When we produce a product in a larger quantity, it is important that each art has reproducible features and properties, i.e. speaking a little more generally, it maintains a constant level of quality. Maintaining the set production quality standards allows to determine, first of all, the useful life of manufactured products and the conditions of their use. Of course, all this for one purpose, to get the full satisfaction of the recipients. If the products have low repeatability, the specified conditions for many of them will be overestimated, which will lead to too fast wear. One of the aspects of maintaining the quality is to maintain an appropriate tolerance of the product parameters, and therefore controlled accuracy, to facilitate this process and to standardize it. Most of such parameters are divided into classes of accuracy, often defined in relevant standards. This is where the issue we are talking about in this article comes from.
Accuracy class – what is it?
Usually they depend on the parameter they concern, otherwise they are determined for electric or electronic elements, and slightly different for the dimensions of machine parts. For the most part, the class of the accuracy of a given parameter specifies the acceptable difference between the assumed and the actual value of the parameter. Usually, this difference is recognized as a percentage or in a unit in which the value is measured and assigned to the appropriate class according to a specific standard.
The standards are established by institutions such as the Polish Committee for Standardization (PN – Polish Standards), German Deutsches Institut für Normung (DIN) or international ISO standards. The standards indicated 18 accuracy classes:
- Defining acceptable shape and position errors, the class with the highest precision is IT01, then IT0. These classes, according to which the accuracy of execution must be extremely high, is used only in very specific scientific applications.
- Next are classes from IT1 to IT4, which are applied to measuring devices and reference elements, as well as to less specific devices for scientific purposes.
- The IT5 to IT11 classes are commonly used in industry and manufacturing. For example, IT5 to IT7 includes most of the mechanical components of motor vehicles, machine tools and even aircraft. The IT8 to IT11 classes are used in heavy equipment and commercial machines, as well as elements of the aforementioned cars and aircraft that are less important for the construction or safety of the vehicles.
- The classes from IT12 to IT18 are called roughing tolerance and are used in workshop practice, manual processing, makeshift structures and other simple structures not subject to high loads. Similar standards also exist for other parameters.
Practical application of accuracy classes
Accuracy classes support efficient production. The fact that they are defined when designing a given product means in practice that one should adjust the class to the assumed final parameters of the device and its assumed price. High accuracy classes are used where it affects the safety, durability and durability of the solution. This is due to the fact that the execution of such elements is more difficult, requires better machines, machine tools or materials, takes more time, and at the quality control stage generates more losses. Therefore, it is necessary at the design stage to match the accuracy class to the devices and subcontractors’ capabilities owned by the production plants, as well as to the purchase price of ready-made elements (eg electronic components).
The second place where we will operate the accuracy classes in the manufacture of the product is the quality control department, which will determine whether the products manufactured have a constant, repeatable and within the scope of parameter tolerance assumed by the designer or customer. In the case of production of subassemblies or components, those with a lower tolerance class can go to cheaper and less demanding devices, and those manufactured with greater accuracy to the final products from higher price levels.
The use of accuracy classes at the stage of both design, production and quality control allows maintaining high quality of production. Additional benefits include facilitating communication between different production departments (using accuracy classes, mistakes can be avoided eg on the line between the constructor and the technologist) and rational production planning.
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