ch2 constrc cnc machine

7/27/2019 Ch2 Constrc CNC Machine

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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:

ch2 constrc cnc machine

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