3 numerical control

NUMERICAL CONTROL

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HISTORICAL DEVELOPMENT• 15th century - machining metal.• 18th century - industrialization, production-type

machine tools.• 20th century - F.W. Taylor - tool metal - HSS

Automated production equipment -Screw machinesTransfer linesAssembly linesusing cams and preset stops

Programmable automation -NCPLCRobots

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A Definition:A Definition:• 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 containing coded alphanumeric data that represents relative positions between a work head (e.g., cutting tool) and a work part.

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MACHINE UNIT

NUMERICAL CONTROLLER

NUMERICAL DATA

(NC CODE)

MANUFACTURINGOPERATOR

PROCESSED PART

Drive Control

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COMPONENTS OF NC MACHINES

MCUMachineTool

CLUDPU

MCU - Machine control unit

CLU - Control-loops unit

DPU - Data processing unit

Hardware Configuration of NC Machine

Machine Control Unit (MCU)• NC machine tool has a main unit, which is known as

Machine Control 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.

A typical Machine Control Unit may consist of the following units :• Input or Reader Unit • Memory• Processor• Output Channels• Control Panel• Feedback Channels

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Machine Tool

•Machine tool is the main components of a numerical control system, which executes the operations.

•It may consist of worktable, cutting tools, jigs and fixtures, motors for driving spindle and coolant and lubricating system.

•The latest development in the numerical control machine tool is the versatile machining center.

•This is a single machine capable of doing a number of operations such as milling, boring, drilling, reaming, and tapping by Automatic Tool Changer (ATC) under the control of tool selection instruction.

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The MCU may be of three types :

•Housed MCU Machine Control Unit may be mounted on the machine tool or may be built in the casing of the machine.

•Swing Around MCUMachine Control Unit is directly mounted on the machine, which can swing around it and can be adjusted as per requirement of the operator’s position.

•Stand Alone MCU Machine Control Unit is enclosed in a separate cabinet which is installed at some remote or same place near to the machine.

OTHER COMPONENTS OF AN NC MACHINE TOOL

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COORDINATE SYSTEMS• Right hand rule

x

y

z

x

y

z

BASIC REQUIREMENT OF NC MACHINE CONTROLa. Preparatory functions: which unit, which

interpolator, absolute or incremental programming, which circular interpolation plane, cutter compensation, etc.

b. Coordinates: three translational, and three rotational axes.

c. Machining parameters: feed, and speed.d. Tool control: tool diameter, next tool number, tool

change.e. Cycle functions: drill cycle, ream cycle, bore cycle,

mill cycle, clearance plane.f. Coolant control: coolant on/off, flood, mist.g. Miscellaneous control: spindle on/off, tape rewind,

spindle rotation direction, pallet change, clamps control, etc.

h. Interpolators: linear, circular interpolation

NC MOTION-CONTROL NC Program

ExecutionSystem

Interpolator &Servo-controlMechanism

Control Logic

Linear Motion

Power

Translator

Relay

Solenoid

CommandsDimensions

NEW NCs or CNCs

•high speed spindle (> 20,000 rpm)

•high feed rate drive ( > 600 ipm)

•high precision ( < 0.0001" accuracy)

Types of Numerical Control•Conventional Numerical Control (NC)

•Direct Numerical Control (DNC)

•Computer Numerical Control (CNC)

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Conventional Conventional Numerical Numerical Control (NC)Control (NC)

• 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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DNC•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

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Direct numerical control (DNC) Distributed numerical control (DNC)

Computer Numerical Control Computer Numerical Control (CNC)(CNC)

• The idea of computer numerical control is to position a computer right at the machine tool.

• Most, if not all machine tools that are numericaly controlled are CNC machine tools.

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Computer Numerical Control (CNC)

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AdvantagesAdvantages

o Reduces time for delivery of part

o 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 o Allows rapid design

changes in part o Less jigs and fixtures

are needed

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Cost-Benefits of NCCosts• High investment cost• High maintenance effort• Need for skilled programmers• High utilization required

Benefits• Cycle time reduction• Nonproductive time reduction• Greater accuracy and repeatability• Lower scrap rates• Reduced parts inventory and floor space• Operator skill-level reduced

CLASSIFICATION OF NUMERICAL CONTROL

• 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 incremental positioning • Hardware and software: Hardware NC software computer numerical control

(CNC)

1.Motion ControlPTP motion-control•To move the machine table or spindle to a specified position so that machining operations may be performed at that point.

• Moving at maximum rate from point to point.

• Accuracy of the destination is important but not the path.

• Drilling is a good application.

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continuous (contouring) control

•To control two or more axes simultaneously to get desired shape.

•To control not only the destinations, but also the paths through which the tool reaches these destinations.

•In the process of machining, the tool contacts the work piece.

•Use linear and circular interpolators

.

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Interpolation Methods1. Linear interpolation

▫ Straight line between two points in space2. 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

Open loop - No position feedback.

Use stepping motor.

motor

table

pulses

2.Control Loops

CLOSED LOOP

• Closed-loop control: to measure displacement of table motion• the advantage of a closed-loop system is its positioning

accuracy.

3.Power Drives•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

4.Positioning system

•Incremental positioning system

•absolute positioning system

Absolute vs. Incremental Positioning

Absolute positioning Move is: x = 40, y = 50

Incremental positioning Move is: x = 20, y = 30.

NC MACHINE RATING•Accuracy

•Repeatability•Spindle and axis motor horsepower•Number of controlled axes•Dimension of workspace•Features of the machine and the

controller.

NC ACCURACY•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.

Note:-• BLU (basic length unit)• BLU is the minimum length distinguishable by

the control unit.

HARDWARE INACCURACIESComponent tolerances:

inaccuracies in the machine elements, machine-tool assembly errors, spindle runout, and leadscrew backlash.

Machine operation: Tool deflection (a function of the cutting force), produces dimensional error and chatter marks on the finished part.

Thermal error: heat generated by the motor operation, cutting process, friction on the ways and bearings, etc. Use cutting fluids, locating drive motors away from the center of a machine, and reducing friction from the ways and bearings

REPEATABILITY

Avg. error

Programmed position

Test result

Repeatability

LEADSCREWS

Leadscrew

Pitch

NutConverting the rotational motion of the motors to a linear motion.

pitch (p): the distance between adjacent screw threads

The number of teeth per inch (n):n = 1 / p

BLU: Basic Length Unit (machine resolution)BLU = p / N

ExampleA 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