Dampers for milling machines

Abstract

A multi-axis milling machine comprises a spindle (3) arranged to receive a tool holder (3) and to rotate the tool holder in the spindle during use, wherein a portion of a spindle housing (11) surrounding the tool holder (3) is provided with one or more damping units (12A, 12B).

Description

Technical Field

[0001] The present invention relates to a processing apparatus, and in particular, but not exclusively, to a processing apparatus for processing superalloy components. Background Art

[0002] Superalloy materials are metal alloys that are extremely hard and durable and are able to withstand high temperatures and stresses. They are used in many applications, including the aerospace industry, where high-performance components often require such materials. These components also often have small geometric tolerances and therefore require careful machining using specialized hardened cutting tools and materials.

[0003] Complex geometries can be produced using a multi-axis machining center, such as a 5-axis machining center of the type manufactured by GROB-WERKE GmbH & Co. KG.

[0004] Multi-axis machining centers are typically computer numerically controlled (CNC) machining centers that allow highly precise control of the cutting head at the high speeds required to economically and accurately form components from superalloy materials. Cutting speeds can typically be in the range of 12,000 revolutions per minute of the cutting tool.

[0005] The inventors have determined that, using non-conventional modifications, the performance of such machinery can be improved in terms of precision, reliability and cutting performance. In addition, the arrangement described herein advantageously significantly reduces the occurrence of machine tool stop alarms. This can significantly increase productivity and reduce machine tool downtime. Summary of the invention

[0006] Various aspects of the invention are described herein.

[0007] Viewed from a first aspect, there is provided a multi-axis milling machine comprising a spindle arranged to receive a tool holder and, in use, rotate the tool holder within the spindle, wherein a portion of the spindle housing surrounding the tool holder is provided with one or more damping units.

[0008] Therefore, providing multiple tuned mass dampers in this configuration advantageously allows for control of vibrations that would otherwise propagate through the machine tool and tool holder. Each machine tool (e.g., a CNC multi-axis milling machine) has its own harmonic or natural frequencies, which are a result of the machine tool design and construction as well as the operating (cutting) conditions. During cutting operations, the natural harmonic frequencies can cause the machine tool to vibrate above a predetermined alarm threshold.

[0009] Each machine tool has its own harmonic "weak spot" in a certain direction or axis (i.e., a specific orientation). Since each machine tool is constructed differently, the exact axis of the harmonic "weak spot" will be different. The term "weak spot" is intended to refer to the axis and location where the natural harmonics of the machine tool are most prevalent, i.e., where the machine tool begins to vibrate excessively.

[0010] Conventionally, unwanted vibrations are addressed by providing the cutting tool holder and machine tool with sufficient mass and stiffness to prevent the generation of unwanted vibrations and harmonics.

[0011] Typically, when cutting superalloys with ceramic tools at high cutting speeds, vibrations are generated in the cutting head and / or the machine tool. These vibrations can exceed the safety limits of the machine tool and / or the cutting tip. Therefore, machine tool manufacturers have introduced vibration alarms that are activated at a predetermined level to stop the machine tool. This thereby prevents damage to the machine tool and / or the tool. The machine tool can then be reset and restarted.

[0012] Such safety systems in conventional machinery advantageously prevent damage to the machine tool or cutting tool, but significantly reduce productivity as the number of alarm activations increases.

[0013] The present invention significantly reduces unwanted alarm activations by identifying "weak" harmonic points on a machine tool and controlling and damping vibrations at or near the points where vibrations are most severe.

[0014] Cutting speeds for ceramic cutting tips can be very high. For example, cutting speeds may exceed 12,000 revolutions per minute. As a result, the natural harmonics of the machine tool can quickly reach the alarm threshold, causing the machine tool to stop operating. In nighttime machining where the operator sets the machine tool to operate overnight, an alarm early in the machining time can result in very low productivity due to excessive machine tool downtime.

[0015] In the arrangements described herein, the one or more damping units may be coupled to the spindle housing at a location corresponding to the location of a resonant frequency of the housing at a predetermined spindle operating speed.

[0016] For a given cutting speed (which is known for the part being machined and the materials involved), a position corresponding to a point at which the machine tool will vibrate resonantly at or near that frequency can be determined. The damping device described herein can then be positioned at or near that position. In addition, the damper can be arranged in line with or parallel to the predicted resonant vibration direction. Resonances that would normally occur for the cutting speed can be damped.

[0017] The or each damping unit may comprise an outer housing enclosing a damping mass, the damping mass being movable within and relative to the housing. Accommodating the damping mass within the housing and allowing the mass to move allows damping to occur. The delay in movement of the mass relative to the housing causes damping to occur.

[0018] The housing and damping mass may be in the form of a concentric cylinder and a surrounding (also cylindrical) housing. The concentric axes of the cylinder and housing may be arranged parallel to the axis along which the spindle housing resonates. The damper is thus arranged to dampen the resonant vibrations of the machine tool in the most effective manner.

[0019] Other cross sections of the damper mass and housing may also be used. For example, the housing and mass may be elliptical, allowing for a larger mass with a smaller vertical height. Likewise, a square or rectangular arrangement may be used. However, a cylindrical arrangement allows for convenient manufacturing and also allows for a more precise way of aligning the mass relative to the housing, making the damper itself dynamically stable.

[0020] The damping mass may be spaced apart from the surrounding housing by one or more elastic / elastomeric couplings.Importantly, the couplings must allow relative movement of the damping mass with respect to the housing to allow damping to occur.

[0021] The elastic coupling may be in the form of a plurality of elastomeric or rubber elements. These may be arranged evenly around the damping mass so as to be in contact with the damping mass on one side and with the housing on the other side. Conveniently, these elements may be in the form of O-ring seals located around the periphery of the damping mass. This allows for convenient mounting and alignment of the member. For example, the member may be arranged in a groove with the housing and / or the damping mass.

[0022] Depending on the vibration performance of the machine tool, a single damping unit may be all that is required to attenuate the resonant frequencies of the machine tool at the cutting frequency / cutting speed and load. Alternatively, one or more pairs of damping units may be arranged around the machine tool, for example on the spindle housing of a milling machine. This advantageously allows the damping mass to be distributed to allow the damper to be conveniently coupled to the body of the machine tool and avoid any problems with the damper body reducing the range of motion of the cutting head relative to the workpiece.

[0023] The dampers may conveniently be circumferentially positioned, and optionally equally spaced, around the spindle housing of the machine tool.

[0024] The elastomeric element or rubber element may be continuous, such as an O-ring seal. Portions of the inner surface of the end face of the damper or of the end face of the damping mass may optionally be provided with a recess aligned with the normal contact line of the elastomeric element and the end face. The term "end face" is intended to refer to the end of the damper which is aligned with the axis of the vibration which the damper with which it operates is attenuated. In a cylindrical arrangement this would be on the end face of a cylinder.

[0025] By providing a recess arranged to coincide with or overlap the normal contact line of the O-ring seal, the O-ring is effectively out of compressive contact with the housing and damper mass at that particular point or over the overlapping length of the recess. Relative motion of the damping mass with respect to the housing then does not compress the elastomer at these points. This reduces the total contact area of ​​the elastomer with the damper mass and housing. For a given input force, the contact area is smaller and the pressure on the remaining elastomer material in contact between the two surfaces is increased.

[0026] Advantageously, the increase in pressure causes the damper to operate (ie, begin damping) at lower input forces than if full contact of the elastomeric body were provided. The increased pressure initiates damping more quickly, thereby producing damping at lower input forces, such as associated with low levels of cutting forces for finishing.

[0027] Advantageously, the recess and the O-ring arrangement are located on either end of the damping unit, ie on the end of the damping unit facing the machine tool and on the opposite end of the damping unit.Vibrations along the damper axis towards the machine tool can then be effectively damped.

[0028] Viewed from another aspect there is provided a damping unit for a multi-axis machining centre arranged to be coupled to a spindle of the machining centre in use and comprising an outer housing enclosing a damping mass movable within and relative to the housing.

[0029] Viewed from yet another aspect, there is provided a multi-axis milling machine comprising a spindle arranged to receive a tool holder and, in use, rotate the tool holder within the spindle, wherein the spindle comprises a housing containing at least one pair of opposed damping units.

[0030] From a further aspect of the invention described herein, there is provided a machining centre comprising a spindle arranged to receive a tool holder and, in use, to rotate the tool holder within the spindle, wherein a portion of a spindle housing surrounding the tool holder is provided with at least one damping unit, the damping unit comprising a damping mass within the housing and spaced apart from the housing by a plurality of elastomeric portions, each elastomeric portion being in contact with the damping mass on a first side and in contact with the housing on an opposite side.

[0031] In such an arrangement, the housing and / or part of the damping mass may be provided with one or more recesses arranged along a contact line between the elastomeric part and the damping mass / housing so as to prevent contact between the elastomeric part and the damping mass / housing across the one or more recesses.

[0032] The recess may be formed on an end face of the damping mass and / or an opposing inner surface of the damping housing. Thus, the increased pressure effect of providing the recess acts only in the direction of the damper which is aligned with or parallel to the direction of primary desired damping.

[0033] Viewed from another aspect, a method for damping a milling machine is provided, the method comprising the steps of: (A) identifying one or more locations and directions of resonant vibrations at a frequency approximately equal to a cutting frequency of a machining operation; and (B) positioning a damper as described herein to the milling machine at the identified locations and directions.

[0034] In summary, the present invention provides in a first aspect a multi-axis milling machine comprising a spindle, which is arranged to receive a tool holder and to rotate the tool holder within the spindle during use, wherein one or more damping units are provided around a portion of a spindle housing of the tool holder, wherein the damping unit or each damping unit comprises an outer housing enclosing a damping mass, the damping mass being movable within and relative to the housing and being spaced apart from the housing by a plurality of elastomeric elements or rubber elements, wherein the elastomeric elements or rubber elements are continuous and portions of the inner surface of an end face of the housing and / or portions of an end face of the damping mass are provided with recesses aligned with a contact line between the elastomeric element and the end face.

[0035] In a second aspect, the present invention also provides a damping unit for a multi-axis machining center, the damping unit being arranged to be connected to the spindle of the machining center during use, and the damping unit comprising a damping mass, the damping mass being located in a shell and being separated from the shell by a plurality of continuous elastomeric portions or rubber elements, each elastomeric portion or rubber element being in contact with the damping mass on a first side and in contact with the shell on an opposite side, wherein the shell and / or a portion of the damping mass is provided with one or more recesses, the one or more recesses being formed on an end face of the damping mass and / or on a relative inner surface of the shell, and the one or more recesses being arranged along a contact line between the elastomeric portion or rubber element and the damping mass / shell so as to prevent contact between the elastomeric portion or rubber element and the damping mass / shell across the one or more recesses.

[0036] The present invention also provides a machining center in a third aspect, which includes a spindle, which is arranged to receive a tool holder and rotate the tool holder within the spindle during use, wherein at least one damping unit is arranged around a portion of the spindle housing of the tool holder, and the damping unit includes a damping mass, which is located within the housing and is separated from the housing by a plurality of continuous elastomeric portions or rubber elements, each elastomeric portion or rubber element being in contact with the damping mass on a first side and in contact with the housing on an opposite side, wherein the housing and / or a portion of the damping mass is provided with one or more recesses, the one or more recesses being formed on an end face of the damping mass and / or on an opposite inner surface of the housing, and the one or more recesses being arranged along a contact line between the elastomeric portion or rubber element and the damping mass / housing so as to prevent contact between the elastomeric portion or rubber element and the damping mass / housing across the one or more recesses.

[0037] The present invention further provides a method for damping a milling machine in a fourth aspect, comprising the following steps:

[0038] (A) identifying one or more locations and directions of resonant vibrations at a frequency approximately equal to a cutting frequency of a machining operation; and

[0039] (B) Positioning the damping unit according to the above second aspect to the milling machine at the identified position and orientation. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Various aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0041] Figure 1 A schematic diagram showing the main components of a computer numerically controlled (CNC) milling station or milling machine;

[0042] Figure 2 The spindle of such a milling machine is shown;

[0043] Figure 3 is a schematic diagram showing the natural frequencies of an exemplary spindle housing;

[0044] Figure 4A and Figure 4B An exemplary spindle including a pair of damping units is shown;

[0045] Figure 5A and Figure 5B An exemplary spindle is shown with and without a pair of damping units;

[0046] Figure 6 A cross section through one damping unit is shown;

[0047] 7A and 7B show a cross section through a damping unit and the positioning of an elastomeric seal, such as an O-ring seal or similar flexible / elastomeric seal;

[0048] Fig. 8A , Figure 8B and Figure 8C shows a further improved arrangement of the damper described herein; and

[0049] Fig. 9A , Fig. 9B and Fig. 9C An alternative configuration of the damping unit relative to the spindle housing is shown.

[0050] Although the present invention is susceptible to various modifications and alternative forms, specific embodiments are shown by way of example in the drawings and described in detail herein. However, it should be understood that the drawings and the accompanying detailed description are not intended to limit the invention to the particular form disclosed, but the invention will cover all modifications, equivalents and substitutes falling within the spirit and scope of the claimed invention.

[0051] It will be appreciated that the features of the aspects of the invention described herein may be used conveniently and interchangeably in any suitable combination.It will also be appreciated that the invention encompasses not only individual embodiments, but also combinations of embodiments that have been discussed herein. DETAILED DESCRIPTION

[0052] Figure 1 is a schematic diagram of a typical multi-axis machining center 1. The example shown is a milling machine comprising a cutting tool 2 coupled to a spindle 3. The spindle 3 contains a tool holder which holds the cutting tool at one end and provides a connection for coupling the tool holder and cutting tool to the spindle 3.

[0053] The spindle is coupled to a spindle drive 4 which is arranged to rotate the spindle (and thus the tool) at high speed. The drive 4 is also arranged so that it can rotate along Figure 1 The x-axis shown on the right moves the spindle and tool in the axial direction.

[0054] At the opposite end of the machining center (drive) is a chamber that contains the part to be machined (workpiece) 5, which is fixed to a movable table 6. The table 6 can be moved in a number of positions (vertical and rotational). In combination with the axial movement of the cutting spindle, the cutting tool can be moved in a number of positions. Figure 1 Each of the axes shown on the right moves relative to the workpiece.

[0055] The machining center includes a large number of sensors that accurately detect the positions of the table 6 and the cutting tool so that accurate machining can be performed during operation.

[0056] The table 6, spindle 3 and drive 4 each have a large mass to prevent excessive vibration during machining operations. Increasing the mass and rigidity of each of the components of the machining center reduces vibration during machining, thereby achieving precise machining. However, a vibration alarm 7 is provided, which deactivates the cutting (milling) operation if the vibration level exceeds a predetermined threshold. In this case, a vibration or "excessive movement" alarm is used, which detects milling machine vibration and compares it to a predetermined threshold or level. The vibration may increase beyond a predetermined level due to a variety of reasons, such as increased tool wear, increased hardness of a portion of the workpiece, or unfavorable cutting characteristics / conditions.

[0057] Figure 2 A simple cross section is shown through a spindle 3, a cutting tool 2 and a workpiece 5. The cutting tool 2 is connected to the spindle 3 by a tapered tool holder 8 which allows the cutting tool to be easily replaced within the spindle 3. During operation of the milling machine, as indicated by arrow 9, the spindle 3, tool holder 8 and tool 2 are rotated at speeds in excess of 12,000 revolutions per minute.

[0058] To allow for the machining of complex shapes and geometries, on some milling machines, the end of the spindle 3 closest to the cutting tool 2 (and thus the workpiece 5) may be tapered so that the tool can cut at an angle relative to the workpiece without the spindle or tool holder connecting or colliding with the workpiece. In other milling machines, no taper may be provided on the spindle. It should be understood that the invention described herein is equally applicable to milling machines with and without tapered heads.

[0059] Every machine tool has a number of natural frequencies. Those skilled in the art will appreciate that in the context of the present invention, a natural frequency is the frequency at which a part of a milling machine will vibrate in the absence of any forces exciting or damping vibration, i.e., it is the frequency at which the part naturally vibrates.

[0060] When a milling machine is caused to vibrate at or near its natural frequency, resonance may occur, which can cause the vibration alarm to be triggered.

[0061] The way a milling machine vibrates during use depends on two stiffness factors, namely:

[0062] (i) static stiffness; and

[0063] (ii) Dynamic stiffness.

[0064] During operation of a milling machine, the damping device described herein can advantageously affect dynamic stiffness. Specifically, when damping is applied in a specific manner as described herein, adding damping increases the dynamic stiffness of a milling machine (and / or a subcomponent of a milling machine).

[0065] All machine tools have a number of natural frequencies in their basic structure. Some of these natural frequencies can be destructive and cause excessive vibration during machining operations. Natural frequencies originate from each part or element of the machine tool. The different vibrations interact (both constructively and destructively) causing the overall vibration performance of a milling machine to be complex and specific to a given milling machine and, importantly, operating or cutting speeds and conditions. For example, different cutting speeds cause a milling machine to exhibit different vibration harmonics.

[0066] In one example, there is a need to improve the dynamic stability of a milling machine at a given cutting speed expressed in revolutions per minute (rpm). For example, for a ceramic cutting tip, this could be a cutting speed of 12,000 rpm and a frequency of 200 Hz.

[0067] By performing vibration analysis on each part of the milling machine using accelerometers and impact hammers, it is possible to confirm how each part of the milling machine vibrates, especially the natural frequency of each part.

[0068] Exemplary equipment that can be used to measure the vibration characteristics of a milling machine is as follows:

[0069] Accelerometer – Kistler Model 8776A50

[0070] Impact hammer – Kistler Model 9726A5000

[0071] The output from the accelerometer and the impact hammer can be transmitted to a conventional computer through an input / output interface of the type manufactured by National Instruments. An analysis of the data can then be formed using commercial software (such as CUTPRO). The vibration characteristics, i.e., the force / vibration characteristics, of each part of the milling machine can be plotted by moving the accelerometer and / or using multiple accelerometers to determine where the vibration occurs and its associated natural frequency. The dampers can then be positioned based on this determination in conjunction with the expected operating speed of the milling machine.

[0072] Specifically, a model can be built and the location and orientation of the component portion having the natural frequency that most closely matches the cutting frequency can be identified. This is referred to herein as the "weakest" portion of the milling machine, i.e., the portion of the milling machine that needs to be damped to prevent resonance at a given cutting speed and frequency (12,000 rpm and 200 Hz in one example).

[0073] Vibration analysis can be performed using conventional accelerometers and impact hammers. Accelerometers are distributed around the milling machine, and predetermined impacts are applied to the milling machine to induce vibrations in the milling machine. The output of the accelerometers can then be recorded, and the location of the milling machine subcomponent whose natural frequency is closest to the cutting frequency (i.e., rpm speed divided by 60) can be identified. Similarly, the accelerometer can be used to determine the direction in which this part of the milling machine is vibrating.

[0074] Therefore, using the accelerometer and the impact hammer it is possible to locate the part of the milling machine that vibrates close to the cutting frequency. This is therefore the part that may resonate during the cutting process, because the cutting frequency causes the resonance, which in turn causes this part of the milling machine to vibrate with increasing amplitude. This in turn causes the vibration alarm of the milling machine to be triggered.

[0075] In one exemplary embodiment, the milling machine has the weakest dynamic stability at 200 Hz in the X direction of the milling machine. The above vibration analysis confirms that a natural frequency of 200 Hz can be found at the milling machine spindle housing in the X direction.

[0076] Once the location and orientation of the "weakest" natural frequency is found, it defines the location and orientation where the Multi-Tuned Mass Damper (MTMD) should be located. In one example, this is the top of the spindle housing.

[0077] Figure 3A graph of frequency versus displacement and associated natural frequencies for an exemplary spindle housing is shown.

[0078] like Figure 3 As shown, the displacement on the y-axis is caused by the vibration of the milling machine. Figure 3 Three separate frequencies are shown.

[0079] The first frequency is a frequency M of about 100 Hz, which reflects the resonance of the machine base, ie the basic structure of the milling machine.

[0080] - The second frequency A is 200 Hz and represents the resonance of the spindle housing.

[0081] - The third frequency B is 1200 Hz and represents the spindle shaft frequency.

[0082] Figure 4A An end view of the spindle 3 is shown, which holds the tool holder 8 and the cutting tool 2 along its central axis. Also shown is the surrounding housing 11 supporting the spindle. Figure 4B , a side view of the housing 11 is shown, illustrating an optional tapering of the housing.

[0083] Back to Figure 4A , shows a modified housing arrangement according to the invention described herein. Specifically, a pair of opposing damping units 12A and 12B are shown. However, a pair of opposing dampers may not be required; for a given milling machine, a single damper may provide sufficient damping to achieve the desired result.

[0084] The damping units 12A and 12B are mounted to the side of the spindle housing 11 and, in the example shown, are arranged to be aligned with the axis 13 (the axis 13 passes through the center of the spindle and the cutting tool). The exact axis may vary depending on the milling machine and the results of vibration analysis, and may not always pass through the center of the spindle shaft. The damping units 12A and 12B are securely connected to the spindle housing, for example using a nut and bolt arrangement. They can advantageously be integrally formed into the spindle housing to form a single housing component containing the pair of opposing damping units or areas. They can also be retrofitted to the outer surface of an existing housing.

[0085] The damping units 12A and 12B are used to interact with the vibrations of the spindle housing passing through the spindle shaft. In effect, the damping units provide the machining centre with a multi-tuned mass damper which allows the machining vibrations to be controlled.

[0086] Now refer to Figure 5A , Figure 5B , Figure 6 7A and 7B describe the damping units 12A, 12B in more detail.

[0087] Figure 5Aand Figure 5B An example spindle housing with and without the pair of damping units 12A and 12B is shown, respectively. Figure 5B As shown, the pair of damping units are directly coupled to the side of the spindle housing 11 by bolt means. This arrangement can be easily retrofitted to the spindle housing.

[0088] Figure 6 Shown through Figure 5B A cross section of a damping unit 12B is shown.

[0089] The outer housing of the damping unit 12B is formed of three parts, defining a central cavity or space for receiving the damping mass. The first part 14 is arranged to be connected to the damping unit 12B. Figure 5B The spindle housing 11 is shown adjoining the two faces. The second part of the housing is the opposite face 15. A cylindrical body 16 extends between the two faces and may be coupled to each face by a flange extending around the periphery of the cylindrical portion 16. The flanges may conveniently be bolted on either side of the central body 16 to define a cavity within the housing and rigidly connect the housing to the spindle housing. The same arrangement may optionally be replicated on the opposite side of the spindle housing for the damping unit 12A.

[0090] The outer housing of the damping unit defines an inner cavity or chamber 17 as described with reference to Figures 7A and 7B.

[0091] The chamber contains a damping mass 18 contained within and surrounded by the housing. The damping mass is also spaced apart from the inner surface of the housing by a plurality of resilient / elastomeric members 19. The resilient members shown are in the form of elastomeric O-ring seals extending around the periphery of the damping mass. Other types of resilient spacers may also be used and may not be in the form of a continuous profile, but may be a plurality of discrete elastomeric sections that provide the same spacing effect as an O-ring seal.

[0092] As shown in Figures 7A and 7B, a first pair of O-ring seals are arranged on the end face of the cylindrical damping mass and a second pair is arranged on the outer peripheral surface of the damping mass. As shown in Figure 7B, two gaps Δx and Δy are thereby provided to space the damping mass from the housing.

[0093] During operation of the machining equipment, vibrations of the spindle housing induce vibrations of the housing of the damping unit. This in turn induces vibrations of the damping mass via the elastomeric O-ring connection, and this produces a damping effect because the movement of the damping mass relative to the damping unit is delayed when the elastomeric coupling compresses and expands asynchronously with the excitation frequency.

[0094] The exact damping level can be optimized by adjusting the gaps Δx and Δy as well as the size and material (durance or elasticity) of the O-ring material (as well as the specific mass of the damping mass itself).

[0095] In one example, the damping unit is configured to provide the desired dynamic stability and damping rate for a given milling machine design and expected machining speed. This defines the frequency of the required damping.

[0096] For example, for a milling machine manufactured by GROB (described above), the parameters determined include:

[0097] Damping mass: Density of each damper 19.0kg / dm 3 , the damping mass of each damper = 4.7kg (2 dampers).

[0098] Material elasticity: O-ring 70x3 Nitril

[0099] O-ring 80x3 Nitril.

[0100] A damping unit with these characteristics is then positioned on the milling machine at a location corresponding to the weakest dynamic stability location (described above). Importantly, this helps to increase the dynamic stability of the spindle housing (which has been identified by frequency analysis). The effect of the increased dynamic stability is that at the desired cutting speeds for superalloy materials, milling machine vibrations are significantly reduced. This in turn prevents those vibration levels where the vibration levels exceed the limits defined by the manufacturer and thereby prevents the milling machine from automatically shutting down (vibration alarm triggering). Furthermore, machining accuracy can also be improved by reducing vibrations and therefore the displacement of the cutting tool relative to the workpiece.

[0101] Advantageously, the performance of the damping unit described herein may be further optimized beyond the advantages described above. Fig. 8A , Figure 8B and Figure 8C This further improvement is shown.

[0102] In particular, the present inventors have determined that creating a discontinuity or interruption in the elastomeric coupling between the damping mass and the damper housing provides a surprising improvement in the dynamic performance of the damper.

[0103] In the above example, a continuous elastomeric O-ring seal is used in the damping arrangement. Therefore, there is a continuous line of contact between:

[0104] (i) O-ring and damper mass (in the damper cavity); and

[0105] (ii) O-ring and housing of the damper (around the perimeter of the damper mass).

[0106] In practice, the O-ring is sandwiched between the damper mass and the damper housing. Compression and expansion of the elastomer allows the damping described herein.

[0107] Fig. 8A The damping performance of the modified damper is shown, with the y-axis being the damping percentage and the x-axis being the input cutting force.

[0108] Fig. 8A Line A in shows the damping performance of the damper of the first embodiment as described above, i.e., a damper having a continuous elastomeric connection between the damping mass and the damper housing. As shown, the damping percentage increases slowly as the input force increases, reaches a peak value at about 1200 Newtons, and then gradually decreases as the input force increases and the damping effect decreases.

[0109] Line B shows the performance of a modified damper that includes a discontinuous connection of the elastomer to the damping mass and the housing. As shown by line B, the percentage of damping increases much faster than the conventional arrangement. As shown, at very low input forces, the percentage of damping increases, thereby providing damping at low processing loads. In addition, for all input loads, the percentage of damping remains higher than the conventional arrangement.

[0110] In operational terms this means that the modified damper can provide effective damping at both the low machining loads associated with finishing and the high machining loads associated with roughing.This wide range of damper performance provided by the damper arrangement described herein is very advantageous in machine tools.

[0111] Figure 8B and Figure 8C One arrangement for achieving these performance improvements is shown. In the example shown, a series of recesses are formed in the damper housing and / or damper mass, coinciding with the contact line of the O-ring seal. The recesses 20 are formed to disengage or disconnect the O-ring 19 from compressive contact with the damper mass 18 and the end plate 14. In effect, the recesses 20 disconnect the O-ring seal from the mass and housing of the damper. This effectively reduces the contact area of ​​the O-ring seal between the damper mass and the housing at either end of the damper. Reducing the contact area increases the pressure on the remaining portion of the O-ring that is in contact with the damper mass on one side and the housing on the other side.

[0112] The inventors have determined that using a recessed portion to reduce the contact area of ​​the elastomeric seal results in Fig. 8A The performance changes shown, namely further improved dynamic damping performance.

[0113] The specific properties of the damper can be selected based on the characteristics of the milling machine and the desired damping performance. For example, the contact area can be modified in combination with the elastomer hardness to achieve the desired performance. In one example, a Shore hardness between 70 and 90 Shore A can be used.

[0114] Figure 8CAn example is shown of how a recess may be incorporated into the end housing of a damper.

[0115] exist Figure 8C On the left hand side of the figure, the connection of the damper to the milling machine is shown and contains a number of (4 in this example) slots. The dotted line represents the normal contact line of the O-ring. Figure 8B As shown, the way in which the recess overlaps this contact line creates a space which results in a break away from the compression contact between the O-ring and the damping mass.

[0116] exist Figure 8C On the right hand side of the diagram, the connection of the damper to the damper end housing is shown. Here, for ease of manufacturing, four semicircles are machined on the end portion of the housing. Similarly, the dotted line represents the contact line of the O-ring overlapping the four semicircles.

[0117] It should be appreciated that the disengagement of the elastomeric contact line can be achieved in a variety of ways and is not limited to the two examples shown. For example, the O-ring itself can be modified to have a reduced thickness, or include recesses or cut-outs that produce the same effect. Similarly, instead of a single continuous O-ring or elastomeric member, multiple discrete elastomeric portions can be provided.

[0118] Figures 9A-9C Alternative arrangements of damping units relative to the spindle are shown. The dampers may be single dampers or dampers arranged in pairs or groups. Importantly, in each case the damping effect is arranged to improve the dynamic stability of the milling machine at the point (and direction) where resonance will occur during the expected cutting parameters.

[0119] like 9A to 9C As shown, the dampers can be arranged at various locations around the spindle housing.

[0120] It should be appreciated that the damping device and method described herein may be located anywhere on the milling machine, depending on natural frequency measurements and desired cutting speeds / frequencies. For a particular 5-axis milling machine, it has been determined that the best location may be on the spindle housing itself as described above.

Claims

1. A multi-axis milling machine comprising a spindle arranged to receive a tool holder and, in use, to rotate the tool holder within the spindle, wherein: One or more damping units are arranged around a portion of the spindle housing of the tool holder, wherein the one or more damping units include an outer housing surrounding a damping mass, the damping mass is movable within and relative to the outer housing, and is spaced apart from the outer housing by a plurality of elastomeric elements or rubber elements, wherein the elastomeric elements or rubber elements are continuous, and a portion of an inner surface of an end face of the outer housing and / or a portion of an end face of the damping mass is provided with a recess aligned with a contact line between the elastomeric element and the end face.

2. The multi-axis milling machine according to claim 1, wherein: The one or more damping units are coupled to the spindle housing at a position corresponding to a position of a resonant frequency of the spindle housing at a predetermined spindle operating speed.

3. The multi-axis milling machine according to claim 1, wherein: The outer housing and damping mass are in the form of concentric cylinders and surrounding housings, and wherein the concentric axes of the concentric cylinders and surrounding housings are arranged parallel to the axis along which the spindle housing resonates.

4. The multi-axis milling machine according to claim 1, wherein: The elastomeric or rubber element is in the form of an O-ring seal positioned about the periphery of the damping mass.

5. A multi-axis milling machine according to any one of claims 1 to 4, comprising one or more pairs of damping units spaced circumferentially around the spindle.

6. A damping unit for a multi-axis machining centre, the damping unit being arranged to be coupled to a spindle of the machining centre in use and comprising a damping mass located within an outer housing and spaced apart from the outer housing by a plurality of continuous elastomeric portions or rubber elements, each elastomeric portion or rubber element being in contact with the damping mass on a first side and in contact with the outer housing on an opposite side, wherein The outer shell and / or part of the damping mass are provided with one or more recesses, the one or more recesses being formed on an end face of the damping mass and / or on an opposite inner surface of the outer shell, and the one or more recesses being arranged along a contact line between the elastomeric part or rubber element and the damping mass / outer shell so as to prevent contact between the elastomeric part or rubber element and the damping mass / outer shell across the one or more recesses.

7. A machining centre comprising a spindle arranged to receive a tool holder and, in use, to rotate the tool holder within the spindle, wherein: At least one damping unit is arranged around a portion of the spindle housing of the tool holder, the damping unit comprising a damping mass, the damping mass being located in an outer housing and being spaced apart from the outer housing by a plurality of continuous elastomeric portions or rubber elements, each elastomeric portion or rubber element being in contact with the damping mass on a first side and in contact with the outer housing on an opposite side, wherein the outer housing and / or a portion of the damping mass is provided with one or more recesses, the one or more recesses being formed on an end face of the damping mass and / or on an opposite inner surface of the outer housing, and the one or more recesses being arranged along a contact line between the elastomeric portion or rubber element and the damping mass / outer housing so as to prevent contact between the elastomeric portion or rubber element and the damping mass / outer housing across the one or more recesses.

8. A method for damping a milling machine, comprising the following steps: (A) identifying one or more locations and directions of resonant vibrations at a frequency approximately equal to a cutting frequency of a machining operation; and (B) Positioning the damping unit according to claim 6 to the milling machine at the identified position and orientation.

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