CNC or computer numeric control is the technology which enables machines to accurately and precisely control the various motors as well as actuators. To be able to understand CNC, we need to understand how and why it was developed. The NC (just Numeric Control) was first invented when the demands for automation of machine tools widely increased. It was just not effective for a human to use various machine tools in accurate fashion while consuming less of the time. Increasing competition led industrialists to seek robust system which can automate use of machine tools

Also see how an aircraft wing is manufactured using CNC

In case you don’t know what machine tools mean, machine tools mean various tools and devices which are used to fabricate various mechanical devices and equipments. Machine tools are simply machines which can make machine as well as assist machine making process. Machine tools includes drilling machines, grinding machines, chisels, ribbeting machines etc.
So you might be wondering how numeric control made automation of these tools possible. Lets start with an example:

Lets say you have to cut a hole of 9mm diameter exactly at the centre of a square plate. If a human where to do it, he will have to follow following steps:
  1. move the spindle of drill machines to the center of square plate.
  2. carefully push the spindle downwards to make the required hole.

These steps might sound easy, but if these tasks have to be repeated for (let’s say) 30 times daily, the precision of the location of hole and quality of hole might fluctuate a lot(unless the worker is an experienced one). Engineers devised a new method to automate it i.e. Numeric Control Machines. In these types of machines, instructions for task to be performed is coded in the punching cards. These cards hold the instructions for motors present in the machine tools.

For ex. ‘rotate this motor for x steps’ and then ‘rotate that motor for y steps’. These are coded in special syntax. This card was used to automate the boring and repetitive process easily as well as efficiently.

Although effective, these cards were not too scalable. If a slight change in the instruction is required, new card has to be made. Similarly, there is no system inbuilt which can check if the instruction in the cards were effectively followed.

Across the time, computers were upgraded. Lots of computing power, small size made computers useful in every field. Computers took over the numeric control machines and thus CNC machines were invented. CNC machines would now use computers and latest technology to guide machine tools across the work piece. Lower latency feedback as well as new manufacturing technologies made CNC machine an important asset for any manufacturing industries.

CNC machines can be used in different ways. Some of the few use cases are listed below:

  • Making circuit boards
  • Making enclosures
  • For computer integrated manufacturing
  • Accurate manufacturing of gears and shafts for automotive industries

Working Principles of CNC Machine

Following diagram gives an overview of working principle of CNC Machine

working principle-CNC

The CNC machine has 6 major components which work together in harmony. They are listed below:

  • Input Device
  • Machine Control Unit
  • Driving System
  • Feedback System
  • Display Unit

The data/program/sketch is fed into machine control unit via input device. Machine control unit converts this data into Gcode. Gcode is nothing but the series of motion commands which tells which motor(on the axis) to run. According to motion commands or (gcode), the MCU sends the control signals to driving system which controls the motion and velocity of machine tools used.

Some CNCs have inbuilt feedback system which prevents machine tools from overshooting. Overshooting is the phenomena where machine tools slightly move more than intended because of various inaccuracies present in the design. To prevent this overshooting, Feedback system is used. In the feedback system, sensors measures the position and velocity of machine tools and then feed them back to Machine control unit. Machine control unit will then compare the data from sensor(i.e. feedback data) to generate motion command in order to compensate the errors.

To get intuition of feedback system, imagine a scenario where machine tool was commanded to move by 2 mm but it moved 1mm far from the intended position. From the current position data fed by the sensor, machine control unit immediately corrects it and reverts the motion.

For more insight on CNC check out this wikipedia link

By Aashiz Poudel

Aashiz is currently a mechanical engineering students pursuing his degrees at IOE,Purwanchal Campus, Dharan. He is not just limited to the field of mechanical engineering. He loves coding and building stuffs.

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