A GRINDING TOOL

Abstract

 Grinding tool (100) comprising a grinding portion (110), wherein the grinding portion (110) is shaped according to a predetermined shape (120) and is substantially rotationally symmetric with respect to a symmetry axis. The grinding tool (100) is manufactured according to a dressing process, the dressing process comprising the steps of (a) rotating the grinding tool (100) about the symmetry axis with a cutting speed; and (b) putting cutting means (300) in engagement with the grinding portion (110) and moving the cuttings means (300) along a given path thereby cutting and shaping the grinding portion (110). The cutting means traverses across the grinding portion (110) with a feed rate. The grinding portion (110) is made of a first material, wherein the first material comprises metal-bonded cubic crystalline boron nitride grains, and the cutting means (300) is made of a second material, wherein the second material comprising at least a cubic crystalline boron nitride material and/ordiamond.

Description

[0001] The present invention refers to grinding tools, and in particularto grinding tools for tooth flank grinding processes. More particularly, the invention refers to grinding wheels, such as, metal-bonded grinding wheels for tooth flank grinding processes.

[0002] In the prior art, grinding tools for tooth flank grinding processing are typically ceramic-bonded corundum grinding tools or ceramic-bonded cubic boron nitride (CBN) grinding tools. In the art, metal-bonded grinding tools are not used for tooth flank grinding processes owing to inherent limitations affecting the known dressing processes (electro-erosive dressing processes, laser dressing processes, dressing processes performed by means of SiC rolls and the like) in the production of such metal-bonded tools. Such dressing processes are indeed complex and costly and are not able to cope with the relatively bad properties of metal-bonded grinding tools in a dressing process. Moreover, the known dressing processes are unsuitable to meet the relatively high requirements on the profile accuracy of grinding tools for tooth flank grinding processing.

[0003] Ceramic-bonded grinding tools have a relatively low wear resistance, which negatively affects the durability and the reliability of the grinding tool. More specifically, the grinding portion of ceramic grinding tools has a relatively high tendency to wear during use, which uncontrollably alters the shape of the grinding portion of the tool thereby jeopardising the precision of the profile of tooth flanks processed with said tools. Frequent re-dressing of such ceramic-bonded grinding tools is thus necessary again rendering the use of these tools uneconomic. Moreover, tooth flanks processed with ceramic-bonded grinding tools exhibit a relatively high surface roughness and may suffer from thermal damages, due to the relatively poor thermal conductivity of the ceramic.

[0004] It is an object of the present invention to provide a grinding tool, in particular a grinding wheel, which at least partially overcomes the drawbacks of the prior art and, in particular, provides a grinding tool for tooth flank grinding processes that provides enhanced accuracy over a longer period as well as reduced wear.

[0005] This object is solved by a grinding tool according to claim 1. Embodiments of the invention are the subject matter of the dependent claims.

[0006] The grinding tool of the invention is characterized by a dressing process applied in the manufacturing thereof using specific dressing tools in a dressing process having specific process parameters with a specific material of the grinding portion of the grinding tool. As such, in the invention, it has been surprisingly found that application of such a dressing process using specific dressing tools is not only possible to manufacture grinding tools having a grinding portions of a specific material but also results in grinding tools exhibiting a significantly enhanced accuracy over a longer period of use as well as a reduced overall wear.

[0007] Accordingly, the invention relates to a grinding tool comprising a grinding portion, wherein the grinding portion is shaped according to a predetermined shape and is substantially rotationally symmetric with respect to a symmetry axis.

[0008] The grinding tool according to the present invention is manufactured according to a dressing process for dressing the grinding portion, the dressing process comprising at least the steps of:

a) Rotating the grinding tool about its symmetry axis with a predetermined cutting speed;

b) Putting the cutting means in engagement with the grinding portion and moving the cuttings means along a given path at a predetermined feed rate, and thereby applying a predetermined shape to the grinding portion.

[0009] The grinding portion and the cutting means are made of a first and of a second material, respectively. The first material comprises metal-bonded cubic crystalline boron nitride grains and the second material comprises at least a cubic crystalline boron nitride material and/or diamond.

[0010] For example, the grinding tool may be a grinding wheel having a rotational symmetry with respect to a symmetry axis. The grinding tool may comprise an inner body, e.g. a metallic body, and a substantially annular outer grinding portion affixed to the inner body.

[0011] The body of the grinding tool may also be asymmetric around the symmetry axis. For instance, the form of the body may not be rotationally symmetric, e.g. the transverse cross section of the body may be square, triangular, or may have another non-rotationally symmetric form. Alternatively or in conjunction with the above, the body may comprise vibration-damping elements and/or through-holes in an asymmetrical arrangement.

[0012] The grinding portion may be arranged at a peripheral portion thereof, the peripheral portion being made of the first material. Said peripheral portion may or may not have a substantially rotational symmetry with respect to the symmetry axis, as long as the surface shape of the grinding portion exhibits such symmetry.

[0013] The predetermined shape of the grinding portion is chosen so as to impart the desired shape to at least a portion of the workpiece, which is to be processed with the grinding tool. In particular, the predetermined shape of the grinding portion may have the substantially negative form of a feature that is to be applied to such workpiece, such as, for example, a dent of a gear wheel.

[0014] The cutting means may for instance be a tip, e.g. an indexable tip connected a tipped tool via e.g. a material, a positive or a non-positive fit. The cutting means, e.g. the indexable tip, may be mounted on a movable support of a dressing machine in order to allow the cutting means to be moved along the given path. The given path of the cutting means includes trajectories described by the cutting means to engage and to shape the grinding portion, e.g. to impart the predetermined shape to the grinding portion.

[0015] The cutting speed in the sense of the present invention is the rotational velocity of the grinding portion. For instance, if the grinding tool is a grinding wheel and the portion is an outer annular region thereof, the rotational velocity is equal to the angular velocity of the grinding wheel multiplied by the peripheral radius of the annular region.

[0016] The feed rate in the sense of the present invention is the rate at which the cutting means traverses the grinding portion; it includes the feed rate along a direction substantially parallel to the symmetry axis of the grinding tool. The feed rate may also have a radial component along the radial direction of the grinding wheel and/or an axial component along the symmetry axis. Moreover, the value of the feed rate, of the axial component thereof, and/or of the radial component thereof may vary as e.g. the position of the cutting means with respect to the grinding tool is moved.

[0017] The grinding portion of the grinding tool of the invention, including the manufacturing steps according to the present invention exhibits a lower surface roughness and thus higher quality compared to known ceramic-bonded grinding tools. Said grinding tool has exhibits an improved wear resistance, which increases the overall durability and the reliability of the grinding tool. In particular, the grinding portion is able to withstand a relatively large amount of mechanic and thermal stress for a relatively high amount of time without altering its shape.

[0018] The dressing method according to the present invention, in conjunction with the specific tools, materials and parameters involved in the process, allows for imparting the predetermined shape to the grinding portion with a degree of accuracy, which is significantly improved over known dressing processes. This feature, among others, distinguishes the metal-bonded grinding tool according to the present invention from ones known in the art where, however, the process for applying the predetermined shape in conjunction with the specific tools, materials and parameters involved in the process represents the main distinction attaining such feature.

[0019] The grinding tool dressed according to this dressing method is thus able to meet improved requirements forthe accuracy of the profile of the grinding portion and is thus particularly suitable for tooth flank processing of gear wheels where continuous improvements in accuracy are of paramount importance.

[0020] According to the present disclosure, the disclosure of a range of values is considered to include all values of the range, including not only the range defining values but in particular also the values intermediary to such range defining values, even in such case where these intermediary values are not expressly noted. Also, it is understood that the range defining values disclosed can be interchanged amongst one another, that is, for example, a first lower range defining value is not only considered disclosed in conjunction with the first upper range defining value but also, where suitable, with a second, or third upper range defining value, or even with a second or third lower range defining value.

[0021] In an embodiment of the present invention, the cutting speed is greater than or equal to 100 m/min and less than or equal to 3000 m/min, preferably greater than or equal to 180 m/min and less than or equal to 2400 m/min, further preferably greater than or equal to 360 m/min and less than or equal to 1800 m/min, further preferably greater than or equal to 520 m/min and less than or equal to 1200 m/min, and further preferably about 600 m/min.

[0022] At such values of the cutting speed, it showed that the manufacturing stress acting on the grinding tool and the manufacturing stress acting on the cutting means are reduced while the quality of the manufacture of the grinding tool and the durability of the cutting means are significantly increased at the same time. In particular, the manufacturing stress acting on the grinding tool and the cutting means comprise both the thermal and the mechanical stresses.

[0023] Alternatively, or in combination with the afore mentioned cutting speeds, in another embodiment of the present invention, the feed rate is greater than or equal to 0,005 mm/rev and less than or equal to 2,5 mm/rev, preferably greater than or equal to 0,1 mm/rev and less than or equal to 2,0 mm/rev, further preferably greater than or equal to 0,2 mm/rev and less than or equal to 1,5 mm/rev, further preferably greater than or equal to 0,3 mm/rev and less than or equal to 1,0 mm/rev, and further preferably about 0,33 mm/rev.

[0024] The feed rate interacts with the cutting speed in such way as to ensure that the manufacturing stress acting on the grinding tool and the manufacturing stress acting on the cutting means remain low. The interplay of feed rate and cutting speed thus allows for an optimized production process being both fast and accurate as well as exerting least possible mechanical and thermal stress on the grinding tool's grinding portion.

[0025] In a particularly preferred embodiment, the value of the cutting speed is chosen in such way that the dress rate ∂, which is defined as the ratio of the cutting speed vs. the feed rate is 1*10⁴1/s to 9*10⁴1/s, more preferably 2*10⁴1/s to 6*10⁴1/s, and further more preferably about 3*10⁴1/s.

[0026] In another embodiment of the grinding tool according to the present invention, the grains comprised in the first material are coated and, in particular metal coated. The coating of the grains leads to an increase of the service life of the grinding tool and results in an increase of the g-ratio of the grinding tool. According to the present invention the g-ratio of a grinding tool is the ratio of the volume of ground material removed from the workpiece by the grinding tool vs. the volume removed from the grinding tool.

[0027] In a further embodiment of the grinding tool, the size of the grains comprised in the first material is comprised within 5 µm and 35 µm. Preferably, the size of the grains comprised in the first material is comprised within 9 µm and 30 µm. More preferably, the size of the grains comprised in the first material is comprised within 12 µm and 30 µm. More preferably, the size of the grains comprised in the first material is about 30 µm.

[0028] Grain sizes within said intervals lead to an increased service life and improved g-ratios of the grinding tool given a reduced thermal stress acting on the surfaces processed using the grinding tool.

[0029] In an embodiment of the grinding tool, the grains comprised in the first material are bonded by a bonding material, e.g. by a metal, with a predetermined hardness. The hardness may be expressed in terms of a Brinell hardness number (HB), for example, determined according to the ISO 6506-1 standard.

[0030] Preferably, the value of the hardness is chosen between 95 HB and 270 HB, preferably between 210 HB and 260 HB, and further preferably between 225 HB and 250 HB. Such values of the hardness of the bonding material result in a relatively low value of roughness of the surfaces ground and similarly as before, lead to an increased service life and improved g-ratios of the grinding tool given a reduced thermal stress acting on the surfaces processed using the grinding tool. In yet another embodiment of the grinding tool accordingto the present invention, the grains comprised in the first material are bronze-bonded.

[0031] If the grains comprised in the first material are weak-bonded, that is, the value of the hardness of the bonding material is between 95 HB and 150 HB, more particularly 110 HB and 135 HB a relatively low value of roughness of the surfaces ground with the grinding tool according to the present invention results and thus the quality of such surfaces processed improve.

[0032] If the grains comprised in the first material are medium-bonded, that is, the value of the hardness of the bonding material is between 160 HB and 210 HB, more particularly 175 HB and 200 HB. The presence of medium-bonded grains leads to an increase of the g-ratio of the grinding tool and a decrease of the thermal stress acting on the surfaces processed therewith are achieved, thus leading to an increased service life.

[0033] If the grains comprised in the first material are hard-bonded, that is, the value of the hardness of the bonding material is between 210 HB and 265 HB, more particularly 225 HB and 250 HB. In this case, a further improvement of the properties of the grinding tool is achieved as well as further improvements on the quality of the surfaces processed.

[0034] According to embodiment of the present invention, the second material comprises polycrystalline cubic boron nitride (PCBN) grains and/or coated PCBN grains, e.g. metal-coated PCBN grains.

[0035] The hardness of materials comprising PCBN grains renders the second material relatively resistant to mechanical stresses and allows for an accurate shaping of the grinding portion of the grinding wheel in the dressing process by means of the cutting means. Alternatively or in conjunction with the above, the second material comprises a polycrystalline diamond (PDC) material. In particular, the PDC material may comprise polycrystalline diamond grains bonded with cobalt.

[0036] In these embodiments, the toughness and the wear resistance of the second material is improved, which indeed, leads to the feasibility of the dressing process for shaping the grinding portion given g-ratios which are suitably large, that is, significantly larger than 1.

[0037] The second material may also comprise a CVD (chemical vapour deposition) thick diamond layer. The CVD thick diamond layer is for instance a CVD diamond layer with a thickness greater than or equal to 0,2 mm and less than or equal to 6 mm. Preferably, the thickness of the CVD thick diamond layer may be greater than or equal to 0,3 mm and may be less than or equal to 5 mm. CVD thick diamond layers exhibit superior hardness and thus large resistance to abrasion. In this case, the cutting means undergoes substantially no damaged in the dressing process itself, which increases its durability and warrants for continuous precision of the dressing of the grinding tools.

[0038] In an embodiment of the grinding tool of the present invention, at least a portion of the predetermined shape substantially conforms to an involute geartooth profile and/orto a cycloidal gear tooth profile. Alternatively or in conjunction with the above, at least a portion of the predetermined shape is a 1A1 profile. The involute geartooth profile and to the cycloidal geartooth profile may in particular comprise at least a portion of the design profile of a to-be ground involute gear and of a to-be ground cycloidal gear, respectively.

[0039] For instance, the involute and/orthe cycloidal geartooth profile may comprise a face of the flank of a first tooth of the design profile of the to-be-ground involute and/or cycloidal gear, respectively. The involute and/orthe cycloidal geartooth profile may also comprise a face of the flank of a second tooth of the to-be-ground gear, wherein the first and the second tooth are in particular adjacent to each other, and wherein the face of the first tooth is opposite to the one of the second tooth. The involute and/orthe cycloidal geartooth profile may also comprise the bottom land between the first and the second tooth, the top land of the first tooth, and/orthe top land of the second tooth. In this case, the tooth flanks of the to-be-ground gear are processed and shaped by "carving" the space between two teeth of the to-be-ground gear.

[0040] The involute gear tooth profile and/or the cycloid gear tooth profile may comprise the two faces of a third tooth of the design profile of the to-be-ground involute and/orcycloid gear, respectively. The involute gear tooth profile and/or the cycloid gear tooth profile may also comprise the two bottom lands adjacent to the third tooth and/orthe top land of the said tooth. This way, the to-be-ground gear is processed by separately shaping each tooth of the said gear.

[0041] In an embodiment of the present invention, the cutting means engages the grinding portion of the grinding tool with a cutting depth. The value of the cutting depth is between 0,05 mm and 0,20 mm. Preferably, the cutting depth is between 0,07 mm and 0,15 mm. More preferably, the cutting depth is about 0,10 mm.

[0042] In another embodiment of the present invention, the cutting means comprises a corner radius, wherein the length of the corner radius is comprised within 0,4 mm and 1,6 mm. Preferably, the length of the corner radius may be comprised within 0,6 mm and 1,2 mm. More preferably, the length of the corner radius may be about 0,8 mm.

[0043] In the present invention, the predetermined path of the cutting means is chosen so that the cutting means engages the grinding portion of the grinding tool with a pulling motion or a with pushing motion. Typically, fractured surfaces and feeding grooves may occur on the grinding portion during the dressing of the grinding tool, especially in proximity of a critical portion of the grinding tool. Said critical portion is for instance the one responsible for the shaping of the bottom lands of the gear. A pushing motion of the cutting means allows for processing the critical portion under mechanical pressure, thereby reducing the occurrence of fractured surfaces and/or feeding grooves.

[0044] The grinding tool according to the present invention, and in particular the embodiments thereof disclosed herein, may be advantageously used for tooth flank processing given the improved durability of the grinding tools, the improved and at the same time constant profiling accuracy thereof and the thus resulting elevated accuracy and quality of the teeth of a gearwheel, such as, for example, a spur gear, e.g. of an involute or a cycloidal spur gear.

[0045] The quality of the surfaces of the gears processed with such grinding tools is improved over ones having surfaces processed with known ceramic-bonded grinding tools. More specifically, metal-bonded grinding wheels have a relatively high thermal conductivity, which limits the negative effects of thermal stress on the surfaces during the grinding process. Contrary to ceramic-bonded tools, metal-bonded grinding tools may comprise grains having a relatively small size, thereby also reducing the roughness of the surfaces ground using said tools.

[0046] The invention also refers to the dressing method to manufacture the grinding tool according to the present invention, and, in particular, embodiments thereof disclosed herein. More specifically, said dressing method comprises at least the steps a) and b) described above to manufacture the grinding tool according to the present invention.

[0047] The dressing method of the present invention allows inter alia to reduce the production costs of the grinding tools of the invention and, indeed, allows for an improved re-processing thereof. Overall, the specific parameters of the dressing method of the invention in conjunction with the materials and tools chosen, allow for the dressing of a grinding tool using a cutting means as per the disclosed process.

[0048] Exemplary embodiments of the invention are described in the following with respect to the attached figures. The figures and corresponding detailed description serve merely to provide a better understanding of the invention and do not constitute a limitation whatsoever of the scope of the invention as defined in the claims. In particular:

Fig. 1a

is a schematic view of the longitudinal cross section of a first embodiment of the grinding tool according to the present invention;

Fig. 1b

is a schematic view of the longitudinal cross section of the first embodiment of the grinding tool during performance of the dressing process;

Fig. 1c

is a schematic view of the transverse cross section of the first embodiment of the grinding tool during the performance of the dressing process;

Fig. 2a

is a schematic view of the predetermined shape of the grinding portion of a second embodiment of the grinding tool;

Fig. 3

is a schematic view of a conventional face milling process, which may be performed with the grinding tool according to the present invention;

Fig. 4a

is a plot depicting the impact of the size of the grains of the first material on the roughness of workpieces processed with grinding tools according to the present invention;

Fig. 4b

is a plot depicting the impact of the size of the grains of the first material on the internal stress of workpieces processed with grinding tools according to the present invention;

Fig. 4c

is a plot depicting the impact of the size of the grains of the first material on the g-ratio;

Fig. 4d

is a plot depicting the impact of the size of the grains of the first material on the specific normal force acting on workpieces processed with grinding tools according to the present invention;

Fig. 5a

is a plot depicting the impact of the hardness of the coating material on the roughness of workpieces processed with grinding tools according to the present invention;

Fig. 5b

is a plot depicting the impact of the hardness of the coating material on the internal stress of workpieces processed with grinding tools according to the present invention;

Fig. 5c

is a plot depicting the impact of the hardness of the coating material on the g-ratio;

Fig. 5d

is a plot depicting the impact of the hardness of the coating material on the specific normal force acting on workpieces processed with grinding tools according to the present invention;

Fig. 6

is a plot depicting the impact of the grain coating on the g-ratio;

Fig. 7a

is a schematic view of a discontinuous profile grinding process, which is performed on an involute gear wheel with a third embodiment of the grinding tool of to the present invention;

Fig. 7b

is a plot depicting the accuracy of the profile of the involute gear wheel ground with the third embodiment of the grinding tool; and

Fig. 8

is a schematic view of an oriented portion of the given path followed by the cutting means during the performance of the dressing process to manufacture a fourth embodiment of the grinding tool of the present invention.

[0049] Figs. 1a shows a schematic view of the longitudinal cross section of the grinding tool according to the present invention. The grinding tool is a grinding wheel 100 comprising an inner body 130 and a grinding portion 110 connected therewith.

[0050] The inner body 130 may for instance be made of metal and may be a substantially circular disk centred on the symmetry axis R. In particular, the body is substantially rotationally symmetric with respect to the symmetry axis R. The inner body 130 comprises a bush 140, which substantially extends along a symmetry axis R. A revolving shaft (not shown) may in particular be inserted inside the bush 140 to put the grinding wheel in rotation about the bush 140 and thus about the symmetry axis R.

[0051] The grinding portion 110 is a substantially annular outer region of the grinding wheel 100 having a substantially rotational symmetry with respect to the symmetry axis R. The grinding portion 110 is shaped according to the predetermined shape 120 and is made of the first material, the first material comprising metal-bonded cubic crystalline boron nitride grains.

[0052] The predetermined shape 120 of the grinding portion 110 may be chosen so as to impart the desired shape to at least a portion of the workpiece, which is to be processed with the grinding wheel 100. In particular, the predetermined shape 120 may conform to the form of at least a portion of such workpiece. For example, if the workpiece is an involute or a cycloidal gear, the predetermined shape 120 may substantially conform to an involute gear tooth profile or to a cycloidal gear tooth profile, respectively. The predetermined shape may for instance be a 1A1 profile.

[0053] Fig. 1b and Fig. 1c depict a transverse and a longitudinal cross section of the first embodiment of the grinding tool 100 during the performance of the dressing process of the present invention, respectively. The grinding wheel 100 rotates about the symmetry axis R with an angular velocity W_A and in particular may be put in rotation by means of a revolving shaft (not shown) inserted inside the bush 140.

[0054] Consequently, the border region of the grinding portion rotates with a given cutting speed Vc, which is equal to the angular velocity W_A multiplied by the radius R_A of the grinding wheel 100. The cutting speed Vc may be between 100 m/min and 3000 m/min, more particularly, the cutting speed may be about 600 m/min.

[0055] The cutting means 300 engages with the grinding portion 110 of the grinding wheel 100 along a given path thereby cutting and shaping the grinding portion 110 to substantially conform to the predetermined shape 120. The cutting means 300 are made of the second material, the second material comprising at least a cubic crystalline boron nitride material and/or diamond.

[0056] The cutting means 300 may in particular be an indexable tip connected a tipped tool 310 via e.g. a material, a positive or a non-positive fit. Such indexable tip 300 may be mounted on a movable arm of a dressing machine (not shown) in order to allow the cutting means to be moved along the given path.

[0057] The cutting means traverses across the grinding portion with a controlled feed rate. For example the feed rate is the rate along the feed direction, the feed direction D being substantially parallel to the symmetry axis R, or the given path. For instance, the value of the feed rate may be between 0,005 mm/rev and 2,0 mm/rev, and more particularly about 0,33 mm/rev.

[0058] Most preferably, the cutting speed and the feed rate are chosen so that the dress rate is between 1*10⁴1/s to 9*10⁴1/s, more preferably 2*10⁴1/s to 6*10⁴1/s, and further more preferably about 3*10⁴1/s.

[0059] Fig. 2 is a schematic view of the predetermined shape 120 of the grinding portion 110 of a second embodiment of the grinding tool 100, wherein the second embodiment may in particular comprise the features of the first embodiment depicted in Figs. 1a to 1c and described above.

[0060] The predetermined shape 120 substantially conforms to an involute or to a cycloidal gear tooth profile 540, which is a portion of the design profile 500 of a to-be ground involute or cycloidal gear (not shown), respectively, wherein this design profile 500 is depicted by a broken line.

[0061] The involute gear tooth profile 540 comprises a face 512 of the flank of the first tooth 510 of the design profile 500 and a face 522 of the flank of a second tooth 520 of said profile 500. The first 510 and the second tooth 520 are in particular adjacent to each other, and the face 512 of the first tooth 510 is opposite to the face 522 of the second tooth 520. The involute gear tooth profile comprises the bottom land 530 between the first 510 and the second 520 tooth, the top land 511 of the first tooth 510, and the top land 521 of the second tooth 520.

[0062] In this case, the tooth flanks of the to-be-ground involute or cycloidal gear are processed and shaped by "carving" the space between two teeth of said gear.

[0063] For later convenience, a conventional face milling process of a to-be-processed metal bar 600 is schematically depicted in Fig. 3. In particular, this face milling process is performed by using a grinding wheel 100, which rotates about the symmetry axis R with an angular velocity W_A and with a speed velocity V_C. The value of the speed velocity V_C is in particular about 30 m/s. The to-be-processed metal bar 600 linearly moves with respect to the grinding wheel 100 with a feed speed V_f, the value of the feed speed V_f being about 150 mm/s. The grinding wheel 100, e.g. the grinding portion thereof 110, engages the to-be-processed metal bar 600 with a width of the cut A_E of about 50 µm.

[0064] Figs. 4a to 4d quantitatively describe the impact of the grains size on the quality of the workpieces processed with the grinding tools of the present invention. These figures allow for comparing the quality of three different workpieces. These workpieces are edge-zone hardened bars made of case-hardened steel with a hardness comprised within 60 HRC and 62 HRC. They are processed according to the process depicted in Fig. 3 and differ from each other in the embodiment of the grinding wheel with which they are milled.

[0065] The three embodiments used to mill the aforementioned workpieces are henceforth called B9-, B12-, and B30-embodiment. They comprise the features of the above-described first embodiment of the grinding tool and differ from each other in the size of the grains comprised in the first material. More specifically, in the B9-, B12-, and B30-embodiment said grain size is about 9, 12, and 30 µm, respectively.

[0066] As shown in Fig. 4a, the value of the roughness average Ra of the milled surface of the three workpieces is relatively small, namely less than 0,4 µm. In particular, the roughness average Ra increases with the grain size and is minimal for the surface of the workpiece processed with the B30-embodiment of the grinding tool 100.

[0067] The thermal stress acting on the milled surfaces of the three workpieces may be evaluated by looking at the internal stress S and at the specific normal force F'_n depicted in Fig. 4b and Fig. 4d, respectively. For instance, the internal stress S may be measured non-destructively by using e.g. X-raydiffractometry. In particular the forces involved in the milling process, e.g. the specific normal force F'_n, may be measured by means of piezoelectric force measurement devices.

[0068] As clearly shown in in Fig. 4b and Fig. 4d, the thermal stress is substantially reduced in case of the workpiece processed with the B12-embodiment of the grinding tool 100 and is further reduced if the face milling process is performed with the B12-embodiment of the grinding tool 100.

[0069] A similartrend may be noticed in the case of the g-ratio G shown in Fig. 4c: the two embodiments of the grinding tool 100 having a grain size within 12 and 30 µm are characterized by an increase of the value of g-ratio G. Said increase is particularly considerable in the case of the B30-embodiment of the grinding tool 100, that is in the case of the embodiment having grains with size of about 30 µm.

[0070] Similarly, the service life of the grinding tool is relatively high forthe B12- and of the B30-embodiment of the grinding tool 100, e.g. forthe embodiments with a grain size comprised within 12 µm and 30 µm. A further increase of the service life is to be noted in the case of the B30-embodiment of the grinding tool 100, that is in the case of the embodiment having grains with size of about 30 µm.

[0071] In particular, the end of the lifetime is determined by a rapid increase of the forces involved in the milling process (e.g. of the specific normal force F'_n) due to the clogging of the chip space of the grinding tool 100.

[0072] Figs. 5a to 5d quantitatively describe the impact of the hardness of the bonding material on the quality of the workpieces processed with the grinding tools of the present invention. These figures allow for comparing the quality of three different workpieces. These workpieces are edge-zone hardened bars made of case-hardened steel with a hardness comprised within 60 HRC and 62 HRC.

[0073] They are processed according to the process depicted in Fig. 3 and differ from each other in the embodiment of the grinding wheel with which they are milled.

[0074] The three embodiments used to mill said workpieces are henceforth called W-, M-, and H-embodiment. They comprise the features of the above-described first embodiment of the grinding tool and differ from each other in the hardness of the bonding material, which bonds the grains of the first material. More specifically, in the W-, M-, and H-embodiment said grains are weak-, medium-, and hard-bonded, respectively. In the W-, M-, and/or H-embodiment the size of the grain may in particular be about 30 µm.

[0075] The value of the roughness average Ra of the milled surface of the three workpieces is relatively small, namely than 0,4 µm (see Fig. 5a). In particular, the roughness average Ra increases with the hardness of the coting material and is minimal forthe surface of the workpiece processed with the W-embodiment of the grindingtool 100, i.e. with the embodiment having weak-bonded grains.

[0076] As can be appreciated by looking at the dependence of the internal stress S and of the specific normal force F'_n on the hardness of the coating material (Figs. 5b and 5d), the thermal stress acting on the milled surfaces of the three workpieces decreases as the hardness of the coating material increases. More specifically, thermal stress is substantially reduced by using medium-bonded grinding tools 100 and is further diminished for workpieces processed with hard-bonded grinding tools 100.

[0077] As can be appreciated from Fig. 5c, the presence of medium-bonded grains leads to an unexpected increase of g-ratio G of the grinding tool 100. The g-ratio G may be further increased by using hard-bonded grinding tools 100, that is by using the H-embodiment of the grinding tool 100.

[0078] The service life of the grinding tool turns out to be relatively high forthe M-embodiment of the grinding tool 100, that is forthe embodiment having medium-bonded grains. A further increase of the service life is to be noted in the case of the H-embodiment of the grinding tool 100, i.e. in the case of the embodiment having hard-bonded grains.

[0079] The plot in Fig. 6 shows how the g-ratio G is affected by the coating of the grains of the first material. More specifically, this figure shows the g-ratio G as measured after processing two workpieces with two embodiments, henceforth called U-and C-embodiment, of the grindingtool of the present invention. These embodiments comprise the features of the above-described first embodiment of the grinding tool and differ from each other in the coating of the grains of the first material. In particular, the grains of the first material of the C-embodiment are coated while the ones of the U-embodiment are not.

[0080] The two workpieces are edge-zone hardened bars made of case-hardened steel with a hardness comprised within 60 HRC and 62 HRC. They are processed according to the process depicted in Fig. 3 and differfrom each other in the embodiment of the grinding wheel used during said process. In particular one workpiece is milled with the U-embodiment while the other one is processed with the C-embodiment.

[0081] As can be taken from Fig. 6, the coating of the grains leads to an increase of the g-ratio G of the grinding tool 100. Moreover, the coating of the grains results in an unexpected increase of the service life of the grinding tool 100, said increase amounting up to a factor of five.

[0082] Fig. 7a is a schematic view of a discontinuous profile grinding process, which is performed to an involute gearwheel500 with a third embodiment of the grinding tool 100 of the present invention.

[0083] The third embodiment of the grinding tool 100 comprises the features of the first embodiment of the grinding tool 100 as disclosed in Figs. 1a to 1c and as described above. Moreover, in this case the first material may comprise bronze-bonded uncoated grains, said grains having a size of about 9 µm. In particular, the predetermined shape 120 of the grinding portion 110 is a 1A1 profile.

[0084] According to the discontinuous profile grinding process, the grinding wheel 100 rotates with an angular velocity W_A about the symmetry axis R. The grinding portion 110 of the grinding wheel 100 engages a border portion 510 of the involute gearwheel 500 to impart to said border portion 510 a predetermined design profile 520, said design profile 520 being depicted in Fig. 7b. The involute gear wheel 500 may be rotatable about a rotation axis R₁ for e.g. changing the portion of the involute gearwheel500 which engages the grinding portion 110 of the grinding wheel 100.

[0085] The design profile 520 of the involute gear wheel 500 is in particular characterised by a normal module of about 2 mm, by a pressure angle of about 20°, and by the number of teeth of the gear wheel 500, number which in this case is equal to 37.

[0086] Fig. 7b shows the design profile 520 of the involute gear wheel 500 together with the actual profile 530 of the involute gearwheel500. More specifically, in Fig. 7b it is shown both the width W_P and the height H_P of the aforementioned profiles 520, 530. The actual profile 530 is shaped by the grinding wheel 100 during the discontinuous profile grinding process and is measured by means of e.g. a coordinate measurement device (not shown).

[0087] As shown in Fig. 7b, the actual profile 530 of the involute gear wheel 500 does not depart significantly from the design profile 520. The substantial match between the actual 530 and the design 520 profile shows that the grinding tools 100 of the present invention is suitable for processing gear wheels 500, since it allows for a relatively high profile accuracy. The quality of the surfaces of the gear wheels 500 processed with said grinding tools is relatively high, since said surfaces having a relatively low roughness and experience a relatively low thermal stress during processing.

[0088] Fig. 8 is a schematic view of an oriented portion 320 of the given path of the cutting means 300 during the performance of the dressing process to manufacture a fourth embodiment of the grinding tool 100 according to the present invention. As shown in this Figure, during the performance of the dressing process to manufacture the fourth embodiment of the grinding tool 100, the cutting means 300 engages the grinding portion 110 of the grinding tool 110 with a pushing motion along the oriented portion 320 of the given path.

[0089] In particular, the fourth embodiment of the grinding tool 100 according to the present invention may comprise the features of the first embodiment described above and shown in Figs. 1a to 1c.

Claims

1. Grinding tool (100) comprising a grinding portion (110), the grinding portion (110) being shaped according to a predetermined shape (120) and being substantially rotationally symmetric with respect to a symmetry axis (R), wherein
the grinding tool (100) is manufactured including to a dressing process for dressing the grinding portion (110), the dressing process comprising at least the steps of:

a) rotating the grinding tool (100) about the symmetry axis (R) with a predetermined cutting speed (V_C), and

b) putting the cutting means (300) in engagement with the grinding portion (110) and moving the cuttings means (300) along a given path at a predetermined feed rat, and thereby applying the predetermined shape (120) to the grinding portion (110),

the grinding portion (110) being made of a first material, wherein the first material comprises metal-bonded cubic crystalline boron nitride grains, and
the cutting means (300) being made of a second material, wherein the second material comprising at least a cubic crystalline boron nitride material and/or diamond.

2. Grinding tool (100) according to claim 1, wherein the cutting speed (V_C) is greater than or equal to 100 m/min and less than or equal to 3000 m/min, preferably greater than or equal to 180 m/min and less than or equal to 2400 m/min, further preferably greater than or equal to 360 m/min and less than or equal to 1800 m/min, further preferably greater than or equal to 520 m/min and less than or equal to 1200 m/min, and further preferably about 600 m/min.

3. Grinding tool (100) according to claim 1 or 2, wherein the feed rate is greater than or equal to 0,005 mm/rev and less than or equal to 2,5 mm/rev, preferably greater than or equal to 0,1 mm/rev and less than or equal to 2,0 mm/rev, further preferably greater than or equal to 0,2 mm/rev and less than or equal to 1,5 mm/rev, further preferably greater than or equal to 0,3 mm/rev and less than or equal to 1,0 mm/rev, and further preferably about 0,33 mm/rev.

4. Grinding tool (100) according to anyone of the preceding claims, wherein the dress rate (∂), defined as the ratio of the cutting speed vs. the feed rate, is 1*10⁴1/s to 9*10⁴1/s, more preferably 2*10⁴1/s to 6*10⁴1/s, and further more preferably 3*10⁴ 1/s.

5. Grinding tool (100) according to anyone of the preceding claims, wherein the value of the cutting speed (Vc) is greater than or equal to 600 m/min and less than or equal to 1200 m/min and the value of the feed rate is greater than or equal to 0,33 mm/rev and less than or equal to 0,5 mm/rev.

6. Grinding tool (100) according to anyone of the preceding claims, wherein the grains comprised in the first material are coated.

7. Grinding tool (100) according to anyone of the preceding claims, wherein the size of the grains comprised in the first material is greater than or equal to 9 µm and less than or equal to 30 µm, preferably greater than or equal to 12 µm and less than or equal to 30 µm, and further preferably about 30 µm.

8. Grinding tool (100) according to anyone of the previous claims, wherein the grains comprised in the first material are bonded by a bonding material having a hardness (h), which is greater than or equal to 95 HB and less than or equal to 270 HB, preferably greater than or equal to 210 HB and less than or equal to 260 HB, and further preferably greater than or equal to 225 HB and less than or equal to 250 HB.

9. Grinding tool (100) according to anyone of the preceding claims, wherein the grains comprised in the first material are bronze-bonded.

10. Grinding tool (100) according to anyone of the preceding claims, wherein the second material comprises polycrystalline cubic boron nitride grains, polycrystalline coated cubic boron nitride grains, a polycrystalline diamond material, and/or a CVD thick diamond layer.

11. Grinding tool (100) according to anyone of the preceding claims wherein at least a portion of the predetermined shape (120) substantially conforms to an involute gear tooth profile and/orto a cycloidal geartooth profile, and/or wherein at least a portion of the predetermined shape (120) is a 1A1 profile.

12. Grinding tool (100) according to anyone of the preceding claims wherein the engagement of the cutting means (300) with the grinding portion (110) has a cutting depth (a_p) being greater than or equal to 0,05 mm and less than or equal to 0,20 mm, further preferably greater than or equal to 0,07 mm and less than or equal to 0,15 mm, and further preferably about 0,10 mm.

13. Grinding tool (100) according to anyone of the preceding claims wherein the cutting means (300) has a corner radius (rε) being greater than or equal to 0,4 mm and less than or equal to 1,6 mm, preferably greater than or equal to 0,6 mm and less than or equal to 1,2 mm, and further preferably about 0,8 mm.

14. Use of the grinding tool (100) according to anyone of the preceding claims for tooth flank processing.

Drawing