Supporting system of a longitudinal-milling machine with a mobile gantry

Abstract

A model of the supporting system for the 6675 longitudinal-milling machine is shown in Fig. 1. The dynamic pliability of this supporting system for a symmetric base configuration relative to the gantry and the middle position of the slide is shown in Fig. 2, in the form of the amplitude‐frequency characteristic for the dynamic pliability of the structure. The resonant oscillations of the supporting system for the initial 6675 milling machine and the trolleys of the portal-supply drive have been experimentally established: (1) under the action of an external force along the O X axis, at resonant frequencies f = 27, 52, and 82 Hz; (2) under the action of a force along the OY axis, at f = 20 and 86 Hz; 3) under the action of a force along the OZ axis, at f = 57 Hz. Examples are shown in Figs. 3, 4, and 5. The slide of the 6675 machine tool is mounted on a rocking guide piece (with two plates for each side of the slide). To increase the rigidity of the guide piece, we consider the use of six plates (three for each side of the slide). Calculation of the dynamic pliability of this design in the OX direction shows that the additional plates have no effect on the resonant amplitude of the dynamic pliability: the amplitude of the first resonant peak varies from 8.263 × 10 ‐8 to 8.251 × 10 ‐8 m/N. Hence, the introduction of additional plates is inexpedient. We now consider the influence of the slide extension on the dynamic pliability of the supporting system. (The slide path is 1 m.) All the previous calculations assume a middle position of the slide. The dynamic pliability of the supporting system with minimum and maximum extension of the slide in the OX direction is shown in Fig. 6, in terms of the amplitude‐frequency characteristic. Table 1 presents the dynamic characteristics of the supporting system for the 6675 milling machine. It is evident from Table 1 that changing the slide position alters the resonant amplitude in the amplitude‐ frequency characteristic of the dynamic pliability. The amplitude of resonant peak 1 varies by no more than 15%. Resonant peak 2 is more sensitive to the slide position: the resonant amplitude increases fourfold as the slide shifts from minimum to maximum extension. In this process, peak 2 becomes larger than peak 1. Analysis of the dynamic characteristics of the supporting system is necessary in order to determine the stability of the milling machine in operation. We will calculate the vibrational stability of the initial design of the 6675 milling machine with maximum extension of the slide, since the dynamic pliability is greatest in this case (Table 1). Table 2 presents the cutting conditions (the material of the blank and the geometry of the end mill). The critical gantry supply s is calculated for cutting depth t = 1, 2, and 3 mm. The corresponding graphs are shown in Fig. 7. It is evident from Fig. 7 that the critical supply s depends on the mill speed n and on the cutting depth t . Variation in n within the working range 200‐350 rpm is accompanied by change in s by up to 40% of its mean value; variation in t from 1 to 3 mm reduces s to less than a quarter of its initial value. However, even with maximum cutting depth t = 3 mm, the maximum critical sup