Single piece flow
Magazine Article, Source : The Manufacturer US
Zone : Manufacturing operations
Published : 08 Feb 2006 18:13
The transition from batch and queue to lean manufacturing involves converting to single piece flow, but it’s not an easy thing to do. Rich Weissman looks at the issues impacting the change
G oing lean is all about transformation. Management and employees must change the way they have been doing things for many years and embrace new and uncertain process changes. Suppliers must also adapt to new business practices and the changing requirements of their customers. The benefits of lean–waste reduction leading to increased levels of operational performance, improved quality and customer service, and better financial performance–are well documented, but there can be considerable stress on the organization making the change.
One of these stresses can be found on the shop floor as manufacturing begins the transition from batch and queue manufacturing to single, or one-piece flow. In traditional manufacturing, specific operations were done in batches by departments that specialized in individual manufacturing tasks like machining, welding, assembly, and test. Through the integration of lean induced cellular manufacturing processes, cross-trained employees produce just the amount of completed products that are required by other internal operations or the end customer. By eliminating complex set-ups, buffer stock, and large batches, lean companies are able to reduce lead times, increase flexibility, reduce inventories, and improve product quality.
One-piece flow has additional benefits, including the simplification of scheduling, a reduction in manufacturing space, and the associated reduction in buffer stock and redeployment of equipment. Transport, storage and queues are reduced, as is the propensity for overproduction. Employees are typically more productive and their skills more easily transferred. An increased amount of employee involvement is critical to the one-piece flow process.
As with the concepts of just-in-time manufacturing and zero inventories, one piece flow may be the goal but not necessarily the reality. However, companies can achieve some level of lean advantage with reduced lot sizes, and many are moving in that direction. Most important is to reduce work in process and evolve from a push system driven by forecast to a pull system driven by customer need. It can be a long road and many companies fall short for a variety of reasons.
In a workplace indoctrinated in the batch processing model, that mindset is the hardest thing to change. “Change management on the shop floor is a big deal,” says Mark Brownhill, product manager for productivity solutions for Charlottesville, VA, based GE Fanuc, the worldwide productivity and automation solutions provider. “Manufacturing companies were taught that cycle time and fixturing were the most important elements in manufacturing and batch processing was just the normal way to manufacture in order to justify those two elements.”
Brownhill feels that larger companies with ingrained infrastructure are at the biggest disadvantage when it comes to realigning manufacturing operations for single piece flow. “Large companies often operate in the mindset that cycle time is everything and set-up is nothing,” says Brownhill. “It is just how they calculate their manufacturing efficiencies.” Brownhill sees that in single piece flow the first pass yield is extremely important, while in batch processing first piece yield problems are just lumped in with the set-up. “Changing that mindset can be a big hurdle.
“The need for customer requested cost savings may push more large manufacturers toward the efficiencies gained from single flow production,” says Brownhill. He sees that once companies begin to document efficiencies and gather efficiency data, the opportunity to generate cost savings will force companies to go to smaller lot sizes, with its increased focus on shorter set-up times and reduced batch processing. “There is a need to get more data driven on the shop floor, rather than operating from gut feel,” says Brownhill. “We also have to work to change the mindset of the engineers who support the shop floor. They often have a batch mentality.”
Among those who have benefited from single flow processing is jet engine manufacturer Pratt & Whitney, having recently introduced lean manufacturing into its turbine module center in East Hartford, CT. It made a transition in the manufacture of turbine blades from batch processing to cellular manufacturing, which had a significant impact on reducing work in process inventory of expensive alloys. It also had an unexpected effect on its equipment suppliers.
“When Pratt & Whitney went to cellular manufacturing we saw an increase in the orders for our shot peening equipment,” says Bill Barker, sales engineer for Grand Rapids, MI based Progressive Technologies, Inc., a designer and builder of custom automated process machinery such as grit blasting and shot peening for aerospace, automotive, and general industry applications. “We had a hard time under-standing why they would buy three or four machines when one was always adequate to do the work.” PTI did need to modify its equipment to meet special ergonomic needs and single piece flow of material requirements. “We worked together to redesign the way our machines were used and operated.”
In the case of Pratt & Whitney, the transition to single piece flow was all about cost. The high cost of the super-alloy material is the major cost component in the manufacture of turbine blades. The cost of the capital equipment, in contrast, was not as significant. By working with capital equipment suppliers such as PTI, Pratt & Whitney was able to reduce raw material costs and improve product throughput. Adds Baker: “Lean is finding its way into our factory also. We used to build our equipment in batches, but with customization like we did for Pratt & Whitney we need to work with smaller lot sizes.”
Integrated testing is an important part of cellular manufacturing, and single piece test flow should not be overlooked when reducing lot sizes. “Test has a batch size just like any production process, but usually it is underestimated,” says Marlin Shelley, president of Salt Lake City, UT based Cirris Systems, a manufacturer of test equipment for the cable and harness manufacturing industry. “If defects do not occur, test batch size is irrelevant, but if defects occur often, test batch size reduction is a critical part of being lean.” Shelley adds that if problems are immediately solved as they occur, the test batch size can be as small as a single piece. But, when parts are batched and tested in later processes, the cumulative batch size grows to include all products not tested, all in- process manufacturing, as well as future batches that will be processed until the error is found.
Shelley feels that well trained and motivated employees are necessary when making the transition from batch processing, and encourages significant training as well as an increase of workforce responsibility to identify defects, determine root cause problems, and collaboratively share problems and solutions. But there can be unanticipated issues. “We were working to set up inline testing equipment with one of our aerospace clients that had a union,” said Shelley. “The trouble was that assembly operators in the work cell were of a different employee classification than those who did the testing. Those building the products could not test them in the work cell due to union rules.” Shelley said that the union did ultimately reclassify the employees so they could do in process testing, but this kind of issue is all too common in the transition of moving towards lean.
The cable and harness manufacturing environment can be at odds with single flow manufacturing, however. While some products, such as complex wire harnesses that are built on harness boards, and are by definition single piece flow, some of the other manufactured products include naturally automated manufacturing processes such as molding and wire preparation. “Sometimes automation may get in the way of lean, especially when it comes to necessary equipment like automated cut, strip, and terminate machines,” says Shelley. He adds that in the industry Cirris services, the key may not be in one–piece flow for all products, but in reducing lot sizes in general. Small companies may have a greater advantage in transitioning to small batch sizes and ultimately single piece flow. “We work with our client companies on changing their long term thinking,” says Sean Jordan, a professional manufacturing advisor with the Randolph Center, VT based Vermont Manufacturing Extension Center (VMEC). “Some companies feel that they save money in producing larger lots, but they need to focus on long term cost savings and that lends itself to more of a single piece flow process.”
Ultimately, in a global manufacturing environment, those companies that do not embrace lean initiatives like single flow manufacturing may place jobs at risk. “Many speak to how employees resist changing the manufacturing environment, but I don’t see it,” says Paul Demers, a VMEC senior professional manufacturing advisor and colleague of Jordan. “We’ve had a 25 percent reduction of manufacturing jobs in Vermont and our employees are ready to do whatever they can to keep their jobs.”
