The cycle time is setup at this step, the initial step. While a cell is organized for flow, the question arises, how fast should the flow go? In other words, how fast should the cell is producing? The answer is it should be produce at the required process cycle time. The other criterion is smooth for flow. It is mentioned as follow.
A cell produces items at a uniform, stable rate that conforms to the desired uniform load. That rate, or takt time rate, is the time between each unit, which is completed in process.
Takt time concept implies a smooth and uniform rate of production, one-piece flow product at a precise time. Takt time is similar to cycle time (definition of Continuous Flow Manufacturing), except takt time refers to the exact cycle time necessary to meet demand of market trend. Generally, takt time is calculated by:
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The first step in designing cell is to change desired daily uniform load to equivalent takt time. For example, suppose that the daily uniform of requirement for a product is 3600 units and the workday is 8 hours that is 70 minutes for lunch and breaks. Thus, we can find the takt time:
Thus, the cell must be design such that on the average of it produces one unit about every 410 second or less. However, there is no provision is made for unscheduled downtime.
Machine breakdowns and interruptions are problems that must be resolved. Not accounting for downtime in the takt time leads to acceptance of problems and no motivation to fix them.
Takt time is the target; it is the rate of production line, which the cell should produce.
In contrast, actual cycle time is the rate at which the cell actually produces, which depends on the time it take to set up machines, performance of operations, the materials, walk form station to station, movement, actions and so on. In designing the cell, the maximum of actually cycle time must not exceed the takt time.
The purpose of this step is to use for cell balancing. Because the actual cycle time of cell is different from each cells. To produce smoothly, the actual cycle time of every cell have to be equal. Assumption these are e cell will be production on combined production line. The time of e cell has to make equally. ACTj is maximum actual cycle time of cell j and the rate of balancing is formula as follow:
The rate of this cycle time is to measure. If the rate is approaching to 1, the best solution is rate equal to one. However, it is very hard to make it equal to one in practical; it is depend on many factors of combined production line.
The other way to calculate the condition is to use the effective of combined production line. We can fine the effective of cell. It is calculated as follow:
Where: ATi is the actual cycle time of Cell i i is number of cell with i = 1,2…..n n is the total cell
Moreover, the lost rate is: Lost rate = 100 - E. Our target is minimum the lost rate and maximum the effective of combined production line.
29 3.2.2. Sequence of production flow
Sequence of product flow is one of constrain of production line. All steps of assembly process are set in order and some of them are changeable but some of them are not. It is very important procedure of one processing of part. This factor is also important thing. Once the sequence changes, the machine of cell will be change correlatively. In addition, the movements of cell will be change respectively. In this thesis, the combine production line is designed for assembly process in case study. This is one sensitive factor for production line.
The procedure of sequence for combined production approach consists of the following sequencing step:
- Determination of a loading sequence with which jobs are introduced by production flow; that sequence is generated by a pure flow-chart, and consequently it is of a permutation type.
- Construction of a working sequence on each station, restricted to the set of jobs actually requiring an operation on that station; to allocate that subset of parts to the workstation. Machine is assigned into this sequence of production flow chart.
The output of the sequencing module is an MxN working sequence matrix. Cell is set in the series by follow chart, which show in figure:
Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 … … … Cell n Then the machine will be arranged as follow in the table 3.1. This table shows the relation between machine and cell. All cells arrange in the sequence and the machine is located in cell. The number tij is mentioned the number of machine j for cell i.
Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 … … … Cell n
Table 3.1 matrix of cell and machine
30 3.2.3. Space of production line
When the change of production line is needed, the space needs to be taken in considering. Because, this is different between Cellular Manufacturing and Continuous Flow Manufacturing, The workstation is concentrate along of production line. In combine production line, the cell will arrange along line, but the space of cell is bigger than workstation. The continuous flow line is longer than combined production line, but the combine production line is wider than continuous flow line. That is why we have to evaluate the space of combined production line.
The space of combined production line is not easy to evaluate. In general, it is evaluated by the practice way or in the designing process. It will be measure in the technical drawing in approximate way. The other way is measure space by practical way when the space is not a big issue for company.