Unqualified fastener product inspection and characteristics

Unless otherwise agreed, it is common practice to include only fasteners with substandard characteristics and different fasteners when estimating ppm values. See Figure A.1.

Figure A.1 Source of unqualified fastener products and methods to achieve the specified ppm values
The incorporation of foreign parts may be caused by the manufacturer's production chain or the production chain of other parts, or other reasons. Parts that are unacceptable in size, geometry, and other characteristics are produced by systemic or non-systematic process characteristics.
Non-systematic process characteristics, such as:
- the raw material has cracks;
- there are solder joints on the wire;
- the chip is stained with debris;
- parts with missing features, such as parts that lack threads or knurls;
- Pitch error:
- The inner wrenching part is too thick and the plating metal is too thick.
Process characteristics of the system, such as measurements and/or tolerances.
Non-system failures cannot be identified by statistical process control (SPC), and system failures can usually be limited by applying statistical process control. Based on statistical principles, the application of statistical process control does not guarantee delivery of unqualified fastener products. For example, if the Cpk value of a single feature is 1.33, it means that 99.994% of the products in this batch of fasteners meet the specifications for this particular property. If the process is symmetrically tolerant, then statistically speaking, every million fasteners may have 0 to 60 products that exceed the public limit.
A.4 inspection
A.4.1 General
In order to achieve the specified ppm value, the following inspection methods can be used. The purpose of the inspection is to check the selected characteristics of the product and reject the non-conforming product.
A.4.2 Automatic inspection
Auto-checking can check single or multiple characteristics. Experience has shown that after an individual inspection, there are still about 10 non-conforming products per million products. During the automatic inspection process, four to five characteristics are usually checked at the same time. Therefore, for an average of one million products, An average of 50 non-conforming products are expected.
A.4.3 Manual inspection
After 100% manual inspection, the number of failures per million products may be 10 times or more of the unqualified data that is automatically checked.
A.4.4 Checking limitations
Because 100% inspection of certain features is not possible, estimating the ppm value of such characteristics for a product or delivery lot is unrealistic for the manufacturer. For example, some characteristics that require damage testing, such as corrosion resistance test, tensile strength test, or torque/tensile relationship test to measure friction coefficient. These characteristics are required to be properly monitored through a written control plan. The process is guaranteed.
If the specified characteristics are particularly important for their intended use, all parties involved should take precautions to ensure zero defects.
A.5 Example of establishing quality standards
Table A.1 lists some of the characteristics that may be included in the agreement between the manufacturer and the user at the specified ppm and/or Cpk values.
Table A.1 Characteristics that may be included

A.6 Limitations of estimating Cpk values
When there are foreign parts or hybrids in a batch of fasteners, it is unlikely to estimate the Cpk value because the characteristics are not statistically distributed at this time. In this case, it is only possible to estimate the value of foreign parts or hybrids per million products.