In most of the machine tools, it is possible to have the relative movements of workpiece and the tool in three directions ; i.e. (i) relative motion of worktable from left to right or vice versa (X-axis), (ii) at right angle to this motion (Y-axis) (these two motions being possible by means of two horizontal slides), (iii) the movement in a third axis at right angles to the first two (Z-axis). In more sophisticated machines two more movements are provided namely, (i) the swivel of the work table with reference the spindle around a vertical axis, (ii) the swivel of the spindle around a horizontal axis.
Point-to-point machine tools are those in which the cutter and the workpiece must be placed at certain fixed relative positions and then the cutter does its work ; e.g. drilling, boring and tapping machines. Numerical control system used for such machines is known as point-to-point or numerical positioning control system. This system first positions the workpiece with respect to the cutting tool and then does the cutting action. It may select the correct tool, insert it into the spindle, advance the tool into cut position, and withdraw it. At the end of the cut it advances the work to the next work point. The path followed from one position to another is closely controlled.
On the other hand contouring machine tools involve motion of the workpiece with respect to the cutter while the cutting operation is taking place. These machine tools include milling and routing machines and flame cutters. The control system for these is known as numerical contouring control (or continuous path system).
In this type of control, two or more motions of the tool or workpiece have to be controlled simultaneously in order to produce complex contours and profiles. These systems are more complex than point-to-point systems and usually require the assistance of a computer because of the vast amount of positional data required.
One additional system of numerical control is sequencing. This is also sometimes known as management information. This is used to select operations and to run the machine in a predetermined sequence of operations ; e.g. on a turret lathe, it may select the spindle speed and the feed rate, engage the correct feed (longitudinal feed, cross feed, or saddle feed), select the turret position and engage the desired feed stop etc.
Thus in a point-to-point control system the dimensional information to be passed to the machine tool consists of a series of required positions for each of the two slides and sometimes, but not always, of the height of the cutter, usually in case of drill. In a simple system the cutter may be brought down and retracted by hand ; however in complicated systems the cutter motion may, at all times, be under the control of the equipment. Sometimes a variety of cutters be fitted in a turret head, and this may also be indexed either manually or under the control of the equipment. It is also’ usual to provide an electrically controlled automatic clamping device to fix the table in the selected position while cutting takes place.
In a contouring system, the relative positions of the workpiece and cutter require to be continuously controlled, since cutting takes place while the positions of the workpiece and cutter are altered.
Thus, the numerical control system must be able to accept a series of numbers, to convert it into a series of velocities and/or positions for the machine slides and to bring the slides to predetermined fixed positions to cut a contour.