Cnc hardware

CNC Hardware

Arvind Deshpande

4/10/2012 1Arvind Deshpande(VJTI)

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CNC

Arvind Deshpande(VJTI)

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CNC Machining Centre


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Components of CNC


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Structure of Machine Tool

Continuous use

Higher cutting speeds and feeds results in

higher cutting forces

High speed of movement

High static stiffness - To reduce deflection

under heavy load.

High stiffness to weight ratio – Better

dynamic response


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CNC Bed

Cast iron – Damping

properties to reduce

vibrations.

Heavy ribbing – To

improve static stiffness.

Concrete – Low cost

and better damping

properties


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CNC Bed


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Spindle

Very important element

Subjected to higher cutting forces (radial and axial)

Deflection to be minimized to get proper surface

finish

Proper supports with little overhang

Proper cooling to remove the heat generated

Oil cooling with heat exchanger

Spindle motor and gearbox can be isolated to avoid

heat transmission to machine tool structure


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Spindle


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Integral Spindle motor and

cooling system


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Rigid spindle assembly


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Bifurcated Column structure

Used in horizontal

machining centre

Increases torsional

rigidity of the support

structure


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Spindle Drives

Large power with infinite variation in speed

DC motors were very popular with speed

control using voltage variation

Nowadays AC motors are used with speed

control using frequency variation.


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Feed Drives

Electric motors are by far the most common component to supply mechanical input to a linear motion system.

Stepper motors and servo motors are the popular choices in linear motion machinery due to their accuracy and controllability.

They exhibit favorable torque-speed characteristics and are relatively inexpensive.

1. Stepper motors – Open loop control system

2. Servomotor – D. C. Motor with feedback elements. Used in closed loop control system.

Used in high capacity CNC machines requiring high torgue.

Sturdy, powerful and reliable


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Open Loop vs. Closed Loop controls


Open loop systems

Programmed instructions are fed into the controller through an input

device.

These instructions are then converted to electrical pulses (signals)

by the controller and sent to the servo amplifier to energize the

servo motors.

Drawback - No feedback system to check whether the program

position and velocity has been achieved.

If the system performance is affected by load, temperature,

humidity, or lubrication then the actual output could deviate from the

desired output.

Generally used in point-to-point systems where the accuracy

requirements are not critical. Very few continuous-path systems

utilize open-loop control.


Closed loop systems Feedback subsystem to monitor the actual output and correct any

discrepancy from the programmed input.

Use position and velocity feed back.

The feedback system could be either analog or digital. The analog systems

measure the variation of physical variables such as position and velocity in

terms of voltage levels. Digital systems monitor output variations by means

of electrical pulses.

To control the dynamic behavior and the final position of the machine slides,

a variety of position transducers are employed.

Majority of CNC systems operate on servo mechanism, a closed loop

principle. If a discrepancy is revealed between where the machine element

should be and where it actually is, the sensing device signals the driving

unit to make an adjustment, bringing the movable component to the

required location.

Closed-loop systems are very powerful and accurate

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Stepper Motors

Rotates in steps (fixed angular increments)

200 steps per revolution i.e 1.80 per pulse

Permanent magnet rotor

Micro step motors – 25000 steps/rev

Advantages

1. Low cost

2. Compact

3. Simple in construction

4. High reliability

5. No maintenance (no brushes)

6. Excellent torgue at high speeds

7. Stepper motors can easily accelerate a load.

8. There is usually no need for clutches and brakes (unless a large

external load is acting, such as gravity).


Stepper Motors

9. Stepper motors are inherently digital. The number of pulses

determines position while the pulse frequency determines velocity.

10. Easily and accurately controlled.

11. They offer excellent heat dissipation

Disadvantages

1. The motor can lose position if the load torque exceeds the motor's

available torque.

2. Open loop stepper motor systems should not be used for high-

performance or high-load applications.

3. Damping may be required when load inertia is very high to prevent

motor shaft oscillation at resonance points.

4. Stepper motors may perform poorly in high-speed applications.


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Stepper Motor


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Stepper motor


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Stepper motor


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Stepper Motor

1. If a stepper motor has 24 poles on the rotor

and 30 poles on the stator, what will be the

step angle of this stepper motor?

2. A stepper motor with a step angle of 1.60 is

attached to the lead screw of machine table.

The pitch of the lead screw is 2 mm. What

will be the axial distance traveled for an

input pulse of 300 to the stepper motor?


Servo Motors

Servo motors are more robust than stepper motors, but pose a more

difficult control problem.

They are primarily used in applications where speed, power, noise

level as well as velocity and positional accuracy are important.

Servo motors are not functional without sensor feedback.

3 common types

The DC brush type servo motors are most commonly found in low-

end to mid-range CNC machinery.

The DC brushless type are typically applied to high-end CNC

machinery.

AC servo motors are another variety that offers high-end

performance


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Linear Motors

Series of magnets attached to machine base

and a set of electrical coils around a steel

laminate attached to moving slide

No mechanical parts in contact

No wear or periodic maintenance required

Not limited in travel like ball screws

Can produce parts with higher accuracy and

close tolerances at higher speeds and feeds


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Hydraulic system


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Lead Screw

To convert rotary motion from drive motor to linear motion of the slides

Square threads

Sliding contact results in higher friction

Low efficiency (35-55%)

Greater power requirement

Backlash

CNC requires high speed of movement where friction will be very high


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Recirculating Ball Screw

Balls circulate between screw and but

Balls are recirculated using deflector or return tube

Preloading to reduce backlash

Advantages

1. Longer life

2. Relatively small wear. Will maintain accuracy through entire life of the screw.

3. Rolling motion reduces friction considerably. Can be used for carrying heavier loads at faster speeds.

4. High efficiency(85-95%)

5. Small power requirement


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Recirculating Ball Screw with

deflector


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Recirculating Ball Screw with

External Return Tube


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A ball screw has lead of 0.125 in/rev. What is

the distance the nut will travel if the screw is

turned 15 degrees?


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Use of Rolling Friction for

Slideways


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Use of Rolling Friction for

Slideways


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Closed Loop Control System


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Feedback Elements

Rotary or linear

Absolute or incremental

LED

Photo transistor or photo

diode


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Absolute Encoder

Sensor detects exact

position of the shaft

Chances of error

whenever 2 bits

change

Grey code used to

reduce error so that

only one bit changes

at a time


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Absolute Encoder


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Incremental Encoder

1 track and 2 sensors

More resolution

Can measure relative

position only

An incremental encoder

has 240 strips

engraved on it. What

will be no. of strips to

be swept for 300

rotation?


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Rotary Feedback Elements


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Linear Feedback Elements


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Linear Feedback Elements


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Mechanical tape reader


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Tooling System


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Tool Presetting

CNC programme is

prepared for tool tip

Reference can be a

fixed point on the tool

holder

Measure the distance

of tool tip from

reference


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Tool Turret

Simple method

Spindle combined with

tool turret

Tool change with tool

change procedure

Tool is identified

directly with tool

position


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Chain type Tool Magazine


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Chain type Tool Magazine


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