In this post we will see about the Classification of numerical control, that is, NC, CNC machines. The main types of NC machines are covered.
CLASSIFICATION OF NC MACHINES
In today's era, variety, complexity of geometry, tolerances, personnel skill and availability of funds considering all factors, NC machines are designed accordingly to meet different requirements within cost constraints. These machines are broadly classified as the following:
(a) Based on feedback control, and
(b) Based on the characteristics of the control system.
BASED ON FEEDBACK CONTROL
In NC machines, to control the position of a machine slide, a group of electromechanical, 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 the 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 moved according to information loaded from the part program into the control system. Therefore, there is no measurement of the slide position and no feedback signal to compare with the input signal. Correct slide movement depends entirely on the ability of the drive systems to move the slide the exact distance required. The most common method of driving the lead screw is by a stepper motor. Stepper motors are the simplest way to convert detailed electrical signals into proportional motion. Since there is no slide position check, the accuracy of the system depends on the motor's ability to travel the exact number of steps given in the input, as shown in fig.
Closed loop system
A closed-loop system sends back a signal to the control unit from a measuring device called a transducer. The transducer is connected to the slide guides. The signal indicates the actual movement and position of the landslides. The control unit continues to adjust the position of the slide until it reaches its destination, this system has feedback. Although more expensive and complex than the open-loop system, these systems provide more accurate positioning. For this type of system, servo motors are used.
open loop and closed loop systemCLASSIFICATION BASED ON CONTROL SYSTEM
Some machine tools require the cutting tool and the workpiece to be placed in certain positions and also moved relative to each other. Based on relative motion, 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 workpiece to be placed in certain fixed relative positions in which they must remain while the cutter does its work. These machines are known as peer-to-peer machines. The control equipment for use with them is known as point-to-point control equipment. Advance rates do not need to be programmed. In these machine tools, each axis is driven separately. In a point-to-point control system, the dimensional information that must be provided to the machine tool will be a series of required positions 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 rearward.
Straight motion control system
NC systems, in which the tool works along a straight line in the direction of a principal coordinate axis, such as along the direction of feed during turning, boring, or milling operations at a controlled rate, are known as control systems. straight line.
Contour or Continuous Path Motion Control System
Other types of machine tools involve movement of the workpiece relative to the cutter while the cutting operation is taking place. These machine tools include milling machines, 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 not be economical to use them unless the workpiece also requires a contouring operation to be performed on it. These machines require simultaneous axis control. On contouring machines, the relative positions of the workpiece and tool must be continuously controlled. The control system must be capable of accepting information about machine slide speeds and positions. Advance rates must be programmed.
contour or continuous path motion control systemCombined Motion Control System
Above all, control systems are combined as groups. Most Jig boring machines, horizontal boring machines and drill presses employ this type of control system. These motion control systems have 3 or 4 linear axes to be controlled continuously and 2 or 3 rotary movements controlled along with the positioning feature. This combination is used for complex contouring operations to be carried out in machining centers.
