W10 Production planning and control Flashcards
Manufacturing operation planning and control
Manufacturing operation planning and control is concerned with managing the day-to-day
activities of the organisation to satisfy customer demands
Planning
Planning is a formalised forecast of
events that will happen in the future. Changes in production variables such as changes in customers
decisions, suppliers not delivering on time, machine breakdowns or other unavailability of resources
(e.g. staff illnesses) can cause things to happen differently to what was planned.
Control
Control is the
process of coping with changes in such variables. Control allows the objectives of the plan to be met,
even when assumptions on which the plan was based are no longer applicable.
Sequencing
Sequencing is determining the order and time in which various jobs should be carried out with
limited resources. Various methods for sequencing include: physical constraints, customer priority,
due date (DD), last in first out (LIFO), first in first out (FIFO), longest operation time (LOT),
shortest operation time (SOT) as well as algorithms such as Johnson’s. These aim to improve
performance objectives of dependability, speed, work in progress inventory, idle time and cost.
Forward scheduling
Forward and backward scheduling is then
used to develop the timing of individual activities. In forward scheduling, tasks are completed as
early as possible. This offers high labour utilisation and flexibility
Backward scheduling
In backward scheduling tasks are
completed as late as possible. This offers lower material costs and less exposure to risks in case of
order changes. Minimising the makespan (the time from the start of the first job to the finishing of
the last job), and the lateness (how far tasks go beyond their deadlines, averaged and summed) are
among important goals in sequencing.
Sequencing rules: Physical constraints
Sometimes the mix of work arriving at a part of an operation may determine the
priority given to jobs. For example, when fabric is cut to a required size and shape
in garment manufacture, the surplus fabric would be wasted if not used for another
product. Therefore, jobs that physically fit together may be scheduled together to
reduce waste
Customer priority
Operations will sometimes use customer priority sequencing, which allows an important
or aggrieved customer, or item, to be processed prior to others, irrespective of the
order of arrival of the customer or item.
Due date (DD)
Prioritizing by due date means that work is sequenced according to when it is due for
delivery, irrespective of the size of each job or the importance of each customer.
Last in first out (LIFO)
Last in first out (LIFO) is a method of sequencing usually selected for practical reasons.
For example, unloading an elevator is more convenient on a LIFO basis, as there is
only one entrance and exit.
First in first out (FIFO)
Some operations serve customers in exactly the sequence they arrive. This is called
first in first out sequencing (FIFO) or sometimes first come, first served (FCFS)
Longest operation time (LOT)
Operations may feel obliged to sequence their longest jobs first, called longest operation
time sequencing. This has the advantage of occupying work centres for long periods
Shortest operation time (SOT)
Most operations at some stage become cash constrained. In these situations, the
sequencing rules may be adjusted to tackle short jobs first, called shortest operation
time sequencing. These jobs can then be invoiced and payment received to ease
cash-flow problems
Johnson’s method
Johnson’s algorithm for sequencing applies to sequencing of n jobs through two workstations (i.e.
determining the order of jobs when there are two serial machines). The algorithm is as follows:
1. Look for the smallest processing time in the remaining jobs;
2. If that time is associated with the first workstation then schedule that job first or as near as
first as possible. If that time is associated with the second workstation, sequence that job last
or as near to last as possible;
3. Delete the scheduled job from the task list; and,
4. If there are any tasks remaining, go to 1
Push Control
A push system where material is moved on to the next stage as soon as it has been processed. Material requirements planning (MRP), Master production schedule
(MPS), Manufacturing resource planning (MRPII) and Enterprise resource planning (ERP) are
examples of push strategies
Pull system
A pull system where material is moved only when the next stage wants it. Just in time (JIT) or Lean Manufacturing
Material Requirements planning (MRPs)
MRP is a push system that utilises the bill of materials (BOM, a listing of all of the raw materials,
parts, subassemblies and assemblies needed to produce one unit of a product) and a master production
schedule (MPS) to make timing and volume calculations to meet forecast demand
In MRP, the BOM is traversed from top to bottom and for each sub assembly the following
items are calculated for each time period.
- gross requirements: the total quantity needed from the item in the time period;
- scheduled receipts: the quantity that will be received due to orders placed before the planning
period; - projected on hand inventory: the amount of inventory of the item that is expected to exist at
the given time period; - planned receipts: the quantity of items to be received in the time period as the result of orders
in the planning period; and, - planned order releases: the quantity that is ordered in the time period. Working in a top-tobottom manner, would allow the orders for all parts of the product to be specified.
Manufacturing resource planning (MRP II)
MRPII is a development of MRP where MRP and the information systems that are related to it
are integrated in a single framework. A single database is thus held for systems such as inventory
management, capacity management and production planning
Enterprise resource planning (ERP)
ERP is the extension of MRPII outside the company. It allows the resources within the entire
enterprise to be managed within a single framework. In ERP, systems such as human resources,
finance, etc. are integrated with production management strategies. The most important advantage
of ERP is perhaps the fact that an enterprise’s ERP can communicate with the ERP in other
enterprises throughout the supply chain. This introduces unprecedented levels of flexibility in supply
chain management
Lean production Basic Working principles
the aim of this approach is to meet demand instantaneously
with perfect quality and zero waste. This involves supplying products with perfect synchronisation
with the demand.
* Discipline - in following work standards, critical for safety and quality;
* Flexibility - to increase people’s responsibility to their abilities;
* Equality - as one person’s problem is really everyone’s. Some lean companies mandate the
same uniform for everyone regardless of rank in the organisation;
* Autonomy - delegating responsibility to people so that they can make direct decisions;
* Development - a more capable workforce will increase productivity;
* Quality of working life - involve people in decision making, increasing job security and
enjoyment of facilities;
* Creativity - an indispensable element of motivation; and
* Total people involvement - staff will take on more responsibility in dealing with the supply
chain, recruitment, quality issues and spending of improvement budgets.
Causes of Waste
- Muda (waste; no value added) Poorly
communicated objectives are often the main cause. Not understanding customer requirements
is an example cause. - Mura (unevenness; lack of consistency) For example, if tasks are not properly documented,
when they are performed by different people the results are different. - Muri (overburden; unreasonable demands) Often resulting from poor planning, these can be eliminated by sequencing, scheduling and considering resource loadings in
production planning.
7 types of waste
- Over-production. Producing more than what is immediately needed.
- Waiting time. Efficiency measures can be used to find out the idle time of resources. Waiting
time of products in the pipeline are somewhat more difficult to calculate. - Transport. Moving items during the operation does not add value and should be minimised.
- Process. Some processes might only exist due to poor design of the product or the process
chain. - Inventory. Keeping raw material, work in progress (WIP) or finished products is a waste of
valuable space. - Motion. An operator who is moving around a lot does not necessary add a lot of value.
- Defectives. Products that do not meet quality requirements are a major waste in production
systems.
Lean synchronisation
In the traditional approach, if a problem occurs in one part of the production system, it would
not be immediately visible in the other parts of the system as these are isolated from each other
by buffer inventories. In the lean approach, if something happens in one part (for example Stage
B in the figure above) the effects will be felt immediately in Stage C and soon after in Stage A.
This would mean that solving the problem in Stage B is no longer the sole responsibility of people
working in that stage and becomes a job for the entire production line. This increased exposure is
one of the defining properties of lean synchronisation as buffers (which are considered as blankets of
obscurity by lean practitioners) no longer protect the other sections from disruptions.
Continuous Improvement
An important principle in Lean is that improvement towards the
ideal ‘instantly meeting demand with perfect quality and no waste’ is a never-ending process. The
Japanese term ‘kaizen’ is often used to express this principle of continuous improvement.
Elimination of Waste
- Streamlined flow
- Matching supply and demand, for example by the use of kanban
- Flexible processes
- Minimising variability through levelling schedules or levelling delivery.
- Mixed modelling
Kanban
- Instruct the preceding stage to send more
- Serve as visual control to show up areas of
over-production - Reducing the number of kanbans becomes
a tool for kaizen.
Better match supply and demand
Hybrid Strategies
MRP and JIT can coexist and be combined in several ways to form a hybrid system. The way in
which they should be combined depends on the complexity of product structures (see below), the
complexity of product routing, the volume-variety characteristics and the level of control required.
In general MRP is better for planning and JIT better for control. Various ways that these systems
can be combined include: using separate systems for different products and using MRP for overall
control and JIT for internal control.
