14238_nc

Ch 7 Numerical ControlSections: 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

2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Numerical Control (NC) DefinedForm 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 workhead (e.g., cutting tool) and a workpart When the current job is completed, a new program can be entered for the next job


Punched tape

Basic Components of an NC System1. 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 motorand 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 SystemsFor flat and prismatic (block-like) parts Milling and drilling operations Conventional Cartesian coordinate system Rotational axes about each linear axis Right hand rule


Coordinate Axis System for Flat and Prismatic Parts


NC Coordinate SystemsFor rotational parts: Turning operations Conventional Cartesian coordinate system, but only x- and z-axes y-axis not needed in turning


Coordinate Axis System for Rotational Parts


Motion Control SystemsPoint-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(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)2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Point-To-Point Control in NC Drilling of Three Holes in Flat Plate


Continuous Path Control in NC Profile Milling of Part Outline


Interpolation Methods1. 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 inside of the nominal curve2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.


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 curve2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.


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 curve2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Absolute and Incremental PositioningAnother 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 system Incremental positioning Locations defined relative to previous position Example: drilling


Absolute vs. Incremental PositioningThe 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 FeaturesCNC 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 sign2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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,2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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.2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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.


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 communication2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.


General Configuration of a Direct Numerical Control System

Connection to MCU is behind the tape reader (BTR). In distributed NC, entire programs are downloaded to each MCU, which is CNC rather than conventional NC2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Distributed Numerical Control ConfigurationsSwitching network


Distributed Numerical Control Configurations

Local area network (LAN)


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 composites2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Automatic insertion m/c

Coordinate measuring m/c

Common NC Machining Operations

Turning2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Common NC Machining Operations

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 starting workpart 5. Many separate machining operations on the part 6. 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 axis2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

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 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


An open-loop positioning system typically uses a stepping motor to rotate the leadscrew. A stepping motor is driven by a series of electrical pulses, which are generated by the MCU in an NC system. Each pulse causes the motor to rotate a fraction of one revolution. called the step angle. The possible step angles must be consistent with the following relationship:

Where -step angle (degrees), and n = the number of step angles for the motor, which must be an integer. The angle through which the motor shaft rotates is given by




Closed-Loop Motion Control System

Uses feedback measurements to confirm that the final position of the worktable is the location specified in the program2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Optical Encoder

Device for measuring rotational position and speed Common feedback sensor for closed-loop NC control2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.


Precision in NC PositioningThree 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 3 of the mechanical error distribution associated with the axis


Definitions of Control Resolution, Accuracy, and Repeatability


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 linch 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. Manual part programming Computer-assisted part programming 2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights Part programming using CAD/CAMreserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Manual data Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover. input

1. 2. 3. 4.

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)


Block FormatOrganization 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 block2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Types of WordsN - 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


Example: Word Address FormatN001 G00 X07000 Y03000 M03 N002 Y06000


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


2.Computer-Assisted Part Programming Manual part programming is time-consuming, tedious, and subject to human errors for complex jobs Machining instructions are written in English-like statements that are translated by the computer into the low-level machine code of the MCU APT (Automatically Programmed Tool) The various tasks in computer-assisted part programming are divided between The human part programmer The computer2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Computer-Assisted Part Programming

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


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


Computer Tasks in Computer-Assisted Part Programming1. 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 MCU2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the book Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

3.NC Part Programming Using CAD/CAM 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


Tool Path Generation Using CAD/CAM 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


Examples of Machining Cycles in Automated NC Programming ModulesPocket milling

Contour turning


Examples of Machining Cycles in Automated NC Programming ModulesFacing and shoulder facing

Threading (external)


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


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