Dimensioning a part drawing
All dimensions applied to the part drawing are conventionally divided into three groups:
• chain (dimensions forming assembly dimensional chains);
• conjugate (dimensions that are simultaneously related to two connected parts);
• free (dimensions not included in chain and in conjugate).
Valuable and conjugate dimensions should be taken from the drawing of the assembly unit. Before drawing dimensions on the drawing of the part, you should choose the method of their installation from the three methods used in engineering:
With the chain method, the dimensions of the individual elements of the part are placed successively as links of the same value (Figure 2.47).
This method is used when dimensioning on interaxal distances; in stepped parts, when it is required to obtain exact dimensions of individual sections between ledges; when machining parts with a set of cutting tools, etc. The error in size, obtained in this case in separate sections, does not depend on the errors of the previous dimensions, which is the advantage of this method.
With the coordinate method, the sizes are set from one base (Figures 2.48, 2.49).
This method is used when you need to accurately position the parts of a part relative to one base, but the size errors between adjacent parts of the part increase somewhat.
Most often use a combined method, which is a combination of chain and coordinate methods.
When the chain (axial) dimensions are inserted, the dimensional chain in the drawing must be unclosed. In the quality of the closing dimension (not shown in the drawing), choose the least important part size.
If the closing size is the overall size, it is shown in the drawing as a reference (Figure 2.50).
Reference is the dimensions that are not subject to the implementation of this drawing and are indicated for greater ease of use of the drawing. The reference dimensions in the drawing are indicated by an asterisk (*), and in the technical requirements they record: * Dimensions for reference (Figure 2.50).
Dimensions on symmetrical products are applied, as shown in Fig. 2.51.
Dimensions with the limiting deviations of the dimensions of the elements processed together are enclosed in square brackets and the specification is placed in the specifications: "The size treatment in square brackets is performed in conjunction with the child. Pos. ... & quot ;, and this requirement is placed on the drawings of both conjugate parts (Figure 2.52).
Dimensioning the drawing of the assembly unit
The drawing of the assembly unit must contain:
• dimensions and limit deviations that must be met and monitored in accordance with this drawing;
• the conjugate dimensions with the designation of landings (in the places of installation on the shafts of gear and worm wheels, couplings, pulleys, sprockets, spacer sleeves, bearings, in the place of installation in the body of the product of bearing caps, glasses, flanges, etc.);
• connecting dimensions (for which this product is attached to other products) with the necessary limit deviations;
• installation dimensions (for which this product is installed at the installation site) with the necessary limit deviations;
• overall dimensions that determine the outline of the product;
• the main dimensions that characterize the product and its main components (for example, for a gearbox with helical cylindrical wheels: axle distance with allowable deviations, line direction, tooth angle, gearing module);
• Required reference dimensions.
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