Cnc hardware

CNC Hardware
Arvind Deshpande

4/10/2012 Arvind Deshpande(VJTI) 1

CNC


CNC Machining Centre


Components of CNC


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

CNC Bed
 Cast iron – Damping
properties to reduce
vibrations.
 Heavy ribbing – To
improve static stiffness.
 Concrete – Low cost
and better damping
properties


CNC Bed


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


Spindle


Integral Spindle motor and
cooling system


Rigid spindle assembly


Bifurcated Column structure
 Used in horizontal
machining centre
 Increases torsional
rigidity of the support
structure


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.


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

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


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.


Stepper Motor


Stepper motor


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


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

Hydraulic system


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

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

Recirculating Ball Screw with
deflector


Recirculating Ball Screw with
External Return Tube


 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?


Use of Rolling Friction for
Slideways


Closed Loop Control System


Feedback Elements
 Rotary or linear
 Absolute or incremental
 LED
 Photo transistor or photo
diode


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


Absolute Encoder


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?

Rotary Feedback Elements


Linear Feedback Elements


Mechanical tape reader


Tooling System


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


Tool Turret
 Simple method
 Spindle combined with
tool turret
 Tool change with tool
change procedure
 Tool is identified
directly with tool
position


Chain type Tool Magazine


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