No mass production system can be continuously responsive to gyrating orders without suffering from muri (overburden of machines, managers, and production associates), and mura (unevenness in productivity and quality). And mura and muri together create muda (waste).Fig.
Relationship Among Predictability, Flexibility and Stability Is Heijunka – When implemented correctly, heijunka provides predictability by leveling demand, flexibility by decreasing changeover time and stability by averaging production volume and type over the long term.
Here is how some Lean Books defines heijunka: “Leveling the type and quantity of production over a fixed period of time. This enables production to efficiently meet customer demands while avoiding batching and results in minimum inventories, capital costs, manpower, and production lead time through the whole value stream.”
From Pitch to Heijunka
Pitch is the bridge connecting customer pack quantity with takt, so it makes no sense to build products in less quantity than the pack size. The same is true for creating production batches that are not multiples of the pack sizes.
Once we have Pitch we can develop a Heijunka Box which tells the operator what to build every pitch interval.Fig.
Heijunka – The Load Leveling Box
Develop a Heijunka Box for a days production from the data below
This Heijunka Box shows lot quantities to produce per Pitch
This Heijunka Box shows pallets to produce per Pitch
The information about what product to build on the next Pitch is placed in the Heijunka box when completed products are picked up.
Since products are picked up from the line at the same rate as the others are scheduled for production Pitch by Pitch, any problem with Flow will prevent pick up and become visible immediately.Fig.
The method that compiles the days production requirement and lets the line determine the sequence is not Heijunka, but it allows higher flexibility in a true high mix / low volume line
Kaizen the Ship Frequency
If the EPEI (or Interval) for all the processes (pacemaker and upstream) is equal to the ship frequency, then the entire value stream will flow
Ideal ship frequency is daily or more frequent to the customer as this will continuously engage the pacemaker and upstream operations to improve
Steadily increasing the ship frequencies is therefore a highly worthy kaizen project for you and the customer.
The facility would be able to convert raw materials into finished goods that can garner cash before the material bills come due.
Response to change in demand.
Rule of thumb:
• Kaizening the process can never finish
• Customer demand always changes
• Design of pacemaker needs to consider changing volumes
• A year-long history of shipment may show periods of stable demand
For increasing demand:
• Smaller day-to-day fluctuations are best handled by use of a finished goods supermarket (allows for takt time and resource planning)
• Run a little overtime each shift
• Toggle the number of operators (may require takt time adjustments due to change of work distribution)
Keeping in mind the core concepts about heijunka will help keep a company heading in the right direction.
• Takt time: The time it takes to finish a product in order to meet customer demand; can be thought of as the customer buying rate. It is the guidance for the entire heijunka implementation.
• Volume leveling: Manufacture at levels of long-term average demand and keep a buffer inventory proportional to variability in demand, stability of production process and shipping speed.
• Type leveling: Essentially, make every product every day and reserve capacity for changeover flexibility; use a heijunka box to visualize the production flow and schedule.
• Heijunka box: A working diagram of type leveling and production schedule.
• Work slowly and consistently: Taiichi Ohno, founder of the Toyota Production System, says it best: “The slower but consistent tortoise causes less waste and is much more desirable than the speedy hare that races ahead and then stops occasionally to doze. The Toyota Production System can be realized only when all the workers become tortoises.”
• Changeover time: Efficiency of changeover is the fulcrum of heijunka; narrowing changeover times helps tighten the value stream between supply and demand.
• Buffer inventory: Having some product ready to ship at the beginning of each production cycle is essential to smoothing production and leveling demand at consistent rates and quality so that resource waste is minimized on the line.
Type standardization: By manufacturing one of each product or service a day, knowledge can be more readily shared across types to benefit every process.