Continuous Flow in Lean Manufacturing
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In the majority of the lean manufacturing production systems, a U-shaped workcell is used. This is because with a worker positioned in the interior of the U, one would save time by minimising movement of both the parts and of the worker by simply turning around and sending it to the next work station.
In the case of very heavy parts, mechanised conveyor belts are usually added to help the parts along from one value adding process to the next at a constant rate, however it is to be noted that manual push or gavity conveyors are the ideal method for moving parts, as their reliability is very high and their down times are low owing to their lack of complexity in their design.
The corners of this workcell can cause alot of problems, as they have a tendency to slow down the production rates by holding some parts in that possible dead space. For this reason, a ball roller transfer section is generally used to combat the dead space areas around the corners of the system.
There may some instances where non-cyclical work has to be done to a part. If this were to be passed through the system like normal, it would cause disruptions to the system. To fix this problem, these operations are done outside the cell by support workers.
To cater for continuous-flow, one-at-a-time manufacturing each work station is made as compact as possible to reduce the waste of space, as well as to reduce possible storing space of parts and sub-assemblies, making batched processes imposible in this confiuration whilst also eliminating useless steps by the worker whilst in operation.
To reduce the ammount of floor space used, one must avoid standardised machines and instead specially designed machine bases must be used. It is important to note that each machine base can vary from workstation to workstation to further optimise assembly subprocesses. For the sake of convienience, modularity and cost, extruded aluminium members are bolted together to form the above mentioned machine bases. Another essential reason why the above set up is very effective is that due to its flexibility, it allows for quick and continuous improvements, which therefore directly impacts the set up time of the machine.
To ensure that no time is wasted looking for misplaced tools, all tools used at a work station has its own designated toolholder. This would ensure the immediate detection of a displaced tool. To further improve on this idea, some plants use modular tools holders to add more flexibility to the work station and therefore increasing the utility of the said workstation.
Another trick used to reduce the downtime is to have backup tools available to the worker, but also out of their way until they are required. To this end, many companies tend to use tool holding structures that can be swung or slid into or out of the workspace with little to no effort required.
So as to help the workers make fast decisions and to put them into effect, an information board is used. The information board is a simple way of supplying relevant information at the workplace with anything from assembly processes, to production targets. This feature takes out the need to chase down a busy supervisor, thus dramatically reducing the production downtime. So as to increase its flexibility in the workplace and reduce waste, it is generally modular and reusable.
To reduce the number of interruptions to a minimum, all parts are generally supplied to each workstation outside of the workcell via gravity feed bins. These bins are designed with simplicity and high reliability in mind to reduce the down times of the production line. To make the bins even more flexible, they are usually designed to be stackable and to be fastened using key studs, thus further increasing the reconfigurability of the production line.
In the case of larger parts however, the above system is shunned and a system of gravity feed conveyors are used instead. On the other hand when parts are very heavy, a case lifter is used to raise the parts to the work station using electricity, pneumatics, or hydraulics.
To reduce the time wasted in the setting up of the line, reconfigurability is a very important factor to take in account when designing a workcell. In light of this, quick-change fixtures are used due to their rapid changeover process. Another perk of using this type of fixture is that since it is very compact, many fixtures can be stored at the workstation, further shortening of the changeover time is experienced.
This arrangement allows for quick reconfiguration and movement to different parts of the workcell to accomodate for a new product, whilst also facilitating the change of the machine or workstation by using lockable casters.
One of the advantages of one-at-a-time manufacturing is the fact that a visual inspection can be done for every assembly. In the case that gages are required, they would be mounted to the workstation while still allowing for quick and easy replacement.
Some of the quality problems encountered may come from a malfuntioning machine or a faulty process. These problems can be quickly solved be restructuring the machine frame system via the replavement of the bolted members.
In the case of a faulty machine, this can also be quickly fixed, by first disconnecting all of the pneumatic or electric lines and then disconnecting the said machine to the frame via the lockable casters, eliminating the reluctance to change and improve the machines. This in turn increases the quality of the assembly appreciably by not allowing anyone to settle for a part that is almost right.
So as to drastically reduce down times due to maintainance, a modular structural framing system is used. This is to ensure that whilst the machine, or the machine frame is being serviced, another can easily take its place, minimising the already small downtime of the reconfigureable defective part in question.
By using this approach, one can standardize the components for most of the machine bases, guards, and workstations. This significantly reduces the number of tools needed to maintain the work cell, which in turn reduces the time needed to maintain the said structures. It is also important to note that since most of the parts are standardised, the company needs less inventory space to house the spare parts needed for maintainance.
Ease of Access
By using aluminium framing system discussed above, all components can be easily accessed and the positions of the various accessories such as the bins, shelves and tool holders can be optimised to increase efficiency.
Another advantage of using the above system is that not only is it possible to insure accessability for every worker by adding and repositioning components, but it also greatly increases the ease of maintainance of the work station by removing the parts in question via simple hand tools.
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