numerical control instructor: dr haris aziz ta: mian wasif

Numerical Control

Instructor: Dr Haris AzizTA: Mian Wasif

Contents

1. Fundamentals of NC Technology

2. Computer Numerical Control

3. DNC

4. Applications of NC

5. Engineering Analysis of NC Positioning Systems

6. NC Part Programming

Numerical Control (NC) Defined

Programmable automation in which the mechanical actions of a ‘machine tool’ are controlled by a program containing coded alphanumeric data

� The alphanumeric data represent relative positionsbetween a workhead (e.g., cutting tool) and a workpart

� When the current job is completed, a new program can beentered for the next job

Basic Components of an NC System

MachineControl Unit

ProgramInstructions

Processing Equipment

1. Program of instructions

� Part program in machining

2. Machine control unit

� Controls the process

3. Processing equipment

� Performs the process

NC Coordinate System

For flat and prismatic (block-like) parts:• Milling and drilling operations• Conventional Cartesian coordinate system• Rotational axes about each linear axis• Right Hand Rule

For rotational parts:• Turning operations• Only x- and z-axes

Motion Control System

Point-to-Point systems• Also called position systems• System moves to a location and

performs an operation at that location (e.g., drilling)

• Also applicable in robotics

Continuous path systems • Also called contouring systems in

machining• System performs an operation during

movement (e.g., milling and turning)

Interpolation Methods

1. 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

motion4. Parabolic and cubic

interpolation– Free form curves using

higher order equations

Absolute vs. Incremental Positioning

Absolute positioning

Move is: x = 40, y = 50

Incremental positioning

Move is: x = 20, y = 30.

Computer Numerical Control (CNC)

• Storage of more than one part program• Various forms of program input• Program editing at the machine tool• Fixed cycles and programming subroutines• Interpolation• Acceleration and deceleration

computations• Communications interface• Diagnostics

Machine Control Unit of CNC

DNC>CNC>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

Direct NC

Distributed NC

NC Applications

� Machine tool applications:

� Milling, drilling, turning, boring, grinding

� Machining centers, turning centers, mill-turn

centers

� Punch presses, thermal cutting machines, etc.

� Other NC applications:� Component insertion machines in electronics

� Drafting machines (x-y plotters)

� Coordinate measuring machines

� Tape laying machines for polymer composites

� Filament winding machines for polymer composites

Common NC Machining Operations

Turning

Milling

Drilling

CNC Horizontal Milling Machine

NC Application Characteristics (Machining)

Where NC is most appropriate:

1. Batch production

2. Repeat orders

3. Complex part geometries

4. Much metal needs to be removed from the startingworkpart

5. Many separate machining operations on the part

6. The part is expensive

Cost-Benefit of NC

Costs• 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

NC Part Programming

1. Manual part programming2. Manual data input3. Computer-assisted part programming4. Part programming using CAD/CAM

Manual Part Programming

Binary Coded Decimal System• Each of the ten digits in decimal system (0-

9) is coded with four-digit binary number• The binary numbers are added to give the

value• BCD is compatible with 8 bits across tape

format, the original storage medium for NC part programs

• Eight bits can also be used for letters and symbols

Creating Instructions for NC

• Bit - 0 or 1 = absence or presence of hole in the tape

• Character - row of bits across the tape• Word - sequence of characters (e.g., y-axis

position)• Block - collection of words to form one

complete instruction• Part program - sequence of instructions

(blocks)

Block Format

Organization of words within a block in NC part program

• Also known as tape format because the original formats were designed for punched tape

• Word address format - used on all modern CNC controllers– Uses a letter prefix to identify each type of

word– Spaces to separate words within the block– Allows any order of words in a block– Words can be omitted if their values do not

change from the previous block

Types of Words

N - sequence number prefixG - preparatory words

– Example: G00 = PTP rapid traverse move

X, Y, Z - prefixes for x, y, and z-axesF - feed rate prefixS - spindle speedT - tool selectionM - miscellaneous command

– Example: M07 = turn cutting fluid on

Example: Word Address Format

N001 G00 X07000 Y03000 M03N002 Y06000

Cutter Off-Set

Cutter path must be offset from actual part outline by a distance equal to the cutter radius

Issues in Manual Part Programming

• Adequate for simple jobs, e.g., PTP drilling• Linear interpolation

G01 G94 X050.0 Y086.5 Z100.0 F40 S800

• Circular interpolationG02 G17 X088.0 Y040.0 R028.0 F30

• Cutter offsetG42 G01 X100.0 Y040.0 D05

Computer Assisted Part Programming

• Write machine instructions using natural language type statements

• Statements translated into machine code of the MCU

• APT (Automatically Programmed Tool) Language� The various tasks in

computer-assisted partprogramming are divided between;

� 1) The human part programmer

� 2) The computer

� Sequence of activities in computer-

assisted partprogramming

Part Programmer’s Job

� Two main tasks of the programmer:

1. Define the part geometry

2. Specify the tool path

Defining Part Geometry

� Underlying assumption: no matter how complex the partgeometry, it is composed of basic geometric elements andmathematically defined surfaces

� Geometry elements are sometimes defined only for use inspecifying tool path

� Examples of part geometry definitions:

P4 = POINT/35,90,0L1 = LINE/P1,P2

C1 = CIRCLE/CENTER,P8,RADIUS,30

Specifying Tool Path and Operation Sequence

� Tool path consists of a sequence of points or connectedline and arc segments, using previously defined geometryelements

� Point-to-Point command:

GOTO/P0

� Continuous path command

GOLFT/L2,TANTO,C1

Other Functions in Computer Assisted 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

Computer Task in Computer Assisted Part Programming

1. Input translation - converts the coded instructions in thepart program into computer-usable form

2. Arithmetic and cutter offset computations - performs themathematical computations to define the part surface andgenerate the tool path, including cutter offsetcompensation (CLFILE)

3. Editing - provides readable data on cutter locations andmachine tool operating commands (CLDATA)

4. Postprocessing - converts CLDATA into low-level codethat can be interpreted by the MCU

NC Part Programming Using CAD/CAM� Geometry definition

� If the CAD/CAM system was used to define the originalpart geometry, no need to recreate that geometry as inAPT

� Automatic labeling of geometry elements

� If the CAD part data are not available, geometry mustbe created, as in APT, but user gets immediate visualfeedback about the created geometry

Tool Path Generation Using CAD/CAM

� Basic approach: enter the commands one by one (similarto APT)

� CAD/CAM system provides immediate graphicalverification of the command

� Automatic software modules for common machiningcycles

� Profile milling

� Pocket milling

� Drilling bolt circles

NC Part Programming using CAD/CAM

Example of Machining Cycle in Automated Part Programming Module

Pocket milling

Contour turning

Example of Machining Cycle in Automated Part Programming Module

Facing and shoulder facing

Threading (external)

Manual Data Input• Machine operator does part

programming at machine– Operator enters program by

responding to prompts and questions by system

– Monitor with graphics verifies tool path

– Usually for relatively simple parts

• Ideal for small shop that cannot afford a part programming staff

• To minimize changeover time, system should allow programming of next job while current job is running

Analysis of NC positioning� Two types of NC positioning systems:

1. Open-loop - no feedback to verify that the actualposition achieved is the desired position

2. Closed-loop - uses feedback measurements toconfirm that the final position is the specified

position

� Precision in NC positioning - three measures:1. Control resolution

2. Accuracy

3. Repeatability

Open loop Motion Control System

� Operates without verifying that the actual positionachieved in the move is the desired position

Example: open loop positioning

The worktable of a positioning system is driven by a leadserew whose pitch =6.0 mm. The leadscrew is connected to the output shaft of a stepping motor through a gearbox whose ratio is 5:1 (5 turns of the motor to one turn of the leadscrew). The stepping motor has 48 step angles. The table must move a distance of 250 mm from its present position at a linear velocity = 500 mm/min Determine (a) how many pulses are required to move the table the specified distance and (b) the required motor speed and pulse rate to achieve the desired table velocity.

(a) the teadscrew rotation angle A correspondingto a distance x = 250 mm,

(b) The rotational speed of the leadscrew corresponding to a table speed of 500 mm/min can

be determined from

Closed Loop Motion Control System

� Uses feedback measurements to confirm that the finalposition of the worktable is the location specified in theprogram

Optical Encoder

� Device for measuring rotational position and speed

� Common feedback sensor for closed-loop NC control

Example: Closed Loop

• An NC worktable operates by closed-loop positioning. The system consists of a servomotor, leadscrew, and optical encoder. The leadscrew has a pitch = 6.0 mm and is coupled to the motor shaft with a gear ratio of 5:1 (5 turns of the drive motor for each turn of the leadscrcw). The optical encoder generates 48 pulses/rev of its output shaft. The encoder output shaft is coupled to the leadscrew with a 4:1 reduction (4 turns of the encoder shaft for each turn of the leadscrew). The table has been programmed to move a distance of 250 mm at a feed rate = 500 mm/min. Determine (a) how many pulses should be received by the control system to verify that the table has moved exactly 250 mm, (b) the pulse rate of the encoder, and (c) the drive motor speed that correspond to the specified feed rate

a

Precision NC positioningThree measures of precision:

1. Control resolution - distance separating two adjacentaddressable points in the axis movement

2. Accuracy - maximum possible error that can occurbetween the desired target point and the actual positiontaken by the system

3. Repeatability - defined as ±3σ of the mechanical errordistribution associated with the axis

Precision

Example: Control Resolution, Accuraq, and Repeatability in NC