Cnc machine tools

UNIT-8
CNC Machines tools
Introduction:-
Production is a process of converting raw material into finished products. The process of production is
achieved by the collective efforts of man and machine using materials and tools. It requires information
and energy to accomplish production by machining. Over the period of time, several techniques and
methods are used in the process. The evolution of new methods of production saw the increasing
involvement of machines in providing the information and energy required for production and the role
played by the humans declined. Prepared programs consisting of information’s and instructions took the
role of controlling the machines instead of manual control. This has led to a manufacturing system of
higher production at lower cost with more accuracy.
Numerical Control (NC):-
The numerical control is the operation of machine tool by a series of coded instructions of numbers,
letters and other symbols to carry out specific task. A combination of manufacturing machine and
numerical control results in numerical control machine (NC machine).
Numerical control can be defined as a system in which actions of a machine tool are controlled by
recorded information’s in the form of numerical data.
Computer Numerical Control (CNC):-
CNC controls are soft wired NC systems as control functions are controlled by software programs. It uses
minicomputer or microcomputer in place of machine control unit used in NC system.
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Workpiece of CNC system:-
The external appearance of CNC system is similar to that of conventional NC system. The tape reader is
used for reading part program into computer memory. The computer hardware interface and servo
system are used to convert part program instructions into machine tool motions through control
algorithm’s contained in the computer. So that machine tool will perform useful work.

Classification of CNC machines:-
1. Milling and machining centers.
2. Lathes and turning centers.
3. Edm machines.
4. Brinding machines.
5. Cutting machines.
6. Fabrication machines.
7. Welding machines.
8. Coordinate measuring machines.
Applications:-
Milling, lathes, lathe turning, grinding, cutting, welding.. etc.
Control systems of a CNC:-
1. Point-to-point system
2. Continuous path control systems.
3. CNC drive systems.

1) Point-to point system:-
CNC controls can be either or point-to-point or continuous path system. The point-to-point control
moves the tool to the programmed point k, normally in rapid traverse, without engaging the workpiece.
PTP total path follow one of the three modes axial path, 45* line and linear path as shown sign.
The PTP system are typically used in drilling, punch presses and spot welders.
2) Continuous path:-
In this motion of the tool and work piece are controlled along many axis simultaneously in the system
and this facilitates machining of different types of curved surfaces and profiles.
3) CNC drive system:-
The structure of CNC control can be either an open loop or closed system. Both are depends on system
of feedback loop to insure heat accuracy of system performance.
a) Open loop control:-
Open loop is characterized by no feedback system in the drive system. Machine control unit provides

control to the drive motors it is being assumed by the system that the machine table will reach the
target position.
There is no way from the machine control unit to know the actual performance of the system.
PTP system usually used open loop drive systems. Where cutting tool does not engage with work piece
during positioning then also be used in light loaded cutting machines.
B) Closed loop controls:-
With closed loop drive systems feedback subsystems are used to monitor the actual output and correct
any discrepancy between the desired and the actual system performance. Feedback systems are 2
types.
i. Analog feedback system
ii. Digital feedback system
In this digital feedback system is used a digital position transducer to measure the position.
Constructional features of CNC machines:-
1. The CNC machine can only be compensate for an error that is detected and communicated to the
control. Deflection, vibrations etc. Can't as at be easily monitored.
2. The capacity for varying the conditions while machines therefore is limited. As far as possible free
condition have to be established as the program is produced.
3. CNC machines are spending more time for shift cutting than conventional machine did in the past.
This higher percentage of cutting sides and transmission systems.
4. In conventional machine tools the operator standing directly in front controlling the machine this is
no longer required for CNC. Since the machine is operating under program control.

CNC controls:-
1. Easy retrace
2. Plate setup and rotation.
3. Feed control.
4. Spindle speed control.
5. Emergency stop.
6. Data logging.
7. Error messages.
Advantages and limitations of CNC:-
1. Reduces production coast.
2. Improved product quality.
3. Facilitate production planning and control.
4. Increased productivity.
5. Greater flexibility.
limitations:-
1. High initial investment.
2. High maintenance.
3. For low production it is costlier process.
ADVANTAGES AND DISADVANTAGES
OF CNC MACHINES
ADVANTAGES:-
1. CNC machines can be used continuously 24 hours a day, 365 days a year and only
need to be switched off for occasional maintenance.
2. CNC machines are programmed with a design which can then be manufactured
hundreds or even thousands of times. Each manufactured product will be exactly the

same.
3. Less skilled/trained people can operate CNCs unlike manual lathes / milling
machines etc.. which need skilled engineers.
4. CNC machines can be updated by improving the software used to drive the
machines
5. Training in the use of CNCs is available through the use of ‘virtual software’. This
is software that allows the operator to practice using the CNC machine on the screen
of a computer. The software is similar to a computer game.
6. CNC machines can be programmed by advanced design software such as
Pro/DESKTOP®
, enabling the manufacture of products that cannot be made by
manual machines, even those used by skilled designers / engineers.
7. Modern design software allows the designer to simulate the manufacture of his/her
idea. There is no need to make a prototype or a model. This saves time and money.
8. One person can supervise many CNC machines as once they are programmed they
can usually be left to work by themselves. Sometimes only the cutting tools need
replacing occasionally.
9. A skilled engineer can make the same component many times. However, if each
component is carefully studied, each one will vary slightly. A CNC machine will
manufacture each component as an exact match.
DISADVANTAGES:-
1. CNC machines are more expensive than manually operated machines, although
costs are slowly coming down.
2. The CNC machine operator only needs basic training and skills, enough to
supervise several machines. In years gone by, engineers needed years of training to
operate centre lathes, milling machines and other manually operated machines. This
means many of the old skills are been lost.
3. Less workers are required to operate CNC machines compared to manually
operated machines. Investment in CNC machines can lead to unemployment.
4. Many countries no longer teach pupils / students how to use manually operated
lathes / milling machines etc... Pupils / students no longer develop the detailed skills
required by engineers of the past. These include mathematical and engineering skills.

Cnc machine tools

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