Production Planning, Scheduling and Control

Introduction and Context

Production planning, scheduling and control (PPC) converts market requirements into coordinated decisions about what to make, how much to make, when to make it, and with which resources. It links sales, product design, purchasing, maintenance, workforce management, inventory, quality, and production operations. In machine technology, PPC is especially important because products often require multiple operations—such as cutting, turning, milling, heat treatment, grinding, inspection, and assembly—performed on limited and sometimes highly specialized equipment.

Planning determines intended future actions. Scheduling assigns those actions to specific time periods, machines, and personnel. Control compares actual performance with the plan and initiates corrective action. These activities are interdependent: an unrealistic schedule cannot be repaired merely by better supervision, and an accurate plan becomes ineffective if materials, capacity, or information are unavailable.

PPC must balance competing objectives:

• customer delivery reliability;
• efficient use of machines and labor;
• low work-in-process and finished-goods inventories;
• short manufacturing lead time;
• stable workloads;
• acceptable quality and safety; and
• responsiveness to engineering or demand changes.

No single plan optimizes all objectives simultaneously. Effective PPC therefore involves explicit priorities, reliable data, appropriate planning horizons, and continuous feedback.

Learning Objectives

After studying this chapter, you should be able to:

1. distinguish strategic, aggregate, master, material, capacity, and shop-floor planning;
2. prepare an aggregate production plan and a master production schedule;
3. construct and interpret a bill of materials;
4. perform basic material requirements planning (MRP);
5. prepare a capacity requirements plan and identify overloads;
6. explain routing, loading, sequencing, dispatching, and Gantt charts;
7. select priority rules for job, batch, mass, and cellular production;
8. apply finite-capacity scheduling concepts; and
9. evaluate the operational, technological, and ethical implications of PPC decisions.

1. The PPC Hierarchy

PPC is commonly organized by planning horizon and level of detail.

The levels are linked. The aggregate plan constrains the MPS; the MPS drives MRP; MRP and routings drive capacity requirements; shop-floor feedback updates all lower-level decisions and may require replanning.

A useful distinction is between independent demand and dependent demand. Demand for finished products, replacement parts, or service items is generally independent because it arises from the market. Demand for components is dependent because it is derived from the quantity and timing of parent-item production. MRP is particularly effective for dependent demand.

2. Aggregate Production Planning

Aggregate planning develops a feasible medium-term plan for product families rather than individual part numbers. It uses forecasts, existing orders, inventory policies, workforce information, and available capacity.

A basic inventory balance for period t is:

where I_t is ending inventory in units in period t, I_{t−1} is beginning inventory, P_t is production, and D_t is demand. The equation assumes that production and demand are measured over the same period and that planned losses, scrap, and backorders are either negligible or represented separately.

If planned backorders B_t are included explicitly, a net inventory position may be written as:

A positive N_t represents available inventory; a negative value represents an unmet requirement of −N_t units. This is a planning identity, not a guarantee that backorders are acceptable.

Common aggregate strategies include:

• Level strategy: maintain a relatively constant workforce and production rate, allowing inventory or backorders to absorb demand variation.
• Chase strategy: vary production approximately with demand by changing workforce, hours, subcontracting, or capacity.
• Mixed strategy: combine regular time, overtime, inventory, subcontracting, and controlled backlog.

A simple cost model is:

where C is total planning cost; R_t is regular-time production; O_t is overtime production; I_t is ending inventory; B_t is backlog; S_t is subcontracted production; F_t is workforce reduction or related adjustment; and c_r, c_o, c_h, c_b, c_s, and c_f are the corresponding unit costs. The summation covers all planning periods. This model is useful only when the cost categories are defined consistently and capacity, labor, and service constraints are also imposed.

Worked Example: Aggregate Plan

A machine shop begins with 100 units in inventory. Forecast demand for three months is 400, 600, and 500 units. Regular production is fixed at 500 units per month, and no backorders are permitted. Ending inventories are:

• Month 1: 100 + 500 − 400 = 200 units.
• Month 2: 200 + 500 − 600 = 100 units.
• Month 3: 100 + 500 − 500 = 100 units.

The plan is feasible under the stated assumptions. It finishes with 100 units, but it may not be economically preferable to a plan that varies production or uses overtime. The calculation also illustrates why an aggregate plan must be checked period by period rather than only over the entire horizon.

3. Master Production Scheduling

The MPS translates aggregate production into a time-phased statement of specific end items. It is not simply a sales forecast. It is a commitment that must be supported by materials, capacity, and delivery requirements.

Typical MPS information includes:

• item or product code;
• time bucket, often week;
• forecast demand;
• customer orders;
• available inventory;
• projected available balance;
• master production quantity; and
• available-to-promise quantity.

A simplified projected available balance is:

where PAB_t is projected available balance, MPS_t is the scheduled production quantity, F_t is forecast demand, and O_t is customer orders in period t. The maximum term is used in a common simplified convention to avoid counting the same demand twice. Organizations may use different order-forecast consumption rules, so the convention must be documented.

The time fence divides the MPS horizon into zones. In a frozen zone, changes require high-level approval because materials and capacity may already be committed. In a slushy zone, limited changes are allowed. In a liquid zone, changes are relatively easy. Time fences reduce nervousness—frequent, disruptive changes to planned orders.

The MPS should be checked against rough-cut capacity planning. If a proposed schedule requires more hours than a critical work center can supply, the planner must revise quantities or dates, use overtime, subcontract, add capacity, or negotiate delivery dates.

4. Bills of Materials and Product Structure

A bill of materials (BOM) is a structured list of all components, subassemblies, quantities, and sometimes revision or effectivity information required to make an item. A single-level BOM lists immediate components; a multilevel BOM represents the complete product structure.

For example, a small machine vise may contain one base, one fixed jaw, one movable jaw, two guide rods, one screw assembly, and fasteners. The screw assembly may itself contain a screw, collar, and handle. The BOM is therefore a product-structure tree, not merely a purchasing list.

A sound BOM must specify:

• item identification and revision;
• quantity per parent;
• unit of measure;
• engineering effectivity dates or serial ranges;
• phantom or non-stock items where applicable;
• scrap or yield assumptions; and
• approved alternatives, if permitted.

BOM accuracy is critical. An incorrect quantity causes systematic shortages or excess inventory; an obsolete revision may produce nonconforming products; and omitted components can stop assembly even when the MRP system reports adequate supply.

5. Material Requirements Planning

MRP is a time-phased method for planning dependent-demand materials. Its principal inputs are:

1. the MPS;
2. the BOM;
3. inventory records, including on-hand balances, scheduled receipts, safety stock, lot sizes, and lead times.

The principal outputs are planned order receipts, planned order releases, rescheduling messages, and exception reports.

MRP Logic

For each item and period, MRP proceeds through these steps:

1. determine gross requirements from the parent schedule and BOM usage;
2. subtract available inventory and scheduled receipts;
3. preserve required safety stock;
4. calculate net requirements;
5. apply the lot-sizing rule; and
6. offset planned receipts by lead time to obtain planned releases.

A basic net-requirement calculation is:

where NR_t is net requirement in period t, GR_t is gross requirement, SS is required safety stock, OH_t is usable on-hand inventory available in the relevant period, and SR_t is scheduled receipt. The formula assumes that units are interchangeable, records are accurate, and scheduled receipts arrive as promised.

For a lot-for-lot policy, planned order receipt equals net requirement. With a fixed order quantity Q, the receipt may be the smallest multiple of Q that covers the net requirement:

where POR_t is planned order receipt and ceil means rounding upward to the next integer. Planned order release is then the receipt shifted earlier by the lead time. If lead time is two weeks and a receipt is required in week 6, the release is scheduled in week 4, subject to calendar and supplier constraints.

Worked MRP Example

A product requires two units of component C. The MPS calls for 50 products in week 4 and 40 in week 5. Therefore, gross requirements for C are 100 units in week 4 and 80 units in week 5. Suppose C has 30 units on hand, no scheduled receipts, zero safety stock, and a one-week lead time. Using lot-for-lot:

• Week 4 net requirement = 100 − 30 = 70 units.
• A planned receipt of 70 is needed in week 4.
• The corresponding planned release is 70 in week 3.
• Week 5 net requirement = 80 units because the week-4 receipt covers only week 4 demand.
• A planned receipt of 80 is needed in week 5, released in week 4.

Thus, the MRP result is a release of 70 units in week 3 and 80 units in week 4. The example assumes the 30 units on hand are usable and available before week 4.

Lot-Sizing Choices

• Lot-for-lot: minimizes planned inventory but may create many orders and setup costs.
• Fixed-order quantity: reflects supplier packs, economic quantities, or process constraints.
• Periodic-order quantity: covers requirements for a selected number of periods.
• Economic order quantity: may be useful when demand is relatively stable, although it is less natural for strongly time-phased requirements.

The classical economic order quantity is:

where D is annual demand in units per 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, and negligible stockout risk. These assumptions often do not hold exactly in manufacturing, so MRP lot sizing should reflect actual setup, supplier, and service conditions.

6. Capacity Requirements Planning

Capacity requirements planning (CRP) converts planned or released orders into workload at specific work centers. It uses routing data, standard times, queue or move times, work-center calendars, and planned order quantities.

A basic load calculation is:

where L_{w,t} is load on work center w in period t; Q_{j,t} is the quantity of job j processed there; p_{j,w} is processing time per unit; and S_{j,w} is setup time. If several batches are processed, setup time should be applied per batch, not mistakenly per unit.

Available capacity can be estimated as:

where N is the number of equivalent resources, H is scheduled hours, U is planned utilization, and E is efficiency relative to the standard. For example, two machines scheduled for 40 hours, with 85% utilization and 90% efficiency, provide 61.2 effective hours: 2 × 40 × 0.85 × 0.90.

The load ratio is:

A ratio above 1 indicates overload under the model. It does not automatically mean that delivery will fail, because existing queues, alternative routings, overtime, and time-bucket aggregation also matter.

Responses to overload include rescheduling, alternate machines, overtime, subcontracting, lot splitting, setup reduction, preventive maintenance changes, cross-training, or capital investment. Responses to underload should be evaluated carefully; idle time may be used for maintenance, training, improvement, or preventive work rather than creating unnecessary production.

7. Routing, Loading, and Shop-Floor Control

A routing specifies the sequence of operations, work centers, standard setup and run times, inspection points, tooling, and sometimes required skills. Routing data connects engineering design with scheduling and costing.

Loading assigns work to machines, work centers, or employees. In infinite loading, all required work is assigned without initially enforcing capacity limits; overloads are then visible for adjustment. In finite loading, assignments are made only within available capacity, so operations are moved to later times, alternate resources, or other shifts when necessary.

Dispatching releases and prioritizes work at the point of production. A dispatch list normally includes job number, operation, due date, quantity, required tooling, priority, and status. Dispatching should not be reduced to a purely mathematical rule: safety holds, quality containment, urgent service parts, and material availability may override a normal sequence.

A Gantt chart displays jobs or activities against time. A machine-oriented chart shows when each work center is occupied, idle, in setup, or under maintenance. A job-oriented chart shows the route and timing of each order. Gantt charts are valuable for communicating a schedule, identifying gaps and overlaps, and monitoring actual progress, but they become difficult to maintain when operations change rapidly unless integrated with a digital planning system.

8. Sequencing and Priority Rules

Sequencing determines the order in which jobs are processed. The appropriate rule depends on the performance objective.

For job j, the critical ratio is commonly written:

where d_j is the due date, t is the current time, and r_j is remaining processing time. A value below 1 indicates that the job is behind schedule if no delay occurs. The rule is dynamic and must be recalculated as conditions change.

For a single machine with all jobs available at time zero and no interruptions, SPT minimizes mean completion time. EDD minimizes maximum lateness under standard single-machine assumptions. These results do not automatically apply to complex job shops, sequence-dependent setups, material shortages, or multiple bottleneck resources.

Lateness and tardiness should be distinguished:

where C_j is completion time and d_j is due date. Lateness may be negative when a job is early; tardiness cannot be negative.

9. Production-System Applications

Job Production

Job production makes relatively low quantities of customized products. Routing varies by order, setup times may be significant, and the system is often a job shop. Dispatching, bottleneck management, accurate status information, and flexible skills are central.

Batch Production

Batch production makes a quantity of one product before changing over to another. The planner must balance setup cost against inventory and responsiveness. Sequence-dependent setups can make the order of jobs strategically important—for example, scheduling similar tool requirements consecutively.

Mass or Repetitive Production

Mass production uses standardized products and stable routes. The central issues are line balancing, rate synchronization, material availability, preventive maintenance, quality at the source, and protection against line stoppage. MPS stability and kanban or other pull signals may complement MRP.

Cellular Production

Cellular manufacturing groups machines and workers to process families of similar parts. It reduces movement and can shorten lead time, but part-family design, machine duplication, cross-training, and workload balance are necessary. A cell may use a hybrid of push planning for material provisioning and pull control for internal replenishment.

10. Finite-Capacity Scheduling

Finite-capacity scheduling recognizes that a machine, operator, fixture, or tool cannot process more than its available capacity during a time interval. It differs from infinite loading, which can place overlapping work on the same resource and reveal the resulting overload later.

A finite schedule must account for:

• machine calendars and planned downtime;
• setup and teardown times;
• labor and skill restrictions;
• tooling and fixture availability;
• material release dates;
• sequence-dependent setup times;
• batch splitting and overlapping operations; and
• preventive maintenance.

Finite scheduling is often represented as a constraint problem. Each operation has a start time, duration, required resource, and precedence relationship. If operation A must precede operation B:

For two operations requiring the same single-capacity machine, their time intervals must not overlap. A schedule is feasible only if all precedence, capacity, material, and calendar constraints are satisfied.

Finite scheduling improves realism but does not eliminate uncertainty. Processing times, breakdowns, absenteeism, rework, and material delays remain stochastic. Excessive utilization can also increase queues dramatically. Consequently, practical schedules normally retain protective capacity, especially at bottlenecks.

11. Integrated Applied Case: Illustrative Machine-Shop Scenario

Background

The following is a fictional illustrative case. Precision Components Ltd. produces two families of hydraulic valve bodies in a cellular job-shop environment. Demand is rising, but deliveries are late. The shop contains a machining cell, a heat-treatment subcontractor, and a final inspection area.

Problem

The company uses a weekly MPS, but the BOM for one valve family contains an obsolete revision. MRP consequently plans too few sealing components. In addition, the machining center is loaded with 185 hours against 160 effective hours in week 3. The dispatch list uses FCFS, so a long order blocks several short orders with earlier promised dates.

Analysis

The planner first validates the engineering revision and BOM quantities. The corrected MRP shows that sealing-component releases must be advanced by one week because supplier lead time is two weeks. CRP then identifies the machining center as the bottleneck. The team calculates an overload ratio of:

The center is therefore loaded at approximately 116% of effective capacity. The team also compares due dates and remaining work using critical ratios rather than relying only on order arrival time.

Approach and Solution

The company takes five actions:

1. freezes the corrected BOM revision and introduces engineering-change approval;
2. releases missing sealing components immediately and confirms supplier dates;
3. moves an approved operation to an alternate machine;
4. uses one overtime shift at the bottleneck; and
5. sequences short, urgent orders first while grouping compatible tooling to limit setup loss.

A daily Gantt chart is used to compare planned and actual completion. Preventive maintenance is scheduled outside the bottleneck’s committed production window, and inspection capacity is checked so that machining output does not simply create an inspection queue.

Outcome

The corrected material plan prevents assembly stoppages. The alternate machine and overtime remove the week-3 overload, while the revised dispatching rule improves due-date performance without eliminating fairness controls. After the immediate recovery, the company introduces weekly master-schedule review, monthly capacity review, and formal master-data audits.

Learning Points

The case demonstrates that late delivery may arise from interacting causes rather than one bad schedule. Accurate BOMs, feasible capacity plans, appropriate sequencing, and feedback control must operate together. It also illustrates that local utilization is not the same as system performance: maximizing machining hours without checking inspection and assembly can worsen total lead time.

12. Technology, Data, and AI in PPC

Enterprise resource planning (ERP) systems integrate orders, inventory, purchasing, BOMs, routings, and financial data. Manufacturing execution systems (MES) provide more detailed shop-floor status, machine data, quality records, and labor information. Advanced planning and scheduling systems may apply finite-capacity algorithms, constraint programming, mixed-integer optimization, or heuristic search.

AI and machine learning can support demand forecasting, estimated processing times, predictive maintenance, anomaly detection, and schedule adjustment. However, AI does not remove the need for process knowledge. Poor master data, biased historical records, unrecorded downtime, or unstable engineering revisions can produce confidently wrong recommendations. Human review is especially important when recommendations affect safety, labor conditions, quality release, or customer commitments.

Good governance requires traceability of schedule changes, access control, revision management, cybersecurity, data-quality ownership, and protection of personal workforce data. Automated systems should not conceal capacity shortages or encourage unsafe overtime merely to satisfy a numerical delivery target.

13. Challenges, Limitations, and Best Practices

PPC models are abstractions. They may assume deterministic demand and processing times even though actual operations are variable. Forecast error, inaccurate inventory, unreported scrap, unreliable lead times, downtime, engineering changes, and supplier disruption can invalidate an otherwise correct calculation.

Best practices include:

• maintain one controlled source for BOM and routing revisions;
• measure inventory accuracy and cycle-count critical items;
• separate forecast, order, and safety-stock logic;
• protect bottlenecks with realistic calendars and buffers;
• use time fences to control unnecessary MPS changes;
• include setup, yield, scrap, and quality-hold effects;
• review exception messages rather than treating every system recommendation equally;
• coordinate material, capacity, quality, and maintenance plans;
• measure both local and end-to-end performance; and
• involve operators in validating standard times and practical sequences.

Useful performance measures include schedule adherence, on-time delivery, throughput, average flow time, work-in-process, inventory accuracy, capacity utilization, changeover time, and first-pass yield. Measures should be balanced. For example, utilization alone may reward large batches and excessive inventory, while on-time delivery alone may encourage costly overtime or unsafe work.

Chapter Summary

Production planning, scheduling, and control form a hierarchy linking medium-term aggregate decisions with daily shop-floor execution. Aggregate planning balances demand, production, workforce, inventory, subcontracting, and backlog. The MPS specifies end items by time period and must be checked against rough-cut capacity. BOMs describe product structure, while MRP converts the MPS into time-phased component requirements using inventory, lead times, and lot-sizing rules. CRP translates planned work into loads at specific work centers and identifies capacity overloads.

Routing defines how work is performed; loading assigns work to resources; sequencing determines order; dispatching releases work; and Gantt charts communicate timing and progress. Priority rules such as FCFS, SPT, EDD, and critical ratio are useful under stated assumptions but must be adapted to real constraints. Job, batch, mass, and cellular systems require different planning emphases. Finite-capacity scheduling improves feasibility by respecting resource limits, but uncertainty and data quality remain decisive. Modern ERP, MES, optimization, and AI tools can strengthen PPC when supported by accurate master data, human judgment, ethical governance, and continuous feedback.

Key Terms

Aggregate planning: Medium-term planning of product families, capacity, workforce, inventory, and subcontracting.

Available-to-promise: Quantity and timing that can be committed to customers based on supply and planned production.

Bill of materials: Structured list of components and quantities required for a parent item.

Capacity requirements planning: Time-phased calculation of workload and available capacity at work centers.

Critical ratio: Priority index calculated from time remaining divided by work remaining.

Dependent demand: Demand derived from the planned production of another item.

Dispatching: Releasing and prioritizing work at the shop-floor level.

Finite-capacity scheduling: Scheduling that does not assign overlapping work beyond available resource capacity.

Gantt chart: Time-based visual representation of activities, resources, and progress.

Independent demand: Demand arising directly from markets, customers, or service requirements.

Loading: Assignment of jobs or operations to resources.

Master production schedule: Time-phased schedule for specific finished products or end items.

Material requirements planning: System for calculating dependent-demand material requirements and planned orders.

Net requirement: Quantity still needed after considering available inventory and scheduled receipts.

Priority rule: Procedure for selecting the next job to process.

Routing: Sequence of operations, resources, times, and requirements for manufacturing an item.

Sequencing: Determining the order in which jobs are processed.

Time fence: Planning-horizon boundary controlling the degree of schedule change permitted.

Work-in-process: Partially completed inventory within the production system.

Review Questions

Short Answer Questions

1. What is the difference between aggregate planning and master production scheduling?
2. Distinguish independent demand from dependent demand.
3. What information is required as input to an MRP system?
4. Why is a routing important in capacity requirements planning?
5. Define finite-capacity scheduling.

Long Answer Questions

1. Explain the hierarchy of PPC from aggregate planning through shop-floor control.
2. Compare level, chase, and mixed aggregate-planning strategies, including their operational consequences.
3. Explain MRP logic and discuss how BOM errors, lead-time errors, and inventory inaccuracies affect its results.
4. Compare FCFS, SPT, EDD, and critical-ratio sequencing rules and identify situations in which each may be useful.
5. Discuss how PPC requirements differ among job, batch, mass, and cellular production systems.

Analytical / Application Questions

1. A product requires three units of component X. The MPS requires 40 products in week 5 and 30 in week 6. Component X has 20 units on hand, no scheduled receipts, zero safety stock, and a one-week lead time. Using lot-for-lot planning, calculate the planned order releases for X.
2. A work center has three machines, each scheduled for 35 hours. Planned utilization is 80% and efficiency is 90%. A set of orders requires 82 effective hours. Calculate available effective capacity and the load ratio, then recommend two possible corrective actions.
3. Four jobs are available at time zero on one machine. Their processing times are 4, 7, 2, and 5 hours, with due dates of 10, 12, 8, and 15 hours respectively. Determine the SPT sequence and calculate each completion time. Explain why the sequence may not be appropriate if due-date performance is the dominant objective.

Multiple Choice Questions with Answers

Question 1

Which document specifies the time-phased quantities of specific finished products?

A. Bill of materials

B. Master production schedule

C. Routing sheet

D. Dispatch list

Correct Answer: B. Master production schedule

Question 2

Demand for a purchased component that is calculated from the production plan for its parent product is called:

A. Independent demand

B. Random demand

C. Dependent demand

D. Seasonal demand

Correct Answer: C. Dependent demand

Question 3

A product requires two units of component X. If 50 products are scheduled, what is the gross requirement for X?

A. 25 units

B. 50 units

C. 52 units

D. 100 units

Correct Answer: D. 100 units

Question 4

In lot-for-lot MRP, the planned order receipt is normally equal to the:

A. Gross requirement

B. Net requirement

C. Safety stock

D. Lead time

Correct Answer: B. Net requirement

Question 5

A work center has 120 hours of available capacity and a calculated load of 150 hours. Its load ratio is:

A. 0.80

B. 1.00

C. 1.25

D. 2.70

Correct Answer: C. 1.25

Question 6

Which priority rule generally minimizes mean completion time on a single machine when all jobs are available at time zero and assumptions are satisfied?

A. Shortest processing time

B. First come, first served

C. Longest processing time

D. Random selection

Correct Answer: A. Shortest processing time

Question 7

If a job has a completion time of 18 hours and a due date of 15 hours, its tardiness is:

A. −3 hours

B. 0 hours

C. 3 hours

D. 33 hours

Correct Answer: C. 3 hours

Question 8

Which activity assigns a sequence of manufacturing operations and their required work centers?

A. Routing

B. Forecasting

C. Demand smoothing

D. Inventory counting

Correct Answer: A. Routing

Question 9

What is the principal feature of finite-capacity scheduling?

A. It ignores machine calendars

B. It permits unlimited overlapping work

C. It respects available resource capacity

D. It eliminates processing-time variability

Correct Answer: C. It respects available resource capacity

Question 10

A machine shop has 100 units in inventory, produces 300 units, and faces demand of 250 units in a period. Ending inventory is:

A. 50 units

B. 150 units

C. 350 units

D. 650 units

Correct Answer: B. 150 units