# Numerical Control Instructor: Dr Haris Aziz TA: Mian Wasif 2.

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Numerical Control Instructor: Dr Haris Aziz TA: Mian Wasif 2

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 2

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 positions between a workhead (e.g., cutting tool) and a workpart  When the current job is completed, a new program can be entered for the next job 3

Basic Components of an NC System
1. Program of instructions  Part program in machining 2. Machine control unit  Controls the process 3. Processing equipment Program Instructions Machine Control Unit  Performs the process Processing Equipment 3

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 4

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) 5

Interpolation Methods
Linear interpolation Straight line between two points in space Circular interpolation Circular arc defined by starting point, end point, center or radius, and direction Helical interpolation Circular plus linear motion Parabolic and cubic interpolation Free form curves using higher order equations 6

Absolute vs. Incremental Positioning
Absolute positioning Move is: x = 40, y = 50 Incremental positioning Move is: x = 20, y = 30. 7

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 8

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

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 Drilling Milling 11

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 starting workpart 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 12

NC Part Programming Manual part programming Manual data input Computer-assisted part programming 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 prefix G - preparatory words
Example: G00 = PTP rapid traverse move X, Y, Z - prefixes for x, y, and z-axes F - feed rate prefix S - spindle speed T - tool selection M - miscellaneous command Example: M07 = turn cutting fluid on

N001 G00 X07000 Y03000 M03 N002 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 interpolation G02 G17 X088.0 Y040.0 R028.0 F30 Cutter offset G42 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 part programming are divided between;  1) The human part programmer  2) The computer

 Sequence of activities in computer-assisted part programming

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 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

Specifying Tool Path and Operation Sequence
 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,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 the part program into computer-usable form 2. Arithmetic and cutter offset computations - performs the mathematical computations to define the part surface and generate the tool path, including cutter offset compensation (CLFILE) 3. Editing - provides readable data on cutter locations and machine tool operating commands (CLDATA) 4. Postprocessing - converts CLDATA into low-level code that can be interpreted by the MCU

 Geometry definition  If the CAD/CAM system was used to define the original part geometry, no need to recreate that geometry as in APT  Automatic labeling of geometry elements  If the CAD part data are not available, geometry must be created, as in APT, but user gets immediate visual feedback about the created geometry

 Basic approach: enter the commands one by one (similar to APT)  CAD/CAM system provides immediate graphical verification of the command  Automatic software modules for common machining cycles  Profile milling  Pocket milling  Drilling bolt circles

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 actual position achieved is the desired position 2. Closed-loop - uses feedback measurements to confirm 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 position achieved 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 corresponding to 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 final position of the worktable is the location specified in the program

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 positioning
Three measures of precision: 1. Control resolution - distance separating two adjacent addressable points in the axis movement 2. Accuracy - maximum possible error that can occur between the desired target point and the actual position taken by the system 3. Repeatability - defined as ±3σ of the mechanical error distribution associated with the axis

Precision

Example: Control Resolution, Accuraq, and Repeatability in NC

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