Case study : [Line Balancing in a Manufacturing Plant]

This study concerns a former division of a major UK-based manufacturer of railway rolling stock and equipment. The plant manufactures a range of bogies, which are the supporting frames and wheel sets for rail vehicles. The company has a history of supplying the passenger train market in the UK but over a period of time low demand and increased competition had led it to enter new markets including European inner-city transport and the supply of freight bogies to Far East countries. The need to compete on a global basis led the company to re-evaluate its manufacturing facility with particular emphasis on the need to increase output, reduce lead times and increase flexibility. To meet these demands management had identified areas where substantial investment was required.

The Production Process

The facility layout is on a product-line basis with the manufacturing process consisting of six main stages of fabrication, welding, frame machining, paint, fitting and quality audit. Each stage must be completed in order before the next stage can begin. The stages are now briefly described:

Fabrication. The fabrication stage prepares the bogie frame sections from sheet steel and bought-in castings. A custom template is designed from which the parts required are cut from sheet steel to standard batch sizes. Parts not needed immediately are held in storage. Processed parts and castings are brought together to form a bogie 'kit', which is assembled on a jig and taken to the subsequent welding stage.

Welding.

A bogie subassembly is manually welded on a jig at a workstation to form a main bogie frame.

Frame machining. The main bogie frame is then transferred to a CNC [computer numerically controlled machine] center for the machining of any holes or bores needed for the fixing of subassemblies such as the braking and suspension systems. Bogies are fixed to a slave table and the machine processes the frame according to a preset operation sequence. Paint. The frame is then manually painted while being suspended from an overhead moving circular track.

Fitting. Manufactured subassemblies and bought-in components such as motors are then assembled on the bogie frame. The frames are placed on supports and are moved along a line at different stages of assembly with overhead cranes. Quality audit. Final inspection is carried out to ensure all bogies meet the required specification. It is usual that a certain amount of paint touch-up work is required at this stage due to damage caused to the paint finish during the fitting stage.

The Line Balancing Study

The focus of the study was on product layout design with the main objective being to ensure that the performance of the whole manufacturing system would meet required output levels. The output level was converted into a target cycle time (time between manufacture of products or output rate). As stated, the product layout consists of six main stages with the product passing through each stage in turn. This means that the effective cycle time for the whole system is determined by the stage with the longest cycle time. The study objective was to obtain a balanced line (all cycle times equal), which would enable a smooth parts flow through the production stages facilitating the introduction of a pull-type just-in-time (JIT) production control system.

A simulation model (Chapter 8) was used to estimate the cycle time at the main manufacturing stages. The graph (Figure 4.16) shows clearly where management effort needed to be directed to achieve the target cycle time. The quality audit stage was set at a nominal amount by management. Significant problems had occurred at this stage with the spray finish on the bogie frames being damaged during the subassembly fitting stage. This had to be rectified by a manual touch-up process, which could take longer than the original spray time. The paint area would also need to be reconfigured due to new environmental controls. Management had recognized the problems and an investment in an epoxy paint plant producing a hard-wearing finish was planned. The bogie frame machining center had previously been recognized by management as a bottleneck process. The bogie frame went through a number of preprogrammed steps on the machine and the cycle time was dependent on the capability of the machining center itself. Consequently a major part of the planned investment was a new machining center with a quoted cycle time below the target. An investigation of the fabrication processes revealed that although the cycle times were above target, the majority of this time was used for machine setup. Figure 4.17 shows the effect on cycle time of a reduction in setup time of 10% to 90%.

From Figure 4.17 it is clear that a setup reduction of 50% is required to achieve the target cycle time. A team was assembled to achieve this target and it was met by the use of magnetic tables to hold parts ready for processing. The simulation was rerun and the results (Figure 4.18) show the system achieving the required performance. It can be seen that a further reduction in fabrication setup times and a reconfiguration of the welding line would reduce the overall cycle time further, producing a more balanced line and increasing capacity utilization.

By implementing the changes outlined in the study the simulation was able to predict the following improvements in performance (Table 4.6).

In the table cycle efficiency = 100%-%idle time where %idle time = idle time per cycle/total cycle time.

These are substantial improvements in performance and meet the output targets set by management. However, the results in Figure 4.18 show that further reductions in the cycle times for the fabrication and weld stages would lead to a further increase in cycle efficiency, reflecting a more balanced line, and thus a further increase in output.

Source: Excerpt from Greasley, A. (2004) "The case for the organisational use of simulation''. Journal of Manufacturing Technology Management, Vol. 15, No. 7, pp. 560-566, Emerald Group Publishing. Reproduced with permission.

Question:

1. Identify the process type used in this organisation and explain the rationale to balance a production line of a manufacturing plant. ?

2. Explain the strategies that are used to balance a production line in this case study. ?

3. Explain alternative strategies to balance a production line in a manufacturing plant.