Numerical Control (NC) Sections: 1. Fundamentals of NC Technology 2. Computer Numerical Control (CNC) 3. DNC 4. Applications of NC 5. Engineering Analysis of NC Positioning Systems 6. 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 by a program program containing coded alphanumeric alphanumeric data ▪
The alphanumeric data represent relative positions between a workhead (e.g., cutting cutting tool) and a workpart
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When the current job is completed, 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
NC Coordinate Systems For flat and prismatic (block-like) parts ▪
Milling and drilling operations
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Rotational axes about each linear axis
Coordinate Axis System for Flat and Prismatic Parts
NC Coordinate Systems For rotational parts: ▪
Turning operations
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Conventional Cartesian coordinate system, but only x - and z-axes
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y-axis not needed in turning
Coordinate Axis System for Rotational Parts
Motion Control Systems Point-to-Point systems ▪
Also called position systems
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System moves to a location and performs an operation at that location (e.g., drilling)
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Also applicable in robotics Continuous path systems
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Also called contouring systems in machining
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System performs an operation during movement (e.g., milling and turning)
Point-To-Point Control in NC Drilling of Three Holes in Flat Plate
Continuous Path Control in NC Profile Milling of Part Outline
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 or der equations
Absolute and Incremental Positioning Absolute positioning ▪
Locations defined relative to origin of axis system Incremental positioning
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Locations defined relative to previous position
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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, positioning, the move is specified by x
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= 40, y = 50
In incremental positioning, positioning, the move is specified by x = 20, y = 30.
Computer Numerical Control (CNC) Is defined as an NC system whose MCU is based on a dedicated microcomputer r ather than on a hardwired controller
Computer Numerical Control (CNC) – Features ▪
Storage of more than one part program
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Various forms of program input
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Program editing at the machine tool
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Fixed cycles and programming subroutines
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Interpolation
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Acceleration and deceleration computations
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Communications interface
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Diagnostics
Machine Control Unit MCU
CNC software There are three types of software programs: 1) Operating system software 2) Machine interface software 3) Application software
DNC ▪
Direct numerical control (DNC) – control of of multiple machine tools by a single (mainframe) (mainframe) computer computer through direct connection
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Distributed numerical control – network consisting of of central computer connected to to machine tool MCUs, which are CNC ▪
Present technology
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Two way communication
General Configuration of a Direct Numerical Control System
Connection to MCU is behind behind the tape reader (BTR). In distributed NC, entire programs are downloaded to each MCU, which is CNC rather than conventional NC
Distributed Numerical Control Configurations Switching network
Local area network (LAN)
Applications of NC ▪
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Machine tool applications: ▪
Milling, drilling, turning, boring, grinding
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Machining centers, turning centers, mill-turn centers
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Punch presses, thermal cutting machines, etc.
Other NC applications: ▪
Component insertion machines
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Drafting machines
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Electrical wire wrap machines
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Tape laying machines
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Filament winding machines
Common NC Machining Operations
CNC Horizontal Milling Machine
NC Application 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 workpart 5. Many separate machining operations on the part 6. The part is expensive
Advantages of NC ▪
Nonproductive time is reduced
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Greater accuracy and repeatability
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Lower scrap rates
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Inspection requirements are reduced
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More complex part geometries are possible
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Engineering changes are easier to make
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Simpler fixtures
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Shorter lead times
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Reduce parts inventory
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Operator skill-level requirements are reduced
Disadvantages of NC ▪
Higher investment cost ▪
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Higher maintenance effort ▪
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CNC machines are more expensive
CNC machines are more technologically sophisticated
Part programming issues ▪
Need for skilled programmers
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Time investment for each new part
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Repeat orders are easy because part program program is already available
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 verify that the actual position achieved is the desired position 2. Closed-loop - uses feedback feedback measurements measurements to confirm that the final position is the specified position
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Precision in NC positioning -three measures: 1. Control resolution 2. Accuracy 3. Repeatability
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Operates without verifying that the actual position position achieved in the move is the desired position
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Uses feedback measurements to confirm that the final position of the worktable is the location specified in the program
Optical Encoder ▪
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Device for measuring rotational position and speed Common feedback sensor for closed-loop NC control
Precision in NC Positioning Three measures of precision: 1. Control resolution - distance separating two two adjacent addressable points in the axis movement 2. Accuracy - maximum possible error that can can occur between the desired target point and the actual position taken by the system 3. Repeatability - defined as ±3 s of the the mechanical mechanical error distribution associated with the axis
NC Part Programming NC part programming consists of planning and documenting the sequence of processing steps to be performed on an NC machine. The NC programming methods are: 1. Manual part programming 2. Computer-assisted Computer-assisted part programming 3. Part programming using CAD/CAM 4. Manual data input
Manual part programming Binary Coded Decimal System: ▪
Each of the ten digits digits in decimal system is is coded with four-digit binary number
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The binary numbers are added to give the value
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BCD is compatible with with 8 bits across tape format, format, the original storage medium for NC part programs
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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
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Character - row of bits across the tape
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Word - sequence of characters (e.g., y-axis position)
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Block - collection of words to form one complete complete instruction
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Part program -
sequence of instructions (blocks)
Types of Words N - sequence number prefix G - preparatory words ▪
Example: G00 = PTP rapid traverse move X, Y, Z - prefixes for x , y , and zaxes
F - feed feed rate prefix prefix S - spindle speed T - tool selection M - miscellaneous command ▪
Example: M07 = turn cutting fluid on
Example: Word Address Format N001 G00 X07000 Y03000 M03 N002 Y06000
Computer-Assisted Part Programming ▪
Manual part programming is time-consuming, tedious, tedious, and subject to human errors for complex jobs
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Machining instructions instructions are written in English-like statements that that are translated by the computer computer into the low-level machine code of the MCU
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APT (Automatically Programmed Tool)
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The various tasks in computer-assisted computer-assisted part programming are divided between ▪
The human part programmer
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The computer
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
Cutter Offset Cutter path must be offset from actual part outline by a distance equal to the cutter radius
NC Part Programming Using CAD/CAM Is a computer interactive graphics system equipped with software to accomplish certain tasks in design and manufacturing and to integrate the design and manufacturing functions ▪
Geometry definition ▪
If the CAD/CAM system was used to define the original part geometry, no need to recreate recreate that geometry geometry as in APT ▪
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Automatic labeling of geometry elements
If the CAD part data are not available, available, geometry must must be created, as in APT, but user gets immediate immediate visual feedback about the created created geometry
Tool Path Generation Using CAD/CAM ▪
Basic approach: enter the commands commands one by one (similar to APT) ▪
CAD/CAM system provides provides immediate graphical graphical verification of the command
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Automatic software software modules for common machining machining cycles ▪
Profile milling
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Pocket milling
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Drilling bolt circles
Examples of Machining Cycles in Automated NC Programming Modules
Pocket milling
Contour turning
Facing and shoulder facing
Threading (external)
Manual Data Input
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Machine operator does part programming at machine ▪
Operator enters enters program by responding responding to prompts and questions by system
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Monitor with graphics verifies tool path
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Usually for relatively simple parts
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Ideal for small shop that that cannot afford a part programming programming staff
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To minimize changeover changeover time, system should should allow programming of next job while current job is running
1. Type three of the NC advantages?
Simpler fixtures
Shorter lead times
Reduce parts inventory
2. What are the Basic Components of an NC System?
Program of instructions
Machine control unit
Processing equipment
3. What are The NC programming methods?
Manual part programming
Computer-assisted Computer-assisted part programming
Part programming using CAD/CAM
Manual data input
4. What is the Computer Numerical Control (CNC)? Is defined as an NC system whose MCU is based on a dedicated microcomputer rather than on a hard-wired controller
