Why Implement Flow Cells?
Flow is at the heart of lean. A flow cell is the most efficient way of producing in discrete manufacturing.
Outcomes from introducing flow hit every business priority for today’s manufacturers:
- Increased cash flow
- Increased productivity
- Drastic reduction in lead time
- Flexibility to match capacity to demand
- Improved quality
- Reduced footprint
- Foundation for continuous improvement
What is a Flow Cell?
Physically bringing process steps together to produce in one-piece-flow with operators focused on value-added work within the cell.
- All value-added work takes place inside the cell
- Cell width allows multiple operators to work with no interference
- Operators move and carry the product
- Number of operators can be scaled up and down to meet takt
Design Rules for Manual Cells
If a cell is for purely manual operations such as assembly (can contain some small machinery with changeover times less than takt time), use the following design rules:
- Arrange the workstations in the most compact U-shaped layout possible, leaving enough space between the two “branches” that two people can comfortably work back-to-back or cross each other while carrying the product, all without interference.
- The number of operators needed is equal to Σ(OCT)/TT. The numerator is the workload needed to complete all the work elements in the cell, and the denominator is the Takt time for the cell (i.e., cell available time per day divided by daily demand).
- In most cases, you will round up and allocate all the “waiting time” to the operator at the head of the cell—the one working at the first and last stations of the cell.
- If the remainder (number after the decimal) is 4 or less, you should multiply it by the Takt time and perform kaizen to reduce the sum of operator CTs by at least that amount, and round down. Only if this kaizen proves unattainable will you round up.
- Distribute the work among the number of people in the cell in such a way as to minimize walking distances. Try out various configurations and let the operators choose the one they prefer.
- Place Standard WIP locations where operators pass the product to one another. There should be only enough space to store one product per location.
- Make sure nothing interferes with free movement within the entire cell; operators should be able to help each other if needed, and the cell should work with only one operator if needed.
- Make any equipment or work table as small as possible, and easy to move (e.g., with rollers).
- The work must follow One-Piece-Flow: process only one product at a time in each workstation.
- Material must be within easy reach and not require extra motion such as orientation, unwrapping, untangling, etc. (that work should be performed by a material handler).
- Pay attention to ergonomics: the product should not have to be lifted; if the product is heavy, it should be moved on a slide or track—not carried; there should be no twisting, bending or straining in order to perform the work.
- A cell always has a Team Leader, who fulfills many roles:
- Jump in and help if an operator is struggling to meet Takt (for example, if rework needs to be done, it will be done by the TL while the operator goes back to the start position). A TL must know how to perform every job in a cell!
- Replace operators who step away from the cell (e.g. bathroom break).
- Act as backup in case no one can be found to replace an absent operator.
- Observe the cell and check for deviations from standards.
- Identify ways to improve, encourage kaizen within the team, implement team’s ideas.
- A cell typically has a Material Handler. In some cases, this role can be performed by the Team Leader, but since it requires travel away from the cell, this may be detrimental to the quick support role described in 12(a) above. The Material Handler is responsible for:
- Ensuring the cell never runs out of materials.
- “Reconditioning” materials (e.g., opening boxes, “breaking” large containers into smaller ones that go into the cell, unwrapping components, placing components in the right orientation on flow racks that feed the operators).
- Resupplying components from the warehouse.
- (optional) Carrying away finished goods from the cell and delivering raw materials to the cell (these are usually in pallets), though that is often performed by a milk run person.
Design Rules for Machine Cells
If a cell is mostly made up of machines with long changeover times, use the following design rules:
- Machines in a cell must have CT (including loading and unloading) < TT. The exception is when it’s possible to feed two machines with very little or no difference in operator CT. For example, if the machines are very small or the load/unload activities can be performed from a single point. To save on MCT and OCT, try to automate the part ejection at the end of the machine cycle: it’s cheap and remarkably improves productivity!
- Machines should be highly reliable, since any downtime or defect affects the entire cell!
- Remember that the changeover time of the cell will be that of the machine with the longest changeover time.
- Machines in a cell should be easily moved. If maintenance needs to be performed on a machine, it may need to be moved to access certain places. Routine inspection and maintenance should be feasible with the machine in place.
- Arrange the workstations in the most compact U-shaped layout possible, leaving enough space between the two “branches” that two people can comfortably work back-to-back or cross each other while carrying the product, all without interference.
- The number of operators needed is equal to Σ(OCT)/TT. The numerator is the workload needed to complete all the work elements in the cell, and the denominator is the Takt time for the cell (i.e., cell available time per day divided by daily demand).
- In most cases, you will round up and allocate all the “waiting time” to the operator at the head of the cell—the one working at the first and last stations of the cell.
- If the remainder (number after the decimal) is 4 or less, you should multiply it by the Takt time and perform kaizen to reduce the sum of operator CTs by at least that amount, and round down. Only if this kaizen proves unattainable will you round up.
- Distribute the work among the number of people in the cell in such a way as to minimize walking distances. Try out various configurations and let the operators choose the one they prefer.
- Place Standard WIP locations where operators pass the product to one another. There should be only enough space to store one product per location.
- Make sure nothing interferes with free movement within the entire cell; operators should be able to help each other if needed, and the cell should work with only one operator if needed.
- Make any equipment or work table as small as possible, and easy to move (e.g., with rollers).
- Material must be within easy reach and not require extra motion such as orientation, unwrapping, untangling, etc. (that work should be performed by a material handler).
- Pay attention to ergonomics: the product should not have to be lifted; if the product is heavy, it should be moved on a slide or track—not carried; there should be no twisting, bending or straining in order to perform the work.
- In machine cells, the product is often oily and dripping with fluid. To keep the cell clean, gutters are installed in between the machines, over which the part is transported.
- Safety is always the top priority. Since flow cells operate on the principle of “separation of man and machine”, it is necessary to ensure that the operator can’t get hurt by contact with the moving/cutting/hot parts of the machine. Use guards, moving guards, light curtains and/or pressure pads to this effect.
The separation principle is achieved through “one-touch switches”, which activate the machine cycle when the operator is moving away. The rule for the operator is to always move forward and never turn back, so that the machine cycle is completed by the time the operator returns.
Examples
Observe these videos carefully for the items previously described:
- “One-touch-switch”
- Ergonomics
- Auto-eject
- No interference
- Separation of Man and Machine
- Gutters / chutes
Procedures for Cell Design
- Look across the entire value stream and identify what process steps could be grouped into a cell. Pay particular attention to the steps furthest downstream (closest to the customer).
- If for a machine cell, use Production Capacity Sheet to identify any kaizen need:
- Design the cell layout using the design rules.
- Try out alternative designs using actual machines or cardboard simulation, and address any issue found during observation.
- Collect data for each SKU used in the cell:
- Supplier data: vendor name, vendor location (country), lead time, MOQ
- Packaging data: Units per box/container/pallet, LxWxH dimensions, Weight
- Usage data: Average demand, Count per shipped unit, Scrap or defect rate
- Locations: is the material stored in a warehouse? Inside store/supermarket? Line-side? Flowrack?
- Material handler work: will the material need to be transferred to a different container before reaching the operator? Unwrapped? Inspected? Individually placed in a flowrack?
- Components are typically presented to the cell operators on a flowrack. Determine if the original container can be used and what the flow rack dimensions are.
- The cyclical stock is the maximum WIP in the flow rack minus the min. (resupply point). Multiply the quantity by Takt time to determine the frequency at which this SKU needs to be replenished. The CT for this circuit can’t be less than the smallest cyclical stock time.
- The circuit walk time is the time it takes to walk through every point of the material handler’s path (without stopping or doing any work).
- Play with the variables such as cyclical stock, units per container, etc. to determine the best Circuit CT and total daily workload. If the workload is below 50% of the available time per day, it’s likely you can serve two cells with one material handler: copy a second template below the first one and add a field to show the combined workload for both cells.
- Build the cell. If you’re using modular tube systems (Creform, Trilogiq, FlexPipe…), order early and count 6 to 8 labor hours per workstation if it includes flow racks.
Takt Time/Cycle Time Bar Chart
A preferred alternative to the production capacity sheet, the takt time/cycle time bar chart is a visual representation of capacity in your value stream:
- The bar height is the capacity of each process step, ie, its adjusted combined cycle time
- The CT of the mill is 24s per part, but since there are 2 mills, the combined CT is 12s
- Takt time is the red line
- Separate MCT (grey) and OCT (green)
- The adjusted CT is CT/OEE
- Assembly and the weld machine are the constraints: since the bars exceed Takt, we can’t meet customer demand and kaizen is required
- Always indicate in label if more than 1 resource
- Adjust the MCT by showing the OEE losses in red.
- The gap between the bar and Takt line is unused capacity
What’s Next?
Your work is not done when you go live with the cell; in fact, you’re only getting started.
Put at least one full-time resource to support the team for the first week. The first week is key to setting the right habits for all those who work there, because there will always be things that need “debugging.”
Make sure to have a visual management system in place (hour-by-hour board and daily accountability review) and always keep the standards up to date!
You must manage the cell as a communication asset as well as a production asset.
- Highlight the “before and after” improvements in simple, factual terms
- Show the safety and ergonomic benefits of the cell.
- Develop a sense of community within the cell team; they are the pioneers.
- Have top leadership visit the cell early and often to learn from it
After a couple of weeks, the cell should be hitting most of its targets. Encourage other teams to visit and even work in the cell. In this way, you will create a “pull” from those who were more cautious. The most resistant people may be the last ones to pull for cells to be established in their areas, but they’ll pull (or see the writing on the wall and decide to leave).
