Rules for drawing drawings of cast parts
Dimensioning on drawings of cast parts
Dimensioning on drawings of cast parts must be carried out in accordance with the location of the casting bases and machining bases, and also taking into account the dimensional deviation.
The following basic rules for the casting of castings •.
• The dimensions of the untreated surfaces should be attached to the foundry base directly or through other dimensions;
• the original machining base must be linked to the rough foundry base;
• All other dimensions of the machined surfaces - to the base of the cutting process directly or through other dimensions.
Bind the casting dimensions to the dimensions of the surfaces being processed
cutting, and binding the dimensions of the surfaces processed by cutting to the foundry dimensions is unacceptable, except when the casting base and the machining base are the same.
These rules must be observed for all three coordinate axes of casting.
In Fig. 8.4 shows the options for drawing the dimensions of the cast piece. Below are the typical errors allowed in this case.
Dimensioning in Fig. 8.4, but it is not true. Distance between machined planes tied to unprocessed surfaces through dimensions c; B and n, in this case varies widely, together with the variations in the dimensions of the untreated surfaces.
The same mistake is made on 8.4, b, where the distance between the surfaces being processed is given by the sum of the sizes b and a.
When applying the dimensions in Fig. 8.4, in the distance between the planes to be machined, b is kept within the necessary rather narrow limits (within the tolerance for machining by cutting). The error lies in the fact that the black surfaces are tied to adjacent machined flat surfaces (sizes c and a). It is almost impossible to sustain such coordination; the position of the black surfaces depends on the accuracy of the casting, the same accuracy is determined by the oscillation of the distance to the machined planes.
In Fig. 8.4, g, the error is aggravated by the fact that the thickness of the upper horizontal wall (defined in the previous cases directly by the size (I)) is determined by the size of the inner cavity (1 y specified relative to the processed lower surface plane.) Thus, In this case, the thickness of the wall will fluctuate over a wide range.
In the sizing system in Fig. 8.4, d the position of the lower machined flat surface is given by two dimensions from the untreated surfaces of the part with the height of the cup and the size of the flange. It is almost impossible to sustain such coordination.
In Fig. 8.4, e dimensions are correct. As the roughing base, the upper unprocessed surface of the flange is selected. To it the size of the cutting base is tied - the bottom surface of the flange, to which, in turn, the processed upper flat surface (dimension e) is tied. The top untreated surface is coordinated from the casting base (dimension b) and from it - the thickness of the upper wall (size c1). The distance A between the upper machined surface and the upper untreated wall becomes the closing link of the dimensional chain and serves as the compensator of the deviations in the arrangement of the surfaces obtained by casting.
The size A in the drawing is not specified, so it is not taken into account when controlling the part. Of course, the nominal value of A should be greater than the maximum possible displacement of the upper wall as a result of inaccuracy in casting.
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