Dr. J. Ramkumar

Professor, Department of Mechanical Engineering

Micro machining Lab, I.I.T. Kanpur

Micro machining Lab, I.I.T. Kanpur

Basic NC and CNC

Outline

1. Introduction to CNC machine

2. Component and Function of CNC

3. Coordinate System

1.Introduction to CNC machine

CNC = Computerized Numerical Control

History and Development of Technology

Conventional M/C

NC M/C

(1948 US Air force, MIT 21 months )

CNC M/C

1997 → 1st commercial NC m/c “CINCINNATIC HYDROTEL VERTICAL-SPINDLE MACHINE “

History and Development of Technology

Conventional vs. CNC machine

Machine Structure

The CNC machine tools are basically built in the same

way as conventional machine tools. The difference lies

in the fact that the machine components relevant for

turning and milling processes are controlled by

computers.

Conventional vs. CNC machine Function

Conventional vs. CNC machine

Conventional vs. CNC machine

Conventional machine

→ eyes, hands, brain, skill

CNC machine

Program→ Control unit→Motor→Motion

Measuring & Reflection Unit

No skill is required for operating CNC m/c .

Conventional machine

CNC machine

Difference between Conventional M/C & CNC M/C

Item Conventional

machine

CNC

machine

1. Movement Acme screw Ball screw

2. Feed Manual Motor

3. Measurement Manual Linear scale

Advantages of CNC

• Flexible, high accuracy

• Short production time

• Complex shapes

• Short setting time

• No skill requirement

• Short inspection time/ high quality product

• Low cost

Disadvantages of CNC

• High machine cost

• Complicated maintenance

• Skill & training are required for programming and maintenance.

• Parts are imported from aboard.

• High tooling cost

• Temperature, humidity & dust must be controlled.

Why CNC?

HUMAN ➔ LIMITATION

Components of Traditional NC Systems

Direct Numerical Control (DNC)

Direct Numerical Control (DNC)

▪ Feed drive

▪ Measuring system → Direct / Indirect

▪ Work spindle → hydraulic

▪ Cooling system → reduce heat

▪ Tool turret

Component and Function of CNC

Feed drive

Measuring System

Tool change facilities

Coordinate System

Axes on a CNC lathe

Axes on a CNC milling machine

Zero and reference points on CNC

Zero Point of machine on a CNC lathe

Machine Zero Point and Work part zero point on CNC milling machine

Classification based on the motion type

Classification based on the motion type

Classification based on the motion type

Classification based on the control loops

Classification based on the control loops

Classification based on the control loops

Classification based on the control loops

Classification based on the number of axes

Classification based on the number of axes

Classification based on the number of axes

Classification based on the number of axes

Classification based on the power supply

Driving System

➢The requirement is that the driving system has to response accurately according to the programmedinstructions.

➢The motor is coupled either directly or through a gear box to the machine lead screw to moves themachine slide or the spindle.

Three types of electrical motors are commonly used:

1. Stepping motor

2. DC Servo motor

3. AC Servo moto

1. Stepping Motor

➢The stepper motor is known by its property to convert a train of input pulses (typically square wave pulses)into a precisely defined increment in the shaft position.

➢Each pulse moves the shaft through a fixed angle.

➢Multiple "toothed" electromagnets arranged around a central gear-shaped piece of iron.

➢The electromagnets are energized by an external driver circuit or a micro controller. In that way, the motor canbe turned by a precise angle.

➢To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts thegear's teeth.

➢When the gear's teeth are aligned to the first electromagnet, they are slightly offset from the nextelectromagnet.

➢This means that when the next electromagnet is turned on and the first is turned off, the gear rotatesslightly to align with the next one.

➢From there the process is repeated. Each of those rotations is called a "step", with an integer number ofsteps making a full rotation.

What does Stepper means?

Stepper Motor / Electro magnet

Rotor

Stator

Coils

2

1

S

N

1

2

Outside Casing

Stator

Rotor

Internal components of a Stepper Motor

2 2

1

N

S

1

S

N

Stators

Rotor

Cross Section of a Stepper Motor

Four Steps per revolution i.e. 90 deg. steps.

Full Step Operation

Eight steps per. revolution i.e. 45 deg. steps.

Half Step Operation

2 2

1

1

S

N

S

N

NN

S S

1

a b

Winding number 1

2

a b

Winding number 2

One

step6 pole rotor

How many steps are required for one complete revolution?

Six pole rotor, two electro magnets

The top electromagnet (1) is turned

on, attracting the nearest teeth of a

gear-shaped iron rotor. With the

teeth aligned to electromagnet 1,

they will be slightly offset from

electromagnet 2

The top electromagnet (1) is

turned off, and the right

electromagnet (2) is energized,

pulling the nearest teeth slightly

to the right. This results in a

rotation of 3.6° in this example.

Practical Stepper motor operation

The bottom electromagnet (3)

is energized; another 3.6°

rotation occurs.

The left electromagnet (4) is enabled,

rotating again by 3.6°. When the top

electromagnet (1) is again enabled,

the teeth in the sprocket will have

rotated by one tooth position; since

there are 25 teeth, it will take 100

steps to make a full rotation in this

example.

Stepping Motor to move read-write head

Stepper motor applications

Paper feeder on printers

CNC lathes

Stepper motors

Stepper motor applications

Rotor

Stator coils

CNC Stepping Motor

Step 1 0 0 1 1

Step 2 1 0 1 0

Step 3 1 1 0 0

Step 4 0 1 0 1

+

CW CCW

Control sequence to turn a stepper motor

Advantages:-

Low cost for control achieved

Ruggedness

Simplicity of construction

Can operate in an open loop control system

Low maintenance

Less likely to stall or slip

Will work in any environment

Disadvantages:-

Require a dedicated control circuit

Use more current than D.C. motors

High torque output achieved at low speeds

Advantages / Disadvantages

Open Loop Positioning SystemsStepper Motor calculations

➢It uses a stepper motor to rotate the lead screw. A stepper motor is driven by series ofelectrical pulses generated by MCU.

➢For each pulse the motor rotates a fraction of revolution called Step Angle, it is given by

Where, ns = Number of step angles for the motor (an integer).

➢If np is the pulses received by the motor then angle through which motor rotates is

Stepper Motor calculations

❖Lead Screw is connected to the motor shaft through a gear box.

❖Angle of the lead screw rotation taking the gear ratio into account is given by

rg = Gear ratio

= Am/A= Nm/ N

Nm= RPM of motor, N= RPM of lead Screw

The linear movement of worktable is given by

p = pitch of lead screw

Stepper Motor calculations

❖Total number of pulses required to achieve a specified x-position increment is calculated by:

Where ,ns = 360/ α

Control pulses are transmitted from pulse generator at a certain frequency which drives the work table at

the corresponding velocity.

The rotational speed of lead screw depends on the frequency of

the pulse train

N = RPM of lead screw, fp = frequency of pulse train (Hz, Pulses/sec)

Equation (1)

Stepper Motor calculations

➢The table travel speed in the direction of lead screw axis is determined by:

Where, Vt = Table travel speed (mm/min)

fr = Table feed rate (mm/min)

p= Lead screw pitch (mm/rev)

The required pulse train frequency to drive the table at a specified

linear travel rate by combining equations (1) and (2):

Equation (2)

2. DC Servo Motor➢ The principle of operation is based on the rotation of an armature winding in a permanently energized

magnetic field.

➢ The armature winding is connected to a commutator, which is a cylinder of insulated copper segments

mounted on the shaft.

➢ DC current is passed to the commutator through carbon brushes, which are connected to the machine

terminals.

Servo Motor Detail

+ 5V

Actuator

Reduction gear

Position feedback

Potentiometer

(closed loop system)

Small electric DC motor

3. AC Servo Motor➢ In an AC servomotor, the rotor is a permanent magnet while the stator is equipped with 3-phase

windings.

➢ The speed of the rotor is equal to the rotational frequency of the magnetic field of the stator, which

is regulated by the frequency converter.

CNC Programming

➢Programming consists of a series of instructions in form of letter codes

Preparatory Codes:

➢G codes- Initial machining setup and establishing operating conditions

➢N codes- specify program line number to executed by the MCU

➢ Axis Codes: X,Y,Z - Used to specify motion of the slide along X, Y, Z direction

➢ Feed and Speed Codes: F and S- Specify feed and spindle speed

➢ Tool codes: T – specify tool number

Miscellaneous codes – M codes For coolant control and other activities

Programming Key Letters

O - Program number (Used for program identification)

N - Sequence number (Used for line identification)

G - Preparatory function

X - X axis designation

Y - Y axis designation

Z - Z axis designation

R - Radius designation

F – Feed rate designation

S - Spindle speed designation

H - Tool length offset designation

D - Tool radius offset designation

T - Tool Designation

M - Miscellaneous function

Table of Important G Codes

G00: Rapid Transverse

G01: Linear Interpolation

G02: Circular Interpolation, CW

G03: Circular Interpolation, CCW

G17: XY Plane, G18: XZ Plane,G19: YZ Plane

G20/G70: Inch units

G21/G71: Metric Units

G40: Cutter compensation cancel

G41: Cutter compensation left

G42: Cutter compensation right

G codes are instructions describing machine tool movement

G43: Tool length compensation (plus)

G44: Tool length compensation (minus)

G49: Tool length compensation cancel

G80: Cancel canned cycles

G81: Drilling cycle

G82: Counter boring cycle

G83: Deep hole drilling cycle

G90: Absolute positioning

G91: Incremental positioning

Table of Important M codes

M Codes are instructions

describing miscellaneous

functions like calling the tool,

spindle rotation, coolant on/off

etc.,

Basic CNC program for turning operation

%

N10 T104 M06

N20 G97 S2000 G95 F0.1 M03

N30 G00 X18 Z2 M08

N40 G01 Z-22N50 G01 X26

N60 G00 X200 Z200 M09

N70 M30

Basic CNC program for turning operationO0001

N5 M12

N10 T0101

N15 G90

N20 G0 X50 Z50

N25 M3 S600

N30 M8

N35 G1 X50 Z0 F600

N40 Z-30 F200

N45 X100 Z-50 F150

N50 G0 X50 Z50

N55 T0100

N60 M5

N65 M9

N70 M13

N75 M30

N80 %

Basic CNC program for milling operation

N5 G90 G71

N10 T1 M6

N15 X-100 Y86 Z95

N20 G00 X0 Y0 S2500 M3

N25 Z12.5

N30 G01 Z-12.5 F150

N35 X-20 Y30

N40 G02 X10 Y100 R80

N45 G01 X140 Y60

N50 G02 X150 Y0 R50

N55 G01 X0 Y0

N60 G00 Z12.5

N65 G91 G28 Z0 M5

N70 G91 G28 X0 Y0

N75 M30

Basic CNC program for milling operation: Circular interpolation

N2 G17 G71 G90 G94 G54

N4 T1 L90

N6 G00 Z5 D5 M3 S500 X20 Y90

N8 G01 Z-2 F50

N10 G02 X60 Y50 I0 J-40

or, N10 G02 X60 Y50 R40

N12 G03 X100 Y50 I20 J0

or, N12 G03 X100 Y50 R20

N14 G00 Z100

N16 M02

Basic CNC program for drilling operation

N1 T16 M06

N2 G90 G54 G00 X0.5 Y-0.5

N3 S1450 M03

N4 G01 Z-0.375 F9

N5 G00Z 5

N6G00 X 1.5Y-0.5

N7G01Z-0.375 F9

N8 G00Z 5

N9 G00 X1.5 Y-1.5

N10Z-0.375F9

N11 G00Z 5

N12 G00X0.5Y-1.5

N13 Z-0.375F9

N14 G00 Z5 X0Y0

N15 M30

Sample problem: Milling

G90

AbsoluteX Y G91

IncrementX Y

P0 X0 Y0 P0 X0 Y0

P0-P1 X-20 Y-20 P0-P1 X-20 Y-20

P0-P2 X12 Y-20 P1-P2 X32 Y0

P0-P3 X20 Y-12 P2-P3 X8 Y8

P0-P4 X20 Y14 P3-P4 X0 Y26

P0-P5 X12 Y20 P4-P5 X-8 Y6

P0-P6 X-20 Y20 P5-P6 X-32 Y0

Exercise: Write CNC program for the following sequence of milling operation

Thank You