DEMAND FLOW MANUFACTURING CONCEPTS AND METHODS
Dan Reed - LeanMean-Manufacturing.com
LeanMean-Manufacturing.Com is dedicated to providing a one stop exchange of information on manufacturing software systems consisting of: ERP, CAD, CAM, CAE, Demand Flow, Manufacturing Engineering, and Logistics. The combination of these software technologies with proven "Lean manufacturing methods" has created a new manufacturing business model. This new model encompasses two components: Lean Manufacturing methods, and standards based Computer-Integrated Manufacturing technologies.
This paper introduces the concepts of flow manufacturing. Flow manufacturing is a pull-driven strategy. At its core is the principle that demand can be synchronized with a daily production rate by properly sequencing items on a flow line that is replenished frequently by suppliers. In this way, inventory is kept to a minimum, goods are made to demand, cycle times fall within the required order-to-deliver response times, and constraints are minimized.
Manufacturers have been working on one of two new scheduling methods: constraint-driven planning and scheduling and demand-driven flow manufacturing. Both are focused in different ways on plant floor performance issues. The first assumes constraints need to be monitored, managed, and regulated with highly sophisticated planning systems. The second attempts to eliminate as many constraints as possible to streamline the flow of production, to produce any product on any day (mixed model production) and to manage production with simplified techniques and integrated systems.
Major Shift Under Way - Lean Manufacturing
Call it demand flow, lean manufacturing, or agile, it has been a "competitive secret" of many manufacturing companies considered leaders in their industry. Most notable are original equipment manufacturers in: automotive, electronics, and aerospace industries, that have pushed their supplier chains to adopt just-in-time (JIT) and Lean manufacturing practices for the last decade. At one time it was primarily process engineers who championed the use of flow techniques. However, during the last decade a growing number of corporate executives have become committed to demand flow manufacturing as a competitive business strategy.
The principles of demand flow manufacturing are based on eliminating waste, streamlining processes, building to demand, and implementing continuous improvement concepts adopted from proven Japanese methods, and made famous in the Toyota Production System.
The demand flow concept is based on the theory that by "properly designing production lines" and balancing the mix of products to a daily demand rate, quality products can be produced as ordered, and at a rate that falls within the required order-to-deliver response time. As a result, the entire supply process is pulled, and sequenced from actual demand; it is not pushed, or rescheduled to meet customer promise dates.
Fundamentals of Lean Techniques
The most critical element of demand flow manufacturing is establishing the correct design of each flow line. The first step is to "Map" your manufacturing processes to determine process commonalties. Process Mapping defines product families that will be built together on a given line. Based on demand patterns, and what is known about future growth expectations, a line is designed so that processes and materials flow at a consistent rate to meet the daily demand.
The TAKT time of one unit every "X" minutes (drumbeat or rhythm), and total product cycle time (TPCT), are used as the baseline to make further line design decisions. These include how primary, feeder and independent lines will flow; which sequence of events will occur where and when; how many and what types of Kanbans should be used; and what other resources and personnel are needed.
To address slower moving operations, more time-consuming parts production, or highly engineered machine processes, the line is synchronized through use of "in-process Kanbans", calculated to balance the line throughput. The Kanbans act as buffers in the line; they provide the mechanisms to add capacity for a specific operation in order to bring the line up to the daily rate, or to hold a batch between a feeder and a primary line.
More than a line design results from this process. Many companies also rationalize their products and processes as they look for ways to reduce cycle times, product complexity, inventories and extra steps in their flow lines. It is not uncommon for manufacturers to re-design their products with fewer parts, more common and modular components across a product family (Group technology) and more standardized materials and parts across their lines. For those that traditionally design products to order, this rationalization allows them to sell highly configured products and still meet their customers' needs. This reduces the complexity; inventories levels and frequent line design changes for one-off engineered products.
Flow lines, by their nature, change the way products are built and tracked. They remove subassembly operations, replacing them with feeder lines, thereby eliminating intermediate component identification and inventory stocking.
Flow-Driven Line Design and Synchronization
Demand Smoothing across lines is a procedure used to calculate the daily production rates for every product on flow lines, based on a given day's demand, not on a fixed daily production rate. Using the new rates for that day, the appropriate mix of products for each line is determined and sequenced. The result is considerable flexibility on the line to match changing demand characteristics in the marketplace. As daily demand varies, the items produced vary and the resources, such as operators, move around. Demand smoothing is significantly more flexible, and more adept at managing daily variations, then rate based scheduling methods found in repetitive ERP systems.
Sequencing differs considerably from constraint-based scheduling, which regulates the flow around a constraint and schedules orders in priority fashion on the basis of due dates. Pull-driven sequencing on the other hand, factors in each product's total product cycle time (TPCT) and daily demand volume to determine the best sequencing pattern for optimal throughput. Sequencing is focused on optimizing the current order backlog, "current demand," while grouping all other demand variables into flex fences. It also groups like products together, to maximize set-ups.
Whether booking configured or standard products, customers always request a promise date. In a demand flow environment, as in any other, the order taker needs visibility of product availability and any capacity or material shortages for products that have lengthy order cycle times. To provide such an immediate available to promise (ATP) response, the flow manufacturing system needs to be fully integrated with other business systems. Within months of initiating a demand flow pilot program, most companies find they need better systems than those they started with; typically, a series of spreadsheet programs used independently of each other. Most agree they would have been better off with an integrated, automated system from the start; one linked to their suppliers as well as to their ERP and order management systems. Such a system can maintain all the data in one place, and ensure data integrity.
Kanbans are visual methods for replenishing materials as they are consumed during the manufacturing process. Kanbans, limit the amount of inventory kept in raw-in-process or on a production line. Kanbans produce the effect of holding materials, in a non-converted form, until the last possible moment, before committing material it to production.
There are three types of material Kanbans. Raw material Kanbans are replenished when commodity stock is consumed by demand. Kanbans that are replenished only when orders are booked, and a demand signal is generated. And in process Kanbans are added to a line to buffer operations between feeder and primary flow lines or to add capacity to a process as needed.
It is quite common to have inventory on demand flow lines, which is maintained in a two-bin system. When one bin is empty, the second is used, while the first is replenished to the empty bin, on the line by suppliers. In that way, the line doesn't run out of materials and a Kanban mechanism - an empty bin triggers the signal for replenishment. In some cases, this is used to interface demand flow methods with MRP planning methods.
Engineering change control is not new to manufacturing systems. However, it is different within demand flow environments. Instead of associating the change directly with an end item, it is associated with processes involved in producing the item. In demand flow, the engineering change is tied directly to a process definition, sequence of events, associated resources, and operational method sheets. As a result, when a change in material goes into effect, just the associated process is changed; the material information is rolled up into the end item for production reporting.
Parametric (3D Solid Modeling) design functionality. Once the initial design concept is sketched, simply selecting the subject dimension, which controls that geometry, and type in a new value, can make any future geometry adjustments to those sketches. The geometry will automatically update, corresponding to the new dimension, as well as any other geometry affected by the dimensional change. This feature allows for blistering fast design changes, as well as development of new parts. Parametric design represents the "critical element" required to achieve rapid customizations, inherent within the short cycle, customer oriented, demand driven "Lean manufacturing" environments.
Operational Method Sheets are essential for producing high quality products. They are graphically produced work instructions for line operators. Unlike traditional work papers created for a given work order, method sheets are created for a specific process performed on the flow line. They are also designed to be more visual than work papers, to provide more guidance and to direct proper quality control and verification procedures for each process event. In effect, operational method sheets serve as; quality assurance communications, between the customer, engineering, management, and plant floor personnel!
Operational Method sheets are based on engineered product specifications and drawings, and they must be updated when a process undergoes engineering changes. Integrating the creation of method sheets with the bill of materials, and engineering change functions produces considerable time savings and quality measure improvements.
Backflushing. A demand flow environment simplifies production reporting. When the production process is finished, the resulting products are reported as complete. All the materials consumed in producing those finished products are automatically deducted from raw and in-process inventory through backflushing techniques. Any exception issues are handled with a scrap code. Backflushing techniques very often accommodate multiple product revision reporting, regardless of when or where they occur, to address the engineering changes.
Exception reporting is always needed, whether it refers to products or process variances. Demand flow lines are no different; they are just easier to monitor. Deviations from the daily rate are reported as production variances, and line bottlenecks are reported as production linearity exceptions.
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