Industrial Technologies

ROUGH DESIGN OF A TRANSFER LINE (SINGLE-MODEL)

– define the technological routing and operations of the product type: identification of

STEP 1

the product type, to be manufactures, estimating the required production volumes (yearly demand) and

defining its technological routing (comprising all information for the operations therein).

– identify all the machine types that are needed and balance the line:

STEP 2 26

based on the analysis of the technological routing. The

the required machine types are identified

§ most relevant choice at this step is between specialized machines, dedicated to single or few

operations (minus) but providing high speed performance (plus), versus machining centres that

means general purpose machines, that can process a wider set of operations (plus) at lower speed

performance (minus). (another plus) They can be used to configure the line, by replicating the same

types of machining centres along the line (as a “module”, to which different operations are assigned)

or, when the line is re-configured for new products (at the end of life of a product for which the line

was previously configured). They can be re-used to enable the facility life extension (the line does

not become obsolete because the demand of the product has ceased after some year (of course there

are no other factors, cost reduction of new technologies … which make the machining centres

themselves obsolete); (allocation of operations to work-stations/machines) is then resolved:

line balancing problem

§ machining centres provides more degree of freedom for the allocation of operations, thanks to their

flexibility, thus helping in line balancing; specialized machines are more constrained.

calculate the theoretical production capacity:

STEP 3 –

As previously discussed, once it is known the allocation of operations to workstations, it is also known

the CT = max working time (of the bottleneck machine) as well as the PC, defined here as theoretical

value because no problem occurrence is considered in regard to the line functioning. Considering the

typical unit of measurement of the CT in seconds/piece (very short time between exits of two successive

work-pieces from the line), 3600 is used to convert to proper unit of measurement of PC/TPC (p/h).

– calculate the actual production capacity

STEP 4

Actual production capacity is a different value (lower than TPC) because of two main reasons: scraps

and failures.

Therefore, the model includes:

i) 1-SR as the percentage share of good products, materials within the tolerance limits

(respecting target quality limits). TPC is then reduced simply considering the share of good

products exiting the line during an hour, hence TPC x (1-SR).

ii) Failures happen leading to a machine downtime, which subsequently lead to a line

downtime. In fact, if a machine stops in a transfer line, due to the rigid interconnection

(through integrated, common material handling system) each machine has to stop. This

functional behaviour is named, in system reliability analysis, as series logic. It can be

demonstrated that, within the series logic, the Line Availability (time share of the system is

UP respect to total time UP and DOWN) can be calculated as a multiplication of Availability

of each equipment: Machine Availability X Material Handling System) thus the APC is the

TPC realized in output only during the time the Line is available. Thus, APC = TPC x A (share

of time the line is available). SR and 1-A are considered as coefficients representing

independent events and so they both reduce the TPC.

compare the actual production capacity and the demand. If necessary, modify the line

STEP 5 –

and go back to step 2.

If APC < D (demand of the product = demand rate, pieces/hours), re-design is needed, going back to

step 2 i) by including additional machines along the line which induce the need to resolve again

the line balancing problem. This is then expected to lead – if well done - to reduce the

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maximum working time (of the bottleneck) > CT reduction > TPC enhancement > but

there is a trade off: with additional machines, more machines in series logic equals to a

reduction of Availability.

ii) by parallelization of more machines for the same operations. It potentiates CT reduction,

TPC enhancement, A enhancement due to more reliable system structure.

iii) Changing the machine types (with different speed performance, different working times

… etc.).

iv) Other decisions may consider, for example, the effect of enhancing preventive

maintenance policies, aimed at increasing A beyond the inherent A (only with corrective

maintenance policies), or of the repair policies, to achieve reduced downtimes, hence,

again, enhancement of A.

ROUGH DESIGN OF A TRANSFER LINE (MULTI-MODEL)

Assumptions:

The transfer line is used according to batch manufacturing approach: batch A of a product, then

1) batch B of another product, etc. To change from one batch to another, it is required a setup.

Set-up times are considered sequence independent: this could be either because in reality there

2) is no sequence dependence (negligible), or it is the result of an optimization procedure that

enabled to identify best batch sequencing (first batch A, then batch B, etc.) to reduce set-up

times. In this last case, set-up times are given by assuming the scenario of optimal sequence.

– identify the production mix (estimating the required production volumes (yearly

STEP 1

demand) define the technological routing and operations of the product types (in the production

STEP 2 –

mix) – identify all the machine types that are needed and balance the line (for each product

STEP 3

type). The line processes one product type at a time (multi-model), therefore the line balancing problem

is solved for each single product (as before). The only difference is due to the fact that there is a

constraint, that is: if N product types are considered in the production mix, N line balancing problem

should be solved keeping fixed the machine types selected at each workstation of the transfer line: they

are the same for all products.

– calculate the cycle time for each product type j

STEP 4

Calculate the cycle time as an outcome of the bottleneck machine identification. This is again based on

the hypothesis that the line processes one product type at a time (multi-model) and the line balancing

problem is solved for each single product, hence, bottleneck machine is fixed/identified when the

product type is identified (at a given workstation h).

calculate the whole time to produce a batch of product type j.

STEP 5 –

Calculation of the time required to manufacture a batch of the single product j, in case of synchronous

part transfer. This results from summing up the times spent at three phases of the batch manufacturing

process:

a) Setup time, to prepare the line to produce product j (a first phase, seconds/batch).

b) Time needed to complete the first work-piece of product j. There is in fact a load transient phase,

due to the line throughput time of the first work-piece, between the load time and the exit time

of the first work-piece in/out of the line; 28

c) Time needed to complete the whole batch, except the first work-piece, of product j, paced by the

bottleneck constraint/hence the CT (CT x (Qj – 1)). Since CT is normally a low value, the

approximated formula is properly applicable (and preferable for its simplicity).

calculate the time needed for a set of batches (within a production campaign - remind,

STEP 6 –

the campaign might be expressed as a duration of time, a campaign of three weeks or as a certain amount

of production, a campaign of 22 batches).

calculate the average theoretical production capacity.

STEP 7 –

In this case, it is an average for the batch manufacturing during the production campaign of the line (the

total quantities [pieces/set of batches]/total time for the campaign [seconds/set of batches] [p/h]

à

after conversion seconds to hours). It is worth remarking that the average is weighted by the batch

quantities of products j; as such, TPC depends on the production mix assumed within the production

campaign (batch quantities Q j change and TPC change).

àT

– calculate the actual production capacity

STEP 8 – compare the actual production capacity and the demand. If necessary, modify the line

STEP 9

and go back to step 3. 29

MODELLING METHODS: SYSTEM AVAILABILITY

PROBLEM SETTING

In manufacturing systems, the occurrence of (failure and breakdown are terms used as

failures

synonyms) leads to performance losses at each machine and, subsequently, at system level; the impact

of failures can be different depending on the system.

configuration of the

The can be evaluated by analysing the configuration of the system, based on the

system availability

availability of each single machine.

The occurrence of a failure leads to consequences in a single machine/station: considering the

production flow, the work-pieces cannot visit the machine for some period of time when the machine is

under repair after the failure: machine not functioning, unexpectedly not available for production so the

machine is in a state of “downtime”, under maintenance intervention.

When the machine is in a “downtime” state, then the performances of the machine are lost: zero

production by the machine. Not necessarily the production capacity is affected at the manufacturing

system level or, at least, the impact at system level may be partial, e.g. not totally leading to a zero

production capacity; of course, there are cases that the system goes totally down (and this happens soon

or immediately) when a machine breaks down: production capacity is totally lost for a given period (as

downtime of the machine).

Only when the machine is repaired and its service to production is reactivated, the machine, and the

system, are again supporting the production capacity at standard performance (max, nominal level of

the production capacity).

Problems:

How many machines do we need to meet demand?

Ø What happens if a machine breaks down?

Ø What is the impact of a failure in the production capacity of the system?

Ø

Study: to analyse the function carried out by the machines for the production capacity of the whole

Ø system;

to evaluate the of the machines on the availability/unavailability of the

impact of unavailability

Ø manufacturing system, thus on the whole production capacity (in order to meed the demand).

Bear in mind that each configuration has different strengths and weaknesses:

job shop: is the system characterized by low impact if breakdowns;

Ø transfer line: is the system characterized by high impact of breakdowns;

Ø manufacturing cell: is the system in between.

Ø

These differences are mostly due to different degree of flexibility: routing flexibility and the presence of

buffer