Product inspection and process characteristics of unqualified fasteners

Unless otherwise agreed, the usual practice is to include only fasteners with unqualified characteristics and the mixing of different fasteners when estimating the ppm value.
The mixing of foreign parts and miscellaneous products can be caused by the manufacturer's production chain or the production chain of other parts, or other reasons. Parts that fail in size, geometry, and other characteristics are produced by the process characteristics of the same system or non-system.
Non-systematic process characteristics, such as:
There are cracks in the raw materials;
There are solder joints on the wire;
There are crumbs on the extrusion die;
Parts with missing features, such as missing threads or knurled parts;
Pitch error:
The plated metal of the inner wrench is too thick, etc.
The process characteristics of the system, such as measurement and/or tolerance.
Non-system failures cannot be distinguished by statistical process control (SPC), while system failures can usually be restricted by the application of statistical process control. Based on statistical principles, the application of statistical process control cannot guarantee that there will be no unqualified fastener products in the delivery. For example, if the Cpk value of a single characteristic is 1.33, it means that 99.994% of the products in this batch of fasteners meet the specifications for this particular attribute. If this process is symmetrical tolerance, then statistically speaking, 0-60 products per million fasteners may exceed the common limit.
A.4 Inspection
A.4.1 General Rules
In order to achieve the specified ppm value, the following inspection methods can be used. The purpose of the inspection is to inspect the selected characteristics of the product and eliminate non-conforming products.
A.4.2 Automatic check
Automatic check can check single or multiple features. Experience has shown that after a single feature is automatically inspected, there are still about 10 unqualified products per million products. During the automatic inspection process, four to five features are usually checked at the same time. Therefore, on average, per million products, It is expected that there will be an average of 50 nonconforming products.
A.4.3 Manual inspection
After 100% manual inspection, the number of unqualified products per million pieces may be 10 times or higher than the unqualified data after automatic inspection.
A.4.4 Inspection limitations
Because it is impossible to check 100% of certain characteristics, it is unrealistic for manufacturers to estimate the ppm value of such characteristics of a certain product or delivery batch. For example, certain characteristics that need to be inspected by destruction test, such as corrosion resistance test, tensile strength test, or torque/tensile relationship test for measuring friction coefficient, etc. These characteristics require a written control plan for proper monitoring Process to ensure.
If the specified characteristics are particularly important for their intended use, all interested parties should take prudent measures to ensure zero defects as much as possible.
A.5 Examples of characteristics for establishing quality standards
Table A.1 lists some characteristics that may be included when the manufacturer and the user reach an agreement under the specified ppm value and/or Cpk value.
A.6 Limitations of Cpk Estimation
When there are foreign parts or miscellaneous products in a batch of fasteners, because the characteristics are not distributed according to the statistical law at this time, it is impossible to estimate the Cpk value. In this case, it is only possible to estimate the value of foreign parts or miscellaneous products per million products.