CAD/CAM/CIM

CAD/CAM/CIM Unit IV

Lecture Notes, Prashanth N, Dept of Mechanical Engg | 1

Material Handling and Storage

Introduction to Material Handling: The purpose of Material handling in a factory is to move raw materials, work in process (WIP), finished goods & supplies from one location to another to facilitate the overall operation of manufacturing. The handling of the materials must be performed safely & efficiently at low cost in a timely manner accurately (The right materials in right quantities should be moved to the right location) without any damage to the materials.

Overview of Material Handling Equipment:

A great variety of material handling equipment is available commercially. Material handling

Equipment includes:

(1) Material Transport equipment: Material transport includes equipment that is Used to

move materials inside a factory, warehouse. Eg: Industrial Trucks (Manual & Powered),

Monorails, AGV’s, Conveyors, Hoists & Cranes etc

(2) Storage systems: Although it is generally desirable to reduce the storage of materials

in manufacturing. It seems unavoidable that raw materials and work-in-process will spend

some time being stored, even if only temporarily. And finished products are likely to spend

some time in a warehouse or distribution center before being delivered to the final

customer. Accordingly, companies must give consideration to the most appropriate

methods for storing materials and products prior to, during, and after manufacture.

Eg: Racks, shelves, bins, AS/RS, Carousel storage system etc

(3) Unitizing Equipment: The term unitizing equipment refers to (1) containers used to hold

individual items during handling and (2) equipment used to load and package the

containers. Containers include pallets, boxes, baskets, barrels, pails, and drums etc. The

second category of unitizing equipment, loading and packaging equipment, includes

palletizer, designed to automatically load cartons onto pallets and Shrink-wrap plastic film

around them for shipping.

(4) Identification and Tracking systems: Material handling must include a means of keeping

track of the materials being moved or stored. This is usually done by affixing some kind of

label to the item, carton, or unit load that uniquely identifies it. The most common label

used today consists of bar codes that can be read quickly and automatically by bar code

readers.

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Types of Material Handling Equipments

1. Industrial trucks: Industrial trucks divide into two types: non-powered and powered.

Non powered trucks (also called as Hand Trucks) are platforms or containers with

wheels that are pushed or pulled by human workers 10 move materials. Powered

industrial trucks are steered by human workers. They provide mechanize movement

of materials. Examples for both is shown below

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2. Cranes, monorails & hoists: These are usually heavy equipment/ material lifting

devices and used to move only for a limited distance. These are generally power

operated with automatic controls for ease of operation. For example: Jib Cranes,

under sling cranes, over head monorails, powered & hand operated hoists, etc.

3. Conveyors: Conveyors are used when material must he moved in relatively large

quantities between specific locations over a fixed path. The fixed path is

implemented by a track system, which may be in-the-Floor. Above-the-Floor, or

overhead. Conveyors divide into two basic categories: (I) powered and (2) non

powered. In non powered conveyors. Materials are moved either manually by

human workers who push the loads along the fixed path or by gravity from one

elevation to a lower elevation.

Types of Conveyors

a. Roller and Skate Wheel Conveyors: These conveyors have rolls or wheels on which the loads ride. Loads must possess a flat bottom surface of sufficient area to span several adjacent rollers. Pallets. tote pans. or cartons serve this purpose well. These are shown below

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b. Belt Conveyors: Belt conveyors consist of a continuous loop: Half its length is used for delivering materials, and the other half is the return run. The types of belts used are Flat belt or Troughed belts. An Example of Flat belt is shown in below figure.

c. Conveyors Driven by Chains and Cables: The conveyors in this group arc driven

by a powered chain or cable that forms an endless loop. Various types of Conveyors under this category are as follows: 1) Chain: Chain conveyors consist of chain loops in an over-and-under

configuration around powered sprockets at the ends of the pathway. (2) Slat: It uses individual platforms called slats. Connected to a continuously moving chain, although the drive mechanism is powered chain, it operates much like a belt conveyor. (3) in-floor towline: These conveyors make use of four-wheel carts powered by moving chains or cables located in trenches in the floor.

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(4) Overhead trolley: A trolley in material handling is a wheeled carriage running on an overhead rail from which loads can be suspended. An overhead trolley conveyor consists of multiple trolleys. Usually equally spaced along a fixed track. The trolleys are connected together and moved along the track by means of a chain of cable that forms a complete loop. (5) power-and-free overhead trolley: A power-and-free overhead trolley conveyor is similar to the overhead trolley conveyor. Except that the trolleys arc capable of being disconnected from the drive chain, providing this conveyor with an asynchronous capability.

4. Automated Guided Vehicle Systems (AGVS): Thes are usually battery operated, automatically steered vehicles, which follow a preset path. Generally they are interfaced with automated systems to derive maximum benefits of integrated automation & manufacturing. Types of AGV’s include Driver less trains, Pallet Trucks, Unit Load carriers. NOTE: AGV’s are discussed in detail in future topic

5. Industrial Robots: These are programmed material handling machines that perform very efficiently and economically. They are capable of performing loading, unloading and movement operations in the manufacturing line.

Other Material handling equipment might include, Dial indexing tables, Elevators etc.. Principles of Material Handling: The important principles of Material handling in any industry are as follows:

1. Unit Load Principle: Material to be moved should be arranged into a large unit size & unit size should be same for all materials. The materials are placed in a pallet or other standard sized container for convenience in handling. The Materials & container together are referred to as unit load. The unit load should be as large as possible.

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2. Avoid Partial Loading: Transport the full unit load whenever possible rather than partial loads. i.e. Load the material handling equipment to its maximum safe limit.

3. Shortest distance principle: Movement of material should be over the shortest

distance. Realization of this principle depends heavily on plat layout design.

4. Straight line flow rule: The material handling path should be in a straight line from the point of origin to the point of destination, thus this principle is in consistent with the shortest distance principle.

5. Minimum terminal time principle: Movement of a unit load consist of the move time + the time required for loading unloading & other activities that do not involve actual transportation of the material. Hence minimize these non-move times.

6. Gravity Principle: Use gravity to assist the movement of the materials to certain extent as possible, at the same time give consideration to safety & risk of the product damage (Prevent product damage while transport).

7. Carry loads both ways: The handling system should be designed & scheduled to the extent possible to carry loads in both direction, i.e. return trips with empty load are wasteful.

8. Mechanization Principle: Manual handling of materials should be avoided. The handling process should be mechanized where it is possible to increase efficiency & economy.

9. Systems Principle: Integrate material handling system in the facility including, receiving, inspection, storage, production, assembly packaging, shipping & Transportation.

10. Systems flow Principle: Integrate the flow of material with the flow of information in handling the storage systems. The information for each item moved should include identification, origination point & destination point.

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11. Part orientation principle: In automated production system the orientation of the work part should be established & maintained throughout the material handling process.

Selection of material Handling Equipment: Following are the important factors that are to be considered while selecting material handling equipments for an industry.

1. Type of Material: Material Shape, Size, Weight to be considered 2. Plant Layout: Space available, Machine arrangement etc 3. Type of Production 4. Type of Production Machine 5. Type of Material flow 6. Other Factors: Include Cost of material handling, operation & Maintenance costs,

life of the equipment etc.

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Automated Guided Vehicle Systems (AGVS)

An automated or automatic guided vehicle system (AGVS) is a materials handling system that uses independently operated, self-propelled vehicles that are guided along defined pathways in the floor. The vehicles are powered by means of on-board batteries that allow operation for several hours (8 to 16 hours is typical) between recharging. The definition of the pathways is generally accomplished using wires embedded in the floor or reflective paint on the floor surface. Guidance is achieved by sensors on the vehicles that can follow the guide wires or paint. Automated guided vehicles (AGVs) increase efficiency and reduce costs by helping to automate a manufacturing facility or warehouse. There are a number of different types of AGVS all of which operate according to the preceding description. The types can be classified as follows:

1. Driverless trains: This type consists of a towing vehicle (which is the AGV) that pulls one or more trailers to form a train. It was the first type of AGVS to be introduced and is still popular. It is useful in applications where heavy payloads must be moved large distances in warehouses or factories with intermediate pickup and drop-off points along the route.

2. AGVS pallet trucks: Automated guided pallet trucks are used to move palletized

loads along predetermined routes. In the typical application the vehicle is backed into the loaded pallet by a human worker who steers the truck and uses its forks to elevate the load slightly. Then the worker drives the pallet truck to the guide patn, programs its destination, and the vehicle proceeds automatically to the destination for unloading. The capacity of an AGVS pallet truck ranges up to 6000 Ib, and some trucks are capable of handling two pallets rather than one. A more recent introduction related to the pallet truck is the forklift AGV. This vehicle can achieve significant vertical movement of its forks to reach loads on shelves.

3. AGVS unit load carriers. This type of AGVS is used to move unit loads from one

station to another station. They are often equipped for automatic loading and unloading by means of powered rollers, moving belts, mechanized lift platforms, or other devices. Variations of the unit load carrier include light-load AGVs and assembly line AGVs. The light-load AGV is a relatively small vehicle with a corresponding light load capacity (typically 500 Ib or less). It does not require the same large aisle width as the conventional AGV. Light load guided vehicles are designed to move small toads (single parts, small baskets or tote pans of parts, etc.) through plants of limited size engaged in light manufacturing. The assembly line AGVS is designed to carry a partially completed subassembly through a sequence of assembly workstations to build the product.

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Vehicle guidance and routing There are several functions that must be performed to operate any automated guided vehicle system successfully. These functions are: 1. Vehicle guidance and routing 2. Traffic control and safety 3. System management We describe these functions in this and the following two subsections. The term guidance system refers to the method by which the AGVS pathways are defined and the vehicle control systems that follow the pathways. AGV Guidance There are three common types of vehicle guidance systems:

1. Embedded guide wires 2. Paint Strips 3. Self Guidance

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1. Embedded Guide wire Method: In the guide wire method the wires are usually embedded in a small channel cut into the surface of the floor. The channel is typically about 1/8 in. wide and 1/2 in. deep. After the guide wires are installed, the channel slot is filled so as to eliminate the discontinuity in the floor. An alternative but less permanent way to install the guide wires is to tape them to the floor. A frequency generator provides the guidance signal carried in the wire. The signal is of relatively low voltage (less than 40 V), low current (less than 400 mA), and has a frequency in the range 1 to 15 kHz. This signal level creates a magnetic field along the pathway that is followed by sensors on-board each vehicle.

When the vehicle is moving along a course such that the guide wire is directly between the two coils, the intensity of the magnetic field measured by each coil will be equal. If the vehicle strays to one side or the other, or if the guide wire path curves, the magnetic field intensity at the two sensors will be different. This difference is used to control the steering motor, which makes the required changes in vehicle direction to equalize the two sensor signals, thereby tracking the defined pathway.

2. Paint Strips Method: In this system, a strip of paint is made along the pathway that should be followed by the AGV. The strip can be either painted or taped, which has a special character being recognized by an optical sensor mounted on the underside of the AGV. When the vehicle is loaded and programmed to move, the optical sensor tracks the paint strip and moves along it. Another type of paint controlled guidance system uses a paint strip which contains fluorescent particles. An ultraviolet (UV) light source on the AGV acts as the sensor, as the fluorescent particles reflect the UV LIGHT. The vehicle thus moves along the paint strip. Paint strip guided systems are useful in electronic industries, where the use of embedded wire system may cause EMI/EMC (Electromagnetic interference/compatibility) problems.

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3. Self Guidance: Self guided vehicle (SGV) is a class of AGV’s which do not require any embedded wire or paint strip to define their pathways. Such vehicles are guided by the microprocessor controlled system on board the vehicle. It uses a combination of dead reckoning and Beacons located throughout the plants, wherever the AGV has to travel. Dead reckoning refers to the capability of the vehicle to move in a route that has no defined pathways (Such as the strips or embedded wire).

The vehicle guidance is handled by the on board microprocessor, taking the inputs from the wheel rotations, and the rotation of steering motor that controls the angles to which the wheels are steered. Laser sensors on the vehicle identify the beacons and move in the desired path.

AGV Routing

Routing an AGV is the process of making the vehicle to select the right path, among the available pathways, when planned deviations are to be taken. When the vehicle approaches branched paths, each one of them leading to different destinations, the control system on the vehicle has to select the right path to reach its designated destination. There are two methods available to route the AGV in a correct pathway

1. Frequency Based Method 2. Path Switch Method

1. Frequency Based Method

In this method, the embedded guide wires are operated at different frequencies for each of the deviated path from the main line. The guide wire of the main line will have one frequency, while the deviated paths will have different frequencies. Hence, the system requires 2 or more frequency generators depending upon the number of destination and pathways required. In operation, if the AGV from main path has to deviate left or right at this junction, the sensor on the vehicle reads an identification code on the floor and selects a path which has this code. The microcomputer in the AGV is programmed/preset by the loading operator to read the particular code and select that pathway, hence the vehicle moves in the selected direction.

2. Path Switch Method In this system, all the wire guides embedded in the floor operate at the same frequency, but the routing is decided based on which path is active (Powered ON). That means, of all the available pathways, only the required pathways is kept powered ON, and all other guide ways are switched-OFF.

Traffic control and safety The purpose of traffic control for an AGVS is to prevent collisions between vehicles travelling along the same guide path in the layout. This purpose is usually accomplished by means of a control system called the blocking system. The term "blocking" suggests that a

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vehicle travelling along a given guide path is in some way prevented from hitting any vehicle ahead of it. There are several means used in commercial AGV systems to accomplish blocking. They are:

1. On-board vehicle sensing

2. Zone blocking On-board vehicle sensing and zone blocking are often used in combination to implement a comprehensive blocking system.

1. On-board vehicle sensing (sometimes called forward sensing) involves the use of some form of sensor system to detect the presence of vehicles and carts ahead on the same guide wire. The sensors used on commercial guided vehicles include optical sensors and ultrasonic systems. When the on-board sensor detects an obstacle (e.g, another guided vehicle) in front of it, the vehicle stops. When the obstacle is removed, the vehicle proceeds.

2. Zone Blocking: The concept of zone control is simple. The AGVS layout is divided into

separate zones (as shown in below Fig), and the operating rule is that no vehicle is permitted to enter a zone if that zone is already occupied by another vehicle. The length of a zone is sufficient to hold one vehicle (or a train in driverless train systems) plus an allowance for safety and other considerations. These other considerations include the number of vehicles in the system, the size and complexity of the layout, and the objective of minimizing the number of separate zone controls. When one vehicle occupies a given zone, any trailing vehicle is not allowed into that zone. The leading vehicle must proceed into the next zone before the trailing vehicle can occupy the given zone.

AGV Safety Systems Though traffic control is a means of optimising the vehicle movement and avoidance of collisions, the AGV’s are fitted with various safety devices, they include: 1. Blinking & Rotationg Red Lights 2. Beeper/Warning Bells 3. Contactless type obstacle sensor 4. Contact type obstacle sensor 5. Bumper

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AGV System Management In large organizations having number of AGV’s, Managing AGV becomes an important task. Thus some of the important methods of AGV system management are as follows:

a. On-board control panel b. Remote Call stations c. Central computer control system

a. On-board Control Panel: All the AGVs will have control panel connected to the vehicle

control processor. The control of vehicle movement using the on board control panel is the simplest method.

b. Remote Call Stations: Remote call stations can be at a centralized location or at each of

the load/unload point. In this, a sensor at the load/unload point activated (by the control panel operation at the point or from the central call station), sends a signal to a moving vehicle in that pathway so as to stop there and load/unload goods.

c. Central Computer Control System: Central Computer control system is a fully automated, computer controlled centralized AGVs management system. This is most suitable for very large industries, with a big fleet of AGVs. The Central computer has a program with a database of all the AGVs, number of stations, pathways, regular load/unloading information and remote link with the onboard control system of the AGVs. This is more sophisticated, fully automated and efficient system.

AGV Applications Automated Guided Vehicles can be used in a wide variety of applications to transport many different types of material including pallets, rolls, racks, carts, and containers. AGVs excel in applications with the following characteristics:

Repetitive movement of materials over a distance Regular delivery of stable loads Medium throughput/volume When on-time delivery is critical and late deliveries are causing inefficiency Operations with at least two shifts Processes where tracking material is important Driverless train operations Storage/distribution Assembly line operations FMS Mail delivery in offices Hospitals Raw Material Handling Work-in-Process Movement Pallet Handling Finished Product Handling Trailer Loading Roll Handling

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Analysis of Material Transport System

1. Total Cycle time per delivery per vehicle, Tc

𝑇𝑐 = 𝑇𝑙 + 𝑇𝑑 + 𝑇𝑢 + 𝑇𝑒

𝑇𝑐 = 𝑇𝑙 +𝐿𝑑

𝑉𝑐+ 𝑇𝑢 +

𝐿𝑒

𝑉𝑐

Where, Tc= Total cycle time per delivery, min/delivery

TL= Time to load, min

Td= Travel time to destination, min

Tu= Unloading time, min

Te= Empty Travel Time, min.

Ld= Distance between load and unload stations, meters

Le= Distance travelled empty, meters

Vc=Velocity of the vehicle, m/min

2. Number of Deliveries per hour per vehicle, Dh

𝐷𝑕 =60 𝐹𝑡

𝑇𝑐

Where, Ft= Traffic Factor (Fraction)

Tc= Cycle Time per Delivery

3. Number of vehicles, Ns (Without worker efficiency and other factors)

𝑁𝑣 =𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑖𝑒𝑠/𝑕𝑜𝑢𝑟

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑖𝑒𝑠/𝑕𝑜𝑢𝑟/𝑣𝑒𝑕𝑖𝑐𝑙𝑒

4. Efficiency of Handling System, Eh

Eh =

𝐿𝑑𝑉𝑐 𝑋 𝐹𝑡 . 𝐸𝑤 𝑋 𝐴

𝑇𝑐

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5. Work Load on the System, WL

WL= Dt X Tc min/hr

Where, Dt= Total Number of Deliveries Required /hr

6. Useful Time Available (in min) per vehicle per hour, TA

TA= 60 X A X Ft X Ew

Where, A= Availability Factor

Ft= Traffic Factor

Ew= Worker Efficiency

7. Number of Vehicles, Nv (with various factors considered)

𝑁𝑣 =𝑊𝐿

𝑇𝐴

Problem1:

In a plant it is proposed to use AGVs for material handling. The following data is provided:

Deliveries required= 40 deliveries/hr

Velocity of the vehicle=50m/min

Average distance to be travelled per delivery=150m

Average time to load at pickup point=1min

Average unload time at destination=1.5min

Average distance travelling empty=100m

Traffic factor=0.9

Determine a) The no. of AGVs required

b)Efficiency of the material handling system

Solution:

Given:

Dt= 40 del/hr

Vc=50 m/min

Ld= 150m

TL= 1min

Tu=1.5min

Le=100m

Ft=0.9

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To Find?

Nv=?

Eh=?

a) Number of AGVs Required, Nv

w.k.t

𝐷𝑕 =60 𝐹𝑡

𝑇𝑐



𝐿𝑒

𝑉𝑐

𝑇𝑐 = 1 +150

50+ 1.5 +

100

50

𝑻𝒄 = 𝟕. 𝟓 𝒎𝒊𝒏

𝐷𝑕 =60𝑋 0.9

7.5= 7.2 𝑑𝑒𝑙 𝑝𝑒𝑟 𝑣𝑒𝑕𝑖𝑐𝑙𝑒/𝑕𝑟

𝑁𝑣 =𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑖𝑒𝑠/𝑕𝑜𝑢𝑟

𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑖𝑒𝑠/𝑕𝑜𝑢𝑟/𝑣𝑒𝑕𝑖𝑐𝑙𝑒=

40

7.2= 5.55 𝑣𝑒𝑕𝑖𝑐𝑙𝑒𝑠 = 6 𝑣𝑒𝑕𝑖𝑐𝑙𝑒𝑠

b) Efficiency of handling system, Eh

Eh =

𝐿𝑑𝑉𝑐 𝑋 𝐹𝑡 . 𝐸𝑤 𝑋 𝐴

𝑇𝑐

Let us assume Ew & A equal to 100% or 1

Eh =

15050

𝑋0.9

7.5= 0.36 = 𝟑𝟔%

Problem 2:

An AGV is used to satisfy material flows indicated in the from-to chart in the table

below:

To: 1 2 3 4

From: 1 0/0 9L/90 7L/120 5L/75

2 5E/90 0/0 0/NA 4L/80

3 7E/120 0/NA 0/0 0/NA

4 9E/75 0/NA 0/NA 0/0

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The table shows deliveries per hour between stations (above the slash) and distances in

meters between station (below the slash). Moves indicated by ‘L’ are trips in which the

vehicle is loaded. While ‘E’ indicates moves in which the vehicle is empty. It is assumed

that availability=0.90, Traffic Factor=0.85, and worker efficiency =1.0. Speed of an AGV

=0.9m/s. If load handling time per delivery cycle=1.0 min. What is the number of

vehicles needed to satisfy the indicated deliveries per hour?

Solution:

Given:

Vc= 0.9 m/s = 54m/min

Th= 1min (=TL+Tu )

Ft= 0.85

A= 0.9

Ew= 1

To Find?

Nv= ?

Wkt,



𝐿𝑒

𝑉𝑐

We also know that (Th=TL+Tu= 1 min )

Using these relation and the data given, the cycle time and the number of deliveries for

different routes are computed as below.

Route Cycle time, min No. of deliveries

1→2→1 Tc= 1.0+(90+90)/54= 4.33 min 5

1→3→1 Tc=1.0+(120+120)/54=5.44 min 7

1→4→1 Tc=1.0+(75+75)/54=3.78 min 5

2→4→1 Tc=1.0+(80+75)/54=3.87 min 9-5=4

1→2* Tc=1.0+90/54=2.67 min 9-5=4

* Assuming vehicles on route 1→2 are used to make deliveries on route 2→4→1.

𝐴𝑣𝑎𝑒𝑟𝑎𝑔𝑒 𝑇𝑐 =5 4.33 + 7 5.44 + 4 3.87 + 4(2.67)

5 + 7 + 5 + 4 + 4

= 𝟒. 𝟏𝟗𝟐𝐦𝐢𝐧𝒑𝒆𝒓𝒅𝒆𝒍𝒊𝒗𝒆𝒓𝒚 𝒄𝒚𝒄𝒍𝒆

Total no. of deliveries,

Dt= (5+7+5+4+4)=25

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Number of AGVs, Nv

W.k.t

WL = Dt X Tc =25X4.192 min= 104.8min/hr

Useful Available time,

TA= 60 X A X Ft X Ew = 60 X 0.9 X 0.85 X 1 = 45.9 min/hr/vehicle

Therefore, 𝑁𝑣 =𝑊𝑙

𝑇𝐴=

104.8 min 𝑝𝑒𝑟 𝑕𝑟

45.9 min 𝑝𝑒𝑟 𝑣𝑒𝑕𝑖𝑐𝑙𝑒= 𝟐. 𝟐𝟖 = 𝟑 𝒗𝒆𝒉𝒊𝒄𝒍𝒆𝒔 𝒑𝒆𝒓 𝒉𝒓

Storage system performance

The performance of a storage system in accomplishing its function must be sufficient to

justify its investment and operating expense. Various measures used to assess the

performance of a storage system include:

(1) storage capacity,

(2) density,

(3) accessibility, and

(4) throughput.

In addition, standard measures used for mechanized and automated systems include

(5) utilization and

(6) reliability.

1. Storage capacity can be measured in two ways: (1) as the total volumetric space

available or (2) as the total number of storage compartments in the system available

for items or loads In many storage systems, materials are stored in unit loads that

are held in standard size containers (pallets, tote pans, or other containers). The

standard container can readily be handled, transported, and stored by the storage

system and by the material handling system that may be connected to it. Hence,

storage capacity is conveniently measured as the number of unit loads that can be

stored in the system.

2. Storage density is defined as the volumetric space available for actual storage

relative to the total volumetric space in the storage facility.

3. Accessibility refers to the capability to access any desired item or load stored in the

system. In the design of a given storage system, tradeoffs must be made between

storage density and accessibility.

4. System throughput is defined as the hourly rate at which the storage system (1)

receives and puts loads into storage and/or (2) retrieves and delivers loads 10 the

output station. In many factory and warehouse operations, there are certain periods

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of the day when the required rate of storage and/or retrieval transactions is greater

than at other times. The storage system must be designed for the maximum

throughput that will be require " during the day

5. Utilization defined as the proportion of time that the system is actually being used

for performing storage and retrieval operations compared with the time it is

available. Utilization varies throughout the day, as requirements change from hour

to hour. It is desirable to design an automated storage system for relatively high

utilization, in the range 80-90%. If utilization is too low, then the system is probably

overdesigned. If utilization is too high, then there is no allowance for rush periods or

system breakdowns.

6. Availability is a measure of system reliability, defined iI-S the proportion of time that

the system is capable of operating (not broken down) compared with the normally

scheduled shift hours. Malfunctions and failures of the equipment cause downtime.

Reasons for downtime include computer failures, mechanical breakdowns, load

jams, improper maintenance, and incorrect procedures by personnel using the

system.

Automated Storage & Retrieval System (AS/RS)

An automated storage & retrieval system (AS/RS) can be defined as a storage system that performs

storage and retrieval operations with speed and accuracy under a defined degree of automation.

The basic AS/R$ consists of a rack structure for storing loads and a storage/retrieval mechanism

whose motions are linear (x-y-z motions).

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Components of an AS/RS:

1. Storage structure

2. S/R machine

3. Storage modules (Eg: Pallets for unit loads)

4. One or more pickup and deposit stations.

In addition, a control system is required to operate the AS/RS.

S/R (storage /Retrieval) machines are used to deliver materials to the storage racks and to

retrieve materials from the racks. Each AS/RS aisle has one or more input/output stations

where materials are delivered into the storage system or moved out of the system. The

input/output stations are called pickup-and-deposit (P&D) stations in AS/RS terminology

P&D stations can be manually operated or interfaced to some form of automated handling

system such as a conveyor or AGVs.

AS/RS Types

Several important categories of automated storage/ retrieval system can be distinguished.

The following are the principal types:

Unit load AS/RS: The unit load AS/RS is typically a large automated system designed

to handle unit loads stored on pallets or in other standard containers. The system is

computer controlled, and the SIR machines are automated and designed to handle

the unit load containers. A unit load AS/RS is shown in above figure.

Deep-lane AS/RS: The deep-lane AS/RS is a high-density unit load storage system

that is appropriate when large quantities of stock are stored, but the number of

separate steer types (SKUs) is relatively small.

Mini/Load AS/RS: This storage system is used to handle small loads (individual parts

or supplies) that are contained in bins or drawers in the storage system. The SIR

machine is designed to retrieve the bin and deliver it to a P&D station at the end of

the aisle so that individual items can be withdrawn from the bins. The P&D station is

usually operated by a human worker. The bin or drawer must then be returned to its

location in the system. A miniload AS/R system is generally smaller than a unit load

AS/RS and is often enclosed for security of the items stored.

Man onboard AS/RS: In this system, a human operator rides on the carriage of the

S/R machine. Whereas the mini load system delivers an entire bin to the end-of-aisle

pick station and must return it subsequently to its proper storage compartment. The

man on- board system permits individual item, to he picked directly at their storage

locations. This offers an opportunity to increase system throughput.

CAD/CAM/CIM Unit IV


Automated item retrieval system: These storage systems are also designed for

retrieval of individual items or small product cartons: however. The items are stored

in lanes rather than bins or drawers. When an item is retrieved. It is pushed from its

lane and drops onto a conveyor for delivery to the pickup station. The operation is

somewhat similar to a candy vending machine, except that an item retrieval system

has more storage lane, and a conveyor to transport items to a central location.

Vertical lift storage modules (VLSM): These are also called vertical lift automat ed

storage/retrieval systems (VL-ASJRS). AIl of the preceding ASJRS types are designed

around a horizontal aisle. The same principle of using a center aisle to access loads is

used except that the aisle is vertical.

Carousel Storage System

A carousel storage system consists of a series of bins or baskets suspended from an

overhead chain conveyor that revolves around a long oval rail system, as depicted in below

figure

Carousels can be classified as horizontal or vertical. The more common horizontal

connguranon. as shown in above fig, comes in a variety of sizes, ranging between 3 m (10ft)

and 30 m (100 ft) in length. Carousels at the upper end of the range have higher storage

density, but the average access cycle time is greater. Accordingly most carousels are 10-16

m (30-50 ft) long to achieve a proper balance between these competing factors.

CAD/CAM/CIM Unit IV


The carousel can be either an overhead system (called a top-driven unit) or a floor-mounted

system (called a bottom-driven unit). In the top-driven unit. A motorized pulley system is

mounted at the top of the framework and drives an overhead trolley system. The bins are

suspended from the trolleys. In the bottom driven unit, the pulley drive system is mounted

at the base of the frame, and the trolley system rides on a rail in the base. This provides

more load-carrying capacity for the carousel storage system. It also eliminates the problem

of dirt and oil dripping from the overhead trolley system in top-driven systems.

Vertical carousels are constructed to operate around a vertical conveyor loop. They occupy

much less floor space than the horizontal configuration, but require sufficient overhead

space. The ceiling uf the building limits the height of vertical carousels, and therefore their

storage capacity is typically lower than for the average horizontal carousel.

Work In Process (WIP) Storage System

Work-in-process (WIP) storage is a more recent application of automated storage

technology. While it is desirable to minimize the amount of work-in-process, it is also

important to effectively manage WIP that unavoidably does exist in a factory, Automated

storage system either automated storage/retrieval systems or carousel systems, represent

an efficient way of storing materials between processing steps, particularly in batch and job

shop production, In high production, work-in-process is often carried between operations b

conveyor systems, which thus serves both storage and transport functions.

The merits of an automated WIP storage system for batch and job shop production can best

be seen by comparing it with the traditional way of dealing with work-in-process. The typical

factory contains multiple work cells, each performing its own processing operations on

different parts. At each cell orders consisting of one or more parts are waiting on the plant

floor to be processed, while other completed orders are waiting to be moved 10 the next

cell in the sequence. It is not unusual for a plant engaged in batch production to have

hundreds of orders in progress simultaneously, all of which represent work-in process.

The disadvantages of keeping this entire inventory in the plant include: (1) time spent

searching for orders, (2) parts or even entire orders becoming temporarily or permanently

lost, sometimes resulting in repent orders to reproduce the lost parts, (3) orders not being

processed according to their relative priorities at each cell, and (4) orders spending too

much time in the factory, causing customer deliveries to be late. These problems indicate

poor control of work-in-process.

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Automated storage systems help to regain control over WIP. Reasons that justify the

installation of automated storage systems for work-in-process include:

1. Buffer Storage in Production

2. Support of Just in Time (JIT) delivery

3. Kitting of parts for assembly

4. Compatible with automated part identification system

5. Computer control & tracking of materials

6. Support for factory wide automation

CAD/CAM/CIM Unit IV


Group Technology (GT) & Flexible Manufacturing

System (FMS)

Group technology is a manufacturing philosophy in which similar parts are identified and

grouped together to take advantage of their similarities in design and production. Similar

parts are arranged into part families. Where each part family possesses similar design

And/or manufacturing characteristics. Grouping the production equipment into machine

cells, where each cell specializes in the production of a part family is called cellular

manufacturing.

Conditions for the application of Group Technology (GT)

The plant currently uses traditional batch production and a process type layout and

this result in much material handling effort, high in-process inventory, and long

manufacturing lead times.

The parts can be grouped into part families. This is a necessary condition, each

machine cell is designed to produce a given part family, or limited collection of part

families, so it must be possible to group parts made in the plant into families.

However, it would be unusual to find a mid-volume production plant in which parts

could not be grouped into part families.

There are two major tasks that a company must undertake when it implements group

technology. These two tasks represent significant obstacles to the application of GT.

1. Identifying the part families. If the plant makes 10,000 different parts, reviewing all

of the part drawings and grouping the parts into families is a substantial task that

consumes a significant amount of time.

2. Rearranging production machines into machine cells. It is time consuming and

costly to plan and accomplish this rearrangement and the machines are not

producing during the changeover.

Benefits of Group Technology

GT promotes standardization of tooling, fixturing. and setups.

Material handling is reduced because parts are moved within a machine cell rather

than within the entire factory.

Process planning and production scheduling are simplified

Setup times are reduced, resulting in lower manufacturing lead times.

Work-in-process is reduced.

Worker satisfaction usually improves when workers collaborate in a OT cell.

Higher quality work is accomplished using group technology.

CAD/CAM/CIM Unit IV


PART FAMILIES

A part family is a collection of parts that are similar either because of geometric shape and

size or because similar processing steps are required in their manufacture. The parts within

a family are different, but their similarities are close enough to merit their inclusion as

members of the part family.

The two parts shown in below fig are very similar in terms of geometric design, but quite

different in terms of manufacturing because of differences in tolerances, production

quantities, and material.

The biggest single obstacle in changing over to group technology from a conventional

production shop is the problem of grouping the parts into families. There are three general

methods for solving this problem. All three are time consuming and involve the analysis of

much data by properly trained personnel. The three methods are;

(1) Visual inspection,

(2) Parts classification and Coding, and

(3) Production flow analysis.

The visual inspection method is the least sophisticated and least expensive method. It

involves the classification of parts into families by looking at either the physical parts or

their photographs and arranging them into groups having similar features.

PARTS CLASSIFICATION & CODING

This is the most time consuming of the three methods. In parts classification and coding,

similarities among parts are identified, and these similarities are related in a coding system.

Two categories of part similarities can be distinguished:

(1) Design attributes, which are concerned with part characteristics such as geometry, size,

and material; and

CAD/CAM/CIM Unit IV


(2) Manufacturing attributes, which consider the sequence of processing steps required to

make a part. While the design and manufacturing attributes of a part are usually correlated,

the correlation is less than perfect. Accordingly, classification and coding systems are

devised to include both a part's design attributes and its manufacturing attributes.

Reasons for using a coding scheme include:

Design retrieval. A designer faced with the task of developing a new part can use a

design retrieval system to determine if a similar part already exists. A simple change

in an existing part would take much less time than designing a whole new part from

scratch.

Automated process planning. The part code for a new part can be used to search for

process plans for existing parts with identical or similar codes.

Machine cell design. The part codes can be used to design machine cells capable of

producing all members of a particular part family, using the composite part concept.

To accomplish parts classification and coding requires examination and analysis of the

design and/or manufacturing attributes of each part. The examination is sometimes done by

looking in tables to match the subject part against the features described and diagrammed

in the tables. An alternative and more-productive approach involves interaction with a

computerized classification and coding system, in which the user responds to questions

asked by the computer. On the basis of the responses, the computer assigns the code

number to the part. Whichever method is used, the classification results in a code number

that uniquely identifies the part's attributes.

PRODUCTION FLOW ANALYSIS

Production flow analysis (PFA) is a method for identifying part families and associated

machine groupings that uses the information contained on production route sheets rather

than on part drawings. Work parts with identical or similar routings are classified into part

families. These families can then be used to form logical machine cells in a group technology

layout. Since PFA uses manufacturing data rather than design data to identify part families,

it can overcome two possible anomalies that can occur in parts classification and coding.

First, parts whose basic geometries are quite different may nevertheless require similar or

even identical process routings. Second, parts whose geometries are quite similar may

nevertheless require process routings that are quite different.

PFA consists of the following steps:

1. Data collection. The minimum data needed in the analysis are the part number and

operation sequence, which is contained in shop documents called route sheets or

operation sheets or some similar name. Each operation is usually associated with a

CAD/CAM/CIM Unit IV


particular machine, so determining the operation sequence also determines the

machine sequence. Additional data such as lot size, time standards, and annual

demand might be useful for designing machine cells of the required production

capacity.

2. Sortation of process routings. In this step, the parts are arranged into groups

according to the similarity of their process routings.

3. PFA Chart: To create a PFA chart which is a tabulation of the process or machine

code numbers for all of the part packs.

4. Cluster analysis. From the pattern of data in the PFA chart. related groupings are

identified and rearranged into a new pattern that brings together packs with similar

machine sequences.

CELLULAR MANUFACTURING

Cellular manufacturing is an application of group technology in which dissimilar machines or

processes have been aggregated into cells, each of which is dedicated to the production of a

part or product family or a limited group of families. The typical objectives in cellular

manufacturing are similar to those of group technology:

To shorten manufacturing lead times,

To reduce work-in-process inventory

To improve quality.

To simplify production scheduling.

To reduce setup times.

MACHINE CELL DESIGN

Design of the machine cell is critical in cellular manufacturing. The cell design determines to

a great degree the performance of the cell.

Types of Machine Cells and Layouts

GT manufacturing cells can be classified according to the number of machines and the

degree to which the material flow is mechanized between machines. Four common GT cell

configurations are as follows:

1. Single machine cell

2. Group machine cell with manual handling

3. Group machine cell with semi-integrated handling

4. Flexible manufacturing cell or flexible manufacturing system

Single machine cell consists of one machine plus supporting fixtures and tooling. This type

of cell can be applied to work parts whose attributes allow them to be made on one basic

type of process, such as turning or milling.

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Group machine cell with manual handling is an arrangement of more than one machine

used collectively to produce one or more part families. There is no provision for mechanized

parts movement between the machines in the cell. Instead, the human operators who run

the cell perform the material handling function. The cell is often organized into a U-shaped

layout, as shown in below fig.

Group machine cell with semi-integrated handling uses a mechanized handling system,

such as a conveyor, to move parts between machines in the cell.

The flexible manufacturing system (FMS) combines a fully integrated material handling

system with automated processing stations. The FMS is the most highly automated of the

group technology machine cells.

CAD/CAM/CIM Unit IV


FLEXIBLE MANUFACTURING SYSTEMS

A flexible manufacturing system (FMS) is a highly automated OT machine cell consisting of a

group of processing workstations (usually CNC machine tools), interconnected by an

automated material handling and storage system, and controlled by a distributed computer

system. The reason the FMS is called flexible is that it is capable of processing a variety of

different part styles simultaneously at the various workstations, and the mix of part styles

and quantities of production can be adjusted in response to changing demand patterns. The

FMS is most suited for the mid-variety, mid-volume production range.

Components of FMS

There are several basic components of an FMS:

(1) Workstations,

(2) Material handling and storage system, and

(3) Computer control system.

In addition, even though an FMS is highly automated,

(4) People are required to manage and operate the system.

Following are the types of workstations typically found in an FMS.

1. Load/Unload Stations: The load/unload station is the physical interface between the

FMS and the rest of the factory. Raw workparts enter the system at this point, and

finished parts exit the system from here. Loading and unloading can be

accomplished either manually or by automated handling systems. Manual loading

and unloading is prevalent in most FMSs today. The load/unload station should be

ergonomically designed to permit convenient and safe movement of work parts. For

parts that are too heavy to lift by the operator, mechanized cranes and other

handling devices are installed to assist the operator.

CAD/CAM/CIM Unit IV


2. Machining Stations: The most common applications of FMSs arc machining

operations, the workstations used in these systems are therefore predominantly

CNC machine tools. Most common is the CNC machining centre.

3. Assembly: Some FMSs are designed to perform assembly operations. Flexible

automated assembly systems are being developed to replace manual labor in the

assembly of products typically made in batches. Industrial robots are often used as

the automated workstations in these flexible assembly systems. They can be

programmed to perform tasks with variations in sequence and motion pattern to

accommodate the different product styles assembled in the system.

4. Other Stations and Equipment: Inspection can be incorporated into an FMS either

by including, an inspection operation at a processing workstation or by including a

station specifically designed for inspection. Coordinate measuring machines (CMM)

or Machine Vision can be used for the purpose of Inspection.

Material Handling and Storage System

Functions of the Handling System: The material handling and storage system in an FMS

performs the fol1owing functions:

1. Random, independent movement of workparts between stations: This means that

parts must be capable of moving from anyone machine in the system to any other

machine. To provide various routing alternatives for the different parts and to make

machine substitutions when certain stations are busy.

2. Handle a variety of workpart configurations: For prismatic parts, this is usually

accomplished by using modular pallet fixtures in the handling system. The fixture is

located on the top face of the pallet and is designed to accommodate different part

configurations by means of common components, quick-change features, and other

devices that permit a rapid build-up of the fixture for a given part. For rotational

parts, industrial robots are often used to load and unload the turning machines and

to move parts between stations.

3. Temporary storage: The number of parts in the FMS will typically exceed the

number of parts actually being processed at any moment. Thus, each station has a

small queue of parts waiting to be processed. This helps to increase machine

utilization.

4. Convenient access for loading and unloading workparts: The handling system must

include locations for load/unload stations.

5. Compatible with computer control: The handling system must be capable of being

controlled directly by the computer system to direct it to the various workstations,

load/unload stations, and storage areas.

CAD/CAM/CIM Unit IV


Material Handling Equipment w.r.t FMS:

The types of material handling systems used to transfer parts between stations in an FMS

include a variety of conventional material transport equipment (which were discussed

already at the beginning), inline transfer mechanism, Industrial Robots.

The material handling function in an FMS is often shared between two systems:

(1) A primary handling system

(2) A secondary handling system.

The primary handling system establishes the basic layout of the FMS and is responsible for

moving work parts between stations in the system.

The secondary handling system consists of transfer devices, automatic pallet changers. And

similar mechanisms located at the workstations in the FMS. The function of the secondary

handling system is to transfer work from the primary system to the machine tool or other

processing station and to position the parts with sufficient accuracy and repeatability to

perform the processing or assembly operation. Other purposes served by the secondary

handling system include: (1) reorientation of the workpart if necessary to present the

surface that is to be processed and (2) buffer storage of parts to minimize work change time

and maximize station utilization.

COMPUTER CONTROL SYSTEM

The FMS includes a distributed computer system that is interfaced to the workstations,

material handling system, and other hardware components. A typical FMS computer system

consists of a central computer and microcomputers controlling the individual machines and

other components. The central computer coordinates the activities of the components to

achieve smooth overall operation of the system. Functions performed by the FMS computer

control system can be grouped into the following categories:

1. Workstation control: In a fully automated FMS, the individual processing or

assembly stations generally operate under some form of computer control. For a

machining system, CNC is used to control the individual machine tools.

2. Distribution of control instructions to workstations: Some form of central

intelligence is also required to coordinate the processing at individual stations. In a

machining FMS, part programs must be downloaded to machines, and Direct

Numerical Control System (DNC) is used for this purpose, The DNC system stores the

programs, allows submission of new programs and editing of existing programs as

needed.

3. Production control: The part mix and rate at which the various parts are launched

into the system must be managed. Input data required for production control

CAD/CAM/CIM Unit IV


includes desired daily production rates per part. Numbers of raw work parts

available, and number of applicable pallets.

4. Traffic control: This refers to the management of the primary material handling

system that moves work parts between stations. Traffic control is accomplished by

actuating switches at branches and merging points. Stopping parts at machine tool

transfer locations, and moving pallets to load/unload stations.

5. Shuttle control: This control function is concerned with the operation and control of

the secondary handling system at each workstation. Each shuttle must be

coordinated with the primary handling system and synchronized with the operation

of the machine tool it serves,

6. Workpiece monitoring: The computer must monitor the status of each cart and/or

pallet in the primary and secondary handling systems as well as the status of each of

the various workpiece types.

7. Tool control: In a machining system, cutting tools are required. Tool control is

concerned with managing two aspects of the cutting tools such as Tool location &

Tool Life Monitoring.

8. Performance monitoring and reporting: The computer control ~ystem i,

programmed to collect data on the operation and performance of the FMS. This data

is periodically summarized, and reports are prepared for management on system

performance.

9. Diagnostics: This function is available to a greater or lesser degree on many

manufacturing systems to indicate the probable source of the problem when a

malfunction occurs. It can also be used to plan preventive maintenance in the

system and to identify Impending failures. The purpose of the diagnostics function is

to reduce breakdowns and downtime and increase availability of the system.

Human Resource in FMS

One additional component in the FMS is human labor. Humans are needed to manage the

operations of the FMS. Functions typically performed by humans include:

(1) Loading raw workparts into the system,

(2) Unloading finished parts (or assemblies) from the system.

(3) Changing and setting tools.

(4) Equipment maintenance and repair,

(5) NC part programming in a machining system,

(6) Programming and operating the computer system, and

(7) Overall management of the system.

CAD/CAM/CIM Unit IV


FMS PLANNING

The initial phase of FMS planning must consider the parts that will he produced by the

system. The issues are similar to those in GT machine cell planning, they include:

Part family considerations: Any FMS must be designed to process a limited range of

part (or product) styles.

Processing requirements: The types of parts and their processing requirements

determine the types of processing equipment that will be used in the system. In

machining applications. Rotational parts are produced by machining centers, milling

machines, and like machine tools: rotational parts are machined by turning centers

and similar equipment.

Physical characteristics of the workparts: The size and weight of the parts

determine the size of the machines at the workstations and the size of the material

handling system that must be used.

Production volume: Quantities to be produced by the system determine how many

machines Will be required. Production volume is also a factor in selecting the most

appropriate type of material handling equipment for the system.

After the part family, production volumes, and similar part issues have been decided. Design

of the system can proceed. Important factors that must be specified in FMS design include:

Types of workstations.

Variations in process routings.

Type of Material handling system.

Work-in-process and storage capacity.

Tooling.

Pallet fixtures.

Computerized Manufacturing Planning System

PROCESS PLANNING

Process planning involves determining the most appropriate manufacturing and assembly

processes and the sequence in which they should be accomplished to produce a given part

or product according to specifications set forth in the product design documentation.

COMPUTER AIDED PROCESS PLANNING (CAPP)

There is much interest by manufacturing firms in automating the task of process planning

using computer-aided process planning (CAPP) systems.

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The benefits derived from computer-automated process planning include the following:

1. Process rationalization and standardization: Automated process planning leads to

more logical an consistent process plans than when process planning is done

completely manually. Standard plans tend to result in lower manufacturing costs and

higher product quality.

2. Increased productivity of process planners: The systematic approach and the

availability of standard process plans in the data files permit more work to be

accomplished by the process planners.

3. Reduced lead time for process planning: Process planners working with a CAPP

system can provide route sheets in a shorter lead time compared to manual

preparation.

4. Improved legibility: Computer-prepared route sheets are neater and easier to read

than manually prepared route sheets.

5. Incorporation of other application programs: The CAPP program can be interfaced

With other application programs, such as cost estimating and work standards.

Types of CAPP

Computer-aided process planning systems are designed around two approaches. These

approaches are called:

(1)Retrieval CAPP systems and

(2) Generative CAPP systems.

1. Retrieval CAPP System: A retrieval CAPP system, also called a variant CAPP system, is

based on the principles of group technology (GT) and parts classification and coding, In

this type of CAPP, a standard process plan (route sheet) is stored in computer files for

each part code number. The standard route sheets are based on current part routings in

use in the factory or on an ideal process plan that has been prepared for each family.

A retrieval CAPP system operates as illustrated in Figure 25.3. Before the system can be

used for process planning, a significant amount of information must be compiled and

entered into the CAPP data files. It consists of the following steps:

(1) Selecting an appropriate classification and coding scheme for the company,

(2) Forming part families for the parts produced by the company; and

(3) Preparing standard process plans for the part families.

It should be mentioned that steps (2) and (3) continue as new parts are designed and

added to the company' design data base.

After the preparatory phase has been completed, the system is ready for use, for a new

component for which the process plan is to be determined. the first step is to derive the

GT code number for the part. With this code number, a search is made of the part family

file to determine if a standard route sheet exists for/he given part code. If the file

CAD/CAM/CIM Unit IV


contains a process plan for the part it is retrieved (hence. the word "retrieval" for this

CAPP system} and displayed for the user. The standard process plan is examined to

determine whether any modifications are necessary. It might be that although the new

part has the same code number. There are minor differences in the processes required

to make it. The user edits the standard plan accordingly. This capacity to alter an existing

process plan is what gives the retrieval system its alternative name: variant CAPP

system.

If the file does not contain a standard process plan for the given code number, the user

may search the computer file for a similar or related code number for which a standard

route sheer does exist. Either by editing an existing process plan, or by starting from

scratch, the user prepares the route sheet for the new part. This route sheet becomes

the standard process plan for the new part code number The process planning session

concludes with the process plan formatter, which prints out the route sheet in the

proper format.

CAD/CAM/CIM Unit IV


2. Generative CAPP System: Generative CAPP systems represent an alternative approach

to automated process planning. Instead of retrieving and editing an existing plan

contained in a computer data base, a generative system creates the process plan based

on logical procedures similar to the procedures a human planner would use.

In a fully generative CAPP system, the process sequence is planned without human

assistance and without a set of predefined standard plans. The problem of designing a

generative CAPP system is usually considered part of the field of expert systems, a

branch of artificial intelligence. An expert system is a computer program that is capable

of solving complex problems that normally require a human with years of education and

experience.

Process planning fits within the scope of this definition. There are several ingredients in

a fully generative process planning system.

1. Creation of Knowledge Base

First the technical knowledge of manufacturing and the logic used by successful process

planners must be captured and coded into a computer program. In an expert system

applied to process planning, the knowledge and logic of the human process planners is

incorporated into a so-called "knowledge base."The generative CAPP system then uses

that knowledgebase to solve process planning problems {i.e., create route sheets).

2. Computer Compatible part description CAD model

The second ingredient in generative process planning is a computer-compatible

description of the part to be produced. This description contains all of the pertinent data

and illustration needed to plan the process sequence, two possible ways of providing

this description are:

(1) The geometric model of the part that is developed on a CAD system during product

design and

(2) (2) A GT code number of the part that defines the part features in significant detail.

3. Inference Engine

The third ingredient in a generative CAPP system is the capability to apply the process

knowledge and planning logic contained in the knowledge base to a given part

description. In other words, the CAPP system uses its knowledge base to solve a specific

problem planning the process for a new part. This problem-solving procedure is referred

to as the "inference engine” in the terminology of expert systems. By using its

knowledge base and inference engine, the CAPP system synthesizes a new process plan

from scratch for each new part it is presented.

CAD/CAM/CIM Unit IV


PRODUCTION PLANNING AND CONTROL

Production planning and control (PPC) is concerned with the logistics problems that are

encountered in manufacturing, that is, managing the details of what and how many

products to produce and when, and obtaining the raw materials, parts, and resources to

produce those products. In a very real sense.PPC is the integrator in computer integrated

manufacturing.

PRODUCTION PLANNING

Production planning IS concerned with: (1) deciding which products to make, how many of

each, and when they should he completed: (2) scheduling the delivery and/or production of

the pans and products: and (3) planning the manpower and equipment resources needed to

accomplish the production plan. Activities within the scope of production planning include:

1. Aggregate production planning. This involves planning the production output levels

for major product lines produced by the firm. These plans must be coordinated

among various functions in the firm, including product design, production, marketing

and sales

2. Master production planning. The aggregate production plan must be converted into

a master production schedule (MPS) which is a specific plan of the quantities to be

produced of individual models within each product line.

3. Material requirements planning (MRP) is a planning technique, usually implement

by computer- that translates the MPS of end products into a detailed schedule for

the raw materials and parts used in those end products

4. Capacity planning is concerned with determining the labor and equipment resources

needed to achieve the master schedule.

Production planning activities divide into two stages: (1) aggregate planning which results in

the MPS, and (2) detailed planning. This includes MRP and capacity planning.

PRODUCTION CONTROL is concerned with determining whether the necessary resources to

implement the production plan have been provided, and if not. It attempts to take

corrective action to address the deficiencies. This Includes

1. Shop floor control. Shop floor control systems compare the progress and status of

production orders in the factory to the production plans (MPS and parts explosion

accomplished by MRP)

2. Inventory control: Inventory control includes it variety of techniques fur managing

the inventory of a firm. One of the important tools in inventory control is the

Economic order quantity formula.

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3. Manufacturing resource planning. Also known as MRP II. manufacturing resource

planning combines MRP and capacity planning as well as shop floor control and

other functions related to PPC.

4. Just-In-time production systems. The term "just-in-time" refers to a scheduling

discipline in which materials and parts are delivered to the next work cell or

production line station just prior to their being used. This type of discipline tends to

reduce inventory and other kinds of waste in manufacturing.

MATERIAL REQUIREMENT PLANNING (MRP)

Material requirements planning (MRP) is a computational technique that converts the

master schedule for end products into a detailed schedule for the raw materials and

components used in the end products.

The detailed schedule identifies the quantities of each raw material and component item. It

also indicates when each item must be ordered and delivered to meet the master schedule

for final products. MRP is often thought of as a method of inventory control. Even though it

is an effective tool for minimizing unnecessary inventory investment, MRP is also useful in

production scheduling and purchasing of material.

Inputs to MRP System:

To function the MRP program must operate on data contained in several files. These files

serve as inputs to the MRP processor they are:

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(1) MPS,

(2) Bill of materials (BOM) file and other engineering and manufacturing data. And

(3) Inventory record file

1. The MPS lists what end product, and how many of each are to be produced and

when they are to be ready for shipment.

2. The bill of materials (BOM) file is used to compute the raw material and component\

requirements for end products listed in the master schedule. It provides information

on the product structure by listing the component parts and subassemblies that

make up each product. The structure of an assembled product can be illustrated in

below fig

3. The inventory record file is referred to as the item master file in a computerized

inventory system. The types of data contained in the inventory record are divided

into three segments:

a. Item master data: This provides the item's identification (part number) and

other data about the part such as order quantity and lead times.

b. Inventory status: This gives a time-phased record of inventory status. In

MRP, it is important to know not only the current level of inventory, but also

any future changes that will occur against the inventory.

c. Subsidiary data: The third file segment provides subsidiary data such as

purchase orders, scrap or rejects, and engineering changes.

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MRP Outputs & Benefits

The MRP program generates a variety of outputs that can be used in planning and managing

Plant operations. The outputs include:

(1) Planned order releases. Which provide the authority to place orders that have heen

planned by the MRP system

(2) Report of planned order releases in future period,

(3) Rescheduling notices, indicating changes in due dates for open orders:

(4) Cancelation notices, indicating that certain open orders have been cancelled because of

changes in the MPS;

(5) Reports on inventory status;

(6) Performance reports of various types. Indicating costs. Item usage, actual versus planned

lead times and so on:

(7) Exception reports. Showing deviations from the schedule, orders that are overdue,

scrap, and so on: and

(8) inventory forecasts. Indicating projected inventory levels in future periods

Benefits:

Benefits reported by users include the following

(1)Reduction in inventory,

(2) Quicker response to changes in demand than is possible with a manual requirements

planning system.

(3)Reduced setup and product changeover costs,

(4) Better machine utilization,

(5) Improved capacity to respond to changes in the master Schedule, and

(6) As an aid in developing the master schedule.

CAPACITY PLANNING

Capacity planning is concerned with determining what labor and equipment resources are

required to meet the current MPS as well as long-term future production needs of the firm

Capacity planning also serves to identify the limitations of the available production

resources so that an unrealistic master schedule is not planned.

Capacity planning is typically accomplished in two stages, as indicated in Figure below

1. when the MPS is established: and

2. when the MRP computations are done.

In the MPS stage. A rough-cut capacity planning (RCCP) calculation is made to assess the

feasibility of the master schedule. Such a calculation indicates whether there is a significant

violation of production capacity in the MPS. On the other hand, if the calculation shows no

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capacity violation, neither does it guarantee that the production schedule can be met. This

depends on the allocation of work orders to specific work cell in the plant.

Accordingly a second capacity calculation is made at the MRP schedule is prepared called

capacity requirements planning (CRP). These detailed calculations determine, whether

there is sufficient production capacity in the individual departments and work cells to

complete the specific parts and assemblies that have been scheduled by MRP. If the

schedule is not compatible with capacity, then adjustments must be made either in plant

capacity or in the master schedule.

Capacity adjustments can be divided into short term adjustments and long-term

adjustments. Capacity adjustments for the short term include:

1. Employment levels: Employment in the plant can be increased or decreased in

response to changes in capacity requirements,

2. Temporary workers: Increases in employment level can also be made by using

worker from a temporary agency. When the busy period is passed, these workers

move to positions at other companies where their services are needed

3. Number of work shifts: The number of shifts worked per production period can be

increased or decreased.

4. Labor hour. The number of labor hours per shift can be increased or decreased,

through the use of overtime or reduced hours.

5. Inventory stockpiling. This tactic might be used to maintain steady employment

levels during slow demand periods

6. Order backlongs: Deliveries of the product to the customer could be delayed during

busy periods when production resources are insufficient to keep up with demand.

7. Subcontracting: This involves the letting of jobs to other shops during busy periods.

or the taking in of extra work during slack periods.

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Capacity planning adjustments for the long term include possible changes in production

capacities that generally require long lead times. These adjustments include the following

types of decisions

1. New equipment Investments: This involves investing in more machines or more

productive machines to meet increased future production requirements, or investing

in new types of machines to match future changes in product design.

2. New plant construction: Building a new factory represents a major investment for

the company. However. it also represents a significant increase in production

capacity for the firm.

3. Purchase of existing plants from other companies: Acquisition of existing

companies. This may be done to increase productive capacity). However. There are

usually more important reasons for taking over an existing company, for example, to

achieve economies of scale that result from increasing market share and reducing

staff.

4. Plant closings: This involves the closing of plants that will not be needed in the

future.

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