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CNC Machines
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.pdf  1462975902-1CNCMachinesSeminarreport.pdf (Size: 891.08 KB / Downloads: 3)



INTRODUCTION
Numerical control for machines .tools were introduced in 1950’s by Prof.John T Parsons. The first NC
machine was built at the Massachusetts institute of Technology in 1953 by joint efforts of US Air Force, the
MIT and parson’s cooperation. NC is control by numbers .NC is control recorded information called part
program, which is set of coded instructions given as numbers for automatic control of am machine in a predetermined
sequence.
Numerical control can be defined as a technique of controlling a machine tool by the direct insertion of
numerical data at some point of the system .The functions that are controlled on the machine tool are
displacement of the slide members, spindle speeds ,tool selection etc.At first ,the numerical control was used
to produce geocentrically complex parts ,but later used for added efficiency in medium batch production of
turned and milled parts presently, Numerical control is employed in all sectors of production .
Rapid development in the field of electronics such as integrated circuit, large scale integrated circuits and
development of minicomputer lead to the development of minicomputers based CNC systems. Further
development and the electronic “chip” revolution have ushered in the current generation “compact and
powerful” Microprocessor based CNC systems.
Development of computer numerically controlled (CNC) machines is an outstanding contribution to the
manufacturing industries. It has made possible the automation of the machining process with flexibility to
handle small to medium batch of quantities in part production.
Initially, the CNC technology was applied on basic metal cutting machine like lathes, milling machines, etc.
Later, to increase the flexibility of the machines in handling a variety of components and to finish them in a
single setup on the same machine, CNC machines capable of performing multiple operations were
developed. To start with, this concept was applied to develop a CNC machining centre for machining
prismatic components combining operations like milling, drilling, boring and taping. Further, the concept of multi-operations was also extended for machining cylindrical components, which led to the development of
turning centers.
Computer Numerical Control (CNC) is a specialized and versatile form of Soft Automation and its
applications cover many kinds, although it was initially developed to control the motion and operation of
machine tools.
Computer Numerical Control may be considered to be a means of operating a machine through the use of
discrete numerical values fed into the machine, where the required 'input' technical information is stored on a
kind of input media such as floppy disk, hard disk, CD ROM, DVD, USB flash drive, or RAM card etc.
The machine follows a predetermined sequence of machining operations at the predetermined speeds
necessary to produce a workpiece of the right shape and size and thus according to completely predictable
results. A different product can be produced through reprogramming and a low-quantity production run
of different products is justified.
ADVANTAGE OF CNC MACHINES
 Higher flexibility
 Increased productivity
 Consistent quality
 Reduced scrap rate
 Reliable operation
 Reduced non productive time
 Reduced manpower
 Shorter cycle time
 High accuracy
 Reduced lead time
 Just in time (JIT) manufacture
 Automatic material handling
 Lesser floor space
 Increased operation safety
 Machining of advanced material



CNC SYSTEMS
INTRODUCTION
Numerical control (NC) is a method employed for controlling the motions of a machine tool slide and its
auxiliary functions with input in the form of numerical data. A computer numerical control (CNC) is a
microprocessor-based system to store and process the data for the control of slide motions and auxiliary
functions of the machine tools. The CNC system is the heart and brain of a CNC machine which enables the
operation of various machine members such as slides, spindles, etc. as per the sequence programmed into it,
depending on the machining operations.
The main advantage of a CNC system lies in the fact that the skills of the operator hitherto required in the
operation of a conventional machine is removed and the part production is made automatic.
The CNC systems are constructed with a NC unit integrated with a programmable logic controller (PLC)
and some times with an additional external PLC (non-integrated). The NC controls the spindle movement
and the speeds and feeds in machining. It calculates the traversing path of the axes as defined by the inputs.
The PLC controls the peripheral actuating elements of the machine such as solenoids, relay coils, etc.
Working together, the NC and PLC enable the machine tool to operate automatically. Positioning and part
accuracy depend on the CNC system's computer control algorithms, the system resolution and the basic
mechanical machine accuracy. Control algorithm may cause errors while computing, which will reflect
during contouring, but they are very negligible. Though this does not cause point to point positioning error,
but when mechanical machine inaccuracy is present, it will result in poorer part accuracy


Computer Numerical Control (CNC) is a specialized and versatile form of Soft Automation and its
applications cover many kinds, although it was initially developed to control the motion and operation
of machine tools.
Computer Numerical Control may be considered to be a means of operating a machine through the use of
discrete numerical values fed into the machine, where the required 'input' technical information is stored on
a kind of input media such as floppy disk, hard disk, CD ROM, DVD, USB flash drive, or RAM card etc.
The machine follows a predetermined sequence of machining operations at the predetermined speeds
necessary to produce a work piece of the right shape and size and thus according to completely predictable
results. A different product can be produced through reprogramming and a low-quantity production run of different products is justified.



Control Systems
Open Loop Systems :
Open loop systems have no access to the real time data about the performance of the system and therefore no
immediate corrective action can be taken in case of system disturbance. This system is normally applied
only to the case where the output is almost constant and predictable. Therefore, an open loop system
is unlikely to be used to control machine tools since the cutting force and loading of a machine tool is never
a constant. The only exception is the wirecut machine for which some machine tool builders still prefer to
use an open loop system because there is virtually no cutting force in wirecut machining.

Close Loop Systems:
In a close loop system, feedback devices closely monitor the output and any disturbance will be corrected in
the first instance. Therefore high system accuracy is achievable. This system is more powerful than the open
loop system and can be applied to the case where the output is subjected to frequent change. Nowadays,
almost all CNC machines use this control system.



Input Devices
a. Floppy Disk Drive
Floppy disk is a small magnetic storage device for CNC data input. It has been the most
common storage media up to the 1970s, in terms of data transfer speed, reliability,
storage size, data handling and the ability to read and write. Furthermore, the
data within a floppy could be easily edited at any point as long as you have the proper
N
C
P
L
C
Servo Drive Servo Motor
Spindle Head
Work piece
Table
Encoder
Position Feedback
Tacho
Generator Velocity
Feedback
Tape Reader
Tape Punch
Other Devices
Machine
Elements
Inputs
Outputs
Lead
Screw
Command
value
 Proximity switches
 Limit switches
 Relay coils
 Pressure switches
 Float switches
CNC machines
http://www.pediain.com
program to read it.
However,this method has proven to be quite problematic in the long run as floppies
have a tendency to degrade alarmingly fast and are sensitive to large magnetic fields
and as well as the dust and scratches that usually existed on the shop floor.
b. USB Flash Drive
A USB flash drive is a removable and rewritable portable hard drive with compact
size and bigger storage size than a floppy disk. Data stored inside the flash drive are
impervious to dust and scratches that enable flash drives to transfer data from
place to place. In recent years, all computers support USB flash drives to read and
write data that make it become more and more popular in CNC machine control
unit.
c. Serial communication
The data transfer between a computer and a CNC machine tool is often
accomplished through a serial communication port. International standards for serial
communications are established so that information can be exchanged in an orderly way.
The most common interface between computers and CNC machine tools is referred to the
EIA Standard RS-232. Most of the personal computers and CNC machine tools have built
in RS232 port and a standard RS-232 cable is used to connect a CNC machine to a
computer which enables the data transfer in reliable way. Part programs can be
downloaded into the memory of a machine tool or uploaded to the computer for
temporary storage by running a communication program on the computer and setting
up the machine control to interact with the communication software.
CNC machines
http://www.pediain.com
Direct Numerical Control is referred to a system connecting a set of numerically
controlled machines to a common memory for part program or machine program
storage with provision for on-demand distribution of data to the machines. (ISO
2806:1980) The NC part program is downloaded a block or a section at a time into the
controller. Once the downloaded section is executed, the section will be discarded to
leave room for other sections. This method is commonly used for machine tools that do
not have enough memory or storage buffer for large NC part programs.
Distributed Numerical Control is a hierarchical system for distributing data between
a production management computer and NC systems. (ISO 2806:1994) The host
computer is linked with a number of CNC machines or computers connecting to
the CNC machines for downloading part programs. The communication program
in the host computer can utilize two-way data transfer features for production data
communication including: production schedule, parts produced and machine utilization etc.
 

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