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Flexible Assembly in a Lean World




In designing lean cells for flexibility, the typical cell design is either U-shaped, J-shaped or L-shaped with stations interlinked by manual roller conveyors.
In the midst of a global recession, it's critical to examine manufacturing processes for waste and to pursue strategies that eliminate it. Some process review takes place naturally during tough times as companies cut costs and reduce staffing levels. Work gets shifted around, and work practices are changed for the better as companies look for new ways to get the work done. But it's also important to look beyond the recession to more normal production volumes and to put systems in place that can respond to upswings in the business cycle as well as downturns. This is where lean manufacturing principles come in. By providing a systematic way to identify waste and remove it, lean manufacturing is more than just an extra tool in the manufacturing toolbox. It's a way of life. To be good at lean, companies must commit to it and pursue lean techniques vigorously—and permanently.


Are there any special challenges to implementing lean principles for medical device manufacturers? Although the short answer is no, a good place to start is to look at the flow of material through the production process. Typically, this is the source of the greatest waste: employees hunting for parts, build-up of work-in-process (WIP) inventory, and long travel distances. The pace of innovation in medical products is also extreme—items that start out in low-volume production may quickly become high-volume products. This presents the age-old challenge: getting better products to market faster while maintaining profit margins and lowering manufacturing costs. To do this, medical manufacturers must optimize material flow and build as much flexibility as possible into their assembly and manufacturing systems. But how does a manufacturer decide on the most appropriate technologies and systems? This article examines how medical manufacturers can implement lean practices. Itoffers a matrix for determining the most appropriate methods to implement lean assembly and material flow processes in their operations.


Lean Production System Matrix


Figure 1. (click to enlarge) A lean production system matrix.

A helpful approach to choosing the right assembly technologies is to think in terms of production volume and product mix, and to chart these factors in a matrix (see Figure 1).


In the most unpredictable assembly environment—low volumes, high product mix—manual assembly is usually the most reasonable choice because most companies would never recoup their investment in automated equipment. But as volumes increase, automation can help to increase efficiency and quality and reduce waste. The lean production system matrix shown in Figure 1 can give manufacturers a critical head start in identifying the best options for their specific material flow and assembly situation.


Sometimes, it is suggested that automated assembly and lean production are contradictions in terms. Indeed, lean production is often associated with manual production systems, many of which have been integrated into assembly operations with outstanding results. Sometimes, however, automated assembly is the only way to achieve the desired quality and productivity improvements. For example, high-precision, repetitive assembly operations may require conveyors for positioning repeatability and robotics for assembly speed. In other circumstances, handling of sensitive components may be too difficult to achieve with manual assembly. In these and other situations, the lack of an automated parts-routing strategy may lead to a loss of valuable time, resulting in increases in per-piece costs, lower margins, and less profit.


It's worth noting here that the country that is most known for its lean companies—Japan—also has the highest deployment of robotic technologies in the world, according to the Robotic Industries Association. (The United States has the second-highest rate of robot use.) This serves as an example that lean and automation are not mutually exclusive.

The Simplest Case: Manual Production Systems


For manufacturers with a high degree of product variation and either low or unpredictable product volumes, the most flexible approach is manual assembly, because a device company can simply add workers to a well-designed system to increase production. If a company manufactures medical equipment, for example, and sells 50 pieces each year, much of the assembly is likely manual. Or if a device is relatively new to the market and demand is growing but still unpredictable from month to month, manual assembly is probably the right choice there, too. But what is a well-designed system? Material flow, people flow, and information flow are just as critical in a manual environment as they are in mixed or fully automated assembly systems.


Achieving the optimal flow of material, people, and information begins and ends with a properly designed cell. For starters, a typical cell design is either U-shaped, J-shaped, or L-shaped, with stations interlinked by manual roller conveyors.


There's no set rule for the size and exact shape of a cell because production requirements vary so widely. But these shapes eliminate wasted space and enable the operator to move swiftly from station to station with no excess steps or energy. They also keep the value-added operations on the inside of the cell.


Before any rearranging of equipment, however, the first critical step is to analyze the assembly process and segment it logically into time-balanced increments of work. Because a work cell should be designed to optimize the flow of product through the cell, the defined work segments become steps in the assembly process, which in turn may become stations in the cell. If a company is making a large machine, many of the same rules apply, although the company may want to focus on shortening parts supply routes and reducing process-related waste (painting, welding, shipping, and other commonly performed tasks). Regardless of the situation, the goal is to pare the assembly process down to value-added processes only. Anything not value-added is waste.


The cell should be laid out so that the work can be divided into time-balanced increments for one, two, or three operators, which allows the company to shift existing resources into and out of the cell as demand dictates. In a well-designed cell, adding one person, for example, doubles the output in the available time. Good cell design helps optimize the material flow.


Assuming that the processes and work cells are well designed, assembly costs in a manual system remain relatively constant. This is because time-balancing the operations allows a company to add workers to the cell only as it adds demand. However, as production speeds increase, automation might become the appropriate method because companies can often increase the speed of the conveyors and other machines without spending beyond their original investment. In other words, with automation, costs go down as speed increases. So with extremely fast production cycle times and high-volume production, a more mechanized approach may be the right choice, especially if conveyor systems and other automated technologies are designed specifically for assembly flexibility.

A Word About Outsourcing


Figure 2. (click to enlarge) A comparison of costs per transfer cycle of various systems.

Many companies outsource their assembly operations to countries with low-cost manual labor, with the idea that it's easy to add workers to increase production as needed instead of investing in expensive automated equipment. The chart in Figure 2 compares the costs per transfer cycle of manual transfer, a pick-and-place system, and an automated system. As cycle time goes down (i.e., the speed of manufacture increases), automated transfer begins to make greater economic sense—even compared with manual assembly in low-wage countries.


Outsourcing often carries with it unexpected or unplanned costs, as many manufacturers of toys, pet food, baby formula, and other products are now beginning to understand. The cost of manufacturing may be lower, but it may not be easy to resolve process issues or quality problems across great distances and considerable language barriers. For medical products, government rules and regulations come into play, too. When thinking about lean, it's worth remembering that travel distance is a kind of waste. That's why companies that practice lean as a culture try to manufacture as close to their customers as possible and insist that their suppliers do the same.

Individual Automated Work Cells in a Manual Production Environment


Some production environments may need one or more automated processes and the rest of the assembly steps are performed manually due to volume or mix issues. Highly sensitive medical subassemblies, for instance, may require assembly in a cleanroom environment before they can be added to the larger assembly operation. Certain high-precision operations may also require robotics, automated welding, or other equipment. Unlinked automated cells are also frequently used in packaging, inspection, and other postassembly operations. But even here, lean practices apply: Keep travel distances short, avoid overproduction, and eliminate waste wherever possible.


Due to the cost of robots and other automated production equipment, individual automated production cells are most effective with:


• High-volume processes with a predictable product mix.


• High-speed or precise assembly.


• Heavy or repetitive operations.


• Non-value-added load and unload.


Nonlinked automated production cells can dramatically reduce cost per unit produced, improve quality, and make more efficient use of human capital.

The exact type of automated assembly tool depends on the application. For example, modular systems built using a combination of electromechanical and pneumatic actuators are often a good choice because they are less expensive, more modular, and offer a broader range of work envelopes than a pedestal type four- or six-axis robot. But these latter types offer higher speeds along with a more humanlike arm movement. The key is to make sure that the technology used is the one that eliminates the most waste. In the right application, automated pick-and-place and robotic systems can dramatically reduce cost per unit produced, improve quality, and allow device firms to make more efficient use of their human capital.


Flexible, Modular Production Systems


Moving from a manual production system to one that incorporates automation, such as automated conveyor systems, is typically determined by production pace and product mix or type. Conveyor systems are generally classified into two basic types: nonsynchronous and synchronous. Nonsynchronous conveyor systems provide independent movement of parts from station to station on an as-needed basis as they become ready for the next operation. Work can be routed independently along a flexible path to optimize material flow through each assembly process. Also, nonsynchronous systems can accommodate the full range of product sizes and weights. These types of conveyors are best for:


• Medium- to high-volume production.


• Predictable product mix.


• Any combination of manual, automated, or mixed production lines.


• Electrostatic discharge–safe transport of sensitive components.


• Safe transport of heavy components.


Additionally, there are virtually no limitations on the number or complexity of assembly steps with nonsynchronous systems. Manual tasks can be readily integrated with automated operations because the system allows for varying station cycle rates. In addition, a company can easily add buffers as needed to balance assembly line work flow. The ability to smoothly integrate manual and automated operations, as well as alter the mix as market forces or product changes demand, lets manufacturers remain flexible and still stay lean.


A Hybrid: Nonsynchronous Conveyors and Lean Production


A typical nonsynchronous, pallet-based conveyor system is made up of modular components.

The drive to become lean, even in a mostly automated assembly environment, has recently led clever manufacturers to develop a kind of hybrid system that incorporates lean ideas. Such hybrid manufacturing systems combine the economies of a manual system with the safety and efficiency of an automated system. The result is a system in which some assembly is done manually while more dangerous, ergonomically difficult, or other nonmanual tasks are accomplished by machine.


In a hybrid system, workstations or cells may be combined with nonsynchronous assembly conveyor systems to achieve desired production goals. For example, automated nonsynchronous assembly conveyors can be situated as feeder lines to a central U-, L-, or J-shaped manual lean cell. The powered conveyors transfer parts between automated operations behind the scenes, so to speak, then supply subassemblies or parts to the lean cell. Machine loading and unloading, a non-value-added task, is also automated in this model to allow the workers in the cell's manual assembly area to exclusively concentrate on value-added work. This is an ideal combination if volumes are high enough to merit automation. The powered conveyors add value by delivering parts on an as-needed basis to the cell, while the high-value-add manual workforce is not tasked with wasteful activities.


For many medical device companies, such hybrid systems offer a promising alternative that would allow them to redeploy existing conveyors in a lean configuration or to add automation where needed if growth demands it. Flexible assembly conveyors based on a T-slotted aluminum frame (which most now are) allow for easy assembly, disassembly, and reconfiguration since they are simply bolted together. These conveyors also come in a wide range of sizes, styles, and even cleanroom capabilities to handle products from pacemakers and home defibrillators to wheelchairs and hospital beds.

Synchronous Conveyors and Fully Automated Production Systems


A fully integrated automated production system allows manufacturers to make better use of high-value human resources.

Synchronous conveyor systems are most commonly found in fully automated assembly systems, especially in very high-speed, high-volume assembly where there is little or no variation from part to part. Medical products such as syringes, catheters, tubing, and other products not requiring much additional assembly might benefit from synchronous conveyor systems, provided that volumes are high enough.


Synchronous conveyor systems use indexed movement of parts from station to station, along a fixed path, and at a fixed cycle rate. Examples include rotary dial machines and CAM-operated in-line machines. Short cycle rates, standardized production, and a high level of automation are features of the synchronous system.


Synchronous systems have limitations in assembly applications. System throughput, for example, must be geared to the slowest operation on the assembly line. And there is no provision for cycle independence; all parts of the production line move in lock step, similar to Lucille Ball in the famous “I Love Lucy” chocolate factory episode. This is a fundamental characteristic of fully automated production.


To offset these disadvantages, many manufacturers accomplish fully automated systems by using modular nonsynchronous conveyors, Cartesian robots, and other automated systems for assembly. Such systems are easier to reconfigure than purpose-built synchronous conveyor systems. Modular flexible chain conveyors can also be used for high-speed synchronous product transport, especially in the system's packaging operations. The key to all of this is the use of modular technologies. It's possible for a company to grow into full automation more easily if it has started with modular nonsynchronous conveyor technologies. The manufacturer can simply add robotics, additional conveyors, vision systems, or other required components to its original investment.


In any case, full automation only makes sense when a device company has long product life cycles and high production runs. Only then can it take full advantage of its investment in sophisticated equipment. Full automation for certain processes also allows a manufacturer to make better use of high-value human resources by deploying them in new lean initiatives.


A fully integrated automated production system is the best choice for manufacturing that has the following features:


• High-volume, predictable mix.


• High-speed assembly that doesn't require manual efforts.


• Transport of sensitive components.


• Safe transport of heavy components.


• Complex, linked assembly sequences.




To stay competitive in today's economy, manufacturers must seize every opportunity to increase productivity and throughput, reduce costs, eliminate waste, and improve product quality and reliability. They must achieve this while managing change on an almost daily basis.


Lean manufacturing principles, when effectively applied, can make these benefits an ongoing reality for medical manufacturers. It is important to remember that the technology a company uses to get lean very much depends on its assembly requirements. By looking at the best way to optimize material flow, people flow, and information flow—even if the manufacturer is sourcing products or components from overseas—it's possible to take big steps to eliminate waste.


Normally, there's a best way to manufacture any product. That way might be simple manual workstations, sophisticated lean manufacturing cells, or semi- or fully automated production. No matter which system is used, manufacturers should be committed to continuous improvement and to reevaluating and retooling their assembly system as needs change.




Kevin Gingerich is director of marketing services for Bosch Rexroth Corp. (Buchanan, MI).


Copyright ©2009 Medical Device & Diagnostic Industry


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