CNC machine tool and system
3. Automation
The word ‘Automation’ is derived
from Greek words “Auto” (self) and “Matos” (moving). Automation therefore is
the mechanism for systems that “move by itself”. However, apart from this
original sense of the word, automated systems also achieve significantly
superior performance than what is possible with manual systems, in terms of
power, precision and speed of operation.
Definition:
Automation is a set of technologies that results in operation of machines
and systems without significant human intervention and achieves performance
superior to manual operation
Automation of production systems can be classified into three basic
types:
1. Fixed automation (Hard Automation)
2. Programmable automation
3. Flexible automation (Soft Automation).
1. Fixed automation (Hard
automation): Fixed automation refers to the use of
special purpose equipment to automate a fixed sequence of processing or
assembly operations. Each of the operation in the sequence is usually simple,
involving perhaps a plain linear or rotational motion or an uncomplicated
combination of two. For example feeding of a rotating spindle. It is
relatively difficult to accommodate changes in the product design. This is also
called hard automation.
Advantages:
1. Low unit cost
2. Automated material handling
3. High production rate.
Disadvantages:
1. High initial Investment
2. Relatively inflexible in accommodating product changes.
2. Programmable automation: In
programmable automation, the production equipment is designed with the
capability to change the sequence of operations to accommodate different
product configurations. The operation sequence is controlled by a program,
which is a set of instructions coded. So that they can be read and interpreted
by the system. New programs can be prepared and entered into the equipment to
produce new products. Examples of programmable automation include numerically
controlled (NC) machine tools, industrial robots, and programmable logic
controllers.
Advantages:
1. Flexible to deal with design variations.
2. Suitable for batch production.
Disadvantages:
1. High investment in general purpose equipment
2. Lower production rate than fixed automation.
3. Flexible Automation: (Soft
automation): Flexible automation is an extension of
programmable automation. A flexible automation system is capable of producing a
variety of parts with virtually no time lost for changeovers from one part style
to the next. There is no lost production time while reprogramming the system
and altering the physical set up. Examples of
flexible automation are the flexible manufacturing systems for performing machining
operations.
Advantages:
1. Continuous production of variable mixtures of product.
2. Flexible to deal with product design variation.
Disadvantages:
1. Medium production rate
2. High investment.
3. High ‘unit cost relative to fixed automation.
Reasons or
goals or Advantage of Automations
1. To increase labor productivity- Automating
a manufacturing operation usually increases production rate and labor
productivity. This means greater output per hour of labor input.
2. To reduce labor cost-
Higher investment in automation has become economically
justifiable to replace manual operations. Machines are increasingly being
substituted for human labor to reduce unit product cost.
3. To
improve worker safety- By automating a given operation and transferring the
worker from active participation in the process to a supervisory role, the work
is made safer. The safety and physical well-being of the worker has become a
national objective which has achieved through automation.
4. To improve product
quality- Automation not only results in higher production rates
than manual operations, it also perform manufacturing process with great
uniformity and conformity to quality specification.
5. To reduce manufacturing
lead lime. Automation helps to reduce the elapsed time between
customer order and product delivery, providing a competitive advantage to the
manufacturer for future orders. By reducing manufacturing lead time, the
manufacturer also reduces work-in-process inventory
3.1 Numerical Control (NC)
Numerical control of
machine tool may be defined as methods of programmable automation in which
various function of the machine tool are controlled by letters, numbers and
symbols. Basically an NC machine runs on a program fed to it. The program
consists of instruction about the method of what tool are to be used, at what
speed, at what feed and to move from which point to point in which path etc.
Since program is the controlling point for product manufacturing, the machine
becomes versatile and can be used for any part. All the function of an NC
machine tool is therefore controlled electronically, hydraulically and
pneumatically.
In NC machines tool one
or more of the following functions may be automatic:-
a)
Starting
and stopping of machine-tool spindle
b)
Controlling
the spindle speed
c)
Positioning
the tool at desired locations
d)
Controlling
the rate of movement of the tool tip (i.e. feed rate)
e)
Changing
of tool in spindle
Numerical control can be
applied to a wide variety of processes. The application is generally divided
into two categories
a)
Machining
tool application such as drilling, milling, turning, grinding etc
b)
Non
machine tool application such as assembly, drafting and inspection.
Basic component of an NC system
An
NC system consists of three basic components
I.
A
program of instruction
II.
Machine
control unit (MCU)
III.
Processing
Equipment or Machine tool
Fig.1: Basic component of an NC
system
I) Program of
instruction:
The program of instruction is the detailed step by step commands that direct
the processing equipment. In machine tool applications, the program of
instruction is called a part program and the person who prepares the program is
called a part programmer. The program is coded in numerical or symbolic form on
some type of input medium that can be interpreted by the controller unit. For
many years, the common input medium was 1-inch wide punch tape. Today punch
tape has been replaced by newer storage technologies in modern machine shops.
These technologies include magnetic tape, diskettes and electronic transfer of
part programs from a computer.
II) Machine control unit
(MCU): It
is the second basic component of NC system. The machine control unit (MCU)
consists of a microcomputer and related control hardware that store the program
of instruction and executes it by converting each command into mechanical
actions of the processing equipment one command at a time. The related hardware
of the MCU includes components to interface with the processing equipment and
feedback control element. The typical element of MCU includes the tape reader,
a data buffer, signal output channel and feedback channel.
The tape reader is an
electro mechanical device for winding and reading the puchtape containing the
program of instruction. The data contained on the tape are reads into the data
buffer. The signal output channels are connected to the servo motors and other
controls in the processing equipment. Through these channels, the instructions
are sending to the processing equipment from the machine tool unit.
III) Processing equipment: The third basic component of an NC system is the processing equipment that performs useful work. Its operation is directed by MCU, which in turn is driven by instruction contained in the part program. In the common example of NC machining the processing equipment consists of work table and spindle as well as the motors and controllers to drive them. Varity of machining operation means that a verity of cutting tool is required. The tool is kept in a tool drum or other holding devices, when the tape called a particular tool the drum rotates to position the tool for inserting into the spindle chuck. The machining table or worktable can orient the job so that it can be machined o several surface as per requirement.
Different
steps used in NC manufacturing or The NC procedure
1. Process planning:
It is
concerned with the preparation of a route sheet. The route sheet is a listing
of the sequence of operation which must be performed on the workpiece. It is
called a rout sheet because it also lists the machine through which the part
must be routed in order to accomplish the sequence of operation.
2. Part programming: A part programmer plans
the process for the portion of the job to be accomplished by NC. Part
programming is the sequence of machining steps to be performed by NC. There are
two ways to program for NC.
I.
Manual
part programming
II.
Computer
assisted part programming
3. Tape Preparation: A punch tape is
prepared from the part programmers NC process plan. In manual part programming,
punch tape is prepared directly from the part program manuscript or a
typewriter like device equipped with tape punching capabilities. In computer
assisted part programming the computer interprets the list of part programming
instruction performs the necessary calculation to convert into a detail set of
machine tool motion commands.
4. Tape verification: After punch tape has
been prepared, a method is usually provided for checking the accuracy of tape.
Sometimes the tape is checked by running it through a computer program which
plots the various tool movements on paper.
5. Production: The final step in NC
procedure is to use NC tape in production. The machine tool operator’s function
during production is to load raw workpiece in machine. The NC system then takes
over and machined the part according to the instruction on tape. When the part
is completed the operator removes it from the machine and loads the next part.
3.2 NC coordinate system ( or axes of NC
machine tool)
To
program NC processing equipment a standard axes system must be defined by which
the position of the cutting tool related to the workpiece can be specified.
There are two axis system used in NC.
I.
For
flat and prismatic work part
II.
For
rotational parts
Both
axis systems are based on Cartesian coordinate system. Axes system for flat and
prismatic parts consists of three linear axes (x, y, z) and three rotational
axes (a, b, c) as shown in fig.2: below
Fig.3:
coordinate system for rotational parts
The
coordinate axes for a rotational NC system are illustrated in Figure 3. These systems
are associated with NC lathes and turning centers. Although the work rotates,
this is not one of the controlled axes on most of these turning machines.
Consequently, the y- axis is not used. The path of the cutting tool relative to
the rotating workpiece is defined in the x-z plane, where the x-axis is the
radial location of the tool, and the z-axis is parallel to the axis of rotation
of the part.
3.3 NC Motion control system
In
order to accomplish machining process, cutting tool and workpiece must be moved
relative to each other. In NC there are three basic types of motion control
system:
1.
Point-to-Point (PTP)
2.
Straight cut
3. Contouring
Point-to-point systems represent
the lowest level of motion control between the tool and workpiece. Contouring
represents the highest level of control.
1.
Point-to-point control: Point-to-point
(PTP) is also sometimes called a positioning system. In PTP, the objective of
the machine tool control system is to move the cutting tool to a predefined
location. The principle function of the PTP is to position the tool form one
point to another within coordinate system. Each tool axis is controlled
independently, therefore; the programmed motion always in rapid traverse. Once
the tool reaches the desired location, the machining operation is performed at
that position. NC drill presses are a good example of PTP systems. The spindle
must first be positioned at a particular location on the workpiece. This is
done under PTP control
Fig
1: Point to point NC System
2.
Straight cut: Positioning systems are the simplest
machine tool control systems and are therefore the least expensive of the three
types. However, for certain processes, such as drilling operations, tapping,
riveting and spot welding, PTP is perfectly suitable. Straight-cut control
systems are capable of moving the cutting tool parallel to one of the major
axes at a controlled rate suitable for machining. It is therefore appropriate
for performing milling operations to fabricate workpiece of rectangular
configurations. An example of a straight cut operation is shown in Figure (2).
An NC machine capable of straight cut movements is also capable of PTP
movements.
Fig 2:
Straight cut
3.
Contouring
(continuous) Path CNC System: Contouring is the
most complex, the most flexible, and the most expensive type of machine tool
control. It is capable of performing both PTP and straight-cut operations. In
addition, the distinguishing feature of contouring NC systems is their capacity
for simultaneous control of more than one axis movement of the machine tool.
The path of the cutter is continuously controlled to generate the desired
geometry of the workpiece. For this reason, contouring systems are also called
continuous-path NC systems. All NC contouring system have the ability to
perform linear and circular or parabolic interpolation features which recorded
in the NC computer under a (G preparatory code). Figure (3) illustrates the
versatility of continuous path NC. Milling and turning operations are common
examples of the use of contouring control.
Fig 3: Contouring
3.4 Advantage, Disadvantage and
application of NC machine
Advantages of
NC
1. Reduce non
productive time: It reduce non productive
time in NC machine tools in the following ways
·
By reducing set up time
·
Bt reducing workpiece handling time
·
By reducing tool changing time
Due to reduction in non productive
time the production of machine increases.
2. Greater accuracy and repeatability: Compared
with manual production methods, NC reduces or eliminates variations that are
due to operator skill differences, fatigue, and other factors attributed to
inherent human variability. Parts are made closer to nominal dimensions, and
there is less dimensional variation among parts.
3.
Inspection requirement are reduced: Less
inspection is needed when NC is used because parts produced from the same NC
part program are virtually identical. Once the program has been verified, there
is no need for the high level of sampling inspection.
4.
More-complex part geometries are possible: NC technology
has extended the range of possible part geometries beyond with manual machining
methods. This is an advantage in product design in several ways: (1) More
functional features can he designed into a single part, thus reducing the total
number of parts in the product and the associated cost of assembly (2)
mathematically defined surfaces can be fabricated with high precision and (3)
the space is expanded within which the designer's imagination can wander to
create new part and product geometries.
5. Reduce Fixturing: NC requires fixture
which are simpler and less costly to fabricate because the positioning is done
by the NC tapes rather than by jigs and fixture.
6. Reduce manufacturing lead time: As
the job can be set up more quickly with NC and few steps are generally required
with NC. The lead time to deliver a job to the customer is reduced.
7. Reduced floor space requirements:
Since one NC machine center can often accomplish the production of several
conventional machines, the amount of floor space required in an NC shop is
usually less than a conventional shop
Disadvantages of NC
1. Higher initial
investment: The cost of NC machine tool is much higher compared
to conventional machining tool. The cost is often 5 to 10 times and also the
cost of tool is high so there is very high initial investment. All these make
the machine hourly rate high. As a result it is necessary to utilize the
machine tool for a large percentage of time.
2. Higher maintenance cost: As NC is a
complex and sophisticated technology it requires higher investment foe
maintenance in terms of wages of highly skilled personnel and expensive spares.
3. Part programming: NC
equipment must be programmed. To be fair it should be mentioned that process
planning must be accomplished for any part whether or not it is produced on NC
equipment. However NC part programming is a special preparation step in batch
production that is absent in conventional machine shop operations
4. Higher utilization of
NC equipment: To maximize the economic benefits of an NC machine tool,
it usually must he operated multiple shifts. This might mean adding one or two
extra shifts to the plants normal operations, with the requirement for
supervision and other staff support.
Computer
Numerical Control (CNC)
CNC is the short form
for computer numerical control. We seen that the NC machine works as per the
program of instruction fed to the controller unit of the machine. The CNC
machine comprises of the mini computer or the microprocessor that acts as the
controller unit of the machine. While in the NC machine the program is fed into
the punch card in CNC machines the program of instruction is fed directly into
the computer via a small board similar to the traditional keyboard.
In CNC machine the
program is stored in the memory of the computer. The programmer can easily
write the code, and edit the program as per the requirement. Compared to NC
machine the CNC offers great additional flexibility and computational
capability. New system can be incorporate into the CNC controller simply by
reprogramming the unit. Because of its capacity and the flexibility the CNC
machines are called as “Soft-wired” NC.
Advantages of CNC Machine
- CNC machines can be used continuously 24×7
throughout the year and only need to be switched off for occasional
maintenance.
- CNC machines are programmed with a design
which can then be manufactured hundreds or even thousands of times. Each
manufactured product will be exactly the same.
- Less skilled/trained people can operate CNC
machines unlike manual lathes / milling machines etc. which need skilled
engineers.
- CNC machines can be updated by improving the
software used to drive the machines
- Training for correct use of CNC machines is
available through the use of ‘virtual software’. This software is like a
computer game that allows the operator to practice using the CNC machine
on the screen of a computer.
- Modern design software allows the designer to
simulate the manufacture of his/her idea. There is no need to make a
prototype or a model. This saves time and money.
- One person can supervise many CNC machines as
once they are programmed they can usually be left to work by themselves.
Only the cutting tools need replacement occasionally.
CNC Machine Disadvantages
- CNC machines are more expensive than manually
operated machines, although costs are slowly coming down.
- The CNC machine operator only needs basic
training and skills, enough to supervise several machines. In years gone
by, engineers needed years of training to operate centre lathes, milling
machines and other manually operated machines. This means many of the old
skills are being lost.
- Fewer workers are required to operate CNC
machines compared to manually operated machines. Investment in CNC
machines can lead to unemployment.
- Many countries no longer teach pupils /
students how to use manually operated lathes / milling machines etc…
Pupils / students no longer develop the detailed skills required by
engineers of the past. These include mathematical and engineering skills
APT
PROGRAMMING
APT stands for
Automatically Programmed Tool. It is a language that defines the tool path with
respect to the part geometry, and often forms the basis for post-processor
generated NC files. The APT language consists of four types of statements. Geometry statements will be used to
specify the elemental features defining the part shape. Motion statements are used to specify the path taken by the tool. Post-processor statements control the
machinery, controlling coolants as well as the feeds and speeds. Auxiliary statements complete the
picture, specifying the part, required tools, etc. The following sections
describe each of the APT statements
1.
Geometry Statements
- All geometric elements must be defined before tool motion may be programmed.
Geometry statements associate a symbol with a description of the geometric
element and its parameters. The general form for a geometry statement is:
Symbol =
geometric type/parametric description
The symbol consists of up to six
alpha-numeric characters. The geometric type describes these features are
POINT, LINE, PLANE, and CIRCLE are valid APT geometric types.
To specify a point:
P0 = POINT/1.0, 1.2, 1.3 specifies a point at XYZ coordinates
1.0, 1.2, and 1.3,
respectively.
P1 = POINT/INTOF L1, L2 specifies a point at the
intersection of lines L1 and L2, which
must
have been defined prior to the statement.
To specify
a line:
L1 = LINE/P0, P1 specifies a line by two points, previously
defined.
L1 = LINE/1.0, 1.2, 1.3, 2.0, 2.1,
2.3 specifies a line by two
points, given as explicit coordinates.
To specify
a plane:
PL0 = PLANE/P0, P1, P2 specifies a plane
through three, non-collinear, previously
defined
points.
PL1 = PLANE/P3, PARLEL, PL0 specifies a plane through a point
P3 parallel to a plane PL0.
To specify
a circle:
C0 =
CIRCLE/CENTER, P0, RADIUS, 1.0 specifies
a circle of radius 1 from a center point of P0.
2. Motion
Statements: The format for motion commands
follows the pattern:
motion/description
GO/TO, L1, TO,
PL1, TO, L2 specifying that the tool should use
line L1 as the drive surface, plane P1
as the part surface,
and line L2 as the check surface
Some motion commands are: GOTO,
GO/TO, ON, PAST, TANTO
3.
Post-Processor Statements: These statements provide processing
parameters to the post-processor program. Typical programs will require
parameters for feeds, speed, and other tool/spindle/machine controls. Examples:
SPINDL/600 specifies
the spindle to be 600 rpm.
FEDRAT/6.0 specifies
a feed rate of 6 inches per minute.
TURRET/T2 specifies
loading tool # 2 in the turret.
MACHIN/MILL,2 specifies a mill
machine type, and controller type 2
4.
Auxiliary Statements:
These statements complete the APT programming language, and include the FINI
statement to mark the end of the program as well as statements to define the
width of the tool. An example is:
CUTTER/0.25 specifies a
quarter-inch cutter diameter
FINI
end program
QUESTION: Write an APT program for the profiling of the
part in Figure 1 is to be generated. The processing parameters are: (a) feed
rate is 5.39 inches per minute; (b) spindle speed is 573 revolutions per
minute; (c) a coolant is to be used to flush the chips; (d) the cutter diameter
is to be 0.5 inches, and (e) the tool home position is (0, -1, 0).
SOLUTION:
APT
geometry and tool path for the given workpiece is as follows:
APT Programming is:
PARTNO EXAMPLE
MACHIN/MILL, 1
selects the target machine and controller type
CUTTER/0.5000 specifies
the cutter diameter
P0 = POINT/0, -1.0, 0
P1 = POINT/0, 0, 0
P2 = POINT/6.0, 0, 0
P3 = POINT/6.0, 1.0, 0
P4 = POINT/2.0, 4.0, 0 geometry
statements to specify the
L1 = LINE/P1, P2
pertinent surfaces of the part
C1 = CIRCLE/CENTER, P3, RADIUS, 1.0
L2 = LINE/P4, LEFT, TANTO, C1
L3 = LINE/P1, P4
PL1 = PLANE/P1, P2, P3
SPINDL/573 sets
the spindle speed to 573 rpm
FEDRAT/5.39 sets
the feed rate to 5.39 ipm
COOLNT/ON turns
the coolant on
FROM/P0 gives the starting position for the
tool
GO/PAST, L3, TO, PL1, TO, L1 initializes contouring motion; drive, part.
GOUP/L3, PAST, L2
GORGT/L2, TANTO, C1 motion
statements to contour the part
GOFWD/C1, ON, P2
in a clockwise direction
GOFWD/L1, PAST, L3
RAPID move
rapidly once cutting is done
GOTO/P0 return
to the tool home position
COOLNT/OFF turn
the coolant off
FINI end program
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