March 1, 2005
By Darren Dolcemascolo
After you’ve mapped your value streams, you are ready to setup continuous flow manufacturing cells. Most cells that have been set up in the past ten years do not have continuous flow; most changes to cells have been a layout change only. That is, machines were moved in a cellular arrangement and nothing more was changed. A change in layout alone does not create continuous flow. This article will discuss seven steps to creating continuous flow manufacturing cells.
1. Decide which products or product families will go into your cells, and determine the type of cell: Product-focused or Group Technology (mixed model). For product focused cells to work correctly, demand needs to be high enough for an individual product. For mixed model or group technology cells to work, changeover times must be kept short.
To learn more about the types of cells and their appropriate application, read our article entitled, “Types of Manufacturing Cells.”
2. Calculate Takt Time. Takt time, often mistaken for cycle time, is not dependent on your productivity- it is a measure of customer demand expressed in units of time:
Takt Time = Available work-time per shift / Customer demand per shift
Ex: Work time/Shift = 27,600 seconds
Demand/Shift = 690 units
Takt Time = 27,600/690 = 40 sec.
The customer demands one unit every 40 seconds. What if your demand is unpredictable and relatively low volume? Typically, demand is unpredictable; however, aggregate demand (that is, demand of a group of products that would run through a cell) is much more predictable. Takt time should generally not be adjusted more than monthly. Furthermore, holding finished goods inventory will help in handling fluctuating demand.
3. Determine the work elements and time required for making one piece. In much detail, document all of the actual work that goes into making one unit. Time each element separately several times and use the lowest repeatable time. Do not include wasteful elements such as walking and waiting time.
4. Determine if your equipment can meet takt time. Using a spreadsheet determine if each piece of equipment that will be required for the cell you are setting up is capable of meeting takt time.
5. Create a lean layout. More than likely, you will have more than one person working in your cell (this depends on takt time); however, you should arrange the cell such that one person can do it. This will ensure that the least possible space is consumed. Less space translates to less walking, movement of parts, and waste. U-shaped cells are generally best; however, if this is impossible due to factory floor limitations, other shapes will do. For example, I have implemented S shaped cells in areas where a large U-shape is physically impossible.
6. Balance the cell. This involves determining how many operators are needed to meet takt time.
Number of Operators = Total Work content / Takt time
Ex.: Total work content: 49 minutes
Takt time: 12 minutes
Number of operators: 49/12 = 4.08 (4 operators)
If there is a remainder term, it may be necessary to kaizen the process and reduce the work content. Other possibilities include moving operations to the supplying process to balance the line. For example, one of my clients moved simple assembly operations from their assembly line to their injection molding operation to reduce work content and balance the line.
7. Determine how the work will be divided among the operators. There are several approaches. Some include:
Splitting the work evenly between operators
Having one operator perform all the elements to make a complete circuit of the cell in the direction of material flow
Reversing the above
Combinations of the above
After you’ve determined the above 7 elements, you will have gathered much of the necessary data required to begin drawing and laying out your continuous flow manufacturing cell.
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