Classification of Numerical Control CNC Machines

In this post we will see about Classification of numerical control i.e. NC ,CNC  machines . Major types of NC machines are covered.

CLASSIFICATION OF NC MACHINES

In present era, variety, complexity of geometry, tolerances, skill of personnel and availability of funds by considering all factors, the NC machines are designed according to meet different requirements within the cost constraints. These machines are broadly classified as the following :
(a) Based on feedback control, and
(b) Based on control system features.

BASED ON FEEDBACK CONTROL
In the NC machines, to control the position of a machine slide, a group of electro-mechanical, pneumatic or hydraulic components are used which are collectively known as Servo Mechanism. The output from the data handling equipment is passed through separate channels to servo system, which in turn drives the machine slides. This servo system, based on feedback control, can be approached in three ways :

Open Loop System

In an open loop system the machine slides are displaced according to the information loaded from the part program into the control system. Hence there is no measurement of slide position and no feedback signals for comparison with the input signal. The correct movement of slide entirely depends upon the ability of the drive systems to move the slide through the required exact distance. The most common method of driving the lead screw is by a stepper motor. The stepper motors are the simplest way for converting detail electrical signals into proportional movement. As there is no check on the slide position, the system accuracy depends upon the motors ability to step through the exact number of steps provided at the input as shown in fig.

Closed Loop System

A closed loop system is sends back a signal to the control unit from a measuring device called as transducer. The transducer is attached to the slide ways. The signal indicates the actual movement and position of the slides. The control unit continues to adjust the position of the slide until it arrives it’s destination, this system has feedback. Although more costly and complex than open loop system, these system gives more accurate positioning. For this type of system, servomotors are used.

open loop and closed loop system
open loop and closed loop system

 

CLASSIFICATION BASED ON CONTROL SYSTEM

Some machine tools require that the cutting tool and work piece shall be placed at certain positions and also be moved relative to each other. Based on the relative motion, the NC machines can be classified as :

Point to Point Motion Control System
Some machine tools for example drilling, boring and tapping machines etc, require the cutter and the work piece to be placed at a certain fixed relative positions at which they must remain while the cutter does its work. These machines are known as point-to-point machines. The control equipment for use with them is known as point-to-point control equipment. Feed rates need not to be programmed. In theses machine tools, each axis is driven separately. In a point-to-point control system, the dimensional information that must be given to the machine tool will be a series of required position of the two slides. Servo systems can be used to move the slides and no attempt is made to move the slide until the cutter has been retracted back.

Straight Line Motion Control System
The NC systems, in which the tool works along a straight line in the direction of a major coordinate axis, such as along the direction of feed during turning, boring or milling operation at a controlled rate, are known as Straight line control system.

Contouring or Continuous Path Motion Control System
Other type of machine tools involves motion of work piece with respect to the cutter while cutting operation is taking place. These machine tools include milling, routing machines etc. and are known as contouring machines and the controls required for their control are known as contouring control. Contouring machines can also be used as point-to-point machines, but it will be uneconomical to use them unless the work piece also requires having a contouring operation to be performed on it. These machines require simultaneous control of axes. In contouring machines, relative positions of the work piece and the tool should be continuously controlled. The control system must be able to accept information regarding velocities and positions of the machines slides. Feed rates should be programmed.

ontouring or continuous path motion control system
contouring or continuous path motion control system

Combined Motion Control System
The above all the control systems are combined as groups. Most of the Jig boring, horizontal boring and drilling machines employ this type of control system. This motions controls systems are having 3 or 4 linear axes to be controlled continuously and 2 or 3 rotary movements controlled along with the positioning facility. This combination is used for complex contouring operations, to be carried out on machining centers.

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