Basic of Supply Chain Management
Session 4: Material Requirements Planning 物料需求计划
Material requirements planning (MRP) is the bridge between master planning and production in the factory. This session addresses the basic concepts of the MRP process, which relies on the use of planning application software to support a set of business processes for the day-to-day management of the priority plan. MRP has two major business functions:
Base on the master production schedule (MPS) quantities and due dates for end items, create a time-phased plan that determines what components are needed to make a product and when. This is called a priority plan for order releases and receipts for materials purchased from suppliers and manufacturers of components, or subassemblies, and end items by the factory.
Maintain and adjust the priority plan to account for changes in customer orders and forecasts, material shortages, and production problems.
Learning Objectives 学习目标
Material Requirements Planning Environment 物料需求计划环境
Explain the role of dependent versus independent demand in MRP.
Describe the relationship between MRP and three other functions in the manufacturing planning and control (MPC) hierarchy.
Bill of Material 材料清单
Define parent and component in the bill of material (BOM), and explain their relationship.
Perform a simple BOM explosion.
Explain the use of the planning bill.
Differentiate between where-used reports and pegging reports.
Material Requirements Planning Logic 物料需求计划逻辑
List four major inputs to MRP.
Describe the role of lead-time offsetting and exploring in creating the priority plan.
Explain the origin of the gross requirements in an MRP record.
Calculate the net requirements for the end item and components.
Offset a planned order release from a planned order receipt.
Using and Managing the Priority Plan 使用和管理优先计划
Differentiate between a scheduled receipt and a planned order receipt.
Differentiate between an open order, a planned order, and a firm planned order.
Explain the role of MRP software in the release of planned orders and managing the priority plan.
List the events of following the release of a planned order.
List at least two techniques that can be used to maintain component and end-item due dates in the priority plan.
Material Requirements Planning Environment 材料需求计划环境
In this section we will address the underlying concepts of MRP:
- nature of demand 需求的性质
- linkages to other manufacturing planning and control functions 与其他生产计划和控制职能的联系
- MRP objectives MRP的目标
- inputs to MRP MRP的输入
Nature of Demand 需求的性质
From an MPC standpoint, there are two types of demand: independent demand and dependent demand.
The table shown in Visual 4-6 is an independent demand item, where demand for the item is independent of any other demand. The decision on how many tables to make depends on either a forecast, customer orders, or both. The components of the table, however, are dependent demand items.
All end items, such as the table, have a BOM -- which we will soon discuss in more detail -- that lists the description and quantity of all of its components. Once the forecast or order quantity is known, MRP calculates the number of all the components required. The demand for components is dependent on the demand for the table.
Linkages with Other Manufacturing Planning and Control Functions 与其他制造计划和控制功能的联系
MRP is the last of the priority planning levels. It is directly linked to three other MPC functions:
Master scheduling: The MPS establishes specific end-item due dates and quantities. MRP is the component planner. It calculates a time-phased priority plan. This plan consists of (1) order release and receipt dates for the purchase and manufacture of components and (2) the final assembly operation. MRP determines the materials and quantities needed to make a product and when.
Capacity requirements planning (CRP): The MRP is valid only if there is sufficient production capacity when it is needed. The check against capacity availability is called CRP.
Production activity control (PAC): MRP is the input to PAC. It plans the release and receipt dates for orders, but it is the responsibility of purchasing and PAC to implement and control the orders.
Objectives of MRP MRP的目标
What does MRP do? It has two major objectives: determine requirements and keep priorities.
Requirements -- MRP determines requirements by addressing the following questions:
What components are required to manufacture an end item? Some of these will need to be purchased, some already are in inventory, and some will be made as part of the end-item production process.
What quantity of each component will be required?
When will orders be released and received for (1) purchased components from suppliers and (2) for components that will be manufactured or assembled during the production process?
Priorities -- The order and release dates for purchased and manufactured components listed above constitute the priority plan, but MRP is affected by unplanned events such as changes in the demand, shortages of production capacity, late arrival or completion of components, and excessive scrap. Material planners keep the priority plan current in the face of changing customer sales orders and due dates by actions that include the following:
Expediting or de-expediting work orders -- also called shop orders or manufacturing orders -- or purchase orders.
Changing work orders or purchase orders, such as quantities and due dates
Adding or canceling work orders or purchase orders
MRP Process Inputs and Outputs MRP过程的输入和输出
There are four major data inputs to the MRP process, as shown in the table below.
|MPS||Planned and scheduled orders for end items 计划和排程的成品订单||End-item quantities 成品数量
End-item due dates 成品交期
|Product structure file 产品结构文件||BOMs||Components required 所需组件
Quantity of each component per parent 每个父组件的数量
|Inventory record file 库存记录文件||Inventory status 库存状态||Component and end-item quantities on hand 组件和成品的在手数量
Component and end-item quantities allocated 组件和成品的已分配数量
Component and end-item quantities on order 组件和成品的预订数量
|Item master file 物料主文件||Planning factors 计划因子||Lot size 批量大小
Lead time 交货时间
Yield and scrap factors 产量和废品因子
Safety stock 安全库存
The section on the MRP logic will cover outputs in some detail. MRP creates a priority plan consisting of planned order receipts and releases. Planners, with the support of MRP software, release planned orders that authorize the following:
the release of purchase orders to suppliers
the release of manufacturing orders, or shop orders, to the factory
Because of the volume and complexity of calculations, most companies rely on application software for MRP. Software applications are a lot faster and more accurate in performing MRP calculations needed to create the priority plan. Example of such calculations are:
Exploding BOMs for determining an end item's component quantities, down to the component and raw material level
Checking inventory records for quantities on hand, on order, or allocated
Offsetting order release and receipt dates for net component and raw material requirements from the end-item due dates, which will be covered later in this section.
In addition, MRP software keeps track of due dates in the priority plan and the status of materials and production. It also
advises planners on when to release planned orders
advises when to expedite or take other actions on maintain the due dates in the priority plan
replans MRP on a periodic basic to update material requirements, due dates, and inventory status.
Bill of Material 物料清单
bill of material (BOM) -- A listing of all the subassemblies, intermediates, parts, and raw materials that go into a parent assembly showing the quantity of each required to make an assembly. 所有组成父组件的部件、中间体、零件和原材料的清单，显示每个组件所需的数量。 -- APICS Dictionary
The authors of Introduction to Materials Management,7th ed. (Arnold et al.), state that the BOM is one of the most important documents in a manufacturing company. They refer to it as the major building block of MRP. BOM data on the raw materials and components needed to produce end items are critical inputs to MRP. Variants of the BOM include recipes for baked goods and formulas for chemicals.
《物料管理导论》(Introduction to Materials Management，第7版)的作者Arnold等指出，BOM是制造企业中最重要的文件之一。他们称其为MRP的主要组成部分。生产最终产品所需的原材料和部件的BOM数据是MRP的关键输入。BOM的变体包括烘焙食品的配方和化学配方。
Summarized BOM and the Indented Bill 汇总物料清单和多级物料清单
The BOM usually is presented in list form. Visual 4-12 is an example of a summarized BOM, which is a list of all the components in an assembly. For each component, a bill shows a unique part number, a description, the quantity needed in the assembly of its parent, and the unit of measure.
The indented bill in Visual 4-13 is more commonly used and gives an indication of an assembly or end-item product structure. Note the part-number indentation showing that table 100 actually consists of two subassemblies: base 300 and top 025, and a hardware kit 822. The latter is a purchased component because the bill does not show components going into it during production; therefore it is not made by the manufacturer and has to originate with a supplier.
Base 300 and top 025, as subassemblies, consist of other components. The components of base 300 include legs 306, leg bolts 326, and frame 357. The latter is a subassembly made up of wooden sides 311 and wooden ends 433.
What components in the indented bill, in addition to hardware kit 822, are purchased components?
How many assemblies/subassemblies are in the bill?
With respect to quantities, let's use as an example legs 306 that go into base 300. One unit of base 300 requires four legs 306. Note that the unit of measure is EA(each), so that the quantity required would be 4*1 = 4. If the unit of measure were one dozen, table 100 would be an unconventional 48-legged table.
关于数量，让我们以底座300中的腿306为例。底座300的一个单位需要四条腿306。注意，度量单位是EA(每个)，因此所需的数量是4*1 = 4。如果以一打为单位，那么桌子100将是一张非常规的48条腿的桌子。
Note: In practice, glue might often be purchased as an operating supply item and not be included in the BOM. Tracking the specific amount of glue actually used could be impractical. This practice enables the costs to be included as part of the overhead.
Single-Level BOM 单级BOM
This visual shows the BOM in what is called a product tree format to facilitate learning and understanding.
We mentioned earlier that the APICS Dictionary definition refers to the BOM as a listing of all components that go into making an assembly. The visual illustrates that a BOM for an end item such as table 100 actually is made up of several single-level bills for its assemblies.
At the top of the visual is a single-level bill for table 100. The bill shows table 100 as the parent and right below it are components needed for its final assembly. We pointed out earlier that two of the components -- base 300 and top 025 -- also are assemblies, but because they are components of another assembly (table 100), they are called subassemblies. The third components is hardware kit 822 consisting of purchased hardware specific to table 100.
Multilevel BOM 多级BOM
If you examine the multilevel BOM for table 100, you should be able to see that it actually is made up of two subassemblies and the final assembly. The point is that end items usually consist of several logical groupings of components, or assemblies, based on the way the product is assembled.
The multilevel BOM for an end item such as table 100 is an example of the grouping of single-level BOMs for the subassemblies that go into it.
It is important to note that BOM applications software stores information for each assembly and subassembly as a separate and unique single-level bill. When BOM data for table 100 are needed for MRP purposes, the BOM application will assemble the single-level bills for all of the assemblies into a multilevel bill for table 100. The subassembly single-level bills can be used for any end item that requires them.
Parent-Component Relationship 父组件关系
We have alluded to the parent-component relationship is single-level BOMs. The multilevel bill illustrates the fact that subassemblies and purchased components are components of the assemblies that they go into, but they also are parents of the subassemblies and purchased components that go into them.
BOM: Summary 总结
These are the key points about the BOM:
All manufactured end items have a BOM.
An end item usually has a multilevel bill consisting of two or more single-level bills.
All single-level bills consist of a parent and components below it.
For each subassembly made by the manufacturer and not purchased from a supplier, there also will be a single-level bill.
The parent in a subassembly single-level bill also is a component of a final assembly or subassembly.
If a part has no other parts going into it on the BOM, then it is assumed to be a purchased part.
In summary: 总之
A multilevel BOM is a collection of single-level bills.
A subassembly parent also can be a component of an assembly or another subassembly.
If a component has no other components going into it, it cannot be a subassembly and therefore is an item purchased from a supplier.
Planning Bill 计划单
A BOM lists the components necessary to build an end product. A planning bill, on the other hand, plans material requirements for a family of end products for which there are several variations based on options for one or more components.
Take the example of a bicycle, which has several options for three main components.
|Component||Number of options|
The company takes 18 different variations which these components options (332 = 18), each with its own BOM to maintain. The planning bill simplifies the 18 bills by showing, on one bill, the percentage split -- totaling 100 percent -- for each of the three components based on a forecast or historical usage data.
公司采用18种不同的组件选项(332 = 18)，每个组件都有自己的BOM来维护。计划单简化了18个清单，在一个清单上显示了基于预测或历史使用数据的三个组件的百分比分割(总计100%)。
The planning bill in this case represents the average bicycle. It does not represent a buildable product but is used to simplify forecasting, MPS, and MRP. If 10,000 bicycles were forecasted, you could calculate the number for each of the 18 variations to plan for and ultimately make and ship to distributors. For example, if 10,000 bicycles were forecasted, you would order and stock 2,500 small, 5,000 medium, and 2,500 large-size frames and use the same reasoning for ordering colors and wheels. These the can be assembled when orders come in.
Where-Used and Pegging Data 用途和资料溯源
Where-used data are used in where-used reports. A where-used report is an inversion of a BOM information.
A BOM lists the components of a parent item.
A where-used report lists all of the items, or parents, in which a component is used whether or not there is a demand for the parent.
Data may be single level (traced to the immediate parent) or full level (traced to the end-item parent).
Where-used information has the following uses:
identifying the parent items affected by an engineering change and assessing the impact of the change
assessing the aggregate impact of costing changes for a component
assessing the effects of component scarcity
assessing the benefits and costs of standardizing similar components, preventing storekeeping unit proliferation
A pegging report is similar to a where-used report except that it shows only those parents for which there is an existing requirements. These parents are currently in the priority plan and might be affected by a delay in the delivery of a component. The pegging report shows the following:
the parents creating demand for components 父项创建组件的需求
the quantities required 所需数量
when those quantities are required 何时需要这些数量
where-used logic to identify current sources of demand 用于确定当前需求来源的用途逻辑
may be single level or multilevel 可为单层或多层
Uses of the BOM
There are many uses for the BOM, starting as early as the design stage of a product and continuing through the various stages of the product management cycle: engineering change control, MRP, order management, manufacturing, service and repair, and cost accounting. This places a premium on a accuracy and being up-to-date, as errors in the bills can have broad ripple effects.
|Type of use||Description of use|
|Product definition||The BOM specifies the components needed to make the product. Product definition first occurs early in the product design process as product and process design engineers discover the best way to design and make a product.
|Engineering change control||Product design engineers use the BOM to document changes made in the design of a product and the components used. The recording and control of changes to the product and the BOM are absolutely essential.
|Planning||The BOM is used by material planners to identify the materials that have to be scheduled to make an end product and what components have to be purchased or made to satisfy the MPS.
|Customer service||Customer service uses BOMs when configuring make-to-order products during order entry.
|Manufacturing||The BOM lists the parts needed to make or assemble a product.
|Service and repair||The BOM determines the replacement parts necessary to repair a broken assembly.
|Costing||Cost accounting uses the BOM to determine the quantity of direct materials consumed in making specific products.
Resource : Introduction to Materials Management,7th ed.,Amold et al., p.70
Material Requirements Planning Logic 物料需求计划逻辑
Approach to this Section
In this section, we are going to learn about the mechanics, or logic, that MRP uses to develop the priority plan.
We will explain the MRP logic that creates the priority plan through
exploding and offsetting requirements 需求展开和补偿
calculating gross and net requirements 计算毛需求和净需求
This will bring us to the point in the planning process where a developed priority shows the time-phased dates and quantities for planned order releases and receipts for both purchased components (or purchase orders) and manufactured components (or manufacturing orders).
In the next section, we will address the basics of using and putting the material requirements plan into action.
MRP Logic Overview
As discussed earlier and as summarized in Visual 4-23, the MRP process uses key inputs to determine and end item's required components (what); the quantities of those components (how much); and a time-phased plan (when) for the operations that will convert the end item into a finished product.
This time-phased plan, or priority plan, expresses the necessary actions in the form of planned order releases and receipts. At the appropriate time, the planned orders are released; this authorizes purchase orders to be released to suppliers for materials and manufacturing orders to the factory.
Lead-Time Offsetting and Exploding 提前期补偿和展开
To create a priority plan, MRP uses a process called lead-time offsetting and exploding. Three of the important components are
- lead time, or the time it takes to make a subassembly or component or receive delivery of a purchased component. Manufacturing lead-time data are found in the item master record.
- offsetting, which is the process of determining when a planned order release is needed in advance of the planned order receipt date of the item.
- exploding, which is the process of determining the total of each component needed for a parent.
Below is the procedure for offsetting and exploding the planned order releases for D.
Visual 4-25 illustrates the offset procedure for parent B and its purchased component, D.
- Parent B has a lead time of 1 week. Its planned order receipt date is the beginning of week 5, so a planned order release for B must be offset to the beginning of week 4.
- Parent B is an assembly of components C and D. This means planned order receipts for these components need to be available at the beginning of week 4, as they are needed when the planned order release of B occurs.
- Because the lead time for D is 1 week, a planned order release needs to be scheduled for the beginning of week 3 to meet the week 4 due date.
Exploding, also called BOM explosion, is the process of multiplying a parent item's need for components based on the BOM. We already have covered this in the section on the BOM in Problem 4.1. We'll briefly illustrate this with the visual.
- Each parent in the BOM requires one unit of a component. Because the requirement for end-item parent B is 50 units and because D is a component of B, there is a requirement for 50 Ds in order to make 50 units of B. A single B requires 1 C, so the requirement for C = 1 * 50 units = 50 units.
- The requirement for parent B of 50 units is in week 5. This means the planned order receipt for B is at the beginning of business on the first day of week 5.
- A planned order release for D has been offset by one week from a planned order receipt due in week 4 per the lead-time information from the item master record.
- The planned order release quantity was exploded, per the assumed BOM requirement that one unit of parent B requires one unit of D, so 50 units of B require 50 units of D.
How many Ds would have to be ordered if the week 5 requirement for B was 100 units and the required quantity of D per parent item B was two units?
Gross and Net Requirements 毛需求和净需求
The next step in the MRP calculation logic is the determination of gross and net requirements. In previous exercises, we assumed that if a parent B has a component requirement of 50 Ds, you would need to schedule a receipt of 50 units of D, offset by the appropriate lead time.
If you have items in inventory, they must be taken into account and subtracted from the gross requirement to determine a net requirement. This also is called gross to net, or netting.
Recognizing Available Inventory 识别可用库存
The convention is that the projected available inventory shown in a period represents the period-ending inventory. This means it is available at the very beginning of the next period.
To relate this to the visual, look at the 20 units of projected available shown for B in week 4. This is the week 4 end-of-period inventory that is available during week 5.
Note that the inventory available in week 1 for end item B (20) and component C (10) are shown in the cells before week 1 to indicate that they were the on-hand inventory at the end of the period that preceded week 1.
What is the inventory available during week 3 for component C?
Calculation of Net Requirements
The following summarizes the process for determining net requirements. Refer to the visual.
Find the gross requirement for an end item, such as B (50 units), which is determined by the MPS. It is shown in and is available at the beginning of week 5.
Refer to the equation for net requirements, which is as follows:
Net requirements = gross requirements - available inventory 净需求=总需求-可用库存
Based on the MRP record shown in the visual, the net requirement for B in period 5 is 30.
A receipt of 30 then is planned based on lot-size policy.
What is the net requirement for component C in week 4?
Component Availability and Offsetting 组件可用性和偏移量
Gross requirements, net requirements, and planned order receipts are considered to be available at the beginning of the week in which they are displayed.
For example, the gross and net requirements of end item D are shown in week 4, so a planned order receipt for D is shown in week 4 and is expected to be available at the beginning of the week. A planned order release for D is shown in week 3, based on a one-week lead time.
Basic MRP Record 基本的MRP记录
Salient Features 显著特性
Up until this point, we have not taken the time to discuss the salient features and terminology pertaining to a typical MRP record. There is one for every component of an end item, including the end item itself.
If there were a time stamp on this record, it would be at the beginning of week 1.
Week 1 is the action bucket, though no actions are necessary because there are no scheduled receipts or planned order releases; however, when week 3 rolls to week 1, there is the need for a planned order release.
The lead time establishes the offset of the planned order release from the planned order receipt.
The top row shows time periods, called time buckets -- or weeks, in this example.
The number of periods in the record is called the planning horizon; it should be as long as the cumulative lead time, which is equal to longest lead time among all the BOM paths below the end item.
An item is available at the beginning of the time bucket in which it is required.
The quantity shown in the projected available row is the projected available balance at the end of the period; all other quantities are for the beginning of the period.
Scheduled Receipts 计划接收量
The example shown above adds yet another row of data called scheduled receipts.
Scheduled receipts are planned orders that, before week 1, already have been released or placed with suppliers and the factory and have receipt dates within the planning horizon.
These orders are called open orders.
In periods in which there are scheduled receipts, their quantities are added to the projected available balance. (See the addition of 20 units of scheduled receipts to projected available in week 2.) Eventually the quantities influence the gross to net requirements calculation.
The new formula for net requirements = gross requirements - scheduled receipts - projected available.
净需求量的新公式 = 总需求量 - 计划接收量 - 预计可用量。
Class Problem 4.5:
Given the following product tree BOM, explode, offset, and calculate the gross and net requirements. All lead times are one week except for K, where the lead time is two weeks. The component quantities per parent are shown in parentheses and the lot-size policy is as required. The MPS calls for completion of 50 Js in week 5. There is a scheduled receipt of 47 Ks in week 2. There are 85 Ls and 135 Ns on hand.
Capacity Requirements Planning 能力需求计划
In master scheduling, rough-cut capacity planning (RCCP) is used to check the MPS against available capacity. In MRP, another capacity check is performed -- this time focusing on resources available to meet due dates in the priority plan at the component level as opposed to the end-item level.
This will be addressed in Session 5: Capacity Management and Production Activity Control.
Using and Managing the Priority Plan 使用和管理优先计划
Role of MRP Software MRP软件的角色
MRP software applications are used in most manufacturing companies and also are used in non-manufacturing industries such as retail, distribution, and service. They perform two major functions:
Perform gross to net, lead-time offset, and explosion calculations.
Given an end-item due date from the MPS, MRP software can perform the gross to net, offsetting, and explosion calculations necessary to create the priority plan for end items and their components. It does these fast, accurately, and in very high volumes.
Create action and exception messages.
Action and exception messages are critical to planners in managing the priority plan. MRP software is integrated with inventory and other systems. The software also keeps track of due dates in the priority plan and the status of materials and production. For example, it issues action messages when a planned order becomes due for release, or tools into the action bucket; it issues exception messages when material availability is not sufficient to meet a planned order receipt date for a component or an end item.
It is important to note that MRP software is not normally programmed to release planned orders. That is the role of the planner. On the basis of action and exception messages, the planner will release planned orders, expedite or de-expedite component schedules, or change order quantities.
Order Control 订单控制
In managing the priority plan, the material planner works with three main types of orders as shown in the MRP record: planned orders, firm planned orders, and released orders -- or scheduled receipts.
Planned Orders 计划订单
Planned orders are created by planning software when it encounters net requirements. The net requirement generates planned order receipts in the system, and planned order releases appear in a prior period based on the lead-time offset. Note the planned order receipt of 5 units in week 3 and the planned order release in week 1.
Firm Planned Orders 已确认计划订单
A firm planned order is a tool that enables the planner to override the MRP logic. Note the firm planned order release of 35 units in week 2, three weeks instead of two before the planned order receipt. This could be the result of the planner's decision to allow another week of lead time for the receipt due in week 5, possibly because production has a temporary quality problem and requires another week.
Released Orders 已发出订单
Planners are responsible for releasing planned orders, aided by action messages from the MRP system. Once released, a planned order becomes on open order to the factory or supplier. It appears as a scheduled receipt of 20 units as in week 2 of the MRP record shown in the visual.
A released order also is under the control of the planner and is subject to expediting, de-expediting, change in order quantities, and even cancellation.
In what week was the planned order released for the scheduled receipt for week 2?
What types of orders are under the control of the planner?
Releasing Planned Orders 发出计划订单
The release of an order by a planner, with assistance of the MRP application logic, ensures that work is done in time to meet net requirements.
As shown in the visual, the release of the order involves a number of events.
- Releasing a planned order 发布已计划好的订单
- Scheduling a receipt 安排接收
- Creating an open order 开单
- Allocating components to the order 分配组件到订单
Note that the open order is an authorization to:
- purchasing, to buy the necessary material, or to
- manufacturing, to make the component
Maintaining Priorities and Replanning 维护优先级和重新计划
Releasing planned orders is only one of several responsibilities of the planner. The planner has the general responsibility of managing the current priority plan, such as planned orders, open orders, and end-item due dates. The planner also deals with the numerous events that might affect order priorities.
These include 这些包括
external events such as 外部事件如：
- changes to customer orders 客户订单变更
- supplier shortages 供应商短缺
- late deliveries and lower than required order quantities from suppliers 迟交及供应商低于所要求的订货数量
internal events such as 内部事件如：
- lower yields or higher scrap factors than planned 收益率低于计划或废品系数高于计划
- late or early open order completions 延迟或提前完成公开订单
- capacity or resource constraints 产能或资源限制
- material shortages caused by inventory record inaccuracies that hide shortages or surpluses of components or end items and lead to the need to expedite or de-expedite.
There basically are two sets of actions available to the planner:
- Maintain the priority plan by taking actions to maintain the scheduled due dates for the production of components or the end item. These would include expediting or de-expediting.
- Replan the due date if an event does not enable the maintaining of a due date. Class Problem 4.6 illustrates the options available to the planner when the receipt of a component is less than planned and affects the schedule of not just one but two parent items that use the same component.