Lab 6

The Big Chill
Refrigerator Factory

Introduction

This lab deals with designing an assembly line and arranging a schedule so that a production process runs as efficiently as possible. As you address this problem, you will be expected to use the scientific method (problem, question, hypothesis, test, decision). The scientific method will support and guide your thinking process as you work on the lab.

Technology Problem

Imagine that you are an efficiency engineer who has been asked to assist in retooling an assembly line for building refrigerators. You need to design the line with maximum efficiency in mind. To do this, you will need to determine the space required for each part to be installed, the order in which parts will be installed, where bins of parts are to be placed for each station, where empty bins are to be stored, and finally, the number of fork truck operators (FTOs) that are required.

You will want to hire enough FTOs to keep the line operating smoothly, but you do not want to hire too many. Each FTO costs the company about $65,000 per year with benefits. Hiring 10 FTOs will cost the company well over a half a million dollars per year!

The first two steps of the scientific method (problem, question) suggest that you give the problem more definition and ask questions about it. You can accomplish this by getting information directly from this lab and from doing bibliotechnology research.

Figure 1 on the next page shows a layout of the plant and the existing assembly line. Due to the expense and time involved, the company does not want to change the location and/or length of the assembly line.

Your Job

You need to design the layout for the retooling of an existing assembly line and then determine the number of FTOs needed to keep the line running smoothly. You will need to consider the following components of the assembly process

• The line speed
• The number of parts bins each station requires per day
• Location of the full and empty bins for each station
• The time required to move bins to a station (using rectilinear distance)
• The number of FTOs required.

Keep in mind that part of the definition of an efficient line is a line running at minimum cost!

Bibliotechnology Research

Background

When a new assembly line is installed, or an existing one is renovated, the company's primary concerns are safety and cost. When the line is designed, the amount of space required by a particular station to install a part needs to be determined. For example, if a part takes two minutes to install and the line moves 15 feet per minute, that station would require at least 30 feet of line space. Once this is computed, the appropriate number of workers needed to work the line and provide support for the line is determined. In this problem we will focus on the location of the parts storage bins and the number of FTOs needed to keep the line supplied with the necessary parts.

The FTO uses a fork truck (a tractor-type vehicle) to deliver parts to the specific stations on the assembly line. The FTOs play important roles in keeping an assembly line moving. If a particular station runs out of parts, the line will be stopped. This can be very expensive. When the line stops, nothing is being made and all of the line workers are standing around idle. Line workers average about $25 per hour with benefits. Benefit costs include health insurance, contributions to retirement accounts, vacation pay, sick time, etc. Suppose there are 100 workers on the line. If the line is down for 15 minutes while a part is restocked, this idle time costs the company $625.

When parts are delivered to a station by an FTO, the parts are in a standard sized bin. If the part is small, a bin can hold considerably more parts than when the part is large. Consequently, some stations will need to have bins delivered more often than other stations. Not only do some stations require more bins of parts, some stations are located further from the supply area than are other stations. This means that it will take an FTO longer to deliver some bins of parts.

As you begin work on the lab, it is important to research topics that will aid you in the process. Relevant topics might include operations research, safety in manufacturing, and job scheduling. As you do the research, be sure to cite your sources and keep a record that is sufficiently detailed to allow a reader to repeat your work step by step. Create a glossary of terms that are used in assembly line design and scheduling. If you find any tables, diagrams, or charts that are useful, include them also. As you proceed through the lab, it is imperative that you organize a record of all your work in a model portfolio. At the end of the course, you will present the model portfolio to an audience and deliver a defense of its embedded thesis.

The following are web sites that you may find useful:

• http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/routing.htm
• http://www.geocities.com/CapitolHill/lobby/5544/vrp.html
• Operations research resource
  http://wpweb2k.gsia.cmu.edu
• Operations research /management science
  http://www.informs.org
• Free downloadable job scheduling software
  http://www.espSoftware.com
• Regulations for occupational safety
  http://www.osha.gov

Mathematics Tools

As you work on this lab you will need to select and use mathematics tools that help you to develop a model of the technology problem. Mathematics topics and habits of thought that support the process of designing an assembly line and scheduling events will be most useful. The mathematics tools for solving the technology problem include: methods for constructing scale drawings; the time-rate-distance formula (d = rt); addition, subtraction, multiplication, and division; the distance formula, and methods for creating timing diagrams.

By observing the problem and question steps of the scientific method, you have established a framework or context for investigating the technology problem. The next step of the scientific method suggests developing a hypothesis about the real-world situation being considered. In this context you can think of the assembly line that you propose as a model or hypothesis. The model constitutes a "statement" about the assembly line. It should describe, explain, and yield a solution of the technology problem.

Model Development

Part A
Line Speed and Spacing

The first step in setting up the assembly line is to determine how much space is required for each station in the assembly process. This depends on how fast the line is moving and how long a particular process takes to perform. There are two separate processes in assembling refrigerators. One is assembling the left door with the ice/water dispenser. The other process is assembling the various parts to actually build the refrigerator. The time to perform each step, as well as the order in which the steps are performed, are given in Table 1 on the next page. The times in the column labeled "Left Door" are the times required by each station to assemble the ice/water dispenser door. The times in the column labeled "Refrigerator" are the times to assemble, test and package the refrigerator.

In any installation area there are several people involved in the process. No worker needs to walk more than 10 feet to return to the start of his/her own area. Consequently, no large amounts of time are ever required for a worker to return to the start of his/her installation area. As a result, no additional time needs to be added to the times in Table 1.

Due to safety issues, however, additional line space (buffers) must be added between stations. In this plant, allow 10 feet between stations in the door assembly line and 20 feet between stations in the refrigerator assembly line.

Using a rate of 6 feet per minute for the line and the information about space required between stations, complete Table 2 and Table 3.

Now that the distances are determined, make a scale drawing of the plant layout shown in Figure 1, several pages back. Indicate the length of assembly line required for each workstation on your drawing. Be sure to label each station clearly.

Part B
Number of Bins Required

The company assembles refrigerators during the 7 – 4 shift. The line actually runs until 12:00 and then has a half-hour lunch break and continues until 4. During the time the line is running, a refrigerator is completed every minute. This means a refrigerator enters each station every minute. Determine how many refrigerators are assembled during a shift.

Knowing the number of refrigerators made per day, you can now determine how many of each type of refrigerator part is required in a day. Remember the main part of each refrigerator requires 2 sides and 4 refrigerator shelves. Fill in the values in the "number of parts per day" column in Table 4 and Table 5.

Now that we know how many parts are required at each station per day, we can determine the number of bins that need to be delivered during the shift. Parts are delivered in bins that are 6 feet by 4 feet by 3 feet. Due to the various sizes of parts, the quantity of a specific part that fits in a bin will also vary. Use the "number of parts per bin" column to determine how many bins per day must be delivered to each station. Once this is determined, compute how often (frequency) a bin needs to be delivered to each station. Record these values in Table 4 and Table 5.

Part C
Complete Cycle of Work

As stated previously, fork trucks are used to make deliveries of parts in the plant. At the beginning of the day each truck must be driven from the fork truck parking area (FTP) to the appropriate full storage bin location (FSL). From that point, the truck goes to the designated assembly line (DAL) which is the station that requires the part. The full bin is exchanged with the empty bin at the station. The empty bin is then taken to the area where empty bins are stored, the empty storage location (ESL), to be refilled at a later point. The fork truck is then driven back to the FSL to start the next delivery. This process is repeated throughout the day as stations need additional parts. This loop from the FSL to the DAL to the ESL back to the FSL is called a complete cycle (CC). At the end of the day, each fork truck must be returned to the parking area from the ESL. This whole process is shown in Figure 2.

Determining the shortest path each FTO travels for deliveries will help minimize plant costs. On your scale drawing decide what sections of the bin storage will be used for full bins and empty bins. You will also need to decide where a particular type of part (e.g. compressors) will be stored. Lastly, you will need to measure the rectilinear distances in feet from the FSL to the DAL to the ESL back to the FSL for each delivery.

On the scale drawing of the plant, indicate the aisles to be used by the FTOs. A two-way aisle needs to be 20 feet wide. The workstations must include a distance of at least four feet back from the line for the workers to work and carry parts. An additional four feet must also be included for the parts bin(s). (Figure 3)

Clearly mark the shortest distance (in feet) from full storage bins (FSL) to the designated assembly line (DAL), then on to the empty storage bin (ESL) and back to the full storage bin (FSL). Measure the distance traveled for each leg of a complete cycle of work and record the distance in Table 6 and Table 7. (Measure all distances from the center of the bin area in which the part is stored to the delivery location at the beginning of the workstation.) At the beginning of the day the fork truck must be driven from the FTP area to the FSL before going to the specified station. At the end of the day each fork truck will need to be returned from the ESL area to the FTP. List these distances in Table 6 and Table 7.

The fork trucks travel an average of 5 mph. This includes lifting the boxes, reversing the direction, and heading out from the storage area or assembly line location. You will need to convert this speed to feet per minute.

Using the distances you listed in Table 6 and Table 7, and the average fork truck speed, you can compute the time required for an FTO to travel each part of a complete cycle. In Tables 8 and Table 9 record the time required in minutes for each phase of a complete cycle.

Part D
Scheduling Fork Lift Operators

It is now time to determine the number of operators required to staff the facility. You will then need to work out an assignment for each operator. That is, assign each operator specific stations that are his/her primary responsibility. FTOs need to be available at the end of the door assembly line to take bins containing left doors to the FSL to be retrieved later for use on the main assembly line. FTOs also need to be available at the end of the main line to remove complete refrigerators and defective refrigerators. Remember that a refrigerator needs to be removed every minute. Complete and defective refrigerators are taken to separate areas of the refrigerator storage area (which you must designate). On average, there are 3 defective refrigerators per shift (differentiate by a large, red tag).

When making assignments you must keep several things in mind.

• Each operator is available for 480 minutes per shift.
• Assignments for a particular operator should be within the same section of the plant.
• In some cases an operator may have a split assignment. An operator may have 440 minutes of work scheduled. In the remaining 40 minutes, this operator is free to do some other task – perhaps to deliver 10 bins of compressors. This will leave the remaining six bins for another operator to deliver.
• Be sure to include a 15-minute break in the morning and afternoon. This should be scheduled only when the stations have adequate inventory to continue production, or if another operator can temporarily handle the deliveries.
• Block out the 30-minute lunch break.
• Determine what deliveries must be made before the line starts at the beginning of the day. It may be that FTOs need to deliver to several of the first stations immediately.

Create a timing diagram indicating the tasks each operator must perform. Be sure to label each task (What bin of parts is being delivered? Is the FTO delivering a complete refrigerator? Is the FTO returning to the parking area?, etc.). You should consider the research you have done on job/task scheduling to aid you in the decision-making process.

It is now time to apply the fourth step of the scientific method – to test the model. In the context of this technology problem, your test will involve determining whether your model satisfies all the conditions of the problem and especially the criterion of efficiency. On the basis of your testing, you will decide, in accordance with the fifth step of the scientific method, to retain, reject, or modify the model.

As part of your solution of the technology problem, you will need to create a well-organized model portfolio that documents all of your work on the lab.

Model Portfolio

The model portfolio must include a scaled floor layout with details of the assembly line, storage bin locations, completed refrigerator locations (complete and defective), and the fork truck routes. Include detailed explanations that justify the layouts (include calculations and any formulas that were used). The model portfolio must include the timing diagram for the FTOs' schedules as well as a detailed explanation of how the diagram was constructed and verified for feasibility. You must also include relevant information you discovered in your research concerning assembly lines, job scheduling, and/or safety standards for manufacturing facilities. The model portfolio should be sufficiently detailed to allow the reader to follow your work and thought processes step by step.

Thesis Defense

You will be asked to give an oral presentation of your model portfolio. This is called the thesis defense. It may help to imagine that you, an efficiency engineer, are presenting the results of your research – a proposed model of an assembly line – to an audience of refrigerator company decision-makers.