Critical Path Method, MRP, PERT, OPT, And Lean Manufacturing

Introduction to Critical Path Method

 The critical and non-critical tasks in a project consisting of an array of activities that interact in a complex manner can be determined by use of the critical path method. CPM was first put to action in 1957 and has since grown to become an important tool for overseeing large undertakings, such as major construction projects. It enhances efficiency through dividing activities that are complicated into a series of smaller tasks with the resources and costs associated with each task. This enables manufacturing engineers to see areas that can experience potential breakdowns as a result of time constraints.

Projects with multiple tasks can easily be handled efficiently by the respective project managers through the creation of human networks and building of teams through the use of CPM. It also binds the entire team together, helps projects be completed efficiently, and accurate project duration estimation and costing

The use of CPM in big projects can prove to be complicated. In addition to this, the critical path is not always clear, and its population can be time consuming. The CPM is also limited in that it cannot create and control schedules of people working in a project.

MRP is a system for managing the resources in the manufacturing processes of a company. Initially, MRP was created in 1964 to support the Polaris program as a response to the Toyota Manufacturing Program. Black & Decker was its first user and had been implemented in 700 firms by 1975. It was developed to MRP II in 1983 by Oliver Wight to bring together rough cut capacity planning, master scheduling, and planning for capacity requirements. The MRP II software formed a third of all software sold in the US software sector by 1989.

Some of the strengths of MRP are that the integration, centralization and processing of information through the system leads to design engineering, management of inventory, scheduling and cost control. MRP also ensures materials needed for production are availed on time, results in little or no excess inventory, leads to timely deliveries of finished products, and uses manufacturing resources optimally.

Requires precise data to be entered and clean records to be kept, it also just provides a framework that has be used effectively, and if data is not accurate, a lot of problems can be experienced. 

PERT which is normally used in conjunction with CPM in project management is a statistical tool used for analysis. It is a scheduling tool used commonly in project management, Was first used in the late 1950s in the US Navy in the Polaris missile project and uses a system of numbering events in sequences of tens or hundreds.

1. Strengths of PERT:

Applications of Critical Path Method

Allows for analyses of activities, improves coordination in departments by improving planning and decision making, and enables analysis of what-if situations

1. Weaknesses of PERT:

It is a subjective analysis tool that requires new project activities to be identified and then arranged in sequence. It is basically a method focused on time and is resource intensive.

First, it requires activities to be identified, the activities sequence determined, and a network of the activities created. The completion time for all identified activities are then entered and the longest possible path (the critical path) to complete all the activities is then identified. The progress of the CPM should be updated regularly.

This is a proprietary computer software system of scheduling. OPT starts with performance measurement, planning projects, and identifying the software and hardware requirements. This is followed by plant analysis where manufacturing processes and their management are analyzed. Bottlenecks are then analyzed and computer modeling of the system done, followed by definition of data (data to be fed into system). Th outputs are then defined to create the MPS (master production schedule): this is achieved by planning for constraint capacity. The OPT is then operated through the OPT software to control complex manufacturing processes.

In this strategy, capacity is added in expectation of demand. This is usually done by medium to large companies and is associated with large investments. This is a high capital intensive strategy

This a capacity strategy where capacity is only added after the materialization of demand. This entails adding capacity only when existing capacity is exhausted or exceeded and is a more conservative strategy used by small to medium organizations. The investment in this case is small to medium

Match capacity strategy: A strategy for capacity in which a balance between lag and lead capacity strategies is struck by avoiding periods of high over- or under- utilization. This is a moderate strategy with very low to low-medium investment implications and is used, or is suitable for very small organizations to medium firms, or even startups.

The carbon-fiber aluminum laminate used in the manufacturing of aircrafts was initially utilized in secondary components of aircrafts. Their desirable properties of strength and weight has made their usage more prominent in aircrafts since they are stronger than steel and lighter than aluminum. The composites are increasingly being used in primary structures such as wings and fuselages. The laminates can also be formed into any shape making the actualization of computational fluid dynamics in the modeling of aircrafts possible.

Introduction to MRP

The carbon fiber manufacturing process goes through various stages as outlined below;

Polymerization – a precursor which provides the backbone of the fiber is chemically reacted to form polymer chains or three dimensional networks. The quality of the finished product is dependent on the type of precursor used in the polymerization process.

Spinning – this is the process through which pan fibers are formed. A liquid coagulation bath is used to wet spin the polymers. The end result is the formation of a PAN precursor fiber. A finishing oil is applied the clumping of tacky filaments. The PAN fibers are dried and wound into bobbins.

Oxidation – specialized ovens are used to achieve the cross-linking of polymer chains. The temperature ranges from 200? – 300 degrees Celsius. The temperatures in the oxidation oven are controlled for each precursor since it provides stabilization of the reaction which is exothermic in nature.

Carbonization – this process is conducted in furnaces that are oxygen-free. The oxygen molecules that are in the process remove a portion of the fiber, thus the need to expel them from the purge chambers. The purge chambers are located at the entrance of furnaces to prevent oxygen intrusion.

The carbon-fiber aluminum laminate is thus produced through the following processes;

1. The treating of the metallic surface in order to increase the bonding between the metallic layer and the carbon fiber.
2. Depositing of material

• Preparation of the cure

1. Curing of the material
2. The post stretching in order to reserve residual stress 

1. Laminate composite material

The laminated composite (carbon-fiber aluminum) is produced as follows:

	Activity	Time in seconds
Laminate Production	Metallic surface treatment	60
	Depositing of material	300
	Preparation of the cure	120
	Curing of the material	150
	Post stretching	300
Trimming and Cutting	Surface treatment and Sizing	60

Laminate composite production takes 990 seconds, forming of the 48″ x 96″ or 48″ x 120″ plates.

1. Drilling takes 90 seconds. Assembly and inspection take 120 seconds. Total time (CPM) is 3600 seconds (20 minutes) for the manufacture of a 48″ x 96″ or 48″ x 120″ carbon-fiber aluminum plate. At 12 hours per day for 6 days, the capacity using a single line is 216 plates as shown in the image below. 

• Process plan

Process Plan For carbon-fiber aluminum laminate Production
Part Name			Changes
Materials			Approval
				Date
No.	Operation	Workstation	Setup	Tool	Time(s)
1	Surface treatment	Treatment Section	Pour-in	Treatment Unit	60
2	Material Deposition	Deposition Section	Place-in	Deposition Unit	300
3	Cure Preparation	Preparation Section	Place-in	Mixing Unit	120
4	Material Curing	Curing Section	Place-in	Curing Unit	150
5	Post stretching	Stretching Section	Place-in	Stretching Unit	300
6	Trimming/Cutting	Trimming Section	Place-in	Cutter	60
7	Drilling	Drilling Section	Set in place	Drill	90
8	Inspection	Air-drying Section	Assembly	Visual	120

On a monthly basis, the production plant can manufacture 864 units per month, which is 216 per week and 36 per day based on a 12 hour work day.

• With increased efficiency and implementation of a 14 hour day schedule, the unit can manufacture 250 units a week. This totals to 1000 units per month when utilizing a single production line. The number to be manufactured per month is limited in number by the capacity of the plant and the activities in the production. Surface treatment takes at least one minute, material deposition takes at least five minutes. The critical part of the manufacturing is the post stretching which slows the process as it takes at least five minutes to complete. 

• Activity nodes 

• Floor layout is shown below 

• Power Point
• Principles of Lean Manufacture

Lean manufacture is a systematic production method aimed specifically at minimizing waste in a manufacturing system while maintaining productivity at high levels. Lean takes cognizance of wastes created via uneven workload and those created through over burdening. Lean manufacturing seeks to exalt what adds value while reducing or eliminating what does not add value. Lean is based on various principles that include identifying value, mapping the value stream, creating flow, establish pull, and seeking perfection: these are discussed in the following sections

The needs of the customer for a specific product define its value and so it is imperative for each company to determine what value customers place on products and services. Value dictates the amount the customer is willing to pay and the result is a top down approach to target costing. Target costing places focus on what the customer can willingly pay for certain features, products, and services and based on these, the product costs are determined. The business is then tasked with reducing costs and eliminating wastes to meet the customer price while also profiting greatly.

Strengths of MRP

This refers to the entire flow of the life cycle of a product from the raw materials origin to the cost to the customer in using the product and its ultimate disposal of the product. Critically studying this stream and identifying its wastes and value-add will help the company fully understand wastes associated with production and delivery of the product. Based on this, there should be a stronger partnership between customer and manufacturer in the entire stream.

After all wastes have been eliminated from the value stream, it is imperative to ensure the remaining steps during production flow smoothly bereft of any interruptions, bottlenecks, or delays. The value chain must keep flowing forward, and this is a critical point in lean manufacturing so the product and its attendant steps (raw materials, parts, sub-assemblies) never stop during the production process.  There must be full synchronization in each and every aspect of the production and delivery with all other elements. Carefully designing the flow across the value chain has a tendency to increase value and minimize waste.  

The pull approach ensures that there is no build up in the work in process inventory when things are made ahead of time. The traditional approach tends to use a process such as ERP where production is pushed through the manufacturing process based on a schedule and forecast. The pull approach requires nothing to be made until it is ordered by the customer. This requires a high level of flexibility and cycle times that re very short to achieve. Each step within the value chain is informed of requirements every single day based on the needs of the customer.

The aim of Lean manufacturing is perfection through total quality management that is achieved through the systematic and continuous removal of the root causes for poor quality from the processes of production. This is so that there is a continuous move towards perfection in the plant and the products being made in it. An organization aiming for lean manufacturing develops the attitude of relentless pursuit of perfection.

References 

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