Numerical Control System

A Numerical Control System may be defined as a system in which machines are controlled with the help of some sort of alpha-numerical and symbolic data in an appropriate format. This format is known as Part Program.

Further, EIA (Electronic Industrial Association) Define NC System as

“A System in which actions are controlled by direct insertion of numerical data at some point. The System must automatically interpret at least some portion of this data.”

Note:- NC System comes under the category of programmable automation.

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Historical Background

John T. Parsons is regarded as the father of Numerical Control System. John T. Parsons first demonstrated his idea regarding NC system to US Air force and developed the World’s first Numerical Control Milling machine in collaboration of MIT in 1948. It was a retrofitted version of the old milling machine with limited control features. It used a punch card as a medium to transfer the part program in MCU.

It was 1952 when a more advance version of NC system was developed and introduced. It used a punch tape as a medium to transfer part program in MCU.

Note:- Punch card stored a limited number of instructions due to its size. A number of punch card was needed for a single part program. This was a major drawback as several punch card had to be changed.

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Elements of NC System

A NC system mainly has the following three elements:-

• Part Program
• MCU
• Controlled Machines

Part Program

A Part Program consists of a set of instructions to be followed by machine. It tells the machine.

• What to do?
• How to do?
• When to do?

The person, who writes a part program, is known as Part Programmer and is expected to have thorough knowledge about the different manufacturing process.

MCU

This one is the main element of any NC system. It is also known as the Brain of Machine as it sends the signal to the machine as a brain sends to the body. There are at least five main components of an MCU.

Tape Reader

Tape Reader is the main component of MCU and responsible for reading the part program.

Data Buffer

The function of the data buffer is to store the set of instruction received from the tape reader in a logical sequence.

Signal Output Channels

The only function of the Signal Output Channel is to send the instructions to the machine from the data buffer.

Feedback channels

The Feedback channel established a connection from machine to MCU to check whether the machine is working according to the sent instructions.

Sequence controller

The Sequence controller ensures the proper working of all the above components logically.

Controlled Machines

The machines/processes that are controlled by NC System are known as Controlled machines/processes.

• NC Turning Machine
• NC Drilling Machine
• NC Milling Machine

How Does It Works?

A NC System works in at least steps:-

Step 01. Preparation of Part Program

The first step to operate an NC System is the preparation of a part program. A part program includes several instructions, in an appropriate format, to be followed by machine for manufacturing a product. The Commonly used format is the G & M codes programming.

Step 02. Feeding of Part Program in MCU

After the preparation of the part program for a given product, it is feed into the machine control unit of the respective machine. The main function of MCU is to read the program and convert it into a machine-readable format. Other functions of MCU include sending the signals/instructions to the controlled machine and established a proper feed-back channel.

Step 03. Execution of Part Program

This step is dedicated to the execution of the part program. In this step respective machine runs according to the command received from the MCU.

Types of Numerical Control System

A Numerical Control System may be classified under the follow head.

Control System

A NC System may be classified into two category on the basis of control system. These are:-

Open-Loop NC System

An open-loop NC system is one in which output is independent of the input. It means the output of the machine/process is not compared with the given inputs. Besides this, an open-loop NC system has the following features:-

• Simple in Construction & Working
• Less Costly
• Accuracy is quite low.
• No Sensors are used.

Closed Loop – NC System

In a closed-loop NC system, the output is dependent on the input. It means the output of the machine/process is compared with the given inputs. A closed-loop NC system also has the following features:-

• Complex in Construction & working
• Costly
• Accuracy is high.
• Sensors are used.

Motion Control

On the basis of motion control, A NC System may be classified as:-

Point-to-Point NC System

In a Point-to-Point NC system, the machine tool moves from one point to another point with a set predefined speed and through a shortest possible path (the shortest possible path between two points is a straight line). The PTP NC system has the following features:-

• Speed and path of the tool is Predefined
• less Costlier
• Used in discrete manufacturing processes like Drilling, Spot Welding, etc.

Straight Cut NC System

Straight Cut NC System offers the capability to move the machine tool along/ parallel to any major axis with a controlled feed and speed. Straight Cut NC system shows the following features:-

• Feed, speed and path of the tool (only along/parallel to any major axis) can be changed according to the requirements.
• Costlier than PTP – NC System
• Use in Continuous manufacturing processes like Milling, Turning, etc.
• The function of PTP – NC System can also be performed with Straight Cut NC System.

Contouring NC System

Contouring NC System has the capability to move machine tool along /parallel to any complicated path. Contouring NC System has the following features:-

• Feed, speed and path of the tool can be changed according to the requirements.
• Costlier than PTP & Straight Cut NC System
• Use in Continuous manufacturing processes like Milling, Turning, etc.
• The function of PTP & Straight Cut NC System can also be performed with Straight Cut NC System.

Number of Axis

On the basis of number of axis, A NC System may be classified as:-

2 Axis NC System

A 2 axis NC System offers the tool movement in any two directions.

Examples:- A NC Turning machine offers movement on tool in X and Z directions.

3 Axis NC System

A 3 axis NC System offers the tool movement in three directions.

Examples:- A NC Milling machine offers tool’s movement in X,Y & Z direction.

Advantages

• Improved product quality
• Reduced cost of the product
• Reduced non-productive time that further lead to lower the manufacturing lead time (MLT)
• Increased Productivity due to better utilization of available resources
• Increased manufacturing flexibility to cope up the changes
• Safer working environment for the workers
• Competitive advancement to the manufacturers, etc.

Disadvantages

A Numerical Control System faces the following limitations:-

• Higher Initial Investment (due to higher cost of automated machines or control system)
• Skilled Labor Requirements ( as the functioning of automated machines or control system needs special skills)
• Higher Unemployment rate ( because of the higher working rate of automated machines or control system )
• Higher maintenance cost ( because of complex circuits involved in automated machines or control systems)

Drawbacks of NC System

A Numerial Control system had the following drawbacks that pave the way of further development.

MCU

MCU was the least reliable element of a NC system. It was hardwired due to which it suffered a lot of problems.

Punched Tape

The punched tape got wear and tear after a number of use due to its direct contact with the tape reader.

Part Programmer mistakes

A part programmer mistakes were so common in Conventional Numerical Control System that a large portion of time got consumed in part programming.

Non Optimal feed & speed

Conventional NC System didn’t come with the online part program editing feature. Due to this, the optimization of the feed and speed was not possible during the process.

No self-diagnosis

Conventional NC System comes with no self-diagnosis system. A service engineer had to check every possible component of the machine to find out the actual error/fault.

Further Developments

The advancement in the field of electronic and introduction of computer in manufacturing field lead to the development of Direct numerical Control (DNC), Computer Numerical Control (CNC) and Adaptive Control System.