CAM Notes Group technology

1 INTRODUCTION 

Group technology (GT) signifies a philosophical tool, which attempts to analyze and to arrange the parts into groups to take advantage of their similarities according to design and production process. On the basis of groups, families can be established for rationalizing the manufacturing process in the area of small and medium batch sizes of mass production of a large product mix. In the cut throat competition in the global market to meet the needs of customers, manufactures are forced to adopt the small batch production with large production mix as compared to from mass production paradigm. To accomplish these needs, manufactures have to adopt the method of GT to produce small volume batches consisting of complex parts in a short production period. GT philosophy is applied to organize a large portion of the manufacturing systems into cells, which leads to cellular manufacturing system.

GT can be defined as “It is the realization that many problems are similar, and that by grouping similar problems, a single solution can be found to a set of problems thus saving time and effort”    

The definition of GT is although quite broad, one usually relates GT to production flow analysis only. GT can be applied in different area of production system. For component design, it is obvious that many components have a similar shape. These similar components can be grouped into design families and a new design can be created by simply modifying an existing design from the same family. By using this concept, composite components can be identified. 

For manufacturing purposes, GT presents a greater importance than simply a design philosophy. Similar manufacturing processes may be required for those components which are not similar in shape. And such those components can be grouped into a cell. The set of similar components can be called as a part family. Since all family members require similar processes, a machine cell can be built to manufacture the family. This makes production planning and control much easier because only similar components are considered for each cell. Such a cell-oriented layout is called a group-technology layout or cellular layout.      

2 CODING AND CLASSIFICATION

Coding is a process of establishing symbols to the parts used for communicating media and at the same time manufacturing details of the product. Classification refers to the process of categorization of a set of parts into part families. Coding and classification both are closely related i.e. classification requirements must be considered while the construction of a coding scheme and coding can be used for classification process. A survey of all components must be completed before a coding scheme can be constructed. The application of the coding scheme decides the selection of relevant features. For example, tolerance is not important for design retrieval; therefore, it is not a feature in a design oriented coding scheme but in a manufacturing-oriented coding system, it is an important feature.

The code structure affects its length; the accessibility and the expandability of a code and thus, these coding structures differ form each other:

1.      In terms of the symbols they employ such as numeric, alphabetic, or alphanumeric. 

2.      In the assignment of these symbols to generate codes.

However, the variations in codes resulting from the way the symbols are assigned can be categorized into three different type of code structure 

1.      Mono code or Hierarchical code 

2.      Chain (matrix) code or Poly code 

3.      Hybrid code or Mixed code 

7.2.1 Mono code or Hierarchical code  

The structure of these codes is like a tree in which each symbol is qualified by the preceding characters. Figure 7.1 depicts the monocode generation scheme. The first digit (from 0 to 9) divides the set of parts into major groups such as sheet metal parts, machined parts, purchased parts, and raw materials, and so forth. The second subsequent digits further partition the set into subgroups for each of these groups. For example, the second digit partitions the machined parts into rotational (0) and nonrotational (1) parts. Consider a code 100 in figure 7.1. It represents a machined rotational part with a length to diameter ratio of less than 0.5. The digit 1 in the first place of code has different meaning and different information. Therefore, the digits in a monocode cannot be interpreted independently; the interpretation depends on the information contained in the preceding symbol.

Advantage: 

•      It can represent a large amount of information with very few code positions. 

•      The hierarchical nature of the code makes it useful for storage and retrieval of design related information such as geometry, material, and size as depicts in figure7.1. 

Disadvantage:

•      A drawback is related to the complexity of the coding system. 

•      The applicability of these codes in manufacturing is limited, as it is difficult to cover information on manufacturing sequences in hierarchical manner

Figure 1.1 Example of Monocode.

2.2 Chain code or Poly code   

In polycode the code symbols are independent of each other. Each digit in specific location the code represents a distinct bit of information. In table 7.1, a chain-structured coding scheme is presented. Numeral 3 in the third position always means axial and cross hole no matter what numbers are given to position 1 and 2.

Advantages:

•      Chain codes are compact and are much easier to construct and use.

Disadvantage:

•      They cannot be as detailed as hierarchical structures with the same number of coding digits. 

Digit position	1	2	3	4
Class of features	External shape	Internal shape	Holes	…
Possible value
1	Shape 1	Shape 1	Axial	…
2	Shape 1	Shape 1	cross	…
3	Shape 1	Shape 1	Axial and cross	…
. .	. .	. .	. .	. .

Table 1.1 chain structure

2.3 Mixed code or Hybrid code  

Mixed code is mixture of the hierarchical code and chain code (Figure 7.2). It retains the advantage of both mono and chain code. Therefore, most existing code system uses a mixed structure. One good example is widely used optiz code 

 Figure 7.2 A hybrid structure

4 Group Technology Coding Systems 

Too much information regarding the components sometimes makes the decision very difficult proposition. It would be better to provide a system with an abstract kind of thing, which can summarize the whole system with the necessary information without giving great details.

Group technology (GT) is a fitting tool for this purpose. Coding, a GT technique, can be used to model a component with necessary information. When constructing a coding system for a component’s representation, there are several factors need to be considered. They include

•      The population of components (i.e. rotational, prismatic, deep drawn, sheet metal, and so on)  Detail in the code.

•      The code structure.

•      The digital representation (i.e. binary, octal, decimal, alphanumeric, hexadecimal, and so on).  

In component coding, only those features are included which are variant in nature. When a coding scheme is designed the two criteria need to be fulfilled (1) unambiguity (2) completeness.