Machinery's Handbook, 31st Edition
1396 CAD/CAM Table 1. Standard Sizes of Mechanical Drawing Lettering ANSI/ASME Y14.2-2014
Inch
Metric
Use For
Min. Letter Heights, (in.)
Min. Letter Heights, (mm)
Drawing Size D, E, F, H, J, K A, B, C, G
Drawing Size
0.24 0.12 0.24 0.24 0.10 0.12
6 3 6 6
A0, A1
Drawing title, drawing size, CAGE Code, drawing number, and revision letter a Zone letters and numerals in borders Drawing block headings Section and view letters
A2, A3, A4
All All All All
All All All All
2.5
All other characters
3
a When used within the title block. Drawing Exchange Standards.— The ability to transfer working data between different CAD, CAD/CAM, design analysis, and NC/CNC programs is one of the most important requirements of engineering drawing programs. Once an engineer, designer, draftsman, or machinist enters relevant product data into his or her machine (computer or machine tool), the information defining the characteristics of the product should be available to the others involved in the project without recreating or reentering it. In view of manufacturing goals of reducing lead time and increasing productivity, concurrent engineering, and improved prod- uct performance, interchangeable data are a critical component in a CAD/CAM program. DXF stands for Drawing Exchange Format and is a pseudo-standard file format used for exchanging drawings and associated information between different CAD and design analy- sis programs. Nearly all 2D and 3D CAD programs support some sort of drawing exchange through the use of DXF files, and most can read and export DXF files. There are, however, differences in the drawing features supported and the manner the DXF files are handled by each program. For example, if a 3D drawing is exported in the DXF format and imported into a 2D CAD program, some loss of information results, because all the 3D features are not supported by the 2D program. Most common drawing entities (lines, arcs, etc.) will transfer, although other problems may occur. For example, entities treated as a single object (such as blocks, hatch patterns, and symbols) may be divided into hundreds of components when converted into a DXF file, making the drawing more difficult to edit. IGES stands for Initial Graphics Exchange Specification and is a means of exchanging or converting drawings and CAD file for use in a different computer graphics system. Normally, a drawing prepared on the computer graphics system used by company A would have to be redrawn before it would operate on a different computer graphics system used by company B. However, with IGES, the drawing can be passed through a software package called a pre- processor, which converts it into a standardized IGES format that can be saved on a storage device, such as a portable hard drive or memory stick. A postprocessor at company B is then used to convert the standard IGES format to that required for their graphics system. Most graphics software companies have or can assist with IGES programs for file conversion. STEP stands for Standard for the Exchange of Product Model Data and is a series of existing and proposed ISO standards written to allow access to all the data surrounding a product. It extends the IGES idea of providing a geometric data transfer to include all the other data that would need to be communicated about a product over its lifetime, and facilitates the use and accessibility of the product data. Although STEP is a new standard, software tools have been developed for converting data between IGES and STEP formats. STL, an abbreviation for standard tessellation language, is the format primarily used to send CAD drawings to additive manufacturing machines. Many full-featured CAD sys- tems support translation of drawing files into STL format. CAD programs without STL file format capability can use the IGES or DXF file format, but this process can be time- consuming and expensive, because added steps may be needed to recreate features lost in conversion; also unwanted changes may occur. The safest route, reducing unexpected errors, is to create a CAD model and export it directly into the STL format. Reverse STL generators are available to display a file saved in STL format or convert it into a form that can be imported into a CAD program. For more on this topic, see Metal Additive Manu- facturing on page 1555.
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