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67040-Ch07

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Ch 7 Numerical Control
Sections:
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) Defined
Form of programmable automation in which the mechanical
actions of a machine tool or other equipment are
controlled by a program containing coded alphanumeric
data
ƒ The alphanumeric data represent relative positions
between a workhead (e.g., cutting tool) and a workpart
ƒ When the current job is completed, a new program can be
entered for the next job
©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.
Basic Components of an NC System
1. Program of instructions
ƒ Part program in machining
2. Machine control unit
ƒ Controls the process
3. Processing equipment
ƒ Performs the process
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Basic Components of an NC System
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
NC Coordinate Systems
For flat and prismatic (block-like) parts
ƒ Milling and drilling operations
ƒ Conventional Cartesian coordinate system
ƒ Rotational axes about each linear axis
ƒ Right hand rule
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Coordinate Axis System for
Flat and Prismatic Parts
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
NC Coordinate Systems
For rotational parts:
ƒ Turning operations
ƒ Conventional Cartesian coordinate system, but only x- and
z-axes
ƒ y-axis not needed in turning
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Coordinate Axis System for
Rotational Parts
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Motion Control Systems
Point-to-Point systems
ƒ Also called position systems
ƒ System moves to a location and performs an operation at
that location (e.g., drilling)
ƒ Also applicable in robotics
Continuous path systems
ƒ Also called contouring systems in machining
ƒ System performs an operation during movement (e.g.,
milling and turning)
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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
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Continuous Path Control in NC
Profile Milling of Part Outline
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Interpolation Methods
1. Linear interpolation
ƒ Straight line between two points in space
2. Circular interpolation
ƒ Circular arc defined by starting point, end point, center
or radius, and direction
3. Helical interpolation
ƒ Circular plus linear motion
4. Parabolic and cubic interpolation
ƒ Free form curves using higher order equations
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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.
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 curve
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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.
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
©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.
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
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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 Positioning
Absolute positioning
ƒ Locations defined relative to origin of axis system
Incremental positioning
ƒ Locations defined relative to previous position
ƒ Example: drilling
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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 vs. Incremental Positioning
The workhead is presently
at point (20, 20) and is to be
moved to point (40, 50)
ƒ In absolute positioning,
the move is specified by x =
40, y = 50
ƒ In incremental positioning,
the move is specified by x =
20, y = 30.
©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.
Computer Numerical Control (CNC) –
Additional Features
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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
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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
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Configuration of
CNC Machine Control Unit
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
DNC
ƒ Direct numerical control (DNC) – control of multiple
machine tools by a single (mainframe) computer
through direct connection and in real time
ƒ 1960s technology
ƒ Two way communication
ƒ Distributed numerical control (DNC) – network
consisting of central computer connected to machine
tool MCUs, which are CNC
ƒ Present technology
ƒ Two way communication
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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 NC
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Distributed Numerical Control
Configurations
Switching network
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Distributed Numerical Control
Configurations
Local area network (LAN)
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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.
Applications of NC
ƒ Machine tool applications:
ƒ Milling, drilling, turning, boring, grinding
ƒ Machining centers, turning centers, mill-turn centers
ƒ Punch presses, thermal cutting machines, etc.
ƒ Other NC applications:
ƒ Component insertion machines in electronics
ƒ Drafting machines (x-y plotters)
ƒ Coordinate measuring machines
ƒ Tape laying machines for polymer composites
ƒ Filament winding machines for polymer composites
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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
Turning
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
Common NC Machining Operations
Milling
Drilling
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Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.
CNC Horizontal Milling Machine
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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.
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
©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.
Advantages of NC
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Nonproductive time is reduced
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
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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.
Disadvantages of NC
ƒ Higher investment cost
ƒ CNC machines are more expensive
ƒ Higher maintenance effort
ƒ CNC machines are more technologically sophisticated
ƒ Part programming issues
ƒ Need for skilled programmers
ƒ Time investment for each new part
ƒ Repeat orders are easy because part program is
already available
ƒ Higher utilization is required
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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.
NC Positioning System
ƒ Typical motor and leadscrew arrangement in an NC
positioning system for one linear axis
ƒ For x-y capability, the apparatus would be piggybacked on
top of a second perpendicular axis
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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
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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
©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.
Open-Loop Motion Control System
ƒ Operates without verifying that the actual position
achieved in the move is the desired position
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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.
Closed-Loop Motion Control System
ƒ Uses feedback measurements to confirm that the final
position of the worktable is the location specified in the
program
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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
ƒ Device for measuring rotational position and speed
ƒ Common feedback sensor for closed-loop NC control
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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 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
©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.
Definitions of Control Resolution,
Accuracy, and Repeatability
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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.
NC Part Programming
1.
2.
3.
4.
Manual part programming
Computer-assisted part programming
Part programming using CAD/CAM
Manual data input
©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.
Binary Coded Decimal System
ƒ Each of the ten digits in decimal system is coded with
four-digit binary number
ƒ The binary numbers are added to give the value
ƒ BCD is compatible with 8 bits across tape format, the
original storage medium for NC part programs
ƒ Eight bits can also be used for letters and symbols
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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.
Creating Instructions for NC
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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
ƒ Part program - sequence of instructions (blocks)
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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.
Block Format
Organization of words within a block in NC part program
ƒ Also known as tape format because the original
formats were designed for punched tape
ƒ Word address format - used on all modern CNC
controllers
ƒ Uses a letter prefix to identify each type of word
ƒ Spaces to separate words within the block
ƒ Allows any order of words in a block
ƒ Words can be omitted if their values do not
change from the previous block
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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 Words
N - sequence number prefix
G - preparatory words
ƒ 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
ƒ Example: M07 = turn cutting fluid on
©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.
Example: Word Address Format
N001 G00 X07000 Y03000 M03
N002 Y06000
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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.
Issues in Manual Part Programming
ƒ Adequate for simple jobs, e.g., PTP drilling
ƒ Linear interpolation
G01 G94 X050.0 Y086.5 Z100.0 F40 S800
ƒ Circular interpolation
G02 G17 X088.0 Y040.0 R028.0 F30
ƒ Cutter offset
G42 G01 X100.0 Y040.0 D05
©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.
Computer-Assisted Part Programming
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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
ƒ The human part programmer
ƒ The 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.
Computer-Assisted Part Programming
ƒ Sequence of activities in computer-assisted part
programming
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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.
Part Programmer's Job
ƒ Two main tasks of the programmer:
1. Define the part geometry
2. Specify the tool path
©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.
Defining Part Geometry
ƒ Underlying assumption: no matter how complex the part
geometry, it is composed of basic geometric elements and
mathematically defined surfaces
ƒ Geometry elements are sometimes defined only for use in
specifying tool path
ƒ Examples of part geometry definitions:
P4 = POINT/35,90,0
L1 = LINE/P1,P2
C1 = CIRCLE/CENTER,P8,RADIUS,30
©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.
Specifying Tool Path and
Operation Sequence
ƒ Tool path consists of a sequence of points or connected
line and arc segments, using previously defined geometry
elements
ƒ Point-to-Point command:
GOTO/P0
ƒ Continuous path command
GOLFT/L2,TANTO,C1
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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.
Other Functions in Computer-Assisted
Part Programming
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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
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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.
Cutter Offset
Cutter path must be
offset from actual
part outline by a
distance equal to
the cutter radius
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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 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
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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.
NC Part Programming Using
CAD/CAM
ƒ Geometry definition
ƒ If the CAD/CAM system was used to define the original
part geometry, no need to recreate that geometry as in
APT
ƒ Automatic labeling of geometry elements
ƒ 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
©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.
Tool Path Generation Using
CAD/CAM
ƒ Basic approach: enter the commands one by one (similar
to APT)
ƒ CAD/CAM system provides immediate graphical
verification of the command
ƒ Automatic software modules for common machining
cycles
ƒ Profile milling
ƒ Pocket milling
ƒ Drilling bolt circles
©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.
Examples of Machining Cycles in
Automated NC Programming Modules
Pocket milling
Contour 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.
Examples of Machining Cycles in
Automated NC Programming Modules
Facing and shoulder facing
Threading (external)
©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.
Manual Data Input
ƒ Machine operator does part programming at machine
ƒ Operator enters program by responding to prompts and
questions by system
ƒ Monitor with graphics verifies tool path
ƒ Usually for relatively simple parts
ƒ Ideal for small shop that cannot afford a part programming
staff
ƒ To minimize changeover time, system should allow
programming of next job while current job is running
©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.
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