Cutting tool with sensor

Abstract

A cutting tool (1) comprises a tool body (2) having at least one holder (3) for receiving a tool holder (4) or a cutting insert (5), where the holder (3) comprises at least one support surface (6). The cutting tool body (2) comprises a sensor retention structure (7) adjacent to the support surface (7) and a piezoelectric sensor (8) arranged in the sensor retention structure (7). According to the invention, the four-flat market cutting tool (1) comprises a clamping device (9) for fixing the tool holder (4) or the cutting insert (5), and the piezoelectric sensor (8) is an elastically deformable piezoelectric sensor (8) having a central longitudinal axis (24) and a sensitivity direction perpendicular to the central longitudinal axis (24). The piezoelectric sensor (8) comprises a measuring section (10) along which the central longitudinal axis (24) is substantially parallel to the support surface (6). The piezoelectric sensor (8) is elastically deformed in the sensor retaining structure (7) into a pre-stressed initial state (19) and can be deformed into a plurality of further stressed measuring states (21).

Description

Technical Field

[0001] The present invention relates to a cutting tool, and more particularly, to a cutting tool equipped with a sensor. Background Art

[0002] Nowadays, there is a general trend towards increased monitoring of metal cutting processes. One way to do this monitoring is to place sensors on or inside the cutting tool. These sensors can be of various types, such as sensors for measuring temperature, strain, vibration, etc. The data from these sensors can be used to monitor the state of the cutting process and the state of the tool, and if the sensors are connected to the numerical control system of the machine tool, interventions can also be made in the cutting process to improve the quality of the manufactured part or avoid failures. An example is the measurement of cutting forces during a metal cutting operation. Changes in cutting forces can be caused by vibrations, wear or even failure of the cutting tool. Such cutting force data can provide important information about the cutting operation. Tools equipped with sensors are significantly more expensive than traditional tools, and it is also difficult to integrate sensors into cutting tools without affecting their performance. Sensors can weaken the tool body, for example reducing rigidity, or reduce the accuracy of the tool.

[0003] A type of sensor that is more common in various fields, but less commonly used in cutting tools, is the so-called piezoelectric sensor. This type of sensor is based on the piezoelectric effect, i.e. the accumulation of electrical charge in some solid materials in response to mechanical stress.

[0004] In EP1984182, a cutting tool using a piezoelectric sensor is disclosed. The piezoelectric sensor is used to measure the cutting force during operation of the cutting tool. The problem with this solution is that the cutting tool is expensive and complex, and also not so reliable and accurate.

[0005] These deficiencies have led to a need for a tougher, more reliable solution that can provide a cutting tool with greater reliability and cutting accuracy, and that is less complex and less costly. Summary of the invention

[0006] It is an object of the present invention to address at least some of the above problems and issues. It is an object of an embodiment of the present invention to provide a tougher cutting tool including a piezoelectric sensor. Another object of the present invention is to provide a reliable cutting tool including a piezoelectric sensor. Yet another object of the present invention is to provide a more economical cutting tool including a piezoelectric sensor, both because it is cheaper to manufacture and because it is easier to maintain. Yet another object is to provide a cutting tool equipped with a piezoelectric sensor, which deviates as little as possible in structure and function from a standard non-sensored tool and is therefore more accurate.

[0007] According to the present invention, at least one of the above-mentioned objectives is achieved by a cutting tool, which includes a tool body, the tool body having at least one tool seat, the at least one tool seat being used to receive a tool holder or a cutting blade, wherein the tool seat includes at least one support surface. The cutting tool body includes a sensor retaining structure adjacent to the support surface; and a piezoelectric sensor arranged in the sensor retaining structure. The cutting tool also includes a clamping device for fixing the tool holder or the cutting blade, and the piezoelectric sensor is an elastically deformable piezoelectric sensor having a central longitudinal axis and a sensitivity direction perpendicular to the central longitudinal axis. The piezoelectric sensor includes a measuring section, along which the central longitudinal axis is substantially parallel to the support surface. The piezoelectric sensor is elastically deformed in the sensor retaining structure to a prestressed initial state, and is capable of deforming to a plurality of additional stressed measurement states.

[0008] In the prior art, when designing a cutting tool incorporating a piezoelectric sensor, it is necessary to use a very robust sensor in order to avoid excessive deformation or even breakage during machining, as this would mean a loss of precision in the cutting process. The risk of expensive, rigid, crystalline, piezoelectric sensors is that they wear out faster than the tool itself, which also affects the performance of the tool and can lead to costly failures or repairs. In order to ensure the correct positioning of the cutting blade, the required geometric tolerances of the sensors in the prior art are very high. A damaged sensor also means that the tool itself can no longer operate. In addition, the piezoelectric sensor must be specifically designed for various combinations of tool and blade sizes. All these factors make the cutting tool less precise, less reliable and more expensive.

[0009] Due to the sensor retention structure adjacent to the support surface and the prestressing of the elastically deformable piezoelectric sensor to an initial state, high forces on the sensor can be avoided, allowing the use of inexpensive universal sensors instead. Mechanical stresses caused by the cutting forces on the tool body deform the sensor retention structure and thereby the sensor, which are registered by the sensor. Due to the prestressed initial state of the elastically deformable piezoelectric sensor, the sensor will detect stresses caused by compression tension or shear in the sensor retention structure, i.e. a plurality of additional stress states of the sensor, within the preferred measuring range of the sensor. Thus, the above-mentioned problems of previous solutions can be avoided.

[0010] According to the present invention, the cutting tool is preferably used in a CNC machine tool, i.e., a computer numerical control machine tool, or a similar machine tool, and the cutting tool may be a milling tool, a turning tool, a drilling tool, or another cutting tool using a tool holder or a cutting insert. The cutting tool is preferably used to machine a workpiece, most commonly a metal workpiece, but may also be a workpiece formed of fiber, ceramic, polymer or composite materials and other similar materials.

[0011] In the present invention, the cutting tool comprises a cutting tool body which is preferably made of a high-strength material such as steel, aluminum or titanium.

[0012] The cutting tool body comprises at least one seat for receiving a toolholder or a cutting insert. A toolholder, also called a sleeve, is a removable part of the tool body and comprises at least one insert seat. It is usually used to facilitate the replacement of multiple cutting inserts at one time, or to allow the replacement of damaged insert seats without replacing the entire cutting tool body. In some cases, they are used to adjust the position of the insert in the tool body. If the cutting insert is received directly by the tool body without a toolholder, the seat is a insert seat.

[0013] The tool holder comprises at least one supporting surface. According to the previous description, the supporting surface is a tool holder supporting surface or a blade supporting surface for accurately holding the tool holder or the cutting blade. The cutting tool also comprises a clamping device, which is used to fix the tool holder or the cutting blade by pressing the tool holder or the cutting blade into the tool holder and the at least one supporting surface to ensure its precise positioning. The use of more than one supporting surface allows the fixing of the tool holder or the cutting blade to be improved by supporting the tool holder or the cutting blade in more than one direction. The at least one supporting surface is advantageously arranged in the tool body so that it absorbs the main cutting force or one or more main components of the cutting force in the best possible way. This means that, compared with the direction of the cutting force or one or more main components of the cutting force, the at least one supporting surface is preferably facing in the opposite or substantially opposite direction. The main component of the cutting force can be, for example, a tangential component or a radial component, and the main cutting force can be the resultant of these cutting force components.

[0014] In the cutting tool body, the sensor retaining structure is adjacent to at least one supporting surface. In this context, "adjacent" means that the sensor retaining structure is intersected by the plane of the supporting surface, or it is in close proximity to the supporting surface in the cutting tool body. The sensor retaining structure can be a channel in the tool body. Preferably, the sensor is arranged in the sensor retaining structure less than 3mm or more preferably less than 2mm from the supporting surface. Even more preferably, the sensor retaining structure is within 1mm from the supporting surface. Most preferably, the sensor retaining structure is within 0mm from the supporting surface, i.e. the sensor retaining structure is intersected by the plane of the blade supporting surface, i.e. the retaining structure is an open channel in the supporting surface, i.e. a groove. The sensor retaining structure can also continue to extend away from the supporting surface, either as a closed channel or an open channel, so that the piezoelectric sensor can be connected to a signal processing circuit or a connection point arranged in another component of the cutting tool.

[0015] In one embodiment of the invention, the seat comprises more than one support surface. Preferably, each of these support surfaces faces in a direction opposite to the direction of the main cutting force component, such as a tangential support surface and a radial support surface.

[0016] In one embodiment of the present invention, the sensor retention structure may be adjacent to more than one support surface in each seat.

[0017] According to one embodiment, the cutting tool may include a plurality of seats, each seat having at least one support surface.The sensor retention structure may be adjacent to one or more support surfaces.

[0018] The cutting tool also includes a piezoelectric sensor arranged in or retained in or assembled in the sensor retention structure. The piezoelectric sensor is an elastically deformable piezoelectric sensor having a central longitudinal axis and a main sensitivity direction perpendicular or substantially perpendicular to the central longitudinal axis. Elastically deformable (also called flexible, bendable) piezoelectric sensors are typically used in more general applications, such as intrusion detection, detecting rain or hail, monitoring patients in hospital beds, etc. These sensors are different from those sensors typically used in cutting tools because they are not enclosed in a rigid housing. Therefore, they can be arranged, for example, to follow the perimeter of an arbitrary object.

[0019] The main sensitivity direction of a piezoelectric sensor is the main direction in which the sensor is designed to measure force or is optimized to measure force. Although piezoelectric materials generate charge regardless of the direction of stress, the sensor can be constructed so that it will not record stress in some directions, or only record stress in a poor way. Factors that will affect the sensitivity direction are, for example, the geometric properties of the piezoelectric material, the properties of the housing of the sensor, the orientation of the piezoelectric material relative to the housing, etc. Some sensors can be omnidirectional, that is, they measure stress equally well in all directions, in which case the main sensitivity direction is all directions. In the present invention, the main sensitivity direction is at least in a direction perpendicular to or substantially perpendicular to the central longitudinal axis of the sensor.

[0020] The piezoelectric sensor further comprises a measuring section along which a central longitudinal axis of the piezoelectric sensor is parallel or substantially parallel to the support surface and along which the piezoelectric sensor is elastically deformable in the retention structure to a prestressed initial state and is capable of deforming to a plurality of additional stressed measurement states. The measuring section may be the entire length of the sensor or, preferably, only a portion of a longer sensor, such as a sensor cable connected to a signal processing circuit, wherein the measuring section is only in that portion of the sensor adjacent to the support surface. The presence of a prestressed initial state of the piezoelectric sensor, wherein the piezoelectric sensor is adjacent to the support surface, and the absence of the prestressed initial state when stress measurement is not required, enables the sensor to be configured to measure only in a preferred area.

[0021] Thus, the measurement segment is a segment of the piezoelectric sensor along which the main measurement will be performed. Along this measurement segment, the central longitudinal axis of the piezoelectric sensor and the central longitudinal axis of the sensor retaining structure are parallel. Although the piezoelectric sensor can detect forces in other segments of the piezoelectric sensor (i.e., those segments of the sensor retaining structure that are not adjacent to the support surface and are not subjected to force), these may (but not necessarily) be regarded as noise in the measurement signal. Since those segments of the piezoelectric sensor that do not require stress measurement are not pre-stressed, the signals from these segments are much weaker. If necessary, the technician can filter out this noise using signal processing methods known in the art. Along the measurement segment, the central longitudinal axis is parallel to or substantially parallel to the support surface, and the sensitivity direction is perpendicular to or substantially perpendicular to the support surface. In other words, the sensitivity plane is perpendicular to or substantially perpendicular to the support surface. Therefore, the sensor responds in all radial directions or substantially radial directions along the central longitudinal axis in the sensitivity plane.

[0022] Due to the sensor retention structure, the piezoelectric sensor is protected from full clamping forces and is also protected from full cutting forces during use. The sensor retention structure, or in embodiments with an open channel in the support surface, the retention structure together with the toolholder or cutting insert also protects the piezoelectric sensor from wear, such as chip wear or cutting fluid wear.

[0023] The prestressed initial state is the state of the piezoelectric sensor in which the piezoelectric sensor reaches a level such that the additional stress on the sensor retaining structure and thus on the piezoelectric sensor will generate a preferred charge. Due to the characteristics of the piezoelectric material, the piezoelectric sensor will only generate a charge when the stress (i.e. the cutting force) changes. Therefore, this prestressed initial state will not generate a constant current from the piezoelectric sensor.

[0024] When used in a cutting process, the cutting tool may be subjected to slight deformation due to the cutting force. As part of the cutting tool body, the sensor retaining structure will also deform. For example, the shape or size of the sensor retaining structure (such as a channel in the tool body) may change when viewed in a cross section perpendicular to the central longitudinal axis. The sensor retaining structure preferably has a cross section such that when the piezoelectric sensor is arranged in the sensor retaining structure, the sensor retaining structure at least partially surrounds the piezoelectric sensor, thereby generating a stress field and achieving the initial state of the prestress. Here, "partially surround" means that when the piezoelectric sensor is arranged in the sensor retaining structure, the piezoelectric sensor touches the sensor retaining structure at least at two points when viewed from a cross section perpendicular to the central longitudinal axis of the piezoelectric sensor. In an embodiment in which the sensor retaining structure is intersected by a support surface plane (i.e., it is an open channel), one of these points may be located on the tool holder or cutting blade, so that when the tool holder or blade holder is clamped in place, there are at least two points in total pressing on the piezoelectric sensor, as seen in the cross-sectional view. During the cutting process, the changing cutting forces will generate changing stresses in the tool body and the sensor retaining structure, resulting in changing deformations, which will deform the piezoelectric sensor to multiple force measurement states and generate changing output signals from the piezoelectric sensor. Preferably, the signal is processed and interpreted in suitable circuits, such as charge amplifiers and suitable telemetry or wireless sensor nodes.

[0025] Preferably, the cross section of the piezoelectric sensor and the cross section of the retaining structure are adapted to each other. The cross section of the piezoelectric sensor is preferably circular or substantially circular, but other shapes are also possible, such as elliptical, polygonal, etc. Therefore, in order to achieve a good adaptation to the piezoelectric sensor, the cross section of the sensor retaining structure should preferably be rectangular, square, u-shaped, curved with one radius or curved with several radii, depending on the exact shape of the cross section of the piezoelectric sensor, so as to fully surround the piezoelectric sensor. The purpose of the adaptation of the sensor retaining structure and the piezoelectric sensor or vice versa is to enable a preferred prestressed initial state to be achieved.

[0026] Due to the combination of an elastically deformable piezoelectric sensor and a prestressed initial state, the tolerances of the sensor retention structure can be much looser than would have to be in the prior art, which makes manufacturing easier and cheaper.

[0027] In one embodiment of the invention, the cutting tool comprises a shim, which is a plate-like structure providing a support surface. The shim is placed in the seat and is therefore a replaceable component of the cutting tool body. The shim protects the cutting tool body from damage that can occur during the cutting process. If a malfunction occurs during the cutting process, such as a breakage of the cutting blade, the shim may be damaged, but other parts of the cutting tool body will not be damaged. The shim can then be replaced and the cutting tool can be used again. In one embodiment of the invention, the sensor retention structure is obtained in the shim.

[0028] In one embodiment, the sensor retention structure is intersected by the support surface. In other words, the sensor retention structure is a channel, groove or recess in the cutting tool body that intersects the plane of the support surface. In this embodiment, the sensor retention structure can be easily machined in the cutting tool body using, for example, a milling tool. As has been previously described, the shape of the cross-section of the groove parallel to the support surface can vary. The tool used in manufacturing can affect this shape. For example, if a milling tool is used, the cross-section of the groove can be square, rectangular or U-shaped and machined in one simple operation. The positioning of the sensor retention structure adjacent to the support surface can be selected to facilitate continued extension of the retention structure, or to make the retention structure where there is more space.

[0029] In one embodiment, the sensor retaining structure is embedded in the cutting tool body in close proximity to the support surface. For example, the sensor retaining structure can be in the form of a drilled hole in the tool body, spaced a certain distance from the support surface. As previously mentioned, the distance can be greater than 0 and less than or equal to 3 mm, or greater than 0 and less than or equal to 2 mm, or greater than 0 and less than or equal to 1 mm. The stress in the cutting tool body will vary with the distance from the support surface, and the output of the piezoelectric sensor will also vary accordingly. This must be taken into account when processing the signal. In this embodiment, the initial state of prestressing can be achieved by press-fitting the piezoelectric sensor into the sensor retaining structure. If the force to be measured is very large, the stress on the sensor may exceed its stress limit. In this case, the distance between the sensor retaining structure and the support surface can be increased. If necessary, it can also be increased to a distance greater than 3 mm.

[0030] In one embodiment, the initial state of prestressing is achieved by a clamping device. The clamping device can be used to fix the tool holder or cutting blade to the cutting tool body. Preferably, the clamping device is a screw connection (i.e., a threaded hole in the cutting tool body that intersects the support surface, and a screw), or a top clamp (i.e., a threaded hole in the cutting tool body that does not intersect the support surface, a clamp and a screw). In this embodiment, the sensor retaining structure intersects the support surface plane, i.e., a groove in the tool body that intersects the support surface plane. The piezoelectric sensor is arranged in the sensor retaining structure. The size of the sensor retaining structure is adjusted so that when the tool holder or cutting blade is clamped into the tool holder and the at least one support surface, the piezoelectric sensor will be pressed into the sensor retaining structure and deformed to the initial state of prestressing. Preferably, the recommended clamping force (which can be expressed as the torque required on the clamping device screw to fix the tool holder or blade) is the same as the force required to achieve the initial state of prestressing of the piezoelectric sensor.

[0031] In one embodiment of the invention, the piezoelectric sensor includes a measurement range. This may be a range between two stress levels. Within this range, the relationship between the stress in the sensor and the accumulated current can be described by a strictly monotonic function. In other words, an increase in stress level will result in an increase in charge. In a more specific embodiment, the strictly monotonic function is an increasing linear function.

[0032] In one embodiment, the initial state of prestressing causes the sensor to reach above the lower end point of the measuring range. This means that any further stress on the sensor will be within the measuring range, thereby simplifying signal processing. Preferably, the prestressing causes the piezoelectric sensor to be above the lower end point of its measuring range. More preferably, the prestress is just above the lower end point of the measuring range. This saves as much measuring range as possible for additional stress. If the prestress is too large, the stress generated by the cutting force will exceed the upper end point of the measuring range, or even worse, exceed the stress that the piezoelectric sensor can withstand without breaking. When a load (such as a cutting force) is applied to the cutting tool, a sufficient level of prestress can be confirmed by studying the sensor output, as it will be manifested within the preferred measuring range according to a function describing the relationship between the stress in the sensor and the accumulated current.

[0033] In one embodiment of the present invention, the piezoelectric sensor is a piezoelectric sensor cable. Preferably, it is a coaxial cable having, starting from the center, a central core, a piezoelectric material around the central core, a metal braid around the piezoelectric material, and finally an outer sheath. Preferably, the piezoelectric material is a copolymer. Advantageously, the piezoelectric sensor cable is a so-called annular sensor, which means that it is not made in discrete pieces, sensor by sensor. Instead, the cable can be obtained in large quantities, for example by the meter on a cable roll, and cut into suitable lengths for specific uses. This greatly reduces costs. The piezoelectric sensor cable also acts as its own signal conductor, so that the signal can be transmitted to other circuits in or outside the cutting tool without the need for a separate cable as in the prior art. This reduces the number of potential failure points because there are fewer joints.

[0034] In one embodiment, the cutting tool includes a temperature sensor. Preferably, the temperature sensor can be placed in the tool body so that it can accurately measure the temperature around the piezoelectric sensor. Using a temperature sensor in this way has advantages. One reason is that the temperature around the sensor can be used to calculate thermal effects on the sensor, such as thermal expansion of the sensor retaining structure and / or the piezoelectric sensor itself. Using this information, these thermal effects can be filtered out of the signal. Another advantage is that the temperature can be monitored so that the temperature limit of the piezoelectric sensor is not exceeded. Preferably, the temperature sensor is a thermistor. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In the following, exemplary embodiments will be described in more detail with reference to the accompanying drawings, in which:

[0036] Figures 1a to 1d is a view of a turning tool, showing a first embodiment of the present invention;

[0037] Figures 2a to 2b is a view of a turning tool showing a second embodiment of the present invention;

[0038] Figures 3a to 3c is a view of a milling cutter showing a third embodiment of the present invention;

[0039] Figures 4a to 4c is a view of a milling cutter showing a fourth embodiment of the present invention;

[0040] Figure 5 is a view of a piezoelectric sensor cable used in the aforementioned embodiment;

[0041] Figures 6a to 6d is a cross-sectional view of a cutting tool showing the preload used in the aforementioned embodiments. DETAILED DESCRIPTION

[0042] Reference Figures 1a to 1d , describes a first embodiment of a cutting tool equipped with a sensor. The first embodiment of the invention is in the form of a turning tool. The cutting tool (1) comprises a cutting tool body (2) having a seat (3) for receiving a cutting insert (5), wherein the seat (3) comprises three support surfaces (6). The cutting tool (1) further comprises a sensor retaining structure (7) adjacent to one of the support surfaces (6), and a piezoelectric sensor (8) arranged in the sensor retaining structure (7). The cutting tool (1) comprises a clamping device (9) for fixing the cutting insert (5). The piezoelectric sensor is an elastically deformable piezoelectric sensor (8) having a central longitudinal axis (24) and a main sensitivity direction perpendicular to the central longitudinal axis (24). The piezoelectric sensor comprises a measuring section (10), along which the central longitudinal axis (24) is substantially parallel to the support surface (6), and the piezoelectric sensor (8) is elastically deformed in the sensor retaining structure (7) to a prestressed initial state (19), and can be deformed to a plurality of further stressed measurement states (21). The prestressed initial state (19) is achieved by clamping the blade with a clamping device (9), pressing the blade into the blade seat (3) with a clamping force (20) and pressing it against the support surface (6), thereby pressing the piezoelectric sensor (8) into the sensor retaining structure (7).

[0043] Reference Figures 2a to 2b , describes a second embodiment of the present invention. This embodiment is also a turning tool, but has some differences compared to the first embodiment. Here, the support surface 6 and the sensor retaining structure (7) are formed in the gasket (17). The gasket (17) serves as a replaceable support surface (6) for the cutting blade (5). The cutting tool (1) according to the second embodiment also includes a temperature sensor (23) arranged in the tool body (2). As in the previous embodiment, the temperature sensor (23) and the piezoelectric sensor (8) can be connected to the circuit (11) and the power supply (12). The clamping device (9) is here in the form of a top clamp and a screw connection. The initial state (19) of the prestressing is achieved in the following way: the blade is clamped with the top clamp (9), thereby pressing the gasket (17) and the cutting blade (5) into the tool seat (3) with the clamping force (20). The piezoelectric sensor is pressed into the sensor retaining structure (7) on the gasket (17) by the cutting blade (5). In an alternative embodiment, similarly, the sensor retention structure (7) may be adjacent to a support surface on the tool body (2) in the tool seat (3) for receiving the shim (17).

[0044] Reference Figures 3a to 3c, a third embodiment of the invention is described. The cutting tool is a milling tool comprising six tool seats (3), each tool seat having three support surfaces (6). Here again, a shim (17) is used, but the sensor retaining structure (7) is adjacent to the support surface (6) that supports the blade via the shim (17), rather than adjacent to the shim itself. The cutting tool (1) has significant tangential cutting forces (22) during machine operation, so the piezoelectric sensor (8) is arranged in the sensor retaining structure (7) adjacent to the support surface (6), which faces substantially opposite to the direction of the tangential force. In this embodiment, the main cutting force is in the tangential direction because this is a rotating cutting tool. The support surface (6) adjacent to the sensor retaining structure (7) faces substantially in the direction of rotation, i.e. opposite to the direction of the main cutting force (22). However, the blade (5) is clamped by a clamping force (20) in a substantially radial direction relative to the axis of rotation of the cutting tool.

[0045] Reference Figures 4a to 4c , a fourth embodiment is described. This embodiment is a milling tool comprising eight tool seats (3) for receiving tool holders (4). The tool holder (4) is a detachable component of the cutting tool body (2). Each tool holder (4) comprises a tool seat for receiving a cutting blade (5). According to this embodiment, the sensor retaining structure (7) is adjacent to the support surface (6) in the tool seat (3) for receiving the tool holder (4). However, in an alternative embodiment, similarly, the sensor retaining structure (7) can be adjacent to the tool seat (3) for receiving the cutting blade (5), or in a further alternative embodiment, the sensor retaining structure can be adjacent to some or all of the support surfaces of both the tool holder (4) and the blade (5) tool seat (3).

[0046] Reference Figure 5, describes a piezoelectric sensor (8) used in the above embodiments. The piezoelectric sensor (8) is elastically deformable and is in the form of a cable. It comprises an outer sheath (16), a metal braid (15), a piezoelectric copolymer (14) and a central core (13). The outer sheath (16) makes the sensor less accurate in the axial direction because it resists axial tension and compression. However, in the radial direction, the outer sheath (16) is easily deformable and the sensor (8) therefore has a main sensitivity direction in all directions perpendicular to the central longitudinal axis (24), i.e. the radial direction. The cable (8) used in all the above embodiments has a diameter of 2.7 mm and a circular cross section. The sensor is arranged in a sensor retaining structure (7) in all the above embodiments, which has a square or nearly square cross section when viewed in a direction perpendicular to the central longitudinal axis (24). In order to achieve the preferred prestressed initial state (19), the side length of the square cross section of the sensor retaining structure is 2.6 mm. During the machining operation, as the cutting force (22) causes the cutting tool body (2) to elastically deform, the sensor retaining structure (7) deforms accordingly, thereby pressing on the cable (8).

[0047] According to an embodiment, the measuring range of the piezoelectric sensor (8) has a relationship between the stress on the piezoelectric sensor (8) and the charge generated in the sensor (8) defined by an increasing linear function. This means that a greater stress always generates a greater charge, and that a double stress generates a double charge. An increasing linear function is a subset of a strictly increasing monotonic function.

[0048] The cutting tool (1) according to this embodiment comprises a clamping device (9) in the form of a screw connection for clamping the tool holder (4) or the cutting blade (5), the second embodiment being combined with a top clamp. As is common in the industry, there is usually a recommended torque for the clamping device (9) screw recommended by the manufacturer to firmly clamp the tool holder (4) or the cutting blade (5). For example, in the first and second embodiments of the present invention, the torque is 5.0 Nm and 6.4 Nm respectively. The clamping force achieved by the torque on the clamping device screw (9) fixes the blade in the tool holder by pressing the blade into the supporting surface and also generates a stress field in the sensor that is higher than 4 MPa, which is the lower end point of the measuring range of this particular sensor (8). Due to this preload (19), the sensor (8) detects all cutting forces (22) within the measuring range.

[0049] The cutting tool according to the embodiment is not used for machining operations that produce stress fields in the piezoelectric sensor above 37MPa, which is the upper end of the measurement range. Although the cutting tool can be used above this range, the signal processing will have to be adjusted for the potential nonlinear relationship between stress and charge.

[0050] The cutting force applied to the cutting blade will cause an increase in stress in the tool body and thus on the piezoelectric sensor, and the piezoelectric sensor will generate a charge. In the cutting process, the cutting force varies greatly due to cutting data (such as feed amount, cutting depth, material, etc.). For example, in the cutting process using the first embodiment of the present invention, the average cutting force is about 800N. The change of the cutting force around the average level produces a change of about + / -125mV in the output voltage of the charge amplifier, or a charge change of about 2pC at the sensor.

[0051] In other cutting processes, the cutting forces may vary over a larger range, such as 1-10 kN. For the largest cutting forces, the sensor signal may indicate that the stress field in the sensor has exceeded the upper limit of the measurement range and that the signal is no longer linearly related to the stress. If this is the case, the sensor retention structure may be adjusted for this situation, such as by widening the sensor retention structure relative to the sensor, thereby reducing the stress in the sensor for a given cutting force.

[0052] One particularly beneficial application of the present invention is for milling operations (shown in the third and fourth embodiments) because the cutting insert has interrupted or intermittent cutting and a continuous pass in and out of the workpiece. The same applies to turning processes with interrupted surfaces, such as a cylinder including grooves in the lateral surface. Interrupted cutting is more difficult for the cutting tool because the variation in cutting forces is greater, so the benefit of using a tool equipped with a sensor is also greater.

[0053] To measure the cutting force, an integrator is preferably used to convert the charge generated in the piezoelectric sensor, which is proportional to the force, into a voltage signal, which is proportional to the force. This type of circuit (11) is known in the art as a charge mode amplifier or charge amplifier.

[0054] The circuit (11) can also provide amplification, low pass filtering and drift compensation. In terms of its overall transfer characteristic, this can be described as a band pass characteristic. The low cut-off frequency is determined by the drift compensation and is set to about 1 Hz, as typical cutting processes have dynamic cutting forces well above this frequency range. As explained previously, static forces are not within the range of this sensor type.

[0055] The passband gain is set to optimize the desired output signal dynamic range for the intended application. This parameter can be easily adjusted by changing the feedback capacitor in the circuit, making it very versatile and cost-effective.

[0056] The high cut-off frequency can be configured to reject unwanted high frequency signal components, preferably determined by the telemetry technology used. Preferably, this limit does not interfere with the cutting force signal. In most practical applications, these are in the range from 10 Hz to 100 Hz. In this case, the cut-off frequency is preferably set to about 1 kHz.

[0057] Each sensor can provide a signal proportional to the force, which is preferably processed by the telemetry technology so that the signal can be accessed by the user. In this case, the telemetry technology can provide a sampling rate of up to 9.5kS (kilosamples) and share a power supply (12) circuit for the telemetry circuit and the charge amplifier circuit.

Claims

1. A cutting tool (1), comprising: - a cutting tool body (2), said cutting tool body (2) comprising at least one seat (3) for receiving a tool holder (4) or a cutting insert (5), wherein said seat (3) comprises at least one support surface (6), - a clamping device 9 for fixing the tool holder (4) or the cutting insert (5) in the tool holder (3), in - the tool body (2) comprises a sensor retaining structure (7) adjacent to the support surface (6), - a piezoelectric sensor (8) arranged in the sensor retaining structure (7), and - the piezoelectric sensor (8) is an elastically deformable piezoelectric sensor, the piezoelectric sensor (8) having a central longitudinal axis (24) and a main sensitivity direction perpendicular to the central longitudinal axis (24), Features The piezoelectric sensor (8) comprises a measuring section (10), along which - the central longitudinal axis (24) is substantially parallel to the support surface (6), and The piezoelectric sensor (8) is elastically deformed in the sensor retaining structure (7) into a prestressed initial state (19), and the piezoelectric sensor (8) is deformable into a plurality of further stressed measuring states (21).

2. The cutting tool (1) according to claim 1, wherein the sensor retention structure (7) is a channel in the tool body (2).

3. The cutting tool (1) according to claim 2, wherein the channel is an open channel in the support surface (6).

4. The cutting tool (1) according to claim 3, wherein when the tool holder (4) or the cutting insert (5) is fixed in the tool seat (3), the tool holder (4) or the cutting insert (5) is pressed against the support surface (6) by the clamping device (9), and the sensor is deformed to the prestressed state (19).

5. The cutting tool (1) according to any of the preceding claims, wherein the piezoelectric sensor (8) is a piezoelectric sensor cable.

6. The cutting tool (1) according to any of the preceding claims, wherein the piezoelectric sensor is a coaxial cable comprising a piezoelectric copolymer.

7. The cutting tool (1) according to any of the preceding claims, wherein the piezoelectric sensor comprises a measuring range within which the relationship between the charge generated by the sensor and the mechanical stress on the sensor can be described by a strictly monotonic function.

8. The cutting tool (1) according to any one of the preceding claims, wherein the prestressed initial state (19) is equivalent to a mechanical stress on the piezoelectric sensor (8) being greater than a lower end point of the measuring range.

9. The cutting tool (1) according to claim 8, wherein the prestressed initial state of the piezoelectric sensor, in combination with the measured state (21) of the further stress caused by the cutting force, is equivalent to a stress lower than an upper limit of the measuring range.

10. The cutting tool (1) according to any of the preceding claims, wherein the seat comprises a shim (17) and the support surface (6) is positioned on the shim (17).

11. The cutting tool (1) according to any of the preceding claims, wherein the at least one support surface faces oppositely or substantially oppositely to a direction of a main cutting force (22) or a direction of one of its components.

12. The cutting tool (1) according to any of the preceding claims, wherein a temperature sensor (23) is arranged in the insert seat (3).

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