One-Piece Flow is a fundamental element of becoming lean. To think of processing one unit at a time usually sends a shudder through the organisation which has batch manufacturing as its lifeblood. The word “one” does not necessarily have a literal meaning. It should be related to the customers’ requirements and could be one unit of order. However, what it does mean is that the organisation should only process what the customer wants, in the quantity he wants and when he wants it.
One-piece flow (also commonly referred to as continuous flow manufacturing) is a technique used to manufacture components in a cellular environment. The cell is an area where everything that is needed to process the part is within easy reach, and no part is allowed to go to the next operation until the previous operation has been completed. The goals of one-piece flow are: to make one part at a time correctly all the time to achieve this without unplanned interruptions to achieve this without lengthy queue times. One-piece flow describes the sequence of product or of transactional activities through a process one unit at a time. In contrast, batch processing creates a large number of products or works on a large number of transactions at one time – sending them together as a group through each operational step. One-piece flow focuses on employees’ efforts on the manufacturing process itself rather than on waiting, transporting products, and storing inventory. It also makes the production process flow smoothly, one piece at a time, creating a steady workload for all employees involved. One-piece flow methods need short changeover times and are conducive to a pull system.
There are many advantages to incorporating the one-piece flow method into your work processes. These include the following:
- It reduces the time that elapses between a customer order and shipment of the finished product.
- It prevents the wait times and production delays that can occur during batch processing.
- By reducing excess inventory, one-piece flow reduces the labour, energy, and space that employees must devote to storing and transporting large lots or batches.
- It reduces the damage that can occur to product units during batch processing.
- It reveals any defects or problems in product units early in the production process.
- It gives your organization the flexibility to meet customer demands for a specific product at a specific time.
- It reduces your operating costs by making non-value-added work more evident. This enables you to eliminate waste.
Difference between a push system and a pull system
“Fat” organizations use a push system. In such a system, goods are produced and handed off to a downstream process, where they are stored until needed. This type of system creates excess inventory. Lean organizations, on the other hand, use a pull system, in which goods are built only when a downstream process requests them. The customer then “pulls” the product from the organization. The final operation in a production process drives a pull system. Customer-order information goes only to the product’s final assembly area. As a result, nothing is produced until it is needed or wanted downstream, so the organization produces only what is needed. A pull system streamlines the flow of materials through your production process. This greatly improves your organization’s productivity by doing the following:
- It reduces the time that employees spend in nonvalue-added steps, such as waiting and transporting product units.
- It reduces downtime caused by product changeovers and equipment adjustments.
- It reduces the distances that materials or works in progress must travel between assembly steps.
- It eliminates the need for inspection or reworking of materials.
- It bases your equipment usage on your cycle time.
Achieving one-piece flow
While many are familiar with the terminology, there is still a significant amount of confusion regarding what one-piece flow means and, more importantly, how to achieve it. Let us begin by stepping back and attempting to understand the concept of “connected flow.” Achieving connected flow means implementing a means of connecting each process step within a value stream. In a typical MRP batch-and-queue manufacturing environment as illustrated below, parts move from functional area to functional area in batches, and each processing step or set of processing steps is controlled independently by a schedule.
There is little relationship between each manufacturing step and the steps immediately upstream or downstream. This results in:
- Large amounts of scrap when a defect is found because of large batches of WIP,
- Long manufacturing lead time,
- Poor on-time delivery and/or lots of finished goods inventory to compensate,
- Large amounts of WIP.
When we achieve connected flow, there is a relationship between processing steps: That relationship is either a pull system such as a supermarket or FIFO lane or a direct link (one-piece flow). As illustrated below, one-piece flow is the ideal method for creating connected flow because the product is moved from step to step with essentially no waiting (zero WIP).
The basic condition for achieving one-piece flow works best when your production process and products meet certain requirements.To be good candidates for one-piece flow, we must have the following conditions:
- Processes must be able to consistently produce a good product. If there are many quality issues, one-piece flow is impossible.
- Your product changeover times must be very short; almost instantaneous is best. One-piece flow is impractical when many time-consuming changeover operations are needed during the production process.
- Another requirement is that the products you make must be suitable for one-piece flow. Very small product units are usually not suitable because too much time is required for their setup, positioning, and removal from production equipment. The one-piece flow might be possible for the production of very small product units if you can completely automate their movement through your production process and if your cycle time is short.
- Process times must be repeatable as well. If there is much variation, one-piece flow is impossible.
- Equipment must have very high (near 100 percent) uptime. Equipment must always be available to run. If equipment within a manufacturing cell is plagued with downtime, one-piece flow will be impossible.
- Processes must be able to be scaled to tact time, or the rate of customer demand. For example, if tact time is 10 minutes, processes should be able to scale to run at one unit every 10 minutes.
Without the above conditions in place, some other form of connecting flow must be used. This means that there will be a buffer of inventory typically in the form of a supermarket or FIFO lane between processes; the goal would be to eventually achieve one-piece flow (no buffer) by improving the processes. If a set of processes is determined to a candidate for one-piece flow, then the next step is to begin implementation of a one-piece flow cell.
Implementing one-piece flow
The number of units you produce should equal the number of items your customer’s order. In other words, your selling cycle time should equal your manufacturing cycle time.The first step in implementing a one-piece flow cell is to decide which products or product families will go into the cells, and determine the type of cell: Product-focused or mixed model. For product-focused cells to work correctly, demand needs to be high enough for an individual product. For mixed model cells to work, changeover times must be kept short; a general rule of thumb is that change over time must be less than one tact time. The next step is to calculate tact time for the set of products that will go into the cell. Tact time is a measure of customer demand expressed in units of time and is calculated as follows:
Tact time = Available work-time per shift / Customer demand per shift
Next, determine the work elements and time required for making one piece. In much detail, list each step and its associated time. Time each step separately several times and use the lowest repeatable time. Then, determine if the equipment to be used within the cell can meet tact time. Considerations here include changeover times, load and unload times and downtime. The next step is to create a lean layout. Using the principles of 5-S (eliminating those items that are not needed and locating all items/equipment/materials that are needed at their points of use in the proper sequence), design a layout. Space between processes within a one-piece flow cell must be limited to eliminate motion waste and to prevent unwanted WIP accumulation. U shaped cells are generally best; however, if this is impossible due to factory floor limitations, other shapes will do. For example, I have implemented S-shaped cells in areas where a large U shape is physically impossible. Finally, balance the cell and create standardized work for each operator within the cell. Determine how many operators are needed to meet tact time and then split the work between operators. Use the following equation:
Number of operators = Total work content / Tact time
In most cases, an “inconvenient” remainder term will result (e.g., you will end up with Number of Operators = 4.4 or 2.3 or 3.6 instead of 2.0, 3.0, or 4.0). If there is a remainder term, it may be necessary to kaizen the process and reduce the work content. Other possibilities include moving operations to the supplying process to balance the line.
One-Piece Flow in production
The following illustration shows the impact of batch size reduction when comparing batch and – queue and one-piece flow.
How we can see differences between these both flow systems is very enormous. One-piece flow system saved 18 minutes for to the same batch of 10 pieces. With this system can be produced rather 3 times more than a batch and queue system. Next, the first piece was in processes for only 3 minutes. It means that system or operator can check part immediately in every process (A, B and C). Batch and queue system allowed produce many parts after every process. If will be occurred failure in the system than will be detected too late and many parts will be damaged.
Equipment for one-piece flow
To accommodate one-piece flow, equipment should be correctly sized to meet customer demand. Machines designed for batch production might not be easy to adapt to one-piece-flow cycle times. One-piece flow works best with machines that are smaller and somewhat slower than equipment that is suited for batch processing. Equipment used for one-piece flow also needs to be easy to set up quickly so that you can use it to produce a wide mix of products. Because the volume, capacity, and force requirements are often lower for one-piece-flow production, machines that are suited for it can be smaller. Smaller machines save space and leave little opportunity for waste, such as inventory and defective parts, to accumulate. They are also less expensive to purchase. Slower machines are often sufficient for one-piece flow because the aim is to produce goods according to the manufacturing cycle time. Automated and semi-automated machines work well in one-piece-flow production. They stop and give the operator a signal when a cycle is complete or if any problems occur. They are sometimes also capable of notifying the next operation when to begin processing. And they often unload automatically after processing is done. Synchronize your equipment’s production operations by delaying the start of faster operations rather than speeding up or slowing down the machines. Running production equipment outside of its specified range can reduce product quality or tool life.
To achieve a one-piece-flow method’s full potential, it is important to follow five points with regard to your work-cell layout and employee training. These points are outlined below.
- Simplify the flow of your materials and parts. Below are several guidelines to follow:
- Keep all goods flowing in the same direction.
- Make sure all parts flow from storage through the factory according to the processing sequence.
- Use first-in, first-out, or FIFO stocking.
- Arrange parts for easy feeding into the production line.
- Eliminate any non-value-added space in your work cells.
- Keep all pathways in work areas clear; leave aisles open along walls and windows.
- Make sure that material input and production output are separate operations.
- Position your equipment to allow easy maintenance access.
- Make sure separate work processes are located as close together as possible.
- Set up your production lines to maximize the equipment operators’ productivity. Review the feasibility of both straight-line and U-shaped work cells and their impact on both operator movement and productivity and the flow of work materials. Remember that a U-shaped work cell brings the ending point of a work process close to the beginning point, which minimizes the distance an operator has to move before beginning a new production cycle. This setup is better for some work processes than a straight-line work cell.
- Allot space in the layout of your work cells for regular equipment and product inspection. Remember that the employees working in each cell must be able to easily conduct a full-lot inspection. Such inspections prevent defects by catching any errors and non-standard conditions. This ensures that only defect-free parts are fed to the next step in your production process.
- Minimize your in-process inventory. Predetermine the stock that employees will have on hand for the entire production line. Arrange your work cells to enable an easy flow of materials into and out of all work areas.
- When your equipment is arranged to enable a smooth process flow, equipment operators might need to learn how to run different types of equipment. Such operators usually need to work standing up, instead of sitting down, so they can easily run a number of machines in sequence. Keep this in mind when designing your work cells. Cross-train your employees so that they know how to perform different work functions. Equipment operators are then able to go to other work cells if production is not required at their normal work areas. This also enables an entire work team to take full responsibility for the production process.
Tools to implement a one-piece-flow process
Three tools are necessary for assessing and planning for a one-piece-flow process:
- PQ analysis table
- Process route table
- Standard Operation
- Quick Changeover
PQ analysis table
A PQ analysis table is a tool that helps employees understand the types of products your organization produces and the volume that your customers demand. It also shows whether the majority of your production volume is made up of a small or wide variety of parts. The PQ analysis table enables employees to identify what products are suitable for one-piece-flow production. The P in PQ stands for products; the Q stands for the quantity of production output.
Case example: Quick-Lite’s PQ analysis Quick-Lite conducts a PQ analysis of its spark-plug final-assembly part numbers to see if a wide or limited variety of spark plugs makes up most of the volume. They find that six spark plugs made up 53.3% of the total volume. The manufacturing processes for these six spark plugs are likely candidates for one-piece-flow operations.
Once the Quick-Lite team identifies these products in a PQ analysis table, they create a process route table to determine whether a similar technology is used to manufacture all six types of spark plugs.
A process route table
A process route table shows the machines and equipment required for processing a component or completing an assembly process. Such a table helps you to arrange your equipment in production lines according to product type and to group related manufacturing tasks into work cells. You can also use a process route table to analyze process, function, or task-level activities. The steps for creating a process route table are as follows:
1. Somewhere above the top of the table, write the following:
a. The name or number of the department whose activity is being analyzed.
b. The operation or product that is being analyzed.
c. The name of the person completing the form.
d. The date on which the form is completed.
2. Use the “No.” column on the left for the sequential numbering of the products or operations being analyzed.
3. For each product or operation you are analyzing, enter the item name, machine number, or function.
4. For each product or operation, enter circled numbers in the various resource columns that correspond to the sequence in which the resources are used for that product or operation.
5. Connect the circled numbers with lines or arrows to indicate the sequence of operations. Once you have completed the table, look for items or products that follow the same, or nearly the same, the sequence of the machine and/or resource usage. You might be able to group these machines and/or resources together in the same work cells to improve the efficiency of your operations.
Once your work team a) collects all the data necessary for selecting the products that are suitable for one-piece flow, b) verifies the operations needed and the available capacity, and c) understands the specific task in detail, you can implement the layout of your improved work cells and make one-piece flow a reality in your organization.
A work combination is a mixture of people, processes, materials, and technology that comes together to enable the completion of a work process. The term standard operations refer to the most efficient work combination that a company can put together. When you apply all your knowledge of lean principles to a particular work process to make it as efficient as possible, a standard operation is a result. Employees then use this documented process as a guide to consistently apply the tasks they must perform in that work process. In addition, once you prepare standard operations for your work processes, they serve as the basis for all your organization’s training, performance monitoring, and continuous improvement activities. A big part of making your organization a lean enterprise is identifying different types of waste and finding ways to eliminate them. Ultimately, however, it is the correct combination of people, processes, materials, and technology that enables your organization to create quality products and services at the lowest possible operational cost. Putting together standard operations forces you to break down each of your work processes into definable elements. This enables you to readily identify waste, develop solutions to problems, and provide all employees with guidance about the best way to get things done. Many organizations that have used standard operations report that this lean initiative is the one that has had the biggest impact on their ability to produce better-quality products and services, make their workflow smoother and make their training process more productive. In addition, standard operations enable employees to actually see the waste that they previously didn’t see. The process for developing standard operations involves eight steps.
- Establish improvement teams.
- Determine your takt time.
- Determine your cycle time.
- Determine your work sequence.
- Determine the standard quantity of your work in progress.
- Prepare a standard workflow diagram.
- Prepare a standard operations sheet.
- Continuously improve your standard operations.
Step 1: Establish improvement teams
Some organizations take a top-down approach to the development of standard operations: supervisors alone determine what work tasks are to be performed, by whom, and when. Other organizations believe that only front-line workers should develop standard operations because these employees have keen insight into how things are done. But due to the nature of the steps required to establish standard operations, a team-based approach is best. It is best to have all employees who are impacted by a work process involved in the development of standard operations for that process. Lean organizations understand the need for complete buy-in and support of all work tasks by all the employees involved. It’s also important to coordinate this team effort with your organization’s other lean initiatives.
Step 2: Determine your takt time
Takt time is the total available work time per day (or shift), divided by customer-demand requirements per day (or shift). Takt time enables your organization to balance the pace of its production outputs to match the rate of customer demand. The mathematical formula for determining your takt time is as follows:
takt time = available daily production time/ required daily quantity of output
Step 3: Determine your cycle time
Cycle time is the time it takes to successfully complete the tasks required for a work process. It is important to note that a work process’s cycle time may or may not equal its takt time. A process capacity table is a helpful tool for gathering information about the sequence of operations that make up a work process and the time required to complete each operation. Ultimately, the process capacity table can help you determine machine and operator capacity. Complete a process capacity table before you begin making changes such as moving equipment, changing the sequence of your operations, or moving employees’ positions and/or changing their job responsibilities. It is important to first know what your current capacity is and what it will be in the new process configuration that you plan.
Steps for Creating a Process Capacity Table
- Enter the line/cell name.
- Record the total work time per shift.
- Enter the number of shifts.
- Record the maximum output per shift.
- Enter the sequence number of each processing step being performed on the part or product.
- Record the operation description, which is the process being performed on the part or product.
- Enter the number (if applicable) of the machine performing the process.
- Record the walk time, the approximate time required between the end of one process and the beginning of the next process.
- Enter the manual time, the time an operator must take to manually operate a machine when an automatic cycle is not activated. The manual time includes the time required to unload a finished part from the machine; load a new, unfinished part; and restart the machine.
- Record the automated time, the time required for a machine’s automatic cycle to perform an operation, from the point when the start button is activated to the point when the finished part is ready to be unloaded.
- Calculate the total cycle time by adding the manual time and the
- Enter the pieces per change, the total number of parts or products that a machine typically produces before its tool bits must be changed due to wear.
- Record the change time, the amount of time required to physically change a machine’s tool bits or perform a sample inspection. This is the time required to change tooling due to normal wear during a production run— not the changeover time required to go from making one part or product to making another.
- Calculate the time per piece, the change time divided by the pieces per change.
- Enter the production capacity per shift (also known as the total capacity). This is the total number of units that can be produced during the available hours per shift or per day.
- Record the takt time for the work process in the Takt Time box, using the mathematical formula shown earlier in this chapter.
- Calculate the total capacity of the process by adding the time to finish the process and the time per piece.
Step 4: Determine your work sequence
A work sequence is a sequential order in which the tasks that make up a work process are performed. A work sequence provides employees with the correct order in which to perform their duties. This is especially important for multifunction operators who must perform tasks at various workstations within the takt time. A standard operations combination chart enables your improvement team to study the work sequence for all your organization’s work processes. In such a chart, each task is listed sequentially and broken down into manual, automated, wait, and walk times. Wait time is not included in a process capacity table because worker idle time has no impact on automated activities or the capacity of a process. However, wait time is included in a standard operations combination chart to identify idle time during which a worker could instead be performing other tasks, such as external setup, materials handling, or inspection. The goal is to eliminate all worker idle time.
The steps for completing a standard operations combination chart are described below.
- At the top of a form indicate the following:
- The date that the work process is being mapped.
- The number of pages (if the chart is more than one page long).
- The name of the equipment operator.
- The name of the person entering data on the form (if different from the operator).
- The number and/or name of the part or product being produced.
- The name of the process or activity is mapped.
- The machine number and/or name.
- The work cell number and/or name.
- The required output per designated period (e.g., parts per shift or pounds per day).
- The takt time for the process.
- The total capacity for the process. Ideally, this should equal the takt time that you calculated in step 2.
- The difference between the takt time and the cycle time for the work process.
- It is often helpful to indicate the type of units the work activity is usually measured. Activities are normally measured in seconds, but some are measured in minutes or even longer intervals.
- Number every fifth or tenth line on the graph area to facilitate your recording of activity times. Choose convenient time intervals so that either the takt time or the actual cycle time—whichever is greater—is located near the right side of the graph area.
- Draw a line that represents the activity’s takt time. Trace the line with red so it stands out.
- Sequentially number each operational step in the appropriate column. Steps can include any or all of the following:
- Manual operations.
- Automated operations.
- Time spent walking from one location to another.
- Time spent waiting.
- Provide a brief name and description for each step.
- Note the time required for the completion of each step in the appropriate column.
- Draw a horizontal line on the graph representing each step, using the following guidelines:
- The length of the line should equal the duration of the step.
- The line type should match the action type (see the line key at the top of the sample chart).
- Each line type should be in a different colour, which will make your chart much easier to read.
- Each line you draw should begin at the point on the vertical timeline that corresponds to the actual time the activity begins. It should end at the actual time the activity ends.
For example, if the first step of work activity is an automatic hopper fill that takes fifteen seconds to complete, and the operator assembles a carton for ten seconds during that fifteen seconds, both steps would start at time zero, with the carton assembly ending at time ten and the automatic fill ending at time fifteen. However, if the operator waits until the automatic hopper fill is completed before assembling the carton, the fill would start at time zero and end at time ten, but the carton assembly would start at time fifteen and end
at time twenty-five. Your completed standard operations combination chart should provide you with some useful insights, including the following:
- If the total time to complete the process or activity equals the red takt-timeline, You already have an efficient work combination in place.
- If the total time required to complete the process or activity falls short of the red takt-timeline, you might be able to add other operations to the activity to use your resources more effectively.
- If the total time required to complete the process or activity is longer than the red takt-timeline, there is waste in your process.
Use the following steps to identify where this waste occurs:
1. Look over the steps in your process to see if any of them can be compressed or eliminated. Perhaps one or more steps can be completed during periods when the equipment operator is waiting for automated operations to be completed.
2. Look at the movement of employees and materials. Can you reduce or eliminate any of it by relocating supplies or equipment?
Step 5: Determine the standard quantity of your work in progress
The standard quantity of your work in progress (WIP) is the minimum amount of WIP inventory that must be held at or between your work processes. Without having this quantity of completed work on hand, it is impossible to synchronize your work operations.
When determining the best standard quantity of WIP you should have, consider the following points:
- Try to keep the quantity as small as possible.
- Ensure that the quantity you choose is suitable to cover the time required for your error-proofing and quality-assurance activities.
- Make sure that the quantity enables all employees to easily and safely handle parts and materials between work operations.
Step 6: Prepare a standard workflow diagram
A workflow diagram shows your organization’s current equipment layout and the movement of materials and workers during work processes. Such a diagram helps your improvement team plan future improvements to your organization, such as one-piece flow. The information in your workflow diagram supplements the information in your process capacity table and standard operations combination chart. When combined, the data in these three charts serve as a good basis for developing your standard operations sheet. The steps for completing a workflow diagram are described below.
- At the top of the diagram, indicate the following:
a. The beginning and endpoints of the activity you are mapping.
b. The date the activity is being mapped. The name of the person completing the diagram should also be included.
c. The name and/or a number of the part or product being produced.
- Sketch the work location for the work process you are mapping, showing all of the facilities directly involved with the process.
- Indicate the work sequence by numbering the facilities in the order in which they are used during the activity.
- Connect the facility numbers with solid arrows and number them, starting with 1 and continuing to the highest number needed. Use solid arrows to indicate the direction of the workflow.
- Using a dashed arrow, connect the highest-numbered facility to facility number 1. This arrow indicates a return to the beginning of the production cycle.
- Place a diamond symbol (✧) at each facility that requires a quality check.
- Place a cross symbol (✝) at each facility where safety precautions or checks are required. Pay particular attention to facilities that include rotating parts, blades, or pinch points.
- Place an asterisk (* ) at each location where it is normal to accumulate standard WIP inventory. Adjacent to the asterisk, indicate the magnitude of the inventory— measured in number, weight, volume, and so on.
- Also, enter the total magnitude of the inventory in the “Number of WIP Pieces” box.
- Enter the takt time for the operation in the “Takt Time” box. Calculate the takt time.
- Enter the time required to complete a single cycle of the activity in the “Cycle Time” box. Ideally, this time should equal the takt time.
The workflow diagram provides a visual map of workspace organization, movement of materials and workers, and distances travelled—information not included in either the process capacity table or the standard operations combination chart. You can use this information to improve your workspace organization, re-sequence your work steps, and reposition your equipment, materials, and workers to shorten your cycle time and the overall travel distance. This will help you to achieve your takt time.
Step 7: Prepare a standard operations sheet
Numerous formats exist for standard operations sheets. In general, the layout for your sheet should include the components listed below:
- The header section should contain the following:
- Process name
- Part or product name
- Takt time
- Cycle time
- Approval date
- Revision level
- The work sequence section should contain the following:
- Sequence number
- Description of task
- Manual time
- Automated time
- Walk time
- Inventory requirements
- Key points
- Safety precautions
- Related job procedures
- The workflow diagram section should contain a pictorial representation of the work area.
- The footer section should contain the following:
- Lean enterprise tools applied to the work process
- Safety equipment required
- Page indicator (for multiple-page standard operations sheets)
Step 8: Continuously improve your standard operations
After you complete your standard operations sheet, you should train all employees who are affected by your changes to the work process in question. Don’t be surprised if, during this training, employees discover potential opportunities for even greater improvement. It is through the continuous improvement of your standard operations that your organization can systematically drive out waste and reduce costs. You should review your organization’s standard operations sheet(s) on a periodic basis to ensure all employees are accurately complying with them.
Quick changeover is a method of analyzing your organization’s manufacturing processes and then reducing the materials, skilled resources, and time required for equipment setup, including the exchange of tools and dies.Using the quick-changeover method helps your production teams reduce downtime by improving the setup process for new product launches and product changeovers, as well as improving associated maintenance activities. There are many advantages to using the quick changeover method. These include the following:
- Members of your team can respond to changes in product demand more quickly.
- Machine capacity is increased, which allows for greater production capacity.
- Manufacturing errors are reduced.
- Changeovers are made more safely.
- You can reduce your inventory (and its associated costs) because it is no longer needed for extended downtimes.
- Once you can make changeovers according to an established procedure, you can train additional operators to perform these tasks, which increases the flexibility of your organization.
- Lead times are shortened, improving your organization’s competitive position in the marketplace.
You use the PDCA Cycle to make improvements to your setup and changeover processes. The procedure to implement Quick changeover involves the following steps:
Evaluate your current processes. (Plan)
a. Conduct an overview of your current production process to identify all equipment and processes that require downtime for changeover. Include all processes that require tooling replacement or new dies, patterns, moulds, paints, test equipment, filtration media, and so on.
b. Collect data using a check sheet for each process. Make sure the check sheet includes information about the following:
- Duration of the changeover. This is the time it takes from the start of the changeover process to its completion, including preparation and cleanup.
- The amount of production typically lost during the changeover, including the number of units not produced, the number of hours that operators are not engaged in productive activities lost production time, and rework (measured in hours and units).
- Process events that are constraint operations: these are operations that are long in duration or are critical to completing the manufacturing process.
c. Create a matrix diagram to display this data for each production process (categories might include setup time, resources and materials required, and change over time).
d. Select a process as your target for improvement. A good process to choose is one that has a long downtime, setup time, and/or change over time; is a frequent source of error or safety concerns, or is critical to process output. A constraint operation that requires a changeover during your production operations is often a good first target to select. Choose no more than three targets to work on at one time.
Document all the current changeover activities for the process you have selected. (Plan)
a. Make a checklist of all the parts and steps required in the current changeover, including the following:
- Numeric values for all measurements and dimensions
- Part numbers
- Special settings
b. Identify any waste or problems associated with your current changeover activities.
c. Record the duration of each activity. See the sample data sheet below.
d. Create a graph of your current change over time (in seconds) to establish a baseline for improvement.
e. Set your improvement target. A target of a 50% reduction is recommended.
Identify internal and external process activities. (Plan)
a. Create two categories on your checklist: one for internal processes, and one for external processes.
b. List each task under the appropriate category, making sure to keep the tasks in the correct sequence.
Turn as many internal processes as possible into external processes. (Plan)
Using your checklist, complete the following steps:
a. Identify the activities that employees currently perform while the line or process is idle that can be performed while it is still running.
b. Identify ways to prepare in advance any operating conditions that must be in place while the line is running (e.g., preheating equipment).
c. Standardize parts and tools that are required for the changeover process, including the following:
- Securing devices used.
- Methods of locating and centring objects.
- Methods of expelling and clamping objects.
Streamline the process. (Plan)
a. Use visual management techniques to organize your workplace.
b. Consider ways to error-proof the process.
c. Consider ways to eliminate unnecessary delays in your internal processes by doing the following:
- Identifying the activities that can be done concurrently by multiple employees.
- Using signals, such as buzzers or whistles, to cue operators.
- Using one-turn, one-motion, or interlocking methods.
d. Consider ways to eliminate unnecessary delays in your external processes by making improvements in the following:
- Storage and transportation of parts and tools.
- Automation methods.
- Accessibility of resources.
e. Create a new process map showing your proposed changes to the setup process.
Test your proposed changes to the process. (Do)
a. Consider the feasibility of each proposed change.
b. Prepare and check all materials and tools required for the changeover. Make sure they are where they should be and that they are in good working order.
c. Perform your revised setup activities for the parts and tools. Adjust settings, calibrate equipment, set checkpoints, and so on, as required.
d. Perform a trial run of your proposed changes.
e. Collect data on the duration of the setup time, and update your changeover improvement chart.
Evaluate the results of your changes. (Check)
Take a look at the results of the changes you have made. Did the results meet your target goal? If so, go on to step 8. If not, make adjustments or consider other ways in which you can streamline your changeover activities and make the process external.
Implement your new quick-changeover process and continue to work to improve it. (Act)
- Document the new procedures and train all involved employees on the new procedures.
- Continue to collect data for continuous improvement of the changeover process.
- Create a revised matrix diagram of the change processes and begin the quick changeover process again.
Cellular Manufacturing is a method of producing similar products using cells, or groups of team members, workstations, or equipment, to facilitate operations by eliminating setup and unneeded costs between operations. Cells might be designed for a specific process, part, or a complete product. They are favourable for single-piece and one-touch production methods and in the office or the factory. Because of increased speed and the minimal handling of materials, cells can result in great cost and time savings and reduced inventory. Cellular design often uses group technology, which studies a large number of components and separates them into groups with like characteristics, sometimes with a computer’s help, and which requires the coding of classifications of parts and operation. The cellular design also uses families-of-parts processing, which groups components by shape and size to be manufactured by the same people, tools, and machines with little change to process or setup. Regardless of the cell design (straight line, u-shape, or other), the equipment in the cell is placed very near one another to save space and time. The handling of materials can be by hand, conveyor, or robot. A cell supervisory computer must be used to control movement between equipment pieces and the conveyor when robots or conveyors are used.
The Definition of a Cell
A cell is a combination of people, equipment, and workstations organized in the order of process flow, to manufacture all or part of a production unit. I make little distinction between a cell and what is sometimes called a flow line. However, the implication of a cell is that it:
- Has one-piece, or a very small lot, flow
- Is often used for a family of products
- Has equipment that is right-sized and very specific for this cell
- Is usually arranged in a C or U shape so the incoming raw materials and outgoing finished goods are easily monitored
- Has cross-trained people for flexibility
Objectives of cellular manufacturing:
- To shorten manufacturing lead times by reducing setup, work part handling, waiting times, and batch sizes.
- To reduce Work in Process (WIP) inventory. Smaller batch sizes and shorter lead times reduce work-in-process.
- To improve quality. Accomplished by allowing each cell to specialize in producing a smaller number of different parts. This reduces process variability.
- To simplify production scheduling. Instead of scheduling parts through a sequence of machines in a process-type shop layout, the system simply schedules the parts through the cell.
- To reduce setup times. Accomplished by using group tooling (cutting tools, jigs, and fixtures) that have been designed to process the part family rather than part tooling, which is designed
for an individual part. This reduces the number of individual tools required as well as the time to change tooling between parts.
Steps to Implement Cell Manufacturing
After you’ve mapped your value streams, you are ready to set up continuous flow manufacturing cells. Most cells that have been set up in the past ten years do not have continuous flow; most changes to cells have been a layout change only. That is, machines were moved in a cellular arrangement and nothing more was changed. A change in layout alone does not create a continuous flow. This article will discuss seven steps to creating a continuous flow of manufacturing cells.
- Decide which products or product families will go into your cells, and determine the type of cell: Product-focused or Group Technology (mixed model). For product-focused cells to work correctly, demand needs to be high enough for an individual product. For mixed model or group technology cells to work, changeover times must be kept short.
- Calculate Takt Time.Takt time, often mistaken for cycle time, is not dependent on your productivity- it is a measure of customer demand expressed in units of time:
Takt Time = Available work-time per shift / Customer demand per shift
Ex: Work time/Shift = 27,600 seconds
Demand/Shift = 690 units
Takt Time = 27,600/690 = 40 sec.
The customer demands one unit every 40 seconds. What if your demand is unpredictable and relatively low volume? Typically, demand is unpredictable; however, aggregate demand (that is, the demand for a group of products that would run through a cell) is much more predictable. Takt time should generally not be adjusted more than monthly. Furthermore, holding finished goods inventory will help in handling fluctuating demand.
- Determine the work elements and time required for making one piece. In much detail, document all of the actual work that goes into making one unit. Time each element separately several times and use the lowest repeatable time. Do not include wasteful elements such as walking and waiting time.
- Determine if your equipment can meet takt time. Using a spreadsheet determine if each piece of equipment that will be required for the cell you are setting up is capable of meeting takt time.
- Create a lean layout. More than likely, you will have more than one person working in your cell (this depends on takt time); however, you should arrange the cell such that one person can do it. This will ensure that the least possible space is consumed. Less space translates to less walking, movement of parts, and waste. U-shaped cells are generally best; however, if this is impossible due to factory floor limitations, other shapes will do. For example, I have implemented S-shaped cells in areas where a large U-shape is physically impossible.
- Balance the cell. This involves determining how many operators are needed to meet takt time.
Number of Operators = Total Work content / Takt time
Ex.: Total work content: 49 minutes
Takt time: 12 minutes
Number of operators: 49/12 = 4.08 (4 operators)
If there is a remainder term, it may be necessary to kaizen the process and reduce the work content. Other possibilities include moving operations to the supplying process to balance the line. For example, one of my clients moved simple assembly operations from their assembly line to their injection moulding operation to reduce work content and balance the line.
- Determine how the work will be divided among the operators.There are several approaches. Some include:
- Splitting the work evenly between operators
- Having one operator perform all the elements to make a complete circuit of the cell in the direction of material flow
- Reversing the above
- Combinations of the above
After you’ve determined the above 7 elements, you will have gathered much of the necessary data required to begin drawing and laying out your continuous flow manufacturing cell.
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