Preventive routine, Relative Advantages

Maintenance that is conducted in order to prevent failure. Usually this maintenance is planned some time in advance and has to be conducted within a registered time frame during which the asset is in non-operating condition

Preventative is used to maximise an assets useful lifetime and minimise cost. Its purpose is to avoid unscheduled or unplanned breakdowns, where reactive maintenance is used.

Preventive maintenance strategies can be further divided into usage and condition based maintenance. Under usage based maintenance, the total usage of a part is measured and maintenance is conducted when a certain threshold level has been reached. The usage of parts can be measured in many ways depending on the nature of the equipment. Time in the field is perhaps the most common mean to measure usage.

For vehicles (e.g., rolling stock), mileage is a common measure of usage. The number of on-off cycles is a measure of usage for equipment that is mainly loaded at the end or beginning of on-off cycles. For example, the number of landings is a measure of usage for the landing gear of an aircraft. Since the usage of equipment is usually scheduled, the moment that maintenance is performed can also be scheduled. If there is a large set-up cost associated with maintenance, it can be beneficial to interchange several parts simultaneously (Block replacement and/or overhaul). Otherwise, maintenance can be performed on a single component (Component replacement and/or overhaul).

Advantages:

Less risk factor: Because the equipment and your building are being regularly checked, they are at less risk to breaking down without notice. Therefore, creating a safer working environment for employees.

Follows a schedule: By following a schedule, you are able to keep to a budget while maintaining your building. Also, you will be able to keep track of all your equipment and pin point times when you will need to replace your equipment.

Longer equipment/building life: When equipment is being checked and maintained, it will be kept in its best shape, therefore extending its lifetime. With routine check-ups on building parts such as pipes, boilers, and roofing you’ll extend the life of your building as well.

Money saving: Over time, you will see that less money is being spend because you will not have to replace equipment as much, as well as dealing with last minute break downs. While there still may be some unplanned maintenance needed, the likely hood will go down when the building and equipment are regularly checked. Property wise, you’ll be able to catch roof leaks before they escalate and quickly repair them before mould and debris occur.

Less energy wasting: In general when equipment is not kept in the best conditions possible, it will drain more energy, hiking up your utilities bill. With properly maintained equipment, it will be saving you energy and money. While regularly kept lighting and cooling/heating systems will also help reduce the energy bill.

Less disruptions: With regular checks, you won’t be surprised when something goes wrong. It will be a quick fix because you will know what needs to be done. There will not be problems when it comes to closing down your property and disrupting your workers, if a large problem were to occur.

Disadvantages:

More money upfront: When initially starting a preventative maintenance plan, it will cost you more to regularly maintain equipment and the building, than it would be if you waited for things to simply break down.

Over maintenance: Because there is a regular plan, sometimes items may not need to be checked as often as planned. If this is the case, you can change your maintenance plan to checking the specific equipment or areas less often, while still maintaining a schedule.

More workers: Preventative maintenance require more workers because regular checks are a must. When compared to reactive maintenance, you simply need to call someone in for a onetime fix. Instead this method requires workers to always be on site and perform daily works.

Types of maintenance Breakdown, Spares Planning and control

Planned maintenance

Planned maintenance means that the organization is prepared for a breakdown and even expects it to happen. The equipment runs until it breaks, which initiates a run to failure (RTF) trigger. While RTF triggers can be unplanned, breakdown-maintenance plans use RTF as a way of lowering the cost of maintenance.

This kind of plan needs to be rigorously documented and controlled. Employees should be clear on exactly which parts will break down and which parts will be maintained normally via preventive maintenance. Without these checks, a breakdown maintenance plan can be exploited or run awry.

Unplanned breakdown maintenance

Unplanned breakdown maintenance, on the other hand, occurs when a piece of equipment fails or breaks unexpectedly also called an unplanned downtime event. While some facilities may not utilize a planned maintenance plan, nearly every facility needs resources in place for unplanned maintenance. After all, every piece of equipment will break or fault at some point in its life.

Condition-Based Maintenance

Condition-based maintenance is sometimes considered to be a more advanced alternative to preventive maintenance. Rather than being inspected according to a schedule, machines and systems are carefully observed for changes that could indicate upcoming failure.

With condition-based maintenance, technicians observe the system running and identify variables that could affect functioning, like temperature, vibration speed, power, the presence or absence of moisture, and more.

Corrective Maintenance

Corrective maintenance is initiated when a problem is discovered while working on another work order. With corrective maintenance issues are caught ‘just in time’.

For example, during a scheduled maintenance check or while fixing another issue, a maintenance technician notices that a pipe in a HVAC system is not working as it should. Corrective maintenance is then scheduled for a future date where the problem is repaired or replaced.

Pros

Failed equipment can lead to disastrous consequences, but a clearly-documented maintenance plan can actually have a few significant benefits for an organization.

  • Minimizes maintenance cost by cutting out unnecessary preventive maintenance
  • Lowers cost of replacing disposable items frequently (light bulbs, tools, fuses)
  • Downtime for repairs is consolidated
  • Low staffing needs
  • Simple and easy to understand when maintenance is required

Cons

The downsides of breakdown-maintenance are especially important to weigh given the nature of the maintenance plan. For example, breakdown-maintenance should never be used with safety equipment because a single lapse can cost one or multiple employees their health or their lives.

  • Form of waste in a manufacturing environment
  • Safety issues can occur with unplanned failures
  • Can be costly depending on parts that fail
  • Requires careful planning and execution
  • Can be difficult to pinpoint source of issues

Spares Planning and control

Spare Parts Management purpose is to provide “the right parts, in the right quantity, to the right place, at the right time, with the right level of quality, and at the least total cost to the organization”.

Effective management of maintenance spare parts is a critical contributor to equipment operating performance and to the cost of the maintenance investment.

The scope of Spare Parts Management therefore includes all functions from the supplier through to the point of use. identification and coding, criticality classification, procurement, quality inspection, stocking policies, links to work planning (kitting, staging), supplier management and internal performance.

For managing parts inventory more effectively:

Identify all spare parts: Make sure that all parts required for maintaining the asset are properly identified. If, for example, a part is required for a major overhaul, there may be several items that are purchased for a one-time use. Once the machinery overhaul is completed, there may be unused parts which have future use and should be inventoried. Other equipment in the plant may be able to use the same parts. So, rather than maintenance storing these parts without any correlation with the asset, the items should be added to the inventory control system and placed into parts inventory on the EAMS/CMMS.

Classify all spare parts: Classify the spare parts as per criticality. This classification will support the process to define an effective safety stock:have the right parts, at the right time, with the lowest possible cost at the minimum inventory value.

Utilize and Manage the Bill of Materials (BOM): Having accurate BOMs will support the scheduled preventive maintenance (PM) that is needed on a given piece of equipment. This process will allow the generation of a work order with the scheduled date of the PM.

BOMs will make ordering parts and placing work orders simpler. Additionally, make sure that your BOMs are kept up to date, considering the asset status and modifications.

Use the work order: All spare parts that have been used have to be linked to a work order. For PM, work orders requested in advance can be pulled and staged for pickup or delivered to the requesting department. This will reduce the maintenance department’s wait time at the parts warehouse for their work order to be fulfilled. Work orders need to be created for all parts issuance so that inventory remains accurate.

In case of emergency, it would be possible to understand the historical information related to breakdowns to improve the parts in stock, when this is reasonable.

Limited access to the parts warehouse inventory: Limiting access to the parts warehouse inventory is mandatory to maintain inventory accuracy. Adopt a policy that parts inventories are “off limits” and only parts department employees have access, utilizing badge access to entry and exit points. Allowing everyone to have access can quickly make parts inventories inaccurate.

Optimize the warehouse: Centralize and consolidate parts – Having all your parts centralized (in one or satellite warehouses) and consolidated can make security easier but also makes this potentially large asset easier to control and maintain inventory accuracy.

Use an Inventory control system: By utilizing your ERP’s warehouse functionality or a warehouse management system (WMS) to manage your parts inventory will ensure accuracy and ease of managing your parts warehouse. This should be linked to the EAMS/CMMS.

Using barcodes and scanning functionality in conjunction with the system will improve the efficiency of the management processes in the parts warehouse and inventory accuracy.

Define a stock location for every part: Ensure that stock locations are created at the lowest detail, typically a bin and slot location for each SKU, allowed by your systems.

Implement cycle counting for inventory control: Cycle Counting is a Periodic inventory system audit-practice in which different portions of an inventory are counted or physically checked on a continuous schedule, related with inventory ABC classification. The usual class breakpoints applied are:

A = 10% of line items, gives 65% of turnover

B = 20% of line items, gives 25% of turnover

C = 70% of line items, gives 10% of turnover

Each portion is counted at a definite, preset frequency to ensure counting of each item at least once in an accounting period.

Some variation about these figures is usual, but significant differences can indicate problems with stock holding policy.

Standardize Spare Parts: Standardizing for spare parts management, usually means agreeing that a certain type/model of equipment will be used and with that the spare parts required for each installation will be the same. This is different to industry-wide standardization. This can contribute to improve the MTTR, the negotiations with the Spare Parts Suppliers and the stocks.

Develop Spare Parts Management Processes: Make sure that your Spare Parts Management Processes are developed as per best practices to ensure sustainability and a base for training.

Decide based on accurate data: Use quality data for decision making: develop your reports and KPI´s considering the Spare Parts Management Processes, to support the improvement and base for Spare parts maintenance budget.

Benefits of spare parts management:

  • Allows you to keep spare inventory levels low, reducing carrying and storage costs but providing access to spare parts when needed.
  • Better service to your customers. The longer your operations are up and running, the better you’re able to deliver the services and support your customers demand.
  • Enhanced part visibility so your supply chain can show where the need is and where the parts are headed.
  • Improved equipment uptime as you limit how long equipment is down and don’t have to wait to generate revenue or find a funding source to repair a part when it breaks.
  • Quicker repairs and replacements for defective parts, plus assistance with larger failures.

Waste Management Scrap and surplus disposal, Salvage and recovery

Waste management places an important role in managing operations. Wastes can be categorized into obsolete, surplus and scrap items.

  • Obsolete items: These are those materials and equipments which are not damaged and which have economic worth but which are no longer useful for the Company’s operation owing to many reason such as, changes in product line, process, materials, and so on.
  • Surplus items: These are those materials and equipments which have no immediate use but have accumulated due to faulty planning, forecasting and purchasing. However, they have a usage value in future.
  • Scrap: It is defined as process wastage, such as, turnings, borings, sprues and flashes. They may have an end-use within the plant having commercial values. Hence, should be disposed of periodically.

Disposal of Scrap surplus and Obsolete materials

Disposal of scrap when handled in an imaginative manner can result in handsome returns to the organization. An effective disposal requires a compact disposal organization reporting to the materials manager, continuous market survey on the prices of various categories of scrap generated in the plant and constant touch with the industries which generate similar scrap and with the end-users.

Disposal action follows when the scrap cannot be utilized within the organization. In practice, it has been found that it is profitable to dispose the scrap directly to end-users rather than to middlemen who normally form a cartel of their own which leads to lower returns. Before disposal action it is essential that the scrap is segregated according to metal, size, etc. when the scrap is mixed, the return is even lower than the lowest element in the mixture. This is because the buyer of scrap will have to segregate it at an extra cost. A cursory analysis of scrap prices will reveal that sheet and plate cuttings will fetch fewer amounts per a tonne compared to that of turnings and borings. Also when costly scrap such as copper, aluminum and tungsten are involved, it is imperative that they are segregated as returns are huge and price levels are different. Since scrap is generated process-wise, it comes out in a segregated condition and there should be no difficulty in sorting.

Auction and Tender methods are frequently used for disposal of scrap. Parties in both the cases are normally required to inspect the scrap in the scrap yard and deposit earnest money. Very often the company insists on a basic price depending upon the category of scrap. The disposal section works, in this aspect, in close coordination with the finance department. In many cases the disposal section may try to enter into a long-term contact with end-users such as steel plants.

Many companies have found to their displeasure scrapped components appearing in the market and competing with their parts as ‘original equipment’. This is the price which organizations pay for not dismantling and disfiguring the scrap before disposal. Automobile spare parts and bearings especially are prone to such dangers. For this purpose some organizations go to the extent of requesting vehicle users to demolish filters and plugs before scrapping them. This is very important aspect.

In view of the paucity of raw materials and shortage of credit, it is necessary that optimum usage of materials is made and funds tied up in obsolete surplus and scrap items minimized. This is only possible when top management shows commitment and support. The employees of the organizations are naturally the best people to suggest improvements in materials, processes and new end users for scrap. It is they who can minimize the accumulation of scrap through coordination.

Therefore, top management should work out formal reward systems to promote employee participation in this matter. A few organizations have suggestion box schemes which pay rich dividends to the organization. Employees, too get rewards and recognition in the process.

Following are the reasons for the generation and accumulation of obsolete, surplus and scrap items:

  • Changes in product design: This may lead to some items getting invalid so far as the final product is concerned. Hence, the entire stock of such items as surplus obsolete.
  • Rationalization: Sometimes raw materials are renationalized so as to minimize variety and simplify procurement. The rationalization process renders some items as surplus or obsolete.
  • Cannibalization: When a machine breakdown occurs, sometimes it is rectified using parts of an identical machine which is not functioning due to various reasons. This process of ‘cannibalization’ is not uncommon in many project-based industries. When continued unchecked, this results in obsolete and scrap items.
  • Faulty planning and forecasting: The marketing department may have projected a sales forecast which might be on the higher side. Any material planning has to be based on sales forecasts and this could result in surplus items. Wrong indenting by the user departments also leads to accumulation.
  • Faulty purchase practices: Sub-optimizing decisions like buying in bulk to take care of discounts and transportation economy without taking into account factors such as, shelf life, storage space requirements and technological changes once again lead to the accumulation of surplus and obsolete stocks.
  • Other causes: Many items are held as insurable spares for many years without any consumption. Faulty store-keeping methods, without adequate preservation, lead to spoilage. Inferior materials handling, improper codification and poor manufacturing methods also result in obsolete, surplus and scrap items. Poor maintenance of machine tools may result in excessive tools wear and greater process scrap.

Capacity planning

Capacity planning is the process of determining the production capacity needed by an organization to meet changing demands for its products. In the context of capacity planning, design capacity is the maximum amount of work that an organization is capable of completing in a given period. Effective capacity is the maximum amount of work that an organization is capable of completing in a given period due to constraints such as quality problems, delays, material handling, etc.

Capacity planning is long-term decision that establishes a firm’s overall level resources. It extends over a time horizon long enough to obtain resources. Capacity decisions affect the production lead time, customer responsiveness, operating cost and company ability to compete. Inadequate capacity planning can lead to the loss of the customer and business. Excess capacity can drain the company’s resources and prevent investments into more lucrative ventures. The question of when capacity should be increased and by how much are the critical decisions. Failure to make these decisions correctly can be especially damaging to the overall performance when time delays are present in the system.

The phrase is also used in business computing and information technology as a synonym for capacity management. IT capacity planning involves estimating the storage, computer hardware, software and connection infrastructure resources required over some future period of time. A common concern of enterprises is whether the required resources are in place to handle an increase in users or number of interactions. Capacity management is concerned about adding central processing units (CPUs), memory and storage to a physical or virtual server. This has been the traditional and vertical way of scaling up web applications, however IT capacity planning has been developed with the goal of forecasting the requirements for this vertical scaling approach.

A discrepancy between the capacity of an organization and the demands of its customers results in inefficiency, either in under-utilized resources or unfulfilled customer demand. The goal of capacity planning is to minimize this discrepancy. Demand for an organization’s capacity varies based on changes in production output, such as increasing or decreasing the production quantity of an existing product, or producing new products. Better utilization of existing capacity can be accomplished through improvements in overall equipment effectiveness (OEE). Capacity can be increased through introducing new techniques, equipment and materials, increasing the number of workers or machines, increasing the number of shifts, or acquiring additional production facilities.

Capacity is calculated as (number of machines or workers) × (number of shifts) × (utilization) × (efficiency).

Strategies

The broad classes of capacity planning are lead strategy, lag strategy, match strategy, and adjustment strategy.

Lead strategy is adding capacity in anticipation of an increase in demand. Lead strategy is an aggressive strategy with the goal of luring customers away from the company’s competitors by improving the service level and reducing lead time. It is also a strategy aimed at reducing stockout costs. A large capacity does not necessarily imply high inventory levels, but it can imply higher cycle stock costs. Excess capacity can also be rented to other companies.

Advantage of lead strategy: First, it ensures that the organization has adequate capacity to meet all demand, even during periods of high growth. This is especially important when the availability of a product or service is crucial, as in the case of emergency care or hot new product. For many new products, being late to market can mean the difference between success and failure. Another advantage of a lead capacity strategy is that it can be used to preempt competitors who might be planning to expand their own capacity. Being the first in an area to open a large grocery or home improvement store gives a retailer a define edge. Finally, many businesses find that overbuilding in anticipation of increased usage is cheaper and less disruptive than constantly making small increases in capacity. Of course, a lead capacity strategy can be very risky, particularly if demand is unpredictable or technology is evolving rapidly.

Lag strategy refers to adding capacity only after the organization is running at full capacity or beyond due to increase in demand (North Carolina State University, 2006). This is a more conservative strategy and opposite of a lead capacity strategy. It decreases the risk of waste, but it may result in the loss of possible customers either by stockout or low service levels. Three clear advantages of this strategy are a reduced risk of overbuilding, greater productivity due to higher utilization levels, and the ability to put off large investments as long as possible. Organization that follow this strategy often provide mature, cost-sensitive products or services.

Match strategy is adding capacity in small amounts in response to changing demand in the market. This is a more moderate strategy.

Adjustment strategy is adding or reducing capacity in small or large amounts due to consumer’s demand, or, due to major changes to product or system architecture.

Resource optimisation techniques like:

  • Agile: Agile software development refers to a group of software development methodologies based on iterative development, where requirements and solutions evolve through collaboration between self-organizing cross-functional teams.
  • Scrum: Scrum is a subset of Agile. It is a lightweight process framework for agile development, and the most widely-used one.
  • Jira (Tool): Jira is a project management tool used to maintain transparency across all stakeholder’s project members.
  • RACI: This acronym ‘RACI’ stands for the four roles that stakeholders perform in any given project, regardless of its scale. The RACI matrix is a responsibility assignment chart that maps out every task, milestone or key decision involved in completing a project and assigns which roles are Responsible for each action item, which personnel are Accountable, and, where appropriate, who needs to be Consulted or Informed.

Controlling, Scheduling, Routing

Controlling

Production control is the activity of monitoring and controlling any particular production or operation. Production control is often run from a specific control room or operations room. With inventory control and quality control, production control is one of the key functions of operations management.

Types

One type of production control is the control of manufacturing operations.

  • Production planning and control of the when and where.
  • Production control and supply chain management

Management of real-time operational in specific fields.

  • Production control in the television studio in a production control room
  • Master control in television studio
  • Production control in spaceflight in a Mission Operations Control Room

Routing

Routing determines the path from which the raw materials flow within the factory. Once, the sequence is followed, raw materials are transformed into finished goods.

Setting up time for every step is important to measure the overall duration of the production process. Simply saying, routing in manufacturing states the sequence of work and operations. Routing throws light on the quantity and quality of materials to be used, resources involved (men, machine, and material), the series of operations and place of production.

Routing manages “How”, “What”, “How much”, & “Where” to produce in a manufacturing company. It systematizes the process and nurtures optimum utilization of resources to get the best results.

Scheduling

Scheduling is the second step that emphasizes on “When” the operation will be completed. It aims to make the most of the time given for completion of the operation.

As per Kimball and Kimball, scheduling is defined as:

“The determination of the time that should be required to perform the entire series as routed, making allowance for all factors concerned.”

Organizations use different types of schedules to manage the time element. These include Master Schedule, Operation Schedule, Daily Schedule and more.

Corresponding Production Planning

Production planning is the planning of production and manufacturing modules in a company or industry. It utilizes the resource allocation of activities of employees, materials and production capacity, in order to serve different customers.

Different types of production methods, such as single item manufacturing, batch production, mass production, continuous production etc. have their own type of production planning. Production planning can be combined with production control into production planning and control, or it can be combined with enterprise resource planning.

Production planning is the future of production. It can help in efficient manufacturing or setting up of a production site by facilitating required needs. A production plan is made periodically for a specific time period, called the planning horizon. It can comprise the following activities:

  • Determination of the required product mix and factory load to satisfy customer’s needs.
  • Matching the required level of production to the existing resources.
  • Scheduling and choosing the actual work to be started in the manufacturing facility”
  • Setting up and delivering production orders to production facilities.

Types of planning

Different types of production planning can be applied:

  • Advanced planning and scheduling
  • Capacity planning
  • Master production schedule
  • Material requirements planning
  • MRP II
  • Scheduling
  • Workflow

Tips to improve production planning

Forecasting demand

Before production planning, the first action to take is forecasting demands for your products. While this may not be accurate to the last digit, getting rough estimates rolling is important to allocate resources. Forecasting can be done based on factors like historical order data and market trends/demands. Drawing out proper forecasts helps planning the type and quantity of materials to be produced and also the planning of raw material procurement.

Control inventory

Both, inventory shortage and inventory surplus are undesirable states. You can’t proceed with production when there’s a shortage and you waste space and money when there’s a surplus. Efficiently controlling inventory involves reordering when current inventory dips below a certain level, calculating the lead times to order items with long lead times earlier, and factoring in storage conditions. A well-controlled raw material inventory helps run a smooth production line and outputs finished goods inventory on time.

Plan for everything and everyone

Often, when making production plans, some machine or some person is unaccounted for. The problem here is that that machine may go down or the worker may be on leave, or worse, working on something else. Hence, plan for every machine, raw material, workstation, warehouse, and employee.

Monitor

Once the production plan is final and work orders are handed out, the manufacturing process begins. At this point, things may go wrong, machines pause, or items may get misplaced. Constantly monitoring the factory floor with supervisors or with IoT devices ensures that all the pieces are moving as planned.

Adapt

Despite your best planning, things go wrong on the factory floor. Anything can happen from suppliers making late deliveries to workers falling sick to machines failing. It’s important to be flexible and adapt to these changes quickly so that the planned quantities can be delivered on time. Ideally, you should also plan for any such risks beforehand.

KPIs for production planning

A few key performance indicators to track in production planning are:

  • Production cost: This is essentially the monetary cost involved in producing the item. Costs include raw materials, electricity, fuel, worker salaries, rent, etc.
  • Capacity utilization rate: It’s the percentage of actual manufacturing output against the total possible manufacturing output. If many machines and workers are sitting idle, your capacity utilization is low. Ideally, you want it high but never full.
  • Projected versus actual hours: When planning, you may allocate a certain number of hours for completion of the production plan. But, it may take longer due to delays from workers or unexpected tasks. This KPI gives you a picture of how much time it was supposed to take and how much it did.
  • Employee utilization (productivity): You want workers to be working properly during the punch in and punch out. Nobody wants to be a machine by working to the dot but working 4 hours out of 8 is also not reasonable.
  • Takt time: Takt time is a lean manufacturing concept. It is the time taken to produce a single unit of item.

Accepting Sampling Procedures

Acceptance sampling is a statistical measure used in quality control. It allows a company to determine the quality of a batch of products by selecting a specified number for testing. The quality of this designated sample will be viewed as the quality level for the entire group of products.

A company cannot test every one of its products. There may simply be too high a volume or number of them to inspect at a reasonable cost or within a reasonable time frame. Or effective testing might result in the destruction of the product or making it unfit for sale in some way.

Acceptance sampling solves these problems by testing a representative sample of the product for defects. The process involves first, determining the size of a product lot to be tested, then the number of products to be sampled, and finally the number of defects acceptable within the sample batch.

Products are chosen at random for sampling. The procedure usually occurs at the manufacturing site the plant or factory and just before the products are to be transported. This process allows a company to measure the quality of a batch with a specified degree of statistical certainty without having to test every single unit. Based on the results how many of the predetermined number of samples pass or fail the testing the company decides whether to accept or reject the entire lot.

The statistical reliability of a sample is generally measured by a t-statistic, a type of inferential statistic used to determine if there is a significant difference between two groups that share common features.

Acceptance sampling uses statistical sampling to determine whether to accept or reject a production lot of material. It has been a common quality control technique used in industry. It is usually done as products leaves the factory, or in some cases even within the factory. Most often a producer supplies a consumer a number of items and a decision to accept or reject the items is made by determining the number of defective items in a sample from the lot. The lot is accepted if the number of defects falls below where the acceptance number or otherwise the lot is rejected.

In general, acceptance sampling is employed when one or several of the following hold:

  • Testing is destructive;
  • The cost of 100% inspection is very high; and
  • 100% inspection takes too long.

A wide variety of acceptance sampling plans are available. For example, multiple sampling plans use more than two samples to reach a conclusion. A shorter examination period and smaller sample sizes are features of this type of plan. Although the samples are taken at random, the sampling procedure is still reliable.

Acceptance sampling for attributes

A single sampling plan for attributes is a statistical method by which the lot is accepted or rejected on the basis of one sample. Suppose that we have a lot of size M; a random sample of size  N<M is selected from the lot; and an acceptance number B is determined. If it is found the number of nonconforming is less than or equal to B, the lot is accepted; and if the number of nonconforming is greater than B, the lot is not accepted. The design of a single sampling plan requires the selection of the sample size N and the acceptance number B.

MIL-STD-105 was a United States defense standard that provided procedures and tables for sampling by attributes (pass or fail characteristic). MIL-STD-105E was cancelled in 1995 but is available in related documents such as ANSI/ASQ Z1.4, “Sampling Procedures and Tables for Inspection by Attributes”. Several levels of inspection are provided and can be indexed to several AQLs. The sample size is specified and the basis for acceptance or rejection (number of defects) is provided. MIL-STD-1916 is currently the preferred method of sampling for all Department of Defense (DoD) contracts.

Control charts for Attributes and Variables Charts

Control charts are statistical tools used in quality control to monitor manufacturing and service processes. They help in identifying variations in processes and distinguishing between common causes (natural variations) and special causes (assignable variations). Control charts are broadly classified into Attribute control charts and Variable control charts based on the type of data being analyzed.

1. Attribute Control Charts

Attribute control charts are used when data can be categorized into discrete groups such as pass/fail, defective/non-defective, or good/bad. These charts help in monitoring quality characteristics that cannot be measured on a continuous scale but can be counted.

Types of Attribute Control Charts

  1. p-Chart (Proportion Defective Chart)

    • Purpose: Monitors the proportion of defective items in a sample.
    • Application: Used when sample sizes vary, and each item can be classified as defective or non-defective.
    • Example: Monitoring the percentage of defective smartphones in a production batch.
    • Formula: p = x / np

 Where:

      • = proportion of defectives
      • x = number of defective units
      • n = sample size
  1. np-Chart (Number of Defectives Chart)

    • Purpose: Tracks the number of defective items rather than the proportion.
    • Application: Used when the sample size remains constant.
    • Example: Counting the number of defective bulbs in a fixed sample of 100 bulbs per day.
    • Formula: np = n × p

Where:

      • np = number of defective items
      • n = sample size
      • p = proportion of defectives
  1. c-Chart (Count of Defects Chart)

    • Purpose: Monitors the number of defects per unit, rather than defective items.
    • Application: Used when a single unit can have multiple defects (e.g., a car with multiple scratches or dents).
    • Example: Counting the number of surface defects in a sheet of glass.
    • Formula: c = ∑(number of defects)
  2. u-Chart (Defects Per Unit Chart)

    • Purpose: Tracks the average number of defects per unit when sample sizes vary.
    • Application: Used when each sample has a different number of inspected units.
    • Example: Monitoring the number of defects per meter of fabric in textile production.
    • Formula: u = c / n

 Where:

      • u = average defects per unit
      • c = total defects found
      • = total number of inspected units

Advantages of Attribute Control Charts

  • Useful when measurement data is unavailable.
  • Easy to implement for inspection processes.
  • Provides insights into product quality trends.

Limitations of Attribute Control Charts

  • Less precise compared to variable charts.
  • Requires larger sample sizes for accurate conclusions.

Variable Control Charts

Variable control charts are used when data can be measured on a continuous scale such as weight, height, temperature, or time. These charts help in monitoring the variability and central tendency of a process.

Types of Variable Control Charts

  1. X̄-Chart (Mean Chart)

    • Purpose: Monitors the average value of a process over time.
    • Application: Used when multiple observations are taken per sample.
    • Example: Monitoring the average weight of chocolate bars in a factory.
    • Formula: Xˉ=∑X / n

 Where:

      •  = sample mean
      • X = individual measurements
      • n = sample size
  1. R-Chart (Range Chart)

    • Purpose: Measures process variability by tracking the range within a sample.
    • Application: Used alongside X̄-Charts to ensure consistent production quality.
    • Example: Monitoring variations in the thickness of metal sheets.
    • Formula: R = Xmax − Xmin
    •  Where:
      • R = range of sample
      • Xmax = largest observation
      • Xmin = smallest observation
  2. s-Chart (Standard Deviation Chart)

    • Purpose: Tracks process variability using the standard deviation of sample data.
    • Application: Used when monitoring small variations in a stable production process.
    • Example: Controlling the uniformity of tablet weights in a pharmaceutical company.
    • Formula: s = √(∑(X−Xˉ)^2 / n−1)

Where:

      • s = standard deviation
      • X = individual observations
      •  = sample mean
      • = sample size
  1. X̄-s Chart (Mean and Standard Deviation Chart)

    • Purpose: Combines X̄-Charts and s-Charts to analyze both central tendency and variability.
    • Application: Preferred when sample sizes are larger than 10.
    • Example: Ensuring precision in aerospace manufacturing processes.

Advantages of Variable Control Charts

  • Provides greater accuracy than attribute charts.
  • Helps detect both small and large variations.
  • Effective for monitoring continuous improvement.

Limitations of Variable Control Charts

  • More complex and expensive to implement.
  • Requires trained personnel for accurate interpretation.

Key Differences Between Attribute Control Charts and Variable Control Charts

Aspect Attribute Control Charts Variable Control Charts
Data Type Discrete (pass/fail, defective/non-defective) Continuous (measurement-based)
Purpose Monitors proportion, count, or rate of defects Tracks central tendency and variability
Examples p-chart, np-chart, c-chart, u-chart X̄-chart, R-chart, s-chart
Inspection Complexity Easier to implement Requires skilled personnel
Cost Lower cost Higher cost
Accuracy Less precise More precise
Best used for High-volume inspection, service industries Manufacturing, engineering, pharmaceuticals

 

Operating characteristics curves

O.C. curves quantifies manufacturer’s (producer’s) risk and consumer’s (purchaser’s) risk. This is a graph of the percentage defective in a lot versus the probability that the sampling plan will accept a lot.

An O.C. Curve drawn for sampling plan of n = 300 and C = 10 at Fig. 60.1 indicates the following:

AQL = 0.02 or 2%

Manufacturer’s risk = 0.05

Consumer’s risk = 0.10

LTPD = 0.05 or more defectives.

All practical sampling plans have an operating characteristics curve, briefly called O.C. curve.

Following points need emphasis regarding O.C. curves:

(i) There is some chance that good lots will be rejected.

(ii) There is some chance that bad lots will be rejected.

(iii) These risks can be calculated by the theory of probability and depends on the number of samples inspected, the acceptance number, and the percent defectives in the lot offered for sample inspection. Given the amount of risks which can be tolerated, a sampling plan can be devised to meet these requirements.

(iv) The larger the sample used for inspection, the nearer the O.C. curve approaches the ideal. However beyond a certain point, the added cost in inspecting a large number of parts far exceeds the benefit derived.

In any acceptance sampling plan, three parameters are specified. The first parameter is number of articles N in the lot from which sample is to be drawn. The second parameter is the number of articles n in the random sample drawn from the lot, and the third is the acceptance number C.

This acceptance number C is the maximum allowable number of defective articles in the sample. If more than C defectives are found in a sample the lot is liable to be rejected. Since the lot size has little affect on the probability of acceptance, therefore lot size is generally ignored in deriving a sampling plan.

O.C. curve of an acceptance sampling plan (i.e. for a particular combination of n and C) shows how well the sampling plan discriminates between good and bad lots. In order to exam­ine the suitability of an acceptance sam­pling plan, it is necessary to compare their performance over a range of pos­sible quality levels of the product.

The graph of this performance is known as operating characteristic curve. Fig. 60.2 below shows an ideal O.C. curve where it is desired to accept all lots having 3% or less defectives, and to reject all lots having more than 3% defectives.

In this curve, all lots with less than 3% defectives have a probability of accep­tance of 100%, while all lots with more than 3% defectives have a probability of acceptance as 0%. However, such a plan does not exist in reality.

Zones of O.C. Curve:

O.C. curve can be divided into following 3 zones:

(a) Acceptance Zone:

In this zone all the batches are accepted, therefore, the O.C. curve should be so selected that its acceptance zone accepts what is considered to be satisfactory lot.

(b) Rejection Zone:

In this zone, all the batches are rejected. Hence the O.C. curve selected should be such that it rejects what is considered to be an unsatisfactory lot.

(c) Zone of Indecision:

This is the zone where there is no purity that whether any particular batch or lot will be accepted or rejected. This problem can be solved either by adopting 100% inspection or by taking larger sample, but these will increase the inspection costs.

A batch or lot in this zone is worse than acceptable lot, and better than those what is considered as unaccept­able. Thus its quality is border-line, and practically does not matter much whether a lot is finally accepted or rejected from this zone.

Credit: https://www.businessmanagementideas.com/production-management/operating-characteristic-o-c-curves/6960

Cost Factor in Plant location

Availability of Raw Materials:

Proximity of sources of raw materials is the obvious explanation of the location of majority of sugar mills in Uttar Pradesh. This means that the raw material should be available within the economical distance. Easy availability of supplies required for maintenance and operation of the plant should also be considered.

Proximity to Markets:

Cost of distribution is an important item in the overhead expenses. So it will be advantageous to be near to the center of demand for finished products. Importance of this is fully realized if the material required for the manufacturing of products are not bulk and fright charges are small.

Transport Facilities:

Since freight charges of raw materials and finished goods enter into the cost of production, therefore transportation facilities are becoming the governing factor in economic location of the plant. Depending upon the volume of the raw materials and finished products, a suitable method of transportation like rail, road, water transportation (through river, canals or sea) and air transport is selected and accordingly plant location is decided. Important consideration should be that the cost of transportation should remain fairly small in comparison to the total cost of production.

Location in Proximity of Cities:

First tendency is to locate the industries or enterprises in the proximity of cities rather than in rural or urban areas. These sub-urban sites offer today practically all advantages, facilities and services available in cities and towns with the added advantage of land required for future expansion on cheap rates.

Planned Industrial Centres:

While industrial towns may be planned and developed by big industrial houses or govt., the late trend is to develop areas as industrial estates and sell these to people interested in starting their units at various places. Noida and Faridabad are the examples of this type of development.

Competition for Development of Industries:

In order to generate the employment opportunities the state and central govt. offer concessions to attract industrialists to set up industries in their states or territories.

Appropriate Site Selection:

Appropriate site selection is important because of the following:

(i) A good location may minimize the cost of production and distribution to a considerable extent. Such reduction in the cost of production helps in elevating either the competitive strength or the profit margin of the business.

(ii) Initiation of an enterprise involve a relatively large permanent investment. If the selected site is not proper, all the money invested on factory building, installation of machinery etc. will go waste and the owner will have to suffer a great loss.

(iii) Location put constraints for the physical factors of the overall plant designs heating, ventilation requirements, storage capacity for raw materials, transportation requirements for input material and finished products, energy requirements cost of labour, taxes and construction costs.

(iv) Location of plant decides the nature of investment cost to be incurred.

(v) Government policies sometimes play an important role in site selection.

(vi) Probably no location is so perfect as to guarantee success but locations can be so bad as to bankrupt an enterprise.

Nearness to Markets:

It reduces the cost of transportation as well as the chances of the finished products getting damaged and spoiled in the way (especially perishable products). Moreover a plant being near, to the market can catch a big share of the market and can render quick service to the customers.

Availability of Labour:

Stable labour force, of right kind, of adequate size (number), and at reasonable rates with its proper attitude towards work are a few factors which govern plant location to a major extent. The purpose of the management is to face less boycotts, strikes or lockouts and to achieve lower labour cost per unit of production.

Availability of Fuel and Power:

Because of the wide spread use of electric power, in most cases fuel (coal, oil, etc.) has not remained a deciding factor for plant location. Even then steel industries are located near source of fuel (coal) to cut down the fuel transportation costs.

It is of course essential that electric power should remain available continuously, in proper quantity and at reasonable rates.

Availability of Water:

Water is used for processing, as in paper and chemical industries, and is also required for drinking and sanitary purposes. Depending upon the nature of plant, water should be available in adequate quantity and should be of proper quality (clean and pure). A chemical industry should not be set up at a location which is famous for water shortage.

Climatic Conditions:

With the developments in the field of heating, ventilating and air- conditioning, climate of the region does not present much problem. Of course, control of climate needs money.

Financial and Other Aids:

Certain states give aids as loans, feed money, machinery, built up sheds, etc., to attract industrialists.

Land:

Topography, area, the shape of the site, cost, drainage and other facilities, the probabil­ity of floods, earthquakes (from the past history) etc., influence the selection of plant location.

Community Attitude:

Success of an industry depends very much on the attitude of the local people and whether they want work or not.

(11) Presence of related industries.

(12) Existence of hospitals, marketing centres, schools, banks, post offices, clubs, etc.

(13) Local bye-laws, taxes, building ordnances, etc.

(14) Housing facilities.

(15) Security.

(16) Facilities for expansion.

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