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Numerical Control is a system in which

actions are controlled by the direct insertion

of numerical data at some point. The

system must automatically interpret at least

some portion of the data

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Data is sent to the machine tool by means of

punch cards or tapes. The reader at the

machine performs no calculations or

interpolations.

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•A numerical control, or “NC”, system controls many machine functions and movements which were traditionally performed by skilled machinists.

•Numerical control developed out of the need to meet the requirements of high production rates, uniformity and consistent part quality.

•Programmed instructions are converted into output signals which in turn control machine operations such as spindle speeds, tool selection, tool movement, and cutting fluid flow.

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• By integrating a computer processor, computer numerical control, or “CNC” as it is now known, allows part machining programs to be edited and stored in the computer memory as well as permitting diagnostics and quality control functions during the actual machining.

• All CNC machining begins with a part program, which is a sequential instructions or coded commands that direct the specific machine functions.

• The part program may be manually generated or, more commonly, generated by computer aided part programming systems.

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• CNC : Computer and Numeric Control

• Conventionally, an operator decides and adjusts various machines parameters like feed , depth of cut etc depending on type of job , and controls the slide movements by hand. In a CNC Machine functions and slide movements are controlled by motors using computer programs.

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MACHINE UNIT

NUMERICAL

CONTROLLER

NUMERICAL

DATA

(NC CODE)

MANUFACTURING

OPERATOR

PROCESSED

PART

Drive Control

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All computer controlled machines are able to accurately and repeatedly control motion in various directions. Each of these directions of motion is called an axis. Depending on the machine type there are commonly two to five axes.

Additionally, a CNC axis may be either a linear axis in which movement is in a straight line, or a rotary axis with motion following a circular path.

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Absolute Coordinate System Incremental Coordinate System 4/4/2014 Hareesha N G, Dept of Aero Engg, DSCE,

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•Each axis consists of a mechanical component, such as a slide that moves, a servo drive motor that powers the mechanical movement, and a ball screw to transfer the power from the servo drive motor to the mechanical component.

• These components, along with the computer controls that govern them, are referred to as an axis drive system.

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• Using a vertical mill machining center as an example, there are typically three linear axes of motion. Each is given an alphabetic designation or address. The machine table motion side to side is called the “X” axis. Table movement in and out is the “Y” axis, while head movement up and down the column is the “Z” axis.

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If a rotary table is added to the machine table, then the fourth axis is designated the “b” axis.

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•The method of accurate work positioning in relation to the cutting tool is called the “rectangular coordinate system.” On the vertical mill, the horizontal base line is designated the “X” axis, while the vertical base line is designated the “Y” axis. The “Z” axis is at a right angle, perpendicular to both the “X” and “Y” axes. •Increments for all base lines are specified in linear measurements, for most machines the smallest increment is one ten-thousandth of an inch (.0001). If the machine is graduated in metric the smallest increment is usually one thousandth of a millimeter (.001mm). •The rectangular coordinate system allows the mathematical plotting of points in space. These points or locations are called “coordinates.” The coordinates in turn relate to the tool center and dictate the “tool path” through the work.

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Advantages:

• High Repeatability and Precision e.g. Aircraft parts

• Volume of production is very high

• Complex contours/surfaces need to be machined. E.g. Turbines

• Flexibility in job change, automatic tool settings, less scrap

• More safe, higher productivity, better quality

• Less paper work, faster prototype production, reduction in lead times

Disadvantages:

• Costly setup, skilled operators

• Computers, programming knowledge required

• Maintenance is difficult

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• Lathes for metal and plastics

– Used to produce 3D product shapes and moulds for plastic products.

• Milling machine for mould making and surface milling.

– Used to produce dies for

die cutting printed products.

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• Automated version of a manual lathe.

• Programmed to change tools automatically.

• Used for turning and boring wood, metal and plastic.

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• Has 3 to 5 axes.

• Used for wood, metal and plastic.

• Used to make 3D prototypes, moulds, cutting dies, printing plates and signs.

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• Controlled by G and M codes.

• These are number values and co-ordinates.

• Each number or code is assigned to a particular operation.

• Typed in manually to CAD by machine operators.

• G&M codes are automatically generated by the computer software.

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• The tool or material moves.

• Tools can operate in 1-5 axes.

• Larger machines have a machine control unit (MCU) which manages operations.

• Movement is controlled by a motors (actuators).

• Feedback is provided by sensors (transducers)

• Tool magazines are used to change tools automatically.

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• Most are made from

high speed steel (HSS),

tungsten carbide or ceramics.

• Tools are designed to direct waste away from the material.

• Some tools need coolant such as oil to protect the tool and work.

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• Tool paths describes the route the cutting tool takes. • Motion can be described as point to point, straight cutting or

contouring. • Speeds are the rate at which the tool operates e.g. rpm. • Feeds are the rate at which the cutting tool and work piece

move in relation to each other. • Feeds and speeds are determined by cutting depth, material

and quality of finish needed. e.g. harder materials need slower feeds and speeds.

• Rouging cuts remove larger amounts of material than finishing cuts.

• Rapid traversing allows the tool or work piece to move rapidly when no machining is taking place.

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Is a method where a single computer

controls many numerical control machine

tools. These machine tools may or may not

be of a similar nature

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Computer Numerical Control (CNC)

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o Reduces time for

delivery of part

Reduces scrap rate of

material

o Reduces tooling

costs

o Reduces layout time

o Increases machine

and tool life

o Reduces storage

problems

o Less setup time

o Reduces actual

machining time

Allows rapid design

changes in part Less

jigs and fixtures are

needed

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Top View

Front View

Tool Home

1.) X & Y Rapid To Hole Position

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Top View

Front View

2.) Z Axis Rapid Move

Just Above Hole

3.) Turn On Coolant

4.) Turn On Spindle

.100”

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Top View

Front View

5.) Z Axis Feed Move to

Drill Hole

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Top View

Front View

6.) Rapid Z Axis Move

Out Of Hole

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Top View

Front View

9.) X&Y Axis Rapid

Move Home

7.) Turn Off Spindle

8.) Turn Off Coolant

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Top View

Front View

Tool At Home

O0001 N005 G54 G90 S600 M03 N010 G00 X1.0 Y1.0 N015 G43 H01 Z.1 M08

N020 G01 Z-.75 F3.5

N030 G91 G28 X0 Y0 Z0 N035 M30

N025 G00 Z.1 M09

Here’s The CNC Program!

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Top View

Front View

Tool At Home

O0001 O0001

Number Assigned to this program

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Top View

Front View

Tool At Home

O0001 N005 G90 S600 M03

N005 Sequence Number

G90 Absolute Programming Mode

S600 Spindle Speed set to 600 RPM

M03 Spindle on in a Clockwise Direction

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Top View

Front View

O0001 N005 G90 S600 M03 N010 G00 X1.0 Y1.0

G00 Rapid Motion

X1.0 X Coordinate 1.0 in. from Zero

Y1.0 Y Coordinate 1.0 in. from Zero

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Top View

Front View

O0001 N005 G90 S600 M03 N010 G00 X1.0 Y1.0

N015 Z.1 M08

Z.1 Z Coordinate .1 in. from Zero

M08 Flood Coolant On

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Top View

Front View

O0001 N005 G90 S600 M03 N010 G00 X1.0 Y1.0

N015 Z.1 M08 N020 G01 Z-.75 F3.5

G01 Straight Line Cutting Motion

Z-.75 Z Coordinate -.75 in. from Zero

F3.5 Feed Rate set to 3.5 in./min.

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Top View

Front View

O0001 N005 G90 S600 M03 N010 G00 X1.0 Y1.0

N015 Z.1 M08 N020 G01 Z-.75 F3.5

G00 Rapid Motion

Z.1 Z Coordinate .1 in. from Zero

M09 Coolant Off

N025 G00 Z.1 M09

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Top View

Front View

O0001 N005 G54 G90 S600 M03 N010 G00 X1.0 Y1.0 N015 G43 H01 Z.1 M08

N020 G01 Z-.75 F3.5

N030 G91 G28 X0 Y0 Z0 G91 Incremental Programming Mode

G28 Zero Return Command

X0, Y0, Z0

X,Y,& Z Coordinates at Zero

N025 G00 Z.1 M09

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Top View

Front View

O0001 N005 G54 G90 S600 M03 N010 G00 X1.0 Y1.0 N015 G43 H01 Z.1 M08

N020 G01 Z-.75 F3.5

N035 M30 N030 G91 G28 X0 Y0 Z0 N025 G00 Z.1 M09

M30 End of Program

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Example: A Milling Operation

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X

Z Y

(0,0,0)

NC CODE (Word Address Format) N50 G00 X15 Y12.5 Z0 N55 M03 N60 G01 Z-2.5 F500 M08 N65 G01 X50 N70 G01 Y45 N75 G01 X15 N80 G01 Y12.5 N85 G00 Z0 M09 N90 M04

SPINDLE STARTED !

SPINDLE STOP !

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