Spindle modes
Cutting power – and more!
Spindle power is only one factor of productivity
The decision with respect to the main or workspindle of a machine tool is often made based on the expected machining power. However, spindles used in conjunction with modern motor and drive control technology offer many more application options which do not depend solely on the power parameters.
Speed control for constant cutting speed
The main task of a spindle drive is to provide the cutting power that is required for machining. From the point of view of the CNC and the drive technology, the spindle motor is in speed control mode. The task of the spindle drive is to keep the spindle speed, which is defined by the CNC programmer as the S value in the CNC program, at a constant level, and consequently the cutting speed.
Position control and continuous-path mode
Certain applications may require that the spindle in the form of a rotary axis be changed from speed control mode to position control or continuous-path mode. A simple example is the tool change in a milling machine. The spindle must be rotated through a defined number of degrees in order to clamp the tool correctly in the gripper arm and in the magazine. The groove provided in the tool holder for this purpose is oriented (e.g. 45 degrees) such that the tool can be positively locked in the magazine location. The CNC changes over from speed control mode to position control mode in this case.
Thread cutting – now with spindle and feed axis in position control
In the past, thread cutting operations (G33) were performed with a speed-controlled spindle. The feed axis, usually the Z axis, was therefore synchronized with the rotating spindle. This combination of speed-controlled spindle and position-controlled feed axis was stretched to its performance limits when tapping fine threads however. This is why the spindle is now changed over to position control for thread cutting. This takes place automatically by means of the CNC language commands G331 and G332.
Synchronous operation of main spindle and counterspindle
A further application for spindles in position control comes up in the context of turning machines: the process whereby a turning workpiece is transferred from the main spindle to the counterspindle. Due to the workpiece geometry, the counterspindle must assume a particular defined angular position in relation to the main spindle, as the machining operations of the front and rear faces would not otherwise match. A multi-edged component at the gripping point of the counterspindle is a good example here. The so-called synchronous spindle mode makes angle-synchronous transfer possible even when the spindles are rotating. Both spindles are in position control in this case.
Multi-edge machining: Synchronous mode with transformation ratio (gearbox mode)
Multi-edge machining or polygon turning is a special form of synchronous spindle mode. In this case, a rotating turning tool machines a workpiece which is rotating synchronously with it. This process for producing key surfaces saves a huge amount of time compared with milling. The transformation ratio of the driven tool and the workpiece in the main spindle determines the number of key surfaces.
End face machining and peripheral surface machining
The kinematic transformations TRANSMIT and TRACYL must also be mentioned in the context of position-controlled spindles. These functions can be used to program contours on the end face or peripheral surface of rotationally symmetrical workpieces in a Cartesian workpiece coordinate system. These contours are then transformed into a movement of the spindle and a linear axis, typically the Z axis in the machine. Here too, the spindle has changed to position control.
Conclusion
The ability to move spindles in position control as well as in speed control mode, i.e. in continuous-path mode, opens up additional fields of application which extend as far as the machining of freeform surfaces on turning machines. In this "supreme discipline", the position-controlled main spindle assumes the task of a highly dynamic orientation axis for 5-axis transformation. When selecting a machine spindle, potential machine buyers should therefore not only consider the power parameters, but should also check whether their processes place further demands on the spindle.
Andreas Groezinger