PROJECT BACKGROUND AND TECHNIQUES

Considering the limitation of generative CAPP in the scope of activities that the rules can cover and in the realisable effectiveness of such systems, a variant type of approach has been adopted in developing the system described here. Under this approach, the plan generation process has been separated from plan refining activities. The system involves user modification with semi-automatic evaluation and error detection using rules, formulae, and decision logic. In some generative systems the manufacturing knowledge base is clustered by the system designer into a static and inflexible

partitions. In the system described here, on the other hand, a relational database is used to store the information intelligently. The system was first of all created to be likened to a variant CAPP systems, it was then extended with external enhancement to automate some of the process plans. Nevertheless, the metallurgical engineers play the key role in problem solving within the system and computers are used as tools to assist the

engineers by doing the jobs requested of them.

3.1 Manufacturing Domain

The system has been developed within Forgemasters Engineering Ltd. - abbreviated from now on as FEL. FEL produces a wide variety of forged components, comprising machined and heat treated forgings and blooms, from carbon, alloy and stainless steel ingots of up to 230 tonnes in weight.

The manufacture of these forgings is a complex activity. The final forged products require good surface finish, tight tolerances and specified internal structure and chemical and mechanical properties. The manufacturing process commences with arc furnace melting of the steel and its refining, partly in the arc furnace itself and partly in a separate ladle furnace and vacuum degassing units such as VAD (Vacuum Argon Degassing), where VCD (Vacuum Carbon Deoxidisation) is carried out for achieving

CHAPTER 3 PROJECT BACKGROUND AND TECHNIQUES

higher purity of steel. This steel is then cast into ingots from which one or more forgings will be made, although some very large forgings may require steel from more than one arc furnace charge to be cast into a single ingot. These steel making and casting processes are earned out in a sister company - Forgemasters Steels Ltd. - but controlled by FEL through a 'steel order' drawn up by FEL's metallurgical engineers. Once the ingots are cast, they are delivered to FEL for forging, heat treatment and machining. Although the forging process must produce a rough shape close to the final design shape, the metallurgical interest in the forging process stems from the effect it has on the internal structure of the ingot. The forging sequence must involve sufficient forging reduction to break up the cast structure and to close any central porosity that remains after casting.

The forgings are heat treated at a number of stages during their manufacture, depending on their size, to the final structure and shape required. Although the bulk of the heat treatment cycles involved are designed to produce specific changes to the microstructure of the forging, the high temperature holding times entailed provide sufficient mobility to the hydrogen dissolved within the forging for it to diffuse out into the atmosphere. Since the precipitation of dissolved hydrogen during service can be major contributor to the cracking of forged components, this diffusion is of considerable importance.

Depending on the hydrogen content when cast, each forging requires a specific holding time at elevated temperature to reduce its hydrogen content to an acceptable level (solid state hydrogen degassing). Hydrogen anneals can thus be required in addition to the microstructure forming heat treatments so that the totality of the high temperature holding times meets the specific hydrogen degassing requirement.

All the process plans, whether they be manufacturing plans, steel order plans or any other plans, are designed to satisfy the steel specifications stipulated by the customer.

CHAPTER 3 PROJECT BACKGROUND AND TECHNIQUES

The knowledge of how to do this is controlled to the work of metallurgists at FEL. In order to assist in the designing of the process plans, specifications are constructed in three parts: the chemical range, the mechanical property range and standard testing procedures. These are then used as the guide for the building of the process plans. In this way, complicated domain knowledge can be treated in a rational fashion. Any given steel composition, herein classified as a chemical range, may correspond to more than one range of mechanical properties. For example, a given chemical range, to meet a specification issue "A", would give a certain set of mechanical properties under the heat treatment adopted in one of the process plans. Another customer, however may require the same chemical range with different mechanical properties. In this case, the same chemical range would be defined as another issue of the chemical range

specification with an issue letter "B" and then related to the new range of mechanical properties and a different set of heat treatments.

After the process planners have put all the information into the process plans, their knowledge is merged into the resulting plan. Thus the relationships developed between the specifications and the process plans need to be built up in the database so as to assist the planners to design the plans consistently and rapidly. For previously used

specifications, the system must be able to access the knowledge already stored in the database about planning to that specification.

3.2 Two-level Hierarchy Control

To control the manufacture processes involved in meeting a specification, several process planning documents are currently carefully drawn up and the manufacturing processes monitored against them. These documents are drawn up either for single individual forgings or for a groups of forgings (multiple forgings) ordered by a single

CHAPTER 3 PROJECT BACKGROUND AND TECHNIQUES

customer even though differences in required geometric shape and mechanical properties may result in slight differences in the manufacturing processes to which individual forgings are subjected.

Chang and Wysk [CW84] analyse the creation of a manufacturing plan as a hierarchical activity comprising 3 stages:

* Production Planning; * Process Planning; * Operation Planning.

In terms of this analysis, the overall manufacturing, or quality plan, drawn up for a forging corresponds to the production planning stage and is the activity carried out at the highest level. The other plans or instructions that stem from these two, such as the steel order or mechanical testing plan, can be seen as existing at the intermediate level. In general we will, hereafter, call these plans process plans and use this hierarchy as the basis for our analysis. The Metallurgical Engineers working within FEL, and using this FEL Technical Database, are not concerned with planning at the manufacturing

operation level. The interrelation between these plans within this manufacturing hierarchy is shown in Fig. 3-1. Each box represents an entity of interest in the process planning system. It follows the actual principles of the manufacturing processes. The lines joining the boxes are to show the flow of data. The system has two levels with regard to the integration of all the process planning and relevant decision making points. The use of the query sub-system associated with the relational database, allows the entire system to be well controlled and maintained, the decision making process proceeding in sequence from the top to the bottom.

CHAPTER 3 PROJECT BACKGROUND AND TECHNIQUES Highest Level (Overall Plans) CDSL Intermidiate Level (Process Sub-Plans) MT. SP. HTP. CPP. FP. Forging Definition Forging Procedures Manufacturing Plan NDT. Tests Reports

Quality Plan

Steel Order

NDT. - None Destructive Test

SP. - Special Procedure