Feed Drives

7/24/2019 Feed Drives

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DRIVES:Drives are employed for systems that requiremotion control .

e.g. transportation system, fans,robots, pumps,machine tools, etc.

In a CNC machine tool there are three major groupsof elements:

Control and ElectronicsElectric drives Electromechanical drives!

"echanical elements table, slide,tool holder etc!

#lso, $ydraulic and %neumatic systems are

integrated &ith the CNC machine tool'he primary function is to cause motion of thecontrolled machine tool member to conform asclosely as possible to the motion commands issuedby the CNC system


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InCNC machines, the metal is removed as a result ofthe movement of the &or(piece and the cutting toolas sho&n in follo&ing )gure:


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CNC machine tool *.'o maintain a constantmetal removal

rate, the spindle and tool movementto be coordinated such that the spindlehas a constant power and the + slide

has a constant torque.In order to ensure a high degree ofconsistency in production, CNC machinesare built up &ith variable speed drives

&ith higher control accuracies.'he electric motor for spindle drive and

feed drive are sho&n in the follo&ing)gures.


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E ectr c Dr ves on a CNClathe


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arious electric motors used inmachine tools are given belo&:

Spindle drive motors

-. DC shunt "otor eparately e/cited!

0. 'hree phase #1c induction motor

Feed drive motors

-. DC ervomotor

0. #C servo motor

2. tepper motor

Note:

pindle drives require constant po&er

3eed drives require constant torque.


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Spindle drivespindle is an important part of the machine tool.

It gets po&er from its drive unit and deliver to the &or( in

case of lathe and to the tool in case of drilling and millingoperations.

pindle is used to

-. Center the &or(piece or! tool

0. $old the &or(piece or! tool

To increase the stifness and minimi4e the torsionalstrain on the spindles, they are designed as sti5 aspossible.

In case of lathe, &hen a chuc( &or( holding device! is

mounted on the spindle, the accuracy of rotation isimportant as it a5ects the roundness o the worpiece.

The rotational accurac! o the spindle is mostlydependent on the design of the bearings used and

preloading


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pindle is used to*.The accurac! and the qualit!, of the

&or(piece depends directly on the geometric

accuracy, running accuracy and sti5ness of thespindle assembly.

'he various types of spindle bearings used inthe design of a spindle for CNC machine tools

are:-. $ydrodynamic bearings

0. $ydrostatic bearings

2. #ntifriction bearings

'he &or( or! tools carrying spindles arerotating at the order of 26+7666 rpm.

'he main function of the drive is to causemotion to the spindle or! slide to e/ecute

motion commands issued by the CNC system.


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requirements #$# spindledrives:

-. E/cellent running smoothness0. $igh rotational accuracy

2. 8ide constant po&er band.

9. 3ast dynamic response .

. $igh speed up to ;666+06666 pm

7. $igh load capacity


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Feed drives

# feed drive comprises of a feed servomotor andan electronic controller.

3eed motor has special characteristics li(econstant torque and positioning.

In continuous path contouring! operations, severalfeed drives have to 8or( simultaneously since the

desired pro)le path has to be follo&ed continuously.'he follo&ing are feed drive used in CNC machinetools.

-. DC servomotors

0. =rush DC servomotors2. #C servomotors

9. tepper motors

. >inear motors


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Requirements o #$# eed drive:

-. 'he CNC feed drive should have constant torquefor overcoming frictional and &or(ing forces.

0. It should be able to provide variable speed &itha speed range of 6.- rpm to 0666 rpm &ithout

&aviness.2. It should be able to position the tools &ith the

smallest increment of -+0 .tm.

9. >o& electrical and mechanical time constants.

$igh torques to &eight ratio.7. $igh torque for quic( responses.


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DC servomotors

DC servomotors are the high performancemotor and are used in CNC machine tools&here the start and stop must be madequic(ly and accurately.

'he light &eight and lo& + inertiaarmatures of DC servomotors respondquic(ly to the e/citation voltage changes.

%rushless D# servomotors

In this type, the construction of iron coredmotor is turned inside out:

'he rotor becomes a permanent magnet

and the stator becomes a &ound iron core.


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Stepper motor# stepper motor steps rotates slo&ly! in+ )/ed

angular increments.tep si4e or! step angle or! angular increment is

determined by the construction of the motor andthe type of drive system used to control it.

Normally the step angle is -.?6. i.e., 066 steps per

revolution step angle@ -.?A!No&adays, micro step motors are capable of giving

angular increment of 6.6-99A step resolution.

#dvantages of stepper motor +

-. >o& cost0. Buggedness

2. implicity in construction +

9. $igh reliability

. No maintenance.


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&inear motors

"ost of the CNC machine tools use+the linear

motors.'he linear motor consists of a series of

magnets attached to the machine base and aset of electrical coils plotted in a steel

laminate core attached to the moving slide.

"achines built up &ith linear motors canproduce parts &ith higher accuracy and close

tolerances at higher speeds and feeds.>inear motors reduce the non machining time

&ith high acceleration and deceleration rates.

Feed Drives

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