8. Group Technology (GT)

8.1 Overview

• Most product models use an exact definition of geometry, or other details

• It can be useful to have a more abstract representation of a part for some tasks,

Storage and recall of designs

Recall of process plans for similar parts

Classification of designs for analysis of production

• GT is used to identify subsets or families of similar parts for the purpose of realizing common features for improved design and process efficiency through standardization.

• GT codes can be used to represent products using any combination of geometry, manufacturing processes, and/or function.

• The advantages of such a system can be found in,

1. Product design: Group technology allows similar designs to be recalled on the computer. Instead of starting from scratch again.

2. Tooling and setups: standard tooling can be developed for a part family, and then standard setup procedures and times can be used.

3. Materials Handling: Factory floor layout can be updated to reflect part families, and reduce part handling time.

4. Production and Inventory Control: The use of GT to set up standard production techniques allows faster production, therefore less inventory, and Work in Process (WIP).

5. Employee Satisfaction: Grouping of machines allows easier tracking of quality (and achievement).

6. Process Planning: Standard plans can be developed for GT part families. The plans can then be altered to fit, instead of producing a new process plan.

• Problems with GT systems are,

1. Not suited to a factory with widely varying products

2. Can have a long setup time, and debugging

3. There are no standard GT codes developed: each GT code application will probably be unique.

4. A GT code may be hard for inexperienced users to read.

8.2 Some Details

• The GT code is made up of a string of digits which identify specific attributes of a part.

• If the digits of a GT code are unrelated, it is a polycode, and each digit may be looked up independently.

• If the digits of a GT code are related, it is a monocode, and they must be looked up in sequence.

• It is possible to have a hybrid GT code which is a combination of polycode and monocode.

• When selecting what the GT digits represent, the guidelines are,

They must differentiate products

Must represent non-trivial features

Only critical features should be encoded

Function should be encoded

Every digit should be significant

• Parts can be encoded using

process flow

tool axis





8.2.1 Optiz Code

• One example system is the popular Opitz code, developed in a German university by H.Opitz.

• This code uses a sequence of 5 digits, 4 digits, and 4 letters, such as ‘11223 4455 ADEA’

The first five digits are the form code (identify shape). See the table for form codes.

The next four digits are the supplementary code: used to represent non-form details such as tolerances, materials, etc.

The last four letters are the secondary code, used to represent production operation types, sequences, or other functions chosen by the manufacturer.

• The Opitz code for a part is constructed from the first digit on, as shown in the tables below.


8.2.2 Decision Tree

• Decision trees are developed to be specific to typical product line, or manufacturing facility.

• To develop one of these trees we draw a tree that shows alternate possibilities for a part, and then number the options (care must be used to leave options not anticipated).

• Part of an example decision tree is given below. This can be expanded as it applies to a particular manufacturer or industry.


8.3 Implementation

• GT should be used when there are a large number of parts that can be divided into groups based upon geometry, function, and/or production.

• implementation is a multistage process,

1. Develop a GT code

2. verify the GT code by coding about 10-20% of the parts in the factory. A good random sample of parts should be used for reliable results.

3. The results of the coding should be reviewed. If too many parts have the same GT code, or there are not enough similarities between codes, then the code should be revised.

4. The remainder of the parts should be coded.

5. An examination of all the parts for the factory will allow the identification of patterns in production, or design. As a result standard production routings, or standard product designs may be selected.

8.4 GT Machine Cells

• After identifying part families, a standard set of production steps can be identified.

• After identifying standard production steps, the factory floor can be reorganized to reflect a more rational layout of machines.

• Various ways to lay out machines for part families are,

Single Machine Cell

Group machine layout

flow line design

• Single Machine Cells are suited to products which may be produced on a single machine, using a single process. This can also involve bringing two machines together. Such as a grinder, on a lathe.

• Group Machine Layout is suited to a small set of operations on a part which cannot be performed on a single machine.

• Flow Line Design: when parts in a family have a number of processes with the same sequence, this system may be set up with a transfer line.

8.5 References

8.1 Ullman, D.G., The Mechanical Design Process, McGraw-Hill, 1997.