Notes Pom Module 6

Production and Operations Management Module 6: Production Scheduling Production scheduling on the shop floor aims to: • Meet customer demands of Quantity • Meet customer requirements of delivery • Achieve maximum utilization resources ( human and infrastructure) • Achieve lowest production cost Production scheduling methods vary depending on type of product, continuous production or batch production, made to order or made to stock etc. Broadly the sequence of realizing the business plan is as follows; 1. Aggregate planning Determines the production requirements over a period of 6 months to 18 months 2. Master production scheduling Converts production plan into specific material and capacity require nets. This is the beginning of all short range planning 3. Material requirement planning Converts MPS into what materials are required , how much and when. 4. Loading Assignment of jobs to various work centers based on future processing, resource utilization and sequence of operations 5. Sequencing Determines what jobs are to be processed when on each work centre. This depends on priorities of jobs and best utilization 6. Detailed scheduling Determines start and finish dates and times for jobs on each machine. 7. Expediting Actions to ensure scheduling is met by coordination for materials, tools, availability of machines and manpower. 8. Production control Information on status of plan and accruals. Information on problems that may be faced in future period. This helps in taking decisions to meet the aim of production planning and scheduling. Master Production Scheduling (MPS) MPS is a plan for future production . The plan unit may be weeks, months or Quarter Inputs for MPS • Customer orders • Market forecasts

• •

Inventory levels Facility loading and capacity information

Objectives of MPS • To schedule products to meet Delivery requirements • To fully utilize capacity at lowest production cost Procedure for preparation of MPS 1. Estimate product quantities and due dates ( based on customer orders and Stocking requirements) 2. Decide time period For MPS ( week, month, Quarter) 3. check existing Inventory 4. Calculate load ( no.. of hours ) at each work centers for the time period 5. Check for over loading or under loading work centers Updating of MPS 1. MPS is made as frozen, firm and open ( frozen –no change, firm –change in exceptional cases, open- major changes may be effected0 2. Firm MPS is released for production planning and control 3. Based on actual production and inventories MPS is updated Poorly Designed MPS results in: • Over loaded facilities • Under loaded facilities • Excessive inventories or frequent shortages • Excessive expediting • Unreliable deliveries Detailed scheduling: Detailed scheduling is concerned with identifying and determining • Product and operations to be carried out on specific work centre • Period ( day, week, month) Scheduling depends on volume of orders, nature of operations and job complexity. Two types of scheduling techniques are used: • Forward scheduling: loading starts as soon as materials are available and work centers are free. No control over when the product may be available. Results in excess inventories. Used where supplier is unable to meet delivery dates. •

Backward scheduling: Here the delivery dates is the starting point and the loading date is determined by the lead time taken in each work center working back wards

Facility loading Facility loading is the process of determining which work centre receives which job Loading depends on capacity determined by load schedules, priority sequencing and work centre utilisation. Finite loading – jobs are assigned to work centers based on required hours and hours available. Infinite loading- Jobs are assigned to work centers without regard to available capacity. This requires decisions regarding overtime, subcontracting or delaying selected orders. Load charts helps in visualizing loading and progress; Dept

Machine shop

Week no

3

Month

Work centre Turning

Mon

Wed

Thu

Fri

Tue

January 2004 Sat

Milling Grinding Drilling

Priority sequencing Question of sequencing arises when several jobs require same work centre for processing. Priority indicates which is to be loaded first. Some single-criterion priority sequencing rules 1. First in first served-Normally applied at service operations like banks, super bazaars 2. Shortest processing time- Job requiring least processing time is taken first. This may ultimately increase throughput time. 3. Longest processing time-Job with longest processing time is taken first. 4. Least slack job first- slack= available time-processing time 5. Earliest due date job first 6. Truncated shortest processing time- Jobs waiting for more than specified time is given priority 7. Preferred customer order- important customers are given priority 8. Random selection 9. Cost over time- based on ration of delay cost and processing cost 10. Least change over cost- change over is based on overall cost of all change over between jobs

Example: Single machine, 6 waiting jobs Jobs Processing time ( mins) A 7 B 6 C 4 D 3 E 2 F 1 Rule: longest processing time first Jobs Processing time ( mins) A 7 B 6 C 4 D 3 E 2 F 1

Total flow time 7 7+6=13 13+4=17 17+3=20 20+2=22 22+1=23

Average flow time = (7+13+17+20+22+23 )/6 = 102/6= 17 mins Rule 2; Shortest processing time first Jobs F E D C B A

Processing time ( mins) 1 2 3 4 6 7

Total flow time 1 1+2 =3 3+3=6 6+4=10 10+6=16 16+7=23

Average flow time = (1+3+6+10+16+23 )/6 = 59/6= 9.83 mins

Sequencing of ‘n’ jobs on 2 machines. ( Johnson’s rule ) • •

Minimum total flow time Minimum idle time

Step 1- List all processing times on two work centers Step2-Scan the processing times

Step 3-Select the job with the shortest time. If the shortest time is at first work center , select the job first. If it is at second work center , select the job last. Step 4- remove the job from the list. Step5- continue step 2 to 4 till all jobs are assigned.. If the processing time is same on work centre 1 and 2 , arbitrarily assign either in the beginning or last. Processing time in hours Job Work center 1 A 2 B 4 C 0.75 D 1.5 E 2.0 F 2.0 Sequence :

Work center 2 1 2.25 2.5 3.0 4.0 3.5

C, D, E ,F , B , A

Calculation of cumulative flow time: 0.75

1.5

2.5

2.0

2.0

3.0

4.0

2.0

4.0

3.5

17 hours Total flow time = 17 hour Idle time Work center 1= 4.75 hrs Idle time Work center 2= 075 hrs Total idle time =5..5 hrs. Total operation time 12.25 + 16.25 = 28.5 hrs. Check Total operation time + total idle time = twice cycle time 28.5 + 5.5 = 2 x 17= 34 hrs.

2.25

1.0

Line Balancing Line balancing deals with apportionment of sequential work activities to work stations in order to gain high utilization of labour and equipment and therefore minimize idle time. Steps in line balancing Step 1- determine tasks to be performed to complete one unit of finished product, determine the sequence . Draw a precedence diagram. Step2- estimate task time Step3- determine cycle time to meet desired production ( hourly/ daily etc) Step4 – assign tasks each task to worker and balance the assembly line Analysis of line balancing ; 1. Determination of number of work stations and time available at each work station 2. Group the individual tasks into approximately equal amounts of work at each work station 3. Evaluate the efficiency Grouping of tasks should be at or slightly less than the cycle time or multiples of cycle times. If multiples are present more than one worker will be doing the same job. •

Cycle time = Available time / out put required

•

Theoretical number of workers required: =( total operation time X out puts per period )/ available time per period per worker.

•

Balance efficiency = Out put of task time / cycle time x no. of work stations( workers) Balance efficiency = Theoretical no. of workers/ Actual no. of workers

Problem: Precedence diagram . operation time in minutes. D 0.20

A 0.65

B 0.40

C 0.30

F 0.400 E 0.45

G 0.30

Operation time = 7 hours per day Output desired is 550 units per day Cycle time = Available time / output = 7 x 60 / 550 = 0.76 mins Theoretical minimum no. of workers= (0.65 + 0.40 + 0.3 + 0.2 0 + 0.45 0.4 + 0.3 )/0.76 = 2.7 / 0.76= 3.552 Workstation 1 : idle time = 0.11 Workstation 2 : idle time = 0.06 Workstation 3 : idle time = 0..11 Workstation 4 : idle time = 0..06 Total idle time = 0.34 mins Balance efficiency = (2.7 x100)/ 4 x 0.76 = 88.81% Also

= 3.552/ 4

X 100 = 88.81

Line of Balance ( LOB) LOB is a technique used for production scheduling and control to ensure committed delivery requirements. There are 5 stages followed on LOB technique. Stage 1 – Preparation of operation program chart. This is based on lead times for each activity. Delivery date is taken as 0 and worked back wards Stage 2- Preparation of completion schedule Stage 3 – Construction of line of balance chart Stage 4 – Construction of program progress chart Stage 5 – Analysis of progress and corrective action. Left part of chart indicates the week numbers and cumulative number of products to be completed Right side chart indicates the operations required At any period line can be drawn from left hand chart and the quantity of products that should have been completed at various operations can be easily read from the Right hand chart

Cumulative completion Quantity

LOB

Q T Y

1

2

3

4

Week no. Operation stages

Benefits of LOB Technique: • LOB is a planning and controlling technique which enables controlling at each stage of production line • Production problems may be spotted early and necessary actions may be taken • When products are to be delivered as per schedule and there are many processing steps and production lead times, LOB has the greatest advantage.