Machine tool for machining workpieces with tools

Abstract

The machine tool is used to machine workpieces with tools and has at least one rotary table with at least one clamping unit with which a pallet holding the workpiece to be machined can be clamped. The clamping device has a clamping element that is firmly connected to a piston. This piston is actuated by a pressure medium, preferably hydraulic medium, and adjusts the clamping element between a release position and a clamping position. In the clamping position, the piston is loaded by the pressure medium located in a pressure chamber, and a line opens into the pressure chamber. This line contains a check valve and is connected to a first pressure relief valve.In order to avoid the risk of damage to the machine tool and / or endangering persons in the area of ​​the machine tool in the event of a crash, a second pressure relief valve is located in the line between the check valve and the pressure chamber. This second pressure relief valve is set to a pressure that is at least 1.5 times higher than the pressure of the first pressure relief valve.

Description

The invention relates to a machine tool for machining workpieces with tools according to the preamble of claim 1.For machining, workpieces are clamped on a round table of the machine tool. The workpiece is machined with the tool, which can be a drill, for example. In use, it may occur that a crash occurs between the workpiece to be machined and the tool. This leads to a very rapid force increase at the collision point due to the large moving masses which have to be decelerated. As a result of the increase in force, the clamping units with which the pallet is connected to the round table break, so that the workpiece with the pallet can be released from the round table. Since a torque is also generated at the collision point between the tool and the workpiece, the workpiece can become detached from the round table in the event of a crash and in the event of a breakage of the clamping units and leave the working space of the machine tool. Not only can damage the tool machine occur in this case, but there is also the risk that persons located in the region of the tool machine may sometimes be seriously injured.The invention is based on the object of configuring the machine tool of the generic type in such a way that, in the event of a crash, the risk of damage to the machine tool and / or risk to persons in the region of the machine tool are avoided.This object is achieved in the generic machine tool according to the invention with the characterizing features of claim 1.The system pressure is adjusted with the first pressure limiting valve in such a way that a secure clamping of the pallet on the round table is achieved. The second pressure limiting valve is set to a pressure that is at least 1 1⁄2 times as high as the pressure of the first pressure limiting valve. The pressure of the second pressure limiting valve depends on how high the mechanical load capacity of the clamping unit is. The second limiting valve can be adjusted to the failure force of the clamping unit, as a result of which it is protected. In the event of a crash, the workpiece with the pallet experiences a translatory force, which leads to the piston pressurizing the pressure medium located in the pressure chamber. The second pressure limiting valve ensures that, when the set pressure is reached, the second pressure limiting valve opens, so that the pressure medium can reach the open air. In this way, the translatory force occurring in the event of a crash can be absorbed almost completely by the clamping unit without it failing. This makes it possible to reduce the rotational energy by means of braking by the motor of the rotary table and by friction between the tool and the workpiece at the collision point.The first and / or the second pressure limiting valve can advantageously be adjusted, so that the system pressure and / or the pressure of the second pressure limiting valve that is decisive for the crash can be matched to the respective application.The check valve is advantageously preceded by a switching valve, which is preferably a 4 / 2-way valve. Depending on the position of the switching valve, the pressure chamber in front of or behind the piston is placed under pressure in order in this way to adjust the clamping element into the clamping position or into the release position.In a simple embodiment, the clamping element is rotatable about an axis between the release position and the clamping position.In order to be able to execute the rotational movement of the clamping element reliably, it is advantageously provided with a rotational device.The rotating device is preferably formed by at least one helical groove and at least one guide part on the rotary table side, which engages in the helical groove. Due to the described configuration of the groove, the clamping element is simultaneously translationally displaced in the direction of the axis of rotation during the rotational movement in order to bring the clamping element into the clamping or release position.In order that the clamping element does not release the pallet and thus the workpiece in the event of a crash, the clamping element is advantageously secured against rotation on the pallet in the clamping position.For the rotation prevention, at least one form-fitting element is advantageously provided, which engages in at least one mating form-fitting element of the pallet.If, in the event of a crash, the workpiece is moved translationally together with the pallet, the form-fit element prevents the form-fit element from rotating relative to the pallet. This ensures that the clamping element is not adjusted from the clamping position into the release position in the event of a crash.The subject matter of the application results not only from the subject matter of the individual patent claims, but also from all the details and features disclosed in the drawings and the description. Although they are not the subject matter of the claims, they are claimed as essential to the invention, provided they are novel, individually or in combination, compared with the prior art.Further features of the invention are evident from the further claims, the description and the drawings.The invention is explained in more detail with reference to an exemplary embodiment shown in the drawings. The following are shown: FIG. 1 shows a schematic illustration of a machine tool having three linear axes and one round axis, FIG. 2 shows, in a representation corresponding to FIG. 1, the machine tool in the event of a crash, FIG. 3 shows a top view of a round table of the machine tool according to the invention with four clamping units, FIG. 4 is a diagram showing the dissipation work generated during a dissipation stroke, FIG. 5 shows a schematic illustration of a hydraulic circuit of a clamping unit of the machine tool according to the invention, FIG. 6 shows a schematic illustration of a clamping unit of the machine tool according to the invention with a swivel clamp, FIG. 7 is a plan view of the clamping claw of the clamping unit of the machine tool according to the invention.The machine tool has a machine bed 1 on which a stand 2 is arranged. It can be moved on the machine bed 1 in the X direction. For this purpose, corresponding guides 3 running in the X direction are provided.On the front side of the stand 2 are arranged guides 4 extending in the Y-direction for a slide 5 which carries a tool spindle 6 with which a tool 7 can be rotatably driven.On the machine bed 1 there are provided further guides 8 extending in the Z-direction for a carriage 9 with which a rotary table 10 can be moved in the Z-direction.Pallets 11 can be securely clamped on the round table 10, on which workpieces 12 to be machined are securely clamped. The workpiece 12 is machined with the tool 7.The round table 10 can be rotated about the axis 13 perpendicular to the Z direction. Due to the rotation capability of the rotary table 10, rotary processes can be carried out on the workpiece 12.The machine tool thus has the three linear axes in the X, Y and Z directions as well as the round axis in the form of the axis of rotation 13.FIG. 2 shows the machine tool in the event of a crash. The case is shown in which the tool 7 located in the spindle 6 collides with the workpiece 12. The result of this is that the pallet 11 is partially lifted off the round table 10. In the round table 10, clamping units 14 are provided in a manner still to be described, which are designed such that no unacceptably high force acts on the clamping units 14.In the event of a crash, this prevents the workpiece 12 from being able to be detached from the round table 10 and thus a risk of machine parts and / or persons working in the region of the machine tool occurs.High forces occur at the collision point 15 between the tool 7 and the workpiece 12, which forces act on the individual components of the machine tool and finally on the workpiece 12 along the indicated force profile 16.Due to the large moving masses, in particular due to the large rotating masses, a very rapid force increase occurs at the collision point 15 in the event of a crash. Forces of approximately 6000 kN can act on the clamping units 14 between the pallet 11 and the round table 10 here, wherein the force increase occurs very rapidly in the event of a crash, for example within 20 ms.FIG. 3 shows the round table 10 in plan view, which in the exemplary embodiment is provided with four clamping units 14, which are advantageously of identical design. The number of clamping units 14 depends on the size of the round table 10. The clamping units 14 are advantageously arranged distributed uniformly over the circumference of the round table 10, so that the pallet 11 can be reliably clamped on the round table 10.The clamping units 14 have a swivel clamp 17 (FIGS. 6 and 7 ) which can be rotated through 90° from a clamping position (solid line) into a release position (dash-dotted line).The swivel clamp 17 has an angular head 18 from which a pin-like guide part 19 projects centrally. It is provided on the circumference with at least one control groove 20, in which a guide element 21 engages.In the embodiment, the guide part 19 has two control grooves 20 which are arranged diametrically opposite one another and in each of which a guide element 21 engages. The guide elements 21 are advantageously balls which enable the swivel clamp 17 to be rotated smoothly about the axis 22 of the guide part 19.The clamping unit 14 is accommodated in a depression 23 in the upper side of the round table 10. A cylinder 25 projects perpendicularly from the bottom 24 of the depression 23, on the inner side 26 of which the guide elements 21 are held.The guide part 19 of the swivel clamp 17 engages with play in the cylinder 25.For displacing the head 18 of the swivel clamp 17 in the axial direction of its guide part 19, the head 18 is firmly connected to a piston 27 which can be displaced by a pressure medium for adjusting the head 18 of the swivel clamp 17. The piston 27 is acted upon on both sides with pressure medium in order to displace the head 18 of the swivel clamp 17 between the clamping and the release position.The piston 27 sealingly abutting an inner wall 28 of the recess 23 projects radially outwards from the lower end of a cylindrical retaining body 29, the end of which remote from the piston 27 extends radially inwards and forms an annular flange 30 which is fastened abutting the underside 31 of the head 18 of the swivel clamp 17. The cylindrical holding body 29 surrounds the cylinder 25 over a part of its height at a distance.In FIG. 6, the maximum stroke (dissipation stroke) s D of the piston 27 is indicated.FIG. 6 shows the swivel clamp 17 in the clamping position, in which the head 18 rests under pressure against a bottom 32 of a receptacle 33 of the pallet 11. The receptacle 33 is formed in a known manner such that the head 18 of the swivel clamp 17 can be inserted through an insertion opening 34 in the underside 35 of the pallet 11. In the exemplary embodiment, the insertion opening 34 extends perpendicularly to the base 32 of the receptacle 33, which forms the clamping surface for the head 18.In order to clamp the pallet 11 on the round table 10, it is placed on the round table 10 in such a way that the head 18 of the swivel clamps 17 reaches the receptacles 33 of the pallet 11 through the insertion openings 34. For clamping, the piston 27 is displaced downward in FIG. 6. In this case, the head 18 is carried along and, via the guide elements 21 and the control grooves 20, is simultaneously pivoted about the axis 22 into the clamping position shown in FIG. 6. It is represented in FIG. 7 by the solid line.For the bracing process, the pressure medium is introduced into a pressure chamber 36, which is located above the piston 27 in FIG. 6. The pressure chamber 36 is sealed off from the outside and is separated by the piston 27 from a lower pressure chamber 37, which is located below the piston 27 in FIG. 6.For the release process, the piston 27 is acted upon by the pressure medium located in the lower pressure chamber 37, as a result of which the head 18 of the swivel clamp 17 is rotated upwards and simultaneously via the guide elements 21 and the control grooves 20 into its release position. The pallet 11 can then be removed easily from the round table 10.FIG. 5 shows the hydraulic circuit diagram for acting on the piston 27 of the clamping unit 14.A pressure line 38 opens into the upper pressure chamber 36, via which the pressure medium is conveyed from a tank 39 by means of a pump 40.Behind the pump 40 is a switching valve 41, with which the passage of the pressure medium to the upper pressure chamber 36 can be blocked or released, depending on the switching position of the switching valve 41.Behind the switching valve 41 in the direction of flow, there is a pilot-operated check valve 42 in the line 38 which prevents the pressure medium from flowing back from the pressure chamber to the tank 39.Connected to the line 38 is an adjustable pressure limiting valve 46, with which the pressure (system pressure) of the pressure medium in the upper pressure chamber 36 can be adjusted.The lower pressure chamber 37 is connected to the tank 39 via a line 44. The switching valve 41 is located in the line 44 in the region between the tank 39 and a transverse line 45 via which the non-return valve 42 is connected to the line 44.In FIG. 5, the clamping position of the swivel clamp 17 is shown by solid lines. The head 18 of the swivel clamp 17 rests under system pressure on the clamping surface 32 of the pallet 11. The piston 27 is under the system pressure of the pressure medium which reaches the upper pressure chamber 36 by the pump 40 via the switching valve 41 located in the first position 1 and the nonreturn valve 42. The pressure medium located in the lower pressure chamber 37 is conveyed back to the tank 39 via the line 44 and the switching valve 41 and a tank line 47.If the pallet 11 on the round table 10 is to be released, the switching valve 41 is switched over into the second position 2, so that the pump 40 delivers the pressure medium from the tank 39 into the line 44 and from there into the lower pressure chamber 37. The piston 27 is displaced upwards by the stroke s D. The check valve 42 is unlocked via the transverse line 45, so that the pressure medium located in the upper pressure chamber 36 reaches the tank line 47, via the line 38, the unlocked check valve 42 and the switching valve 41 located in position 2, via which the pressure medium is supplied to the tank 39.The head 18 of the swivel clamp 17 reaches the position shown by dot-dash lines in FIG. 5 in the release position.By switching over the switching valve 41, the head 18 of the swivel clamp 17 can be adjusted back into its clamping position by conducting the pressure medium in the described manner into the upper pressure chamber 36, while the pressure medium located in the lower pressure chamber 37 reaches the tank line 47 via the line 44 and the switching valve 41 switched into position 1.The system hydraulic pressure prevails in the line 38 and is limited by the pressure limiting valve 46 assigned to the pump 40. This system pressure is, for example, 80 bar. At this system pressure, it is ensured that sufficiently high clamping forces result, so that a secure clamping of the workpiece 12 to be machined is ensured during the machining by the tool 7. The clamping forces are selected such that the workpiece 12 cannot be displaced from its clamping position in the clamping device or even lifted by the machining forces.The second pressure limiting valve 43, which is arranged downstream of the check valve 42, is set to a pressure which is at least 1 1⁄2 times as high as the pressure of the first pressure limiting valve 46. The second pressure limiting valve 43 arranged downstream of the check valve 42 is set to 675 bar in the exemplary embodiment, which corresponds to a factor of 8.4, based on the system pressure of 80 bar.The respective pressure can be set with the two pressure limiting valves 43, 46, depending on the conditions of use.The setting of the pressure at the second pressure limiting valve 43 depends on how high the mechanical load capacity of the clamping units 14 is. The respective clamping unit 14 must not fail during the piston stroke D. This piston stroke is thus referred to as the dissipation stroke.The clamping unit 14 is of course dimensioned such that when the system pressure is applied, failure of the components can never occur.The pressure limiting valve 43 sets the pressure of the hydraulic medium as high as possible, so that as much dissipation energy as possible is generated in a crash situation, which will be described below.If the crash situation illustrated by way of example in FIG. 2 occurs, the swivel clamps 17 are subjected to very severe loading. The described design of the machine tool ensures that, despite the high load on the clamping units 14, the workpiece 12 cannot be displaced from its clamping position on the pallet 11 or even lifted, which would lead to a considerable risk for the machine tool and / or the persons standing around. As can be seen from FIG. 2, the pallet 11 with the workpiece 12 can tilt with respect to the round table 10 in the event of a crash. In this case, the swivel clamps 17 execute the dissipation stroke.The pressure limiting valve 43 ensures that, in the event of a crash, the pressure medium displaced from the upper pressure chamber 36 can reach the open air. The pressure limiting valve 43 has very short switching times. By tuning this quick-switching pressure limiting valve 43 to the failure force of the swivel tensioner 17, it can be protected. At the same time, the collision energy is dissipated in the event of a crash. The forces occurring are explained below with reference to a non-limiting exemplary embodiment.It is assumed that the workpiece 12 has a mass m of 500 kg and a mass moment of inertia J 1 of 25 kgm 2. The round table 10 is provided with four clamping units 14 (FIG. 3 ).For the pallet 11, a mass moment of inertia J 2 of 10 kgm 2 is assumed.The rotating mass is determined by the rotary table and can be determined from the mass moment of inertia of the pallet 11 and of the rotary table 10. For the round table 10 and the pallet 11, a total mass moment of inertia of 12.7 kgm 2 is assumed, which results from the mass moment of inertia J 2 of the pallet 11 of 10 kgm 2 and the mass moment of inertia J 32,7 kgm 2 for the round table 10.The mass moment of inertia J 3 of the rotating mass in the form of the round table 10 loads the swivel clamps 17 in the event of a crash.In the region of the collision point 15, the tool 7 leads to a torque stress on the workpiece 12, by means of which the high forces are also transmitted to the swivel clamps 17. The resulting mass moment of inertia of the entire rotating mass takes up the swivel clamps 17 in the described manner in the event of a crash. It has been shown here that the rotational energy escapes predominantly via the collision point 15, but not via the clamping units 14.The clamping units 14 are additionally subjected to shear by the rotating mass of the round table 10.In the event of a crash, the workpiece 12 is loaded with the pallet 11 upwards (FIG. 2 ). Since the head 18 of the swivel clamps 17 bears against the base 32 of the receptacles 33, this has the result that the respective swivel clamp 17 of the clamping units 14 is loaded upwards from the clamping position. This has the consequence that the piston 27 is correspondingly displaced upwards, whereby the pressure medium located in the pressure chamber 36 is placed under increased pressure. The pressure medium displaced in this case reaches the open air via the pressure limiting valve 43 in the manner described. As a result, the pressure in the pressure chamber 36 can be kept at least approximately constant.So that the head 18 of the swivel clamp 17 is not rotated about its axis when the piston 27 is displaced out of the clamping position, the clamping unit 14 is provided with a rotation prevention means which prevents a rotation of the head 18 of the swivel clamp 17 in the event of a crash.This ensures that the head 18 is not rotated into its release position, as a result of which there would be the risk that the workpiece 12 or the pallet 11 will come free of the round table 10.As can be seen from FIGS. 6 and 7, the head 18 of the swivel clamp 17 is provided on its underside with at least one form-fitting element 48 for preventing rotation, which engages in a depression 49 in the base 32 of the receptacle 33 of the pallet 11. In the event of a crash, this form-fit 48, 49 prevents the head 18 from being able to be pivoted from the clamping position into the release position relative to the pallet 11.The translatory energy W_trans of the described machine tool can be absorbed almost completely by the swivel clamp 17 in the event of a crash without it failing. In the described manner, during the translatory displacement of the piston 27, the pressure in the pressure chamber 36 can be reduced by opening the pressure limiting valve 43. This makes it possible to reduce the rotational energy additionally occurring in the event of a crash by braking with the aid of the rotary table motor and by friction at the collision point 15.On the basis of an exemplary embodiment given below, an energy observation and the magnitude of the dissipation energy takes place, which can be dissipated via the dissipation path of the piston 27 in this way.This dissipation / (piston) travel s D is 20 mm in the example. The piston force and thus the clamping force is assumed to be 2050 N. The dissipation energy of each swivel clamp 17 is further assumed to be 5000 J, i.e. 20 000 J (total W_) for all four swivel clamps 17.In the energy observation, a translatory energy W_trans of 5167 J and a rotatory energy W_rot of 131595 J are assumed in the exemplary embodiment. The total energy W_Spaner_sum of translatory and rotatory energy is thus 136762 J. The proportion of translatory energy W_trans therefore corresponds to 3.78% and the proportion of rotatory energy W_rot corresponds to 96.22%.In the case of the assumed four clamping units 14 of the round table 10, a value of W_Spaner_bund / W_bund of 14.62% is thus obtained for the proportion of the dissipatable energy.As already described above, the translatory energy W_trans can be absorbed almost completely by the swivel clamp 17 without it failing. Thereby, the rotational energy can be built up at the collision point 15 by braking the motor of the rotary table 10 and by the friction between the tool 7 and the workpiece 12.FIG. 4 shows, in a force / pressure dissipation stroke diagram, the dissipation energy W D, which can be absorbed in the event of a crash as a result of the described design of the machine tool, without the risk of the workpiece 12 with the pallet 11 being released from the clamping units 14 or their swivel clamps 17. The piston force F(N) or the pressure p in the pressure chamber 36 is approximately constant over the piston travel s during the dissipation stroke, so that a very large dissipation energy can be absorbed without the clamping units 14 failing.

Claims

Machine tool for machining workpieces (12) with tools (7), having at least one round table (10) which has at least one clamping unit (14) with which a pallet (11) receiving the workpiece (12) to be machined can be clamped and which has a clamping element (18) which is firmly connected to a piston (27) which can be actuated by a pressure medium, preferably hydraulic medium, and adjusts the clamping element (18) between a release position and a clamping position, wherein in the clamping position the piston (27) is loaded by the pressure medium located in a pressure chamber (36) and a line (38) opens into the pressure chamber (36), in which line a check valve (42) is located and to which a first pressure limiting valve (46) is connected, characterized in that a second pressure limiting valve (43) is located in the line (38) between the check valve (42) and the pressure chamber (36), which is set to a pressure which is at least 1 1⁄2 times as high as the pressure of the first pressure limiting valve (46).Machine tool according to claim 1, characterised in that the first and / or the second pressure limiting valve (46, 43) are adjustable.Machine tool according to Claim 1 or 2, characterized in that a switching valve (41), preferably a 4 / 2-way valve, is connected upstream of the nonreturn valve (42).Machine tool according to one of Claims 1 to 3, characterized in that the clamping element (18) is rotatable about an axis (22) between the release position and the clamping position.Machine tool according to one of Claims 1 to 4, characterized in that the clamping element (18) is provided with a rotary device (20, 21).Machine tool according to claim 5, characterised in that the turning device has at least one helical groove (20), in which a guide part (21) on the rotary table side engages.Machine tool according to one of Claims 1 to 6, characterized in that, in the clamping position, the clamping element (18) is secured against rotation on the pallet (11).Machine tool according to Claim 7, characterized in that the clamping element (18) has at least one form-fitting element (48) which engages in at least one mating form-fitting element (49) of the pallet (11).

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