Unit 5 Numerical Control

Sections:1. Fundamentals of NC Technology2. Computer Numerical Control3. Distributed Numerical Control4. Applications of NC5. 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 that represents relative positions between a work head (e.g., cutting tool) and a work part

MachineControl Unit

Power

ProgramInstructions

TransformationProcess

NC Coordinate Systems

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

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

Motion Control Systems

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 motion

4. 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

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

Distributed Numerical Control

MachineControl Unit

TransformationProcess

MachineControl Unit

MachineControl Unit

CentralComputer NC Pgms

BTR BTR BTR

Computer Network

Applications of NC

NC Application Characteristics (Machining)

Batch and High Volume production Repeat and/or Repetitive orders Complex part geometries Mundane operations Many separate operations on one part

Cost-Benefits 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

Precision

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 moveX, 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 Offset

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 interpolation

G02 G17 X088.0 Y040.0 R028.0 F30 Cutter offset

G42 G01 X100.0 Y040.0 D05

Example

NC part program code

N001 G21 G90 G92 X-050.0 Y-050.0 Z010.0; N002 G00 Z-020.0 S1989 M03; N003 G01 G94 G42 Y0 D05 F398; N004 G01 X075.0; N005 G01 X150.0 Y043.02; N006 G01 Y070.0; N007 G01 X080.0; N008 G17 G02 X050.0 Y100.0 R030.0; N009 G01 Y125.0; N010 G01 X0; N011 G01 Y0 N012 G40 G00 X-050.0 Y-050.0 Z010.0 M05; N013 M30;

Comments

Define origin of axes. Rapid to cutter depth, turn spindle on. Bring tool to starting y-value, start cutter offset. Mill lower horizontal edge of part. Mill angled edge at 35 degrees. Mill vertical edge at right of part. Mill horizontal edge leading to arc. Circular interpolation around arc. Mill vertical step above arc. Mill top part edge. Mill vertical edge at left of part. Rapid move to target point, cancel offset, spindle stop. End of program, stop machine.

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

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

Sample Statements

Part is composed of basic geometric elements and mathematically defined surfaces

Examples of statements:P4 = POINT/35,90,0L1 = LINE/P1,P2C1 = CIRCLE/CENTER,P8,RADIUS,30

Tool path is sequence of points or connected line and arc segments

Point-to-Point command: GOTO/P4 Continuous path command: GOLFT/L1,TANTO,C1

NC Part Programming Using CAD/CAM

YouTube

CNC Milling CNC Punching CNC Adhesive Bonding CNC Drug Insertion CNC Bioprocessing CAD/CAM Etc.