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Notes on CNC/DNC
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5/26/2018 Introduction to NC,CNC and DNC
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Introduction to NC machine
DIFINATION:
EIA(Electronic Industries Association) A system in whichactions are controlled by direct insertion of NumericalData at some point. The system must automatically
interpret latest some portion of this data
The control of a machine tool by means ofrecorded information on punched tape or card is
known as Numerical Control Numerical control is defined as a form of software
controlled automation, in which the process iscontrolled by alphanumeric characters or symbols.
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Introduction to DNC machine
Definition
Direct numerical control can be
defined as a manufacturing system in which a
number of machine are controlled by a computer
through direct connection in real time
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Cont
The tape reader is omitted in DNC, thus relieving thesystem as its latest reliable component
Instead of using tape reader, the part programming istransmitted to the machine tool directly from the computer
memory. In principle one large computer can be used to control
more then 100 separate machine.
The DNC computer is designed to provide instruction to
each machine tool on demand. When machine tool need control commands, they are
communicated to it immediately.
DNC also involves data collection and processing fromthe machine tool back to the computer.
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Components of DNC System
Central Computer
Bulk memory
Telecommunication
Lines
Machine Tools
Central
computerBulk memory
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Types of DNC machine
There are two types of DNC machines
1. Behind the tape reader (BTR) system
2. Special Machine control unit
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Types of DNC machine
Behind the tape reader system
In this arrangement computer is linked directly to theregular NC controllerunit.
DNC ComputerBulk memory
NC programs
NC ControllerTwo
Storage
Buffers
Tape reader replaced by
Telecommunication lines
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Cont
The replacement of the tape reader bytelecommunication lines to the DNC computer is whatgives the BTR configuration its name.
The connection with the computer is made betweenthe tape reader and the controller unit
behind the
tape reader.
Except for the source of the command instructions, theoperation of the system is very similar to conventionalNC.
The controller unit use to temporary storage buffer toreceive block of instructions from the DNC computerand convert them in to machine actions.
While one buffer is receiving a block of data, the otheris providing control instructions to the machine tool.
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Cont
Special Machine Control Unit
The other strategy in DNC is to eliminate the regularNC controller all together and replace it with special
machine control unit.
DNC ComputerBulk memory
NC programs
Special MCU
Conventional NC controllerReplaced by special MCU
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Cont
The special MCU is device that is specificallydesigned to facilitates communication between themachine tool and the computer.
The special MCU configuration achieve a superiorbalance between accuracy of the interpolation and fastmetal removal rates then is generally possible withBTR system.
The special MCU is soft wired, while the conventional
NC controller is hard wired.
The advantage of soft wired is its flexibility
Its control function can be altered with relative ease tomake improvements.
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Function of DNC
NC with out punched tape
NC part program storage
Data collection, processing and reporting
Communications
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Advantages of DNC
Elimination of punched tapes and tape
readers.
Grater computational capability and flexibility.Convenient storage of NC part program in
computer files.
Program stored as CLFILE.
Reporting of shop performance.
Establishes the framework for the evolution of
the future computer automated factory.
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Introduction to CNC machine
Definition
CNC is a self-contained NC system for a
single machine tool which uses a dedicated mini
computer, controlled by the instruction stored in its
memory, to perform all the basic numerical control
functions.
As per EIAThe numerical control system
where a dedicated stored program computer is used to
perform some or all basic functions in accordance withcontrol programs stored in Random Access Memory of
the computer
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Cont
The control unit and panel is somewhat similar toNC controls. Only difference being that is worksin on-line mode and NC in batch processingmode.
It can be mounted or stand alone type as in NCs.
A typical CNC may need only the drawingspecifications of a part to be manufactured and
the computer automatically generates partprogram for loaded parts.
CNC offers flexibility and computational capacityalong with option.
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Computerized Numerical Control
Tape Reader
for Initial
Program
Entry
Minicomputer or
Microcomputer
For
Software Functions
Part program
Storage
Computer
Hardware
Interface
And
Servo
System
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Functions of CNC
The Principle Functions of CNC are:
Machine Tool Control
In-process Compensation
Improved programming and Operating feature.
Diagnostics
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Advantages of CNC
The advantages of CNC over NC machines are:
More flexibilityReduce data reading error
Online editing of program
Diagnostic
Conversion of units
Integration with DNC systems
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Disadvantages of CNC
The disadvantages of CNC over NC machines are:
High initial cost
Higher maintenance cost
Skilled CNC personal
Not suitable for long run applications
Machine have to be installed in air-conditioned places