POM Lecture (44)

Unit 3

Scheduling Operations Chapter 13: Scheduling

Lesson 43- OPTIMIZED PRODUCTION TECHNOLOGY And Synchronous

Manufacturing

Learning Objectives

After reading this lesson you will be able to understand

Optimized production technology

Software for OPT

Computerized workforce scheduling systems

Synchronous manufacturing

Dear students, this third lecture focuses solely on OPT, the related software and

other such practical issues.

OPTIMIZED PRODUCTION TECHNOLOGY

There is another approach to the planning and scheduling so far presented is Optimized

Production Technology. This is a computer based system for planning production,

material needs, and recourse utilization. It was first introduced in the USA in 1979 by

Creative Output Inc, a Consulting Form in Milferd, Connecticut.

The key feature of OPT is its emphasis on bottleneck center works- people or machines..

The OPT philosophy is that managing bottlenecks is the key to successful performance:

total system output can be maximized and the in-process inventories reduced.

The OPT software consists of four modules.

1. BUILDNET

2. SERVE

3. SPLIT

4. OPT

The above mentioned modules creates a model of the shop according to the data provided

by the user, how each product is made, its build up sequence, materials, and routing

through the shop), the products time requirements ( setup, run time, schedule delay ), the

capacity availability at each resource ( work center, machine , worker), and the order

quantities.

The initial purpose of SERVE is to create a tentative schedule for the jobs waiting in the

shop. Later it creates a more refined schedule. The crucial information obtained in the

SERVE is an estimate of the percentage utilization of the various shop resources.

The SPLIT module distinguishes critical from non critical resources based on their

percent utilizations calculated by SERVE. Resources that are near or above 100 percent

utilization are the bottleneck operations. These bottlenecks, and the operations that follow

them, are the "critical" operations; all others (those with lower percentage utilizations) are

"non critical."

The OPT module reschedules the critical part of the network using forward

scheduling. Then the program cycles back to SERVE to reschedule the non critical

resources.

The OPT package consists not only of software but of consulting services and

training for implementation as well. The specific details of the procedure, especially of

SERVE and OPT (the detailed scheduling modules), are proprietary (not published and

available to the general public). Consequently, detailed comparative evaluations of its

performance with that of other systems are not available.

The Behavioral elements in intermittent systems are of immense significance. You can’t simply

ignore it. Please pay attention.

Behavioral elements in intermittent systems The next few narrations are about the various behavior aspects of intermittent systems

which relate to the sheer technical complexities of having many, perhaps thousands, of

jobs flowing through many, perhaps hundreds, of work centers. Not only must all these

jobs be processed, but customer deliveries must be on time, and the entire operation must

be smooth and efficient.

A single human being is incapable of accomplishing all this. Our limited mental

capacities prohibit total awareness of current job status and how that status changes over

time. For these reasons, the tools presented in this chapter have great value to managers

of intermittent systems. Gantt load charts and scheduling charts, although simplistic in

concept and appearance, serve as memory supplements. For decision making, priority

sequencing rules playa similar role. By systematically applying priority rules, we get the

simplified process we need.

Although the rules do not ensure optimal system performance, they do help achieve

satisfactory performance, and they are usually better than alternative approaches,

including human intuition.

As a logical extension of the above concept, we now take up for consideration:-

SYSTEM ORGANIZATION AND ROLE RELATIONSHIPS Another behavioral factor is the intra relationships of intermittent system employees,

groups, and work centers or departments. All must be integrated in an effective system.

Behavioral consideration specific to intermittent systems include those related to

individual and group characteristics.

Individual Characteristics You may remember that intermittent systems, compared with

continuous flow systems, comprise a variety of tasks. Different types of employee skills

are necessary. Generally, jobs in intermittent systems are already "enlarged": tasks vary,

and a higher degree of employee responsibility is emphasized in executing the tasks. In

hiring, managers seek employees who are highly skilled and who can work independently

without a great deal of supervision. Through monetary rewards, facilitating group

relationships, and allocating work methodically, management can create a working

environment that helps employees feel secure and fulfills their social needs. Doing so

increases motivation in job performance.

Group Characteristics In discussing facility layout, we pointed out that intermittent

systems comprise work centers sharing common processes. A facility might have several

work centers, each using workers skilled in machining, painting, and photography, for

example. Group affiliatioJ1s are often established among commonly skilled workers,

machinists, for example. There are three reasons for these group affiliations: command

structure, physical proximity, and shared craft interest. As a formal basis for group

affiliation among machinists, for example, the organizational structure may specify that

all machinists report to a machining foreman. Second, the physical proximity of

machinists in the facility, since they usually work near one another, tends to facilitate

interaction and communication, both work-related and personal. Since this is likely to

occur on a regular basis, strong group bonds may form. Finally, an important shared

interest-the craft or skill-is a basis for interaction. Unions facilitate this last affiliation; it

is likely that in a unionized facility of any size, more than one union will represent

differing groups of employees.

A work group significantly affects the operations of the system. Although a group

usually adopts a set of group norms and strives to satisfy member needs, the group norms

mayor may not be consistent with management goals. Group norms can strongly

influence its members' productivity, especially in highly cohesive groups. When the

norms of these cohesive groups are consistent with management goals, the groups tend to

produce at higher levels.

Centralized versus Decentralized Decision Making

A decentralized scheduling system provides an important dimension of managerial

discretion for the first-line supervisor: The supervisor decides which employees will work

on which job orders. This prerogative does not exist in more centralized systems. In an

environment in which wages are hourly and fixed, the decentralized system might be one

of the few devices directly available to the supervisor for rewarding and motivating

employees. In a more centralized system, job assignments are often depersonalized,

handed out by the production control center_ Gains in interdepartmental coordination can

be offset by losses in employee satisfaction and/or productivity.

In many companies, bargaining between subordinates and foremen for job

assignments is a traditionally accepted interpersonal process. Without it, the prestige of

the supervisor and the experienced worker may both diminish. Unless other adjustments

are made, the diminution can lead to frustration and defensive behavior on the part of

supervisor and the subordinates alike, followed by decreased productivity and quality.

Friends, have a little patience.

We are nearing the end of our current discussion.

Before that, a couple of topics must be considered-----

SCHEDULING IN SERVICES

So far we have seen the scheduling systems in manufacturing. Now let us see how the

scheduling system works in service area.

The manufacturing scheduling process is a key element of an integrated supply chain.

APS systems attempt to link to the scheduling process demand data and forecasts, supply-

chain facility and inventory decisions, and the capability of suppliers so that the entire

supply chain can operate as efficiently as possible. The ability to change schedules

quickly while recognizing the implications on the rest of the supply chain can provide a

competitive edge.

One important distinction between manufacturing and services that affects scheduling is

that service operations cannot create inventories to buffer demand uncertainties. A second

distinction is- that in service operations demand often is less predictable. Customers may

decide on the spur of the moment that they need a hamburger, a haircut, or a plumbing

repair. Thus capacity, often in the form of employees, is crucial for service providers. In

this section, we discuss various ways in which scheduling systems can facilitate the

capacity management of service providers.

SCHEDULING CUSTOMER DEMAND One way to manage capacity is to schedule customers for arrival times and definite

periods of service time. With this approach, capacity remains fixed and demand is leveled

to provide timely service and utilize capacity. Three methods are commonly used:

appointments, reservations, and backlogs. .

APPOINTMENTS. An appointment system assigns specific times for service to

customers. The advantages of this method are timely customer service and high

utilization of servers. Doctors, dentists, lawyers, and automobile repair shops are

examples of service providers that use appointment systems. Doctors can use the system

to schedule parts of their day to visit hospital patients, and lawyers can set aside time to

prepare cases. If timely service is to be provided, however, care must be taken to tailor

the length of appointments to individual customer needs rather than merely scheduling

customers at equal time intervals. .

RESERVATIONS. Reservation systems, although quite similar to appointment systems,

are used when the customer actually occupies or uses facilities associated with the

service. For example, customers reserve hotel rooms, automobiles, airline seats, and

concert seats. The major advantage of reservation systems is the lead time they give

service managers to plan the efficient use of facilities. Often, reservations require some

form of down payment to reduce the problem of no-shows.

BACKLOGS. A less precise way to schedule customers is to allow backlogs to develop;

that is, customers never know exactly when service will commence. They present their

service request to an order taker, who adds it to the waiting line of orders already in the

system. TV repair shops, restaurants, banks, grocery stores, and barber shops are

examples of the many types of businesses that use this system. Various priority rules can

be used to determine which order to process next. The usual rule is first come, first

served, but if the order involves rework on a previous order, it may get a higher priority.

Finally, we must tackle the issue of:-

SCHEDULING THE WORKFORCE

This is a important area where the workforce is involved in giving services and therefore,

managing the scheduling system like their on & off periods etc over a period of time for

the workforce becomes important. Similarly assigning postal clerks, nurses, pilots,

attendants, or police officers to specific workdays and shifts. This approach is used when

customers demand quick response and total demand can be forecasted with reasonable

accuracy. In these instances, capacity is adjusted to meet the expected loads on the

service system.

Recall that workforce schedules translate the staffing plan into specific schedules of

work for each employee (see the Aggregate Planning chapter). Determining the workdays

for each employee in itself does not make the staffing plan operational. Daily workforce

requirements, stated in aggregate terms in the staffing plan, must be satisfied. The

workforce capacity available each day must meet or exceed daily workforce

requirements. If it does not, the scheduler must try to rearrange days off until the

requirements are met. If no such schedule can be found, management might have to

change the staffing plan and authorize more employees, overtime hours, or larger

backlogs.

CONSTRAINTS. The technical constraints imposed on the workforce schedule are the

resources provided by the staffing plan and the requirements placed on the operating

system. However, other constraints, including legal and behavioral considerations, also

can be imposed. For example, Air New Zealand is required to have at least a minimum

number of flight attendants on duty at all times. Similarly, a minimum number of fire and

safety personnel must be on duty at a fire station at all times. Such constraints limit

management’s flexibility in developing workforce schedules.

The constraints imposed by the psychological needs of workers complicate scheduling

even more. Some of these constraints are written into labor agreements. For example, an

employer may agree to give employees a certain. number of consecutive days off per

week or to limit employees' consecutive workdays to a certain maximum. Other

provisions might govern the allocation of vacation, days off for holidays, or rotating shift

assignments. In addition, preferences of the employees themselves need to be considered.

One way that managers deal with certain undesirable aspects of scheduling is to use a

rotating schedule, which rotates employees through a series of workdays or hours. Thus,

over a period of time, each person has the same opportunity to have weekends and

holidays off and to work days, as well as evenings and nights. A rotating schedule gives

each employee the next employee's schedule the following week. In contrast, a fixed

schedule calls for each employee to work the same days and hours each week.

Students, this can not be effectively accomplished without:-

DEVELOPING A WORKFORCE SCHEDULE. Suppose that we are interested in developing

an employee schedule for a company that operates seven days a week arid provides each

employee two consecutive days off. In this section, we demonstrate a method that rec-

ognizes this constraint.1 The objective is to identify the two consecutive days off for each

employee that will minimize the amount of total slack capacity. The work schedule for

each employee, then, is the five days that remain after the two days off have been

determined. The procedure involves the following steps.

Step 1. From the schedule of net requirements for the week, find all the pairs of con-

secutive days that exclude the maximum daily requirements. Select the unique pair that

has the lowest total requirements for the two days. In some unusual situations, all

pairs may contain a day with the maximum requirements. If so, select the pair with the

lowest total requirements. Suppose that the numbers of employees required are

Monday: 8 Friday: 7

Tuesday: 9 Saturday: 4

Wednesday

: 2 Sunday: 2

Thursday: 12

The maximum capacity requirement is 12 employees, on Thursday. The pair having the

lowest total requirements is Saturday-Sunday, with 4 + 2 = 6.

Step 2. If a tie occurs, choose one of the tied pairs, consistent with provisions written into

the labor agreement, if any. Alternatively, the tie could be broken by asking the employee

being scheduled to make the choice. As a last resort, the tie could be broken arbitrarily.

For example, preference could be given to Saturday-Sunday pairs.

Step 3. Assign the employee the selected pair of days off. Subtract the requirements

satisfied by the employee from the net requirements for each day the employee is to

work. In this case, the employee is assigned Saturday and Sunday off. After requirements

are subtracted, Monday's requirement is 7, Tuesday's is 8, Wednesday's is 1, Thursday's

is 11, and Friday's is 6. Saturday's and Sunday's requirements do not change because no

employee is yet scheduled to work those days.

Step 4. Repeat steps 1-3 until all requirements have been satisfied or a certain number of

employees have been scheduled.

This method reduces the amount of slack capacity assigned to days having low

requirements and forces the days having high requirements to be scheduled first. It also

recognizes some of the behavioral and contractual aspects of workforce scheduling in the

tie-breaking rules. However, the schedules produced might not minimize total slack

capacity. Different rules for finding the days-off pair and breaking ties are needed to

ensure minimal total slack capacity.

Now let us try to understand the above concepts by an example

Developing Workforce Schedule

The Amalgamated Parcel Service is open seven days a week. The schedule of

requirements is

Day M T W Th F S Su

No of Employees 6 4 8 9 10* 3 2

The manager needs a workforce schedule that provides two consecutive days off and

minimizes the amount of total slack capacity. To break ties in the selection of off days,

the scheduler gives preference to Saturday-Sunday if it is one of the tied pairs. If not, she

selects one of the tied pairs arbitrarily.

Well my dears, the same rules apply.

Solve first.

Tally later.

SOLUTION Friday contains the maximum requirements (designated by an *), and the pair S-Su has

the lowest total requirements. Therefore, employee 1 is scheduled to work Monday-

Friday. The revised set of requirements, after scheduling employee 1, is

Day M T W Th F S Su

Number of employees 5 3 7 8 9* 3 2

TABLE 13.2 Scheduling Days Off

M T W Th F S SuEMPLOY

EE COMMENTS

4 2 6 7 8* 3 2 3

S-Su has the lowest total requirements. Reduce the requirements to reflect a M-F schedule for employee 3.

3 1 5 6 7* 3 2 4

M- T has the lowest total requirements. Assign employee 4 to a W-Su schedule and update the requirements.

3 1 4 5 6* 2 1 5

S-Su has the lowest total requirements. Assign employee 5 to a M-F schedule and update the requirements.

2 0 3 4 S* 2 1 6

M-T has the lowest total requirements. Assign employee 6 to a W-Su schedule and update the requirements.

2 0 2 3 4* 1 0 7

S-Su has the lowest total requirements. Assign employee 7 to a M-F schedule and update the requirements.

1 0 1 2 3* 1 0 8

Three pairs have the minimum requirement and the lowest total: S-Su, M-T, and T-W. Choose S-Su according to the tie-breaking rule. Assign employee 8 a

M-F schedule and update the requirements.

0 0 0 1 2* 1 0 9

Arbitrarily choose Su-M to break ties because S-Su

does not have the lowest total requirements. Assign employee 9 to a T-S schedule.

0 0 0 0 1* 0 0 10

Choose S-Su according to the tie-breaking rule. Assign employee 10 a M-F schedule.

Note that Friday still has the maximum requirements and that the requirements for S-Su

are carried forward because these are employee l's days off. These updated requirements

are the ones the scheduler use, tcr the next employee.

The unique minimum again is on S-Su, so the scheduler assigns employee 2 to a M-F

schedule. She then reduces the requirements for M-F to reflect the assignment of

employee 2.

The day-off assignments for the remaining employees are shown in Table 13.3. In this

example Friday has always has the maximum requirements and should be avoided as a

day off. The schedule for the employees is shown in Table 13.3.

TABLE 13.3 Final Schedule

Employee M T W Th F S Su TOTAL

1 X X X X X off off 2 X X X X X off off 3 X X X X X off off 4 off off X X X X X 5 X X X X X off off 6 off off X X X X X 7 X X X X X off off 8 X X X X X off off

9 off X X X X X off

10 X X X X X off off

Capacity, C 7 7 10 10 10 3 2 50

Requirement

s, R 6 4 8 9 10 3 2 42

Slack C - R 1 3 2 1 0 0 1 8

Decision Point With its substantial amount of slack capacity, the schedule is not unique.

Employee 9, for example, could have Su-M, M-T, or T-W off without causing a capacity

shortage. Indeed, the company might be able to get by with one fewer employee because

of the total of eight slack days of capacity. However, all 10 employees are needed on

Fridays. If the manager were willing to get by with only 9 employees on Fridays or if

someone could work one day of overtime on a rotating basis, he would not need

employee 10. As indicated in the table, the net requirement left for employee 10 to satisfy

amounts to only one day, Friday. Thus, employee 10 can be used to fill in for vacationing

or sick employees.

Can anybody out there tell me something about the:-

COMPUTERIZED WORKFORCE SCHEDULING SYSTEMS Workforce scheduling often has many constraints and concerns. In some types of firms,

such as telephone companies, mail-order catalog houses, or emergency hotline agencies,

employees must be on duty 24 hours a day, seven days a week. Sometimes a portion of

the staff is part-time, allowing management a great deal of flexibility in developing

schedules but adding considerable complexity to the requirements. The flexibility comes

from the opportunity to match anticipated loads closely by using overlapping shifts or

odd shift lengths; the complexity comes from having to evaluate the numerous possible

alternatives. Management also must consider the timing of lunch breaks and rest periods,

the number and starting times of shift schedules, and the days off for each employee. An

additional typical concern is that the number of employees on duty at any particular time

be sufficient to answer calls within a reasonable amount of time.

Computerized scheduling systems are available to cope with the complexity of

workforce scheduling. For example, L. L. Bean's telephone service center must be staffed

with telephone operators 7 days a week, 24 hours a day. The company uses 350

permanent and temporary employees. The permanent workers are guaranteed a minimum

weekly workload apportioned over a 7 week on a rotating schedule. The temporary staff

works a variety of schedules ranging from a full six days week to a guaranteed weekly

minimum of 20 hours. The company uses a computer program to forecast the hourly load

for the telephone work center, translate the work load into capacity requirements, and

then generate week long staffing schedules for the permanent and temporary telephone

operators to meet these demand requirements. The program selects the schedule that

minimizes the sum of expected costs of over and understaffing.

SYNCHRONOUS MANUFACTURING Optimized Production Technology (OPT) was evolved through software. This software was developed by Creative Output Inc USA and the person responsible for it is Dr. Eli Goldratt. Here the scheduling logic is based on the separation of “bottleneck” and non-bottleneck operations. Further Dr Goldratt developed the “Theory of Constraints” (TOC), which has become popular as a problem solving approach that can be applied to many business areas. So let’s go through TOC briefly;

1. Identify the system constraints (No improvement is possible unless the weak link or constraint is found out)

2. Decide how to exploit the system constraints (Make the constraints as effective as possible)

3. Subordinate everything to that decision (align every other part of the system to support the constraints even if this reduces the efficiency of non constraint resources).

4. Elevate the system constraints (If output is still inadequate acquire more of this resource so that it is no longer a constraint)

5. If, in the previous steps, the constraint is broken back, go back to step 1, but do not let inertia become the system constraint (After the constraint problem is solved, go back to the beginning and start all over again. This is a continuous process of improvements

Therefore by removing the constraints and moving forward we get into a situation where the entire operations or production process work in harmony to achieve the ultimate goal of an organization i.e. Profit. From the point of view of operation or production management in order to achieve profit, the goal would be Increase throughput while simultaneously reducing inventory and reducing operating expenses. UNBALANCED CAPACITY Historically (and still typically in most firms) manufacturers have tried to balance capacity across a sequence of processes in an attempt to match capacity with market demand. However, this is the wrong thing to do-unbalanced capacity is better. Consider a simple process line with several stations, for example. Once the output rate of the line has been established, production people try to make the capacities of all stations the same. This is done by adjusting machines or equipment used, workloads, skill and type of labor assigned, tools used, overtime budgeting and so on.

In synchronous manufacturing thinking, however, making all the capacities is viewed as a bad decision. Such a balance would be possible only if the output time of stations were constant or had a very narrow distribution. A normal variation in output causes downstream stations to have idle time when upstream stations take longer to process. Conversely, when upstream stations process in a shorter time, inventor builds up between the stations. The effect of the statistical variation is cumulative. The only way that these stations can be smoothed is by increasing work-in-process to absorb the variation (a bad idea because we should be trying to reduce work-in-process) or increasing capacities. Stream to be able to make up for the longer upstream times. The rule here is that capacity within the process sequence should not be balanced to the same levels. Rather, attempt should be made to balance the flow of product through the system. When flow is balance capacities are unbalanced. What are the limiting factors that prevail? Could you tell me any? Well, BOTTLENECKS AND CAPACITY-CONSTRAINED RESOURCES We have earlier said about constraints and bottlenecks, now let us see its definition and further understanding of the topic. A bottleneck is defined as any resource whose capacity is less than the demand placed upon it. A bottleneck is a constraint within the system that limits throughput. It is that point in the manufacturing process where flow thins to a narrow stream. A bottleneck may be a machine, scarce or highly skilled labor, or a specialized tool. Observations in industry have shown that most plants have very few bottleneck operations.

If there is no bottleneck, then excess capacity exists and the system should be changed to create bottleneck (such as more setups or reduced capacity), which we will discuss later.

Capacity is defined as the available time for production. This excludes maintenance

and other downtime. A non bottleneck is any resource whose capacity is greater than the demand placed on it. A non bottleneck, therefore, should not be working constantly because it can produce more than is needed. A non bottleneck contains idle time.

A capacity-constrained resource (CCR) is one whose utilization is close to capacity and could be a bottleneck if it is not scheduled carefully. For example, a CCR may be receiving work in a job-shop environment from several sources. If these source:; schedule their flow in a way that causes occasional idle time for the CCR in excess of its unused capacity time, the CCR becomes a bottleneck when the surge of work arrives at a later time. This can happen if batch sizes are changed or if one of the upstream operations is not working for some reason and does not feed enough work to the CCR.

Friends, the question that comes to mind is how do we remove these constraints?

Well, the answer is:- We usually can not. Therefore, we must have adequate

METHODS FOR CONTROL The figure below shows how bottleneck and non bottleneck resources should be managed. Resource X and Resource Yare work centers that can produce a variety of products. Each of these work centers has 200 hours available per month. For simplicity, assume that we are, dealing with only one product and we will alter the conditions and makeup for four different situations. Each unit of X takes one hour of production time and the market demand is 200 units per month. Each unit of Y takes 45 minutes of production time and the market demand is also 200 units per month.

The figure shows a bottleneck feeding a non bottleneck. Product flows from Work Center X to Work Center Y. X is the bottleneck because it has a capacity of 200 units (200 hours/l hour per unit) and Y has a capacity of 267 units (200 hours/45 minutes per unit). Because Y has to wait for X and Y has a higher capacity than X, no extra product accumulates in the system. It all flows through to the market.

B part in the figure is the reverse of A, with Y feeding X. This is a non bottleneck

feeding a bottleneck. Because Y has a capacity of 267 units and X has a capacity of only 200 units, we should produce only 200 units of Y (75 percent of capacity) or else work-in-process will accumulate in front of X.

C part in the figure shows that the products produced by X and Y are assembled and

then sold to the market. Because one unit from X and one unit from Y form an assembly, X is the bottleneck with 200 unit_ of capacity and, therefore, Y should not work more than 75 percent or else extra parts will accumulate.

D part in the figure equal quantities of product from X and Y are demanded by the

market. In this case we can call these products "finished goods" because they face independent demands. Here Y has access to material independent of X and, with a higher capacity than needed to satisfy the market, it can produce more products than the market will take. However, this could create an inventory of unneeded finished goods.

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hat part waits not for a resource but for another part so d together.

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gh a bottleneck, queue time is the greatest. As we discuss use the bottleneck has a fairly large amount of work to do it is always working). For a non bottleneck, wait time is ing there waiting for the arrival of other parts so that an

Schedulers are tempted to save setup times. Suppose that the batch sizes are doubled to save half the setup times. Then, with a double batch size, all of the other times (processing time, queue time, and wait time) increase twofold. Because these times are doubled while saving only half of the setup time, the net -result is that the work-in-process is approximately doubled, as is the investment in inventory.

Can you tell me how do we go about FINDING THE BOTTLENECK

FINDING THE BOTTLENECK There are two ways to find the bottleneck (or bottlenecks) in a system. One is to run a capacity resource profile; the other is to use our knowledge of the particular plant, look at the system in operation, and-talk with supervisors and workers.

A capacity resource profile is obtained by looking at the loads placed on each resource by the products that are scheduled through them. In running a capacity profile, we assume that the data are reasonably accurate, although not necessarily perfect. As an example, consider that products have been routed through Resources Ml through M5. Suppose that our first computation of the resource loads on each resource caused by these products shows the following:

Ml 130 percent of capacity M2 120 percent of capacity M3 105 percent of capacity M4 95 percent of capacity M5 85 percent of capacity

For this first analysis, we can disregard any resources at lower percentages because they are non bottlenecks and should not be a problem. With this list in hand, we should physically go to the facility and check all five operations. Note that MI, M2, 'and M3 are overloaded; that is, they are scheduled above their capacities.

We would expect to see large quantities of inventory in front of M I. If this is not the

case, errors must exist somewhere-perhaps in the bill of materials or in the routing sheets. Let's say that our observations and discussions with shop personnel showed that there were errors in M1, M2, M3, and M4. We tracked them down, made the appropriate corrections, and made the capacity profile again: M2 115 percent of capacity M1 110 percent of capacity M3 105 percent of capacity M4 90 percent of capacity M5 85 percent of capacity M1, M2 and M3 are still showing a lack of sufficient capacity, but M2 is the most

serious. If we now have confidence in our numbers, we use Iv'l2 as our bottleneck. If the data contain too many errors for a reliable data analysis, it may not be worth spending time (it could take months) nuking all the corrections. SAVING TIME Recall that a bottleneck is a resource whose capacity is less than the demand placed on it. Because we focus on bottlenecks as restricting throughput (defined as sales), a bottleneck's capacity is less than the market demand. There are a number of ways we can save time on a bottleneck (better tooling, higher-quality labor, larger batch sizes, reducing setup times, and so forth), but:- Just how valuable is the extra time? Very, very valuable! AN HOUR SAVED AT THE BOTTLENECK ADDS AN EXTRA HOUR TO THE ENTIRE PRODUCTION SYSTEM. How about time saved on a non bottleneck resource? AN HOUR SAVED AT /\ NONBOTTLENECK IS A MIRAGE AND ONLY ADDS AN HOUR TO ITS IDLE TIME.

Because a non bottleneck has more capacity than the system needs for its current throughput, it already contains idle time. Implementing any measures to save more time does not increase throughput but only serves to increase its idle time.

Finally, my dear students, we inch towards the end of today’s lecture. But before

we formally close our shops for the day, let’s focus on:-

COMPARISON of SYNCHRONOUS MANUFACTURING with MRP and JIT: (i) MRP uses backward scheduling after having been fed a master production schedule. MRP schedules production through a bill of materials explosion in a backward manner-working backward in time from the desired completion date. As a secondary procedure; MRP, through its capacity resource planning module, develops capacity utilization profiles of work centers. When work centers are overloaded, either the master production schedule must be adjusted or enough slack capacity must be left unscheduled in the system so that work can be smoothed at the local level (by work center supervisors or the work themselves). Trying to smooth capacity using MRP is so difficult and would require many computer runs that capacity overloads and under loads are best left to local decision such as at the machine centers. An MRP schedule becomes invalid just days after it was created.

The synchronous manufacturing approach uses forward scheduling because it focuses on the critical resources. These are scheduled forward in time, ensuring that loads placed on them are within capacity.

The no critical (or non bottleneck) resources are then scheduled to support the critical

resources. (This can be done backward to minimize the length-of time that inventories are held.) This procedure ensures a feasible schedule. To reduce lead time and work-in-process, in synchronous manufacturing the process batch size and transfer batch size are varied-a procedure that MRP is not able to do.

Comparing JlT to synchronous manufacturing, JlT does an excellent job in reducing

lead times and work-in-process, but it has several drawbacks:

1) JIT is limited to repetitive manufacturing. 2) JIT requires a stable production level (usually about a month long). 3) JIT does not allow very much flexibility in the products produced. (Products must be similar with a limited number of options.) 4) JIT still requires work-in-process when used with kanban so that there is "some demand to pull." This means that completed work must be stored on the downstream side each workstation to be pulled by the next, workstation. 5) Vendors need to be located nearby because the system depends on smaller, more frequent deliveries.

Because synchronous manufacturing uses a schedule to assign work to each work station, there is no need for more work-in-process other than that being worked on. The exception is for inventory specifically placed in front of a bottleneck to ensure continuous work, or at specific points downstream from a bottleneck to ensure flow of product

Concerning continual improvements to the system, JIT is a trial-and-error procedure

applied to a real system. In synchronous manufacturing, the system, can be program and simulated on a computer because the schedules are realistic (can be accomplished) computer run time is short.

With that, we have come to the end of today’s discussions. I hope it has been an enriching and satisfying experience.