Types of Control Systems
There are two basic types of control system in numerical andcontouring. In the point-to-point system, also called positioning, each axis of the machine isdriven separately by leadscrews and, depending on the type of operation, at different velocities.
The machine moves initially at maximum velocity in order to reduce nonproductive time butdecelerates as the tool reaches its numerically defined position.
Thus in an operation such as drilling or punching, the positioning and cutting take placesequentially.
After the hole is drilled or punched, the tool retracts, moves rapidly to another position, andrepeats the operation.
The path followed from one position to another is important in only one aspect: The timerequired should be minimized for efficiency.
Point-to-point systems are used mainly in drilling, punching, and straight milling operations.
In the contouring system, also known as the continuous path system, positioning and cuttingoperations are both along controlled paths but at different velocities. Because the tool cuts asit travels along a prescribed path, accurate control and synchronization of velocities andmovements are important.
Linear interpolation for creating a circular path would be quite inappropriate because the programmer would be required to specify the line segments and their respective end points that are to be used to approximate the circle.
Circular interpolation schemes have been developed that permit the programming of a path consisting of a circular arc by specifying the following parameters of the arc: the coordinates of its end points, the coordinates of its center, its radius, and the direction of the cutter along the arc.
The tool path that is created consists of a series of straight-line segments, but the segments are calculated by the interpolation module rather than the programmer.
The cutter is directed to move along each line segment one by one in order to generate the smooth circular path.
A limitation of circular interpolation is that the plane in which the circular are exists must be a plane defined by two axes of the NC system.
Helical interpolation combines the circular interpolation scheme for two axes described above with linear movement of a third axis. This permits the definition of a helical path in three-dimensional space.
Parabolic and cubic interpolation routines are used to provide approximations of free-form curves using higher-order equations.
They generally require considerable computational power and are not as common as linear and circular interpolation.
Their applications are concentrated in the automobile industry for fabricating dies for car body panels styled with free-form designs that cannot accurately and conveniently be approximated by combining linear and circular interpolations.
The most common application of numerical control is for machine tool control. This was the first application of NC and is today the most important commercially.
In this section we discuss the machine tool applications of NC with emphasis on metal machining.