• Author / Uploaded
• Cristian Steven Silva

Citation preview

LEAN SYSTEMS

CHAPTER 6

231

Discussion Questions 1. Compare and contrast the following two situations: a. A company’s lean system stresses teamwork. Employees feel more involved and, therefore, productivity and quality increase at the company. The problem is that workers also experience a loss of individual autonomy. b. A humanities professor believes that all students want to learn. To encourage students to work together and learn from each other—thereby increasing the involvement, productivity, and the quality of the learning experience— the professor announces that all students in the class will receive the same grade and that it will be based on the performance of the group.

2. Which elements of lean systems would be most troublesome for manufacturers to implement? Why? 3. List the pressures that lean systems pose for supply chains, whether in the form of process failures due to inventory shortages or labor stoppages, and so forth. Reflect on how these pressures may apply to a firm that is actually implementing lean philosophy in their operations. 4. Identify a service or a manufacturing process that you are familiar with, and draw a current state value stream map to depict its existing information and material flows.

Problems The OM Explorer and POM for Windows software is available to all students using the 11th edition of this textbook. Go to http:// www.pearsonhighered.com/krajewski to download these computer packages. If you purchased MyOMLab, you also have access to Active Models software and significant help in doing the following problems. Check with your instructor on how best

to use these resources. In many cases, the instructor wants you to understand how to do the calculations by hand. At the least, the software provides a check on your calculations. When calculations are particularly complex and the goal is interpreting the results in making decisions, the software replaces entirely the manual calculations.

Strategic Characteristics of Lean Systems 1. Swenson Saws produces bow, frame, dovetail, and tenon saws used by craft furniture makers. During an 8-hour shift, a saw is produced every 6 minutes. The demand for bow, frame, and dovetail saws is about the same, but the demand for tenon saws is twice the demand for the other three. a. If mixed-model scheduling is used, how many of each saw will be produced before the cycle is repeated? b. Determine a satisfactory production sequence for one unit production. How often is this sequence repeated? c. How many of each saw does Swenson produce in one shift? 2. The Harvey Motorcycle Company produces three models: the Tiger, a sure-footed dirt bike; the LX2000, a nimble café racer; and the Golden, a large interstate tourer. This month’s master production schedule calls for the production of 54 Goldens, 42 LX2000s, and 30 Tigers per 7-hour shift. a. What average cycle time is required for the assembly line to achieve the production quota in 7 hours? b. If mixed-model scheduling is used, how many of each model will be produced before the production cycle is repeated? c. Determine a satisfactory production sequence for the ultimate in small-lot production: one unit.

d. The design of a new model, the Cheetah, includes features from the Tiger, LX2000, and Golden models. The resulting blended design has an indecisive character and is expected to attract some sales from the other models. Determine a mixed-model schedule resulting in 52 Goldens, 39 LX2000s, 26 Tigers, and 13 Cheetahs per 7-hour shift. Although the total number of motorcycles produced per day will increase only slightly, what problem might be anticipated in implementing this change from the production schedule indicated in part (b)? 3. The Farm-4-Less tractor company produces a grain combine (GC) in addition to both a large (LT) and small size tractor (SM). Its production manager desires to produce to customer demand using a mixed-model production line. The current sequence of production, which is repeated 30 times during a shift, is SM-GC-SM-LT-SM-GC-LT-SM. A new machine is produced every 2 minutes. The plant operates two 8-hour shifts. There is no downtime because the 4 hours between each shift are dedicated to maintenance and restocking raw material. Based on this information, answer the following questions. a. How long does it take the production cycle to be completed? b. How many of each type of machine does Farm-4-Less produce in a shift?

Value Stream Mapping 4. Figure 6.12 provides a new current state value stream map for the family of retainers at the Jensen Bearings, Inc., firm described in Example 6.1. This map depicts the value

stream after Kline Steel agrees to accept daily orders for steel sheets and Jensen Bearings continues to deliver the finished goods on a daily basis.

232

PART 1

MANAGING PROCESSES

FIGURE 6.12 ▶ New Current State Value Stream Map at Jensen Bearings, Inc.

4-week Forecast

180/90/60/30/day Forecasts PRODUCTION CONTROL

Kline Steel Co.

Daily Order

GNK Enterprises

Daily Order

2,500 pieces/week –1,500 “L” –1,000 “S” Tray=50 pieces 1 shift

Weekly Schedule

Daily Ship Schedule

1x/Day

PRESS

PIERCE & FORM

I Sheets 1 day

1050 “L” 1200 “S”

a. How many days of raw material does the Bearing’s plant now hold? b. How many days of work in process inventory is held between Press and Pierce & Form? c. How many days of work in process inventory is held between Pierce & Form and Finish Grind? d. How many days of work in process inventory is held between Finish Grind and Shipping? e. What is the new value steam’s production lead time? f. What is the new value stream’s processing time? 5. Anguilla Manufacturing is interested in using the data collected during Value Stream Mapping to evaluate the current state performance of the capacity of its manual assembly line process under different batch size assumptions. The availability of processing per shift, after accounting for breaks and a mid-shift employee meeting, is 436 minutes. The current operating characteristics of each processing step are found in the table below. Note that each step can only process one part at a time and all steps must process the same sized batches. Assemble

Mark

Cycle time 20 seconds 15 seconds 30 seconds 25 seconds 10 seconds per part Setup time 3 minutes per batch

4 minutes

0 minutes

3 minutes

8 minutes

Staging

500 “L” 1200 “S” C/T=35 seconds C/O=45 minutes Uptime=100% 1 Shift 25,200 sec. avail.

c 22 seconds

Calculate each component of the new value stream’s reduced lead time.

Drill

250 “L” 1500 “S”

b 3 seconds

Sand

I 1

C/T=22 seconds C/O=30 minutes Uptime=100% 1 Shift 25,200 sec. avail.

a

SHIPPING

I 1

C/T=3 seconds C/O=2 hours Uptime=100% 25,200 sec. avail. 1 Shift

Saw

FINISH GRIND

I 1

1x/Day

d 35 seconds

Production = e Lead Time Processing = f Time

a. Calculate the average processing time per unit and the capacity at each step assuming batch sizes of: i. 10 units ii. 20 units iii. 30 units iv. 40 units b. Identify the bottleneck operation and the line’s processing capacity for each batch size listed in part a. c. Explain why batch sizes beyond 40 units will not increase the line’s processing capacity further. 6. The manager at Ormonde, Inc., collected the value stream mapping data from the plant’s most problematic manufacturing cell that fabricates parts for washing machines. This data is shown in Table 6.5. Using this data, calculate the current state performance of the cell and answer the following questions. a. What is the cell’s current inventory level? b. What is the takt time for this manufacturing cell? c. What is the production lead time at each process in the manufacturing cell? d. What is the total processing time of this manufacturing cell? e. What is the capacity of this manufacturing cell?

LEAN SYSTEMS

TABLE 6.5

CHAPTER 6

233

OPERATIONS DATA FOR ORMONDE, INC. Average demand: 550/day Batch size: 20 Number of shifts per day: 3

Overall Process Attributes

Availability: 8 hours per shift with a 45-minute lunch break

Process Step 1

Cutting

Cycle time = 120 seconds Setup time = 3 minutes Up time = 100% Operators = 1 WIP = 400 units (Before Cutting)

Process Step 2

Bending

Cycle time = 100 seconds Setup time = 5 minutes Up time = 100% Operators = 1 WIP = 500 units (Before Bending)

Process Step 3

Punching

Cycle time = 140 seconds Setup time = none Up time = 100% Operators = 1 WIP = 200 units (Before Punching) WIP = 1,000 units (After Punching)

Customer Shipments

One shipment of 2,750 units each week

Information Flow

All communications with customer are electronic There is a weekly order release to Cutting All material is pushed

The Kanban System 7. A fabrication cell at Spradley’s Sprockets uses the pull method to supply gears to an assembly line. George Jitson is in charge of the assembly line, which requires 500 gears per day. Containers typically wait 0.20 day in the fabrication cell. Each container holds 20 gears, and one container requires 1.8 days in machine time. Setup times are negligible. If the policy variable for unforeseen contingencies is set at 5 percent, how many containers should Jitson authorize for the gear replenishment system? 8. You are asked to analyze the kanban system of LeWin, a French manufacturer of gaming devices. One of the workstations feeding the assembly line produces part M670N. The daily demand for M670N is 1,800 units. The average processing time per unit is 0.003 day. LeWin’s records show that the average container spends 1.05 days waiting at the feeder workstation. The container for M670N can hold 300 units. Twelve containers are authorized for the part. Recall that r is the average processing time per container, not per individual part. a. Find the value of the policy variable, a, that expresses the amount of implied safety stock in this system. b. Use the implied value of a from part (a) to determine the required reduction in waiting time if one container was removed. Assume that all other parameters remain constant. 9. An assembly line requires two components: gadjits and widjits. Gadjits are produced by center 1 and widjits by center 2. Each unit of the end item, called a jit-together, requires three

gadjits and two widjits, as shown in Figure 6.13. The daily production quota on the assembly line is 800 jit-togethers. ◀ FIGURE 6.13 Components for End Item J

J

G (3)

W (2)

The container for gadjits holds 80 units. The policy variable for center 1 is set at 0.09. The average waiting time for a container of gadjits is 0.09 day, and 0.06 day is needed to produce a container. The container for widjits holds 50 units, and the policy variable for center 2 is 0.08. The average waiting time per container of widjits is 0.14 day, and the time required to process a container is 0.20 day. a. How many containers are needed for gadjits? b. How many containers are needed for widjits? 10. Gestalt, Inc. uses a kanban system in its automobile production facility in Germany. This facility operates 8 hours per day to produce the Jitterbug, a replacement for the obsolete but immensely popular Jitney Beetle. Suppose that a certain part requires 150 seconds of processing at machine cell 33B and a container of parts average 1.6 hours of waiting time there. Management allows a 10 percent buffer for unexpected occurrences. Each container holds 30 parts, and 8 containers are authorized. How much daily demand can be satisfied