Key principles.
Cpk > 1.33
Typical CNC Cpk > 1.33 for designed-tolerance features. Above ±0.025 mm achievable consistently.
Statistical not max
Use statistical (RSS) tolerance analysis at production volumes. Worst case waste tolerance.
Critical
Fixtures locate parts identically. Repeatable < 0.01 mm typical for production fixtures.
Per AQL
Sample inspection per AQL standards (Mil-Std-105). Statistical confidence in process.
Spec material
Specify material per AMS, ASTM. Different mill heats vary. Lot-to-lot variation.
Procedures
Document processes precisely. Same operator, same fixtures, same inspection = same parts.
FAQ
Process capability index. Measures how well process meets tolerance. Cpk > 1.33 = capable.
Worst case: assumes all dimensions at worst limit. RSS: statistical (1/√n reduction). RSS more realistic for production.
First article shows process capability. Cpk requires N=30+ parts to validate.
Tool wear, temperature changes, material variations. SPC control charts detect drift before defect.
Aerospace often demands Cpk > 1.67. Automotive Cpk > 1.33. Consumer products typically Cpk > 1.0.
Prototype or one-off parts: Cpk not relevant. For production parts > 100 units, statistical analysis valuable.
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