CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to US Provisional Application No. 62/289,382, filed on February 1 , 2016.

FIELD OF THE INVENTION

[0002] This invention relates to metal finishing and to an improved apparatus and methods for microfinishing metal cylindrical surfaces with particular applicability for journal bearing surfaces of internal combustion crankshafts.

BACKGROUND

[0003] Numerous types of machinery components must have finely controlled surface finishes in order to perform satisfactorily. For example, fine surface finish control provided by an abrading tool or abrasive media, also referred to as microfinishing, is particularly significant in relation to the manufacturing of journal bearing and cam surfaces such as are found in internal combustion engine crankshafts, camshafts, power transmission shafts, and other machine-finished surfaces. For journal type bearings, very accurately formed journal surfaces are needed to provide the desired hydrodynamic bearing effect which results when lubricant is forced into the small clearance between the journal and the associated bearing shell. Improperly finished bearing surfaces may lead to premature bearing failure and can limit the load carrying capacity and performance of the bearing.

[0004] Currently there is a demand for higher control of crankshaft journal bearing surfaces by internal combustion engine manufacturers as the result of greater durability requirements necessary to offer improved product warranties, the higher operating speeds at which engines (particularly in motor vehicles) are now required to sustain, and the greater bearing loads imposed through increased efficiency and power output capability of engines. Furthermore, there is an increasing demand for improved performance for motor vehicle internal combustion engines in terms of their fuel efficiency capabilities. Moreover, owners and operators of such equipment require long service lives with low warranty claims and maintenance requirements. [0005] In addition to journal bearings, numerous other types of machined components also require finely controlled surface finishes for relatively rotating components, particularly in areas of sliding contact between parts.

[0006] Microfinishing for internal combustion engine crankshaft bearing journals is accomplished presently using various types of machining and tooling systems. Microfinishing operations ordinarily take place after a grinding process which produces the desired journal geometry, but such processes in general do not provide desired surface finish parameters. In typical microfinishing processes for crankshafts, the crankshaft is rotated about its main bearing journal axis and microfinishing tooling is brought to bear against the bearing surfaces, and upon rotation of the crankshaft, the machining action occurs. One approach uses abrasive stone tool inserts which provide the machining action. In another process type, abrasive coated film or tape is used as the machining agent which is pressed against the bearing surfaces using rigid or compliant tool insets such as formed from urethane type compositions or abrasive materials. The applicants have developed significant improvements in the field of journal microfinishing including machining apparatus and methods as described by US patents: 4,682,444; 5,095,663; 5,148,636; and 5,531 ,631 , the disclosures of which are incorporated by reference in their entirety.

[0007] Microfinishing approaches for crankshafts have generally sought to improve the surface finish of a journal bearing surface without changing the contour or geometry provided in the journal by prior machining processes such as, most commonly, grinding operations. For simple hydrodynamic journal bearings, some engine manufacturers specify a slight barrel shape in the journal surface when viewed axially along the journal surface to promote a desired hydrodynamic bearing effect. Although a so-called hourglass type profile in which the journal diameter is minimum at near the center of the axial length of the journal is less frequently desirable, it is specified for certain applications. In other applications an idealized constant diameter cylindrical surface is desired. In yet a further example of specific requirements of engine manufacturers, some engine configurations have a split pin crankshafts in which a single crankshaft journal supports two connecting rods and the journal is therefore comparatively long in length. In those split pin applications, a "double barrel" type journal surface profile may be desired. A crankshaft after grinding and before microfinishing may not have the specified profile configuration. Crankshaft microfinishing operations available today are limited in their capabilities of correcting geometric errors in incoming workpieces and in forcing the workpiece to a desired profile configuration. It is an object of the present invention to provide capabilities during microfinishing operations for maintaining a desired geometry or generating slight changes in journal shape desirable for providing the journal profiles mentioned above pursuant specifications.

[0008] Typical crankshafts have two or more main bearing journals, located on the axis of rotation of the crankshaft, and one or more eccentric connecting rod journals which move in an orbital path relative to the crankshaft axis of rotation. Typically a tool is brought into contact with the bearing journal and is oscillated during machining in the direction of the cylindrical axis of the journal during relative rotation between the tool, the abrasive agent and the journal surface to enhance material removal and eliminate machining debris and abrasive particles. Microfinishing tooling for internal combustion crankshafts must operate within the confines of the axial length of the journal bearing surface being machined since crankshaft structures such as the throws, webs, flanges, and gears present between the bearing surfaces interfere at the axial ends of the journals. Therefore, typical crankshaft microfinishing machines use a pair of arms exending perpendicular to the crankshaft axis of rotation which clamp onto the bearing journals during machining. High production rate machines provide simultaneous microfinishing of multiples or all the the crankshaft bearing journal surfaces of a crankshaft.

[0009] Manufacturers of internal combustion engines and component manufacturers seek to minimize the capital expenditure of machining equipment. This is a factor in terms of the number of machines and tooling configurations which must be operated and maintained. Flexibility of machining systems for microfinishing is a significant advantage, provided that machining performanace and part production rate goals can be simultaneously met. At present, microfinishing tooling of the type described above typically must be designed and produced specifically for workpieces with journals having a given axial length and diameter, and desired surface shape profile. Present crankshaft microfinishing tooling has a width just slightly less than the width of the crankshaft journal, with sufficient clearance to provide desired oscillation of the tool during the microfinishing process. Micromachining workpieces with different dimensional specifications typically requires tooling changeover with the attendant operational downtime and manpower commitments. It is a further object of the present invention to provide enhanced flexibility for microfinishing tooling and machines.

SUMMARY OF THE INVENTION

[0010] In accordance with the present invention, an apparatus and methods specifically suited for crankshaft microfinishing are described providing numerous benefits over current techniques and devices. Two principal features are described, one feature enables providing standardized machines and tooling for a variety of crankshaft configurations. Another area of enhancements according to the invention is related to producing desired journal bearing surface shape. Both features of the invention are realized utilizing microfinishing tooling with shoes having a width significantly less than the axial length of the journal surface to be machined. The narrow tooling width or "thin shoe" design permits journals over a range of axial length to be machined using common tooling. In one implementation of the present invention, a shoe width is less than 50% (and preferably less than 40%) of the journal bearing axial length, while greater than about 20% of the length. Additional benefits are provided through precise control over one or more of various machining parameters during microfinishing including, applied shoe clamping force, tool oscillation, and tool stroking which can be implemented to generate the desired journal profile shapes. These parameters take advantage of the tailored machining cababilities using the "thin shoe" configuration since they permit regions of the journal surface to be machined with a different effect than other regions. Furthermore, with sufficient compliance in the tooling backing, some flexibility of journal diameter can also be accommodated with individual tools.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a pictorial view of a representative crankshaft workpiece loaded into a microfinishing machine in accordance with the present invention showing the driving headstock and tailstock, and illustrating an oscillation mechanism, but shown without the microfinishing clamping arms.

[0012] Figure 2 is a pictorial view of a representative crankshaft workpiece in accordance with the present invention shown with the microfinishing clamping arms.

[0013] Figure 3 is a cross-sectional view taken along line 3-3 from Figure 2 particularly showing the microfinishing tooling and crankshaft journal.

[0014] Figure 4 is a partial enlarged and cutaway section of a representative crankshaft showing a main bearing journal or a rod bearing journal divided into three sub component regions of the axial length of the bearing journal surface.

[0015] Figures 5(A), (B) and (C) are charts illustrating representative schedules for generating desired journal bearing surface profiles by varying shoe clamping pressure during a microfinishing process.

[0016] Figures 6(A), (B) and (C) are charts illustrating representative schedules for generating desired journal bearing surface profiles by varying shoe oscillation frequency during a microfinishing process. [0017] Figures 7(A), (B) and (C) are charts illustrating representative schedules for generating desired journal bearing surface profiles by varying tool stroking schedules during a microfinishing process.

DESCRIPTION OF THE INVENTION

[0018] Figures 1 , 2 and 3 illustrate a microfinishing machine 10 in accordance with the present invention and which is capable of being operated to practice the methods in accordance with the present invention. As shown by the figures, a representative workpiece shown as internal combustion engine crankshaft 12 is supported at opposing ends by machine headstock 14 and tailstock 15 which together cause crankshaft 12 to be rotated about its longitudinal center axis 18. Crankshaft 12 forms a number of cylindrical journal bearing surfaces to be microfinished, including main bearing journals 20 (four shown in Figure 1 ) which are concentric with axis 18, and rod bearing journals 22 (six shown in Figure 1 ) which move in rotational and orbital paths upon rotation of crankshaft 12. Since crankshaft 12 is rotated about its longitudinal axis 18 which is coaxial with main journals 20, their centers remain theoretically stationary during rotation of the crankshaft. Crankshaft 12 includes a number of structures such as throws or webs 24 which provide for the eccentric positioning of the rod bearing journals 22 and provide counterweights for balancing the crankshaft during operation in an engine. Additional structures including pinion shaft 26 and central drive sprocket 28 of the representative crankshaft 12 are shown. The presence of these additional structures limits the axial length within which microfinishing tools must operate. In other words, these structures present obstacles preventing microfinishing tooling from stroking beyond the axial ends of the journal surfaces being microfinished. It is noted that the configurations of crankshaft 12 illustrated in Figures 1 and 2 differ slightly; namely, crankshaft 12 in Figure 2 features five main bearings 20 and four rod bearings 22, and does not include drive sprocket 28. However, these are examples of typical crankshafts subject to machining operations in accordance with the present invention and these differing features do not limit the implementation of the present invention.

[0019] In order to provide oscillation during a microfinishing process, which is described in more detail in the following description, ball screw shuttle mechanism 30 is operated to cause reciprocation of headstock 14 during microfinishing operations. Tailstock 16 is provided with a compliant element such as an air spring (not shown) which allows crankshaft 12 to oscillate in a reciprocating manner along its central axis 18. Oscillation is characterized as small displacements operate at a higher frequency than another type of longitudinal motion described in more detail below termed stroking. Microfinishing machine 10 is operated under computer numerical control by controller 15 and is used with material handling equipment, enabling crankshaft 12 to be loaded into a position between headstock 14 and tailstock 16 which are operated to support the crankshaft for rotation. A rotational drive system (not shown) is provided to drive rotation of crankshaft 12 during microfinishing operations.

[0020] Referring in particular to Figure 3, microfinishing tooling 32 includes upper shoe

34 and lower shoe 36. Shoes 34 and 36 are supported respectively by upper and lower arms 26 and 28. With specific reference to Figure 3, arms 38 and 40 are shown supporting shoes 34 and 36 mounted to the arms. Each of the shoes 34 and 36 forms a semi-cylindrical machining surface 42. A number of tool inserts 44 are mounted to the shoes 34 and 36 and can be formed of hard materials such as ceramics, or compliant materials such as urethane based compounds, as a few examples. Inserts 44 are machined or formed to generate the semi-cylindrical machining surfaces 42. Microfinishing film 46 is pressed against the respective journal bearing surface by the inserts 44 and has a surface coated with an abrasive material. In another implementation, machining surfaces 42 can be formed by an abrasive material which directly acts on the workpiece surface without using and intermediate abrasive film 46. Rotation of crankshaft 12 causes relative motion and machining action between microfinishing film 34 and the journal surfaces 20 and 22. Upper and lower arms 38 and 40 exert a clamping force F, forcing the associated shoes 34 and 36 against film 46 and the journal surface being machined. As previously mentioned, rod bearing journals 22 undergo orbital motion and accordingly the associated upper and lower arms 26 and 28 engaging the rod bearing journals follow this orbital path during machining.

[0021] A representative microfinishing machine 10 has multiples of upper and lower arms 38 and 40, and respective tooling for engagement with each of the rod and main bearing journals 20 and 22. Accordingly, during a machining operation, rotation of crankshaft 12 provides machining action for each of the bearing journals. When using microfinishing film 46, the film is indexed between machining cycles so that a fresh abrasive surface patch acts on the journals during a machining sequence. Upper and lower arms 38 and 40 open following a machining operation to permit unloading of crankshaft 12. Once a new part is positioned between the machine headstock 14 and tailstock 16, arms 38 and 40 are moved to clamp against the journal surfaces under computer numerical control by controller 15. Microfinishing machine 10 further includes stroking linear actuator 48 which causes each of the clamping arms 38 and 40 stroke across the axial width of the respective bearing surfaces 20 and 22 in a precise manner. Preferably such stroking causes all of the clamping arms 38 and 42 acting on the crankshaft to move together. When machining crankshafts 12 having significant axial length differences between their main bearing journals 20 and rod bearing journals 22, separate sequentially operated microfinishing machines 10 may be used, one for the bearing journals having a particular axial length and stroke displacement, and another for the bearing journals having a differing axial length and stroke displacement. An example of such multiple operations is used for crankshafts having so-called split pin rod bearing journals with a single rod journal 22 supporting two connecting rods. Such a sequential process is referred to in the industry as "stitching".

[0022] As best shown in Figure 2, upper and lower shoes 34 and 36 define a width w.

Journals 20 and 22 define an axial width (or length) W, which as mentioned previously may differ between the journals 20 and 22. As shown, journals 20 and 22 typically have accurately machined cylindrical surfaces with the axial ends forming relief or oil grooves 50. Due to interference with the crankshaft throws 24 or other structures, it is possible with these types of workpieces to move shoes 34 and 36 axially only within the confines of the length of the respective bearing journals or until interference with the throws 24 or other structures would occur.

[0023] In addition to microfinishing machine 10 having the capabilities of controlling the clamping pressure F exerted by upper and lower arms 38 and 40, control over the axial position of the arms and tools along the axial length of the journals is also provided by stroking linear actuator 48, again under numerical control by controller 15. Axial motion of the tooling during machining can be provided in two categories; termed oscillation and stroking. Oscillation of the tools mentioned previously is characterized as a high frequency (e.g. 5-300 Hz) and small magnitude relative motion (e.g. 1 -2mm) provided to enhance the cleaning effect for the abrasive agent during microfinishing machining. Typically, liquid machining fluids are used to carry away abrasive particles and metal waste material removed during microfinishing. This oscillation movement is provided by ball screw shuttle 30. Arms 26 and 28 are also controlled by controller 15 to provide a desired stroking motion under control of stroking linear actuator 48, characterized as causing the shoes 34 and 36 to move linearly across the entire or a substantial portion of the axial length of the journal being microfinished.

[0024] In accordance with a feature of the present invention, tooling width w is chosen to be less than 50% and greater than about 20% of the axial length W of journals for a class of crankshafts 12 to be machined (or .2W≤ w≤ .5W). Preferred embodiments have an upper range of width w of equal to or less than about 40% of the axial length W (i.e. .2W≤ w≤ .4W). By choosing w to be less than one-half the journal length W for a journal having the shortest axial length of a class of crankshafts to be machined, a range of greater length bearing journal crankshafts can be machined using the same tooling 32, while still satisfying the above expressed dimensional range. In accordance with this invention, with the narrow "thin shoe" width w defined herein, the shoes 34 and 36 are stroked across the axial length of the journals to provide a machining effect along their entire length. Moreover, by modifying the dynamic position of the tools and/or the clamping force F acting on the tools in a prescribed schedule, desired machining effects can be provided. Shoes 34 and 36 are positioned and moved dynamically throughout a machining cycle under numerical control by controller 15. In order to implement the custom machining effects described herein it is preferred that the tool can be stroked to the axial ends of the journal surface being machined while the axial center of the journal is not machined. This leads to the "less than 50%" parameter mentioned above. When machining split pin rod journals 22, the tooling width w is likely to approach the lower end of the width range mentioned previously.

[0025) In a typical microfinishing process using machine 10 and the processes described herein, a material removal of around 6μ can be achieved for cast iron and forged steel crankshafts. A machining process will typically involve several strokes of the tooling across the axial length of the journal. In one example, six passes or cycles of stroking of the tooling may be provided, with each pass occurring during a period of about 1 second (i.e. stroking frequency of 1 Hz.). During such stroking, oscillation may occurs during the entire machining cycle.

[0026] The tooling and machining system according to the present invention is capable of providing desired journal profile shape in a number of ways which may be used independently or in combination to produce the desired results. Three approaches are described which involve varying or controlling a machining parameter or multiple parameters as a function of the axial positioning of the tooling along the journal surface, including; 1 ) clamping pressure, 2) oscillation, and 3) dwell time (or stroking schedule).

[0027] Figure 4 identifies three positions or areas of the tooling along the axial length of a representative bearing journal (which can be a main bearing journal 20 or a rod bearing journal 22). In a simplified configuration, the journal axial length can be thought of as composed of the three illustrated axial length sub-components; 52, 54, and 56 (with subcomponents 52 and 56 at the axial ends, and 54 at the center). Figure 4 shows the tooling in the three region positions.

[0028] Now with reference to Figures 5(A-C), 6(A-C), and 7(A-C), some of the controlled machining parameters mentioned above are described in more detail. For each of these graphs in Figure 5(A-C), 6(A-C), and 7(A-C), a line is shown which exaggerates the journal profile surface for illustration purposes. Figures 5(A), 6(A) and 7(A) illustrate a so-called "barrel" or convex journal profile in which the middle component 54 (axial center) has a slightly larger diameter than end components 52 and 56. Figures 5(B), 6(B) and 7(B) illustrate a so-called "hourglass" or concave profile form in which end components 52 and 56 have a slightly larger diameter than center component 54. Figures 5(C), 6(C) and 7(C) illustrate a so-called "double barrel" profile form which is equivalent to two of the convex forms of Figures 5(A), 6(A) and 7(A) in a co-linear arrangement, particularly used for so-called split pin journals described previously.

[0029] In Figures 5(A-C) clamping force F exerted on the tooling by clamping arms 38 and 40 is varied as a function of the position of the tooling 32 along the axial length of the bearing journal surface. In these Figures, symbols "+" or "0" are used to show a relatively higher and a relatively lower clamping pressure, respectively. To generate the barrel configuration of Figure 5(A), clamping pressure F is at the higher "+" level when the tooling is in the position of the axial end subcomponents 52 and 56, and reduced while the tooling is at the position of center component 54. Thus clamping pressure F is less at the center section 40 than at the axial ends 38 and 42. With less clamping force F "0" exerted on the tooling while positioned at the center section 54 of the journal, a less aggressive machining action occurs there and accordingly less material is removed. By providing the increased Vclamping force F at the axial ends 52 and 56 with the reduction "0" at the center 54, a desired barrel shape profile can be generated when starting with a cylindrical journal surface. Generating the concave surface profile configuration of Figure 5(B) and the double barrel configuration of 5(C) is achieved using the indicated clamping force F schedules shown in these Figures.

[0030] In Figures 6(A-C) oscillation frequency Hz acting on the tooling 32 provided by ball screw shuttle 30 is varied as a function of the position of the tooling 32 along the axial length of the bearing journal surface. In these Figures, symbols "+" or "0" are used to show a relatively higher and a relatively lower oscillation frequency Hz, respectively. For example the "0" level could be 0 Hz (no oscillation) or some small value e.g. 5 Hz, and the "+" level could be at a higher level e.g. 200 Hz. To generate the barrel configuration of Figure 6(A), oscillation frequency Hz is at the higher "+" level while the tooling is positioned at the axial end subcomponents 52 and 56, and reduced while the tooling is at the position of center component 54. The machining action is increased as oscillation frequency Hz increases. Thus oscillation frequency Hz is less at the center section 54 than at the axial ends 52 and 56. With less "0" oscillation frequency Hz exerted on the tooling 32 while positioned at the center section 54 of the journal, a less aggressive machining action occurs and accordingly less material is removed in that region. Conversely, by providing the increased Voscillation frequency Hz at the axial ends 52 and 56 with the reduction at the center 54, a desired barrel shape profile can be generated when starting with a cylindrical journal surface. In other words, the diameter D of the journal at the axial center region 54 is controlled by controller 15 to be slightly greater than the diameters D measured at the axial ends 52 and 56 of the journal. Generating the concave surface profile configuration of Figure 6(B) and the double barrel configuration of 6(C) is achieved using the indicated oscillation frequency Hz schedules shown in the Figures varied between the relative values "+" and "0".

[0031] In Figures 7(A-C) the stroking schedule acting on the tooling 32 provided by stroking linear actuator 48 is a controlled and varied by controller 15 as a function of the position of the tooling 32 along the axial length of the bearing journal surface. In these Figures, symbols "+" or "0" are used to show a relatively higher and a relatively lower dwell time or inversely stroking axial velocity at the mentioned bearing areas, respectively. Thus the tooling may be stroked along the journal surface and cause to dwell or park at the end positions 52 and 56 for a specified time period before the direction is reversed to traverse across the journal surface. This again results in greater material removal rate at the axial ends to provide the desired profile shape mentioned previously. For example the "0" level could be a dwell time of 1 .0 second and the "+" level could be at 0.5 seconds to generate the barrel configuration of Figure 6(A). The machining action is increased as dwell time increases. Thus dwell time is less at the center section 54 than at the axial ends 52 and 56. With less dwell time "0" exerted on the tooling 32 while positioned at the center section 54 of the journal, a less integrated machining action occurs and accordingly less material is removed. By providing the increased "+" dwell time at the axial ends 52 and 56 with the reduction at the center 54, a desired barrel shape profile can be generated when starting with a cylindrical journal surface. In other words, the diameter D of the journal at the axial center region 54 is controlled by controller 15 to be slightly greater than the diameters D measured at the axial ends 52 and 56 of the journal. Generating the concave surface profile configuration of Figure 7(B) and the double barrel configuration of 7(C) is achieved using the indicated dwell time schedules shown in the Figures varied between the relative values "+" and "0".

[0032] The above desired journal surface profiles are described as being one of barrel, hourglass, or double barrel configurations. It should be noted that an idealized cylindrical i.e. constant diameter surface may also be a desired configuration.

[0033] A preferred feature of the tooling in accordance with this invention is its narrow width w (mentioned previously as a less than 0.5W) which allows the variation in machining effect to be provided. [0034] The capabilities of the present invention can be provided in various manners. For example, a desired bearing journal surface profile specified by a customer can be provided by accurately gauging incoming parts to determine their machined surface characteristics. For example, after a grinding operation, a set of crankshafts 12 may have one of the surface profile configurations described previously, or can exhibit journals with nearly idealized constant diameter cylindrical profile. By utilizing the custom variation capabilities of the present invention, a different surface profile configuration can be impressed in the workpiece journal surface even where the workpieces are provided before microfinishing with a different configuration. Also, the surface profile configuration provided in incoming parts can be precisely preserved using these controllable machining parameters.

[0035] The above approaches are described in relation to discrete journal axial length sub-component regions 52, 54, and 56. It should be noted that in implementing the present invention, the tooling position across the journal surface can be divided into any number of more narrowly defined axial length sub-components (to infinitesimally small subregions). Discussion of the three sub-component regions above was chosen as one way of describing the principles of the present invention. Where such finer subsections of axial length are specified a gradual change between the parameters "+" and "0" would be provided.

[0036] While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.