numerical control

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 15th century - machining metal.
 18th century - industrialization, production-type machine
tools.
 20th century - tool metal – HSS
 Automated production equipment -
 Screw machines
 Transfer lines
 Assembly lines
 Programmable automation -
 NC
 PLC
 Robots
Historical Development

3. What is Numerical control?
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 Numerical Control is a system in which actions are
controlled by the direct insertion of numerical data at
some point.
 In other words Programmable automation in which the
mechanical actions of a ‘machine tool’ are controlled
by a program.

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Components of NC Machines
MCU
Machine
Tool
CLU
DPU
 MCU – Machine Controlled unit
 It consists of some electronic hardware that reads the
NC programme, interprets it and conversely translates
it for mechanical actions of the machine tool.
 Machine Tool
 Machine tool is the main components of a numerical
control system, which executes the operations.
 CLU – Control loops unit
DPU – Data Processing unit
 Units which control the input data and execute the
required operations.

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 Preparatory functions : Which unit, which interpolator, absolute
or incremental programming, which circular interpolation plane,
cutter compensation, etc.
 Coordinates : Three translational, and three rotational axes.
 Machining parameters : Feed, and speed.
 Tool control : Tool diameter, next tool number, tool change.
 Cycle functions : Drill cycle, ream cycle, bore cycle, mill cycle,
clearance plane.
 Coolant control : Coolant on/off, flood, mist.
 Miscellaneous control : Spindle on/off, tape rewind, spindle
rotation direction, pallet change, clamps control, etc.
 Interpolators : Linear, circular interpolation
Basic Requirement of NC Control

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 Convectional Numerical Control (NC)
 Direct Numerical Control (DNC)
 Computer Numerical Control (CNC)
Types of Numerical Control

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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.
Conventional Numerical Control (NC)

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 Direct numerical control (DNC) – control of multiple
machine tools by a single (mainframe) computer through . direct
connection and in real time
 1960s technology
 Two way communication
 Distributed numerical control (DNC) – network consisting of central
computer connected to machine tool MCUs, which are CNC
 Present technology
 Two way communication
DNC

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DNC

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 Computer numerical control (CNC) is
the automation of machine tools by means of computers
executing pre-programmed sequences of machine control
commands.
 In modern CNC systems, the design of a mechanical part and
its manufacturing program is highly automated.
 The part's mechanical dimensions are defined using computer-
aided design (CAD) software and then translated into
manufacturing directives by computer-aided
manufacturing (CAM) software.
 The resulting directives are transformed into the specific
commands necessary for a particular machine to produce the
component, and then are loaded into the CNC machine.
Computer Numerical Control (CNC)

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 Advantages :
 Reduces scrap rate of material
 Reduces tooling costs
 Reduces layout time
 Increases machine and tool life
 Reduces storage problems
 Less setup time
 Reduces actual machining time
 Allows rapid design changes in part
Advantages and Disadvantages
 Disadvantages :
 High investment cost
 High maintenance effort
 Need for skilled programmers
 High utilization required

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• Coordinate systems: for flat and prismatic parts, for rotational parts
• Motion control: point to point (PTP), continuous (contouring) path
• Control loops: open loop and closed loop
• Power drives: hydraulic, electric, pneumatic
• Positioning systems: absolute positioning and incremental
positioning
• Hardware and software: Hardware NC software computer numerical
control (CNC)
CLASSIFICATION OF NUMERICAL CONTROL

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For flat and prismatic (block-like) parts:-
-Milling and drilling operations
- Rotational axes about each linear axis
Coordinate Systems:

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-Turning operations
-Conventional Cartesian coordinate system, but only x and z axes
-y-axis not needed in turning
For rotational parts:-
Radial location of
the tool
Longitudinal axis ( parallel to the
rotation axis )

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Motion Control Systems :-
1-Point-to-Point Systems:-
• To move the machine table or spindle to a specified position
so that machining operations may be performed at that point.
• Accuracy of the destination is important but
not the path .e.g (drilling)

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2 -Continuous Path Systems :-
• To control not only the destinations, but also the paths through
which the tool reaches these destinations.
• Use linear and circular interpolators. e.g.(milling and turning)
y

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NC Interpolation methods for continuous path
Control :
1. Linear interpolation
Straight line between two
points in space
2. Circular interpolation
Circular arc defined by starting
point, end point, center or radius,
and direction
3. Helical interpolation
Circular plus linear motion
4. Parabolic and cubic interpolation
Free form curves using higher
order equations

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Control Loops:-
1-Open loop:-
No position feedback.
2-Closed-loop control:-
To measure displacement of table motion. the advantage of a
closed-loop system is its positioning accuracy.

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• Electric motor
-widely used
-small size, ease of control, low cost
• Hydraulic drive
-much larger power/size ratio
• Pneumatic drive
-rarely used in NC positioning system
-can be used to drive the auxiliary devices
Power Drives:-

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Positioning systems:-
Positions are defined Relative to the origin of the
coordinate System (axis system)
[x=40, y=50]
Absolute:-
Incremental:-
Positions are defined relative to the
previous location of the tool (or
next position relative to the present)
[X=20, y=30]

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Part Programmer's Job
Two main tasks of the programmer:
1.Define the part geometry
2.Specify the tool path

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• Underlying assumption: no matter how complex the
part geometry, it is composed of basic geometric
elements and mathematically defined surfaces
• Geometry elements are sometimes defined only for
use in specifying tool path
Examples of part geometry definitions:
P4 = POINT/35,90,0
L1 = LINE/P1,P2
C1 = CIRCLE/CENTER,P8,RADIUS,30
Part Programmer's Job…
1.Define the part geometry

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• Tool path consists of a sequence of points or connected
line and arc segments, using previously defined
geometry elements
Point-to-Point command:
GOTO/P0
Continuous path command
GOLFT/L2,TO,C1
2.Specify the tool path
Part Programmer's Job…

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Other Functions in Part Programming
• Specifying cutting speeds and feed rates
• Designating cutter size (for tool offset calculations)
• Specifying tolerances in circular interpolation
• Naming the program
• Identifying the machine tool

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• Accuracy =control resolution and hardware accuracy.
• Control resolution: the minimum length distinguishable by
the control unit (BLU).
• Hardware inaccuracies are caused by physical machine
errors (Component tolerances, Machine operation, Thermal
error).
Note:-
• BLU (basic length unit)
NC ACCURACY

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Example-
A machine has 1 BLU = 0.001".To move the table 5" on
X axis at a speed (feed rate) of 6 ipm. Calculate pulse
rate and pulse count.
• pulse rate = speed/BLU = 6 /0.001
= 6,000 pulse/min
• pulse count = distance/BLU
= 5/0.001 = 5,000 pulses

28. Thank
You
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