From Fixed to Mixed
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INDUSTRIAL ENGINEER – VOLUME 47: NUMBER 3
By Tiago Simões, AnabelaCarvalho Alves and Paula Carnerio
Today’s market demands customized products that meet all of your customers’ needs, which requires companies to adopt a set of strategies that can achieve this efficiently and effectively.Lean tools have become increasingly popular as manufacturing and service firms aim to do more with less.The company was dealing with increased demand for its units and needed to add production capability without using more resources. The lean team examined the data, identified the current system’s problems and proposed a set of solutions.
The initial analysis
This project was developed in the assembly section of a company that manufactured machine tools like guillotines and press brakes. The production system was a fixed-station layout, meaning that the guillotines and press brakes each were assembled on their own sides of the assembly line corridor.In terms of customization, the models could be very different in the finishing process, but high demand for the products made it necessary to design the basic models from one standard.Manufacturing these models required a long list of operations. For example, assembling a guillotine took 71 operations, and building a press brake took 59 operations. Because many of these operations were repetitive, the team selected the ones that were the most irregular and complex: 15 for the guillotine and 12 for the press brake.
Ten operators worked to build the machine tools. The section had space for 16 models, eight of each type in each corridor. The area occupied by the 16 models was approximately 500 square meters. In Figure 1, these spots are numbered eight and nine.Using the above tools revealed that one of the main problems was the long distance traveled by the operator to assemble a guillotine. It was noted that, for example, the operator walked about 609 meters just to get materials for the guillotine. In addition, preparation, such as the operation to prepare metal tubes, required the operator to perform too many movements.
It also turned out that sometimes operators were placed in workstations without being trained on what to do, forcing them to seek advice from more experienced operators. Work instructions also were missing in the workstations. Another problem observed was the disorganization of the workstations, which made operators lose an average of five minutes each time they had to search for things they needed.
Devising a plan
Some of the above problems were critical enough to risk the success of designing a mixed assembly line. Solving such issues involved organizing the workplace and training the operators, not only in technical skills but also in organizational skills like 5S and operations balancing. To cut down on disruptions of the workplace, it was proposed to apply the 5S tool to arrange and organize the necessary tools so that operators wouldn’t lose five minutes every time they needed a different tool.
A formation matrix was created for the operators, and an ABAB rotation program was implemented to increase the versatility of the operators. ABAB implies that every two hours the operator rotates between workstations, making a total of four changes per day. This will improve their efficiency and break up their repetitive and monotonous tasks.So the design of the assembly line started by calculating the takt time for the line. The client demand would be one model per day, be it a guillotine or press brake. Therefore, the takt time for the assembly line was calculated to be 7.5 hours, which took up the entire eight-hour workday with two breaks of 15 minutes.
Operations were categorized as preparation, standard or customizable. Preparation operations were those needed to prepare components and/or subassemblies, such as preparing tubes.The preparation and standard operations should be placed in the assembly line and could be done in parallel. Customizable operations should be carried out of line. These operations were studied in order to eliminate all activities that added no value.
Therefore, in order not to exceed takt time, it was proposed that the line have six workstations (39.63 / 6 = 6.6 hours < 7.5 hours for guillotine and 39.11 / 6 = 6.5 hours < 7.5 hours for press brake). This made it possible to calculate the number of operators for the line and conclude that the line needed six operators.Next, the layout for the assembly line was studied, taking into account the location and area where it would be implemented. The six workstations were distributed in order to make possible the implementation of the mixed assembly line. As the objective is to have the model under construction be pulled, an automatic conveyor that could hold at least 30 tons was needed. This conveyor also needed to operate at a speed of 0.7 meters per hour because of the takt time and the size of each workstation.
Results and discussion
It is important to say that most of these proposals, especially those related to the new mixed assembly line, were not implemented because the facilities were not ready in time. However, some proposals, like the training actions, rotation program and the formation matrix, could be implemented in the current production unit.Under the new formation matrix, operators who reached level five in all operations would never have to ask other employees how to do their work. The rotation program helped the work be less monotonous and exhausting, allowing operators to acquire more flexibility. The organization of workstations (5S) would bring benefits by reducing the time wasted during the employees’ search for tools, as well as eliminating excessive movements.
In the fixed-station production system, 87 hours were required to assemble a guillotine. The assembly line would reduce that to only 47 hours. The press brakes required a manufacturing time of 71 hours in the fixed-station production system, and the assembly line would reduce that to only 42 hours. So the proposal for changing to a mixed assembly line would decrease production times by 45.9 percent for guillotines and 40.8 percent for press brakes.
The number of operators needed in the mixed assembly line also would decrease from 10 to eight, a decrease of 20 percent. Meanwhile, annual production of guillotines would increase from 72 to 112, or 55.6 percent. And instead of producing 144 press brakes each year, the facility could produce 186, a 29.2 percent increase.
Generally, the new mixed assembly line would allow elimination of unnecessary movements, cut out the time wasted searching for materials and organize the workstations. The assembly line would standardize operations and reduce lead-times, allowing the company to meet its customers’ deadlines for delivery.
Of course, more important than reducing wastes or changing a production system is the change in the attitude of employees after being involved with and participating in the project.