1. CNC MACHINESCNC MACHINES AND ITSAND ITS
( Mechanical Engineering / Special Machines / Unit 5)( Mechanical Engineering / Special Machines / Unit 5)
v CNC Machines
vComponents of CNC machine
Dr.S.Cartigueyen, M.Tech., Ph.D.
2. v CNC MACHINES
o Modern manufacturing systems are advanced automation systems. The numerical
control (NC) and computer numerical control (CNC) machines are an integral part of
the automation systems.
o CNC machining is capable of meeting the tightest tolerances, and producing the most
accurate, precise products over and over again. First ‘CNC Machining’ means :
Machining : Process of removing metal with the assistance of mechanical machines.
NC – Numerical Control : Governing the machines automatically through a set of
instructions (letters, numbers, symbols).
CNC – Computer Numerical Control : Process by which an operator can write, adjust
and implement instructions using a computer comfort.
q Numerical Control (NC)
o NC is defined as control of machine slide movements and various functions by
means of letters, numbers and symbols
o Letters, numbers and symbols forming some sequence is called Part Program.
o The part program is arranged in the form of blocks of information. Each block
contains the alphanumerical data required to produce one segment of the work piece.
o The part program is translated into the appropriate electrical signals for input to
motors that run the machine.
3. q Components of NC System
o Three basic components are 1. Program of instructions 2. Machine control
unit (MCU) and 3. Machine tool.
o The coded instructions are to cause the machine to perform a specific series
of operations. Basically, it is control of machine tools by numbers.
Functions automated by NC system
- Starting and stopping of machine tool spindle.
- Controlling the spindle speed.
- Positioning the tool tip
- Guiding the tool in desired paths
• Reduces non productive time
• Reduces manufacturing lead time
• Greater manufacturing flexibility
• Improves quality control
• Reduced inventory
• High investment cost
• High maintenance effort.
• Part programming
• Higher utilization of NC equipmentsBasic Components of NC System
5. q Working Principle of a CNC System
o In CNC systems the part program is fed into the computer. Then the
instructions are read and interrupted by the controller to convert it into signals
that activate the machine tool.
o The signals are sent to the servo control system. This servo control system
control the machine tool for performing different operations.
o The feed back transducer checks the actual position of the machine tool slide
and passes it to CPU.
o If there is any difference between given input and actual position achieved, the
CPU takes corrective action to achieve the given input values by suitable
instructions to the machine tool.
o Closed loop CNC system has both velocity and position control loops.
o Software of CNC system has three major programs
Part Program: It has part dimensions, Spindle speed, Feed rate, etc.
Service Program: Used to check , edit, correct the part program.
Control Program: Accepts the part program as a input data to produce
signals to drive the axes of motion.
6. Machine Control Unit (MCU)
o It consists of programmable logic controller (PLC), processor, memory devices. PLC
controls the spindle ON/OFF, coolant ON/OFF, tool path, speed, feed and tool changes.
o It converts the program commands into signals.
Operator Control Panel
o It provides the interaction between user & CNC system. Also it consists of monitor,
feather touch keyboard, machine control panel.
q Schematic diagram of a CNC machine tool
7. Working Principle of a CNC System …
Servo Control Unit & Drives
o PLC gives command signal to servo drives , this command signal values are
converted into actual movement on the machine by servo drives.
o PLC receives feedback signal about actual movement of the machine tool
through feedback devices. Feedback devices are transducers & encoders. This
feedback signals are compared with the Input signals and error correction is done.
q Features of CNC Machines
1. Storage of more than one part program
- CNC machine can store multiple programs. It can be used again and again.
2. Various forms of program input
- Multiple data entry capabilities such as magnetic tape, floppy discs, manual
data input, communication ports, etc.
3. Program editing at the machine tool
- Helps in testing and correcting a program entirely at the machine tool.
- Output can be viewed without doing actual machining
8. Features of CNC Machines…
4. Fixed cycles and programming sub-routines
- Fixed cycle : Frequently used machining cycle as Macros, stored in memory and can
be called into every other programs.
- Helical, parabolic and cubic interpolations are executed only through CNC (as a
stored program algorithm)
6. Positioning features for set up
- Position set feature (software option) in CNC is used to set the work piece or fixture
with respect to the machine tool axes.
7. Cutter length and size compensation
- Compensations like tool nose radius, cutter radius etc are automatically made in the
computed tool path.
8. Acceleration and deceleration calculations
- Feed rate is smoothly decelerated and then accelerated back after direction change.
9. Communications interface
- Machine can be linked to other computers & computer-driven devices (like robots)
- On-line diagnostics capability to detect mal-functions / system breakdowns etc.
10. Advantages of CNC Machines
q Higher accuracy
q Reduce lead time
q Higher flexibility
q Reduce scrap rate
q Reliable operation
q Consistent quality
q Reduced manpower
q Increased productivity
q Reduced non productive time
q Shorter cycle time
q JIT Manufacture
q Lesser floor space
q Increased operational safety
q Machining advanced materials
Disadvantages of CNC Machines
§ High machine cost
§ Complicated maintenance
§ Parts are imported from abroad
§ High tool cost
§ Temperature, humidity and dust must
Applications of CNC System
• Parts needed in a hurry
• Parts with complicated contours
• Parts requiring expensive jigs and fixtures
• Parts those have several engineering changes
• Cases where human errors can be extremely
• Parts requiring close tolerance or good
11. q CNC Turning Centre / CNC Lathe
- CNC Turning centre is a machine tool capable of performing various turning and
related operations in one setup under CNC system.
- CNC turning centers are designed mainly for machining shaft-type work pieces
which are supported by a chuck. It is capable of producing cylindrical parts in
greater volume with high accuracy.
- Main parts of the CNC turning centre are bed, head stock, tailstock, turrets, servo
system and MCU.
- The various work holding devices are, self-centering chuck , collect chuck,
counter centrifugal chuck.
- CNC turning centre tool turret have the capacity of 8, 12 or 16 tools.
Two axis control
Z axis - Parallel to spindle axis,
X axis - Perpendicular to spindle axis
(a) Horizontal spindle turning centre
(b) Vertical spindle turning centre
12. (a) Horizontal Spindle Turning Centre
- CNC chucking centre is designed to machine most work that is held in a chuck.
- Chucking machines usually have shorter beds and a single saddle with single
drum type turret or two independent saddles with turret as shown in the figure.
- Checking the dimensions of
work piece after machining.
- Sensing the tool whether is
worn-out or not.
- Automatic tool changing
when tools have broken.
- Automatic work piece
changer at the completion of
Horizontal Spindle Turning Centre
14. (b) Vertical Spindle Turning Centre
- Vertical CNC lathes widely used for machine heavy components.
- Some of these machines can also be used for milling operations. Such machines
are sometimes known as turn -mill centers.
- The tool turret is placed in a plane above the spindle.
Rotary Table Vertical Spindle Turning Centre
Machine Axes Convention
15. q Machining Centre / Milling Centre
- A machining centre (MC) is capable of performing multiple operations like
milling, drilling, reaming, tapping, boring and allied operations.
Automatic tool changer,
Automatic work piece positioning and
Automatic pallet changer.
Five axis control
X axis, Y axis, Z axis - Linear movements.
A axis - Spindle Tilt and
B axis - Table rotation.
(a) Horizontal spindle machining centre
(b) Vertical spindle machining centre
(c) Universal machining centre
- Main parts of CNC machining centre are bed, column, spindle, table, servo
mechanism, tool magazine, automatic tool changer (ATC) and automatic pallet
16. (a) Horizontal Spindle Machining Centre
- HMC is a machining center with its spindle in a horizontal orientation.
- X axis - Table longitudinal movement
- Y axis - Spindle up and down movement
- Z axis - Table movements towards spindle
- HMC have rotary table with axis parallel to Y axis and is called B axis
- Generally these are single spindle machines with ATC & APC
Machine Axes Convention
17. Horizontal Spindle Machining Centre (HMC)…
- This machining center design favours uninterrupted production work.
- One reason for this is that the horizontal orientation encourages chips to fall away,
so they don't have to be cleared from the table.
- Horizontal design allows a two-pallet work changer which saves time.
- Work can be loaded on one pallet of an horizontal machining center while
machining occurs on the other pallet.
- Used for heavier work piece with large metal removal.
Horizontal Spindle Machining Centre
18. (b) Vertical Spindle Machining Centre (VMC)
• VMC has its spindle on vertical axis.
• Used for heavier work piece with large
• X axis - Table longitudinal movement
• Y axis - Table cross wise movement
• Z axis - Spindle up & down movement
• VMC have rotary table with axis parallel
to X axis & is called A axis
•Tool magazine have up to 60 tools
• Generally these are single spindle with
ATC or multi spindle with turret head.
(c) Universal Machining Centre (UMC)
- Having both vertical and horizontal spindles
- Capable of machining vertical, horizontal and diagonal surfaces.
- Changing the spindle configuration from horizontal to vertical and vice versa is
Vertical Spindle Machining Centre
19. q Machine Axes Conventions – Co-ordinate System
• Each ‘axis of motion’ has a separate driving device ( stepper motor / servo motor)
• Z axis is always in rotating ‘ Spindle’.
• Three main axes of motion will be referred to as the X, Y and Z axes.
• Thumb – X axis ; Index finger – Y axis ; Middle finger – Z axis
20. q Turning Centre Axes Conventions qMachining Centre Axes Conventions
o Basically CNC turning centre has two axes namely X and Z axis.
o CNC Machining centre has three linear axes namely X, Y and Z.
o Axis designation appear with positive (+) or negative (-) signs for direction of travel.
21. q Co-ordinate Measuring Machine (CMM)
v CMM is an electro mechanical system designated to measure several features of
a work piece using ‘ Probe (Sensor) ’ as given below:
o to locate a surface
o to locate the centre of a hole, cylindrical shafts
o to locate geometric centre of a pocket/spigot
o to measure angles
o to measure a series of coordinate points relative to a given axis
Various Parts of Bridge type CMM
22. Co-ordinate Measuring Machine (CMM)…
o CMMs are built rigidly and very precise. They are equipped with digital read-out
or can be linked to computers for online inspection of parts.
o CMM consists of a measuring table to hold the work piece. The probe can be
moved in three directions and brought into contact with the surface to be measured.
o CMM records even complex profiles with high sensitivity and speed.
o Probe can be position to a resolution of 1 micron.
1. Probe head and probe
2. Mechanical structure
3. Drive system and
4. Digital computer with
Bridge type CMM
23. q Construction of Coordinate Measuring Machine (CMM)
o Based on the construction & mechanical structure of CMM, it can be classified as
24. Coordinate Measuring Machine (CMM)...
Specifications of CMM
- Measuring range (X, Y & Z axis)
- Axis guide method (air bearing)
- Drive speed
- Maximum measuring speed
- Maximum measuring acceleration
- Maximum work piece length
- Maximum work piece weight
- Temperature range
- Temperature variation
- Probe type
- Table design
Benefits of CMM
- High production rate
- Less man power required (automated)
- High precision
- Reduced operator errors
- High integrity with FMS
Disadvantages of CMM
-Table and probe may not be in perfect
- Probe may have run out.
- Probe moving in Z-axis may have some
- Probe will move X and Y direction but not
be square to each other.
- Chances of errors in digital system
26. v COMPONENTS OF CNC MACHINE
o The major components of a CNC machine are controller, drives and mechanical
Drives Primary function of drives is to cause motion of the spindle and slide ways
as per the command.
Types of Drives (i) Electrical drives (ii) Hydraulic drives
1. Spindle drives (constant power) – AC/DC motors
2. Feed drives (constant torque) – AC/DC Servo motors, Stepper motors , Linear
Spindle (constant power) drive Feed (constant torque) drive
27. q Slide Movement Elements
o Precise positioning and repeatability of machine tool, slides are the major
functional requirements of CNC machines.
o The inaccuracies that caused are mainly due to the stick slip motion when plain
slide ways (metal to metal contact) are used.
o To eliminate stick-slip, there are different slide ways systems such of rolling
friction slide ways and slide ways with low friction are required.
Requirement of Good Slide Ways
• High accuracy
• Low wear
• Negligible stick-slip
• High rigidity
• Low price
• Low co-efficient of friction properties
• Good surface finish on slide way surface
• High Stiffness at the sliding joint
• Good damping capacity
Types of Slide ways
1. Friction type slide ways
- V/flat/round/dove tail slide ways
- Plastic coated slide ways ,
2. Antifriction slide ways
- Linear motion bearings (ball, roller)
- Recirlculating ball screw and nut
28. q Friction Slide Ways
- Removes ‘stick-slip’ and reduces frictional resistance between sliding parts.
- To improve load bearing properties, hardened steel inserts are used.
- Flat slide ways have better load bearing capabilities than other guide ways.
q Plastic Coated Slide Ways
- Plastic or non metallic inserts are mostly used.
- Polymer composite inserts are used.
- Inserts are thermoplastics or thermosetting types.
- These inserts reduce coefficient of friction,
increase strength, wear resistance and load bearing
capacity. They also have self-lubricating property.
- The worn-out strip can be replaced easily
without the need for any machining of bed ways.
Plastic Coated Slide Way
29. q Antifriction Slide ways
- It involves intermediate rolling member (balls or rollers) between the sliding members,
thus reducing the friction.
- Less wear, low friction, no stick slip, high load capacity but low damping capacity.
(a) Linear motion bearings
- Balls or rollers are used to get antifriction linear motion in slide ways. The unit
consists of a bearing block and rail.
- The unit is constructed in such a manner that each of the rows of balls, rolling over the
rail comes into contact with the race-way at an angle of 45 degree.
- The race is in line contact rather than the conventional point contact.
- The system is capable of withstanding equal load in any direction.
Linear Ball Bearing Linear Roller Bearing
30. Antifriction Slide ways..
(b) Recirculating ball screw and nut
- Balls rotate between the screw and nut.
- It converts the sliding friction into rolling friction.
- Ball screws are primarily employed in feed mechanism of CNC machine tools.
- The balls provide only physical contact between the nut and the lead screw. The
balls are re-circulated from one end to the other by return tubes.
- The connection between the screw and nut is achieved by an endless stream of
recirculating steel balls.
Recirculating ball screw and nut
31. (b) Recirculating ball screw and nut…
§ Low co-efficient of friction
§ Less wear and long life
§ Requires less power
§ No stick-slip effect
§ No backlash
§ High efficiency (90%)
§ Used for heavier loads at
Elimination of Backlash
o Backlash is the sum of the axial clearance and deformation caused by axial or
o By making the carriage movement accurately in both the direction, backlash
between screw and nut can be avoided.
o Backlash can be avoided by preloading. It is achieved by fitting two nuts.
Recirculating ball screw and nut
32. qTool Magazines
-The system of arrangement which holds large number of tools is called tool
magazine. It is used in CNC turning centres and machining centres.
- It is specified by storage capacity and shape.
- Storage capacity ranges from 12 - 200 tools.
- It keeps the tools clean and free from damage. Also it keeps track of which tools
are there by coding.
Types 1. Tool turret 2. Drum/Disc type 3. Chain type
(1) Tool Turret
o It is the simplest form of tool magazine
o It consists of tool storage without tool changer
o Turret is indexed in required position for
o Tool can be easily identified
o Tool change time is more
33. Tool Magazines…
(2) Drum/Disc type
o It is the rotates to get the desired tool in position
with tool change arm
o It carries 12 - 50 tools
o If diameter of the disc is large, more number of
tools can be hold
o It have pockets where the tools can be inserted
Drum/Disc Type MagazineTool Turret type Magazine
34. Tool Magazines…
(3) Chain type Tool Magazine
o It is capable of storing more number of tools.
o Used in machining centre.
o Tools are inserted into their pockets which are
attached to the chain.
o Chain moving on sprocket and sprockets are
driven by motors.
Chain type Tool Magazine
35. q Automatic Tool Changer (ATC)
- All CNC machining centers should have the capability of changing the tools
- The device used to pick up a tool from tool magazine and replace it with the tool
in the spindle with in 3 to 7 seconds, is called ATC.
- ATC reduce the idle time during tool changing operations.
Types of ATC
1. Turret head
2. 180º rotation
3. Pivot insertion
5. Spindle direct
q 180º Automatic Tool Changer
- Simplest ATC
- Upon receiving a tool change command:
- MCU sends the spindle to its fixed tool
- At the same time, the tool magazine is
indexed to the proper position.
180º Automatic Tool Changer
37. Working of 180º Automatic Tool Changer…
1. ATC arm rotates through 90º from the rest position.
2. One end grips tool in the magazine & another end grips tool in the spindle.
3. Both tools are pulled out.
4. ATC arm rotates through 180º.
5. It inserts new tool to the spindle & old tool to the magazine.
6. ATC arm rotates through 90º and goes to the rest position.
Characteristic of Tool Magazines
o Tools magazine has to be compact and as simple as possible.
o Interchange of tools should not interfere with the work piece space and tool space
o Easier & safer manual exchange of tools in tool magazine during loading/ unloading.
o All preparatory works for tool exchange should be made during machining.
o Capable of holding enough tools needed for machining.
• Minimal motion involved
• Fast tool changes
• Tools must be stored in a plane parallel to the spindle
• Chances of getting chips, coolants on the tool holders.
38. q Feed Back Devices
oThe electronic devices such as Transducer and Encoders used for giving the
position and velocity of the machine elements are called feedback devices.
o Feed back is required to close the loop.
o Based on feedback control the systems are classified as
1. Open Loop Control System
2. Closed Loop Control System
(1) Open Loop Control System
o In this control system, there is no feed back devices to compare the actual
position of the cutting tool or work piece with the input signals are called open loop
Open Loop Control System
39. Feed Back Devices…
(2) Closed Loop Control System
o The machine tool control system in which there are feed back devices to compare the
actual position of the cutting tool or work piece with the input signals are called open
loop control systems.
Types 1. Velocity feed back 2. Position feed back
(1) Position Feed Back
- Measuring the slide movement by linear transducer.
(2) Velocity Feed Back
- Tacho-generator gives voltage output proportional to its speed. Output voltage is used
as feed back to monitor the motor speed. It can be done by rotary transducer.
Closed Loop Control System
40. Feed Back Devices…
Open Loop Control System Vs Closed Loop Control System
Open Loop Control
- Less maintenance
Closed Loop Control
-Accuracy is more
- More cost
44. q In-Process Probing (Process Inspection)
-The process of inspection is called Probing and the device used for inspection is
called Inspection Probe. Probes (Sensors) may be contact type & non-contact type.
-The process of performing the inspection procedure during manufacturing
operation using probes is called In-process probing. Here manufacturing as well as
inspection are done simultaneously.
Types of Probes 1. Single tip probe 2. Multiple tip probe 3. Touch trigger probe
• Loading / unloading time and inspection time will be reduced.
• Costly inspection equipment / devices will be eliminated.
• Over processing of defective components will be eliminated.
• Component quality and process control will be improved.
45. q Tool Materials
- Various cutting tool materials have been used in the industry for different applications.
- The cutting tools can be classified on the basis of the following :
1. On the basis of setting up of cutting tool
(a) Preset tools (b) Qualified tools (c) Semi qualified tools
2. On the basis of cutting tool construction
(a) Solid tools (b) Brazed tools (c) Inserted bit tools
3. On the basis of cutting tool material
(a) High speed steel (HSS) (b) High carbon tool steel (HCS)
(c) Cast alloy (d) Cemented carbide (e) Ceramics
(f) Boron nitride (g) Diamond (h) Sialon
High Speed Steel (HSS)
- 18:4:1 steel :18% Tungsten , 4% Molybdenum, 1% Vanadium.
- Good hot hardness, abrasion resistance, high toughness. It can be used up to 600ºC.
- Tungsten carbide powder + Cobalt (as binder) + Sintering
- Improved toughness, impact strength. Available in different shapes.
46. Tool Materials…
- Cemented carbide + Coating materials
- Coating materials : 5 micron of tin, titanium carbide, aluminium oxides, etc.
- Life of the coated tool is 2 to 3 times the life of uncoated tools.
- Alumina based high refractory materials. Used for high speed.
- It can withstand high temperature, chemically more stable & high wear resistance.
q Tool Life
v 50% increase in cutting speed results 90% decrease in tool life
v 50% increase in feed rate results in 60 % decrease in tool life
v 100 % increase in depth of cut results in 25% decrease in tool life.
v For longer tool life , the tool material should have
- Hot hardness , wear and abrasion resistance, increased thermal conductivity
- High impact and toughness, lower coefficient of thermal expansion
- Easy to form, grind and sharpen to the required geometry.
47. q Tool Inserts
- CNC Tools are very costly. They cannot be re-sharpened frequently.
- So, Tool Tips called Tool Inserts are produced by powder metallurgy process.
- Tool inserts are available in various shapes.
- Tool inserts can not be re-sharpened but they have multiple cutting edges.
- Tool inserts are used in CNC to minimize the tool change and tool setting time.
- Mechanically clamped tool tips are known as “ Indexable Inserts”.
- Inserts with built-in chip breakers can be used by the way of clamping arrangements.
Shapes of Inserts Tool Inserts Clamping Tool Inserts