A guide to cellular based manufacturing processes
Cellular or small group manufacturing practices have many advantages over traditional line-based manufacturing practices and are an integral part of lean manufacturing which has its roots in the automotive industry. Each group or cell can handle multiple families of parts, incorporate greater operator involvement across multiple processes rather than just one in traditional line-based manufacturing, and handle fewer parts for more precision work.
Increasingly, cellular based manufacturing is being used in the aerospace and medical device industry for processes which require flexibility for future product development without changing assembly lines.
Companies like Best Technology are developing part cleaning, passivation, and metal finishing equipment – products that enable cellular-based manufacturing.
“Cellular environments are excellent for products that require variability and customization and a loose coupling of steps,” said Robby Slaughter, AccelaWork. “Ultimately, you wind up with more options. And in a rapidly changing economy, you need to adapt to constantly changing customer demands.”
“Many aerospace and medical device industry companies have transitioned to cellular based manufacturing to reduce work in progress (WIP) and increase quality controls by having specific employees own an entire manufacturing cell. This way, the employee sees the part throughout a specific set of manufacturing processes and can easily observe when process has changed and quality has slipped,” said Mike Bangasser, Best Technology. “Another reason these types of companies have cellularized is due to the high precision nature of manufacturing. Many times, a manufacturing step will require ultra-high precision machining and the next process could be state-of-the-art laser welding. Splitting up these process steps allows an employee to become an expert at one process rather than just slightly knowledgeable about multiple processes.”
One of the goals of lean manufacturing is to create a system to deliver products on-time, at the lowest possible unit cost, with the flexibility to accommodate customer demand or design changes.
The basic building blocks of work groups or cells include:
* Cellular-based manufacturing machines
* Tools, gauges, and fixtures
* Material storage
* Materials handling
Many believe workgroups of between five and seven people are optimal. Cells with 10 or more work stations are less common. If the products are very complex, a multi-station assembly line is typically used, with work cells potentially acting as feeder cells.
For companies considering cellular-based manufacturing, time and money will need to be devoted to retrain people, move equipment into workgroups and create signage and operating instructions for each tool, instrument, and machine.
The first step in implementing cellular manufacturing is to break down the various items produced by the company into a number of part sets or families. The grouping process (group technology) involves identifying items with similarities in design characteristics or manufacturing characteristics, and grouping them into part families. Design characteristics include size, shape, and function. Manufacturing characteristics or process characteristics are based on the type and sequence of operations required.
The objective of cellular manufacturing is to design cells in such a way that some measure of performance is optimized. This measure of performance could be productivity, cycle time, or some other logistics measure. Measures seen in practice include pieces per man hour, unit cost, on-time delivery, lead time, defect rates, WIP, and percentage of parts made cell-complete.
One surgical device manufacturer has established cells which include nine different part numbers based on customer demand. Cells consist of all the work centers, wash stations, inspection equipment, heat treat ovens, deburring equipment, passivation units, coating equipment, supplies, enclosures, and benches necessary to complete the product from raw material to finished goods. Basically the raw material is delivered to the work centers.
Products are machined, inspected and then moved to secondary operations at the end of each shift. Secondary operations consist of mechanical deburring, heat treating, passivation, coating, and putting product in clean packages. Part flow is vital to meet customer demand each day. Cell performance, safety, delivery and quality are monitored and discussed at meetings.
By breaking the factory into small, homogeneous and cohesive productive units, production and quality control is made easier. Cells that are not performing according to volume and quality targets can be easily isolated, since the parts/products affected can be traced to a single cell. Also, because the productive units are small, the search for the root of problems is made easier.
Quality parameters and control procedures can be dovetailed to the particular requirements of the parts or workpieces specific to a certain cell. By focusing quality control activity on a particular production unit or part type, the cell can quickly master the necessary quality requirements. Control is always enhanced when productive units are kept at a minimum operating scale, which is what cellular manufacturing provides.
Once identified, similar items can be classified into families. Then a system is developed that facilitates retrieval from a design and manufacturing database. For example, the system can be used to determine if an identical or similar part exists before a completely new part is designed. If a similar part is found, it may be that a simple modification would produce satisfactory results without the expense of new part design. Similarly, planning the manufacturing of a new part after matching it with an existing part family can eliminate new and costly processing requirements.
Frequently, machines are grouped in an efficient U-shaped configuration. Since each machine operates on its own for much of the cycle, few workers may be needed, and even then only for a limited number of steps.
Some plants in advanced stages of cellular manufacturing utilize what is known as a ‘mini-plant’. The cell does the manufacturing and has its own support services, including its own industrial engineer, quality manager, accountant, and marketing representative and/or salesperson. An entire facility can be broken down into a number of mini-plants, each of which operates as an independent profit center.
Cellular-based manufacturing requires a high degree of collaboration, communication, and consistency to work properly. For many companies and their communications, the approach is worthwhile because products are delivered on time and according to specifications.