As for the size of castings in ship inspection, there is no clear division in relevant standards so far. The basic size and dimensional tolerance of castings are as follows:

1) Basic size of casting: the size of the blank casting before machining, including the necessary machining allowance.

2) Dimensional tolerance is the allowable variation of dimensions. The tolerance is equal to the absolute value of the algebraic difference between the larger limit dimension and the smaller limit dimension; Also equal to the absolute value of the algebraic difference between the upper deviation and the lower deviation.

According to the provisions of GB / T 6414-1999, the dimensional tolerance code of castings is CT, and the tolerance level is divided into 16 levels. For the dimension with general tolerance, the tolerance code shall be uniformly marked on the drawing, such as "general tolerance GB / T 6414-ct12"; For the dimension not suitable for general tolerance, individual tolerance shall be specified, and individual tolerance shall be marked after the basic dimension on the drawing.

The tolerance zone shall be symmetrically distributed with respect to the basic dimension, i.e. half above the basic dimension and half below the basic dimension. For special reasons, the tolerance zone can also be asymmetrically distributed upon the agreement between the foundry and the purchaser. In this case, the tolerance shall be separately marked after the basic dimension.

The dimensional tolerance grade of casting die produced by different production scale and production mode is different. For the mass repeated production mode, it is possible to achieve a more precise tolerance level than shown in the table through careful adjustment and control of the core position; When sand casting method is used for small batch and single casting production, it is usually impractical and uneconomic to achieve small tolerance by adopting metal pattern and developing equipment and casting process. The table shows the wider tolerance applicable to this production method.

Many dimensions of castings are affected by parting surface and core, so it is necessary to increase dimensional tolerance. Since it is not necessary for the designer to know the layout of the mold and core used, these tolerance increases have been included in the table.

Wrong mold (wrong box) because of the dislocation during mold closing, one part of the casting and the other part are staggered at the parting surface. When it is necessary to further limit the stagger, the larger stagger value shall be indicated on the drawing, such as "general tolerance larger stagger".

There are many classification methods for castings: according to the different metal materials used, they are divided into steel castings, iron castings, copper castings, aluminum castings, magnesium castings, zinc castings, titanium castings, etc. Each type of casting can be further divided into different types according to its chemical composition or metallographic structure. For example, iron castings can be divided into gray iron castings, ductile iron castings, vermicular iron castings, malleable iron castings, alloy iron castings, etc; According to the different molding methods, the castings can be divided into ordinary sand mold castings, metal mold castings, die castings, centrifugal castings, continuous castings, investment castings, ceramic mold castings, electroslag remelting castings, and bimetal castings. Among them, ordinary sand casting is widely used, accounting for about 80% of the total casting output. And aluminum, magnesium, zinc and other non-ferrous metal castings are mostly die castings.