MACHINE TOOL FOR GRINDING DISCS

Methods and machine tools for grinding discs. The machine tool comprises two grinding units with respective grinding spindles on which grinding wheels are arranged and a disc unit that comprises a disc spindle on which the disc to be grinded is arranged. The rotation axes of the grinding spindles are perpendicular to the disc spindle rotation axis. The grinding wheels comprise respective grinding surfaces which are perpendicular to the rotation axis of the disc spindle. The grinding surfaces have a width that is equal or greater than the width of the main surfaces of the disc. The two grinding units are configured to simultaneously move relative to the disc unit in an axis that is parallel to the rotation axis of the disc spindle and in opposite directions such that, in use, the grinding surfaces simultaneously contact opposite surfaces of the disc to grind them down.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority of European Patent Application No. 21382752.0 filed Aug. 9, 2021. The content of the referenced application is incorporated into the present application by reference.

TECHNICAL FIELD

In general, the present invention relates to the field of machine tools, and more particularly, to machine tools which have been designed for grinding discs, e.g., brake discs or other pieces with similar geometries, such as circular knives or circular rotary blades, having a high degree of hardness. These discs may be integrally made of hard materials or more preferably, they may be coated with hard materials. The machine tool comprises a disc unit on which the disc to be grinded is to be arranged and two grinding units for simultaneously grinding both main surfaces of the disc. The two grinding units move tangentially and in opposite directions relative to the disc unit to carry out the grinding process.

STATE OF THE ART

Discs are conventionally used in many industrial processes and common applications. These discs may be made of different metals or metal alloys, for example, iron, steel, etc., or may be also made of composite materials such as reinforced carbon-carbon or ceramic matrix composites. For example, the discs may be brake discs for being installed in means of transport such as vehicles or trains, circular knives, circular rotary blades or any other disc-shaped piece designed for industrial or domestic applications. These discs may be also made of or may be coated (hard-coated) with materials with a high degree of hardness to improve their resistance to corrosion and to high temperatures, to increase their mechanical strength, etc.

For the particular case of the brake discs, wear and tear of cast iron brake discs generate higher amounts of brake dust than hard-coated brake discs. Brake dust is mainly made up of iron particles and is caused by the grinding of the cast iron brake disc caused by the brake pads. According to studies, brake dust is up to a 55% of total mass of particles amongst non-exhaust road traffic emissions in urban environments. Thus, the most recent environmental restrictions and higher requirements for wear and corrosion resistance of the brake discs are leading the automotive industry to the replacement of the conventional cast iron brake discs with the hard-coated brake discs.

The hard-coated brake discs offer several advantages over conventional cast iron brake discs. For example, hard-coated brake discs offer improved brake response, high thermal stability, high abrasion resistance, and longer life. They are also more resistant to deformation or warping at high temperatures and, unlike cast iron brakes, do not corrode even when in contact with water or salt during the winter seasons. Besides, as they have higher mechanical resistance the amount of brake dust they emit is significantly lower than conventional cast iron brake discs.

Resurfacing discs is a well-known technique for extending their operational life. Discs are turned or machined, grinding down their surfaces to make them smooth and even so there is very little wear. However, existing solutions for grinding discs made of hard materials or hard-coated discs present complex designs, low productivity rates and allow grinding one single disc per operational cycle of the grinding tool. Moreover, these existing solutions generate extremely high temperatures on the grinded surfaces of the discs during grinding operations that may damage said surfaces and may leave contouring marks that may affect to their performance. For example, in the case of grinding braking discs the contouring marks may reduce the braking capacity of the resurfaced brake discs. Besides, for the hard-coated discs the elevated temperatures reached during the resurfacing operation may also provoke the detachment of the coating from the core of the disc.

Therefore, there is a need in the state of the art for a machine tool for grinding discs, in particular hard-coated discs, that presents a simpler design, provides higher productivity and efficiency rates, that is able to maintain the temperature reached by the disc during the grinding operation within a range that is low enough to ensure that no damages are produced on their main surfaces and that does not leave contouring marks on said main surfaces.

DESCRIPTION OF THE INVENTION

A first object of the invention is a machine tool for grinding discs. The machine tool comprises a first grinding unit and a second grinding unit. The first grinding unit comprises a first grinding spindle having at least one first grinding wheel arranged thereon and a first motor assembly to rotatably actuate the first grinding spindle. The second grinding unit comprises a second grinding spindle having at least one second grinding wheel arranged thereon and a second motor assembly to rotatably actuate the second grinding spindle. The first and second grinding wheels may be made of different abrasive materials, such as diamond (C), cubic boron nitride (cBN), silicon carbide (SiC), etc. In turn, the first and second motor assemblies may be formed by direct spindle motors or AC motors with pulley or gear transmission and means for attaching the grinding units to the surface on which they are to be mounted.

The machine tool further comprises at least one disc unit having a disc spindle on which the disc to be grinded is to be arranged and a third motor assembly for rotatably actuating the disc spindle. The third motor assembly may be formed by servomotors to actuate the disc spindle and means for attaching the disc unit to the surface on which it is mounted. The rotation axes of the two grinding spindles are substantially perpendicular to the rotation axis of the disc spindle.

The discs may be any disc-shaped piece, whether these discs are new or they are not and their main surfaces are subjected to wear and tear, that need to be resurfaced. As used herein, the term “disc” generally refers to any disc-shaped piece, in other words, to any piece having a substantially thin circular geometry with two main surfaces. These main surfaces are preferably planar surfaces which are perpendicular to the axis of the disc although they may have other geometries. The discs may incorporate hubs, cutting teeth, contact surfaces, etc., may have different dimensions and may be made of different materials depending on the application for which they have been designed. They may have been designed for applications in which they rotate around its rotation axis or for being fixed. For example, the discs may be brake discs for conventional vehicles, light, medium or heavy-duty vehicles, trains, or any other means of transport, circular knives, circular rotary blades or any other disc-shaped piece designed for industrial or domestic applications.

As used herein, the term “hard material” refers to materials with a hardness higher than 750 HV (Vickers). Similarly, the expression “hard-coated discs” refers to discs formed by a core, that may be made of cast-iron or other materials, and whose main surfaces are coated with a layer of hard material (hardness higher than 750 HV) such as Tungsten carbide-based surface coatings and new generation metallic materials (Ni, Co or Fe based alloys, able to protect the disc against corrosion and high temperatures of the braking process) plus cost efficient and ecological ceramic phase (VC, TiC, SiC, Al2O3) that improves the wear resistance of such coatings. In some examples, the layers of hard material can be deposited on the main surfaces of the core using techniques such as laser material deposition or high-velocity oxygen fuel (HVOF) deposition, among many other technologies for depositing of materials. As used herein, the “main surfaces” of the disc are those surfaces subjected to wear and tear which are susceptible of being grinded.

The grinding wheels comprise respective grinding surfaces which are substantially perpendicular to the rotation axis of the disc spindle. These grinding surfaces are those surfaces of the grinding wheels that contact the main surfaces of the discs to grind them down. These grinding surfaces will be preferably planar although may have a different geometry depending on the geometry of the surface of the disc to be grinded. The grinding surfaces have a width that is equal or greater than the width of the surface of the disc to be grinded, the width of the surface of the disc to be grinded being measured in a radial direction of the disc. The width of the grinding surfaces may be measured in a direction parallel to the rotation axis of the grinding spindles. The first grinding unit and the second grinding unit are configured to simultaneously move relative to the disc unit in an axis that is parallel to the rotation axis of the disc spindle and in opposite directions such that, in use, the grinding surfaces of the grinding wheels simultaneously contact opposite surfaces of the disc along the entire width of the surface of the disc to be grinded.

The grinding units are displaceable relative to the disc unit in a direction that is perpendicular to the surfaces of the disc to be grinded between an operative position in which the grinding wheels contact the disc to grind it down, and an inoperative position in which the grinding wheels does not contact the disc and they are positioned far away from the disc. The grinding units are preferably located at both sides of the disc unit.

The material removal from the disc by the abrasive grinding wheels is mainly given by the relatively high tangential peripheral speed between the grinding wheels and the disc (the tangential speed ratio between the disc and grinding wheels may range, for example, between 1-30 and 1-180, although these ratios may be also out of these ranges), in combination with the substantially perpendicular movement (feed motion) of the grinding wheels relative to the disc. The tangential material removal mechanism due to the tangential peripheral speed of the grinding wheels relative to the disc reduces the temperature reached on the braking surfaces during grinding operation. Thus, plastic deformations, cracks and wear in the discs are avoided or at least minimized. Besides, by having grinding surfaces whose width is equal or greater than the width of the surface of the disc to be grinded there is no need to move the grinding wheels in a direction that is perpendicular to the rotation axis of the disc spindle during the grinding operation, avoiding creating contouring marks on the main surfaces of the brake disc. The combination of the perpendicular movement of the grinding wheels relative to the disc and the grinding wheels having a width that is equal or greater than the width of the surface of the disc to be grinded reduces the time required to grind the disc.

In some embodiments, the first grinding unit is mounted on a first platform and the second grinding unit is mounted on a second platform, the first and second platforms comprising means for moving the platforms relative to the at least one disc unit in an axis that is parallel to the rotation axis of the disc spindle and in opposite directions, respectively. Preferably, the first platform and the second platform comprise respective ball screw drives or linear motors to move the platforms perpendicularly with respect to the at least one disc unit.

In some embodiments, the at least one disc unit is fixedly mounted on a bench and the first and second platforms are movably mounted on the same bench. In this way, while the disc unit remain fixed, the two platforms move relative to the disc unit.

In some embodiments, the first grinding spindle and the second grinding spindle are configured to rotate in the same direction or in opposite directions.

In some embodiments, the grinding spindles are cantilevered grinding spindles or twin-grip grinding spindles. The cantilevered grinding spindles are preferred when the width of the surface to be grinded is relatively small (e.g., less than 250 mm) while the twin-grip grinding spindles is preferred when the width of the surface to be grinded is great (e.g., greater than 250 mm) since they provide a higher structural rigidity.

In some embodiments, the disc to be grinded is a disc made integrally of a hard material or a hard-coated brake disc. For example, the disc may be coated with Tungsten carbide-based surface coatings and new generation metallic material (Ni, Co or Fe based alloy, able to protect the disc against corrosion and high temperatures of the braking process) plus cost efficient and ecological ceramic phase (VC, TiC, SiC, Al2O3) that improves the wear resistance of such coatings.

Preferably, the disc to be grinded may be selected from a group comprising brake discs, circular knives and circular rotary blades although it may also be any other disc-shaped piece designed for industrial or domestic applications.

In some embodiments, the first grinding unit comprises a first dresser having a first dressing tool. The first dresser is configured to dress the first grinding wheel. In turn, the second grinding unit comprises a second dresser comprising a second dressing tool. The second dresser is configured to dress the second grinding wheel. The dressers are preferably configured to dress the grinding wheels when the grinding units are in their inoperative position, i.e., when the grinding wheels do not contact the disc and are retracted from the disc unit. Alternatively, the dressers may be configured to dress the grinding wheels when the grinding units are in their operative position, i.e., when the grinding wheels are grinding the disc. The dressers may be stationary dressers (e.g., diamond dressers, dressing stones, etc., having a diamond or abrasive stone as dressing tools) or may be rotary dressers (e.g., disc dressers, crushing disc dressers, silicon carbide dressers, etc., having abrasive discs or wheels as dressing tools). These dressers are actuated from time to time in order to clean the grinding surfaces of the grinding wheels from grinding dust, to provide the required geometry and to expose abrasive grains.

Preferably, each one of the dressers comprises first means for moving the dressing tool in a direction perpendicular to the rotation axis of the grinding spindles and towards the grinding surfaces of the grinding wheels and second means for moving the dressing tool in a direction parallel to the rotation axis of the grinding spindles and along the width of the grinding wheel. The first and second means may be any mechanism able to move the dressing tool in both directions. The width of the grinding wheel is measured in a direction parallel to the rotation axis of the grinding spindles on the grinding surface.

In some embodiments, these dressers may be located in correspondence with a plane formed by both grinding wheels rotational axes, behind the grinding wheels and in proximity to the location of said grinding wheels.

In some embodiments, the machine tool comprises a first disc unit comprising a first disc spindle on which a first disc to be grinded is to be arranged and the third motor assembly for rotatably actuating the first disc spindle. The machine tool further comprises a second disc unit comprising a second disc spindle on which a second disc to be grinded is to be arranged and a fourth motor assembly for rotatably actuating the second disc spindle. These third and fourth motor assemblies can be formed by stepper motors and servomotors and means for attaching the grinding units to the surface on which they are to be mounted. In such embodiments, the first grinding unit comprises two grinding wheels arranged on the first grinding spindle and the second grinding unit comprises two grinding wheels arranged on the second grinding spindle such that a first grinding wheel of the first grinding unit is located in correspondence with a first grinding wheels of the second grinding unit to simultaneously grind the first disc and a second grinding wheel of the first grinding unit is located in correspondence with a second grinding wheel of the second grinding unit to simultaneously grind the second disc.

In some embodiments, the machine tool comprises a dresser with a dressing rotary tool, e.g., a disc or wheel, located between the first and second grinding units, the dresser being movable in a direction parallel to the rotation axis of the grinding spindles such that a dressing surface of the dressing rotary tool simultaneously contacts the grinding surfaces of the pairs of grinding wheels of the first and second grinding units located in correspondence to each other. Preferably, the first and second grinding units are configured to firstly move from their inoperative position towards their operative position (until a dressing position that may match their operative position or may be different) and then, the dresser moves in the direction parallel to the rotation axis of the grinding wheels until the dressing surfaces of the dressing disc contact the grinding surfaces of the grinding wheels to dress them down.

A second object of the invention is a method for grinding discs that uses the machine tool previously described. The method comprises the steps of:

arranging a disc to be grinded in the disc spindle of the disc unit;

actuating the disc spindle of the disc unit, the first grinding spindle of the first grinding unit and the second grinding spindle of the second grinding unit by the respective motor assemblies;

simultaneously moving the first grinding unit and the second grinding unit in an axis that is parallel to the rotation axis of the disc spindle and in opposite directions towards the disc until the grinding surfaces of the grinding wheels simultaneously contact opposite surfaces of the disc along the entire width of the surface of the disc. In other words, the grinding units simultaneously move from their inoperative position in which they do not contact the disc to their operative position in which they contact the disc in a direction that is tangential to the main surfaces of the disc;

simultaneously grinding both surfaces of the disc by the grinding wheels of the first and second grinding units during a predefined period of time (target cycle time). This predefined period of time may depend on a target productivity rate to remove a given amount of material. In turn, this target productivity rate will depend on whether under the current operational conditions, the amount of material to be grinded can be achieved within the predefined dimensional tolerances, surface finish (roughness) and integrity conditions (thermal damage, discolouring, cracks, spalling, etc.) of the disc;

simultaneously retracting the first grinding unit and the second grinding unit. The grinding units move from their operative position to their inoperative position; and

removing the disc from the brake spindle of the disc unit.

The steps of arranging and removing the disc on/from the brake disc unit can be performed manually by an operator or automatically by a robotic arm or any other loading/unloading mechanism.

In some embodiments, particularly when the grinding surfaces of the grinding wheels need to be cleaned, smoothed or homogenized, the method comprises the steps of:

actuating at least one of the first dressing tool and the second dressing tool of the first and second dressers, respectively. This dressing operation may be required in only one of the two grinding wheels or in both. Thus, the dressing operation of the first grinding wheel can be carried by the first dressing tool independently or jointly and severally with the dressing operation of the second grinding wheel by the second dressing tool;

moving, by the first means, the corresponding dressing tool in a direction perpendicular to the rotation axis of the respective grinding spindle until a dressing surface of the dressing tool contacts the grinding surface of the grinding wheel; and

moving, by the second means, the dressing tool in a direction parallel to the rotation axis of the grinding spindle and along the width of the grinding wheel to dress its grinding surface, the width of the grinding wheel being measured in a direction parallel to the rotation axis of the grinding spindle on the grinding surface.

In some embodiments, the method comprises the steps of:

arranging a first disc to be grinded in the first disc spindle of the first disc unit;

arranging a second disc to be grinded in the second disc spindle of the second disc unit;

actuating the disc spindles of the disc units, the first grinding spindle of the first grinding unit and the second grinding spindle of the second grinding unit by the respective motor assemblies, wherein the first grinding unit comprises two grinding wheels arranged on the first grinding spindle and the second grinding unit comprises two grinding wheels arranged on the second grinding spindle such that a first grinding wheel of the first grinding unit is located in correspondence with a first grinding wheel of the second grinding unit and a second grinding wheel of the first grinding unit is located in correspondence with a second grinding wheel of the second grinding unit;

simultaneously moving the first grinding unit and the second grinding unit in an axis that is parallel to the rotation axis of the disc spindle and in opposite directions towards the discs until the grinding surfaces of the grinding wheels simultaneously contact opposite surfaces of the discs along the entire width of the surface of the disc;

simultaneously grinding both surfaces of the discs by the grinding wheels of the first and second grinding units during a predefined period of time;

simultaneously retracting the first grinding unit and the second grinding unit; and

removing the discs from the disc spindles of the disc units.

The machine tool and the method for grinding discs of the present invention present several advantages over the prior art. The present solution provides a grinding operation that is thermally efficient, is carried out at low-temperature, ensures the mechanical integrity of the disc and guarantees the reaching the required surface finish within the dimensional tolerances of the piece while high productivity rates are obtained. The machine tool presents a simple structure in which the disc remains fixed and only the grinding wheels move in a perpendicular direction. The productivity is maximized by grinding the entire width of the disc in one penetration operation of the grinding wheel. The width of the grinding wheels is large enough to grind discs of different widths. Besides, by simultaneously contacting both surfaces of the disc with the grinding wheels, deflections and loss of productivity that would be generated if the brake disc were grinded only on one side is avoided. The present solution does not leave contouring marks on the main surfaces of the disc. It further allows simultaneously grinding two discs.

DETAILED DESCRIPTION OF THE INVENTION

FIG.1shows a plant view of a machine tool100for grinding a brake disc101, according to an embodiment of the invention. It should be understood that the machine tool100ofFIG.1may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the described machine tool100. Additionally, implementation of the machine tool100is not limited to such embodiment.

The machine tool100comprises a first grinding unit102mounted on a first platform103, a second grinding unit104mounted on a second platform105and a brake disc unit106on which the brake disc101to be grinded is mounted. The first grinding unit102comprises a first grinding spindle107, a first grinding wheel108arranged thereon and a first motor assembly109to rotatably actuate the first grinding spindle107. The first grinding spindle107may rotate at a first peripheral speed r1, that for example may range between 100 and 5000 revolutions per minute, around a first rotation axis110. The second grinding unit104comprises a second grinding spindle111, a second grinding wheel112arranged thereon and a second motor assembly113to rotatably actuate the second grinding spindle111. The second grinding spindle111may rotate at a second peripheral speed r2, that for example may range between 100 and 5000 revolutions per minute, around a second rotation axis114. The rotation axes110,114of the first and second grinding spindles107,111are substantially parallel to each other. Preferably, both grinding wheels108,112will rotate in the same direction and with the same peripheral speed although they may rotate in opposite directions and at slightly different peripheral speeds.

The first platform103and the second platform105are movably mounted on a bench115while the brake disc unit106is fixedly mounted on said bench115. For example, the first and second platforms103,105may include servomotors actuating ball screws, linear motors, or similar to move both platforms103,105in a direction that is substantially perpendicular to the rotation axes110,114of the first and second grinding spindles107,111. The first platform103and the second platform105will move at a first speed v1and a second speed v2, respectively, that will be preferably the same speed such that the grinding surfaces108a,112aof the first and second grinding wheels108112simultaneously contact the braking surfaces101a-bof the brake disc101. The speeds v1and v2will vary depending on whether the grinding wheels are contacting the disc or they are not, and may range from 2000 mm/min when the grinding units are moving from their inoperative position to their operative position, to 0.001 mm/min during the grinding operation. By simultaneously contacting both braking surfaces101a-bof the brake disc101deflections and loss of productivity that would be generated if the brake disc were grinded only on one side can be avoided.

The first and second grinding wheels108,112may be made of different material such as diamond (C), cubic boron nitride (cBN), silicon carbide (SiC), a combination of diamond and cBN, etc. The first and second motor assemblies109,113may be formed by direct spindle motors or AC motors with pulley or gear transmission and means for attaching the grinding units102,104to the first and second platforms103,105, respectively. These means for attaching the grinding units102,104to the first and second platforms103,105may be, for example, a coupling structure that may be an integral part of the motor assemblies109,113or being couplable to said motor assemblies109,113and that could be welded or screwed to the platforms103,105.

The brake disc unit106also has a brake disc spindle116on which the brake disc101is arranged and a third motor assembly117for rotatably actuating the brake disc spindle116. The brake disc spindle113may rotate at a third rotational speed r3, that for example may range between 1 and 2000 revolutions per minute, around a third rotation axis118. The third rotational speed r3may be equal or different to the first and second rotational speeds r1, r2. The third motor assembly116may also be formed by a servomotor or stepper motor and means for attaching the brake disc unit106to the bench115. Similarly, these means for attaching the brake disc unit106to the bench115may be, for example, a coupling structure that may be an integral part of the motor assembly117or being couplable to said motor assembly117and that could be welded or screwed to the bench115. The rotation axes110,114of the two grinding spindles108,111are substantially perpendicular to the rotation axis118of the brake disc spindle116.

The grinding surfaces108a,112aof the grinding wheels are substantially planar and perpendicular to the rotation axis118of the brake disc spindle116. These grinding surfaces108a,112ahave a width w1, w2that is equal or greater than the width w3of the braking surfaces101a-bof the brake disc101. The width of the braking surfaces101a-bis measured in the radial direction of the brake disc101. In turn, the width of the grinding surfaces108a,112ais measured in a direction parallel to the rotation axes110,114of the grinding spindles107,111.

FIG.1shows the two grinding units102,104in their inoperative position in which the grinding wheels108,112does not contact the braking surfaces101a-bof the brake disc101and they are positioned far away from it. In use, the platforms103,105move relative to the bake disc unit106such that the grinding units102,104are in their operative position, i.e., the grinding wheels108,112displace relative to the brake disc unit106in a direction that is perpendicular to the brake disc101until the grinding surfaces108a,112acontact the respective braking surfaces101a-balong their entire width. The material removal from the brake disc101by the abrasive grinding wheels108,112is caused by the relatively high tangential peripheral speed between the grinding wheels108,112and the brake disc101, in combination with the relative perpendicular feed motion of the grinding wheels108,112relative to the brake disc10. The tangential material removal mechanism due to the tangential peripheral speed of the grinding wheels108,112relative to the brake disc101reduces the temperature reached on the braking surfaces101a-bduring grinding operation.

Thus, plastic deformations, cracks and wear in the brake discs101are avoided or at least minimized. Besides, by having grinding surfaces108a,112awhose width w1, w2is equal or greater than the width w3of the braking surfaces101a-bof the brake disc101there is no need to move the grinding wheels108,112in a direction that is perpendicular to the rotation axes110,114of the disc spindles107,111during the grinding operation, avoiding creating contouring marks on the main surfaces of the brake disc101that may worse its braking properties. The combination of the perpendicular movement of the grinding wheels108,112relative to the brake disc101with the grinding wheels108,112having a width w1, w2that is equal or greater than the width w3of the surface101a-bof the brake disc101to be grinded reduces the time required to grind the braking surfaces101a-bwhich increases the productivity and efficiency of the resurfacing process. It also avoids creating contouring marks on said braking surfaces101a-bthat may worse the braking properties of the disc101.

The combination of the perpendicular movement of the grinding wheels108,112relative to the brake disc101with the grinding wheels108,112having a width w1, w2that is equal or greater than the width w3of the surface101a-bof the brake disc101to be grinded reduces the time required to grind the braking surfaces101a-bwhich reduces the temperature reached in said surfaces101a-band increases the productivity and efficiency of the resurfacing process.

WhileFIG.1shows the disc unit106with a brake disc101arranged thereon, another disc-shaped piece such as a circular knife or circular rotary blades may be coupled to the disc spindle116of the disc unit106.

FIG.2shows a plant view of a machine tool200for simultaneously grinding two brake discs201,219, according to an embodiment of the invention. It should be understood that the machine tool200ofFIG.2may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the described machine tool200. Additionally, implementation of the machine tool200is not limited to such embodiment.

The machine tool200ofFIG.2is essentially the same machine tool100ofFIG.1but where the first grinding spindle207comprises a first grinding wheel208and a second grinding wheel220arranged thereon and the second grinding spindle211comprises a third grinding wheel212and a fourth grinding wheel221arranged thereon, and having two brake disc units206,221fixedly arranged on the bench215.

The machine tool200comprises a second brake disc unit222having a brake disc spindle223on which the second brake disc219is arranged, e.g., clamped, and a fourth motor assembly224for rotatably actuating the brake disc spindle223. The brake disc spindle223may rotate at a fourth rotational speed r4, that for example may range between 1 and 2000 revolutions per minute, around a fourth rotation axis225. The fourth rotational speed r4may be equal or different to the third rotational speed r3of the other brake disc unit206. The fourth motor assembly224may also be formed by a stepper motor, servomotor or similar, and means for attaching the second brake disc unit222to the bench215. These means for attaching the brake disc unit222to the bench215may be, for example, a coupling structure that may be an integral part of the motor assembly224or being couplable to said motor assembly224and that could be welded or screwed to the bench215. The rotation axis218,225of the two brake disc spindles216,223are substantially parallel to each other. The operation of the machine tool200ofFIG.2is the same than the operation of the machine tool100ofFIG.1.

In this way, the first grinding wheel208is located in correspondence with the fourth grinding wheel221to simultaneously grind the first brake disc201and the second grinding wheel220is located in correspondence with the third grinding wheel212to simultaneously grind the braking surfaces201a-b,219a-bof the first and second brake discs201,219, respectively. This architecture allows grinding two brake discs at the same time increasing the productivity rate of the machine tool200.

While the machine tool200ofFIG.2shows the two brake discs201,219having the same width w3, and thus, the four grinding wheels have also the same widths, the two brake discs201,219may have different widths and thus, the pairs of grinding wheels208,221and grinding wheels212,220may have a different width, each of these widths being adapted to grind the corresponding brake disc.

FIG.3shows a plant view of a machine tool300for grinding a brake disc301including a diamond dresser326for dressing each grinding wheel308,312, according to an embodiment of the invention. It should be understood that the machine tool300ofFIG.3may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the described machine tool300. Additionally, implementation of the machine tool300is not limited to such embodiment.

The machine tool300ofFIG.3is essentially the same machine tool100ofFIG.1but including the diamond dressers326for dressing the grinding surfaces308a,312aof the grinding wheels308,312. The dressing operation is carried out by the diamond dressers326when the grinding units302,304are in their inoperative position. The diamond dressers326are mounted on corresponding platforms327a-bwhich are actuated by respective servomotors (not shown in this figure) to move the diamond dressers326from an inoperative position in which the diamond dressers326do not contact the grinding surfaces308a,312aand they are positioned far away from the grinding wheels308,312and an operative position in which the diamond dressers326contact the grinding surfaces308a,312ato dress them. Specifically, each diamond dresser326comprises a first platform327aconfigured to move the diamond dressers326in a direction that is perpendicular to rotation axes310,314of the grinding spindles307,311and a second platform327bto move the diamond dressers326in a direction that is parallel to rotation axes310,314of the grinding spindles307,311.

In such embodiments, the diamond dressers326are mounted on the platforms303,305and are located in correspondence with a plane formed by both grinding wheels rotational axes and behind the grinding wheels308,312. Moreover, the diamond dresser comprises a diamond328to dress the grinding surfaces308a,312aof the grinding wheels308,312.

The platforms327aare configured to, once the grinding wheels308,312are in their inoperative position, move the diamond dresser326in a direction that is perpendicular to rotation axes310,314of the grinding spindles307,311until the diamond328contacts the grinding surfaces308a,312aof the grinding wheel308,312. Then, the platforms327bare configured to move the diamond dresser326in a direction that is parallel to rotation axes310,314of the grinding spindles307,311along the entire width of these grinding surfaces308a,312ato dress them. Alternatively, this dressing operation may be carried out by the diamond dressers326during the grinding operation of the grinding wheels308,312, i.e., when they are in their operative position.

FIG.4shows a plant view of a machine tool400for grinding two brake discs (not shown in this figure) including one single diamond dresser429, according to an embodiment of the invention. It should be understood that the machine tool400ofFIG.4may include additional components and that some of the components described herein may be removed and/or modified without departing from a scope of the described machine tool400. Additionally, implementation of the machine tool400is not limited to such embodiment.

The diamond dresser429comprises a diamond dressing disc430mounted on a dressing spindle431that is actuated by a servomotor432and it is mounted on a platform433that is configured to move in a direction that is substantially parallel to the rotation axes410,414of the grinding spindles407,411.

In such embodiment, the two grinding units402,404move to a dressing position that is generally located at an intermediate point between their operative and inoperative positions. When the two grinding units402,404are positioned in their dressing position, the distance between the two closest points of their grinding surfaces is substantially equal to the diameter of the diamond dressing disc430. Then, the diamond dresser429is configured to move in a direction that is substantially parallel to the rotation axes410,414of the grinding spindles407,411until it contacts the grinding surfaces420a,412aof the grinding wheels420,412and continuous with the movement along the entire width of the surfaces420a,412auntil said grinding surfaces420a,412ahave been completely dressed. After that, the diamond dresser429is configured to carried out the same operation for dressing the surfaces408a,421aof the grinding wheels408,421.

The architecture and disposition of the diamond dressers ofFIGS.3and4is interchangeable. That is to say, the machine tool300ofFIG.3may incorporate one single dresser placed between both grinding units and being movable in a direction that is substantially parallel to the rotation axes of the grinding spindles and the machine tool400ofFIG.4may comprise one dresser for each one of the grinding wheels as inFIG.3. Besides, the dressers shown inFIGS.3and4may be stationary dressers or rotary dressers with dressing tools made of different abrasive materials.