Manufacturing Technology (ME461) Lecture30

8/12/2019 Manufacturing Technology (ME461) Lecture30

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Review of previous lecture

Kanban Planning. Deterministic and probabilistic model. Cumulative probability function for x

circulating Kanbans. Numerical problems related to Probabilistic

Kanbans.


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Relationship between JIT manufacturing, setup time,and cost:

JIT manufacturing is often understood to mean amanufacturing system with lot size approaching unity. Itis important to understand the conditions under which

this goal may be achieved. The answer may comedirectly from the classical economic productionquantity inventory model.In a JIT manufacturing environment the intent is not to

permit shortages. Accordingly, the total variable costconsisting of setup and holding as a function ofeconomic production quantity Q, is given by


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Example:

Consider a product with the following data:

Unit Cost, C = $100.00Annual inventory carrying cost rate, i= 10%Demand rate, D = 10,000 units/ yearProduction rate, P = 15,000 units/ year.

Determine the optimal lot sizes for various values of setup costs varying from $400.00 to

$0.00016.


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Signal Kanban:

One of the important conditions for the implementation of JIT with kanbans is to have lowsetup times relative to processing times. However, there are situations in manufacturingcompanies, such as forging, die casting, and press operations, in which the setup time is notsmall relative to processing time.

The standard Kanban approach as discussed earlier does not work under these circumstances.We use a special type of Kanban known as a signal Kanban at the work centers with largesetup times.


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Signal KanbanIn effect a Signal Kanban triggers the production of a lot that consists of more than onecontainer at these work centers with large setup times.

However, standard Kanbans at normal work centers concurrently triggers the productionof one container at a time.

Normally, there are two types of signal kanbans. The first is rectangular and is known asraw material ordering kanban. It is used to withdraw materials from the preceeding stage.

The other type is known as the production ordering kanban, is triangular and is used totrigger production of a container.In a normal Kanban process , when a withdrawal is made, a production kanban is sentback to the preceding stage to trigger the production of 1 container.In a signal Kanban system the production Kanban is not sent back to the preceding stageafter every withdrawal of a container to the succeeding station.

A triangular production ordering signal kanban is attached at the reorder point of the lot.

This triangular kanban initiates the setup process to produce the whole lot at the signalkanban work center as soon as the container to which it is attached is withdrawn by thesucceeding stage.


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RawMaterialInventorystage Work

Center

LotSize

Position of rawmaterial orderingsignal kanban

Position ofproductionorderingsignalkanban

AB

A: Signal Kanban receiving postB: Signal Kanban ordering post

Material Signal Kanban

Production Signal Kanban


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Signal Kanban In addition to the production ordering kanban, a rectangular material ordering kanbanmay also be attached to the lot to order material from storage.

As soon as the container with a material kanban is withdrawn, the material signalkanban is sent to the preceding stage to withdraw the required items.

These items will be needed at the work center to process the entire lot.

When raw material reaches the workcenter the production ordering signal kanban maynot be there already to trigger the production.

However, the raw material will be waiting the arrival of the production signal kanbancard to initiate the production setup for the lot.

Normally, the triangular kanban is attached to the next container of the lot after

attaching the Material signal kanban.

There are two important aspects of a signal Kanban system:1. The determination of lot size.2. The position of both production ordering and material ordering signal kanbans.


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Example of signal kanban:

Consider an automotive company that manufactures three types of die casting productsat a die-casting station. Suppose the factory operates on two shifts. The die-casting

machine is utilized at 80% of its capacity of producing parts, leaving 20% operating timefor setup.

Over a period of two shift, the time available for setups = 3.2 hours (2X 8 hr./ shift X 20%).

Suppose the average setup time is 32 mins. A maximum of two setups per part is

possible. (3.2 X60/32X1/3).

If the demand of the part is 1400 per day with = 20 %, the minimal lot size of the partcan be calculated as follows:

Minimal lot size per setup = X 1400 X120% = 840.

If the container capacity is 100 parts. We then need 840/100 = 8.4 ~ 9 containers.

Let us now determine the position of the production ordering signal kanban in the lot of9 containers.The position is determined by the consumption of parts during the time interval from the

point of ordering the lot to the arrival of the lot to its stock location.


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Example of signal kanban:

This time interval is also known as Kanban Cycle Time.

As such Kanban Cycle Time = Kanban waiting time at the receiving post + its transfer time to

the ordering post + its waiting time in the ordering post + lot processing time + containertransfer time to final buffer.

The following formulae can be used to determine the production kanban position in terms ofnumber of containers.

Production signal kanban position = (Average demand) (1+ ) (Kanban cycle time)(Number of parts per container)

Suppose the Kanban lead time is 3 hrs. This is equivalent to 3/16 days based on two shifts of8hrs each.

Production signal kanban position= 1400 (1.20)(3/16) = 3.15 ~ 4 containers

100Similarly we can determine the position of the material ordering signal kanban. Thecomponents of cycle time are the following:Material Ordering signal kanban = waiting time at the receiving post + transfer time to rawmaterial storage + waiting as well as withdrawal time at the raw material storage + materialtransfer time to the work center + lot processing time + container transfer time to final buffer.


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Example of Signal Kanban:

Notice that lot processing time and container transfer time to final buffer are common toboth production and material signal kanbans.

Very often the material ordering signal kanban cycle time is greater than the productionordering signal kanban cycle time. Suppose the material ordering signal kanban cycle timeis 4 hours.

Material ordering signal kanban position = (1400 X 1.2 X 4/16) = 4.2 ~ 5 containers.100

Other types of signal kanbans:

So far we have seen production and conveyence kanbans. However, other types ofkanbans are used in specific situations as follows:

Express Kanban :An express kanban is used when there is a shortage of parts. It must be withdrawn after itsuse. The presence of an express kanban in a red post triggers the following activities:(1) A button for the machining line making the part is switched on, activating a red light

on the light board known as andon for the part.(2) The worker at the location where the light has come on must immediately produce the

part and deliver it personally to the subsequent process.


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Emergency Kanban:This kanban is used as a temporary measure to make up for the defective units and otheruncertainities such as machine failures or fluctuations in daily or weekly production.Through Kanban:In production situations in which two or more work centers are located close to each other,there is no need to exchange production and conveyence kanbans between these work-centers.

Alternative JIT systems:Though JIT is usually understood as a pull system in which the amount of material flow at theimmediately preceding station is determined by the stock consumption at the subsequentstation, there are a no. of alternative methods for JIT production. These are :

(1)Periodic Pull System : In a periodic pull system is one in which the manual informationprocessing time of a kanban method was replaced by an online computerized processing.(2) Constant Work in Process System : In this system in contrast to kanban cards, which are

part no. specific, the CONWIP cards are assigned to the entire production line. Theadvantage of this system over the kanban system is that it can be used in environmentswhere the kanban system is impractical because of a large no. of part types.(3)Long Pull System: In the long pull system, the triggering mechanism works in the sameway as in a pull system. However, the control of the long pull encompasses more than oneworkstation. In this system one unit is allowed to enter the system at the same time that one

unit is pulled at the end of the pull.


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Total Quality Control and JIT:Total quality control is really total in the sense that itpervades all departments of a manufacturingcompany as well as the supplier company.

TQC means an integrated approach to sharingresponsibility for quality production of products; itrequires a commitment from every individual from allfunctional departments in the company to achievethe goal of quality production.

Because defects are not permitted in the JITproduction system, it is important to have a TQCprogram for the successful implementation of JIT

production in the company.


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Robotic Systems Since, the development of the first articulated arm in the 1950s

and subsequent developments in the area of microprocessortechnology, robots have become available in a variety of types,styles and sizes.They are capable of performing a wide variety of types, styles andsizes.In fact the driving force for the purchase of robots is theirapplicability in hostile, strenuous, and repetitive environments aswell as in highly competitive situations with strong economicpressure to perform.

Such applications include welding, painting, and pick-and-placematerial handling, among others.Robotics is now becoming an integral part of automated discreetpart manufacturing system like flexible manufacturing system.


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What is an industrial Robot? According to the definition set by the Robotics institute or America,

An industrial robot is a programmable, multifunctional manipulator designed to movematerials, parts, tools, or special devices through variable programmed motions for the

performance of a variety of tasks.

The developments in the area of robotics since the first articulated arm in 1950 havebeen motivated primarily by the developments in the area of industrial automation inparticular and computer integrated manufacturing systems in general.

An industrial robot consists of a number of rigid links connected by joints of differenttypes, controlled and monitored by a computer..

To a large extent, the physical construction of a robot resembles a human arm.The link assembly mentioned above is connected to the body, which is usually mountedon a base.The link assembly is generally referred to as robot arm. A wrist arm is attachedto the arm. To facilitate gripping or handling, a hand is attached at the end of the wrist. Inrobotics terminology this arm is called an end effector.


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Robot sensors The motion of a robot is obtained by precise movements at its joints and wrist.While the movements are obtained, it is important to ensure that the motion is preciseand smooth.

The drive systems should be controlled by proper means to regulate the motion of therobot. Along with controls, robots are required to sense some characteristics of theirenvironment. These characteristics provide the feedback to enable the control systemsto regulate the manipulator movements efficiently.Sensors provide feedback to the control systems and give the robots more flexibility.

Sensors such as visual sensors are useful in the building of more accurate andintelligent robots. The sensors can be classified in many different ways based on theirutility. In this section we discuss a few typical sensors that are normally used in robots:

1. Position sensors: Position sensors are used to monitor the position of joints.Information about the position is fed back to the control systems that are used to

determine the accuracy of joint movements. Accurate joint movements are reflectedin correct positioning of the end-effectors, which eventually carries out theprescribed task.

2. Range sensors: Range sensors measure distances from a reference point toother points of importance. Range sensing is accomplished by means of televisioncameras or sonar transmitters and receivers. The problem may be reduced byusing a greater number of sensors.


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Robot movement and precision Speed of response and stability are two important characteristics of robotmovement. Speed defines how quickly the robot arm moves from one point toanother.

Stability refers to robot motion with the least amount of oscillation. A good robot isone that is fast enough but at the same time has good stability.

Speed and stability are often conflicting goals. However, a good controlling systemcan be designed for the robot to facilitate a good tradeoff between the two

parameters. The precision of robot movement is defined by three basic features:

1. Spatial resolution, 2. Accuracy, 3. Repeatability

1. Spatial resolution: The spatial resolution of a robot is the smallest increment ofmovement into which the robot can divide its work volume. It depends on thesystems control resolution and the robots mechanical inaccuracies. The controlresolution is determined by the robots position control system and its feedbackmeasurement system. The controller divides the total range of movements forany particular joint into individual increments that can be addressed in thecontroller. The bit storage capacity in the control memory defines this ability todivide the total range into increments. For a particular axis, the number ofseparate increments is give by 2 n


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Numerical Problem A robots control memory has 8 -bit storage capacity. It has two rotational jointsand one linear joint. Determine the control resolution for each joint, if the linear linkcan vary its length from as short as 0.2m to as long as 1.2m.

Control memory = 8 bit.

From the earlier equation, number of increments = 2 8 = 256

(a) Total range for rotational joints = 360 o

Control resolution for each rotational joint = 360/256 = 1.40625 o

(b) Total range for linear joint = 1.2-2 =1.0m

Control resolution for each linear joint = 1/256 = 0.003906m = 3.906mm

Manufacturing Technology (ME461) Lecture30

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