Precision casting accuracy depends on the geometry of machining allowances, casting size and weight with nominal amount of deviation, under other conditions the same occasion, the geometry of the workload and precision machined metal from the amount of two ways, reflecting the precision casting process sophistication of the process.
Size and weight tolerances and machining allowances for castings by GB6414-86GB / T11351-89GB / T11350-89 standards and technical conditions of the decision ordering respectively, while the actual data technical inspection department deviation by the decision. The so-called casting size control is measured in line with the average of the nominal size of castings under the premise of controlling the degree of dispersion measured values. The reason this is usually due to the production of discrete technical conditions and material properties, such as random or systematic errors caused by the change. Precision castings should make judgments and appropriate countermeasures according to the nature of the error and improve the dimensional accuracy to meet the requirements of the standard.
Sometimes, the entire cast only one or two rough size requirements more stringent dimensional tolerances, then there is no need to press the same level of all sizes are specified tolerances. To improve casting accuracy level, generally use a metal mold, you need to be transformed significantly improve the modeling methods, such as using shell casting or other precision casting process, but it will inevitably lead to increased tooling costs, production preparation cycle extension, casting costs multiplied.
When assessing the accuracy of the casting geometry, in addition to the machined surface margin, casting weight deviation need to add provisions, the most representative range of tolerances or deviations wall thickness and muscle thickness should also be given. Wall thickness of the structure is determined by the casting designer required, the minimum allowable wall thickness is determined by the casting process division. In most cases, precision castings, the designer decided to form a wall thickness marked on the pattern. This design wall thickness tolerance between the actual wall thickness is to characterize the geometry precision castings important indicator.