MACHINE TOOL

[0001] The present invention relates to a method for suppressing the phenomenon of chatter in a machine tool, in particular, but not only, in machine tools provided with boring bars.

[0002] The term“chatter” means the occurrence of harmful vibrations during machining of a workpiece, which adversely affect machining quality, in particular in the case of precision machining tasks, the productivity of the machine tool, and can damage the mechanical parts of the machine tool.

[0003] The phenomenon of “chatter” depends on the dynamic performance of the machine tool and on the machining parameters.

[0004] Different techniques are known for reducing or eliminating the phenomenon of “chatter” in a machine tool:

off-line strategies

in-process strategies

passive techniques

active techniques

[0005] The off-line strategies aim to optimize the machining parameters of the machine tool, such as cutting speed, advancement, the type of tool used, the cutting depth, in order to use the machine at maximum performance. The mathematical methods used in these off line strategies, require the so-called lobe diagrams to be estimated, which are stability maps of the machine tool, depending on the cutting speed function and cutting depth, for certain set values of the other constant parameters. The aforesaid off-line strategies require a detailed knowledge of the dynamic performance of the machine tool and the control of all the parameters that affect the reliability of the estimate of the aforesaid set values.

[0006] The in-process strategies aim to modify machining parameters and/or dynamics that are significant for the stability of the machine tool, when the phenomenon of“chatter” is identified by suitable sensors. These are more evolved and effective techniques because they do not require prior knowledge of the particular task to be performed.

[0007] The passive techniques aim to optimize the dynamic performance of the machine tool in the design phase, owing to the use of advanced design techniques and innovative materials. The object consists of maximizing the dynamic stiffness on the spindle, increasing damping and minimizing mass. [0008] The active techniques are the natural evolution of the passive techniques, but the dynamic parameters are optimized by actuators driven by suitable control laws. Applications have been marketed for applications on machine tools.

[0009] As far as the so-called“in-process” strategies are concerned,“Spindle Speed Variation”, i.e. the variation of spindle speed, and“Stiffness Variation” i.e. the variation of the stiffness of the system are very promising. Both techniques can operate according to two principles:

discrete variation, which enables machining to be returned to stable mode, owing to the modification of the cutting speed or the stiffness of the system;

periodic continuous variation, which varies according to a periodic law the cutting speed or the dynamics of the machine. In this manner, during machining, there are periods of stability and instability. When the period of stability is sufficiently long to dissipate the energy accumulated during the unstable period and the duration of the period of instability is sufficiently short to limit the amplitude of the oscillations, machining remains stable. Accordingly, this variation enables machine productivity to be increased.

[0010] In the case of boring bars, studies have been conducted to vary discretely the stiffness thereof by magnetorheological or electrorheological fluids when the machining becomes unstable.

[0011] Other studies have been conducted to vary periodically the stiffness of a boring bar by a magnetorheological fluid.

[0012] The possibility of varying the radial stiffness of the spindle of a machine tool by piezoelectric actuators has also been studied.

[0013] One object of the present invention is to provide a method for varying periodically, simply and effectively the stiffness of a system for machining workpieces by removal of chips, in order to suppress the phenomenon of“chatter” during machining of a workpiece.

[0014] Another object of the present invention is to obtain the variation of the stiffness of the system without having to resort to the use of magnetorheological or electrorheological fluids, or to the use of piezoelectric actuators.

[0015] The objects of the present invention are achieved by a method for varying the stiffness of a system for machining workpieces by removing chips according to claim 1. [0016] Owing to the invention it is possible to obtain simply and effectively the suppression of the phenomenon of“chatter” during machining of a workpiece in a machine tool by removal of chips, by making machining stable without having to intervene on the machining parameters, such as for example cutting speed and cutting depth, obtaining high machining quality and increasing productivity of the machine tool.

[0017] Further advantages and features of the invention can be gleaned from the following description provided merely by way of non-limiting example, with reference to the attached drawings, in which:

[0018] Figures 1 to 3 illustrate schematically a first embodiment of the invention;

[0019] Figures 4 to 9 illustrate schematically a second embodiment of the invention and [0020] Figures 10 to 12 illustrate a third embodiment of the invention.

[0021] According to the present invention, the suppression of chatter is obtained by varying periodically the stiffness of the machine tool during machining of a workpiece, according to a law of preset variation, between a maximum value and a minimum value in a preset interval of time, i.e. in a period. The stiffness is varied as explained in greater detail below, by interpolating displacements along two axes of the machine tool so as to maintain unvaried the position of the tool during machining of a workpiece, or by varying the stiffness of hydrostatic shoe bearings of the machine by varying the fluid supply pressure to the shoe bearings, or by varying the capillary resistance of the supply circuit of hydrostatic shoe bearings.

[0022] The main parameters of the aforesaid law of variation are the form of the function, which discloses how stiffness varies in each period; the frequency, i.e. the number of variation periods of the stiffness in the unit of time and the amplitude of the variation of the stiffness, i.e. the difference between the maximum and the minimum stiffness value in the period.

[0023] From a purely theoretical point of view, the form of function that would ensure most effective suppression of the phenomenon of“chatter” would be a square wave, i.e. a stiffness step variation between a maximum value maintained for a first half of each period and a minimum value maintained for a second half of the period.

[0024] However, in practice it is not possible to obtain the aforesaid waveform, but waveforms that are more or less rounded can be obtained, for example waveforms that approximate a sine wave. [0025] The frequency of variation of the stiffness is linked to the frequency at which the cutting edges of the tool come into contact with the surface of the workpiece to be machined. If the tool has a number n of cutting edges and rotates at a rotation speed equal to m rpm, the contact frequency f of the cutting edges with the workpiece will be equal to f = m/60*n Hz.

[0026] Preferably, the frequency of variation of the stiffness has to be less than the frequency at which the cutting edges of the tool come into contact with the surface of the workpiece to be machined and is limited, as a maximum value, by the dynamics of the system used.

[0027] The amplitude of variation of the stiffness is substantially inversely proportional to the frequency, in the sense that the greater the frequency of variation, the less is the maximum size of variation that it is possible to achieve because of the dynamic limits of the system used.

[0028] In Figure 1, a machine tool is illustrated schematically in which suppression of the “chatter” phenomenon is obtained by interpolating motions along two machine axes.

[0029] The machine tool comprises a machining carriage 1 that is movable along an upright M of the machine tool. The carriage 1 supports a slide 2, called also ram, which is movable with respect to the carriage 1 along a direction X that is perpendicular to said upright M. The ram 2 supports a boring bar 3, that is movable with respect to the ram 2 along said direction X. To the boring bar the spindle is fitted to which a machining tool 4 is fixed with which machining by removal of chips on a workpiece 5 is performed, said machining tool 4 being rotatable around a rotation axis A, parallel to said direction X.

[0030] In order to suppress the phenomenon of chatter, during machining of the workpiece 5, the ram 2 and the boring bar 3 are moved in opposite ways along the direction X so that the movements of the ram 2 and the boring bar 3 are specular, i.e. they are movements that are the same at the same time, but in opposite ways. If with Dci a first displacement of the ram 2 is indicated and with Dc2 a second displacement of the boring bar 3 is indicated the relation Dci+Dc2 =0, i.e. Dci=-Dc2 applies.

[0031] The movements of the ram 2 and of the boring bar 3 determine a variation of the stiffness of the system between a minimum stiffness configuration, illustrated in Figure 1 and a maximum stiffness configuration, illustrated in Figure 2

[0032] As it is not possible to obtain a step law of motion because of the inertia of the axes, because of the limited jerk values, i.e. because of the derivative of the acceleration, of acceleration and speed of the drives and because the components of the machine would be seriously stressed, inducing vibrations, an approximately sinusoidal motion law was chosen, which was obtained with a square wave variation of the jerk, shown in Figure 3.

[0033] In Figure 3, the curve marked by the letter W is the motion law of the ram 2, whereas the curve marked by the letter Z is the motion law of the boring bar 3. In first approximation, if the neighbourhood of a point along one of the curves is considered, the stiffness of the machine varies almost linearly with the position of the ram 2 and of the boring bar 3 along the direction X.

[0034] A typical value of the amplitude of the first displacement Dci and of the second displacement Dc2 is comprised between about 25 and about 150 mm, in a time that varies between 0.2 s and 1 s, i.e. at a frequency comprised between 1 Hz and 5 Hz. This means that this method for suppressing chatter is usable in machining tasks at a contact frequency between the cutting edges of the tool 4 and the workpiece 5 comprised, indicatively, between 20 Hz and 40 Hz.

[0035] In Figures 4 to 8 a second embodiment is illustrated of the present invention that is applied to a machine tool in which a machining tool 4 is fitted to a spindle 6 that is in turn fixed to a ram 2, which is movable along a direction X, inside a machining carriage 1. The machine tool 4 is rotatable around a rotation axis A parallel to said direction X. The ram 2 is movable with respect to the machining carriage 1 on at least one pair of hydrostatic shoe bearings 7, 8, supplied by a pressurized fluid.

[0036] The ram 2 with the hydrostatic shoe bearings 7, 8 is schematizable as a prismatic beam 9, supported by two yielding supports 10 and 11, with variable stiffness (see figure

5).

[0037] The stiffness of hydrostatic shoe bearings 7, 8 can be varied by modifying the pressure of the fluid that supplies the hydrostatic shoe bearings.

[0038] In Figure 6, a first mode of varying the pressure of the fluid that supplies the hydrostatic shoe bearings is illustrated schematically.

[0039] In Figure 6, a hydrostatic shoe bearing 7 is shown schematically that is supplied by a pump 12 through a block of capillary resistances 18 of the supply circuit of hydrostatic shoe bearings. The pump 12 is driven by a motor 19. The delivery pressure Pa of the pump 12 is adjusted by a maximum pressure valve 20. This valve can be controlled proportionally or by steps. [0040] If it is not possible to vary the supply pressure of the entire hydrostatic plant, but it is desired to vary only the pressure of some shoe bearings, the diagram of Figure 7 is used. In Figure 7, a hydrostatic shoe bearing 7 is shown schematically that is supplied by a pump 12 through a block of capillary resistances 18 of the supply circuit of the hydrostatic shoe bearings. The pump 12 is driven by a motor 19. The delivery pressure Pa of the pump 12 is maintained constant by a maximum pressure valve 20 associated with the pump 12. At the outlet of the pump 12, a pressure-reducing valve 21 is provided, by means of which, proportionally or by steps, the pressure Pt is adjusted of the fluid that is sent to the hydrostatic shoe bearings.

[0041] By controlling appropriately the pressure-reducing valve 21, it is possible to obtain a set stiffness variation function.

[0042] The relation between the value of the supply pressure and the stiffness of hydrostatic shoe bearings 7, 8 is shown in Figure 8, from which it is seen that the stiffness of two opposite hydrostatic shoe bearings and which are not subjected to outside loads is linearly dependent on supply pressure, so it is well controllable.

[0043] The constraints on the law of variation of the stiffness are linked to the drive dynamics of the proportional valve and to the dynamics of the supply hydraulic circuit. It is possible to manage to obtain a law of variation of the stiffness that approximates a step variation, that has tilted stiffness increase and reduction ramps and transitions that are less sudden than in the ideal step function.

[0044] With the technique of varying the stiffness of hydrostatic shoe bearings that is disclosed above it is possible to obtain a variation in stiffness of 5-10%, with a relatively high frequency of variation of the stiffness, up to about 50Hz.

[0045] In Figure 9, a second mode of varying the pressure of the fluid that supplies the hydrostatic shoe bearings is illustrated schematically.

[0046] In this case, instead of the pressure-reducing valve 21, a three-way valve 22 is provided, with an inlet 23 connected to the delivery of the pump 12, a first outlet 24 connected directly to the supply circuit of the hydrostatic shoe bearings and a second outlet 25 connected to the supply circuit of the hydrostatic shoe bearings by a hydraulic resistance 26 that when traversed by the fluid that supplies the hydrostatic shoe bearings, generates a pressure drop Dr.

[0047] By switching the three-way valve so as to send the pressurized fluid coming from the pump 12 to the first outlet 24 or to the second outlet 25, the pressure Pt can be varied at the inlet of the supply circuit of the hydrostatic shoe bearings between the value Pa of the delivery pressure of the pump and a value Pa-Dr. In this manner, a function can be obtained for varying the stiffness of hydrostatic shoe bearings that approximates a step function.

[0048] In Figures 10 to 12, a third embodiment of the present invention is illustrated in which the stiffness of hydrostatic shoe bearings is varied by modifying the capillary resistance of the circuit that supplies the hydrostatic shoe bearings 7, 8.

[0049] One example of modification of the capillary resistance is illustrated in Figure 10. The modification of the capillary resistance can be obtained by supplying the hydrostatic shoe bearings 7, 8 by a first branch of supply circuit 13 comprising a first capillary resistance 15 of a length LI, or by a second branch of supply circuit 14 arranged parallel to the first branch of supply circuit 13 and comprising a second capillary resistance 16 of a length L2>L1. The first branch of supply circuit 13 and the second branch of supply circuit 14 are supplied by the supplying pump 12 at constant pressure. Between the supplying pump 12 and the two branches of supply circuit 13, 14 a three-way valve 17 is inserted with an inlet 27 connected to the delivery of the pump 12, a first outlet 28 connected to the first branch of supply circuit 13 and a second outlet 29 connected to the second branch of supply circuit 14. By switching the three-way valve 17, the delivery of the pump 12 can be connected to the first branch of supply circuit 13, or to the second branch of supply circuit 14, alternatively. By driving the three-way valve at a preset switching frequency, periodic variation of the pressure of the fluid that supplies the hydrostatic shoe bearings is obtained and, consequently, periodic variation of the stiffness of hydrostatic shoe bearings between two discrete values. Switching frequency means the frequency at which the three-way valve 15 connects alternatively the supplying pump 12 to the first branch of supply circuit 13 and to the second branch of supply circuit 14.

[0050] Also in this case it is possible to manage to obtain a law of variation of the stiffness that is approximately a step law of variation, that has tilted stiffness increase and reduction ramps and transitions that are less sudden than in the ideal step function.

[0051] With this technique of varying the stiffness of hydrostatic shoe bearings, it is possible to obtain a variation of stiffness of the order of 5-10%, with a medium low frequency of variation up to about 10Hz.

[0052] With regard to the lengths LI and L2 of the capillary resistances, choosing for LI a value up to about 500 mm and for L2 a value up to about 4000 mm can be hypothesized. These values are approximate and depend on the type of machine, because they are influenced by the mass thereof, by the supply pressure of the hydrostatic plant and by many other parameters.

[0053] In Figure 11, a further embodiment is illustrated schematically of modification of the stiffness of hydrostatic shoe bearings 7, 8 by varying the capillary resistance of the circuit supplying the hydrostatic shoe bearings 7, 8.

[0054] In the embodiment in Figure 11, the hydrostatic shoe bearings are supplied through a supply circuit 30 supplied in turn by the pump 12 at constant delivery pressure Pa. The supply circuit 30 comprises a first branch 31 of supply circuit, that has a first capillary resistance 35 having a length L3 and a second capillary resistance 36 having a length L4, arranged in series, and a second branch 32 of supply circuit, by means of which it is possible to exclude the second capillary resistance 36 from the supply circuit 30. The second branch 32 of supply circuit is provided with a normally closed non-return valve 33, which can be opened by a pressure signal Pp sent to the valve 33 through a pilot circuit 34. Sending the pressure signal Pp causes the non-return valve 33 to be opened so that the pressurized fluid that supplies the hydrostatic shoe bearings passes through the second supply branch 32 without passing through the second capillary resistance 36.

[0055] Driving the non-return valve 33 at a set drive frequency, periodic variation can be obtained of the length of the capillary resistance of the supply circuit 30 of hydrostatic shoe bearings between two discrete values, i.e. between a value L3+L4, when the non return valve 33 is closed and a value L3, when the non-retum valve is open.

[0056] The periodic variation of the length of the capillary resistance of the supply circuit 30 involves a periodic variation of the pocket pressure of the hydrostatic shoe bearings and, consequently, a periodic variation of the stiffness of the hydrostatic shoe bearings between two discrete values.

[0057] The relation between the stiffness of hydrostatic shoe bearings 7, 8 and the length of a capillary resistance is not linear, as can be seen in the graphic of Figure 12 that shows the variation in stiffness in function of the length of the capillary resistance. The stiffness rapidly increases up to a maximum for low values of the length of the capillary resistance in order to then diminish progressively with a further increase of the length of the capillary resistance.

[0058] Another method of variation of the capillary resistance, which is not illustrated, provides for the use of an actuator, for example of piezoelectric type, which drives a plug, for example tapered, configured so as to occlude in a variable manner the passage section of the fluid that supplies the hydrostatic shoe bearings, thus varying the capillary resistance of the supply circuit of the shoe bearings.

[0059] The periodic and continuous variation techniques of the dynamics of a machine tool according to the present invention enable machine productivity to be increased in particular during the roughing step. The interpolation technique of two parallel axes and varying the hydrostatic stiffness by modulation of the supply pressure and variation of the capillary resistance have different dynamics, so that combined use of the aforesaid techniques is conceivable, in order to maximize the stabilization effect in a milling machining.