Computer Numerical
Control
Eastern Mediterranean University
Computer Integrated Manufacturing
MENG 364
Fall 2013
Computer Numerical Control
Numerical Control:
Definition: A form of programmable automation in which the process is
controlled by numbers, letters and symbols.
Historical Background
• In late 1940s, John T Parsons conceived a method of using punched
cards containing coordinate position data to control m/c tool. The m/c
was directed to move in small increments, thus generating desire
surface of an airfoil.
• Later Parsons demonstrated his work to US air force . The force
sponsored a series of projects at MIT. The 1st development at MIT was
a prototype NC milling m/c by retrofitting a conventional tracer mill
m/c with position servomechanisms for three m/c tool axes.
• The first demonstration was held in 1952. In 1953, the usefulness had
been proven. Shortly thereafter, m/c tool builders launched projects for
developing commercial NC units, thus creating a revolution.
• Further sponsorship of air force devised a NC language called APT.
Though it is considered lengthy and complex, many advanced
languages are based on APT.
Basic Components of NC System
An NC system consists of following 3 basic components:
• Program of instructions
• Controller Unit, also called m/c control unit (MCU)
• M/C tool
General relationship among three components is shown in Fig.
Basic Components of NC System
(1. Program of Instructions)
•
This is detailed step by step set of directions which will tell the m/c tool what to do.
•
The most common input medium used is 1 inch wide punched tape. Other input
mediums are punched cards, magnetic tape
Punched tape
Punched card
Magnetic tape: plastic film
with magnetic coating
Basic Components of NC System
(2. Controller Unit)
• Controller unit consists of electronics hardware that read
and interpret the program of instructions and convert it
into mechanical actions of m/c tool.
Basic Components of NC System
(3. Machine Tool)
•
It is the part of NC system which performs useful
work.
•
Milling m/c tool is an example of NC system
•
Fixtures, cutting tools, auxiliary equipment
needed in machining operation are part of m/c
tool
•
A machining center can perform several
functions:
(a) It is capable of performing various operations:
drilling, tapping, reaming, milling, boring
(b) It has capacity to change tool automatically
under tape command.
(c) Several tools are kept in magazine. The magazine
rotates and position the desired tool. The automatic
tool changer automatically picks the tool and inserts
into spindle.
NC Procedure
Process Planning: The drawing of work part must
be interpreted in terms of manufacturing processes to
be used. This step is referred as process planning and it
is concerned with route sheet. The route sheet is listing
of the sequence of operations which must be performed
on work part. It is called rout sheet because it also lists
machines through which the part must be routed .
NC Procedure
Part Programming: The programmer plans the sequence
of machining steps to be performed by NC and to
document these in in a special format. The are two ways
of programming:
•Manual part programming: The machining instructions are
prepared manually on a form called program manuscript.
•Computer assisted part programming: Machining
instructions are created with the aid of computer. This saves
much time and is useful for complex parts.
NC Procedure
Tape Preparation:
In manual programming, the punched tape is prepared
directly from the part program manuscript on a typewriter like device equipped with the tape punching
capability.
In computer-assisted programming, the computer
interprets the list of part
programming
instructions, performs the necessary calculations to
convert this into a detailed set of m/c tool motion
commands, and then controls a tape punch device to
prepare tape for specific NC m/c.
NC Procedure
4. Tape Verification: After the punch tape has been prepared, a method is
usually provided for checking accuracy of tape. Some time the tape is checked
by running it through a computer program which plots various tool movements
(or table movements). In this way, major errors in tape can be discovered.
Another method is Acid test. In this test, foam or plastic material is used to
make a part. Then the error can be detected easily.
5. Production: The final step in NC procedure is to use NC tape in
production. Raw material is clamped in fixture, tools are selected and holded
in spindle. The zero point on job is selected and machining is begun.
NC Coordinate Systems
• In order for the part programmer to plan the sequence of positions and
movements of the cutting tool relative to work piece, it is necessary to establish a
standard axis system by which the relative positions can be specified.
• Ref Fig, two axes x & y are defined in plane of table and z axis is perpendicular to
this plane; and movement in z direction is controlled by vertical motion of spindle.
• In addition to 3 axes, the machines (especially NC milling) can posses capacity to
control one or more rotational axes. These are a, b and c, which specify angles
about x, y & z axes, respectively.
NC Coordinate Systems
(Fixed Zero & Floating Zero)
The programmer must determine position of tool relative to origin (zero
point) of coordinate system. NC machines have two methods for
specifying zero point:
Fixed Zero: In this case the origin is always located at the same position on
m/c table. Usually this is the southwest corner of table and all tool
locations will be defined by positive x & y coordinates.
Floating Zero: The second and more common feature on modern NC
machines allows the m/c operator to set zero point at any position on
m/c table.
NC Coordinate Systems
(Absolute and incremental positioning)
1. Absolute Positioning: In this method, the tool position is always defined with
respect to zero point/origin of coordinate system.
2. Incremental Positioning: In this method, the tool position is defined with
respect to previous position of tool on work-piece.
NC Motion Control Systems
(Point-to-point NC)
•
•
Point to point is also sometimes called positioning system
In PTP, the objective of m/c tool control system is to move cutting tool to a predefined location
• The speed or path by which this movement is accomplished is not important in
PTP NC.
• Once the tool reaches the desired location, machining operation is performed
at that location.
Example: NC drill presses are good example of PTP system. The spindle must be
positioned at a particular location on w/p. This is done under PTP control.
G-Code Programming
Coordinate System
Y-axis
A (X,Y)
(1,.875)
X-axis
Origin
3D Coordinate System (X,Y,Z)
Z-axis
Y-axis
X-axis
Basic Machine Axes: 3 axis
• Milling Machines: 3 axis
X – axis (table left and right)
Y – axis (table in and out)
Z – axis (usually the spindle
axis)
Additional Axes
•
•
•
•
•
•
A – axis (angular axis about X - axis)
B – axis (angular axis about Y – axis)
C – axis (angular axis about Z – axis)
U – axis (secondary axis parallel to X)
V – axis (secondary axis parallel to Y)
W – axis (secondary axis parallel to Z)
G - Code Programming
• G – Code Programming
•
Originally called the “Word Address”
programming format.
• Processed one line at a time sequentially.
Common Format of a Block
Sequence
#
N50
Preparatory
Function
G90 G01
Dimension
Words
X1.40Y2.25
Feed
Rate
Spindle
Function
F10
S1500
Individual Words
Tool
Function
T01
Misc.
Function
M03
Word Address 1
• N – Sequence or line number
•
A tag that identifies the beginning of a block of
code. It is used by operators to locate specific lines of a
program when entering data or verifying the program
operation.
• G – Preparatory function
•
G words specify the mode in which the milling
machine is to move along its programmed axes.
Word Address 2
• Dimension Words
X – Distance or position in X direction
Y – Distance or position in Y direction
Z – Distance or position in Z direction
• M – Miscellaneous functions
•
M words specify CNC machine functions not
related to dimensions or axial movements.
Word Address 3
• F – Feed rate (inches per minute or millimeters per
minute)
•
Rate at which cutting tool moves along an axis.
• S – Spindle speed (rpm – revolutions per minute)
•
Controls spindle rotation speed.
• T – Tool number
•
Specifies tool to be selected.
Word Address 4
• I – Circular cutting reference for x axis
• J – Circular cutting reference for y axis
• K – Circular cutting reference for z axis
Manual Part Programming Methods
(Groups of G codes)
Commonly used G codes in form of groups:
1. Coordinate system Group (G90, G91)
Absolute programming using G90:
- G90 is used for specifying tool position in absolute
coordinate system.
- Ref. Fig, suppose the tool has to pass through the route: OA-B-C. Then blocks will be:
N001 G90 G01 X5.0 Y10.0 F200
N002 X25 .0 Y15.0
N003 X35.0 Y5.0
- This system is generally advised for programming because
there are few chances of errors.
- Whatever the route of move, programmed X &Y values of
each position remain the same.
Incremental programming G91:
- G91 is generally default mode and is cancelled with G90.
End of program words (M02, M30) also cancel them.
- Ref to Fig, the incremental programming for path O-AB-C:
- N001 G91 G01 X5.0 Y10.0 F200
- N002 X20.0 Y5.0
- N003 X10.0 Y-10.0
Manual Part Programming Methods
(Groups of G codes)
Linear and Rapid interpolation (G01 &G00)
• This code is used when material needs to be cut using feed rate
• Say material needs to be cut at feed rate of 250mm/min from point D-E (Fig.), the
program of block will be:
N01 G90 G00 X20.0 Y100.0 (Absolute Programming)
N02 G01 X110.0 Y30.0 F250 (F function is added)
Manual Part Programming Methods
(Groups of G codes)
-
(c) Circular interpolation:
• When an arc is to be traversed in a plane,
the G02 (CW) or G03 (CCW) is used.
• Ref Fig, the program block for F-G:
N01 G90 G00 X65.0 Y60.0 (CCW- Absolute)
N02 G03 X15.0 Y30.0 I-50.0 J-40.0 F 200.0
(I, J can be replaced by R).
- R can not be used for full circle. In such case,
I, J need to be used.
- If motion is CCW from G-F
50