Avoiding vibration in slewing gear pieces for SINUMERIK 828D/840D sl
Avoiding vibration in slewing gear pieces
When machining long and slender workpieces which are often very sensitive to vibrations, the speed in critical areas should be set to a harmonic oscillation in a controlled manner during the machining process. The aim should be to change the spindle speed such that the machining process is not impaired and no chatter marks are formed on the rotating surface. To avoid this and to ensure a continuous working process, critical areas need to be overcome as quickly as possible. This application provides an option of how such a vibration occurring at the workpiece can be avoided.
The figure on the left shows an example of a workpiece which is made to vibrate by the unfavourable state of stress during the turning process.
Change in speed of the master spindle can eliminate any vibrations
Permanent change in speed of the master spindle can eliminate any vibrations occurring in the slewing gear piece. When manually changing the spindle speed by using the override, it is very difficult to create a harmonious vibration speed of the master spindle. Therefore, a cycle which consists of two part subprograms (.SPF files) should be created. By selectively switching the cycle on and off, a harmonic vibration speed of the master spindle is forced. Both subprograms of the cycle form the basis for future use of the "OSZI" command, which can be safely used in the program GUIDE (G-code).
You can find the cycles and a detailed description of the application for download in the Service and Support Portal. The link can be found at the end of this page.
This application provides the following benefits:
Avoiding chatter marks on the workpiece
Precision turning process on the workpiece is still possible
The subprograms OSZI.SPF and OSZIOFF.SPF, which are used to turn the harmonic vibration at set speed on and off will be stored in the password protected area of the system data / user cycles. This area is accessible only to authorized, qualified personnel. The end user should be responsible, act deliberately and cautiously, as the cycles described require changes to system data.
The Figure shows the use of the vibration prevention function (OSC) in the SINUMERIK. OSC and OSZIOFF can be inserted into the respective workpiece program as high-level language commands. Only the percentage change in speed is entered within the parentheses. A change in the user cycles is not necessary from the perspective of the machine operator. Please note that this feature is intended only for use in the programGUIDE and does not apply to ShopTurn.
Vibrations on the workpiece can be achieved by varying the constant speed. Therefore, the set value of the spindle changes constantly. Thus, the set value of the spindle speed fluctuates between a lower and upper limit back and forth within a defined period of time. In the current example, the selected set value of the spindle speed ranges between 90% and 110% of rated speed selected. The time interval was fixed at 2 seconds at an initial spindle speed of 1000 rpm.
Principle of the ideal speed behavior
Vibration acts only on the active master spindle and depends on its dynamics. In order to reduce vibrations, the set value of the spindle speed is reduced or increased by the percent factor after each time factor. The result is a vibration that is strongly dependent on the dynamics of the spindle and the workpiece mass. There are no tables of values for this vibration behavior, thus the required parameters (spindle speed, time factor) for the material used must be determined experimentally.
The desired spindle behavior is illustrated in the upper figure. Constant fluctuation of the spindle speed between upper and lower limits specified by you results in a sinusoidal wave.
The other figure shows a change in the spindle speed with a very large time factor selected. The spindle speed lingers for a moment on the upper or lower limit of the selected set value of the spindle speed; this may again lead to the rattling of the workpiece.