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Tuesday, 20 November 2018

What are Numerical Control Machine? What are NC Machines?

          Numerical control, popularly known as the NC’. Numerical control is defined as the form of programmable automation, in which the actions are controlled by the direct insertion of numerical data such as numbers, letters, and symbols. In case of the machine tools, these programmable automation machines are used for the operation.
          In other words, the numerical control machine is defined as a mechanical machine that is controlled by the set of instructions (input information) called as a program via punched paper tapes or magnetic tapes. The name Numerical control is given to this type of programming, since numbers form the basic program instructions. When the type of job changes, the program instructions of the job also change. Writing new instructions for each job is easy, hence NC offers a lots of flexibility in its use and also more economical for producing a single or a large number of parts.
          The NC machines are used in variety of applications like inspection, assembly, sheet metal work etc., but it is mainly used for machining operations like drilling, milling, turning etc.,

The main elements of a NC machine tool are,
1. The control unit, also known as console or director - it sends the command signals to the drive units.
2. The drive units.
3. The position feed back package.
4. Magnetic box - It acts like electrical control cabonet
5. Manual control - To perform functions manually. 

Classification of NC Machine Tools:

          According to various features, NC machine tools are classified as the following
1. According to the type of power to the drives
1. Electrical
2. Hydraulic
3. Pneumatic
2. According to motion control system of slides
1. Point- to – point system -- Used for drilling machines
2. Straight line system -- Used for turning machines
3. Contour (or) continuous path system -- Used for milling machines
3. According to the feedback system
1. Open loop system -- In this system, there is no ‘feed back’ and no return signal to indicate whether the tool has reached the correct position at the end of the operation or not.
2. Closed loop system -- A feed back is built into system, which automatically monitors the position of tool.
4. According to axis identification
1. 2- axis
2. 3- axis
3. 4- axis
4. 5- axis

Method of Listing Coordinates of Points in NC/ CNC System

Coordinates of Points
          Two types of co-ordinate systems are used to define and control the location of the tool in relation to the work piece. Each system has its own applications and the two co-ordinate systems can be used independently or may be mixed within a NC part program based on the machining requirements of the work piece. The co-ordinate systems used are
1. Absolute co-ordinate system and
2. Incremental co-ordinate system.

(a) Absolute Co-ordinate System: In this system the co-ordinates of a point are always referred with respect to one reference point that is, datum. The datum positions in the X-axis, Y-axis and Z-axis are defined by the user / programmer before starting the operation on the machine. A main advantage of using absolute system is that it is very easy to check and fix a program written using this method. If a mistake is made in the value of any dimension in a particular block, it will affect that dimension only and once the error is corrected there will be no further problems.

(b) Incremental Co-ordinate System: In the incremental system the co-ordinates of any point are calculated with reference to the previous point i.e. the point at which the cutting tool is positioned is taken as datum point for calculating the coordinates of the next point to which movement is to be made. It is difficult to check a part program written in incremental dimension mode.

Absolute and Incremental Programming

Point
Absolute system  
Incremental System 
P1
1,3
1,3
P2
3,2
2,-1
P3
4,2
1,0
P4
4,3
0,1

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