material defects and plating defects were recorded as separate dispositions so that assembly-fit behavior could be evaluated on both the total-inspection basis and the effective-fit basis.
Full inspection record
| Dimension level | Fit OK | Loose fit | Material defect | Plating defect | Total | Loose / total |
|---|---|---|---|---|---|---|
| 37.50 | 21 | 30 | 4 | 1 | 56 | 53.57% |
| 37.55 | 29 | 2 | 5 | 6 | 42 | 4.76% |
| 37.60 | 24 | 17 | 1 | 8 | 50 | 34.00% |
Effective fit comparison
Because material and plating defects do not provide an independent fit judgment, the primary comparison uses only parts classified as fit OK or loose fit.
| Dimension level | Fit OK | Loose fit | Effective fit sample | Loose-fit rate |
|---|---|---|---|---|
| 37.50 | 21 | 30 | 51 | 58.82% |
| 37.55 | 29 | 2 | 31 | 6.45% |
| 37.60 | 24 | 17 | 41 | 41.46% |
Statistical result: Pearson chi-square = 22.13, p = 0.0000156 and Cramer’s V = 0.424. Under these sampled conditions, dimension level and loose-fit result show a statistically significant, medium-to-strong association.
DOE Interpretation and Decision Boundary
- 37.55 produced the lowest effective loose-fit rate at 6.45%, compared with 58.82% at 37.50 and 41.46% at 37.60.
- The odds of a loose fit were approximately 20.7 times higher at 37.50 than at 37.55, and approximately 10.3 times higher at 37.60 than at 37.55.
- The difference between 37.50 and 37.60 is not conclusive on the effective-fit basis because its 95% confidence interval includes 1.
- The response is non-linear: the middle level performed best, so the data do not support a simple rule that increasing or decreasing the dimension continuously increases looseness.
- Combined material and plating defect rates also varied by group: 8.93% at 37.50, 26.19% at 37.55 and 18.00% at 37.60. This variation indicates possible batch, cavity, polishing or plating confounding.
Engineering conclusion: the current data support an association between the tested dimension levels and loose-fit outcome, but they do not prove that dimension alone caused the difference. The 37.55 level is the best candidate among the three tested settings and should be treated as a validation center point, not yet as a released production tolerance.
Recommended Confirmation DOE
Analyze the confirmation run with logistic regression including linear and quadratic dimension terms, with lot or cavity used as blocking factors. This will test whether the apparent middle optimum is repeatable and separate dimension influence from process variation.
Tolerance Review Direction
| Dimension | Current tolerance | Proposed review tolerance | Engineering note |
|---|---|---|---|
| 14.3 | 14.3±0.1 | 14.3+0.25/-0.1 | This raises the upper limit to 14.55, but the observed sample maximum is 14.75. Outliers and full distribution still require review. |
| 37.34 | 37.34+0.15/-0.1 | 37.34+0.25/-0.1 | This raises the upper limit to 37.59, but the observed sample maximum is 37.65. Additional validation is needed before formal drawing release. |
The proposed tolerance change should be treated as a drawing-review direction, not as a final approval by itself. Before release, XSD recommends confirming plastic-part dimensions, assembly stack-up, step protrusion gauge criteria, and Cpk/Ppk or batch-distribution evidence.
Improvement Actions
- Measure the mating plastic part and confirm whether shrinkage is present across different cavities, batches or suppliers.
- Create an assembly stack-up record connecting metal dimensions, plastic dimensions and accepted step protrusion results.
- Use a dedicated fitting gauge or visual limit sample for the step protrusion acceptance decision.
- Separate true dimensional outliers from tolerance limits that may be too narrow for the functional assembly requirement.
- Only revise the drawing tolerance after sample distribution, assembly validation and customer acceptance criteria are aligned.
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