introduction to manufacturing system and cnc machines

By

V. THULASIKANTHAssistant Professor

Mechanical Engineering Department

Manufacturing Systems & CNC Machines

Email Id: [email protected]

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mailto:[email protected]

Manufacturing SystemManufacturing Systemwhat is system.what is system.A system has number of components, A system has number of components, combined together to achieve a goal.combined together to achieve a goal.

ManufacturingManufacturing is conversion of raw is conversion of raw material into final goods and services.material into final goods and services.

Any manufacturing organization can Any manufacturing organization can also be called as a Production system.also be called as a Production system.

Manufacturing SystemManufacturing System There are 3 main components in a There are 3 main components in a

manufacturing system.manufacturing system.

INPUT TransformationProcess OUTPUT

ManufacturingManufacturing Production System - Production System -

DefinitionDefinition Collection of people, equipment, Collection of people, equipment,

and procedures organized to and procedures organized to accomplish the manufacturing accomplish the manufacturing operations of a company. operations of a company.

AimAim: : identify and locate the most identify and locate the most efficient method to produce a product.efficient method to produce a product.

ObjectiveObjective: reduce time to market, : reduce time to market, increase quality, reduce costincrease quality, reduce cost

Manufacturing SystemsManufacturing Systems

Manufacturing Support SystemsManufacturing Support SystemsManufacturing SystemsManufacturing Systems

Manufacturing SystemsManufacturing SystemsRelationship between Manufacturing and its Support systemsRelationship between Manufacturing and its Support systems

Manufacturing SystemsManufacturing Systems

Components ofComponents ofManufacturing SystemManufacturing System

Production MachinesProduction Machines

Material Handling SystemMaterial Handling System

Computer Control SystemComputer Control System

Human ResourcesHuman Resources

Production MachinesProduction Machines In virtually all modern manufacturing systems, In virtually all modern manufacturing systems,

most of the actual processing or assembly most of the actual processing or assembly work is accomplished by machines or with the work is accomplished by machines or with the aid of toolsaid of tools

Classification of production machines:Classification of production machines:1.1. Manually operated machines are controlled or Manually operated machines are controlled or

supervised by a human workersupervised by a human worker2.2. Semi-automated machines perform a portion of the Semi-automated machines perform a portion of the

work cycle under some form of program control, work cycle under some form of program control, and a worker tends the machine the rest of the and a worker tends the machine the rest of the cyclecycle

3.3. Fully automated machines operate for extended Fully automated machines operate for extended periods of time with no human attentionperiods of time with no human attention

Manually Operated MachineManually Operated Machine

Manually operated machines are controlled or supervised by a human worker. The machine provides the power for the operation and the worker provides the control. The entire work cycle is operator controlled.

Semi-Automated MachineSemi-Automated Machine

A semi-automated machine performs a portion of the work cycle under some form of program control, and a worker tends to the machine for the remainder of the cycle. Typical worker tasks include loading and unloading parts

Fully-Automated MachineFully-Automated Machine

Machine operates for extended periods (longer than one work cycle) without worker attention (periodic tending may be needed).

Material Handling SystemMaterial Handling System In most manufacturing systems that process In most manufacturing systems that process

or assemble discrete parts and products, the or assemble discrete parts and products, the following material handling functions must be following material handling functions must be provided:provided:1.1. Loading work units at each stationLoading work units at each station2.2. Positioning work units at each stationPositioning work units at each station3.3. Unloading work units at each stationUnloading work units at each station4.4. Transporting work units between stations in multi-Transporting work units between stations in multi-

station systemsstation systems5.5. Temporary storage of work unitsTemporary storage of work units

Loading - Positioning - Loading - Positioning - unloading unloading

Loading involves moving the work unit into Loading involves moving the work unit into production m/c or processing equipment.production m/c or processing equipment.

Positioning is accomplished by workholderPositioning is accomplished by workholder

A workholder is a device that accurately locates and A workholder is a device that accurately locates and orients the part for the operation and resist any orients the part for the operation and resist any forces that may occurring during processingforces that may occurring during processing

After completion of production process the work After completion of production process the work unit may Unloaded.unit may Unloaded.

Work Transport Between Work Transport Between StationsStations

Two general categories of work transport Two general categories of work transport in multi-station manufacturing systems:in multi-station manufacturing systems:1.1. Fixed routingFixed routing

Work units always flow through the same Work units always flow through the same sequence of workstationssequence of workstations

Most production lines exemplify this categoryMost production lines exemplify this category2.2. Variable routingVariable routing

Work units are moved through a variety of Work units are moved through a variety of different station sequencesdifferent station sequences

Most job shops exemplify this categoryMost job shops exemplify this category

(a) Fixed Routing and (a) Fixed Routing and (b) Variable Routing(b) Variable Routing

Computer Control SystemComputer Control System Typical computer functions in a manufacturing system:Typical computer functions in a manufacturing system:

Communicate instructions to workersCommunicate instructions to workers (receive processing or (receive processing or assembly instructions for the specific work unit)assembly instructions for the specific work unit)

Download part programs to computer-controlled machinesDownload part programs to computer-controlled machines Control material handling systemControl material handling system Schedule productionSchedule production Failure diagnosis when malfunctions occurFailure diagnosis when malfunctions occur and preventive and preventive

maintenancemaintenance Safety monitoringSafety monitoring (protect both the human worker and (protect both the human worker and

equipment)equipment) Quality controlQuality control (detect and reject defective work units (detect and reject defective work units

produced by the system)produced by the system) Operations managementOperations management (manage overall operations) (manage overall operations)

Human WorkersHuman Workers In manufacturing systems, humans perform some In manufacturing systems, humans perform some

or all of the value added work that is or all of the value added work that is accomplished on the parts or products accomplished on the parts or products

These human workers are called laborThese human workers are called labor

Human workers are also needed for automated Human workers are also needed for automated manufacturing systems to manage or support the manufacturing systems to manage or support the system as computer programmers and operators system as computer programmers and operators in the case of CNC machines.in the case of CNC machines.

Manufacturing Systems Manufacturing Systems TypesTypes

Manufacturing System Manufacturing System TypesTypes

Manufacturing Systems Manufacturing Systems TypesTypes

Challenges in Manufacturing

Manufacturing industries strive to reduce the cost of the product continuously to remain competitive in the face of global competition.

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Classification of Manufacturing SystemFactors in Manufacturing Systems Classification Scheme

-Type of operations performed-Number of workstations-System layout-Automation and manning level-Part or product variety

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Classification of Manufacturing SystemTypes of Operations Performed-Processing operations on individual work units-Assembly operations to combine individual units into assembled entities.

Additional parameters:-Type of material processed affect the type of equipment and handling method.-Size and weight of the part or product has an effect on safety hazards-Part or product complexity-Part geometry

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Classification of Manufacturing SystemNumber of Workstations

As the number of stations increases, the amount of work that can be accomplished by the system increases. More stations also mean the system is more complex and therefore more difficult to manage and maintain. The system consists of more workers, machines and parts being handled. The material handling system is more complex in a multi station system as “n” increases. Reliability and maintenance problems occur more frequently.

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Classification of Manufacturing SystemNumber of WorkstationsConvenient measure of the size of the system

Let n = number of workstationsIndividual workstations can be identified

by subscript i, where i = 1, 2,...,nAffects performance factors such as workload capacity, production rate, and reliability

As n increases, this usually means greater workload capacity and higher production rate

There must be a synergistic effect that derives from n multiple stations working together vs. n single stations

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Classification of Manufacturing SystemSystem Layout

An important factor in determining the most appropriate material handling system.-Applies mainly to multi-station systemsn = 1, single station celln >= 2, multi station system-Fixed routing vs. variable routingIn systems with fixed routing, workstations are usually arranged linearlyIn systems with variable routing, a variety of layouts are possible

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Classification of Manufacturing SystemAutomation and Manning LevelsLevel of workstation automation

Manually operatedSemi-automatedFully automated

Manning level Mi = proportion of time worker in

attendance at station iMi = 1 means that one worker must be at

the station continuouslyMi 1 indicates manual operationsMi < 1 usually denotes some form of

automation

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Classification of Manufacturing SystemAutomation and Manning Levels

Utility Workers – not specifically assigned to individual processing or assembly stations. They perform functions such as1.Relieving workers at stations for personal breaks2.Maintenance and repair of the system3.Material handling and4.Tool changing

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Classification of Manufacturing SystemPart or Product Variety

“The degree to which the system is capable of dealing with variations in the parts or products it produces” Possible Variations:-Variations in type and/or color of plastic of molded parts in injection molding.-Variations in electronic components placed on a standard size printed circuit board-Variations in the size of printed circuit boards handled bu a component placement machine-Variations in geometry of machined parts-Variations in parts and options in an assembled product.

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Three cases:1.1. Single-model caseSingle-model case - all parts or products

are identical (sufficient demand/fixed automation)

2.2. Batch-model caseBatch-model case - different parts or products are produced by the system, but they are produced in batches because changeovers are required (hard product variety)

3.3. Mixed-model caseMixed-model case - different parts or products are produced by the system, but the system can handle the differences without the need for time-consuming changes in setup (soft product variety)

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(a) Single-model case, (b) batch model case, and (c) mixed-model case

Current Trends In Manufacturing Engineering

Group Technology

Design for Manufacturing and Assembly

Computer Aided Process Planning

Total Quality Approach

Concurrent Engineering

Computer Integrated Manufacturing

Numerically Controlled Production Process

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Group TechnologyGroup 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.

The improvement is typically achieved by organizing the production facilities into manufacturing cells that specialize in production of certain part families.

Grouping the production equipment into machine cells, where each cell specializes in the production of a part family is called cellular manufacturing.

Parts in the medium production quantity range are usually made in batches.

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Disadvantages of batch production: Downtime for changeovers High inventory carrying costs

GT minimizes these disadvantages by recognizing that although the parts are different, there are groups of parts that possess similarities.

When to Use GT and Cellular Manufacturing

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

This results in much material handling effort, high in process inventory, and long manufacturing lead times.

2. The parts can be grouped into part families:

Each machine cell is designed to produce a given part family, or a limited collection of part families, so it must be possible to group parts made in the plant into families.

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Problems in Implementing GT

1. Identifying the part families Reviewing all of the parts made in the plant and grouping them into part families is a substantial task

2. Rearranging production machines into GT cells It is time-consuming and costly to physically rearrange the machines into cells, and the machines are not producing during thechangeover

Part Family“A collection of parts that possess similarities in geometric shape and size, or in the processing steps used in their manufacture”

Two categories of part similarities can be distinguished:

(1) design attributes, which are concerned with part characteristics such as geometry, size, and material

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(2) manufacturing attributes, which consider the sequence of processing steps required to make a part.

(3) Both design and manufacturing attributes

Ten parts are different in size, shape, and material, but quite similar in termsof manufacturing

All parts are machined from cylindrical stock by turning; some parts require drilling and/or milling

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Traditional Process Layout

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Cellular Layout Based on GT

Each cell specializes in producing one or a limited number ofpart families

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Ways to Identify Part Families

1.Visual inspection

Using best judgment to group parts into appropriate families, based on the parts or photos of the parts

2. Parts classification and coding

Identifying similarities and differences among parts and relating them by means of a coding scheme

3. Production flow analysis

Using information contained on route sheets to classify parts

Parts Classification and Coding

“Identification of similarities among parts and relating the similarities by means of a numerical coding system”

Parts Classification and CodingIdentification of similarities among parts and relating thesimilarities by means of a numerical coding system

•Most time consuming of the three methods,•Must be customized for a given company or industry,•Reasons for using a coding scheme:

•Design retrieval,A designer faced with the task of developing a newpart can use a design retrieval system to determine ifa similar part already exists.Simply changing an existing part would take muchless time than designing a whole new part fromscratch.

Parts Classification and CodingAutomated 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

Features of Parts Classification and Coding Systems

Most classification and coding systems are based on one of the following:•Part design attributes•Part manufacturing attributes,•Both design and manufacturing attributes

Features of Parts Classification and Coding Systems

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Coding Scheme Structures

1.Hierarchical structure (monocode)

Interpretation of each successive digit depends on the value of the preceding digit

2. Chain-type structure (polycode)

Interpretation of each symbol is always the same No dependence on previous digits

3. Mixed-code structure

Combination of hierarchical and chain-type structures

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Opitz Classification SystemThe Opitz coding scheme uses the following digit sequence:

12345 6789 ABCD•The first five digits, 12345, are called the form code.

•It describes the primary design attributes of the part, such as external shape (e.g., rotational vs. rectangular) and machined features (e.g., holes, threads, gear teeth, etc.

•The next four digits, 6789are called supplementary code.

•It describes some of the attributes that would be of use in manufacturing (e.g., dimensions, work material, starting shape, and accuracy).

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The extra four digits, ABCD, are referred to as the secondary code

It is intended to identify the production operation type and sequence.

The secondary code can be designed by the user firm to serve its own particular needs.

Second type of coding is MULTY CLASS CODING it is 18 digit

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Flow

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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.

Production flow analysis (PFA)

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Problems from M.P. Groover book on

1.Rank Order Clustering Technique

2. Group Technology Machine Sequence using Hollier Method 1 and 2

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1. In each row of the matrix. read the series of ls and G's (blank entries = D's) from left to fight as a binary number. Rank the rows in 01 del uf decreasing value. In case of a tie, rank the rows in the same order as they appear in the current matrix

2. Numbering from top to bottom, is the current order of rows the same as the rank order

determined in the previous step? If yes, go to step 7, If no, go to the following step.3, Reorder the rows in the part-machine incidence matrix by listing them in

decreasingrank order, starting from the top4. In each column ot tbe matrix. read the series of I 's and O's (blank entries = (j's)

fromtop to bottom as a binary number. Rank the columns in order of decreasing value, Incase of a tie. rank the columns in the same order as they appear in the current

matrix.5. Numbering Irom left to right, is the current order of columns the same as the rankorder determined in the previous step? If yes. go to step 7. If no.go to the following

step.6. Reorder the columns in the part-machine incidence matrix by li~ling them in

decreasingrank order, starting with the left column. Go to step I.7 Stop

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Benefits of Group Technology in Manufacturing

Standardization of tooling, fixtures, and setups is encouraged.

Material handling is reduced.

Parts are moved within a machine cell rather than the entire factory.

Process planning and production scheduling are simplified.

Work-in-process and manufacturing lead time are reduced.

Improved worker satisfaction in a GT cell

Higher quality work

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Flexible Manufacturing SystemA flexible manufacturing system (FMS) is a form of flexible automation in which several machine tools are linked together by a material-handling system, and all aspects of the system are controlled by a central computer.

FMS technology can be applied in situations similar to those identified for group technology and cellular manufacturing; specifically,

• Presently, the plant either (1) produces parts in batches or (2) uses manned GT cells and management wants to automate .

The parts or products made by the facility are in the mid-volume, mid-variety production range.

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Components of FMS Systems Robotics Material Handling / Transport Machines Manual / Automated Assembly Cells Computers Controllers Software Networks

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What are The Features of FMS? An FMS is distinguished from an automated production line by its ability

to process more than one product style simultaneously.

At any moment, each machine in the system may be processing a different part type.

FMS can let us make changes in production schedule in order to meet the demands on different products.

Distinguishing Characteristics: An automatic materials handling subsystem links machines in the system and provides for

automatic interchange of work pieces in each machine

Automatic continuous cycling of individual machines

Complete control of the manufacturing system by the host computer

Lightly manned, or possibly unmanned

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Benefits of FMSFMS systems are intended to solve the following problems: Reduced work in process Increased machine utilization Better management control Reduced direct and indirect labor Reduced manufacturing lead-time Consistent and better qualityReduced inventory

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LAYOUT CONFIGURATIONS OF FLEXIBLE MANUFACTURING SYSTEM

Flexible manufacturing system brings rewards in actual manufacture of products as the process is designed for several products to be run on different machines.

Line layoutAn Automated guided vehicle is most efficient when the movement is in straight-lines along the AGV path in a single-row machine layout.

Machines are arranged only on one side of AGV path and in double row machine layout, machines are arranged on both sides.

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Loop layoutThe loop layout uses conveyor systems that allow unidirectional flow of parts around the loop.

A secondary material handling system is provided at a workstation which permits the flow of parts without anyobstruction.

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Ladder type layoutLadder type layout consists of rungs on which workstations are located. This reduces the average travel distance thereby reducing the transfer time between workstations.

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Carousel layoutIn the Carousel layout configuration, parts flow in one direction around the loop. The load, unload stations are placed at one end of loop

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If a handling robot is used in a Flexible manufacturing system cell , the machines are laid out in a circle, such a layout is called circular layout.

Robot centered cell

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CIM is the manufacturing approach of using computers to control the entire production process.

The integration allows individual processes to exchange information with each other and initiate actions.

Through the integration of computers, manufacturing can be faster and less error-prone, although the main advantage is the ability to create automated manufacturing processes.

Typically CIM relies on closed-loop control processes, based on real-time input from sensors.

Computer-integrated manufacturing (CIM)

“CIM is a management philosophy in which the functions of design and manufacturing are rationalized and coordinated using computer, communication and information technologies.”

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•In a CIM system functional areas such as design, analysis, planning, purchasing, cost accounting, inventory control, and distribution are linked through the computer with factory floor functions such as materials handling and management, providing direct control and monitoring of all the operations.

•Some factors involved when considering a CIM implementation are the production volume, the experience of the company or personnel to make the integration, the level of the integration into the product itself and the integration of the production processes.

CIMS Benefits:

•Increased machine utilization•Reduced direct and indirect labour•Reduce mfg. lead time•Lower in process inventory•Scheduling flexibility.

CIM represents a new production approach that will allow the factories to deliver a high variety of products at a low cost and with short production cycles.

Historical overviewHistorical Development 15th century – Machining Metal 18th century - Industrialization, production type machine tools 20th century - F.W. Taylor tool metal, HSS

Automated production equipment Screw machines Transfer lines Assembly lines ------using cams and preset shops

Programmable automationNC PLCRobots

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Introductory concepts

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THOSE USING SINGLE POINT TOOLS

THOSE USING MULTI POINT TOOLS

THOSE USING ABRASIVE TOOLS

lathesshapersplanersboring machinesetc.

drilling machines

milling machines

broaching machines

hobbing machines

etc.

grinding machines

honing machines

etc.


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Computer Control of Manufacturing SystemsNUMERICAL CONTROL (NC) A form of programmable automationNumbers, letters, and symbols are coded to define

a program of instructions for a particular work part or job.

Two categories of numerical control applications: Machine tool applications (drilling, milling, etc.) Non-machine tool applications (assembly, drafting, etc.)

Basic Components of NC1. Program of instructions2. Machine control unit3. Processing equipment

Drive system Machine tool Feedback system

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Computer Control of Manufacturing Systems

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flexo writer


The number of pulses transmitted to each axis determines the

incremental axis position, and the frequency of these pulses regulates

the axis speed.

Miscellaneous control functions are a set of on/off signals to

implement the control of the speed and direction of the spindle

rotation, control of coolant supply, selection of cutting tool,

automatic clamping and unclamping, etc.

NC is often referred to as the older generation of numerical control

technology. NC systems are hard-wired controls in which most

functions are implemented by electronic hardware based upon digital

circuit technology.

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Introductory conceptsComputer Numerical Control (CNC) Computer numerical control (CNC) is the numerical control system in

which a dedicated, stored program computer is built into the control to perform basic and advanced NC functions.

CNC controls are also referred to as soft-wired NC systems because most of their control functions are implemented by the control software programs.

The control signals in CNC systems are in the form of binary words. Each word contains a fixed number of bits, 32 bits or 64 bits are commonly used. Each bit of data produces one BLU motion in the controlled axis.

Theoretically a 32-bit word could represent one of up to 232 = 4,294,967,296 different axial positions. If the system resolution is, for example, BLU = 0.0001 in., this number can represent up to 429,969 in. possible motions, which is more than enough for all types of applications

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CNC MachinesWhat is a CNC Machine?

Numerical Control Machines.Programs are input thru input devices like tape or punched

cards. NC machines control only the position of job relative to

cutting tool.Feed rate, speed and tools will be selected by the operator.The instruction from the Tape will be converted into

machine movement by MCU units.

Basic components of CNC Machine Program of instructions is detailed step by step command to

that direct the processing unit. Machine Control Unit (MCU) is the brain of the NC machine. Machine Tools or Processing equipment which performs

useful work

1. Program of instructions: Detailed step-by-step commands that direct the processing equipment. The most common medium on which the program is submitted to the machine control unit has been 1-in.-wide punched tape. More recently, magnetic tape cassettes and floppy diskettes and computer via RS-232-C communication are used.

2. Machine Control Unit (MCU): Electronics and control hardware that read and interpret the program of instruction and convert it into mechanical actions of the mach. Tool or other processing equipment.

Implement interpolations (linear, circular, and helical) to generate axis motion commands

Feed axis motion commands to the amplifier circuits for driving the axis mechanisms

Receive the feedback signals of position and speed for each drive axis Implement auxiliary control functions such as coolant or spindle on/off,

and tool change3. Processing Equipment: Component that performs useful work. Ex. Work

table, spindle, motors.

The MCU and other Components of the NC SystemDrive System A drive system consists of amplifier circuits, drive motors, and ball lead-

screws. The MCU feeds control signals (position and speed) of each axis to the amplifier circuits. The control signals are augmented to actuate drive motors which in turn rotate the ball lead-screws to position the machine table.

Machine Tool CNC controls are used to control various types of machine tools. Regardless of

which type of machine tool is controlled, it always has a slide table and a spindle to control of position and speed. The machine table is controlled in the X and Y axes, while the spindle runs along the Z axis.

Feedback System The feedback system is also referred to as the measuring system. It uses

position and speed transducers to continuously monitor the position at which the cutting tool is located at any particular time. The MCU uses the difference between reference signals and feedback signals to generate the control signals for correcting position and speed errors.

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TYPES OF CNC MACHINECNC Machining CentersCNC Turning CentersCNC Drilling MachinesEDM Sinker and wire cut MachinesFlame and Laser-Cutting MachinesWater Jet ProfilersCNC Boring machinesCNC Punching machinesCNC Grinding machinesCNC Welding machinesCNC Bending machines

Basic principle of CNC Each axis consists of a mechanical component,

such as a slide that moves, a servo drive motor that powers the mechanical movement, and a ball screw to transfer the power from the servo drive motor to the mechanical component. These components, along with the computer controls that govern them, are referred to as an axis drive system.

Additionally, a CNC axis may be either a linear axis in which movement is in a straight line, or a rotary axis with motion following a circular path.

Advantages of NC with Small Lot Sizes1. Reduced non-production time2. Reduced fixturing3. Reduced lead time4. Greater manufacturing flexibility5. Easier to accommodate engineering design changes on the work

piece6. Improved accuracy and reduced human error.

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NC is most appropriate for the following conditions1. Frequently processed parts with small to medium lot sizes2. Complex part geometry3. Close tolerances4. Need of much metal removal5. 100% inspection required parts6. Expensive parts where processing mistakes are costly7. Need of many operations on the part8. Likely engineering design changes


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Increased productivity Reduced production costs Facilitation of complex

machining operations Improved production planning

and control Facilitation of flexible

automation High accuracy and repeatability Reduced indirect operating

costs Greater flexibility Lower operator skill

requirement

high initial investment high maintenance

requirement, and not cost-effective for low

productions runs

Advantages Limitations

Advantages and Limitations of CNC The main advantages of using CNC technology are to reduce product cost, improve product quality, and facilitate production planning and control. These benefits can be realized through these nine causes:

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CNC Dual turret center

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CNC vertical Milling Machine

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Double axis machining center

Coordinate System and Machine MotionsCoordinate System -ICoordinate System -IThe purpose is to provide a means of locating the

tool in relation to the work piece.Numerical control coordinate system is defined

with respect to the machine tool table.Depending on the type of NC machine, the part

programmer may have several options for specifying the location. One of these options:

1. *Fixed zero: the origin is always located at the same position on the machine table. All locations must be defined by x and y coordinates relative to that fixed origin. *Floating zero: Modern NC machines allow the machine operator to set the zero point at any position on the machine table.

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Coordinate System -IICoordinate System -II2. *2. *Absolute Positioning: The tool locations are

always defined in relation to the zero point.**Incremental Positioning: The next tool location must be defined with reference to the previous tool location.

Machine Axes Designation & Direction The EIA-267-B standard specifies fourteen axes

for describing the linear and rotary motions of CNC machines. This includes nine linear axes and five rotary axes. The nine linear axes are further divided into the following three groups: (see figure below)

Primary linear axes (X,Y, and Z) Secondary linear axes (U, V, and W) Tertiary linear axes (P, Q, and R)

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The primary axes (X, Y, and Z) are assigned to the primary slide table. The secondary linear axes (U, V, and W) are added to the primary axes for defining the movement of the second moving slide or spindle. Similarly, the tertiary linear axes (P, Q, and R) are used to represent the linear motion of the third slide or spindle.

The five rotary axes consist of three primary rotary axes (A, B, and C) and two special axes (D and E) (right figure). Their definitions are:

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Machine Axes Designation Machine axes are designated according to the

"right-hand rule". When the thumb of right hand points in the direction of the positive X axis, the index finger points toward the positive Y axis, and the middle finger toward the positive Z aixs. The left figure shows the right-hand rule applied to vertical machines, while the right figure applies to horizontal machines.

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Direction of Machine Axes

Direction of Machine Axes CNC controls use the positive (+) and negative (-) sign to

indicate the motion direction of the machine axes. This is how we define the directions.

+Z direction: is the direction which increases the distance between the workpiece and the cutting tool.

-Z direction: is in the opposite direction of +Z. +X direction: (a) In vertical machines, it is the direction to the

right when observed from the spindle toward its supporting column.

(b) In horizontal machines, it is pointed to the right when viewed from the spindle axis toward the workpiece.

-X direction: is in the opposite direction of +X. +Y direction: follows the right-hand rule: when the thumb points

toward the +X axis and the middle finger is directed toward the +Z axis, the index finger points in the direction of the +Y axis.

-Y direction: is in the opposite direction of +Y.

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Reference Zero Points Reference zero points are the base or starting points that are

chosen as the reference for calculating the coordinates of the other points. Also, reference zero points are called the zero points. CNC controls use the following four types of reference zero points to facilitate the programming of tool paths:

Machine zero point Reference return point Work zero point Program zero point Machine Zero Point The machine zero point is the origin of the machine coordinate

system. It is set by the machine tool manufacturer and can not be changed. The machine zero is labeled with an M and represented by this

symbol: The location of the machine zero point varies from manufacturer to

manufacturer.

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For turning machines, the machine zero is normally located at the center of the spindle end face (left figure). In milling machines, the machine zero is usually at the extreme limit of each axis travel (right figure). Normally the machine zero is not directly used as the reference point for writing part programs. It may be used in one of the following three applications:

1. Initial setup of the machine 2. As the reference point for other reference

points such as reference return points, work zeros, and program zeros 3. As the tool change position

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Reference Return Point Reference return points are the locations to which the

machine table or the spindle is returned. They are identified by the letter R and are represented by the symbol

Some CNC controls allow defining up to four reference return points. Normally, the machine zero is set to be the first reference return point in milling machines [(Figure below (left)].

The second, third, and fourth reference return points are specified by setting their parameter values. They can be set at any convenient location within the work envelope. In turning machines, the reference return point is located in the extreme end of the work envelope [Figure below (right)]

The location of the first reference return point is precisely predetermined in each moving axis in relation to the machine zero point. Because of this, it can be used for calibrating and regulating the measuring system of the slide table and spindle.

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Specifically, the reference point is used in four situations1. When the control is powered up, all axes always must be positioned at

the reference return point to calibrate the measuring system.2. The machine must be re-positioned to the reference return point for

reestablishing the proper coordinate values in situations such as losing the current position data due to electrical failure or improper operation.

3. All axes must be retracted to the reference point before the tool change can take place.

4. At the end of the part program, all axes must be retracted to the reference return point to reset the control system for re-running the part program or running a new part program.

121FMS(TGS

)

Work Zero Point Work Zero Point A work zero point is the origin of the workpiece's coordinate

system. It is used to determine the work's coordinate system in

relation to the machine zero point. The work's zero points are often referred to as setup points

because they are the locations for setting up the workpiece on the machine table.

Some CNC controls allow the use of multiple work zero points in one machine setup or operation. The work zero point is labeled by W and represented by the symbol

The work zero point can be chosen by the programmer at any convenient location within the working envelope of the machine. It is recommended that you place the work zero point in a way that it can be easily located and measured on the workpiece.

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)

Work zero pointtwo common methods of choosing the work zero point for turnings (top figure) and the bottom figure shows milling examples.

123FMS(TGS

)

TYPES OF NC SYSTEMSTYPES OF NC SYSTEMS1. Point-to-point: The objective is to move the cutting tool

to a predefined location. Once the tool reaches the desired location, the machining operation is performed at that location.

2. Straight cut: Capable of moving the cutting tool parallel to one of the major axes. Workpieces of rectangular configurations may be fabricated. Angular cuts are not possible.

3. Contouring: The most complex, the most flexible, and the most expensive type of machine tool control. Capable of performing both PTP and straight-cut operations. In addition, simultaneous control of more than one axis movement of the machine tool. Straight or plane surfaces at any orientation, circular paths, conical shapes, most any other mathematically definable form are possible.

124FMS(TGS

)

Types of NC Systems

Position and Motion Control in an NC SystemThe data read into the MCU define machine

table positions.Each axis is equipped with a drive unit which

is connected to the table by means of a lead screw.

The axis positioning system may be designed as either an open-loop or a closed-loop system.

125FMS(TGS

)

CNC Drive SystemsCNC Drive Systems The CNC drive systems can be either open loop

or closed loop type. The main difference between the two systems depends on whether the system has a feedback loop to insure the accuracy of system performance.

Open-loop NC systems make use of stepping motors.Each pulse generated my MCU drives the stepping

motor by a fraction of one revolution, step angle. = Step angle (integer) = number of step angles

126FMS(TGS

)

sn360

sn

P= number of pulses received by the motorfp= pulse rate (frequency of the pulse train)t= duration of the pulse trainS= rotational speed of the stepping motor

angle of rotation=Pangle of rotation=fpt

By controlling the number of pulses to the motor the position of the table is controlled without feedback sensors.

Stepping motors are used on NC systems where the load is relatively small.127FMS

(TGS)

s

p

nf

S60

CNC Drive Systems

Open Loop System No feedback loop is used in open loop drive

systems. The drive motor acts upon the control commands

from the machine control unit (MCU). The system simply assumes the machine table

will reach the target position. There is no way for the MCU to know the actual

performance of the system. An open loop system is very sensitive to the load

resistance. Position and velocity error may occur when a

heavy cutting resistance is encountered.

128FMS(TGS

)

CNC Drive SystemsOpen loop drives are typically used in PTP systems in which the cutting tool does not engage with the work piece during positioning. They can also be used in light-loaded cutting machines. Open loop systems are less expensive, but they are prone to load resistances during machining.

129FMS(TGS

)

A closed-loop system uses position sensors to measure the table position relative to the input value for the axis.

Generally uses dc servomotor or hydraulic actuator.Various feedback sensor devices are used. Ex.

Optical encoder.Optical encoder consists of a light source, a

photodetector,, and a disk which is connected to the rotating shaft whose angular position is to be measured.

As the disk rotates, the openings on the disk cause the light source to be seen as a series of flashes.

The photodetector emits an electrical signal equal to the number flashes which are counted by the MCU.

= angle between the openings in the diskNd= number of openings in the encoder diskangle of rotation= P

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)

sn360

Pulses generated by the optical encoder is compared with the input position command

The error is used to control a dc servo motor, which in turn drives the machine table.

Closed-loop NC systems are more appropriate for processes that generate a significant load during operation. Ex: milling, turning.

Accuracy: A measure of the control system’s capacity to

position the machine table at a desired location .Related to the control resolution of an NC

system.Control resolution is the capability of the MCU to

divide the range of a given axis movement in to closely spaced points. It is the distance between adjacent control points.131FMS

(TGS)

CNC Drive Systems

Closed Loop System With closed loop drive systems, feedback sub-systems

are used to monitor the actual output and correct any discrepancy between desired and actual system performance. Feedback sub-systems may be either an analog or digital type. Analog systems measure the variation of physical systems such as position and velocity in voltage level. Tachometers are typically used to measure the velocity, while resolvers for position.

There are two feedback loops in CNC drive systems: position loop and velocity loop.

The position loop is the outer loop that consists of a comparator, an amplifier circuit, a velocity loop, a resolver, and a resolver interface.

132FMS(TGS

)

n= number of bits for an axisnumber of control points= 2n

Repeatability: Ability of the control system to return to a

given location that was previously programmed into the controller. repeatability errors are caused by mechanical errors.133FMS

(TGS)

n2movement axis of rangeCR

error) mech. of dev. (std. 32

CRaccuracy

error) mech. of dev. (std. 3ityrepeatabilerror) mech. of dev. (std. 6

CNC MachinesWhat is a CNC Machine?

Numerical Control Machines.Programs are input thru input devices like tape or punched

cards. NC machines control only the position of job relative to

cutting tool.Feed rate, speed and tools will be selected by the operator.The instruction from the Tape will be converted into

machine movement by MCU units.

NC applications have two categories1.Machine tool applications

- drilling, milling, turning & other metal working operations

2.Non-machine tool applications- assembly, drafting & inspection.

History: John Parson , Frank Stulen in collaboration with MIT for the USA air force.

NC – predecessor of CNC

Categories of CNC M/c

Basic components of CNC Machine Program of instructions is detailed step by step command to

that direct the processing unit. Machine Control Unit (MCU) is the brain of the NC machine. Machine Tools or Processing equipment which performs

useful work

TYPES OF CNC MACHINECNC Machining CentersCNC Turning CentersCNC Drilling MachinesEDM Sinker and wire cut MachinesFlame and Laser-Cutting MachinesWater Jet ProfilersCNC Boring machinesCNC Punching machinesCNC Grinding machinesCNC Welding machinesCNC Bending machines

Basic principle of CNC Each axis consists of a mechanical component,

such as a slide that moves, a servo drive motor that powers the mechanical movement, and a ball screw to transfer the power from the servo drive motor to the mechanical component. These components, along with the computer controls that govern them, are referred to as an axis drive system.

Additionally, a CNC axis may be either a linear axis in which movement is in a straight line, or a rotary axis with motion following a circular path.

CNC Machine Tools

Point-to-Point systemsAlso called position systemsSystem moves to a location and performs an

operation at that location (e.g., drilling)Also applicable in robotics

Continuous path systems Also called contouring systems when control

of 2 or>2 axe is presentSystem performs an operation during

movement (e.g., milling and turning)

NC Motion Control

NC Motion Control

Point-to-Point systems

Continuous path systems

1.Control system based

It cause the tool to maintain continuous contact with the part as the tool cuts a contour shape. These operations include milling along any lines at any angle, milling arcs and lathe turning.

Point-to-point control systems

Motion ControlPoint-to-point control Linear control Contouring control (continual)

drilling milling machines, lathes machining (milling, turning)

point-to-point motion – tool is moved to specific location, path or speed of move between location is not controlled. (drilling)

Linear control – movement along one of the axes of the machine. Contouring control – movement can be made in two or more axes

simultaneously.

Interpolation Methods1. Linear interpolation

Straight line between two points in space2. Circular interpolation

Circular arc defined by starting point, end point, center or radius, and direction

3. Helical interpolation Circular plus linear motion

4. Parabolic interpolation5. Cubic interpolation

Free form curves using higher order equations

Circular Interpolation Methods

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on only the inside of the nominal curve

Circular Interpolation Methods

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on both the inside and outside of the nominal curve

2.Feedback basedOpen Loop System• Uses stepping motor to create

movement. Motors rotate a fixed amount for each pulse received from the MCU. The motor sends a signal back indicating that the movement is completed. No feedback to check how close the actual machine movement comes to the exact movement programmed.

Closed Loop System• AC, DC, and hydraulic servo motors are

used. The speed of these motors are variable and controlled by the amount of current or fluid. The motors are connect to the spindle and the table. A device called a resolver continuously monitors the movement and sends back a single to MCU to make adjustments.

3.Drives of CNC machine toolHydraulic actuator

high power machine toolStepping motor

small machine due to limited power and torqueDC motor

- excellent speed regulation, high torque, most widely used

1. Absolute . In this mode, the desired target position of the tool for a particular move is given relative to the origin point of the program.

2. Incremental. In this mode, the next target position for the tool is given relative to the current tool position.

4.Coordinate System

NC Coordinate SystemsBased on the Cartesian Coordinate systemFor flat and prismatic (block-like) parts: Milling and drilling operations Conventional Cartesian coordinate system Rotational axes about each linear axis

For rotational parts: Turning operations Only x- and z-axes

Z-axis is always parallel to the axis ofrotation

NC Coordinate Systems

1. Storage of more than one part program-Memory expansions possible

2. Various forms of program input- initially hardwired, magnetic tapes, floppy. Now RS2 32

3. Program editing at the machine tool4. Fixed cycles and programming

subroutines - Macros5. Interpolation

Features of CNC

6. Positioning features for setup- ‘Position set’ feature, references the m/c tool axes to a point or a set of points on the fixture. Operator does not do it.

7. Cutter length and size compensation- cutter sensor used to calculate exact dimensions of tool and cutter offset, as they may be different from that originally planned

8. Acceleration and deceleration computations- to avoid tool marks during direction change

9. Communications interface- RS-232 standards allow m/c tool to link with other computer driven devices for

downloading programs, collecting operational data and interfacing with peripheral equipment like robots, AS/R, AGVs

Features of CNC

Machining centreMachining centre is a machine tool capable of performing several different machining operations on a work part in one setup under program control

capable of milling, drilling, reaming, tapping, boring, facing, and similar operations.

Characterizations of an NC machining center:Automatic tool-changing capabilityAutomatic work part positioning –has a rotary

tablePallet shuttle.

Machining Centre

Three types of machining centres1.Vertical Machining centre2.Horizontal Machining centre3.Universal Machining centre

Machining Centre - types

VerticalFor flat

work pieces that require tool access from the top


Vertical Machining Centre

HorizontalFor cube shaped parts, where access is required all sidesHas spindle on the horizontal axis


Spindle axis can be tilted from horizontal to vertical

Equivalent to 5-axis machining

Universal Machining Centre

CNC Controllers

CNC controller interprets part program instructions and then converts them into two types of control signals:

Motion control signals - a series of electric pulse trains that control position and the speed of machine table

and spindle. - Each pulse activates a motion of one basic length-unit (BLU) which is the minimum

increment size of the NC control systemnumber of pulses transmitted to each axis determines the incremental axis position;

frequency of these pulses regulates the axis speed

CNC Controllers

The control signals in CNC systems are in the form of binary words. Each word contains a fixed number of bits, 32 bits or 64 bits are commonly used.

Each bit of data produces one BLU motion in the controlled axis.

A 32-bit word could represent one of up to 232 = 4,294,967,296 different axial positions.

If the system resolution is, for example, BLU = 0.0001 in., this number can represent up to 429,969 in. possible motions, which is more than enough for all types of applications

CNC Controllers

Miscellaneous control signals-a set of on/off signals to implement the control of 1. speed and direction of the spindle rotation,2. control of coolant supply, 3. selection of cutting tool, 4. automatic clamping and unclamping, etc.

CNC Controllers

Advantages of CNCIncreased productivityReduced production costsFacilitation of complex machining operationsImproved production planning and controlFacilitation of flexible automationHigh accuracy and repeatabilityReduced indirect operating costsGreater flexibilityLower operator skill requirement

High initial costEquipment and installation

High maintenance costSkilled CNC personnel requiredHave to be installed in air-conditioned placesNot suitable for long run applications

Disadvantages of CNC

Direct numerical control (DNC) – control of multiple machine tools by a single (mainframe) computer through direct connection and in real time1960s technologyTwo way communication

Distributed numerical control (DNC) – network consisting of central computer connected to machine tool MCUs, which are CNCPresent technologyTwo way communication

Direct Numeric Control (DNC)

- Bypasses the need for a MCU- Processing of part programs are done on a centralized

computer- Program transmitted to the MCU, one block at a time

(BTR)

Basic componentsMainframe computer

Connected via satellite computers or lengthy cablesBulk memoryCommunications networkNC machine tools


Advantages:Higher reliability than hardwired MCUsElimination or error prone tape and tape-readerControl of multiple machinesImproved computational capability for circular

interpolationsPart programs stored in a central locationComputers located in an environmentally agreeable

location


Types of DNC- based on communication link between

machine tool and computer

1. Dedicated MC- Similar to a CNC as MCU is hardwired

2. Behind the tape reader (BTR) interface- cheaper to use- easily implemented- receives only the program of the part to be manufactured



Connection to MCU is behind the tape reader (BTR). In distributed NC, entire programs are downloaded to each MCU, which is CNC rather than conventional NC

Similar to CNC computersImproved data collection

Distributed Numeric Control (DNC)

DNC –switching network configuration

Limitations1. No of machines2. Frequency of service required for each machine

Direct Numeric Control (DNC)-LAN configuration

“to adapt” means to change a behavior to conform to new circumstances.

An adaptive controller a controller that can modify its behavior in

response to the changes in dynamics of the processes and the disturbances acting on the process.

A self-correcting form of optimal control

Adaptive Control

In machining, it includes automatic adjustment of cutting parameters like speeds, feeds, depth of cut, etc.

Adaptive Control

General Configuration

Adaptive controller performs 3 functions1. Identification

- Identifies the current value of performance index- Functions continuously to be dynamic

2. Decision- decide what changes have to be made to improve system performance

3. Modification- implement the decision

Adaptive Control

Two types of Adaptive controlAdaptive Control with optimization (ACO)Adaptive Control with constraints (ACC)

ACO – attempts to maximize IP – index of performance

IP = MRR/ TWR MRR = Metal Removal Rate; TWR = Tool Wear Rate

usually an economic index of performanceeg: Maximum production rate, minimum production cost

Adaptive Control

Adaptive Control with Constraints (ACC)Machining conditions are maximized within machine

constraints eg: maximum force or torque or powerTwo sensors are employed

Tool vibration sensor – accelerometer mounted on the housing

Spindle torque sensor – strain gauges mounted on the machine spindle

Adaptive Control

introduction to manufacturing system and cnc machines

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