Same part.
Same tolerance.
Process control.
Designing for repeatable production requires statistical process control mindset. Tolerance budgets, capability indices, fixturing, inspection plans.
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
What is Cpk?
Process capability index. Measures how well process meets tolerance. Cpk > 1.33 = capable.
Worst case vs RSS?
Worst case: assumes all dimensions at worst limit. RSS: statistical (1/√n reduction). RSS more realistic for production.
First article reflects production?
First article shows process capability. Cpk requires N=30+ parts to validate.
Drift over time?
Tool wear, temperature changes, material variations. SPC control charts detect drift before defect.
Customer-specific Cpk?
Aerospace often demands Cpk > 1.67. Automotive Cpk > 1.33. Consumer products typically Cpk > 1.0.
When does it not matter?
Prototype or one-off parts: Cpk not relevant. For production parts > 100 units, statistical analysis valuable.
Get an instant quote
Send your CAD — we reply with detailed pricing, lead time, and DFM feedback within 4 working hours.
Start quoteTalk to an engineer
WhatsApp our team directly. Most messages answered within 12 minutes during work hours.
Open WhatsAppExplore all services
CNC, 3D printing, injection molding, sheet metal, casting, finishing — one quality system, one partner.
See all services