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The variety being demanded in view of the varying tastes of the consumer calls for a
very small batch sizes. Small batch sizes will not be able to take advantage of the mass
production techniques such as special purpose machines or transfer lines. Hence, the need for
flexible automation is felt , where you not only get the benefits of rigid automation but are
also able to vary the products manufactured thus bringing in the flexibility. Numerical control
fits the bill perfectly and we would see that manufacturing would increasingly be dependent
on numerical control in future.
Numerical control
Numerical control of machine tools may be defined as a method of automation in
which various functions of machine tools are controlled by letters, numbers and symbols.
Basically a NC machine runs on a program fed to it. The program consists of precise
instructions about the methodology of manufacture as well as movements. For example, what
tool is to be used, at what speed, at what feed and to move from which point to which point in
what path. Since the program is the controlling point for product manufacture, the machine
becomes versatile and can be used for any part. All the functions of a NC machine tool are
therefore controlled electronically, hydraulically or pneumatically. In NC machine tools, 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 tip at desired locations and guiding it along desired paths by
automatic control of motion of slides.
d. Controlling the rate of movement of tool tip ( feed rate)
e. Changing of tools in the spindle.
Functions of a machine tool
The purpose of a machine tool is to cut away surplus material, usually metal from the
material supplied to leave a work piece of the required shape and size, produced to an
acceptable degree of accuracy and surface finish. The machine tool should possess certain
capabilities in order to fulfill these requirements. It must be
a. Able to hold the work piece and cutting tool securely.
b. Endowed the sufficient power to enable the tool to cut the work piece material at
economical rates.
c. Capable of displacing the tool and work piece relative to one another to produce the
required work piece shape. The displacements must be controlled with a degree of
precision which will ensure the desired accuracy of surface finish and size.
Concept of numerical control
Formerly, the machine tool operator guided a cutting tool around a work piece by
manipulating hand wheels and dials to get a finished or somewhat finished part. In his
procedure many judgments of speeds, feeds, mathematics and sometimes even tool
configuration were his responsibility. The number of judgments the machinist had to make
usually depended on the type of stock he worked in and the kind of organization that
prevailed. If his judgment was an error, it resulted in rejects or at best parts to be reworked
or repaired in some fashion.
Decisions concerning the efficient and correct use of the machine tool then depended
on the craftsmanship, knowledge and skill of the machinist himself. It is rare that two expert
operators produced identical parts using identical procedure and identical judgment of speeds,
feeds and tooling. In fact even one craftsman may not proceed in same manner the second
time around.
. Process planners and programmers have now the responsibilities for these matters.
It must be understood that NC does not alter the capabilities of the machine tool. The
With NC the correct and most efficient use of a machine no longer rests with the
operator.Actual machine tool with a capable operator can do nothing more than it was
capable of doing before a MCU was joined to it. New metal removing principles are not
involved. Cutting speeds, feeds and tooling principles must still be adhered to. The advantage
is idle time is reduced and the actual utilization rate is mush higher (compresses into one or
two years that a conventional machine receives in ten years).
Historical Development
1947 was the year in which Numerical control was born. It began because of an urgent
need. John C Parsons of the parson’s corporation, Michigan, a manufacturer of helicopter
rotor blades could not make his templates fast enough. so he invented a way of coupling
computer equipment with a jig borer.
The US air force realized in 1949 that parts for its planes and missiles were becoming more
complex. Also the designs were constantly being improved; changes in drawings were
frequently made. Thus in their search for methods of speeding up production, an air force
study contract was awarded to the Parson’s Corporation. The servomechanisms lab of MIT
was the subcontractor.
In 1951, the MIT took over the complete job and in 1952; a prototype of NC machine was
successfully demonstrated. The term “Numerical Control” was coined at MIT. In 1955 seven
companies had tape controlled machines. In 1960, there were 100 NC machines at the
machine tool shown in Chicago and a majority of them were relatively simple point to point
application.
During these years the electronics industry was busy. First miniature electronic tubes were
developed, then solid state circuitry and then modular or integrated circuits. Thus the
reliability of the controls has been greatly increased and they have become most compact and
less expensive.
Today there are several hundred sizes and varieties of machines, many options and many
varieties of control system available.
Definition:
The simplest definition is as the name implies, “a process a controlled by numbers “.
Numerical Control is a system in which the direct insertions of programmed numerical value,
stored on some form of input medium are automatically read and decoded to cause a
corresponding function on the machine tool which it is controlling.
Advantages of NC machine tools:
1. Reduced lead time:
Lead time includes the time needed for planning, design and manufacture of
jigs, etc. This time may amount to several months. Since the need for special jigs and
fixtures is often entirely eliminated, the whole time needed for their design and
manufacture is saved.
2. Elimination of operator errors:
The machine is controlled by instructions registered on the tape provided the
tape is correct and machine and tool operate correctly, no errors will occur in the job.
Fatigue, boredom, or inattention by operator will not affect the quality or duration of
the machining. Responsibility is transferred from the operator to the tape, machine
settings are achieved without the operator reading the dial.
3. Operator activity:
The operator is relieved of tasks performed by the machine and is free to
attend to matters for which his skills and ability are essential. Presetting of tools,
setting of components and preparation and planning of future jobs fall into this
category. It is possible for two work stations to be prepared on a single machine table,
even with small batches. Two setting positions are used, and the operator can be
setting one station while machining takes place at the other.
4. Lower labor cost
More time is actually spent on cutting the metal. Machine manipulation time
ex.:. Gear changing and often setting time are less with NC machines and help reduce
the labor cost per job considerably.
5. Smaller batches
By the use of preset tooling and presetting techniques downtime between
batches is kept at a minimum. Large storage facilities for work in progress are not
required. Machining centers eliminate some of the setups needed for a succession of
operation on one job; time spent in waiting until each of a succession of machine is
free is also cut. The components circulate round the machine shop in a shorter period,
inter department costs are saved and ‘program chasing’ is reduced.
6. Longer tool life
Tools can be used at optimum speeds and feeds because these functions are
controlled by the program.
7. Elimination of special jigs and fixtures
Because standard locating fixtures are often sufficient of work on machines.
the cost of special jigs and fixture is frequently eliminated. The capital cost of storage
facilities is greatly reduced. The storage of a tape in a simple matter, it may be kept
for many years and manufacturing of spare parts, repeat orders or replacements is
made much more convenient.
8. Flexibility in changes of component design
The modification of component design can be readily accommodated by
reprogramming and altering the tape. Savings are affected in time and cost.
9. Reduced inspection.
The time spent on inspection and in waiting for inspection to begin is greatly
reduced. Normally it is necessary to inspect the first component only once the tape is
proved; the repetitive accuracy of the machine maintains a consistent product.
10. Reduced scrap
Operator error is eliminated and a proven tape results in accurate component.
11. Accurate costing and scheduling
The time taken in machining is predictable, consistent and results in a greater
accuracy in estimating and more consistency in costing.
Evolution of CNC:
With the availability of microprocessors in mid 70’s the controller technology has made a
tremendous progress. The new control systems are termed as computer numerical control
(CNC) which are characterized by the availability of a dedicated computer and enhanced
memory in the controller. These may also be termed “soft wired numerical control”.
There are many advantages which are derived from the use of CNC as compared to NC.
• Part program storage memory.
• Part program editing.
• Part program downloading and uploading.
• Part program simulation using tool path.
• Tool offset data and tool life management.
• Additional part programming facilities.
• Macros and subroutines.
• Background tape preparation, etc.
The controls with the machine tools these days are all CNC and the old NC control do not exist any more.