5 computer control
TRANSCRIPT
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History
1955 - John Parsons and US Air Force define a need to
develop a machine tool capable of machining complex and
close tolerance aircraft parts with the same quality time
after time (repeatability). MIT is the subcontractor and
builds the machine for the project.
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History: Continued
1959 - MIT announces Automatic Programmed Tools
(APT) programming language
1960 - Direct Numerical Control (DNC). This eliminates
paper tape punch programs and allows programmers tosend files directly to machine tools
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History: Continued
1968 - Kearney & Trecker machine tool builders market
first machining center
1970s - CNC machine tools & Distributed Numerical
Control
1980s - Graphics based CAM systems introduced. Unixand PC based systems available
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History: Continued
1990s - Price drop in CNC technology
1997 - PC- Windows/NT based Open Modular
Architecture Control (OMAC) systems introduced to
replace firmware controllers.
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Numerical Control
Sections:
1. Fundamentals of NC Technology
2. Computer Numerical Control
3. Distributed Numerical Control4. Applications of NC
5. NC Part Programming
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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
Machine
Control Unit
Power
Program
Instructions
Transformation
Process
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What is numerical control (NC)?
NC has been defined by the Electronic Industries Association (EIA)
asa system in which actions are controlled by the direct insertion of
numerical data at some points. The system must automatically
interpret at least some portion of this data
The term NCis used to describe the control of the various functions
of a machine using numeric data. In the early age of NC, machines
were fed with information by means of the punched tape. An Electro-
mechanical tape reader was used to load a machine tape into the
controller.
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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
Onlyx- and z-axes
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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)
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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, anddirection
3. Helical interpolation
Circular plus linear motion
4. Parabolic and cubic interpolation
Free form curves using higher orderequations
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Absolute vs. Incremental Positioning
Absolute positioning
Move is: x= 40, y= 50
Incremental positioning
Move is: x= 20, y= 30.
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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
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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
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Distributed Numerical Control
Machine
Control Unit
Transformation
Process
Machine
Control Unit
Machine
Control Unit
Central
Computer NC Pgms
BTR BTR BTR
Computer Network
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The EIA definition of DNC
A system connecting a set of numerically controlledmachines to a common memory for part program ormachine program storage with provision for on-demanddistribution of data to machines.
In DNC, several NC machines are directly controlled bya computer, eliminating substantial hardware from theindividual controller of each machine tool. The part-program is downloaded to the machines directly (thusomitting the tape reader) from the computer memory.
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DNC
DNC is a computer control system.
DNC is the method of controlling a CNC with a separate
computer.
This is used when the CNC's memory is not sufficient
enough to handle the machining program.
If a separate computer is used to control various CNC's
then the control system is called as Distributed Numeric
Control else it is called Direct Numeric Control.
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General Configuration of aDirect 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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Distributed Numerical Control
Configurations
Switching network
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Distributed Numerical Control
Configurations
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Applications of NC
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
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Common NC Machining Operations
TURNING
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Common NC Machining Operations
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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
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Advantages of NC
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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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
Repeat orders are easy because part program isalready available
Higher utilization is required
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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
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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 positiontaken by the system
3. Repeatability - defined as 3of the mechanical error
distribution associated with the axis
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Precision
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NC Part Programming
1. Manual part programming
2. Manual data input
3. Computer-assisted part programming
4. Part programming using CAD/CAM
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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
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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 completeinstruction
Part program - sequence of instructions (blocks)
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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
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Types of Words
N - sequence number prefix
G - preparatory words
Example: G00 = PTP rapid traverse move
X, Y, Z - prefixes forx, y, and z-axes
F - feed rate prefix
S - spindle speed
T - tool selection
M - miscellaneous command Example: M07 = turn cutting fluid on
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Example: Word Address Format
N001 G00 X07000 Y03000 M03
N002 Y06000
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Cutter Offset
Cutter path must be
offset from actual
part outline by a
distance equal tothe cutter radius
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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
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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 stopEnd of program, stop machine.
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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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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
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Computer-Assisted Part Programming
Sequence of activities in computer-assisted partprogramming
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Sample Statements
Part is composed of basic geometric elements and
mathematically defined surfaces
Examples of statements:
P4 = POINT/35,90,0
L1 = LINE/P1,P2
C1 = CIRCLE/CENTER,P8,RADIUS,30
Tool path is sequence of points or connected line and arcsegments
Point-to-Point command: GOTO/P4
Continuous path command: GOLFT/L1,TANTO,C1
C
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NC Part Programming Using
CAD/CAM
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YouTube
CNC Milling
CNC Punching
CNC Adhesive Bonding
CNC Drug Insertion
CNC Bioprocessing
CAD/CAM
Etc.