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CONSTRUCTIONAL DETAILSOF CNC MACHINES
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The main function of a machinetool is to control the workpiece-
cutting tool positional relationshipin such a way as to achieve a
desired geometric shape of the
workpiece with sufficientdimensional accuracy.
Machine tool
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Machine tool provides:work holding
tool holding
relative motion between tool
and workpiece
primary motion
secondary motion
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Primary motion
Relative motion
between tool and
workpiece
Secondary motion
Cutting motion
Cutting speed
Feed motion
Feed rate
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m a c h i n e c o n t r o l u n i t
p o s i t i o n t r a n s d u c e r s
w o r k h o l d i n g d e v i c e
t o o l h o l d i n g d e v i c e
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CLASSIFICATION OF MACHINE TOOLS
THOSE USING
SINGLE POINT
TOOLS
THOSE USING
MULTIPOINT
TOOLS
THOSE USING
ABRASIVE
TOOLS
lathes
shapers
planers
boring m/cs
etc.
drilling m/cs
milling m/cs
broaching m/cs
hobbing m/cs
etc.
grinding m/cs
honing m/cs
etc.
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NUMERICALLY CONTROLLED M/CTOOLS:
An NC machine tool is functionally the same as aconventional machine tool.
The technological capabilities NC machine tools interms of machining are no different from those ofconventional ones.
The difference is in the way in which the variousmachine functions and slide movements arecontrolled.
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The functions and motions such as;
turning the spindle on and offsetting cutting speedssetting feed rate
turning coolant on and offmoving tool with respect to workpiece
are performed by Machine Control Unit(MCU) in NC machine tools.
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CNC SYSTEM ELEMENTS
A typical CNC system consists of thefollowing six elements
Part program
Program input device Machine control unit
Drive system
Machine tool Feedback system
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NC SYSTEM ELEMENTS
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OPERATIONAL FEATURES of CNC
MACHINES
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PART PROGRAM A part program is a series of coded instructions required
to produce a part. It controls the movement of themachine tool and the on/off control of auxiliary functionssuch as spindle rotation and coolant. The codedinstructions are composed of letters, numbers andsymbols and are arranged in a format of functional
blocks as in the following exampleN10 G01 X5.0 Y2.5 F15.0| | | | || | | | Feed rate (15 in/min)
| | | Y-coordinate (2.5")| | X-coordinate (5.0")| Linear interpolation modeSequence number
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PROGRAM INPUT DEVICE
The program input device is themechanism for part programs to be
entered into the CNC control. The mostcommonly used program input devices arekeyboards, punched tape reader, diskette
drivers, throgh RS 232 serial ports andnetworks.
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MACHINE CONTROL UNITThe machine control unit (MCU) is the heart of a CNCsystem. It is used to perform the following functions:
Read coded instructions
Decode coded instructions
Implement interpolations (linear, circular, and helical) togenerate axis motion commands
Feed axis motion commands to the amplifier circuits fordriving the axis mechanisms
Receive the feedback signals of position and speed foreach drive axis
Implement auxiliary control functions such as coolant orspindle on/off, and tool change
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TYPES of CNC CONTROL SYSTEMS
Open-loop control
Closed-loop control
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OPEN-LOOP CONTROL SYSTEM
In open-loop control system step motors areused
Step motors are driven by electric pulses Every pulse rotates the motor spindle through a
certain amount
By counting the pulses, the amount of motion
can be controlled No feedback signal for error correction
Lower positioning accuracy
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OPEN-LOOP CONTROL SYSTEM
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CLOSED-LOOP CONTROL
SYSTEMS In closed-loop control systems DC or AC
motors are used
Position transducers are used to generateposition feedback signals for errorcorrection
Better accuracy can be achieved More expensive
Suitable for large size machine tools
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CLOSED-LOOP CONTROL
SYSTEMS
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DRIVE SYSTEM
A drive system consists of amplifiercircuits, stepping motors or servomotors
and ball lead-screws. The MCU feedscontrol signals (position and speed) ofeach axis to the amplifier circuits. Thecontrol signals are augmented to actuate
stepping motors which in turn rotate theball lead-screws to position the machinetable.
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STEPPING MOTORS
A stepping motor provides open-loop, digital control
of the position of a work piece in a numerical controlmachine.
The drive unit receives a direction input (cw or ccw)
and pulse inputs.
For each pulse it receives, the drive unit manipulatesthe motor voltage and current, causing the motorshaft to rotate by a fixed angle (one step).
The lead screw converts the rotary motion of themotor shaft into linear motion of the work piece .
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STEPPING MOTORS
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RECIRCULATING BALL SCREWS
Transform rotational motion of the motorinto translational motion of the nut attached tothe machine table.
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RECIRCULATING BALL SCREWS
Accuracy of CNCmachines depends ontheir rigidconstruction, care inmanufacturing, andthe use of ball screwsto almost eliminate
slop in the screwsused to move portionsof the machine.
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POSITIONING
The positioning resolution of a ball screw drivemechanism is directly proportional to thesmallest angle that the motor can turn.
The smallest angle is controlled by the motorstep size.
Microsteps can be used to decrease the motor
step size. CNC machines typically have resolutions of
0.0025 mm or better.
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MACHINE TOOL
CNC controls are used to control varioustypes of machine tools. Regardless of
which type of machine tool is controlled, italways has a slide table and a spindle tocontrol of position and speed.
The machine table is controlled in the Xand Y axes, while the spindle runs alongthe Z axis.
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FEEDBACK SYSTEM
The feedback system is also referred to asthe measuring system. It uses position
and speed transducers to continuouslymonitor the position at which the cuttingtool is located at any particular time.
The MCU uses the difference between
reference signals and feedback signals togenerate the control signals for correctingposition and speed errors.
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CNC MACHINES FEEDBACK
DEVICES
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POTENTIOMETERS
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POTENTIOMETERS
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ENCODERS
A device used to convert linear or
rotational position information into anelectrical output signal.
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ENCODERS
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INDUSTRIAL APPLICATIONS of
ENCODERS
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RESOLVERS
A resolver is a rotarytransformer that
produces an output signalthat is a function of therotor position.
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SERVOMOTOR with RESOLVER
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VELOCITY FEEDBACK
Tachometers:Electrical output is proportional to rate ofangular rotation.
Encoders, Resolvers, Potentiometers:Number of pulses per time is proportionalto rate change of position.
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The safety of the operator is very important
aspect that cannot be overlooked. To answer
safe working condition CNC machine tools are
provided with metallic or plastic guard.
Where it is not possible to provide effective
guards, proximity protection is provided by
pressure mat or light barrier.
3.Safety of the operator:
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The overall guard or perimetery guard serves as an
enclosure for the machine tools.
The perimetery guard protects the operator against
flying swarf and from many accidents by hitting againstthe moving component when the machine is working.
The access to the machine is provided to through large
sliding door for setting up the machine and forloading/unloading of the work piece.
4.Perimeter guard:
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The doors have various types of interlock switches fittedon them. If the door is opened when the machine is
working, the control unit will flash a warning signal or
activate an auditory signal like buzzer.
On some machine the power to the machine may be cut offif the doors are kept open beyond a certain period of time.
During set up period, the operator can cancel the warning
signal. The guards are fitted with transparent window so
that machining area is visible from the operator side.
4.Perimeter guard:
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The pressure mats are used on milling, grinding,
machine where the machine table can move to the
either side of machine.
Since the table moves at the rapid rate it may causesome accident if the operator is standing too close to
the machine.
The pressure mats are placed around the machine andif someone crosses the mat, a warning signal is
generated.
5.Pressure mat:
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Light barriers are also provided on milling, drilling
or grinding machine.
The light barrier consists of a light source, usuallyinfrared, sending a beam to the light sensitive cell.
If anything obstruct light beam, a warning signal is
generated the light barrier are placed around the
machine. The operator if required can make them
in active.
6.Light barrier: