Patent Application
20230415305
The present invention relates to a method for machining workpieces in a gear grinding machine with a grinding tool which is configured as a profile grinding wheel or grinding worm and has vitrified-bonded abrasive grains made of a superabrasive material, in particular cBN, and to a gear grinding machine which is designed to carry out the method.
In gear grinding, a choice can be made between different specifications for the grinding tool. In addition to dressable corundum tools with vitrified bond and non-dressable cBN tools with electroplated bond, dressable cBN tools with vitrified bond are also known. Vitrified-bonded cBN tools exhibit increased flexibility over electroplated bonded tools due to the dressable bond. Due to the high performance cBN cutting grain, vitrified-bonded cBN tools can achieve high material removal rates. As a result, the machined volume between two dressing operations can be increased compared to corundum tools.
A disadvantage of vitrified-bonded cBN tools is that an undesirable grinding-in behavior occurs (Research Report FVA 778 I, IGF No. 18580 N, retrieved on 16.11.2020 from www.fva-net.de). The term “grinding-in behavior” is understood to designate the phenomenon that thermal damage to the edge zone of heat-treated workpieces (so-called grinding burn) can occur immediately after dressing when using a vitrified-bonded cBN tool. For example, during discontinuous profile grinding of gears, where individual gear gaps are ground sequentially, thermal damage to the edge zone is often documented in the first machined gear gaps after dressing. There are various approaches to explaining this grinding-in behavior (insufficient chip space, exposed bond, flattening of the cBN grains).
To overcome this problem, the prior art proposed to condition the grinding tool after dressing by “breaking-in” the grinding tool. Two strategies have been proposed for this. According to a first strategy, after dressing, the first gear gaps or the first workpieces are machined with reduced infeed and/or reduced axial feed rate. This strategy is costly to implement and can result in the properties of the initially machined workpieces deviating from the properties of workpieces machined later. According to a second strategy, after dressing, one or several sacrificial workpieces are machined first and then discarded. This strategy is time-consuming and cost-intensive.
US2005272349A1 discloses a method of conditioning a superabrasive grinding tool in which, after dressing the grinding tool, a plurality of cuts are made in a sacrificial element. The geometry of the sacrificial element corresponds to the geometry of the workpieces subsequently machined with the grinding tool.
It is an object of the present invention to disclose a method which, when using grinding tools with vitrified-bonded superabrasive abrasive grains, ensures uniform workpiece machining without causing thermal damage to the edge zone of the workpieces at the beginning of the tool life and without the need to machine sacrificial workpieces.
This object is achieved by a method according to claim 1. Further embodiments are given in the dependent claims.
Thus, a method is proposed for machining workpieces in a gear grinding machine with a grinding tool comprising vitrified-bonded abrasive grains made of a superabrasive material, in particular cBN. The method comprises the steps:
The process is characterized in that the conditioning kinematics is different from the machining kinematics.
In contrast to the prior art, a sacrificial workpiece is thus not machined with the machining kinematics for conditioning, but conditioning is performed with a conditioning tool that is moved relative to the grinding tool with a special conditioning kinematics. In particular, the conditioning kinematics may correspond to a dressing kinematics such as may be used for dressing the grinding tool. Accordingly, a conditioning tool having a different basic shape than a workpiece, in particular having the basic shape of a dressing tool, may be used for conditioning. For example, if the workpieces are gear-shaped, the conditioning tool is preferably not also gear-shaped. Instead, the conditioning tool may be, for example, a rotating, disc-shaped conditioning tool or a stationary, for example, pin- or tooth-shaped conditioning tool.
Using a different kinematics for conditioning than the machining kinematics results in various advantages. In particular, conditioning can be performed in a much more targeted manner, since the motion sequences can be specifically adapted to achieve an optimum conditioning result. For example, technological parameters such as the infeed of the conditioning tool radially to the axis of rotation of the grinding tool, the rotational speeds of the grinding tool and, if applicable, of the conditioning tool, the direction of action (up-cut or down-cut direction) and, if conditioning is not performed in line contact along the complete working profile of the grinding tool, the contouring feed rate and the degree of overlap can be specifically adjusted. The use of a conditioning tool with separate conditioning kinematics can also reduce unproductive idle time that would otherwise occur after conditioning for replacing a sacrificial workpiece with a workpiece to be machined. Also, unlike a sacrificial workpiece, the conditioning tool can be used multiple times. This significantly reduces material consumption.
The following definitions are used in this document.
In the present document, a “superabrasive material” is understood to be a material whose Vickers microhardness at room temperature is higher than the microhardness of corundum. The class of superabrasive materials includes, in particular, cubic boron nitride (cBN) and diamond. For hard finishing of pre-toothed steel workpieces, cBN is particularly significant because, unlike diamond, it has no chemical affinity for typical gear materials. In this respect, the present invention relates in a particular way to grinding tools whose abrasive body is formed by vitrified-bonded cBN grains.
“Thermal edge zone damage” of a workpiece or “grinding burn” is defined as a damage pattern as specified in ISO 14104:2017-04. The verification of whether or not there is thermal edge zone damage is performed by the surface temper etching method defined in ISO 14104:2017-04. Thermal damage to the edge zone of a workpiece as defined in the present document is present if, according to ISO 14104:2017-04, the workpiece does not meet classification FA/NB2 after a type 3 etching.
In the present document, the term “basic shape” means the geometric shape of an object, abstracted from minor differences in dimensions. For example, two cylindrical gears with the same helix angle, the same module and the same number of teeth are considered to be objects with the same basic shape, even if, for example, the tooth thickness, the profile shape or the flank line of the cylindrical gears differ. Conversely, a disk without cylindrical gear teeth or a fixed pin, tooth or rod are considered to be objects that have a different basic shape than a cylindrical gear.
In the present document, the term “dressing” or “truing” is understood to mean a process by which, on the one hand, a desired geometric shape of a grinding tool is produced or restored and, on the other hand, the grinding tool is sharpened by bringing the rotating grinding tool into engagement with a dressing tool.
In the present document, the term “conditioning” is understood to mean the specific bringing about of a desired wear condition. During conditioning, the geometric shape of the grinding tool, as produced during dressing, is preferably no longer changed. Conditioning can in particular serve to remove bonding agents between the abrasive grains after dressing in order to partially expose the abrasive grains.
The terms “dressing kinematics”, “conditioning kinematics” and “machining kinematics” are respectively understood to mean the sequence of movements performed by the grinding machine during the process of “dressing”, “conditioning” and “machining”, respectively. In particular, a dressing kinematics is understood to be a sequence of movements in which a dressing tool is brought into engagement with the rotating grinding tool to dress the grinding tool. The dressing kinematics may include movements of the grinding tool relative to a machine bed of the grinding machine and/or movements of the dressing tool relative to the machine bed. The dressing kinematics are generated by one or more numerically controlled (NC) axes of the grinding machine. Accordingly, “conditioning kinematics” is understood to mean a sequence of movements in which a conditioning tool is brought into engagement with the rotating grinding tool to condition the grinding tool, and “machining kinematics” is understood to mean a sequence of movements in which the rotating grinding tool is brought into engagement with the workpiece to machine the workpiece.
Two kinematics are considered to be different if the associated movements not only differ in individual parameters such as movement length, speed, etc., but the basic sequence of movements is different. For example, the machining kinematics in continuous generating gear grinding with a grinding worm is different from a dressing kinematics in which the grinding worm is dressed with a rotating dressing wheel. For example, the machining kinematics in continuous generating gear grinding includes a forced coupling of the rotational speeds of the grinding worm and the workpiece to satisfy the rolling condition, while the dressing kinematics does not include such a forced coupling. Also, the machining kinematics in discontinuous profile grinding with a profile grinding wheel is fundamentally different from a dressing kinematics for dressing the profile grinding wheel with a rotating dressing wheel. For example, the machining kinematics requires that the profile grinding wheel be brought into engagement with the next tooth gap after machining one tooth gap. This element is completely missing in the dressing kinematics.
According to the invention, the conditioning kinematics differs from the machining kinematics, i.e. during conditioning a different sequence of movements is performed than the sequence of movements used for machining the workpieces. The conditioning tool is preferably clamped on a conditioning device which is different from the workpiece spindle, i.e., unlike when sacrificial workpieces are used, conditioning is not carried out with the aid of the workpiece spindle, but with the aid of a conditioning device which is separate therefrom. The conditioning device can in particular be integrated into a dressing device or combined with it.
In particular, the basic shape of the conditioning tool can correspond to the basic shape of the dressing tool that is specifically used for dressing the grinding tool, or, when several dressing tools are used, to the basic shape of one of these dressing tools. For example, if a rotary, disc-shaped dressing tool is used for dressing, the conditioning tool may also be disc-shaped and have similar dimensions to the dressing tool. The conditioning kinematics may then correspond to the dressing kinematics for this dressing tool.
However, the basic shape of the conditioning tool may also differ from the basic shape of the dressing tool actually used. For example, the dressing may be performed with a rotating, disc-shaped dressing tool, while the conditioning tool is designed as a stationary element, e.g. as a pin, tooth or rod. Accordingly, the conditioning kinematics may differ from the actual dressing kinematics used. Nevertheless, the conditioning kinematics in this case is also a kinematics such as could also be used for dressing, and in this respect the conditioning kinematics also corresponds to a dressing kinematics in this case.
Preferably, the conditioning tool is made of metal, in particular steel, in a region that comes into contact with the grinding tool during conditioning. Preferably, the steel is a steel with similar properties to the steel from which the workpieces are made. In particular, it may be the same type of steel as used for the workpieces. In particular, the conditioning tool may correspond to the base body, made of steel, of a dressing tool whose hard material coating has been omitted.
As already mentioned, in some embodiments, the conditioning tool is stationary during the conditioning process. In other embodiments, the conditioning tool rotates during the conditioning process, wherein this rotation may be in down-cut (“climb”) or up-cut (“conventional”) direction relative to the rotation of the grinding tool.
In the case of a rotating conditioning tool, the conditioning tool may have the basic shape of a dressing roll, i.e. a disc-shaped basic shape. In particular, the conditioning tool may have the shape of a so-called profile roll or a form roll. The term “profile roll” is to be understood to relate to a dressing roll that is configured to dress the grinding tool in line contact in such a way that a profile shape of the dressing roll is transferred to the grinding tool. The line contact may, for example, only take place in the area of one flank of the grinding tool, it can take place on two adjacent flanks, or it can also include intermediate head and/or foot areas of the grinding tool. On the other hand, the term “form roll” is to be understood as relating to a dressing roll that is provided to dress the grinding tool in point contact. As already explained, the conditioning tool preferably corresponds to the base body of a dressing roll made of steel without hard material coating.
Regardless of whether the conditioning tool is configured to be rotating or stationary, the conditioning tool may generally be in line contact with at least a portion of the working profile of the grinding tool during the conditioning process, or it may be in point contact with a portion of that working profile. Provided that the conditioning tool is not in line contact along the entire working profile of the grinding tool, it may be possible that the gear grinding machine performs a relative movement between the grinding tool and the conditioning tool such that the contact position between the conditioning tool and the grinding tool changes along the profile of the grinding tool during conditioning.
As already mentioned, the present invention allows all workpieces in step c) to be machined with identical machining parameters, in particular with identical infeed perpendicular to the workpiece spindle axis and identical axial feed rate along the workpiece spindle axis. These machining parameters can be selected such that thermal edge zone damage would occur during machining of at least a first workpiece in step c) if step b) were not performed. This is possible because in step b) the conditioning is carried out in such a way that during the machining in step c) precisely no more thermal edge zone damage occurs.
Steps a) to c) can be repeated several times. The conditioning process b) can be carried out several times with the same conditioning tool. Thus, unlike a sacrificial workpiece, the conditioning tool does not have to be discarded after a single conditioning process, but can be reused several times.
The workpiece machining in step c) may in particular be performed by continuous generating gear grinding or by discontinuous profile grinding. The grinding tool may accordingly be a grinding worm or a profile grinding wheel.
The present invention also provides a gear cutting machine particularly configured for carrying out the method disclosed above. The gear cutting machine comprises:
The gear cutting machine is characterized in that it comprises a conditioning device which is different from the workpiece spindle, wherein a conditioning tool can be clamped onto the conditioning device. The control unit is then configured to control the machine axes such that the machine tool performs a method of the type indicated above, such that the conditioning is performed with a conditioning kinematics which is different from the machining kinematics and which preferably corresponds to a dressing kinematics.
Preferred embodiments of the invention are described in the following with reference to the drawings, which are for explanatory purposes only and are not to be construed in a limiting manner. In the drawings,
A dressing device 400 is arranged on the machine bed 100. On a side of the tool carrier 200 facing away from the dressing device 400, a workpiece spindle 500, which is only partially visible in
A machine control 600, shown only symbolically, receives signals from sensors in the machine and controls the linear and pivot axes of the machine, the tool spindle, the workpiece spindle and the dressing device.
A machine concept according to
In
The grinding tool 320 is illustrated in
In particular,
In the context of the present invention, a disk-shaped first conditioning tool 425 is clamped on the second dressing spindle 420 in lieu of a dressing tool. Additionally or alternatively, a stationary second conditioning tool 416 may be provided. The stationary conditioning tool 416 is held in a holder 417, which in the example of
Thus, in the context of the present invention, the dressing device 400 performs the function of a combined dressing and conditioning device. Strictly speaking, only the first dressing spindle 410 with the dressing tool 415 clamped thereon forms the actual dressing device, while the second dressing spindle 420 with the conditioning tool 425 clamped thereon and the holder 417 with the stationary conditioning tool 416 form a conditioning device.
Using NC axes to generate movements with respect to X1, Y1, Z1, A1, X_P, Y_P, C_P1, and C_P2, the grinding tool 320 can be selectively brought into engagement with each of the three dressing or conditioning tools 415, 416, and 425.
While the grinding tool 320 in
To dress the grinding tool 320 or 321, the rotating grinding tool 320, 321 is first brought into engagement with the dressing tool 415, which is also rotating. This produces or restores the desired outer contour of the grinding tool 320, 321 and the grinding tool 320, 321 is sharpened.
In order to avoid or at least reduce the undesirable grinding-in behavior of the grinding tool 320, 321 dressed in this way, as described above, the rotating grinding tool 320, 321 is then brought into engagement with the rotating conditioning tool 425 and/or with the stationary conditioning tool 416. Conditioning is carried out until it is ensured that no thermal damage occurs to the edge zone of the workpieces during subsequent workpiece machining, even if machining is carried out with the same technological parameters for all workpieces.
Instead of a dressing tool and a conditioning tool of the type shown in
The dressing tool 415 may be any dressing tool suitable for dressing an abrasive body made of vitrified-bonded cBN. Such dressing tools are known in the prior art in a variety of embodiments. They can be used for dressing in various ways.
For example, it is known that the dressing of a grinding worm can be performed in line contact between the dressing tool and the grinding tool in order to map the profile of the dressing tool onto the profile of the grinding tool. This is referred to as “profile dressing”. If the dressing tool rotates, it is referred to as a “profile roll”. Depending on the dressing tool and dressing device, each flank of a worm start can be dressed individually during profile dressing, both flanks of a worm start can be dressed simultaneously, or the flanks of two or more worm starts of a multi-start grinding worm can be dressed simultaneously. In addition to the flanks, it is also possible to dress the head and/or foot areas of the worm starts simultaneously or successively. The same dressing tool or another dressing tool can be used for this purpose (cf. e.g. U.S. Pat. No. 6,234,880B1).
It is also known to dress a grinding worm in point contact, whereby the dressing tool is then guided line by line along the flanks of the grinding worm. This is referred to as “form dressing”. If the dressing tool rotates, it is referred to as a “form roll”.
Mixed forms are also known, in which parts of the profile are dressed in line contact and other parts in point contact, either with different dressing tools or with different areas of the same dressing tool (see e.g. U.S. Pat. No. 6,012,972A).
Accordingly, there are a large number of designs of dressing tools. For example, disc-shaped dressing tools (dressing rolls) are known which are driven to rotate about a dressing tool axis for dressing, as is the case with the dressing tool 415. The dressing tool then often has a disc-shaped base body made of steel on which an abrasive coating, for example of diamond grains, is applied. Other types of dressing tools, on the other hand, are configured to be stationary. Such dressing tools may also have a base body of steel which is coated with abrasive material.
Different types of dressing methods and corresponding dressing tools are also known for dressing profile grinding wheels. In particular, a profile grinding wheel can also be dressed in line contact or in point contact. This can again be done with a rotating, disc-shaped dressing tool of the type of dressing tool 415 or with a stationary dressing tool, wherein the dressing tool may have a base body made of steel and an abrasive coating.
An equally large variety of configurations is possible for the conditioning process and the conditioning tool used for this purpose. The conditioning of the grinding tool can also be carried out in line contact or in point contact. The conditioning tool may be configured to be rotating or stationary. In particular, it may be formed by the steel base body of a dressing tool in which the abrasive coating has been omitted, so that the grinding tool is conditioned directly with the steel of the base body.
The conditioning tool may be of the same type as the dressing tool. For example, both the dressing tool and the conditioning tool may be a disc-shaped tool that is rotated during dressing or conditioning. However, the conditioning tool may also be different from the dressing tool. For example, the dressing tool may be rotating while the conditioning tool is stationary.
The decisive factor is that conditioning is not performed with a sacrificial workpiece that is clamped on the workpiece spindle for conditioning, but with a separate conditioning tool. The conditioning tool is not clamped on the workpiece spindle, and conditioning is not performed with a kinematics that correspond to the kinematics used in workpiece machining, but rather conditioning is performed with a kinematics that correspond to the kinematics of a typical dressing operation. While the kinematics used in conditioning may be different from the actual kinematics used in dressing (e.g., because the dressing tool and the conditioning tool are not the same), it is nonetheless a kinematics such as might be used in dressing.
In the examples of
After conditioning, the machining of workpieces takes place. For the sake of completeness, this is illustrated in
In the example of
In the example of
The method described above is summarized in the form of a flow chart in
The invention is not limited to the above embodiments, and further variations are possible. In particular, the invention is not limited to any particular machine design, but can be used with any gear grinding machine that allows both dressing and conditioning.
| Number | Date | Country | Kind |
|---|---|---|---|
| 01589/20 | Dec 2020 | CH | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/084598 | 12/7/2021 | WO |
