Scheduling Techniques

IV ME II SEM Production Planning Control UNIT - 6

UNIT – VI

Scheduling Policies – Techniques, Standard scheduling methods

6.1 Factors affecting scheduling (a) External Factors  Customer demand  Customer delivery dates  Stock of goods already buying with dealers & retailer

(b) Internal factor  Stock of furnished goods from raw materials  Availability of equipment & m/c s  Availability of man power  Availability of material  Additional manufactory facilities  Feasibility of economical products

6.2 Scheduling Policies i. Master Production Scheduling ii. Operator scheduling iii. Order scheduling iv. Product scheduling (a) Master Production Scheduling

 The master schedule, also known as master production schedule (MPS), formalize the production plan and translates it into specific end-item requirements over a short to immediate planning horizon  Master Production Schedule(MPS) is a detailed plan that shows how many end items will be available for sale or distribution during specific periods

 The aggregate production plan must treated into a master schedule, which specifies how unit of each product to be delivered & when.

IV ME II SEM Production Planning Control UNIT - 6

 In turn, this master schedule must be converted into purchase orders for raw materials, orders for component from outside vendors, and production schedules for parts made with in the shop  These events must be timed & conducted to allow delivery of the final product according to the master schedule. Applications are big firms to load entire plant in research & development organization  Specially, the master schedule or master production schedule is a listing of the products to be produced, when they are to be delivered & in what quantities  The scheduling period in the master schedule are typically months, weeks, & days. It consistent in the plant capacity.  Advantages: Easy to understand, less cost to make and maintain, maintained by non technical staff  Disadvantage: providing overall picture , it does not give any details (b) Operation Scheduling

 It is concerned with the problem of assigning specific job to specifies work centers on a weekly, daily or hourly basis  Operation scheduling involves the assignment of start dates to the batches of individual components & the designation of wok center on which the work to be performed  Objectives are i. Meet the required delivery dates for completion of all work on the jobs ii. Minimize the process by reducing lead times iii. Maximize utilization of m/c s & labor resources iv. It can be described as consisting of the following 2 steps i.e., m/c loading and job sequencing- involves determining the order in which the process the job through a given work centre (c) Order Scheduling (O.S)  The purpose of O.S is to make assignments of the orders to the various m/c s in the factory  Inputs to this module consist of the order release & priority control.  Each job order has a certain priority determined by due dates & other factors  The basic document prepared by the order scheduling module is the dispatch list

IV ME II SEM Production Planning Control UNIT - 6

 It reports the jobs that should be done at each work center & certain details about the routing of parts  elaborative technique  Advantages is very detailed earliest possible completion date can be meet  Disadvantage is very costly, difficult to maintain and accurate production is required (d) Production Scheduling

 It involves setting the time of performance for the detailed operation of manufacture i.e, establishing the order of work at each m/c or stage of process in particular of the parts & in the assembly of the products.  Objectives are i. To meet the output goods of the master schedule & to fulfill delivery promises ii. To keep constant supply of work already allocated to each m/c iii. To get out manufacturing orders in the shortest possible time consistent with in economical operation.. Preparation of production schedules is given below

Production Schedule

Model : S15 Month: Apr’06 Part No For assembly Quantities required on dates selected 5 12 19 26 S - 210R 1 500 500 500 500 S - 220 F 2 1000 1000 1000 1000 S- 230 F 4 2000 2000 2000 2000 S- 240F 2

6.2 Scheduling Techniques

Scheduling can be considered as a typical queuing problem with following criteria. 1. Each for the joins the end of the queue with first in first out priority. 2. Job/activity with the longest duration always goes first. 3. Jobs may be tasked according to earliest delivery date. The schedule based on the above criteria can be prepared with the help of following techniques.

IV ME II SEM Production Planning Control UNIT - 6

1. Bar chart 2. Gantt chart 3. CPM & PERT Technique 4. The Run out approach (1) Bar Chart: Henry L Gantt (1900)

 Two coordinate axes horizontal - time elapsed and Vertical - jobs or activities to be performed.  Each bar represents the specific job or activity of the project.  The beginning & end of each bar represents the time of start & time of finish of that activity.  The length of the bar shows time required for the completion of that job or activity. Development of bar chart: The following are important stages in developing a bar chart i.e., Break down: The project into its various activities or jobs or operations.

(1) Gantt chart Gantt charts are usual aids used to depict the sequencing, load on facilities, or progress associated with work effort over a well defined time period

The Gantt chart shows the planned production and actual performance over a period of time for various factors. The Gantt chart indicates how each machine or work centre is planned for work on different job orders. It includes the scheduled start and finish times, time reserved for unavailability of machine, the delay in production and time lost in it, the extent of actual completion of various jobs each day. The information is useful for the management to diagnose the problems, if any, and take necessary action of rescheduling in time.

The Gantt chart is a rectangular chart divided by horizontal and vertical parallel lines. The vertical lines or columns divide horizontal lines into time units which may be hours, days, weeks, months, years, etc. The width of the column indicates

IV ME II SEM Production Planning Control UNIT - 6

the duration of the activity or operation. The flow of time is indicated from left to right. The rows indicate various activities or operations form top to the bottom. The length of the lines drawn show the amount of work completed. So, the Gantt charts are also known as Bar charts. Gantt charts are very easy to prepare and understand. The chart helps in communicating clearly the important shop information. Gantt charts are of two types. Gantt load chart displays graphically the work loads on each machine or work centre. Gantt scheduling or progress chart illustrates the progress of the assigned task

The main limitations of Gantt chart are

(i) The relationship between various operations can not be shown in the chart. ii. When certain modifications are made in a schedule, it is very difficult to incorporate the corresponding changes in the Gantt chart (ii) In spite of these limitations the Gantt charts are widely used in the manufacturing organizations.

(2) CPM & PERT Technique (Refer any OR book for details) The PERT technique provides a measure of the probability of completing the project by the scheduled date. In PERT the assessment of the end event of the project is related to the degree of uncertainty that is associated with the three time estimates.

IV ME II SEM Production Planning Control UNIT - 6

CPM is critical path method is the longest path shows the maximum duration of project completion time

(4) The Run out approach The Run out approach is applied to the process of production, which is geared up to inventory level. It is demand oriented and aims at minimizing stockouts by assigning highest priority to those items which are likely to run out. The run out time for each time can be calculated and a monthly forecast upon which the future needs can be developed. Finally the total number of months that may elapse before the total inventory will run out is calculated. This method is simple, easy, and economical. It helps in minimizing stock outs and contributes towards improving the consumer service. Hence it is widely used in modern manufacturing organizations.

6.2.2 Types of Scheduling

(a) Forward Scheduling is commonly used in job shops where customers place their orders on “needed as soon as possible” basis. Forward scheduling determines start and finish times of next priority job by assigning it the earliest available time slot and from that time, determines when the job will be finished in that work centre. Since the job and its components start as early as possible, they will typically be completed before they are due at the subsequent work centers in the routing. The forward method generates in the process inventory that are needed at subsequent work centers and higher inventory cost. Forward scheduling is simple to use and it gets jobs done in shorter lead times, compared to backward scheduling  Each task is scheduled to occur at the earliest time that the necessary material will be on hand and capacity will be available.  It assumes that procurement of material and operations start as the customers requirements are known  The customers plan their orders as a needed –as-soon-as possible basis.  Used in many companies such as steel mills & m/c tool manufacturers. (b) Backward Scheduling / Reverse Scheduling

IV ME II SEM Production Planning Control UNIT - 6

is often used in assembly type industries and commit in advance to specific delivery dates. Backward scheduling determines the start and finish times for waiting jobs by assigning them to the latest available time slot that will enable each job to be completed just when it is due, but done before. By assigning jobs as late as possible, backward scheduling minimizes inventories since a job is not completed until it must go directly to the next work centre on its routing.  Used assembly type industries & in job shops that consist in advance to specific delivery dates  The jobs start date is determined by setting back from the finished date, the processing time for the job.  By assigning jobs as late as possible, backward scheduling minimizes inventories, since each job is not completed until it is done, but not earlier.

6.3 Standard Scheduling methods

(a) Shop loading methods (b) Index method (c) Assignment problem

6.3.1 Shop loading methods

IV ME II SEM Production Planning Control UNIT - 6

Shop loading can be done using simple charts as shown in fig.

Facility/Work centre 1 2 3 4 Time available (capacity) 90 45 135 45 Job No.: 108 Hours required 9 14 4 10 Hours available 81 31 131 35 117 Hours required 11 9 19 7 Hours available 70 22 112 28 126 Hours required 8 5 11 7 Hours available 62 17 101 21 135 Hours required 4 3 5 5 Hours available 58 14 26 16 144 Hours required 10 7 6 9 Hours available 48 7 90 7 153 Hours required 2 2 5 2 Hours available 46 5 85 5

One may use simple charts for loading purpose:

Work centre No.1 108 117 126 135 144 153 9 11 8 4 10 2

Work centre No.2 108 117 126 135 144 153 14 9 5 3 7 2

Work centre No.3 108 117 126 135 144 153 4 19 11 5 6 5

Work centre No.4 108 117 126 135 144 153 10 7 7 5 9 2

The loading problem needs to be solved simultaneously with accompanying sequencing problem. However when the same jobs can be done by a no. of work centers, the problem is distinctly one of assigning the jobs to work centers based on appropriate criteria such as minimum cost.

IV ME II SEM Production Planning Control UNIT - 6

6.3.2 Index method

Job Work centre 1 2 3 4 Days Days Days Days A 10 9 8 12 B 3 4 5 2 C 25 20 14 16 D 7 9 14 9 E 18 14 16 25 No. of days 20 20 20 20 available

Supposing time is the criterion, ‘indies’ are calculated for the different process times having the base index of 1.0. The lowest index jobs are then assigned to the work centers, keeping in view the limitations of the capabilities of the centers. The next lowest index jobs are then assigned to the work centers and this process is repeated till all the jobs are assigned. This solution may not be optional. In the table below shows jobs that are assigned are underlined. Note that in this example time is the criteria.

Work centre 1 2 3 4 Job Days Index Days Index Days Index Days Index A 10 1.25 9 1.13 8 1.00 12 1.5 B 3 1.5 4 2.0 5 2.5 2 1.00 C 25 1.78 20 1.42 14 1.00 16 1.14 D 7 1.00 9 1.28 14 1.42 9 1.28 E 18 1.28 14 1.00 16 1.14 25 1.78 No. of days 20 20 20 20 available Days assigned 7 14 8 18

In different context, this method is useful when a worker can operate different machines and he can be so assigned.

6.3.3 Assignment problem – See OR book

IV ME II SEM Production Planning Control UNIT - 6

Work centers Job 1 2 3 4 5 A 10 9 8 12 0 B 3 4 5 2 0 C 25 20 14 16 0 D 7 9 10 9 0 E 18 14 16 25 0

Line Balancing, Aggregate planning, Chase planning, Expediting, controlling aspects

7.1 Line Balancing

Line balancing is the production line or assembly line. Suppose there are 3 m/c s or work stations A,B & C which can process 5,10 &15 pieces/unit time respectively and the sequence of the operation is such that the pieces flow from m/c A to B & B to C. Since A as minimum capacity i.e., of processing only 5/unit time. Naturally work station or m/c B will remain idle for 50% of its time & m/c for 66.66% of its time. It shows that the line is unbalanced one way to practically balance the line is to have 3 m/c s of type A, 2 of type B with every m/c of type c. Another approach to balance the line will be to given to some other task to m/c s B & C so that they do not remain idle. The main object of the line balancing is to distribute the tasks evenly over the work station so that idle time of the men & m/c s in minimized. Line balancing aim at grouping the facilities ( or task) & workers in an efficient pattern in order to obtain an optimum or most efficient balance of the capacities & flows of the production or assembly process. Task is grouped so that their total time is preferably equal to or a little lesser than the time available at each work station this reduces the idle time. Advantages are balancing the product line, group the facilities and worker in an efficient pattern, even distribution of tasks

7.2 Aggregate planning

(a) Aggregate planning is an intermediate planning method used to determine the necessary resource capacity a firm will need in order to meet its expected demand

(b) The purpose of aggregate planning is planning ahead because it takes time to implement plans. The second reason is strategic of the company and third aggregate planning help synchronize flow throughout the supply chain; it affects costs, equipment utilization, employment levels and customer satisfaction.

IV ME II SEM Production Planning Control UNIT - 6

 Determine demand for each period.  Determine capacities for each period.  Determine pertinent company policies.  Determine unit cost based on all relevant sources.  Develop alternative plans and calculate the cost for each.  Chose the best overall plan based on company objectives and cost.

(d) Goal of Aggregate Planning

To develop a realistic production plan on an aggregate level that will satisfy organizational goals and customer demand needs at the lowest total cost

Objectives of the Aggregate planning 1. To minimize the investments in the various inventories. 2. To minimize the total cost over the planning horizon. 3. To maximize the customer service. 4. To minimize the changes in the workforce levels. 5. To minimize the changes in the production rates. 6. To maximize the utilization of the plant and the various equipments.

(e) Role of the AP

Integral to part of the business planning process Supports the strategic plan Also known as the production plan Identifies resources required for operations for the next 6-18 months Details the aggregate production rate and size of work force required

IV ME II SEM Production Planning Control UNIT - 6

(f) Developing the Aggregate Plan

7.3 Types of Aggregate Plans 7.3.1 Level Aggregate Plans  Maintains a constant workforce  Sets capacity to accommodate average demand  Often used for make-to-stock products like appliances

IV ME II SEM Production Planning Control UNIT - 6

 Disadvantage- builds inventory and/or uses back orders

7.3.2 Chase planning

 A chase production plan is the first of three different approaches to manufacturing production we will discuss. A chase strategy is used when the demand varies throughout the year. In this strategy, the production capacity is increased and decreased to match, or chase, the demand. Some also refer to this as a matching strategy

IV ME II SEM Production Planning Control UNIT - 6

 Produces exactly what is needed each period  Sets labor/equipment capacity to satisfy period demands  Disadvantage- constantly changing short term capacity

(a) Principles of the Chase Method

The chase method helps firms match production and demand by hiring and firing workers as necessary to control output

(b) Chase demand strategy • Cost of strategy – hiring and firing workers • This strategy would not be feasible for industries which require highly skilled labor or where competition for labor is fierce. • This strategy would be cost effective during periods of high unemployment or when low-skilled labor is acceptable.

IV ME II SEM Production Planning Control UNIT - 6

 Reduced inventory costs.  High levels of worker utilization

(d) Hershey’s use of Chase Strategy Demand for chocolate is high during the winter months. Facilitated by the location of Hershey’s manufacturing facility, the company hires farmers from the surrounding areas to aid in meeting demand When demand drops in the spring and summer months the farmers are let go and thus able to return the their fields 7.4 Expediting Expediting means taking action if the progress reporting indicates a deviation of the plan from the originally set targets. Even with the best plans & schedules, things go wrong due to machine breakdowns, proper tooling not available, quality problems etc. It is the expeditor’s job to compare the actual progress of the order against the predetermined schedule. For orders that fall behind the schedule, the expeditor recommends corrective action. This may involve rearranging the sequence in which orders are to be done on a certain machine, coaxing the foreman to tear down one setup so that another order can be run 7.5 Controlling aspects  The aim of production control is to produce the products of correct quantity, in best quality, at the right time by using the best & least expensive methods.  The supervision of all the relevant operations with help of control mechanism that feed backs the progress of the work.  Comparing the actual performance with the present standards, if any deviation in the performance is found then these deviations are controlled & analyzed.  Control phase makes sure that the programmed production is constantly maintained