Process Planning, Production Control and Lean Operations

Introduction

Manufacturing performance depends not only on machine capability but also on how work is planned, released, coordinated and improved. Process planning converts a product design into an economically feasible sequence of operations. Production control coordinates materials, machines, labour and information so that orders are completed on time. Lean operations then seeks to remove activities that consume resources without creating customer value.

These activities are strategically important because they influence delivery reliability, cost, quality, flexibility, inventory investment and employee safety. Modern practice combines traditional production planning and control with MRP/ERP systems, just-in-time (JIT) methods, visual control, statistical improvement and constraint management.

Learning Objectives

After studying this chapter, the learner should be able to:

1. Prepare a process plan and routing for a manufactured component.
2. Distinguish loading, sequencing, scheduling and dispatching.
3. Calculate basic capacity, inventory and scheduling measures.
4. Explain MRP, ERP, JIT and Kanban systems.
5. Apply value-stream mapping, 5S, SMED, TPM, Six Sigma and the Theory of Constraints.
6. Evaluate implementation difficulties, ethical issues and improvement priorities.

1. Process Planning and Routing

Process planning is the systematic determination of the operations, machines, tools, fixtures, inspection methods, labour skills and parameters needed to manufacture a part. Its principal document is the route sheet, which normally contains:

• part number, revision and quantity;
• operation sequence and work-centre identification;
• machine, tool, fixture and gauge requirements;
• setup and run times;
• quality characteristics and inspection points;
• material handling and safety instructions; and
• standard quantities, scrap allowances and authorization.

A process plan must satisfy three conditions: it must produce the required specification, be feasible with available resources, and be economical at the expected volume. A design change should therefore trigger controlled review of the routing, tooling, inspection plan and associated ERP master data.

1.1 Operation sequence

A typical sequence is:

1. review drawing, tolerances, material and customer requirements;
2. select raw material and determine blank dimensions;
3. identify major transformation operations;
4. select machines and work centres;
5. decide operation precedence and datum strategy;
6. estimate setup, processing, inspection and transport times;
7. specify tooling, fixtures and quality controls;
8. calculate capacity and cost; and
9. release and maintain the controlled route.

Operation precedence is important. For example, a reference surface may need to be machined before dimensions based on that surface are produced. Heat treatment may be required before finish grinding, while cleaning may be required before coating.

1.2 Time and capacity calculations

The planned time for a batch can be estimated as:

Here, T_batch is total planned time, T_setup is setup time, Q is batch quantity, T_cycle is time per unit, T_move is material-handling time, and T_inspection is inspection time. All times must use the same unit. The equation assumes that cycle time is constant and does not explicitly model breakdowns or quality losses.

If a machine is available for A hours per period and planned maintenance, breaks and other unavailable time total D hours, effective available time is:

For a workload of W hours, the approximate capacity utilization is:

Utilization above 1, or 100%, indicates that the planned load exceeds available capacity. In practice, a sustainable operation normally requires a margin for variability, preventive maintenance and urgent work.

Worked example: routing time and capacity

A batch of 80 shafts requires a 2.0-hour setup, 6 minutes machining per shaft, 0.5 hour inspection and 0.5 hour handling. The planned time is:

T_batch = 2.0 + (80 × 6/60) + 0.5 + 0.5 = 11.0 h

If the work centre has 40 scheduled hours, of which 6 hours are unavailable for maintenance and breaks, then:

C_available = 40 − 6 = 34 h

The utilization for this batch is 11/34 = 0.324, or approximately 32.4%. This result concerns the batch workload only; it is not a measure of the entire factory’s utilization.

2. Production Planning and Control

Production planning determines what should be produced, in what quantity and when. Production control ensures that the plan is executed and corrected when actual conditions differ. The hierarchy commonly includes:

2.1 Loading, sequencing and scheduling

Loading assigns work to machines or work centres. Sequencing determines the order in which jobs are processed. Scheduling assigns jobs to resources and time periods, considering precedence, availability and due dates. Dispatching authorizes the next job to begin.

Common sequencing rules include:

• first-come, first-served: simple and perceived as fair;
• earliest due date: reduces lateness risk;
• shortest processing time: often reduces average flow time;
• critical ratio: prioritizes jobs according to remaining time relative to remaining work.

For job i, the critical ratio is:

where d_i is the due date, t is the current time and p_i is remaining processing time. A value below 1 indicates that the job is behind its nominal schedule. The rule is a priority aid, not a guarantee of optimality.

A schedule should account for setup sequence, material availability, operator qualifications, maintenance windows, quality holds and transportation. A mathematically attractive schedule that cannot be executed is not a good production plan.

2.2 Performance measures

For a job completed at time C_i and due at d_i, lateness is:

Positive lateness means completion after the due date; negative lateness means early completion. Tardiness is the non-negative part:

Flow time is completion time minus release time. These measures should be interpreted with customer priorities and inventory consequences. Maximizing machine utilization at every work centre can increase queues and lead time, because high utilization leaves little capacity to absorb variability.

3. Inventory, Materials and Resource Planning

Inventory includes raw materials, work-in-process (WIP), finished goods, maintenance supplies and purchased components. It can protect against uncertainty and enable economies of scale, but it also ties up cash, hides defects, increases handling and may become obsolete.

The basic economic order quantity (EOQ) model is:

where D is annual demand in units/year, S is ordering or setup cost per order, and H is annual holding cost per unit. The model assumes relatively constant demand, instantaneous replenishment, no quantity discounts, known costs and no stockouts. It is a benchmark, not a universal prescription.

The reorder point under constant demand and fixed lead time is:

where d is demand per time unit and L is lead time in the same unit. With uncertainty, safety stock is added:

SS is safety stock. Its determination should reflect demand variability, lead-time variability and the required service level; it should not be selected merely as an arbitrary percentage of demand.

Worked example: EOQ and reorder point

Annual demand is 12,000 units, ordering cost is $50 per order and annual holding cost is $2 per unit. Thus:

Q* = √((2 × 12000 × 50)/2) = √600000 ≈ 775 units

If daily demand is 40 units and replenishment lead time is 6 working days, with a safety stock of 80 units:

ROP = (40 × 6) + 80 = 320 units

This result assumes that the daily demand and lead time definitions are consistent and that stock is available for use when received.

4. MRP and ERP Systems

Material Requirements Planning (MRP) translates a master production schedule into time-phased requirements for components. Its principal inputs are:

1. the master production schedule;
2. the bill of materials (BOM);
3. inventory records, including on-hand and scheduled receipts; and
4. planning parameters such as lot size, lead time and safety stock.

MRP performs bill-of-materials explosion, offsets requirements by lead time and creates planned order receipts and releases. Net requirements may be represented conceptually as:

Exact system conventions vary, especially in the treatment of safety stock, allocations and projected available balance; therefore the organization’s MRP logic must be understood before interpreting outputs.

MRP is principally a planning and scheduling logic for dependent demand. ERP extends this integration to finance, purchasing, sales, quality, maintenance, human resources and logistics. ERP improves data visibility, but it does not correct inaccurate BOMs, unrealistic lead times, poor inventory records or undisciplined transaction practices. “Garbage in, garbage out” remains a central implementation principle.

5. Lean Operations and the Toyota Production System

Lean operations seeks to provide customer value with less unnecessary resource consumption. The classic wastes are defects, overproduction, waiting, unused talent, transportation, inventory, motion and overprocessing. Waste must be distinguished from necessary non-value-adding work, such as legally required inspection, which may be reduced or redesigned but cannot simply be eliminated.

Two complementary principles are just-in-time, producing and delivering only what is needed when it is needed, and jidoka, stopping or alerting when an abnormality occurs so that defects are not passed forward. Lean is not simply an inventory-reduction program. It exposes process instability, and sustainable improvement requires problem solving, standard work and respect for people.

5.1 Value-stream mapping

A value-stream map shows material and information flows from supplier to customer. It commonly records cycle time, changeover time, uptime, WIP, staffing, demand, production-control signals and information links. The map distinguishes processing time from waiting and identifies bottlenecks, large queues, rework loops and disconnected schedules.

A future-state map should be based on a specific product family and customer demand. It may propose continuous flow, supermarkets, FIFO lanes, pull signals, smaller batches, leveled production and improved information flow. The map is a learning and improvement tool, not a substitute for detailed engineering analysis.

5.2 Takt time and flow

Takt time expresses the pace required to meet demand:

For example, if net production time is 420 minutes per shift and demand is 140 units, takt time is 3 minutes per unit. Takt is not the same as cycle time: cycle time is the actual time between completed units at a process. To meet demand sustainably, a balanced process generally requires cycle time at or below takt, subject to quality, downtime and flexibility constraints.

5.3 Kanban and pull control

Kanban is a replenishment signal, often a card, container, electronic message or marked location. A downstream process withdraws material, and the upstream process replenishes the quantity withdrawn. A simplified container calculation is:

where n is the required number of containers, D is demand per time unit, L is replenishment lead time, α is a safety factor expressed as a fraction, and C is container quantity. The result should be rounded up to a whole container. The formula assumes reasonably stable demand and lead time; quality problems and unreliable processes require corrective action rather than indefinite increases in α.

5.4 5S and standard work

5S consists of sort, set in order, shine, standardize and sustain. It establishes visual control, reduces searching and exposes abnormalities. It is not merely housekeeping: tools, materials, status and safety conditions should have defined locations and standards.

Standard work defines the best currently known method, sequence, standard WIP and timing for a task. It provides a baseline for training and improvement. Standards should be updated when a safer or more effective method is validated.

5.5 SMED and TPM

Single-Minute Exchange of Die (SMED) reduces changeover time, especially the interval between the last acceptable product of one run and the first acceptable product of the next. Improvement steps include separating internal activities, performed while equipment is stopped, from external activities, performed while it is running; converting internal tasks to external tasks; simplifying fastening and adjustment; and using checklists and parallel work.

Total Productive Maintenance (TPM) treats equipment effectiveness as a shared responsibility. It combines preventive maintenance, predictive condition monitoring, operator care, planned maintenance, training and focused improvement. Overall Equipment Effectiveness is often expressed as:

Availability is running time relative to planned production time; performance compares actual speed with an ideal or standard speed; quality is good units divided by total units. The three factors must be calculated over the same defined period, and their definitions must be agreed. OEE is diagnostic, not a complete measure of business success.

6. Six Sigma and the Theory of Constraints

Six Sigma is a data-based approach to reducing variation and defects. DMAIC—Define, Measure, Analyze, Improve and Control—is used for improving an existing process. Define establishes the problem and customer requirement; Measure establishes reliable baseline data; Analyze identifies verified causes; Improve tests and implements solutions; Control maintains gains using standard work, monitoring and response plans.

A defect rate can be represented as:

A unit may contain more than one opportunity for defect. Defects per opportunity is:

These measures require a clear defect definition and complete data. Process capability indices such as C_p and C_pk should only be used when the process is stable and the measurement system is adequate. Capability analysis is not a replacement for process control.

The Theory of Constraints (TOC) argues that system output is limited by at least one constraint. Its five focusing steps are:

1. identify the constraint;
2. exploit it by using existing capacity effectively;
3. subordinate other activities to the constraint;
4. elevate the constraint through added capacity or changed methods; and
5. repeat the process when the constraint moves.

A local efficiency improvement that reduces constraint availability may harm total throughput. TOC therefore encourages system-level rather than department-level optimization.

7. Integrated Implementation and Digital Operations

A practical improvement program begins with customer requirements and a reliable current-state description. Management should establish a product family, baseline lead time, WIP, quality, delivery and cost, then select a constraint or high-impact loss. Cross-functional teams can combine VSM, standard work, SMED, TPM, statistical analysis and Kanban rather than applying tools independently.

Digital manufacturing systems can connect ERP, manufacturing execution systems, sensors, maintenance platforms and quality databases. AI may support demand forecasting, anomaly detection, predictive maintenance and schedule optimization. However, algorithms inherit bias and data-quality problems. A responsible deployment requires human review, cybersecurity, access control, traceability, model validation and a clear response when recommendations conflict with safety or quality requirements.

Ethically, production targets must never encourage bypassing guards, falsifying inspection records, hiding defects or excessive unsafe overtime. Lean systems should include worker participation, ergonomic assessment and protection from retaliation when abnormalities are reported. Environmental performance should include energy, scrap, emissions, water and end-of-life considerations rather than focusing only on labour productivity.

8. Illustrative Case: Improving a Pump-Shaft Value Stream

Background

The following is a fictional illustrative case. A plant makes pump shafts in family batches. Customers complain about eight-day delivery, although total machining time is less than one hour per shaft. The plant has functional departments for turning, milling, grinding and inspection.

Problem and analysis

A current-state map shows 1,200 shafts of WIP between departments. Average changeover is 90 minutes, first-pass yield is 94%, and the grinding department is the constraint. Schedules are released independently by each department. Operators frequently wait for drawings, gauges or material, while completed batches wait for the next operation.

The plant calculates available production time as 420 minutes per shift and customer demand as 140 shafts per shift, giving a takt time of 3 minutes per shaft. Grinding has an effective cycle time of 3.4 minutes and therefore cannot meet demand without improvement or added capacity.

Approach and solution

The team first corrects routing and BOM data, establishes standard work and introduces 5S at the grinding cell. SMED separates external wheel preparation from internal machine adjustment, reducing changeover to 35 minutes. TPM addresses coolant contamination and planned maintenance. A small supermarket is established before grinding, and a Kanban signal limits replenishment. The team uses DMAIC to analyze grinding defects and finds an unstable dressing interval. A controlled dressing standard reduces variation.

Because grinding remains the constraint, upstream departments subordinate production to its pull signals rather than maximizing their own utilization. A second shift is considered only after the process improvements have been verified.

Outcome

After three months, WIP falls from 1,200 to 450 shafts, first-pass yield rises from 94% to 98%, and changeover falls to 35 minutes. Grinding cycle time reaches 2.8 minutes, below the 3-minute takt. Delivery lead time falls to three days. The plant does not claim that the improvements are permanent: control charts, daily management and periodic audits are required to sustain them.

Learning points

The case demonstrates that lead time is often dominated by waiting rather than processing. It also shows why the constraint must be managed at system level, why data integrity precedes sophisticated software, and why lean tools work best when linked to a specific customer requirement.

Challenges and Limitations

Process plans can become obsolete when designs, suppliers or machines change. Schedules are vulnerable to uncertain demand, absenteeism, breakdowns and rush orders. MRP may generate excessive orders when lead times or inventory records are inaccurate. Kanban is unsuitable for every item, particularly highly erratic demand, unique engineered products or unreliable suppliers. Lean programs can fail when management treats them as headcount reduction, when improvement work is added without time or when workers are excluded. Six Sigma can become bureaucratic if measurement is excessive and practical action is delayed. OEE can be gamed through definitions that exclude difficult periods. These limitations require disciplined governance, transparent metrics and engineering judgment.

Chapter Summary

Process planning converts product requirements into controlled operations, resources and inspection activities. Routing, capacity calculation, loading, sequencing, scheduling and dispatching connect design with execution. Inventory decisions must balance service, uncertainty and cost; EOQ and reorder-point formulas are useful models whose assumptions must be checked.

MRP uses the master schedule, BOM and inventory records to plan dependent demand, while ERP integrates manufacturing with wider business processes. Lean operations uses JIT, pull, Kanban, value-stream mapping, takt, 5S and standard work to reduce waste and improve flow. SMED reduces changeover, TPM improves equipment reliability, Six Sigma reduces variation through DMAIC, and TOC directs attention to the system constraint. Successful implementation depends on accurate data, employee involvement, safety, ethical conduct, disciplined standards and continuous learning.

Key Terms

Process planning: Determining operations, resources and methods needed to manufacture a product.

Routing: The ordered path of operations through work centres.

Route sheet: Controlled record of operations, times, equipment and quality requirements.

Loading: Assigning work to resources.

Sequencing: Determining the order of jobs.

Scheduling: Assigning jobs to resources and time periods.

Dispatching: Authorizing work to begin.

WIP: Work-in-process inventory between operations.

EOQ: Benchmark order quantity minimizing ordering and holding costs under stated assumptions.

Reorder point: Inventory position at which replenishment is initiated.

MRP: Time-phased planning system for dependent material requirements.

ERP: Integrated enterprise system linking manufacturing and business functions.

JIT: Producing or delivering only what is needed, when needed.

Kanban: Visual or electronic pull signal for replenishment.

Takt time: Required production pace based on available time and demand.

Value-stream mapping: Visual analysis of material and information flow.

SMED: Method for reducing changeover time.

TPM: Organization-wide approach to equipment effectiveness and maintenance.

Six Sigma: Data-based approach to reducing variation and defects.

Theory of Constraints: Approach that focuses improvement on the system’s limiting constraint.

Review Questions

Short Answer Questions

1. What information is normally included in a route sheet?
2. Distinguish loading, sequencing and scheduling.
3. Why can excessive inventory conceal operational problems?
4. What are the five stages of DMAIC?
5. State the five focusing steps of the Theory of Constraints.

Long Answer Questions

1. Explain how process planning links product design, capacity, tooling and quality control.
2. Compare MRP, ERP, JIT and Kanban in terms of purpose and information flow.
3. Explain value-stream mapping and describe how a future-state map is developed.
4. Discuss the roles of 5S, SMED and TPM in improving manufacturing flow.
5. Critically evaluate the benefits and limitations of using OEE as a performance measure.

Analytical / Application Questions

1. A work centre has 450 scheduled minutes per shift, including 60 minutes of planned downtime. Customer demand is 130 units per shift. Calculate takt time and explain what it implies for a process with a 3.2-minute cycle time.
2. Annual demand is 18,000 units, ordering cost is $40 per order and annual holding cost is $2.50 per unit. Calculate the EOQ, state the model assumptions and explain why the result may need adjustment.
3. A factory has high machine utilization but poor delivery performance and large WIP queues. Using TOC and lean principles, propose a diagnostic sequence and at least four improvement actions.

Multiple Choice Questions with Answers

Question 1

Which document primarily specifies the ordered operations and resources for manufacturing a part?

A. Route sheet

B. Balance sheet

C. Sales invoice

D. Organization chart

Correct Answer: A. Route sheet

Question 2

A machine has 40 scheduled hours and 6 hours unavailable for planned maintenance. What is its available time?

A. 34 hours

B. 40 hours

C. 46 hours

D. 6 hours

Correct Answer: A. 34 hours

Question 3

Which MRP input defines the parent-component relationships?

A. Bill of materials

B. Maintenance log

C. Quality audit report

D. Cash-flow statement

Correct Answer: A. Bill of materials

Question 4

If available production time is 420 minutes and customer demand is 140 units, the takt time is:

A. 2 minutes per unit

B. 3 minutes per unit

C. 4 minutes per unit

D. 5 minutes per unit

Correct Answer: B. 3 minutes per unit

Question 5

What is the primary function of a Kanban signal?

A. Authorize replenishment through a pull system

B. Calculate annual depreciation

C. Replace all preventive maintenance

D. Set product selling price

Correct Answer: A. Authorize replenishment through a pull system

Question 6

Which SMED action is conceptually correct?

A. Convert internal setup work into external work where feasible

B. Increase batch size without analysis

C. Delay tool preparation until the machine stops

D. Remove all setup documentation

Correct Answer: A. Convert internal setup work into external work where feasible

Question 7

OEE is commonly represented as:

A. Availability × Performance × Quality

B. Demand × Price × Profit

C. Quality ÷ Inventory × Sales

D. Setup + Demand + Defects

Correct Answer: A. Availability × Performance × Quality

Question 8

In DMAIC, which phase establishes reliable baseline data?

A. Define

B. Measure

C. Improve

D. Control

Correct Answer: B. Measure

Question 9

According to the Theory of Constraints, what should normally be done first?

A. Identify the system constraint

B. Maximize every department’s utilization

C. Increase all inventories

D. Eliminate all inspections

Correct Answer: A. Identify the system constraint

Question 10

Annual demand is 12,000 units, ordering cost is $50 and holding cost is $2 per unit-year. The EOQ is approximately:

A. 245 units

B. 500 units

C. 775 units

D. 1,200 units

Correct Answer: C. 775 units