ASSEMBLY LINES – 16/12/16
Several activities put in sequence form an assembly line.
Different model of assembly line: general archetypes useful for studying. We have mixed solutions
thanks to the evolution of automation technology.
Assembly stations are put in sequence, operators can perform different assembly operations.
The movement of different pieces and assembly components is defined with a specific time (after
a certain amount of time the worker has to pass the wip to the next station). This movement can
be done in different ways: paced line (there is a physical system, a conveyer, that moves the
pieces), human paced (humans move the pieces). In the past, there was an acoustic signal that
worn the operators when it was time to move the line. In electronic sector it is quite common to
have a convey system that takes the piece from one station to another, even if it seems quite old
fashioned.
Why is it still so common? We know that if the time is fixed, the line will produce a certain
quantity of product with certainty  advantage: it is complex but we know how the line works.
Disadvantage: the system is so rigid, if something goes wrong it’s more difficult to find a solution.
If we have robots, it can be easier because robots can perform the same activities in the same
time. This isn’t true for human beings, it can happen that one operator goes slower than the
previous one or the following one  incomplete situation to solve. We have a probability of non
completion (no enough time to finish the assembly work assigned to that line, incompleteness
problem).
The first solution to be invented and proposed (and still used today) is the unpaced assembly line,
where there is a station that performs on an assembly activities, then the other stations, between
the two there is a physical space to leave materials (called buffer) that decouple the production
assembly flow. Each station is given the same amount of time to finish the work, but physically
there si space in between to accumulate pieces. The operators will work on his assembly piece
until it will be finished, then he puts it in the buffer and the next operator has to take the piece
from the buffer. We may see some WIP inside the line. This solution was invented to provide more
flexibility, requested by the unions before the IIWW. The first exoeriment of this approach was
FIAT plant in Melfi. A problem could be that there is no more direct control in defyining cycle time
to the production phase.
Today unpaced lines are widely used.
The second solution is the continuous flow lines, that is the solution used by car makers nowadays
(also Iveco plant for tracks in Brescia). We have a handling system where operators and
components are mutually “organized”, they work step by step slowly. We still have the concept of
station, but here stations are spaces on which the operators can move following the components
attached to handling system and that is moving slowly. The space of the station is based on the
distance between two phases. Near the stations operators have the components that need to be
assembled. This solution is balancing two weaknesses: the cycle time is given (we have designed
the line in each station, and the line is moving to respect the given time  under control), at the
same time we have some flexibility for the operator, if he doesn’t manage to finish the work in his
space he can follow the piece and work outside the station (if the problem is big they can also stop
the line actually in some cases)  it gives freedom to operators to work on their stuff. So in this
station we are able to reduce the probability of non completion.
These solutions provides opportunities to make an assembly line in a proper way. Said this, our
interest is to work on the design of the line.
The first design is aimed at deciding the number of stations and defying the time for each station,
the second purpose is to design each single stations (by mechanical engineers).
Our design is based on the fact that we have to decide how many stations (working on assembly
operations), which will be the cycle time, and we have to balance the line. The design variable is
the TIME as usual.
Slide 3: minimum set of info to design, that is the list of operations (what we will doing, which are
the assembly operations to include), which is the order of operations (assembly graph, it gives us
the order, sequence, parallelisms and so on, to decide how to distribute the operations in
stations). The assembly graph is not enough since we should know the time needed to perform
activities. Here we have the biggest problem, that is that we don’t know the time. Most of the
assembly activitites are still done by humans that work with different paces. We can describe time
of huamns with a normal distribution (we need the average time and its standard deviation).
Let’s take the example of IKEA chair and let’s imagine to design the line. We have three
operations, and we need to find theirs average and their deviation standards. How to calculate
those times? Trying to build it and measure the time. How many times do we need to register
time? Several times. There are studies that certify times to perform standard activities (motion
time measurement techniques). This info helps to forecast time needed to do an activities without
estimate it. Part of motion time tech is also aimed to find which is the best way to perform
activities to take the minimum amount of time.
Process engineers go in details about the method of how assembly operations has to be
performed.
Once we have this info, we can start with the design of the line, we need to understand how many
stations we need and how to divide the work among stations. In real life we have constraints
space in plants, more or less skilled workers, more or less traditional facilities and so on).
Which are the objectives that we want to achieve? We can be in a situation in which we want ti
have the smallest number of stations given a certain cycle time, that is the time taken by the line
to make a full circle. We can be in a situation where cycle time is given, and It is following from a
strategic decision. We can be in a situation where we want to minimize the cycle time. Or I can be
interested in minizimg the time wasted by the operators (in the handling line), or we want to
minimize the probability of non completion (for a strategic decision or because of a specific
market contests). Or I want to be sure that the cycle time is always respected. Or I can be
interested in minimizing the expected costs, that is the trade off between not finishing the
products and the cost of installing the station itself. We can have many different objectives
according to the situation. The objective should be the first thing to set.
Balancing constraints: vedi slide.
We would like to have a line in which each single station is working within the cycle time and that
all the cycle time is used: in that way all the operators will work for 100% of their time. The
problem is that we are building physical products, we ae dealing with operators, each operations
requires a specific amount of time. In practice, having 100% of efficiency is not the normal life.
We have to distinguish from efficiency of the overall line and utilization of persons.
Efficiency  I consider the line as a single tube.
Utilization of operator  (efficiency on the single station level) I have the 5 min of my cycle time, I
was able to do in that station a lsit of assembly operations till 4,5 min  that operator is almost
fully used.
Let’s imagine that we have a line made by operators, 5 stations (5 operators, one per station) and
22 min of assembly time to divide among the different stations. The line itself will be used 22
min/25 (total cycle time), but how the single operator will be used? This is the kind of problem we
have to solve. The designer of that line has to be sure to designing a line able to finish the work,
knowing that humans can’t work at the maximum possible pace. We may decide that all the
operators won’t work 100% of their time, but 90% of their time  safe coefficient on the
utilization rate (alfa). Or we can accept that one or more operators work more than the other, to
increase the productivity of the lines. How could we decide this? When is it safer to assign less
work? At the beginning or at the end of line? It’s a matter of managing the risk, if next to end of
the line we are not able to finish it is risky, it’s better than the person at the end of the line is less
used than the first ones.
Our game is, given the time allowed for each station, to decide the allocation of operations in
stations, in the proper order. The order to follow is given by the assembly graph.
Remember that the CT in slide 10 is given!!
If we have the amount of time needed to build a component, and the CT we can easily calculate
the number of stations.
We can have different orders because we may have different priority rules. We can have some
bettere guidelines, reference procedures always based on this method of assigning operations but
with a focus on optimization. We’ll see three methods based on technical objectives and based on
info available.
LECTURE 19/12 part 3 – MANCA LA PRIMA PARTE
We need to see other three elements:
Assembly lines can be different (paced, unpaced, continuous). The design method are
independent on the types of line, they can be used anyway. Said this, some peculiarities should be
mentioned. In Paced lines (given cycle time, and then material moved from one station to another
with a certain step time) we have a different level of complexities than in unpaced lines. Unpaced
assembly lunes were invented to be more flexible (for operators and for porudtcion variety),
thanks to buffers that decouple the production flow (partial independence for workers). These
unpaced lines were historically introduced to empower workers and to give them the physical
possibilities to assemble partially different components, all part of the same family of products
(multi model, or mixed model, assembly system). As a consequence, we may have some variability
in the production flow in unpaced lines.
Let’s imagine we have an unpaced line with a decoupling point. Operation is station 1 takes 60”, in
the second station in the 40% of the cases, the activity takes 40”, but in 50% of the cases the
activity takes 72” (for a different component probably), and in the 10% of the cases it takes 120”.
This a multi model assembly line in an unpaced line (lean manufacturing). This case lead to be
unbalanced in some cases (and to solve the unbalance, they put a buffer in the middle). Let’s
imagine that we can’t change the assignment of operations to the station, what can we do? We
have still do decide the dimension of the buffer, important to avoid inefficiency, to avoid blocks in
the system (our buffer should be long and big enough to avoid to block the first station, that
happens in case of 120”) and at the same time to avoid starvation of the second station (in case of
30”)  problems of dynamism of the system. We have an empirical solution and a mathematical
solution for this problem, but not in this course. We have just to remebr that the production
capacity will increase with the increasing of the buffer, but it shouldn’t overcome the production
rate.
The second issue is about continuous line, invented in the 80s n the car industry to merge the
advantages of both paced and unpaced systems. The most important thing to be defined is always
the number of stations, remembering that the cycle time is a matter of the speediness of the line
itself, that needs to be defined as weel. Moreover we have to decide the space, the length of each
station. These lines are usually done with the possibility to change the speediness or the moving
system. If the speed is high, we can expect that station are more (if I want to work faster, I have to
use more resourcs) and the space dedicated to eaxh station is less. In case of fall of demand, I can
reduce the number of station leaving more space to the remaining stations (more more flexible
than unpaced). When the speed is high, the risk is no completion (workers have to work faster in a
limited space). Let’s imagine a continuous flow line, how could we solve the completion? We could
add a “hospital” station, with a skilled person that is able to do all the activities done before the
hospital station (called jolly operators). Hospital station and jolly operator can also be
distinguished, putting the jolly directly on the line.
Another issue is continuous line is that we have to divide the space among the stations, leaving
upstream and downstream space both to let workers work with more flexibility and to be used in
case of changing in speediness.
DESIGN OF ASSEMBLY SYSTEM PART 4
Assembly lines moved from a single model concept, to a multi model – mixed model concept. A
single-model assembly line it’s possible to assemble only one kind of product  useful if the
demand is high with little variation. In case of customization variety, this model is not the most
appropriate one.
Multi-model vs mixed-model:
Multi-model  the line can produces different products but all the batches are made of the same
products; old concept, not very used anymore.
Mixed-model  line in which at the same time I can have in sequence different products of
different types. This is common since the 90s, especially in car industry.
In terms of design, the most famous solution is the U-shaped lines introduce by Toyota, lean
manufacturing, where operators can be easily interchanged. Multi-model and mixed-model
formula have not to be studied!!
Multi-model  we have to calculate the number of the stations, we should work with a design
activities to find a solution feasible for all the products to be done. We have also to consider the
set up time, when designing this system.
Mixed –model  we have different products that require the same opratiosn but with different
cycle time. We have two possibilities, one in the planning phase (to balance the scheduling phase
in control, and to balance the stations), the other is working n the standardization concept (again
from lean manufacturing). BI formula is not asked, just the main concept.
ASSEMBLY SHOP (questions: what is it? What are the advantages?)
Assembly shop are in the middle between fixed position (“stupid” solutions) and assembly line
(much more complex). Assembly shop is the most used solution in this period.
We have assembly operations to perform, instead of having one single fixed station, we decide to
divide the work in shops (fixed position), where operators do some precise activities. Material is
noved station by station, so it is a line. But it could happen that in case of high demand I have
more parallel station for each phase. It was originally used in the car industry.
This solution comes from the past, Volvo was the first one to apply it, then it was adapted to
aeronautic sector, today it is quite fashionable because of a matter of flexibility, productivity.
Today we are into the Industry 4.0, increasing in automation, especially AGV system, flexible
automation. Assembly shops is perfect to implement more automatic solution that combine
flexibility and productivity.
Where assembly can be automatize? Assembly is still today a “manual” activity, because the
human being is the most flexible solution that exists. Since the 70s automation has replaced some
human activities, and this is happening still today.
Some example of automatic solution are the vibrator, and other specialized machines, like the
machines that produces wood products and packages for IKEA (manufacturing AND assembly
automated system). Italy is the first producer in the world of machines that create packaging for
food (near Reggio Emilia). A machine that create packages is manufacturing and assembly system
(this is advanced manufacturing and advanced automation system).