introduction to CNC machines

1. Department of Collegiate and Technical Education
SESSION 2
CNC PROGRAMMING AND MACHINING(20ME42P)
WEEK 1
(IV Semester)
Mechanical Engineering
Mechanical Engineering – 20ME42P

2. OVER VIEW
• Introduction
• Machine structure
• Bed
• Spindle bearings
• Drives
• Axes
• Ball screws
• Outcome
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3. Mechanical Engineering – 20ME42P

4. Introduction
 The construction of CNC machines differs greatly from that of
conventional machine tools.
 This is because of requirement of higher performance levels
 The CNC machines generally employ the different
mechatronics elements that have been developed over the
years. However the quality and reliability of these computer
numerically controlled machines depends on the various
machine elements and other subsystems of the machine.
Mechanical Engineering – 20ME42P

5. Important parts and aspects of CNC machines to
be considered in their design
• Machine structure
• Guide ways
• Feed drives of CNC
• Motors
• Mechanical transmission systems
• Measuring systems
• Spindle and spindle bearings
• Controls, software’s and operator interface
• Tool monitoring.
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6. Machine structure
• The CNC machine structure is the main load carrying and supporting member
of the machine tool.
• All drives, motors, bearings, mechanical transmission systems are rigidly fixed
to the machine structure.
• While working, the machine structure is subjected to both static and dynamic
forces, therefore it is essential that the structure should not deform or vibrate
beyond the permissible limits under the action of these forces.
• All components of the machine structure remain in correct relative positions to
maintain the geometric accuracy irrespective of magnitude and direction of
these forces. Mechanical Engineering – 20ME42P

7. The basic factors which are considered for the design of
machine structure are:
• Static load
• Dynamic load
• Thermal load
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8. Static load
• The static load develops on the machine
structure due to the weights of slides and
the work pieces, and the forces due to
cutting action.
• The structure should have adequate
stiffness and proper structural
configuration to keep the deformation
zero, the structure with in the permissible
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9. Dynamic loads
• The dynamic load is a constantly changing forces acting on the
structures while movement is taking place.
• These forces cause the whole machine system to vibrate.
• The origin of the vibrations are due to the
1. Unbalanced rotating parts
2. Improper machining of gears
3. Bearing irregularities
4. Interrupted cuts while machining (in case of milling)
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10. The effect of vibrations on machine structure can be reduced
by:
1. Reducing the mass of the structure
2. Increasing the stiffness of the structure
3. Improving the damping properties.
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11. Thermal loads
The thermal loads are due to the local heat sources, which set up
thermal gradients within the machine structure.
• Electric motor
• Friction in mechanical drive and gear boxes
• Friction in bearings and guide ways
• Machining processes
• Temperature of surrounding objects
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12. The effect of thermal deformation can be reduced by
• External mounting of drives.
• Removing frictional heat by proper lubrication.
• Efficient coolant and swarf removal system for the dissipation of heat
generated from the machining process.
• Thermo symmetric designing of the structure.
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13. CNC machining bed
The bed is kind of hardened parts of a machine because the tool
turret travels over the CNC lathe bed. The bed of the control
mechanism is the basic support for the entire machine. It is
generally used to place important parts such as guide rails and
headstock to meet the machining accuracy of CNC machine
tools.
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14. Spindle Bearings
As the spindle carrying the tool rotates high speeds, during
the machining operations, it is subjected to torsional
vibrations, bending and sometimes to thrust loading. Hence the
spindle should be supported by bearings such that the spindle
attains more stiffness and the above stresses are reduced.
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15. Types of spindle bearings
• Hydro-dynamic bearing
• Hydro-static bearing
• Anti –friction bearing
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16. Hydrodynamic Bearings
These are the journal bearing with a thin film of oil
maintained between the spindle and the journal.
These are used when the load bearing capacity is low
and frequent starting and stopping of the spindle is
not required. Essential features of these bearing are
their simplicity, good damping properties and a good
running accuracy. The pressure of the oil is created
within the bearing by the rotation of the spindle. Mechanical Engineering – 20ME42P

17. Hydrostatic Bearings
In the hydro static bearing a thick film of oil is
maintained under pressure between the spindle and
bearing. The oil is raised to high pressure by means
of a pump mounted externally to the bearing. After
some time , the oil is exhausted outside , it is
cooled and recirculated. Load carrying capacity of
this bearing is independent of the rotational speed.
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18. Anti friction Bearing
• These bearings are suitable for high speeds and loads,
They are classified into 2 types
• Ball bearings
• Roller bearings
The balls and rollers are enclosed between inner race and outer race. The
main load transferred from the rotating shaft to the bearing by a rolling
contact. But it is important that the races should be perfectly aligned
otherwise the bearing gets damaged. Mechanical Engineering – 20ME42P

19. Drives
Basic function of a CNC machine is to provide automatic and precise motion
control to its elements such work table, tool spindle etc. Drives are used to
provide such kinds of controlled motion to the elements of a CNC machine
tool. The control unit sends the amplified control signals to actuate drive
motors which in turn rotate the ball lead screws to position the machine table
or cause rotation of the spindle.
Drives used in automated system or in CNC system are different types such
as electric, hydraulic or pneumatic Mechanical Engineering – 20ME42P

20. Spindle Drives
• The spindle drives are used to provide angular motion to the
work piece or a cutting tool.
• These drives are essentially required to maintain the speed
accurately with in a power band which will enable machining
of a variety of materials with variations in materials hardness.
• The speed ranges can be from 10 to 20,000 rpm
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21. Feed Drives
These are used to drive the slide or a table. The drive speed
should be extremely variable with a speed range of about
1:20000 which means it should have a maximum speed of
around 2000 rpm and at a minimum speed of 0.1rpm. The feed
motor must run smoothly. The feed drive consists of the
following
• Electric system
• Mechanical Transmission system Mechanical Engineering – 20ME42P

22. Electric System
The electric system uses 2 types of motors
 Servo motor
 Stepper motor
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23. Servo system block diagram
 SERVO MOTORS
• Servomotors are special electromechanical devices that produce
precise degrees of rotation.
• Aservo motor is a DC orAC or brushless DC motor combined
with a position sensing device.
• Servomotors are also called control motors as they are involved in
controlling a mechanical system.
• The servomotors are used in a closed-loop servo system as shown
in FigureAreference input is sent to the servo amplifier, which
controls the speed of the servomotor.
33

24. Servo Motor
The DC servo motors are similar to shunt motors in construction. These motors
are designed to provide a controlled acceleration and de-acceleration. These
motor provide excellent speed regulation with higher torque and higher
efficiency.
In recent years, the DC servo motors are replaced by AC servo motor due to
following features.
• Very quick response.
• Higher dynamic stiffness.
• Supplies constant torque over the entire range of speeds.
• More compact.
• High in reliability.
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25.  STEPPER MOTOR
A stepper motor is a pulse-driven motor that
changes the angular position of the rotor in steps.
Due to this nature of a stepper motor, it is
widely used in low cost, open loop position control
systems.
Types of stepper motors:
o Permanent Magnet
Employ permanent magnet
Low speed, relatively high torque
o Variable Reluctance
Does not have permanent magnet
Low torque

26. Stepper Motor
These motors are used to provide either a continuous or discrete
angular motions. These motors are generally reluctance motor.
The stepper motors are used for accurate control of the position
and the velocity in the axis drive. The working principle of a
stepper motor is receiving a signal called pulse from the control
unit that is microprocessor.
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27. Mechanical Transmission System
These are the elements between the feed drives and slides which
transmits the torque and motion from the feed drives to the slides
The mechanical elements are classified into two types
• Elements which convert the rotary motion into linear motion.
Eg : Rack and pinion, Screw and nut.
• Torque transmitting element. Eg : Gear box and Timing belt
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28. Screw and Nut
• This is the simplest mechanism used to convert rotary motion to linear
motion when the screw is fixed and nut is rotated it moves axially along the
screw axis. The conventional V-threads or square threads are used for the
power transmission.
• Following are the advantages of screw and nut mechanism.
• Simple in construction.
• High load bearing capacity.
• Easy to manufacture. Mechanical Engineering – 20ME42P

29. Torque transmitting elements
Gear Box
A gear box is used to get the desired torque and speed form the
driver. From the gear box it is possible to get high speed
reduction or the velocity ratio of any value can be achieved. The
gear box will also reduce the inertia load on motor shaft .They
are more frequently used where reduction is required between
non coaxial or non -parallel shafts.
Mechanical Engineering – 20ME42P

30. Flexible coupling
These are the couplings which are used when there is a misalignment
between the two shafts. These elastic coupling are also called as elastic
flexible coupling because their elastics properties vary to compensate the
misalignment between the two shafts. Certain amount of flexibility is built in
to the coupling to compensate for these errors.
The following are the example of flexible couplings.
1 Universal coupling
2 Oldham`s coupling
3 Bushed type coupling Mechanical Engineering – 20ME42P

31. CNC AXES
To program the NC processing equipment a standard axis system
must be defined by which the position of the work head relative
to the work part can be specified. There are two axis systems
used in NC, one for flat and prismatic work parts and the other
for rotational parts. Both axis systems are based on the Cartesian
coordinate system.
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32. The axis system for flat and prismatic parts consists of the three
linear axes (x, y, z) in the Cartesian coordinate system, plus three
rotational axes (a, b, c), as shown in Figure
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33. The coordinate axes for a rotational NC system are illustrated
in Figure . These systems are associated with NC lathes and
turning centers-Although the work rotates, this is not one of
the controlled axes on most of these tuning machines.
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34. Ball screws
Ball screws are the ideal lead screw for CNC because they
minimize backlash and friction to the highest degree
possible. They’re called “ball screws” because the circulate
ball bearings in the ball nut. The ball bearings ride in a groove
on the ball screw, and it is the use of ball bearings that makes
the high precision, low backlash and low friction possible.
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35. • However, since the groove in the ball screw is helical, its steel
balls roll along the helical groove, and, then, they may go out of
the ball nut unless they are arrested at a certain spot.
• It consists of a screw spindle, a
nut, balls and integrated ball return
mechanism a shown in Figure .
• The flanged nut is attached to the
moving part of CNC machine tool.
As the screw rotates, the nut
translates the moving part along
the guide ways.
Ball screw / ball bearing screw /
recirculating ballscrew Mechanism
Ballscrew configuration

36. Mechanical Engineering – 20ME42P

37. Outcome
• Identify various components of a CNC machine and use of
those components for any given CNC operation
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38. Thank You
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