Once piece flow manufacturing is basically the opposite of ‘batch & queue’ manufacturing, which is often found when visiting traditional manufacturers.
The difference, and what makes this a lean manufacturing approach, is that one piece flow moves one product (or component, or subassembly) at a time through the different stages of production. At the level of every operation, picking and processing of a new piece starts only once the previous one is finished and has been put down. It means multiple pieces can be in process, but each workstation will only be dealing with one at a time.
If it is well planned (typically by a process engineer or manufacturing engineer), the one piece flow approach can be faster, cheaper, and less wasteful than batch & queue manufacturing.
Fewer products being transformed at any given time. In other words, work in process (WIP) is much lower.
How is one piece flow manufacturing organized?
It can take many shapes. Let’s take one example, the iconic “U cell” that is described in many textbooks:
Source: Courtesy of Quora.com
Let’s look at what allows for a good one-piece flow here:
- There is a clear sequence of steps. It’s been planned. The ‘balancing’ is very important — if, for example, operation 5 is much slower than operations 1-2-3-4, there will be a buildup of inventory in front of it.
- Operation 2 is just behind operation 1, operation 3 is just behind operation 2, etc. and it all makes “flow” possible (i.e. not having materials be stacked up somewhere, wait, and be picked up again later).
- There is physically not enough space to stack up many parts/subassemblies. Operators are forced to take one piece at a time, check an attribute if needed, process the piece, put it down, and start over again.
One piece flow advantages
One piece flow has a number of advantages for manufacturers who’re able to implement it effectively:
- Operators only focus their attention on one item. The layout has to be very clear, for example, “what is on the left is unprocessed, what is on the right is processed, and bad parts go in the red bin”, which prevents mixups. Therefore, operators are less likely to make mistakes.
- Space in the factory is optimized, being tidier, smaller, and less costly for the business.
- Poorly balanced lines, which tend to have lower efficiency, are quite obvious. Managers and engineers will often notice where there is a bottleneck in a matter of seconds, and that can prompt a quick reaction.
- The waste represented by items being left in a queue (during which time they can be damaged or lost), stored (in expensive factory/warehouse space), and transported to the next operation, is eliminated.
- Lead times are shorter. As per Little’s Law, lower WIP allows for a shorter lead time (assuming a constant throughput).
- Inventory levels are typically much lower. Less inventory means the company needs less working capital and will have much more cash in hand. In industries with fast parts obsolescence, like electronics or food, an inventory of 1 million USD costs upwards of 250,000 USD every year!
- Any quality issues are likely to be spotted earlier since fully finished products that can be inspected & tested are available much earlier. Finding a quality issue won’t necessitate a whole batch being quarantined for checking after production is completed. For example, let’s say that a cutting tool is deteriorating and starting to cause problems. If a big batch of thousands of parts has been cut at one time, then all of those pieces need to be reworked or scrapped. Even if one-piece flow is not realistic at the cutting stage, it often makes more sense to cut 50 or 100 parts at a time rather than 5,000.
- If at one point, you decide to automate an assembly process, the work will typically be done one piece at a time. If you have already gone in that direction with manual assembly, taking steps in the direction of full automation will be easier.
One piece flow disadvantages
It isn’t suitable for every factory, though, for these reasons:
- If the factory is producing a very wide variety of different products at the same time, making one piece flow work without confusion necessitates good engineering. (High mix, low volume manufacturing has been implemented in a variety of industries.)
- Good process engineering (or well-trained production leaders & supervisors) is a must. Otherwise, it is difficult to estimate how many workstations and staff you require to set up the process, so mistakes could hinder how effective it is.
- If an operator is constantly situated on one workstation, they risk burnout as they may not be doing a variety of daily tasks, and, ultimately, this could cause boredom, lack of focus, and mistakes. It is also a safety issue, as a lack of variety of motion can lead to injury, especially if the workstation was not designed with ergonomics in mind. Process engineers need to avoid very short cycle times of the same repetitive work.
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