A critical concern of CAD/ CAM is the communication of design and manufacturing data within an engineering organization and between those organizations involved in the manufacture of product. The early development of technology led, however, to a number of software systems and hardware types that were essentially incompatible with each other. Each system vendor used a unique data structure for the storage of the computer models. Each computer manufacturer used a different operating system and, often, there were different rules or protocols for the communication of data between computers, and from computers to terminals and peripheral devices.
The divergence in data formats, and in hardware and operating system specifications, also had the effect of tying companies to particular CAD/ CAM systems, while the software vendors found themselves constrained in their choice of hardware by the variety of protocols and operating systems, and by their penchant for modifying hardware in order to improve its performance or usability.

The wish to improve the accessibility of computer languages led early on to the establishment of standards for these.
What are the standards that apply especially to CAD/ CAM systems and to engineering software-those for computer graphics, for user interface development, and for the exchange of engineering data?

Graphics and computing standards
In 1977, the Graphics Standards Planning Committee of the Special Interest Group on Graphics (Siggraph) of the Association for Computing Machinery (ACM) introduced the Core Graphics System (CORE), and subsequently refined it in 1979. CORE provided a standardized set of commands to control the construction and display of graphics images, and was independent of hardware or of language.

CORE initially provided for line drawing for both 2D and 3D graphics, and in its later versions also included raster operations such as area fill.
A little later, the Graphics Kernel System GKS was developed.
Further important graphics standards are the Computer Graphics Metafile CGM.
The standard itself is essentially a form of computer language comprising a series of commands for graphical operations. The graphical parts of a program may be designed as a series of GKS commands, which are then implemented as procedure, function or subroutine calls within one of the languages to which GKS is bound.
GKS is based on a number of graphical primitives, or elements that may be drawn in an image. The basic set of primitives has the reserved word names POLYLINE (to draw a multi-element line), POLYMARKER (to draw points), FILL AREA (for raster fill operations) and TEXT.
The syntax of commands for these primitives is:

TOLERANCE

Dimensional tolerance – conventional
Geometric tolerance – modern

REASON OF HAVING TOLERANCE
• No manufacturing process is perfect.
• Nominal dimension (the “d” value) can not be achieved exactly.
• Without tolerance we lose the control and as a consequence cause functional or assembly failure.

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