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lOMoARcPSD|428265Samenvatting - college alles
Production Planning & Quality Control (Rijksuniversiteit Groningen)
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Groen = Alleen in de PPP, Blauw = Alleen in het boek (maar staat wel aangegeven als
hoofdstuk wat we moeten kennen), Rood = Zowel in boek als in lecture
Lecture 1
Operations Management Recap
Scheduling chap 13
Planning chap 15
Race without a finish line chap 1
Operations Management Recap
Zie geschreven lecutre
Scheduling Chapter 13
Production Leveling is to prepare production plans to the changes in production levels. The
production level can fluctuate due to promotions, end-of-month quotas, seasonal etc. There
are two ways to deal with this uncertainty:
Create buffers, these can provide a degree of certainty, they keep materials flowing
and work centers productive. However the down side is: high inventory levels.
Level the production schedule: the same quantity is made in each production run
and these runs occur at regularly scheduled intervals. Choosing smaller batch sizes
for this, the smoother the flow will be (and the lower the backlogs, WIP inventory
and leadtimes), and supervisors will have more time to focus on the work at hand.
Requirements for Leveling Production Schedules:
1. Continuous, stable demand: no buffers are desirable. During peak performance,
companies should pay attention to satisfying the first to tiers of customers. First tier:
high volume and common process, second tier: substantial volume is and third tier:
low volume with sporadic orders.
2. Short set up times: from one process to another must be short
3. Production = demand: do not produce more than needed. Also, an aggregate
demand should be made: the total demand for all products in a product group, and
thus not for every product apart. This will lead to a bigger demand, and is thus more
stable
Leveling focus should be on leveling the schedules for the highest-volume products (tier 1
and 2). For one product group the company should keep its production level fixed for as long
as possible, although demand may fluctuate. For multiple products, they must operate in a
similar way, in which the chosen production level should account for preexisting stock and
also should be able to satisfy peak demands.
Level scheduling in pull production
Final assembly schedule. For a pull production system, demand should be stable and
it utilizes only one schedule, for only the last stages of the process.
Mixed model production. For multiple kinds of products, a mixed model production
should be made on one single line. MMP
Batch size. Must be as small as possible, which will result in a small WIP everywhere.
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The MMP schedule
The MPP is producing with a batch size of one, and repeating the sequence of products to
produce. However, one must also look at the change over times. The requirements for MPP
are:
- Continuous demand
- Small setup times
- Demand-driven production (zie bovenaan)
- Flexible workers
- Effective quality assurance
- Small-lot material supply
The advantages of MMP are:
- Low variation in production scheduels
- Low WIP inventory
- Reduced lead times
- Elamination of losses due to line changeover: with MMP an entire line is rarely shut
down for changeovers
- Process improvement: workers rotating through a variety of tasks and operations are
more aware of problems and motivated to eliminate them
- Balanced work loads: even allocation of work
- Fewer losses from material shortages: work continues in other models, and more of
those models are produced until the material arrives.
Production planning and scheduling in different circumstances
There are three main production philosophies:
Make to stock. Companies make products in anticipation of demand, which go into finished
good stocks before being bought to fill customers orders.
Assemble to order
Make parts using a forecast and then put these together into a final combination desired by
the customer.
Make to order
When there is a specified and sporadic demand
Scheduling PPP:
Criteria in scheduling are the goal (total completion time) and the constraints, such as due
date violations and time to create a schedule.
Gantt chart: shows in horizontal lines the amount of work done or production completed in
certain periods of time, in relation to the amount planned for those periods.
How to create a schedule?
by making usage of priority rules:
- First come first serve,
- Shortest processing time (SPT) first. Advantage: many customers surved,
disadvantage: long jobs keep waiting
- Earliest Due Date (EDD). Advantage: less due date violations, disadvantage: perhaps
many small due date violations instead of a single large one
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However, the disadvantage of these priority rules: most are greedy and don’t consider
overall optimum.
Tardiness = projecten die achterlopen!
Maak een table: (this case is FCFS!, so jobs are in normal sequence)
Job Completion time Due date Tardiness
1 11 61 0
2 40 45 0
3 71 31 40
4 72 33 39
5 74 32 42
Total 121
Now,
job nmr = job nmr
completion time = cumulative time for each job to complete (so job 1 takes 11 days, job 2
takes 29 days together at job 2 it will state 40!)
Due date = when the job should be finished (not cumulative)
Tardiness = completion time – due date. When this is a negative number, tardiness is 0.
When it is a positive number, tardiness is that number.
Average tardiness = total tardiness / number of jobs (so 121/5 = 24.2 days)
No. of tardy jobs (=3)
Maximum tardiness (= 42)
Why should we plan?
We can influence waiting
time and/or change over
time. Combining similar
products: less setupts,
more efficient, however
longer waiting time.
Earliest due date: no
batching, no waiting time,
but more setups, less
efficient.
SO: balance between efficiency and speed/flexibility.
But what makes planning difficult?
A lot of possible schedules, cannot compare all of them. We should use:
- Hierarchical planning
- Priority rules
- Mathematical models
- Genetic algorithms
The performance criteria in theory vs in practice
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- Number of constraint violations
- Costs of schedule execution
- Speed of algorithm
- Employees preferences and wishes
- Errors in the schedule
- Understandability of the schedule
- Cost/efficiency of the scheduling process
- Timeliness and reliability of initial release
- Flexibility regarding schedule adaption
- Accessibility of schedulers
- Communication and harmonization quality
Last four are more important if the uncertainty is high
So the interconnections of planning: people have differences in goals (delivery performance,
efficiency, customer service) and differences in goal perceptions (employees have the feeling
that own department strives harder for goal achievement than other)
SO: human traits are needed!! To:
- Change in task from problem solving to correcting errors
- Misfit between mental model and mathematical model
- Memory overload in comparing solutions
Lean = to remove changeover time itself. No batches needed, no effect on efficiency,
however we still need to consider due dates.
Planning chapter 15
Production planning consist out of centralized planning and decentralized planning.
Centralized planning has to accumulate demand information and formulate production
plans and schedules. It concists out of:
- Monthly planning: prepare monthly demand forecast for each product, product
group or item
- Daily scheduling: further decomposition of the monthly planning. Integrating recent
demand information, daily order alterations, material procurement forecast, Kanban
supplier link etc.
Decentralized planning:
- Detailed capacity planning
- Shop floor control
A company that wants to become pull production has to implement:
- Simplified Bills of Material (BOM)
- Stock areas where the workers work
- Postdeduct and deduct lists: keep track of what is made
- Rate-based Master Schedules
Lecture 2
Lean manufacturing, wastes
Focused factories and group technology
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