Unit
- Process: A sequence of interrelated activities that transform inputs into outputs. In manufacturing, a process combines materials, equipment, labor, and energy to create a product with added value. Examples include assembly lines, chemical reactions, and packaging operations.
- Operation: A distinct step within a process where a specific action is performed. Operations are the building blocks of processes and can be classified as value-adding, non-value-adding but necessary, or pure waste. Identifying operation types is the first step in optimization.
- System Boundary: The defined scope of analysis that separates the process from its environment. Setting boundaries helps focus improvement efforts and clarifies what inputs and outputs are considered. A well-defined boundary prevents scope creep in operational studies.
- Throughput: The rate at which a process produces output over a given time period. Measured in units per hour or per shift, throughput is a primary indicator of process efficiency. High throughput does not always mean high quality, so both metrics must be considered together.
- Bottleneck: The operation with the lowest capacity that constrains the overall process throughput. Identifying bottlenecks is critical because improving non-bottleneck operations yields no system-wide benefit. The Theory of Constraints guides systematic bottleneck management.
- Variability: Inherent fluctuations in process parameters such as cycle time, quality, or demand. Variability leads to inefficiencies, waiting times, and rework. Sources include machine breakdowns, material inconsistencies, and human factors. Reducing variability is a core goal of process engineering.
- Standard Work: Documented procedures that define the safest, most efficient way to perform an operation. It provides a baseline for training, auditing, and continuous improvement. Without standard work, processes drift and become unpredictable.
- Key Performance Indicator (KPI): Quantifiable metrics used to evaluate process effectiveness. Common KPIs include Overall Equipment Effectiveness (OEE), first-pass yield, and cycle time. Aligning KPIs with strategic goals ensures that improvement efforts deliver business value.
- Cycle Time: The total time required to complete one unit of work from start to finish. It includes processing, inspection, and any waiting or transport time within the operation. Reducing cycle time improves responsiveness and can lower work in process inventory, but must be balanced against quality and cost constraints.
- Takt Time: The maximum allowable time per unit to meet customer demand, calculated as available production time divided by customer demand. It sets the pace for production and aligns output with demand. When cycle time exceeds takt time, the process cannot satisfy customer requirements without overtime or expediting.
- Work in Progress (WIP): Inventory that has entered the process but is not yet complete. High WIP hides inefficiencies and increases lead time. Lean manufacturing advocates for minimizing WIP to expose problems and improve flow. Tracking WIP levels helps identify unbalanced operations and excessive variability.
Code Example
Python
def calculate_oee(available_time, planned_production_time,
total_units, good_units, ideal_cycle_time):
availability = available_time / planned_production_time
performance = (total_units * ideal_cycle_time) / available_time
quality = good_units / total_units
oee = availability * performance * quality
return round(oee, 4)
# Example calculation
avail = 420 # minutes available after downtime
planned = 480 # planned production minutes
total = 1000
good = 950
ideal = 0.4 # minutes per unit
print('OEE: {}'.format(calculate_oee(avail, planned, total, good, ideal)))This function computes OEE using standard inputs. Availability measures uptime, performance captures speed loss, and quality reflects defect rate.
Quick Revision
- Processes consist of operations that transform inputs to outputs.
- System boundaries define the scope of analysis.
- Throughput and bottlenecks are critical for capacity planning.
- Variability must be reduced for predictable operations.
- Standard work forms the basis for consistent performance.
- KPIs like OEE measure overall process effectiveness.