DESIGN OF MANUFACTURING SYSTEM: JOB SHOP
industries: we are dealing with production systems, with the physical configuration of a
manufacturing plant and of course with the arrangement of available resources in the space available
inside the factory. Two different times: processing industries and manufacturing industries (parts
- = a sort of process that originates goods that can not be split again.
- = we can divide the finished goods. It requires different modelling approach.
The job shop is the first design solution (configuration and layout for a system. In a job shop machines
are grouped on the basis of involved. Similar machines – machines that realize
the same process – are grouped on the same area called shop. The shop becomes a centre of specialized
competences in which similar phases of
different processes take place. For this
reason, each product/part has its own
in the system. In a job shop
materials are moved according to the
required product routings (e.g. from one
department to another). The logistics
(material handling through the factory)
is characterized by flexibility.
In a job shop the labour is divided in
departments according to task
specialization: workers are skilled on the
basis of involved. Operators become expert inside the area and improve their
performances. This is another way of saying that labour is divided in departments according to task
specialisation and workers are over skilled on the technological processes involved. This solution leads
to a functional structure and separation which is favourable to support knowledge transfer between
colleagues of the same unit: the learning process speed because of the exchange of experiences and
practical suggestions from the older workers. Through this configuration I can produce a huge variety
Strengths and weaknesses
One of the most important of job shop is the high flexibility:
- Short to medium-term flexibility - Medium to long-term flexibility
Mix Product (innovation)
Thanks to the basic flexibility in the production system (flexibility derives from):
- Machine flexibility (e.g. in case of breakdown of a machine the capacity will be reduce but there will
be other machines still available to continue with the production)
- Material handling flexibility: the ability to move different products efficiently. Raw materials and
WIP may be transported easily between departments. In the real case lot sizes are physically
transferred to other departments. Here they are stocked in particular area as buffers (decoupling
function between parts of the processes that works at different speeds) between the departments
waiting for the resource availability for next operations
- Routing flexibility: flows can be managed in terms of priorities, level of demand or mix of products
The mayor advantage of this layout is to produce a products. That is the good point but it
big variety of
can be dangerous too: image a huge number of product lots of flow to be planned (the sequence of
different work orders); we need expert resources to manage the complexity of the environment.
Physically is difficult to find where the products are. Which batch has the priority? What am I going to
work? 80 % of the company are organised this way. We need to spend time to manage with problems in
taking decision about scheduling, priorities etc. This is exactly the role of a manager.
Other strengths subsequent to flexibility are:
- Low impact on breakdowns: this is the less vulnerable system at all to face breakdowns; the
production capacity is reduced but with a partial impact on the total capacity that is nominally
installed. This is a consequence of:
Routing flexibility between each department: rebalancing the loads by planning some
o alternatives and temporarily routes in order to reduce the impact of a failure.
The remarkable role of buffers because they decouple the operation of machines through
o stocks of WIP preventing the propagation of a machine breakdowns avoiding material
starvation or blocking.
- Lack of obsolescence: it is different from ageing. Ageing is an endogenous factor, it is related to the
use of machine and their degradation though time. Obsolescence also depends on exogenous factors
due to market needs and opportunities. Assets, resources should be adequate (not aged + consistent
with customers’ desires and capable to provide performances in this way). Obsolescence is when
demand does not meet anymore functions or services deliver in such a way (it may happen because
of a technology gap due an innovation etc.). So why this configuration is the best to face
obsolescence? This fact is due to the possibility to rearrange the plant of the factory according to the
necessitate driven by market’s demand. On the other side, some technologies may become old in
terms of performances: well we don’t need to change and rethink the whole system but it is possible
to operate only where it is needed, I mean replacing the oldest one with something at the state of
There are also some involved in this possible layout:
- Limitations in efficiency (machine efficiency): the complexity of the environment reduce the
efficiency in managing the whole system. With too many flows, batches that cross each other
resources for machines set up operators, it is difficult to reach high performances. Whenever a
portfolio of product orders is planned for a given period, scheduling aims at deciding when the
orders are loaded (loading/timing), how orders are sequenced to be processed at different
machines (sequencing), which alternative plans are scheduled for each product (product routing).
This is a combinational problem and its complexity increases with the high number of machines, the
width of portfolio and the number of alternative process plan for each product. = a multi
objective decision making problem, with purposes often in conflict (example: utilization of
machines: reduce the number of set up vs variety of products and their lead time to customer). There
may also happen some unexpected events that force managers to rethink the routings by
redesigning the plan or just shifting the time-table according to the available resources left and their
- Qualitative characteristics of the product can vary for different pieces: since products need the
“process” of a certain shop, it may happen that a shop produces a batch under quality (because
machine used in the shop can vary because the do the same process), a casual lack in the quality of
the process. It will be our duty to investigate if the problem is casual or permanent and decide
whether act or not. Tolerance guaranteed by different machines could not be the same within the
job shop, according to what we said above. Qualitative characteristic of the product can vary for
different work-pieces changing the machines where the product is processed and obviously more
hours required for production.
- Production management is difficult
High WIP: because of two reasons. It is firstly due to the difficulties in production management
§ to arrange all the routings and tools, operators, materials etc. the difficulty to reschedule because
of an unexpected event. All these situations generate queues in front the machines. Secondly
WIP has an active role in avoiding the spread of breakdowns in the whole system because of
their decoupling role: they provide material to be work whereas the machine is repaired.
Lead times are long (because of queues) and characterized by high variability: the drawback of
§ high WIP is high manufacturing lead times. These entities are strictly directly related. High
queueing times leads to high manufacturing LTs. E.g. long queues in front of the bottle neck the
previous machine can not produce for a certain amount of time. Other cause for the waste of
production time are related to machine interferences. 12
Difficulties in estimating delivery lead times
§ Low utilization rate of machines: 80 % of utilization is a good result. The rest of time could be
§ set up, maintenance or there are some machines interferences: job shop is the perfect place in
which we see interferences. Some issues that reduce the time in which machines are used for
It is difficult to calculate production capacity that depends on:
- Mix of jobs that have to be manufactured - Possibilities to use alternative routings
- Technological characteristics of jobs - Number of machines and their state
- Complexity of pieces (parts) to be - Lot sizes
manufactured - Ability to schedule jobs
Job shop is the easiest system to design. Being the easiest is one of the most difficult to be executed.
To plan, to verify the quality of the product, to size the batch, understand availability. There is a
production manager to manage this complexity. Taking decision on how to schedule the production.
System design – the design method
It is difficult to calculate production capacity that depends on several variables like product mix,
technological characteristics of jobs, complexity of the pieces to manufacture, lot sizes, number of
machines and their states ... It is difficult to calculate/estimate the production capacity available from
the machines installed in the shop. The real dynamics of a system is a combination of all these conditions.
This leads to difficulties in estimating production capacity.
In designing a system there are typical question that we should reply. How many machines do we need
to meet demand? How many operators do we need to meet demand? Where are the bottlenecks? What
happens if a machine break downs? What happens if the production mix changes? What is the effect of
reducing setup time or changing the lot size? What is the effect of adding another machine? ... We must
put ourselves in the worst conditions and create a system able to disintegrate them. The method is
divided in some steps each of them is fundamental for the next one because it allows us to go further
deep in the analysis: from a rough design, an overall picture of the ecosystem, to a very detailed level of
planning and analysing.
STEP 1 – ROUGH DESIGN OF A JOB SHOP
Production mix definition:
a) Identify all the product types: it suffers from uncertainty data - how to define product types?
Through analogies with past experiences, similar plants … and the foresees of future scenario for
production mix and through a sensitivity analysis. Uncertainty: what happens if a scenario shift?
b) Estimate yearly demand for each product type
c) Define the lot sizes for each product type: manage the trade off between set up costs (also time
available for production) and stock holding costs.
STEP 2 – ROUGH DESIGN OF A JOB SHOP
Routing definition: define the main routing for each product type and if possible, define alternative
routings in order to face inconveniences.
STEP 3 – ROUGH DESIGN OF A JOB SHOP
Machine identification: on the basis of routings, it is possible to identify all the machine types that are
necessary to manufacture the production mix. 13
STEP 4 – ROUGH DESIGN OF A JOB SHOP
For each product type, calculate the total time of the operations that have to be done on the same type
of machine. The time includes also the auxiliary movements but it does not include the setup time.
+1 anno fa
I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher franciig_ di informazioni apprese con la frequenza delle lezioni di Industrial technologies e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Politecnico di Milano - Polimi o del prof Macchi Marco.
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