tool die design metal stamping


Tool & Die Making
Bend Radius
Deep Drawn
Fine Blanking
Progression strip layout

Slug pulling prevention
Tool die Design
Tool die design book

Jig and Fixture
Jigs and Fixture Design

Materials treatment
Annealing metal
Case hardening
Electrical Conductivity
heat treatment process
heat treating steel

Thermal Expansion

Alu comparison table
Aluminum Tempering
Carbon Steel
Copper alloy
Low alloys
Material nations & Equivalents
Nickel alloys

Stainless Steel

Stainless Steel comparison table
Tool Steel equivalent
Tungsten Carbide
Wrought stainless Steel

Conversion Charts
Drill Angle
Drill size
Energy Efficiency
Math Area
Machining Cutting Time
Technical articles
Work Done

Geometric Tolerancing
Tolerance chart ISO
Surface Roughness
Surface Texture

Quality Control
Process Capability CPK
Sampling plan
Level I, II, III

Engineering Plastic
Machining of plastic
Selection of plastic


 Process Capability study through CP and CPk measurement.

Process capability and process performance is through the study of the statistical measurements of Cp and Cpk.

Definitions of Cp and cpk

Cp = Represent Process Capability. An indicator to the capability of a particular process.
Cpk = Represent the index of the Process Capability study. A tool to measure the Cp value for the effect of non-centered distribution.

the study of CP

A common Process Capability measure, Cp indicates how well the process distribution fits within its specification limits ( Upper and Lower limit ). In the examples below, processes (a) has Cp greater than one, capable for mass production (b) is equal to one and (c) is less than one. Higher chances to fall out of tolerance.

process capability

The problem with Cp is that it does not measures how well the process distribution is, the process may have high Cp but distributed off the center of its limits, which can result in many rejects. The solution to this is the study of it Cpk.

The study of Cpk

Cpk study is to measures where the Cp value distributed between the upper and lower limits. Cpk only considers the variation half that is closest to the specification limits, as in the figure below.


Cp and Cpk have to be studied together to give a measure of both the potential and centering of the process distribution within the specification limits.

The larger the index, the more stable the process is, the less likely that any item will be outside the specification. CPK value of 1.33 is the industry standard of minimum acceptable level.


CPK Value
Inspection and action needed
Less than 1
100 % inspection may be needed for the production batch. Process improvement is highly recommended.
Between 1 to 3
Normal sampling plan is needed. Process is stable enough for mass production.
Greater than 3
Very capable
No inspection is required, process is very stable.