Ch 7 Numerical Control

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publisher. For the exclusive use of adopters of the book

Automation, Production Systems, and Computer-Integrated Manufacturing, Third

Edition, by Mikell P. Groover.

Ch 7 Numerical Control

Sections:1. Fundamentals of NC Technology2. Computer Numerical Control3. DNC4. Applications of NC5. Engineering Analysis of NC Positioning

Systems6. NC Part Programming







Numerical Control (NC) Defined

Form of programmable automation in which the mechanical actions of a machine tool or other equipment are controlled by a program(through punched tape) containing coded alphanumeric data

• The alphanumeric data represent relative positions between a work head (e.g., cutting tool) and a work part

• When the current job is completed, a new program can be entered for the next job







Basic Components of an NC System

1. Program of instructions– Part program in machining

2. Machine control unit– Controls the process

3. Processing equipment– Performs the process







Basic Components of an NC System

1.Program:

•Is a set of detailed step by step command that direct the action of the processing equipment.

•A person who prepare the program is called part programmer.

•Individual commandrefer position of cutting tool relative to w/p.

•Older days the part program was punched in 1 inch wide punched tape.

•But now, magnetic tape, diskettes and electronic transfer .

2.Machine control unit:Is a micro computer and related control hardware that stores the program of instruction and execute it by converting each command into mechanical action.

•The term CNC is used that all MCU are based on computer technology.

•MCU includes control system software, calculation algorithm, and translation software.

3.Processing equipment:

•That perform the actual productive work(e.g.machining).

•Its operation directed by MCU.

•The processing equipment consist of worktable, and spindle as well as motor and controls to

drive them.

•NC coordinate system:

•First define standard axis system(x,y,z), three rotational axis(a,b,c).

•In most m/c application x,y axis are used to move and position the work table. And z axis for

controlling the tool movement.







NC Coordinate Systems

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







NC Coordinate Systems

For rotational parts:• Turning operations• Conventional Cartesian coordinate system, but

only x- and z-axes• y-axis not needed in turning







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 roboticsContinuous path systems • Also called contouring(when continuous path control is

used for simultaneous control of two or more axes) 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 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







Circular Interpolation

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on only the outside of the nominal curve







Circular Interpolation

Approximation of a curved path in NC by a series of straight line segments, where tolerance is defined on both the inside and outside of the nominal curve







Absolute and Incremental Positioning

Another aspect of motion control is concern with whether position are defined relative to the origin or relative to previous location of the tool.

Absolute positioning• Locations defined relative to origin of axis systemIncremental positioning• Locations defined relative to previous position• Example: drilling







Absolute vs. Incremental Positioning

The workhead is presently at point (20, 20) and is to be moved to point (40, 50) In absolute positioning, the move is specified by x = 40, y = 50 In incremental positioning, the move is specified by x = 20, y = 30.







Computer Numerical Control (CNC) – Additional Features

CNC is defined as an NC system whose MCU is based on micro computer rather than on a hard wired controller.

Features of CNC:• Storage of more than one part program• Various forms of program input(punched,magnetic,floppy

diskettes,RS 232 communication)• Program editing at the machine tool• Fixed cycles and programming subroutines• Interpolation• Acceleration and deceleration computations• Communications interface• Diagnosticsmalfunction sign

Machine control unit for CNC• The MCU consists of the following components and

subsystems:(I) central processing unit(2) memory(3) l/O interface(4) controls for machine tool axes and spindle speed. And(5)sequence controls for other machine tool functions.

These subsystems are interconnected by means of a system bus. which communicates data and signals among the components of network. indicated in the figure,







Central Processing Unit• The central processing unit (CPU) is the brain of the MCU.• It manages the other components in the MCU based on

software contained in main memory.• The CPU can be divided into three sections: (1) control section, (2) arithmetic-logic unit, and (3) immediate access memory.• control section retrieves commands and data from memory

and generates signals to activate other components in the MCU. In short, it sequences. coordinates. and regulates all of the activities of the MCU computer.







• Arithmetic logic unit (ALU) consists of the circuitry to perform various calculations (addition, subtraction, multiplication), counting. and logical functions required by software residing in memory.

• Immediate access memory provides a temporary storage for data being processed by the CPU. It is connected to main memory by means of the system data bus.©2008 Pearson Education, Inc., Upper

Saddle River, NJ. All rights reserved. This material is protected under all copyright












Configuration of CNC 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 Configurations

Switching network







Applications of NC• Machine tool applications:

– Milling, drilling, turning, boring, grinding– Machining centers(almost any CNC milling and drilling machine that

includes an automatic tool changer and a table that clamps the workpiece in place), 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







NC Application Characteristics (Machining)

Where NC is most appropriate:1. Batch production2. Repeat orders3. Complex part geometries4. Much metal needs to be removed from the

starting workpart5. Many separate machining operations on the part6. The part is expensive







Advantages of NC

• Nonproductive time is reduced(e.g.set up time)• Greater accuracy and repeatability• Lower scrap rates• Inspection requirements are reduced• More complex part geometries are possible• Engineering changes are easier to make• Simpler fixtures• Shorter lead times• Reduce parts inventory and less floor space• Operator skill-level requirements are reduced







Disadvantages of NC

• Higher investment cost– CNC machines are more expensive

• Higher maintenance effort– CNC machines are more technologically sophisticated

• Part programming issues– Need for skilled programmers– Time investment for each new part

• Higher utilization is required







NC Positioning System

• Typical motor and leadscrew arrangement in an NC positioning system for one linear axis

• For x-y capability, the apparatus would be piggybacked on top of a second perpendicular axis







Analysis of Positioning NC Systems

• Two types of NC positioning systems:1. Open-loop - no feedback to verify that the actual

position achieved is the desired position2. Closed-loop - uses feedback measurements to

confirm that the final position is the specified position

• Precision in NC positioning - three measures:1. Control resolution2. Accuracy3. Repeatability







Open-Loop Motion Control System

• Operates without verifying that the actual position achieved in the move is the desired position







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







NC Part Programming• NC part programming consists of planning and documenting the sequence of

processing steps to be performed on an NC machine.• The part programmer must have a knowledge of machining (or other processing

technology for which the NC machine is designed) as well as geometry and trigonometry.

• The documentation portion of par! programming involves the input medium used to transmit the program of instructions to the NC machine control unit (MCU).

• The traditional input medium dating back to the first NC machines in the 1950s is l-inch wide punched rape.

• More recently the use of magnetic tape and floppy disks have been used for NC due to their much higher data density.

• Part programming can he accomplished using a variety of procedures ranging from highly manual to highly automated methods.

1. Manual part programming

2. Computer-assisted part programming

3. Part programming using CAD/CAM

4. Manual data input

1.Manual Part Programming• In manual part programming, the programmer prepares the NC code using the low-level machine

language previously described.• The coding system is based on binary numbers• This coding is the low level machine language that can be understood by the MCU.• Whenever higher level languages are used , such as APT, the statements in the program are

converted to this basic code• NC uses a combination of the binary and decimal number system, called BCD system.

• In this coding scheme , each of the ten digits (0-9) in the decimal system is coded as a four digit binary number, and these binary number are added in sequence as in the decimal number system.

• Conversion of the ten digits in the decimal system into binary numbers • Exe: decimal value 1250 would be coded in BCD• Number sequence binary number decimal value• First 0001 1000• Second 0010 200• Third 0101 50• Fourth 0000 0• Sum 1250







Binary Coded Decimal System

• Each of the ten digits in decimal system 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)







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







Part Programmer's Job

• Two main tasks of the programmer:1.Define the part geometry2.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/35,90,0L1 = LINE/P1,P2C1 = CIRCLE/CENTER,P8,RADIUS,30







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







4.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 Tasks 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

Ch 7 Numerical Control

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Transcript of Ch 7 Numerical Control