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Elements of NC (Numerical Control) Machines

As explained in the previous section, the NC system requires the preparation of manuscript (part program) based on the product drawing, preparation of input media (punched cards & punched tapes), the data entry into the control unit, consequent processing and actuation of the machine tool to produce the desired part.

Lay-out of a NC System

In performing all the above operations, the NC machine may have the following elements:

  1. Software
  2. Machine Control Unit
  3. Driving devices
  4. Manual control unit
  5. Machine tool

1. Software

A series of instructions are required to control the actions of a NC machine. These instructions are prepared based on the profile and the material of the part being manufactured. These instructions and their storage media can be called as software.

Software of a NC system can be referred to as items comprising of instructions (programs), languages used to write these programs and a variety of input media.

The instructions given to the NC machine may be dimensional or managerial. They are fed into the control unit as different numbers only. The decimal system of numbering uses 10 digits (0 to 9) to represent any number. The base of the numbers will be 10. But in order to provide information into the control unit or a computer, a convenient system of numbering known as binary format is used. In this system, only two digits (0 & 1) are used and the base is 2. All instructions and information are converted as numbers consisting of digits 0 and 1. It becomes easy to record the information on punched tapes or punched cards in the form of holes (Holes may indicate 1 and no hole may represent 0).

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Input Media

Different types of input media are used to store information and to provide input to various control units of the NC machine. They are

  1. Punched cards
  2. Punched tapes
  3. Magnetic tapes
  4. Floppy disks

Punched cards and punched tapes are useful in storing data in the form a series of punched holes along their length. Punched tapes may be made of paper, a plastic material known as mylar or Aluminium foils. Magnetic tapes are made of plastic material and are coated with Gamma ferric oxide layer. The advantages of magnetic tapes as input media are

(i) their greater strorability and (ii) data can be erased and be reused.

2. Machine Control Unit (MCU)

Machine Control Unit consists of electronic circuits (hardware) that are useful in reading and interpreting the instructions (NC program) fed by means of input media and convert them into mechanical actions of the machine tool.

Generally, the MCU may be of three types

  • Inbuilt type
  • Swing around type
  • Stand alone type
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Inbuilt MCU

When the control unit of the NC machine is housed in the construction of the machine itself, it is known as Inbuilt MCU.

 

In-built MCU

Swing Around MCU

This MCU is a separate unit and is connected to the machine by a swinging arrangement. The unit as a whole can be swung around the machine depending upon the position of the operator.

 

Swing around MCU

Stand alone MCU

Stand alone MCU is designed as a separate unit and placed at a distance from the machine.

Stand alone MCU

3. Driving Devices

Driving devices consist of different types of motors and gear trains. They convert the instructions form the MCU into accurate mechanical displacements of the machine tool slides. The motors may be electrical, hydraulic or pneumatic.

Electrical motors are mainly used as prime movers because of their speed and torque characteristics. A.C. induction motors are cheap and easy to maintain. For easy and effective speed changes, D.C. motors are also used.

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Hydraulic motors are used in some specific types of CNC machines. Hydraulic motors get the drive from oil pumped from a constant delivery hydraulic pump. Hydraulic motors are used where the load is high and wider range of speed is necessary.

Servomotor, stepper motor, synchros and resolvers are different types of motors used as drives in NC machines.

4. Manual Control Unit

Manual control unit consists of dials and switches to be operated by the operator. It may also have a display unit to provide useful informations to the operator. In some machines, the manual control unit may be a part of the MCU (machine control unit)

The operator use the manual contrl unit to

  1. switch on and off the machine
  2. load and unload the workpieces and
  3. change the tools in certain types of machines

5. Machine Tool

It is the element of the NC machine which actually performs the useful work of converting the raw material to finished components. It is designed to perform various machining operations. It consists of a machine table, spindles, cuttingtools, work holding devices such as jigs and fixtures, coolant systems, swarf removal systems and other auxiliary equipments.

Lay out of a NC Machine Tool

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