NUMERIC CONTROLS
Numerical control has caused a virtual revolution
in the discrete metal parts manufacturing industry. The success of NC has led
to number of extensions of Numeric Control concepts and technology.
Four of the important developments are the
following:
Direct numerical control (DNC).
Computer numerical control (CNC).
Adaptive Control.
Industrial robots.
- Direct Numerical Control (DNC)
There are number of problems inherent in conventional
machining which are motivated machine tool builders to seek improvements in the
basic NC system. Among the difficulties encountered in using conventional
numerical control machine are the following Part programming mistakes,Punched
tape, Tape reader, Controller, Management information.
DNC defined:
It can be defined as a manufacturing system
in which a number of machines are controlled by a computer through direct
connection and in real time. The tape reader is omitted in DNC thus relieving
the system of its least reliable component. Instead of using tape reader the part
program is transmitted to the machine tool directly from the computer memory.
In principle,one computer can be used to control more than 100 separate
machines.
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| Direct Numeric Control System |
Figure below illustrates the general DNC
configuration:
The system consists of four components.
1. Central computer
2. Bulk memory which stores the NC part programs.
3. Telecommunication lines.
4. Machine Tools.
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| Configuration of DNC |
The computer calls the part program instructions
from bulk storage and sends data back from the machines. This two way information
flow occurs in real time,which means that machines requests for instructions must
be satisfied almost instantaneously. Similarly, the computer must always be
ready to receive information from the machines and respond accordingly. The
remarkable feature of the DNC system is that the computer is servicing a large
number of separate machine tools,all in real time.
Depending upon the number of machines and computational
requirements that are imposed on the computer it is sometimes necessary to make
use of satellite computers as shown in figure.
- Computer Numerical Control (CNC)
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| CNC Machine |
CNC is a NC system that utilizes a
dedicated, stored program computer to perform some or all of the basic
numerical control functions. As of this writing the typical CNC system uses a minicomputer
as the controller unit. It is expected that in future generations of computer numerical
control, microcomputers will become predominant. Because a digital computer is
used in both CNC and DNC there is often confusion surrounding the two systems.
Figure
below shows the general configuration of CNC Main differences are :
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| Configuration of CNC |
1. DNC computers distribute instructional
data to and collect from a large number of machines.
CNC computers control only one machine or a
small number of machines.
2. DNC computers occupy a location that is typically
remote from the machines under their control.
CNC computers are located very near their
machine tools.
3. DNC software is developed not only to
control individual pieces of production technology,but also to serve as part of
management information system in the manufacturing sector of the firm.
CNC software is developed to augment the
capabilities of a particular machine
tool.
Except for the fact that a digital computer
is used, CNC machines are similar and the part programs are entered in a similar
manner.
Punched tapes are still the common device
for entering the part program into the system.
- Adaptive Control Machining
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| Adaptive Control Machining |
One of the principles reasons for using
numerical control(including DNC and CNC) is that NC reduces the nonproductive
time in manufacturing. This is accomplished through a reduction in the
following elements,which constitute a significant portion of total production
time.
1. Work piece handling.
2. Setup of the job.
3. Lead times between receipt of a order and
production.
4. Tool changes.
5. Operator delays.
For a machining operation the term adaptive
control means a control system that measures output process variables and uses
these to control system that measures certain output process variables nad uses
these to control speed and or feed. Some of the process variables and uses
these to control speed and feed. Some of the process variables that have been
used in adaptive control machining systems include spindle deflection or force,torque,cutting
temperature variation amplitude and horsepower .In other words all the
metal-cutting variables have been tried in experimental control systems.
A number of potential benefits accrue to
the user of an adaptive control machine tool . The advantage gained will depend
upon the particular job under consideration. There are obviously many machining
situations for which adaptive control system cannot be justified.
1. Increased production rates
2. Increased tool life.
3.Greater part protection.
4.Less operator intervention.
5.Easier part programming.
- Industrial Robots
An industrial robot is a general purpose programmable
machine possessing certain anthropomorphic characteristics. It is most typically
used for parts handling tasks but can also be used in conjunction with a
variety of manufacturing processes. The robots can be programmed to carry out a
sequence of mechanical movements. It will perform that sequence over and over
again until re programmed to carry out a sequence of mechanical movements .
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| Industrial robots |
General Physical Configuration:
There are two principal robot configuration
:polar and cylindrical.
The two types are illustrated in figure In
the polar configuration the body of the robot pivots either horizontally
vertically or both. Attached to body pivots is the are that moves by body
motion.
The are of the robot in the horizontal orientation
and can be made to move up and down and in or out with respect to the body.
Arm and body motions
1. Vertical traverse Up and down motion of
the arm
2.Radial traverse Extension and retraction
of the arm
3.Rotational traverse Rotation about the
vertical axis Wrist Motions
4.Wrist swivel Rotation of the wrist
5.Wrist bend Up or down movement of the wrist
which also involves a rotational movements.
6.Wrist saw Right or left swivel of the
wrist.
