Machinery's Handbook, 31st Edition
1394 CAD/CAM Once blocked, the individual components of a block (lines, arcs, circles, surfaces, and text, for example) cannot be changed or edited. To edit them, a copy (instance) of the block must be exploded (unblocked) to divide it into its original components. Once exploded, all of the elements of the block (except other blocks) can be edited. When the required changes have been made, the block must be redefined (redeclared as a block by giving it a name and identifying its components). If the block is redefined using the same name, pre - vious references to the block in the drawing will be updated to match the redefined block. For example, an assembly drawing is needed that shows a mechanical frame with 24 similar control panels attached to it. Once one of the panels is drawn and defined as a block (using the name PANEL , for instance), the block can be inserted (or copied) into the drawing 24 times. Later, if changes need to be made to the panel design, one instance of the block PANEL can be exploded, modified, and redefined with the name PANEL . When PANEL is redefined, every other copy of the PANEL block in the draw- ing is also redefined, so every copy of PANEL in the drawing is updated. On the other hand, if the block was redefined with a different name, say, PANEL1 , existing copies of PANEL would remain unchanged. When redefining a block that already exists in the drawing, be sure to use the same insertion point that was used for the original definition of the block; otherwise, the positions of existing blocks with the same name also will be changed. It is worth noting that many manufacturers distribute drawings of their products in libraries of CAD drawings, usually as DWG or DXF files that can be incorporated into existing drawings. The suitability of such drawings depends on the CAD program, drawing format being used, skill of the technician who created the drawings, and ac- curacy of the drawings. An example, Fig. 6, shows a sewage pump assembly drawing. Libraries of frequently used drawing symbols and blocks are also available from com- mercial sources.
Fig. 6. Available CAD Drawing of a Sewage Pump Assembly Use of Text Attributes to Request Drawing Information Automatically: Text attributes are a useful method for attaching textual information to a particular part or feature of a drawing. An attribute is basically a text variable that has a name and can be assigned a value. Attributes are created by defining attribute characteristics such as a name, location in the drawing, text size and style, and default value. Fig. 7 shows two views of a title block for size A, B, C, and G drawing sheets. The top view includes the title block dimensions per ASME Y14.1-2012, with the names and loca- tions of the attributes (COMPANY, TITLE1, TITLE2, etc.). The first step for creating this title block is to draw the frame of the title block and define the attributes (name, location, and default value for: company name and address, drawing titles [2 lines], drawing size, drawing number, revision number, scale, and sheet number). Then the block must be cre- ated and named with the title frame and the attribute definitions. When the block is inserted into a drawing, the operator is asked to enter the attribute values (company name, drawing title, etc.). The bottom view in Fig. 7 shows a completed title block as it might appear in a drawing. A complete drawing sheet could include several
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