Chapter 4: The Heartbeat - Master Production Scheduling (MPS) |
4.1 The Promise You Make Before Cutting Any Metal |
Every order in a mechanical factory begins with a promise. A salesperson looks at a customer across a desk, or speaks to them over the phone, and says, 'Yes, we can deliver five hundred custom shafts to your warehouse by the fifteenth of next month.' That single sentence sets off a chain reaction. The purchasing department must order steel. The machining center must reserve time on the lathes. The heat treatment subcontractor must be booked. The quality team must plan inspections. The shipping department must arrange a truck. |
If the promise is accurate, the chain reaction ends with a happy customer and a profitable order. If the promise is wrong, the chain reaction ends with expediting costs, late deliveries, penalties, and damaged trust. |
But how does a salesperson know what promise they can makeThey cannot see the future. They cannot know, at the moment they speak, whether the steel supplier will be on time, whether the lathe will break down, or whether a more urgent order will jump the queue. Without a system, the salesperson guesses. They might look at a whiteboard or ask a planner who is too busy to give a careful answer. The result is a promise that sounds good but is often impossible. |
This is the purpose of the Master Production Schedule (MPS) . The MPS is not a vague plan. It is a precise, time-phased statement of what finished products the factory will produce, in what quantities, and on what dates. It is the heartbeat of the entire ERP system. Every other function - purchasing, inventory, shop floor control, finance - beats in rhythm with the MPS. When the MPS is realistic and stable, the factory runs smoothly. When the MPS is fantasy or changes constantly, the factory lurches from crisis to crisis. |
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4.2 The Difference Between MPS and MRP - A Crucial Distinction |
Many people confuse the Master Production Schedule with Material Requirements Planning. They are related but distinct, and understanding the difference is essential. |
The MPS deals with finished products - the things you sell to customers. It answers: What are we going to build, and whenThe MPS is expressed in units of finished goods, such as 'one hundred gearboxes in week twelve' or 'fifty hydraulic cylinders in week fifteen.' |
MRP deals with components and raw materials - the things you buy or make to build the finished products. MRP takes the MPS as an input and then answers: Given the MPS, what do we need to order, and whenMRP explodes the bill of materials, checks inventory, and generates purchase requisitions and work orders. |
Here is an analogy. Imagine you are planning a large dinner party. The MPS is your menu and schedule: 'On Saturday at seven PM, I will serve appetizers. At eight PM, the main course. At nine PM, dessert.' MRP is your shopping list: 'Given that menu, I need to buy beef, potatoes, cream, flour, and wine. I need to start marinating the beef on Friday afternoon and bake the cake on Friday evening.' The MPS sets the rhythm. MRP does the detailed material calculations. |
In a mechanical factory, the MPS is the highest level of planning. It is reviewed by senior management, sales, and production. It is the link between customer demand and factory operations. The MPS is typically expressed in weekly or daily time buckets. It looks into the future anywhere from a few weeks to many months, depending on the product's total manufacturing lead time. |
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4.3 Time Horizons: The Frozen, Slushy, and Liquid Zones |
Not all weeks in the MPS are equal. The factory has different degrees of freedom depending on how far into the future you look. Good ERP systems divide the MPS into three zones, sometimes given colorful names. |
The first zone is the frozen zone. This is the immediate future, typically the next one to two weeks. In the frozen zone, the MPS is fixed. No changes are allowed except in true emergencies. Why frozenBecause materials have already been ordered, work has already begun on the shop floor, and machine capacity has already been allocated. Changing the MPS in the frozen zone would mean stopping work in progress, scrapping partially completed parts, and creating chaos. A typical rule: to change the MPS in the frozen zone, a manager from sales, production, and finance must all sign off. |
The second zone is the slushy zone. This is the medium-term future, perhaps weeks three through six. In the slushy zone, changes are allowed, but they require careful review and have costs attached. The factory has not yet started production, but materials may have been ordered and capacity tentatively reserved. Changing the MPS in the slushy zone might mean expediting some materials or postponing others, but it does not require scrapping work in progress. The name 'slushy' captures the idea: you can still move things around, but it is getting firmer. |
The third zone is the liquid zone. This is the long-term future, beyond about six weeks. In the liquid zone, the MPS is a forecast, not a commitment. Changes are easy and expected. Materials have not been ordered, capacity has not been reserved, and no work has started. The factory uses the liquid zone for long-range planning - ordering raw materials with long lead times, planning major capacity expansions, and negotiating with key suppliers. The name 'liquid' means you can pour the schedule into any shape with little effort. |
The ERP system enforces these zones automatically. If a salesperson tries to change a customer order that falls within the frozen zone, the system either blocks the change or requires a special override with electronic signatures. If the change is in the slushy zone, the system calculates the cost of the change - perhaps showing that expediting fees will be required - and asks for confirmation. If the change is in the liquid zone, the system simply accepts it and updates the forecast. |
This zone structure is not about being rigid for the sake of rigidity. It is about stability. A factory whose MPS changes every day cannot plan anything. Suppliers cannot be trusted. Workers cannot be scheduled. Quality suffers because every change introduces new opportunities for error. The frozen zone protects the factory from its own worst impulses. |
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4.4 The Sales and Operations Planning Meeting - Where the Heartbeat Is Set |
The MPS does not appear by magic. It is created, reviewed, and adjusted in a regular meeting called Sales and Operations Planning (S&OP) . This meeting is the single most important recurring event in a mechanical factory. It typically happens monthly or weekly, depending on the pace of the business. |
Who attends the S&OP meetingSales, production, purchasing, finance, and senior management. Each brings a different perspective. Sales knows what customers are asking for and what promotions are planned. Production knows what the factory can actually build, given machine capacity and worker skills. Purchasing knows what materials are available and what supplier constraints exist. Finance knows the cost implications of different plans. |
The S&OP meeting follows a structured agenda. First, sales presents the demand forecast for the coming months. This forecast is based on actual customer orders, historical patterns, market intelligence, and sales targets. Second, production presents the current capacity situation. Which work centers are overloadedWhich have spare timeAre there planned maintenance shutdowns or upcoming vacationsThird, the team compares demand to capacity. If demand exceeds capacity, they must decide: add overtime, subcontract work, delay some orders, or turn away new business. If capacity exceeds demand, they must decide: reduce work hours, perform maintenance, build inventory for future peaks, or launch a sales push. |
The outcome of the S&OP meeting is an approved MPS for the liquid and slushy zones. The frozen zone is typically not changed at the S&OP meeting unless there is a major disruption, such as a supplier bankruptcy or a sudden customer cancellation. |
The ERP system supports the S&OP meeting by providing what-if scenarios. A planner can create a copy of the current MPS, change some assumptions - increase the forecast for a popular product, add a new customer order, reduce capacity on a bottleneck machine - and see the impact on delivery dates, costs, and inventory levels. The team can compare several scenarios before choosing one. This turns the S&OP meeting from a political negotiation into a data-driven decision process. |
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4.5 Available-to-Promise - The Salesperson's True North |
Every salesperson has been in this situation: a customer asks for a delivery date. The salesperson wants to say yes because they fear losing the order. But saying yes to an impossible date is worse than saying no. An impossible date leads to a late delivery, a disappointed customer, and possibly penalties. |
The ERP system gives the salesperson a powerful tool: Available-to-Promise (ATP) . ATP is a calculation that answers: Given the current MPS and current inventory, what is the earliest date we can promise delivery of a specific quantity of a specific product |
The ATP calculation looks at three things. First, it looks at existing inventory of the finished product. If the product is already sitting on the shelf, the ATP is immediate - today or tomorrow. Second, it looks at planned production from the MPS. If the MPS shows that a batch of the product will be completed in week ten, and today is week eight, the ATP for that batch is week ten. Third, it looks at components. If the finished product is not in stock and not planned, the ATP must account for the lead time to buy materials, manufacture the product, and perform quality checks. |
But ATP is not a simple number. It is a dynamic calculation that respects the frozen and slushy zones. If a customer asks for a quantity that would require changing the frozen zone, the ERP will either show a later date or require a management override. If the customer is willing to pay a premium, the ERP can calculate the cost of expediting - perhaps moving the order into the frozen zone by paying overtime or air freight - and present that cost to the salesperson. |
The most sophisticated ERP systems offer capable-to-promise (CTP) , which is ATP plus capacity constraints. ATP assumes that if you have the materials, you have the capacity to make the product. CTP checks the actual load on the bottleneck machines. It might say: yes, we have the steel, but the only lathe that can make this part is fully booked for the next three weeks, so the earliest delivery is week twelve. CTP is more accurate but requires more data and computing power. Many mechanical factories start with ATP and add CTP as their ERP maturity grows. |
For the salesperson, ATP is liberation. They no longer have to guess or beg a busy planner for an answer. They type the product and quantity into the ERP, and the system returns a date. They can tell the customer with confidence: 'We can deliver by the fifteenth.' And because the date comes from the real MPS and real inventory, it is a promise the factory can keep. |
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4.6 The Rough-Cut Capacity Planning Reality Check |
An MPS can be mathematically valid but practically impossible. The numbers can add up perfectly on paper, but the factory cannot execute the plan because a single machine is overloaded. This is the problem of infinite loading - assuming you have unlimited capacity. The MPS, before capacity checking, is often created with infinite loading. It says what you would like to do if you had unlimited machines, unlimited workers, and unlimited time. |
Rough-Cut Capacity Planning (RCCP) is the reality check. RCCP takes the MPS and applies it against the capacity of key bottleneck resources. It does not check every machine in the factory - that would be too detailed. Instead, it checks the critical resources, the ones that are most likely to be overloaded. For a mechanical factory, these might be the large CNC machining centers, the heat treatment furnace, the final assembly station, and the testing area. |
RCCP works by using a simplified routing. For each product in the MPS, the system knows approximately how many hours it requires on each critical resource. For example, a gearbox might require two hours on the large horizontal machining center, one hour in the heat treatment furnace, and three hours in final assembly. Multiply by the quantities in the MPS, and you get the total load on each resource per week. Compare that load to the available capacity - perhaps the large machining center has eighty hours available per week - and you see the overloads. |
If RCCP shows an overload, the factory has options. They can add capacity: run overtime, add a weekend shift, or subcontract some work. They can reduce load: move some products to earlier or later weeks, or shift products to alternative machines if available. They can change the product mix: substitute a similar product that uses less of the bottleneck resource. Or they can reject new orders until capacity frees up. |
The ERP system makes RCCP interactive. A planner can see a red bar showing an overload on the heat treatment furnace for week nine. They can drag some of the load from week nine to week ten, spreading it out. The system immediately recalculates the impact on delivery dates. The planner can try different strategies - overtime, subcontracting, shifting - and see the cost of each. RCCP turns capacity planning from a guessing game into a visual, quantitative trade-off analysis. |
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4.7 The Danger of the Chase Strategy and the Level Strategy |
Every factory must decide how it will respond to changing demand. This decision is captured in the production strategy that underlies the MPS. There are two extreme strategies, and most factories fall somewhere between them. |
The chase strategy means the factory changes its production rate to exactly match demand. When demand is high, the factory works overtime, hires temporary workers, and runs machines at maximum speed. When demand is low, the factory sends workers home, shuts down machines, and reduces hours. The chase strategy minimizes inventory - you produce only what you sell, when you sell it. But it is brutal on the workforce and on the machines. Workers face unstable schedules and frequent layoffs. Machines suffer from rapid starts and stops. Quality often suffers because the factory is always in a state of hurry or shutdown. |
The level strategy means the factory keeps production constant, regardless of demand. When demand is low, the factory builds inventory. When demand is high, the factory draws down inventory. The level strategy provides stability. Workers have steady hours. Machines run at a consistent pace. Quality can be controlled. But the level strategy requires holding inventory, which ties up capital and creates risk of obsolescence. |
Most mechanical factories use a hybrid. They level production for the core, stable part of their demand - perhaps seventy percent of their volume - and chase the remaining thirty percent. The MPS reflects this hybrid strategy. The ERP system helps by calculating the inventory implications of each strategy. It can show: if you level production at a rate of one hundred units per week, your inventory will rise to five hundred units during the slow season and fall to two hundred units during the peak. You decide whether that inventory level is acceptable. |
The choice of strategy is not purely mathematical. It reflects the factory's values, its workforce agreements, its capital availability, and its market. The ERP does not choose the strategy. But it provides the data to make an informed choice. |
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4.8 The Rolling Horizon - Never Starting from Zero |
A common mistake in factories new to ERP is to think that the MPS is created once and then followed rigidly until the end of time. That is not how planning works. The MPS lives on a rolling horizon. Each week, you look forward a fixed number of weeks - say, twenty-six weeks. You add a new week at the end, and you remove the week that has passed. You update the plan based on new information: new customer orders, supplier performance, machine breakdowns, quality problems. |
The rolling horizon has two critical benefits. First, it keeps the plan current. A plan that is six months old is worse than useless - it is actively misleading. By rolling the horizon weekly, the MPS always reflects the best available information. Second, it provides a natural way to move orders. If a job is late because a machine broke down, you do not delete the job. You roll it forward to a later week when capacity is available. The ERP system shows the ripple effect. Moving one job might push another job later, which might push another, until the entire schedule shifts. |
The length of the rolling horizon should be at least as long as the total cumulative lead time of the longest product. If your longest product takes twelve weeks from raw material order to finished goods, your rolling horizon must be at least twelve weeks. Otherwise, by the time you see a problem, it is already too late to fix it. |
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4.9 The MPS and the Factory's Nervous System |
A factory without a stable MPS is like a human body with a nervous system that fires randomly. Muscles twitch without purpose. Organs receive conflicting signals. The body cannot coordinate its actions. The factory equivalent is chaos: purchasing orders materials that production never uses, workers are rushed to complete jobs that are not urgent, finished goods sit while other orders are late. |
The MPS, when done well, becomes the factory's central nervous system. It sends a consistent, predictable signal to every part of the organization. Purchasing knows what to buy and when. Inventory knows what to keep and what to ship. Production knows what to build and in what sequence. Finance knows what cash will be needed and when. Sales knows what to promise and what to defer. |
But the nervous system is fragile. If the MPS changes too often, the factory experiences nervousness - the ERP term for excessive schedule instability. Nervousness is destructive. Every change to the MPS triggers changes in purchase orders, work orders, and inventory allocations. Suppliers receive revised delivery dates. Workers receive revised assignments. The cost of nervousness is real: expediting fees, idle time, rework, and confusion. |
ERP systems have features to reduce nervousness. One feature is the time fence - another name for the frozen zone. Another feature is order pegging - the ability to see which customer order or forecast drives each production order. When a change is proposed, the ERP shows which customer orders would be affected. A third feature is change impact analysis - the ERP estimates the cost of the change in terms of expediting fees, overtime, and missed deliveries, and presents that cost before the change is approved. |
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4.10 Real-World Example: The Pump Manufacturer's Turnaround |
Consider a manufacturer of industrial pumps. Their pumps have many variants - different motor sizes, different impeller materials, different seal types. Before implementing a robust MPS within their ERP, the company suffered from constant firefighting. The sales team promised aggressive dates to win orders. The production team, unable to push back, simply did their best. The result was a culture of lateness. Eighty percent of orders shipped after the promised date. Customers were angry. Profits were squeezed by expediting costs. |
The company implemented a disciplined MPS process with a three-zone structure: frozen for the next two weeks, slushy for weeks three through six, and liquid for weeks seven through twenty-six. They established a weekly S&OP meeting. The sales forecast was compared to capacity. Conflicts were resolved before they became crises. |
The first change was psychological. Sales learned that they could not promise a date without checking ATP. The ERP was configured so that every sales quote had to be entered into the system, and the system returned the earliest ATP date. Salespeople who tried to promise earlier dates found that the system would not generate a confirmations. Over time, they learned to trust the ATP. |
The second change was operational. Production stopped chasing every change. In the frozen zone, they worked exactly what was planned. If sales wanted to add an urgent order, they had to go through a formal expediting process that added a premium to the order's price. Most urgent requests were not actually urgent enough to pay the premium. The number of changes dropped dramatically. |
The third change was structural. The company shifted from a pure chase strategy to a level strategy for their most popular pump variants. They built a strategic inventory of these variants during slow months and drew it down during peak months. The MPS reflected this level production, which allowed them to smooth their workforce and reduce overtime. |
Within nine months, on-time delivery rose from twenty percent to ninety-five percent. Overtime costs fell by forty percent. Customer satisfaction scores improved dramatically. The MPS did not magically create new capacity. It simply forced the company to make realistic promises and then keep them. |
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4.11 The Human Factors: Discipline, Trust, and Courage |
An MPS is a technical tool, but its success depends entirely on human factors. Three qualities are essential. |
The first is discipline. The frozen zone must be respected. The S&OP meeting must happen every week, even when things are busy. Data must be entered accurately and on time. Discipline feels rigid, but it creates freedom. When everyone follows the same rules, the factory can trust the plan. |
The second is trust. Sales must trust that the ATP date is the best possible date, not a bureaucratic obstacle. Production must trust that the MPS will not change arbitrarily, so they can plan their work. Management must trust that the numbers in the system reflect reality. Trust is built through transparency. When a salesperson sees that the ATP calculation is based on real inventory and real capacity, they trust it. When production sees that the frozen zone is actually frozen, they trust the plan. |
The third is courage. Sometimes the MPS will show that the factory cannot meet the sales forecast. That takes courage to admit. The natural human reaction is to hope for a miracle - a machine will run faster, a supplier will deliver early, a customer will cancel. But hope is not a plan. Courage is accepting the reality of the MPS and then having the difficult conversation with sales, with customers, and with management about what is actually possible. |
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4.12 Summary: The Heartbeat That Holds the Factory Together |
The Master Production Schedule is not the most glamorous part of an ERP system. It does not have the real-time excitement of shop floor control or the analytical depth of inventory intelligence. But it is the heartbeat of the entire operation. Every other process - purchasing, manufacturing, shipping, billing - depends on the MPS. |
When the MPS is realistic and stable, the factory runs like a healthy heart. The beat is regular and strong. Blood flows to every organ in the right quantity at the right time. The body is calm and capable. When the MPS is a fantasy or constantly changing, the factory runs like a heart in fibrillation. The beats are chaotic and weak. Blood does not flow where it is needed. The body is in crisis. |
The ERP system provides the tools to create a strong, stable MPS: time zones, ATP calculations, rough-cut capacity planning, S&OP workflows, and nervousness controls. But the tools are only half the story. The other half is the discipline, trust, and courage of the people who use them. Together, the system and the people create a heartbeat that holds the factory together, even when demand surges, suppliers stumble, and machines break. |
In the next chapter, we will explore how the ERP uses the MPS to drive the detailed calculation of material needs - a process called Material Requirements Planning. But always remember: MRP is only as good as the MPS that feeds it. A bad heartbeat means a dead body, no matter how skilled the surgeon. |
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Key takeaways from Chapter 4: |
1. The Master Production Schedule (MPS) is a precise, time-phased statement of what finished products the factory will build and when. |
2. The MPS is distinct from MRP: the MPS deals with finished products, while MRP deals with components and raw materials. |
3. The frozen, slushy, and liquid zones create stability by limiting changes near the present while allowing flexibility far in the future. |
4. The Sales and Operations Planning (S&OP) meeting is the regular forum where the MPS is reviewed, adjusted, and approved. |
5. Available-to-Promise (ATP) gives salespeople a reliable date they can promise to customers, based on real inventory and planned production. |
6. Rough-Cut Capacity Planning (RCCP) checks the MPS against bottleneck resources to identify overloads before they cause crises. |
7. The choice between chase and level production strategies affects inventory, workforce stability, and cost - most factories use a hybrid. |
8. A rolling horizon keeps the MPS current by adding new weeks and dropping past weeks on a regular schedule. |
9. Schedule nervousness is the destructive effect of too many MPS changes; ERP features like time fences and pegging reduce it. |
10. Real-world success requires discipline to follow the MPS, trust in the system's calculations, and courage to face capacity limits honestly. |
cs@easiersoft.com