Group Technology Workcells are Different!

Taming The Variety Beast

The popularity of Cellular Manufacturing and workcells has grown, fueled by the adoption of the Toyota Production System (TPS) and it's namesake, Lean Manufacturing. But, there are problems.

The developers of TPS, Taiichi Ohno and Shigeo Shingo, were pragmatists. The evolution of TPS was largely trial and error-- long on practice; short on theory. From their writing it is evident that Ohno and Shingo had a wonderful intuitive grasp of the system but their explanations of the why behind it are a bit muddled and somewhat rare.

As a result, much of what came from Toyota is in the form of rules and edicts that reflect what worked and what did not. Rules and edicts also helped propagate the system throughout Toyota and the supplier base.

But, without a strong  theoretical underpinning, it is difficult to know if the rules apply in dissimilar situations. For an example see "When Blitzing Replaces Strategy."

The really big problem involves volume and variety. Toyota's U-shaped, balanced workcells do not work when orders are unpredictable, routings vary and work balance changes. Load leveling is all very well, but customers are not interested in your takt time.

Group Technology workcells manufacture many products that are similar, but not identical. In job shop situations a cell may produce hundreds of different parts. Demand is often erratic and no single part has enough volume for its own cell.

A more fundamental approach to cell design is necessary and the results may look very different than the Toyota model. You can learn more about this from our page on Workcell Design or by downloading the workcell design chapter from Mr. Lee's next book. We also offer a seminar on Cellular Manufacturing.

Workflow

In a GT cell operations and sequence often vary. Some parts require machines 1,2,7 and 9 in that order. The next part may require machines 1,2,3 and 6. The average work time at each machine varies. Work times also change from part number to part number.

Process Technology

GT workcells usually favor smaller-scale, less automated process technologies that changeover easily and fit the lower volumes. Machine shops are an exception. CNC machining fits very well with GT cells. CNC technology does not really cut metal faster, it reduces setup and combines operations.

Batch Size

As in the Toyota system, small batches are better. But the high variety may not allow the kind of setup reductions that we can achieve with only a few products.

Moreover, customers often order in lots and the batch size must reflect this. As a result, production batches tend to be larger and more varied than in a Toyota cell but still much smaller than in functional arrangements. For more on this, see "Lean Lot Sizing."

Transfer Batch

The transfer batch or internal lot size also tends to be larger than in Toyota cells. This is the number of pieces moved together between operations. The inventory in the transfer batch must be large enough to balance operations within the cell.

GT cells are often larger than Toyota cells. The greater distances between workstations require a larger transfer batch to amortize travel time over more pieces. One piece flow may not work.

Layout

GT cells should be designed around the most common workflow routes but they must also accommodate less common routings. This gives rise to unusual arrangements such as L-shapes and X-shapes.