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
 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
 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
Basic Components of an NC System
 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
Coordinate Axis System for
Flat and Prismatic Parts
 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
Coordinate Axis System for
Rotational Parts
 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)
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
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
 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
 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
 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
 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
 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.
 Computer Numerical Control (CNC)
– Additional Features
ƒ Storage of more than one part program
ƒ Various forms of program input
ƒ Program editing at the machine tool
ƒ Fixed cycles and programming subroutines
ƒ Interpolation
ƒ Acceleration and deceleration computations
ƒ Communications interface
ƒ Diagnostics
Configuration of
CNC Machine Control Unit
 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
 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
 Distributed Numerical Control
Configurations

Switching network
Distributed Numerical Control
Configurations

Local area network (LAN)

 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
Common NC Machining Operations

Turning
 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

ƒ Nonproductive time is reduced

ƒ Greater accuracy and repeatability
ƒ Lower scrap rates
ƒ Inspection requirements are reduced
ƒ More complex part geometries are possible
ƒ Engineering changes are easier to make
ƒ Simpler fixtures
ƒ Shorter lead times
ƒ Reduce parts inventory and less floor space
ƒ Operator skill-level requirements are reduced
 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
 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
 Analysis of Positioning NC Systems

ƒ 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
ƒ Precision in NC positioning - three measures:
1. Control resolution
2. Accuracy
3. Repeatability
 Open-Loop Motion Control System

ƒ Operates without verifying that the actual position

achieved in the move is the desired position
 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
 Optical Encoder

ƒ Device for measuring rotational position and speed

ƒ Common feedback sensor for closed-loop NC control
 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
Definitions of Control
Resolution, Accuracy, and
Repeatability
 NC Part Programming
1. Manual part programming
2. Computer-assisted part programming
3. Part programming using CAD/CAM
4. Manual data input
 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
 Creating Instructions for NC

ƒ Bit - 0 or 1 = absence or presence of hole in the tape

ƒ Character - row of bits across the tape
ƒ Word - sequence of characters (e.g., y-axis position)
ƒ Block - collection of words to form one complete
instruction
ƒ Part program - sequence of instructions (blocks)
 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
 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
 Example: Word Address Format
N001 G00 X07000 Y03000 M03
N002 Y06000
 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
 Computer-Assisted Part Programming

ƒ Manual part programming is time-consuming, tedious,

and subject to human errors for complex jobs
ƒ Machining instructions are written in English-like
statements that are translated by the computer into
the low-level machine code of the MCU
ƒ APT (Automatically Programmed Tool)
ƒ The various tasks in computer-assisted
part programming are divided between
ƒ The human part programmer
ƒ The computer
 Computer-Assisted Part Programming

ƒ Sequence of activities in computer-assisted part

programming
 Part Programmer's Job

ƒ Two main tasks of the programmer:

1. Define the part geometry
2. Specify the tool path
 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
 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
 Other Functions in Computer-
Assisted Part Programming

ƒ Specifying cutting speeds and feed rates

ƒ Designating cutter size (for tool offset calculations)
ƒ Specifying tolerances in circular interpolation
ƒ Naming the program
ƒ Identifying the machine tool
 Cutter Offset

Cutter path must be

offset from actual
part outline by a
distance equal to
the cutter radius
 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
 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
 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
 Examples of Machining Cycles in
Automated NC Programming Modules

Pocket milling

Contour turning
 Examples of Machining Cycles in
Automated NC Programming Modules

Facing and shoulder facing

Threading (external)
 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