HomeOne Piece Flow - Pros and Cons
Background: One piece flow is considered one of the key elements of any lean process and to some extent a "holy" grail.
The Pros: There are many benefits to achieving one piece flow as follows:
1: Improves safety. Research shows that overexertion is one of the main sources of injury in the workplace. When we transition to one piece flow we limit the need to lift heavy pallets and containers of material and parts in repetitive unconnected processes. A host of ergonomic benefits are achieved which reduce the risk of repetitive strain injuries, increase productivity and improve the bottom line.
2: Builds in Quality. When we “make one, move one” defects are detected immediately (usually the next work station) forcing immediate corrective action. This in itself is a form of error proofing. Even if there are small buffers between adjacent work stations, the occurrence of a defect results in the scrapping of few parts compared to the amounts associated with disconnected processes having larger batch quantities.
In contrast, when batches of material are produced piles of scrap may result when a defect is detected downstream. Why? The entire batch may have the same defect.
Since the circuit boards were produced in batches (very typical in the surface mount technology world I might add) they were not tested until the end of the assembly line. Often defect problems will go undetected for years and decades due to poor communications between processes where the scrap runs at a low level of a few per cent. Workplace cultures may become inured to the presence of small of scraps and do not bother to report the problems or have given up reporting it due to a lack of response from management or employees in the downstream process.
3. Improves Flexibility. One piece flow is faster than batch and queue. This speediness factor allows us to wait longer to schedule the order (and still deliver on time).
Subsequently, we are better able to respond to last minutes changes from the customer.
4: Improves scalability. With one piece flow, equipment can be designed smaller and at lower cost since the need to produce huge batches of material at breakneck speed is no longer required. Capacity may be increased at low cost simply by adding labour to the cell.
5: Reduces inventory. With one piece flow, work in process (WIP) is reduced practically to nil. Working capital is reduced for raw material, work in process and finished goods is reduced dramatically..
6: Improves productivity. Many of the wastes so inherent with batch and queue production (e.g. motion, transportation, waiting) are greatly reduced with one piece flow. As a result, productivity increases.
7: Simplifies material replenishment. One piece flow paced at takt time allows for material delivery to be done by timed milk runs or set quantity deliveries.
8: Frees up floor space. As already discussed, one piece flow reduces the amount of WIP stored on the floor. In order for one piece flow to function, work stations must be connected and not isolated on their own island. The reduction in floor space can be half or less of what would be needed for a traditional batch and queue process. Given that floor space can have a marginal cost of $200.00 at today's rates for an addition, this is a huge capital benefit.
9: Makes kaizen take root. One piece flow is hard since the buffers and buffers of inventory are gone. Buffers hide operational problems and waste. Further, quality must constantly improve, machine reliability must increase, changeovers must be shortened, etc. In short, kaizen must take root. Continuous improvement takes easily because the problems are visible to everyone. In a traditional batch and queue process the problems are hidden or forgotten due to time lags with the result that continuous improvement takes place at a glacial rate.
10: Improves morale. Employees want to do good work. They want to see progress. They want be involved.
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One Piece Flow Disadvantages: Like every good idea there are times when one piece flow can cause severe problems. The above picture schematically shows a simple engine block machining transfer line.
A large automotive OEM engine plant installed in the mid 1990's an ultra modern CNC flexible ("agile", able to machine aluminum and ferrous material) engine block transfer line. While there were numerous issues with the line including its "agile" feature which was eventually abandoned as not workable, the main issue was that actual output of the line fell far short of planned capacity for the line.
The intent of the line was to have one piece flow and achieve all of the touted advantages. Accordingly the line was built with no storage buffers between each machining center. The line which had about 10 machining centers, with each experiencing typical up time of 60% for tool changes and quality inspections. Statistically a process having 10 steps each with 60% up time will have a net up time of only 0.6%.
An assessment of the line and why it did not produce as intended, quickly established the lack of between process buffers between each machine tool. Needed buffer sizes were calculated based on the expected machine tool up time of each process, and retroactively installed at great expense and a substantial delay in the platform launch. Subsequently, this line proceeded to perform at the required throughput level.
One piece flow will not work very well where individual processes have structural downtime associated with them. Where processes operate in batches, such as tumble blast shot machines, one piece flow will result in destruction of the machine. For a tumble shot blast machine to function efficiently a reasonable batch size must be buffered in front of the machine.
Final Comments: One piece flow or the goal of achieving flow obviously works best in a manufacturing or processing environment where it is easy and makes sense to connect processes together. Achieving flow in an indirect business process, is still a worthwhile objective for the reason that lead time can be compressed to the absolute minimum.
For example in a foundry there were shipping delays every month waiting for inspection quality documents, due to bureaucratic spreading of the work to two employees. The process was redesigned to have the lab technician prepare his own documents using a computer with document templates created for this purpose. The problem of late shipments was eliminated for this reason. The second individual who was doing only non-valued added work was reassigned to a value added administrative task (accounts receivable).
