THE INFLUENCE OF CUTTING PARAMETERS ON THE DYNAMIC STABILITY PERFORMANCE OF FINISHING CNC MACHINES

The paper describes the influence of the cutting regime parameters on the dynamic stability reserve of CNC finishing machines. For this purpose it is used an experimental stand comprised of transducers for measuring tool and workpiece rotation speeds, as well as a measuring device for the dynamic components of the cutting force (Ft, Fx). The processed materials are steel, bronze and cast iron. Finally there is presented the variation pattern of these parameters and their influence on the finishing process productivity.


INTRODUCTION
CNC finishing machines are one of the most significant cases for the treatment of vibration problems. This is due to the fact that surface finishing operations are usually solved by finishing [1].
Compared to conventional machining (with defined tools) where by choosing appropriately depending on the type of machine tool and the given operation, the cutting regimes, the process remains stable for a long time, when finishing, the process becomes unstable in a relatively short time. This causes the process to be interrupted in order to straighten the abrasive tool.
Among the rectification operations, the finishing process occupies an important place due to the special conditions in which it takes place [2].
In order to study the action of the parameters of the cutting regime on the dynamic stability reserve of the CNC finishing machine, we further reproduce the structure and connection schemes that are established between the main components in the technological process of finishing ( fig. 1).
The experimental stand used for this purpose consists of transducers for measuring tool and workpiece speeds, as well as a sensor for measuring the dynamic components of the cutting force (Ft, Fx). Using for each measuring channel adequate connection schemes, at the level of the central computer, the variation of the two components Ft, Fx is obtained according to the other parameters of the cutting regime, with the help of an interface for which special programs have been built such as: control of the acquisition board, processing of the measured signals, writing to files on the hard disk and displaying the signals on the screen.

DETERMINATION OF THE INFLUENCE OF CUTTING REGIME PARAMETERS ON THE DYNAMIC STABILITY OF FINISHING CNC MACHINE TOOL IN THE EVENT OF A STEEL TEST PART
The variation diagrams of Ft and Fx are drawn for an optimal cutting regime corresponding to fig. 2, 3:  Figure 5: Analysis of the dynamic stability reserve for a given cutting regime when processing cast iron parts

CONCLUSIONS
From the analysis of the evolution of the components of the cutting forces it is found that the highest values occur in the case of steel finishing, while in the case of finishing cast iron and bronze they are significantly reduced, the measured values being relatively close [3].
Increasing the cutting depth has a negative effect on the dynamic stability reserve of the finishing machine. The feed rate and the main cutting speed admit value ranges in which their action is generally positive in a middle range of the cutting speed nS = 17500 r / m and in a higher range of advance variation f2 = 1600 mm / min and f3 = 3000 mm / min.
The best stability reserve was obtained for small cutting depths aP1 = 0.005 mm, average advances f2 = 1600 mm / min and a circular advance of the part increased to nP2 = 400 r/m.
Usually if the increase of the stability reserve is imposed by the increase of the advance, of the main cutting speed or of the circular advance of the part, these do not always present effective measures to increase the stability [4], [5], [6].
In the case there is no direct kinematic connection between the main kinematic chain and the feed kinematic chains, so that the speed of the main shaft is independent of the feed size, so that as the speed of the main shaft increases, the feed rate remains constant, the feed decreases, i.e. the positive influence of the increase of the main cutting speed is partially compensated by the negative effect of the corresponding decrease of the longitudinal or circular advance thus canceling the effectiveness of the adopted measure, therefore it is necessary that in these cases, with the increase of the main axis speed, the same direction and the longitudinal and circular advance of the part, which is possible in the case of machine tools provided with adaptive controls [2].
Due to these limitations regarding the measures of increase of nS and f or nP, in order to ensure the dynamic stability, it remains certain the decrease of the cutting depth, following that the loss of productivity of the technological operation, caused by the variation of the cutting depth, is partially avoided by the corresponding increase in the circular feed of the part, which generally leads to an increase in the stability of the machine [1].

SOURCES OF FUNDING
None.