NC Machine Tools

At present, the CNC machining center appears to be the most capable and versatile automatic machine tool that can perform drilling, milling, boring, reaming and tapping operations. The general objective behind the development of NC machine tools continues to remain the reduction of cost of production by reducing the production time. This in turn is directed towards the avoidance of non-productive time which is mainly due to the number of setups, setup time, workpiece handling time, tool change time and lead time. The performance of a variety of machining operations on the same machining center eliminates the non-productive waiting time that occurs if such operations are performed on different machines. Provision of automatic tool changing, indexing of tables and several pallets add to the productivity of the machining centers.

The basic information that has to be input into the system consists of the part geometry and cutting process parameters followed by the cutting tools used. This part program is then entered into the controller of the machine, which in turn runs the machine tool to make the part.

The command received from the operator is communicated to the corresponding axis driving system for execution. The axis motion control system operates in a feed back loop with suitable transducers such as linear scales and/or rotary encoders to get the appropriate position or velocity feed back. Most of these systems have a very high response with good resolution of the order of 1 micron or less.

The controllers have a number of modes in which they operate. There can be four possible modes in which the controller can function in relation to a machining center. The first shows a typical drilling machine operation termed point-to-point mode. In this, the control has the capability to operate all three axes, but not necessarily simultaneously. As a result, it would be possible to move the tool to any point(in X and Y-Axes) at the fastest possible speed and carry out the machining operation in one axis(Z-axis) at that point. This would be useful for drilling and punching machines. The second type is an improvement over this in which in addition to the point-to-point mode, the machine tool has the capability to carry out a continuous motion in each of the axis directions. This would help in obtaining the milling in a straight line along any of the axes. The third type shows a control system, which improves the previous type by adding the simultaneous motion capability in any two axes. This is what is required in most of cases. Any 3D profiles to be machined can be completed using the concept of 2.5D mode, in view of the limitation of the machine.

The last one is the highest form of control that is generally found in most of the current day control systems. This gives the capability of simultaneous three or more axes motion. This would be useful for machining most of the complex 3D profiles encountered in industrial practice such as aerospace components, moulds and dies.