construction of cnc machine-3 unit

8/13/2019 Construction of CNC Machine-3 Unit

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Construction features of

CNC machine


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Operational features of CNC


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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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DRIVE SYSTEM

A drive system consists of amplifiercircuits, stepping motors or servomotorsand ball lead-screws. The MCU feeds

control 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, digitalcontrol of the position of a workpiece in anumerical control machine. The drive unitreceives a direction input (cw or ccw) and pulse

inputs. For each pulse it receives, the drive unitmanipulates the motor voltage and current,causing the motor shaft to rotate bya fixed angle(one step). The lead screw converts the rotarymotion of the motor shaft into linear motion ofthe workpiece .


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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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MACHINE TOOL

CNC controls are used to control varioustypes of machine tools. Regardless ofwhich type of machine tool is controlled, it

always has a slide table and a spindle tocontrol of position and speed. Themachine table is controlled in the X and Y

axes, while the spindle runs along the Zaxis.


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FEEDBACK SYSTEM

The feedback system is also referred to asthe measuring system. It uses positionand speed transducers to continuously

monitor the position at which the cuttingtool is located at any particular time. TheMCU 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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ENCODERS A device used to convert linear or rotational position information

into an electrical output signal.


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Anatomy of Rotary OpticalAbsolute Encoder


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Anatomy of AbsoluteMagneto Linear Encoder


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ENCODERS


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INDUSTRIAL APPLICATIONS of

ENCODERS


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RESOLVERS

A resolver is a rotarytransformer thatproduces an output signal

that 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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CNC CUTTERS

Turning center cutters

Machining center cutters


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TURNING CENTER CUTTERS

Types of cutters used on CNC turningcenters

Carbides (and other hard materials) insertturning and boring tools

Ceramics

High Speed Steel (HSS) drills and taps


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STANDART INSERT SHAPES

Vused for profiling, weakestinsert, 2 edges per side.

Dsomewhat stronger, used forprofiling when the angle allows it,2 edges per side.

Tcommonly used for turningbecause it has 3 edges per side.

Cpopular insert because thesame holder can be used forturning and facing. 2 edges perside.

Wnewest shape. Can turn and

face like the C, but 3 edges perside. SVery strong, but mostly used

for chamfering because it wontcut a square shoulder. 4 edgesper side.

Rstrongest insert but least

commonly used.


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TYPICAL TURNING,THREADING and PARTING TOOLS


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MACHINING CENTER CUTTINGTOOLS

Most machining centersuse some form of HSS orcarbide insert endmill asthe basic cutting tool.

Insert endmills cut manytimes faster than HSS,but the

HSS endmills leave a

better finish when sidecutting.


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MACHINING CENTER CUTTINGTOOLS (contd)

Face mills flattenlarge surfaces quicklyand with an excellent

finish. Notice theengine block beingfinished in one passwith a large cutter.


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MACHINING CENTER CUTTINGTOOLS (contd)

Drills, Taps, and Reamers

Common HSS tools such asdrills, taps, and reamers arecommonly used on CNCmachining centers. Note that a

spot drill is used instead of acenterdrill. Also, spiral point orgun taps are used for throughholes and spiral flute for blindholes. Rarely are hand tapsused on a machining center.


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TOOL HOLDERS

All cutting tools must be held in a holderthat fits in the spindle. These include endmill holders (shown), collet holders, facemill adapters, etc. Most machines in theUSA use a CAT taper which is a modifiedNST 30, 40, or 50 taper that uses a pullstud and a groove in the flange. Themachine pulls on the pull stud to hold theholder in the spindle, and the groove inthe flange gives the automatic toolchanger something to hold onto. HSK tool

holders were designed a number of yearsago as an improvement to CAT tapers,but they are gaining acceptance slowly.

construction of cnc machine-3 unit

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