In order to draw thediagram, the following process parameters must be selected after the casting scheme is determined:
(1) Machining allowance the size of castings increased for machining is called machining allowance. Its size depends on the type of alloy, the size of the casting, the production batch, the distance between the machining surface and the datum plane, and the position of the machining surface during pouring. Due to the high pouring temperature of steel castings, the surface of castings is not smooth enough, so the machining allowance should be relatively large; in mass production, the machining allowance can be small due to the mechanical modeling and complete process equipment; in single piece and small batch production, the machining allowance should also be increased due to the large manual modeling error.
The lower and middle limits of machining allowance values are used for mass production, and the upper limits are used for single piece and small batch production. Whether the holes and grooves to be machined on the casting are cast depends on the size of the holes and grooves, the production batch, the type of alloy and other factors. Generally speaking, under the condition of single piece and small batch production, the machining holes with the diameter of iron castings less than 25 mm and the diameter of steel castings less than 35 mm can not be cast, because it is economical to drill directly during machining.
(2) Shrinkage after the casting is cooled, the size of the casting is smaller than that of the mold cavity due to the shrinkage of the alloy. In order to ensure the proper size of the casting, when manufacturing the model, the shrinkage of the steel and the shrinkage of the sand mold are placed in advance, and the size of the model is enlarged proportionally. Therefore, a “scale” including the shrinkage of the alloy should be used to draw the model diagram. The shrinkage of the alloy varies with the type of alloy and the size, shape and structure of the casting. Generally, cast steel is about 1.5-3.0%.
(3) Draft angle in order to make the model (or core) easy to take out from the mold (or core box), where the vertical wall is perpendicular to the parting surface, a certain gradient must be left when manufacturing the model, which is called draft angle or casting angle. The size of draft angle depends on the height of vertical wall, modeling method, model material and surface finish, etc. generally, the higher the vertical wall is, the smaller the angle is; the machine modeling should be smaller than the manual modeling. The inclination of the inner wall of the casting should be larger than that of the outer wall. Generally, the specific value of the draft angle can be found in the relevant manual.