Introduction

In this article we will understand the functioning of the CNC machine and their parts. We will also learn the control circuitry of the CNC machine.

Block diagram of CNC MachineCNC Machine Block Diagram

A CNC system consists of  6 major elements:

  1. Input Device
  2. Machine Control Unit
  3. Machine Tool
  4. Driving System
  5. Feedback Device
  6. Display Unit

Input Devices

The  data  transfer  between  computer  and  CNC  machine  is  through a serial communication port. RS-232 cable is used  for data transfer between CNC  machine  and  computer. Part programs from computer is transfer to the input devices for storage and then  transfer  to the machine control unit.Input devices used are Floppy Disk, CD-ROM,Flash etc.

Machine Control Unit (MCU)

MCU  is the heart of the CNC system. Machine control unit has two sub units: the Data Processing Unit (DPU) and the Control Loop Unit (CLU).

Data Processing Unit

On receiving part programme, the DPU  encodes the part programme into  machine  codes.  DPU  then calculate the intermediate positions of the motion in terms of BLU (basic length unit) which is the smallest unit of  length that can be handled by the controller. The calculated data are passed to CLU further.

Control Loop Unit

The data from the DPU are converted into electrical signals by CLU to control the  driving  syatem.  Other  functions  such  as machine spindle, coolant, tool clamp ON/OFF are also controlled by this unit according to the internal machine codes.

Machine Tool

. In order to obtain high accuracy, the design of the machine slide and the driving lead screw of a CNC machine is very important. The slides are machined to high accuracy and coated with anti-friction material in order to reduce the stick and slip.

Driving System

It is an important component of a CNC machine as the accuracy and repeatability depend very much on the performance of the driving system.  The driving system has to response accurately according to the programmed instructions. Driving  system usually uses electric motors although hydraulic motors are sometimes used for large machine tools. This motor is coupled either directly or through a gear  to the machine lead screw to moves the machine slide or the spindle. Electrical motors that are commonly used.: AC,DC and Stepping motor.

Feedback Device

In order to make CNC machine operating accurately, the positional values and speed of the axes need to be constantly updated For this, two types of feedback devices are normally used positional feedback device and velocity feedback device.

Positional Feed Back Devices

There are two types of positional feedback devices:a) linear transducer for direct positional  measurement  and b)  rotary  encoder  for  angular  or  indirect  linear measurement.

Velocity Feedback Device

The actual speed of the motor can be measured in terms of voltage i.e generated from a tachometer mounted at the end of the motor shaft.. The voltage   generated is compared with the motion command voltage corresponding to the desired speed. The difference of the voltages can is  used to actuate the motor to eliminate the error.

Display Unit

The Display Unit use as an interactive device between the machine and the operator.  When  CNC  running,  this  Unit  displays  the  present status like position of the machine slide, spindle RPM, the feed rate, the part programmes. In an advanced CNC machine, the Display Unit also show the graphics simulation of  the  tool . Much other important  information  about the  CNC system can also displayed for maintenance such as machine parameters, programmer controller, error massages and diagnostic data.

 APPLICATIONS OF CNC MACHINES:

These days the CNC machines are found in almost all industries. Everyone involved in the manufacturing should know what a CNC machine can do for  company. Due to many applications of CNC machines in various industries, there is a good demand of the CNC programmers

Industries for removing metal : Metal removing industries remove the metal from the raw material as per the requirements. Automotive industries for making the shafts, gears, and other parts. It can be manufacturing industries for making various rounded, square, rectangular, threaded  jobs. All these metal removal works are performed by different machine like lathe, milling , drilling, boring, shaping machine, reamer, etc. Earlier these machines are operated by the operators, but these machines with CNC are now used automatically.

Industries for Fabricating Metals: In many industries, steel plates are required. In fabrications industry  the machining operations are performed on such plates. In these industries,the CNC  are used for various operations like  plasma cutting, punching, laser cutting and welding and other applications. To give final shape to plates CNC lasers and CNC plasma cutters are used . To punch the holes in the plates of all sizes CNC tpunch presses are used. And if we want to bend the plate so as to give it a final shape, we can use CNC press brakes.

Electrical Discharge Machining Industry: The EDM machines remove the metal by  the sparks that burn the metal. There are two types of EDM along t CNC automation – Vertical EDM and Wire EDM.  Vertical EDM needs an electrode of the shape and size of the cavity that is made in the job. Wire EDM is used to make punch for the dies set that are used in the industries where fabrication is done.

 Other Industries:

CNC  machines are also used  in the wood working industries to perform various operations like routing and drilling. CNC machines also used for the electrical industry such as coil winders and soldering machines

Now-a-days in every industry we find the CNC machine. The increment in progress made by the manufacturing sector is mainly due to the progress  in the CNC technology.

CNC machine used to produce the following types of product:

  • Parts which have complicated contours
  • Parts which requiring close tolerance.
  • Parts which requiring   expensive   jigs   and   fixture when   produced   on conventional  machines
  • Parts that may have several engineering changes.
  • In operations where human errors could be extremely costly
  • Parts that is to be completed quickly.
  • Use where Small batch lots or short production require.