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 Machine
A CNC system consists of 6 major elements:
- Input Device
- Machine Control Unit
- Machine Tool
- Driving System
- Feedback Device
- Display Unit
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.
. 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.
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.
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.
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.
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.