Chapter 6 Computer Control of Manufacturing Systems

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Chapter 6Computer Control of Manufacturing
Systems
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Agenda

• Metal cutting machines
• NC part programming
• Loading the program
• Computer-aided part programming
• CAD/CAM-based part programming
• PLC
• Generalized process control

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Metal-Cutting Machines

• Metal-cutting process excess material from a
  raw workpiece using a cutting tool.
• The removed material
• Machine tool control the workpiece-cutting tool
  relationship?shape, accuracy
• Functions to perform a machining operation
• Determining the on the workpiece
• Controlling the cutting
• Controlling the at which the path is
  traversed
• Controlling the

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Metal-Cutting Machines

• NC Machines various machine and are
  controlled by a machine control unit (MCU). Types
  of NC control systems
• Open-loop control stepping motor
• MCU ?current pulses? motor
• motors position (controlled by number of
  revolutions)
• Cheap, simple, easier to maintain, small machines
• because of no feed back to the controller
• Closed-loop control Servos motor (Variable speed
  DC motor)
• The position information is to the
  controller by a sensor (resolver)
• Cannot move in very precise amounts as stepping
  motor
• Correct errors, better accuracy
• Large machines
• More complex, more expensive to buy and to
  maintain

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Metal-Cutting Machines

• Programming for NC machines writing a
  commands ?MCU ? machine action
• Motion and Coordinate system nomenclature for NC
  machines Cartesian coordinate system
• x,y,z axes
• Supplementary linear motions (U,V,W) to the
  x,y,y motions
• Rotary motion (A, B, C) a to advance
• Spindle rotation .

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NC Part Programming

• Basics
• Assumption the tool undergoes the primary motion
  (x,y,z)
• Tool position cutting tip( turning), center of
  the cutter (drilling, milling)
• Reference point origin? determined by programmer
• Absolute positioning tool potion is given
  relative to the
• Incremental positioning the next tool position
  is given relative to the tool position
• Modal commands effect , e.g. feed rate
• Nonmodal commands effective for only at the .
• Blocks (statements) one or more machine ?each
  operation
• A command in a block word

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NC Part Programming

• Block format the arrangement of words
• Fixed sequential format 0050 00 0025400
  0012500 0000000 0000 00
• 0060 01
  0025400 0012500 -0010000 0500 08
• 0070 01
  0025400 0012500 0000000 0500 09
• Tab sequential format
• 0050 TAB 00 TAB 0025400 TAB 0012500 TAB TAB
  TAB
• 0060 TAB 01 TAB TAB TAB -0010000 TAB 0500
  TAB 08
• 0070 TAB 00 TAB TAB TAB 0000000 TAB 0000
  TAB 09
• Word address format N4G2X43Y43Z43R43F40S40T
  2M2I43J43K43
• e.g.
• N50 G00 X25400 Y12500 Z0 F0
• N60 G01 Z-10000 F500 M08
• N70 Z0 M09

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Example
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NC Part Programming

• Classes of command check the MCU manual
• Preparatory functions G-commands, e.g.
  G01linear interpolation, controlled feed rate
  G02 circular interpolation, CW
• I, J, K location of the to the
  of the arc
• Canned cycles (fixed cycles) sub-function,
  canceled G80.
• E.g N50 G81 X25400 Y12500 Z-10000 F500 M08
• Axis motion commands motion distance
• I, J, K relative to x,y,z axes
• Cutter compensation G41 and G42, canceled uses
  G40
• Feed and speed commands G92-G98,F0maximum feed
  rate
• Identification commands identify in
  the program, e.g. T01 tool in no. 1 slot is
  used, N increments of 5 or 10 between
  consecutive blocks
• Miscellaneous commands remaining commands to
  control machines operations, e.g. M03 spindle
  on CW
• Special characters , EOB, , etc.
• Advanced features scale program, macro, etc.

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NC Part Programming
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NC Part Programming
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Example

• Write an NC program to machine a 0.5 in.-wide L
  shapeed slot in a mild steel workpiece with
  dimensions 3x2.75x0.75 in. as shown in the
  following figure
• Assumptions
• The top lower left corner will be used for
  program zero
• Machining motion will start in the indicated
  position
• The tool is 0.25in. above the top surface of th
  part prior to start machining
• The tool diameter is 0.5in. So only one pass is
  required
• A cutting speed of 500rpm and feed rate of 10
  in./min are used
• Machine specification N3G2X43Y43Z43R43F4S4T2M
  2

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Solution
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Loading the Program

• Conventional NC use punched tape
• Direct NC a central computer ?NC machines one
  block at a time
• CNC use PC
• Distributed NC a central computer ?CNC machines
  several blocks at a time.

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Computer-Aided Part Programming

• Computer-aided programming systems (part
  programming languages) English-like command
  rather than MG codes
• Advantages
• Reduce errors,manual calculations.
• Path simulation save machining time
• Less time to prepare, less space to store
• Easy to learn, understand and program, trace and
  correct errors

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Computer-Aided Part Programming

• Automatically Programmed Tools(APT) the first
  part programming language.
• Structure of the APT system
• APT language basics
• Statement what?how? Tool? Up to 4092 elements
• Word lt6 alphanumeric characters, the first is
  letter
• Key words standard words, letters only
• Symbols words are used to define geometric
  entities, store numeric values
• Label special word, reference statements
• Mathematical operations and functions
• Punctuation characters / , . ) ()
• Major word/ some minor words, data

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Computer-Aided Part Programming
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Computer-Aided Part Programming

• APT types of statements
• Initialization and termination
• e.g. PARTNO part1
• MACHIN/MILL
• END
• Geometry definition statements
• e.g. P1 POINT/ 1, 20, 1
• L1 LINE/P1,(POINT/(2,3,4))
• Tool definition and motion statements
• Point-to-point motionto target without regard to
  the path
• Continuous-path tool motion ds, ps, cs, rel
  concepts
• e.g. LOADTL/1
• CUTTER/20
• RAPID GOTO/P1

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Computer-Aided Part Programming

• Postprocessor commands
• Convert to machine tool coordinate system
• Check for violation of machine tool slide travel
  limitations
• Check for violation of available spindle speeds
  and feed rates
• Select available spindle speeds and feed rates
• Develop motion commands using available MG codes
• Determine slide acceleration and deceleration
  avoiding overshoots
• Determine other MCU requirements
• Process machine-specific functions
• Provide output including diagnostics, NC tape,
  operator instructions
• E.g. RAPID
• FEDRAT/500

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CAD/CAM-Based Part Programming

• The CAD/CAM system generates and
  from the information and
• Output CLFILE or APT program ?postprocessing ?
  NC program ?machine
• Advantages
• Tool path generation ? check errors
• Using CAD data data consistency, easy to
  accommodate design changes
• Disadvantage no path optimization

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Beyond Postprocessors

• Eliminate postprocessor?
• BCL NC processing (32-Bit Binary CL Exchange
  Input Format for NC Machines)

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Programmable Logic Controllers

• PLC a special-purpose computer ? control
  specified processes by implementing program
  
• Structure
• PLC vs. PC
• Dedicated application
• Using small control program and rapid ,
  simultaneous execution of the individual steps in
  the program

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PLC

• Logical control
• Decision by attributes vs. decision by variables
  (yes-no) vs. (values)
• Decision by attributes ? automation using logical
  control
• Logical relationships AND, OR, NOT, etc.
• Logic devices perform logical relationships in
  PLCs
• represents the logic in a PLC control
  program Each rung ? output

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PLC

• Programming the PLC
• (an external unit) ? program ?memory
• Programming languages BASIC, FORTRANT, Boolean
  functions,
• Counters and timers handle variable-type
  questions, may have more than one output

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Example

• Consider a production line in which parts
  requiring processing are brought to a machine by
  a conveyor. A robot is used to load parts from
  the conveyor onto the machine and, after the part
  has been processed, unload the part from the
  machine and place it on a pallet. The cycle time
  for processing each part is 10min. The robot is
  to palletize the parts by placing 125 parts on
  each pallet. The parts are to be arranged on the
  pallet in 5 layers of 25 parts each. Once a
  pallet is complete, it is dispatched and a new
  pallet is started. An allowance of 30s is to be
  made for pallet dispatching and presentation of a
  new pallet. Construct a ladder logic diagram to
  effect the required control.

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Solution
We will use the following I/O assignments
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Generalized Process Control

• ? processes, equipments in a manner
• PLC ? single process
• NC ? single machine
• Online control the control computer communicate
  directly with the process, equipment