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- A.1.1 Status of mass production
With the increasing customer demand for fastener delivery quality, manufacturers are beginning to consider special elements in mass production. Modern fastener manufacturing machines (eg, cold heading machines, thread rolling machines) can produce between 60 and 800 pieces per minute depending on the type of machine, the size of the machine and the cycle of the machine used. A fastener manufacturer can produce millions of workpieces per day. Therefore, special measures are required to ensure process capability. In addition, during the manufacturing process, the workpiece is loaded into the container multiple times and transported to each processing point. The number of shipments varies with the length of the production chain (eg cold heading, rolling, cleaning, quenching and tempering, surface treatment, corrosion protection). Depending on the processing, etc., more than 20 times is also normal. Special measures are required during transportation to prevent mechanical damage (especially sharp corner collisions) and to avoid mixing.
Under the current state of production technology, the quality goal of zero defects is impossible to achieve. In order to achieve the goal of zero defects, a transitional goal was established by specifying criteria for evaluating the quality of delivered fasteners. The conditions set by this standard are based on the number of non-conforming products per million products and/or the process capability value (Cpk) of the specified characteristics.
A.1.2 Product Process Impact
The measurement and tolerance requirements for certain characteristics are based on the product process used (cold forming process, metal cutting process, heat treatment process). Here are some examples:
In the fastener manufacturing process, many production processes are performed by cold forming. The stable retention of tolerances depends not only on the materials used, but also on many other factors; for example, the contours that are freely formed in the opening and closing molds are not as accurate as the contours that are pressed into the closure. Some cold forming processes inevitably have sharp corners. In this case, it is often not possible to give clear and defined tolerances or limits.
In threaded fastener products, threading methods can affect thread geometry, tolerances, surface roughness, folding on threads, etc., which can vary greatly depending on the method of threading.
For screws with a large ratio of length to diameter, the possibility of bending increases (especially when the final process includes heat treatment quenching and tempering). For individual parts and machining methods, the corresponding tolerances should be considered.
When the workpiece is subjected to an anticorrosive coating treatment, uneven thickness of the plating layer is inevitably caused. This is related to the size of the workpiece and the method of coating. Uneven thickness may affect the corrosion resistance of the workpiece in the salt spray test. In this case, depending on the type and geometry of the fastener, its corrosion resistance is largely affected by the surface condition of the fastener. It is often impractical to insist on a salt spray test before delivery, as the test time often exceeds hundreds of hours. To this end, it is necessary to adopt appropriate real-time control of production management methods to ensure corrosion resistance. It is not possible to take a measure of the ppm value for the quality of this property. This also applies to the application of lubricating oil to fasteners to comply with the specified coefficient of friction. Maintenance like this range (eg μtot = 0.08 to 0.14) can only be guaranteed by a suitable real-time controlled production management method, which is checked by random inspection.
These examples illustrate the need for close collaboration between manufacturers, distributors, and users.
A.2 Problems with changing packaging
Changing the packaging increases the risk of mixing with miscellaneous items or other parts and causing mechanical damage.
A.3 Sources of non-conformities and methods for meeting the specified ppm values
There are three sources of non-conformities:
- fasteners with unqualified characteristics;
- have other parts or miscellaneous items (not identical fasteners);
- Non-fasteners (metallic or non-metallic) mixed in.
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