Patent Application
20080008550
The invention relates to a method for machining the bearing seats of shafts, especially of crankshafts, wherein the bearing seats are subjected to a plurality of machining operations after a shaft has been initially formed by forging or casting.
Conventionally, the bearing seats of shafts, such as the seats of the main bearings and of the thrust bearings of crankshafts, are machined by multi-stage chip-removing processes. According to a typical machining sequence in this regard, the forged or cast crankshaft is first rough-machined by turning, milling or turn broaching, then semifinish-machined by grinding and finally subjected to finish machining. In particular, now that it has become standard practice to harden the bearing seats, not least to protect them from damage in the course of further manufacture, the machining routinely includes rotary grinding after hardening, since for many years grinding was the only usable method for reducing bearings to their final dimensions after they had been hardened.
Rotary milling of the bearing seats of shafts has also been proposed repeatedly, as has already been done in DE 212950 and AT 286067, for example. In such a case, a milling cutter turning around an axis of rotation disposed perpendicular to the axis of the bearing seat to be machined is moved with tangential feed along the rotating workpiece. More recently, such suggestions have reappeared, especially to avoid wet grinding and the expense associated with disposal of the waste products produced during grinding.
For example, WO 97/32680 A1 (this document will be abbreviated as D1 hereinafter) proposes a method for machining bearing seats of crankshafts without wet grinding, wherein the crankshaft is rotated at approximately 20 to 100 rpm by means of a holding fixture in which it is clamped and mounted so as to be driven in rotation around its longitudinal axis, which coincides with the C axis of the machine, and is machined with a milling tool, which can be rotated around its A axis (which is parallel to the X direction of the machine) in a tool spindle, can be infed to the workpiece along its axis and can be fed perpendicular to its axis (in Y direction of the machine; tangentially relative to the workpiece). The corresponding machining takes place in two stages.
According to the explanations in D1 (page 11, lines 11 to 25), in order to allow for the different stresses and strains on the cutting tool and the quality to be achieved by the machining operation, the tangential feed rate is controlled in such a way that the circumferential cutters of the milling tool have optimal cutting conditions during the rough-machining phase (roughing) and that the end blades can act on the entire running seat during finish-machining (smoothing) of the running seat. According to page 4, line 27 to page 5, line 9 of D1, the high cutting speeds necessary for high-speed cutting can be achieved by appropriately high speeds of revolution of the rotary milling tool, while the crankshaft can nevertheless be rotated at the usual speed of up to approximately 100 rpm, as is also used during grinding in order to achieve high quality of the workpiece. Because of the high cutting speeds, the stresses and strains on the workpiece are supposedly small and very good three-dimensional geometry can be achieved. With a tool matched to the contour of the bearing seat, the tangential feed of the rotary milling tool permits machining of the entire width of the bearing seat in one work cycle. The conventionally provided steps of rough machining and grinding machining of the bearing seats could be combined—in one machine and with one clamping of the crankshaft—in one working step, and so considerable streamlining could be achieved. On the milling tool, there are provided three cutting tips made of common types of cutting materials and equipped respectively with an end cutter and a circumferential cutter. The cutting tip geometry is adapted to the geometry of the bearing seat to be machined as regards transition radius, flat shoulders, etc. (see D1 page 6, line 33 to page 7, line 5).
In view of the throw height of the crankshaft, the known rotary milling tool must be very elongated (protruding length) to ensure that it can undertake machining of the bearing seat (see D1 page 8, lines 18 to 20).
In summary, there are proposed in D1 a method and a device that supposedly permit short cycle times and very good quality during machining of the (unhardened) bearing seats of crankshafts by using high-speed rotary milling, so that the machining step of grinding can be made completely unnecessary (see D1 page 15, lines 22 to 27).
From EP 1030755 B1 (this document is abbreviated as D2 hereinafter) there is known the machining of crankshafts by the following sequence of machining steps: chip removal—hardening—chip removal—finishing. In this connection, it is explained that the bearing faces are hardened in their near-surface zones, especially in the case of steel crankshafts. This serves the purpose of increasing the wear resistance of the bearing locations, preventing damage to these bearing faces during handling throughout the entire manufacturing process, and influencing the strength characteristics of the crankshaft (see D2 column 1, lines 23 to 30). The technical starting point of the ideas in D2 is the conventional machining of crankshafts in four steps: The first step is chip-removing machining with specific cutters; in this connection there is also mentioned rotary milling, and in particular high-speed milling. In the subsequent second machining step, the bearing face of the crankshaft is hardened. The third step relates to grinding by means of a hard, massive grinding tool, such as a grinding wheel. Finally, in the fourth step, finishing is achieved by a grinding belt or grindstone, which is usually stationary and is pressed against the outer circumference of the rotating bearing location of the crankshaft. The material allowance abraded during finishing ranges from 1 to 10 μm (see D2 column 2, line 32 to column 3, line 15).
In order to lower the costs of crankshaft machining, it is endeavored according to D2 to reduce the machining of the bearing locations from four to three different machining steps (see D2 column 3, lines 21 to 24). By omitting machining by grinding, the machining sequence is reduced from four to only three machining methods that are different in principle. Consequently, disposal problems for elimination of grinding sludge should cease to exist, investment costs for grinding machines and costs for tool consumption would no longer be incurred, and last but not least, a larger inventory of workpieces would no longer be needed to compensate for the prolonged turnaround time of the workpieces caused by grinding. In contrast, disposal of the chips from chip-removing machining causes no problems, since either dry cutting will be possible (high-speed milling) or complete separation of chips and oil can be achieved because the specific surface of the chips is much smaller than that of grinding dust (see D2 column 4, lines 21 to 33).
A further consideration relevant to the grinding of bearing locations that has been practiced heretofore is that the roundness deviations resulting from chip-removing rough-machining are usually reduced only in their absolute size but not in their nature by grinding. Thus grinding would not reduce long-period roundness deviations to short-period roundness deviations, but instead the number of undulations in the shaft would either remain the same or decrease, with the consequence that further improvement of the roundness deviations by finishing, considered as an improvement in the result per unit time, would actually become more difficult during finishing (see D2 column 7, line 49 to column 8, line 3).
According to D2, hardening is followed by more material removal by chip-removing machining, especially the second chip-removing machining step (finish chip removal) of a two-stage metal-cutting operation (see D2 column 9, lines 20 to 24).
What is common to the comments in D1 and D2, therefore, is that multi-stage rotary milling machining of the bearing seats by wet grinding should be avoided. In this way, as explained, the quality of manufacture could be increased and the costs incurred due to elimination of the grinding sludge could be lowered for both unhardened (see D1) and hardened (see D2) bearing seats. Because of this, the unanimous, known interpretation that multi-stage rotary milling machining should be integrated into rough-machining. This interpretation is not invalidated even by the fact that, as the possible combination of machining sequences up to the ready-to-use condition of a crankshaft, there is mentioned the following: chip removal—hardening—chip removal—finishing (see D2 column 4, lines 50 to 54).
The present invention is based on the knowledge that, starting from the structural complexity of crankshafts and other shafts with bearing seats, especially with eccentric bearing seats, optimization of manufacturing costs and working result cannot be achieved with the known measures alone. Its object is to provide a method for machining the bearing seats of shafts in a manner than can contribute to manufacture of shafts satisfying stringent quality requirements at comparatively low costs.
This object is achieved according to the invention by a method for machining the bearing seats of shafts, wherein the bearing seats are subjected to the following machining operations after a shaft has been initially formed by forging or casting:
During rotary milling machining, the axis of the rotary milling tool is offset relative to the axis of the bearing seat to be machined by an eccentricity, which is retained or in other words is constant during the corresponding machining stage, preferably including the phases of immersion and retraction of the tool. By the fact that a feed movement of the milling cutter transverse to its longitudinal axis, or in other words in Y direction or tangentially relative to the workpiece, does not occur during rotary milling machining, the machining speed (relative feed) is guided alone by the circumferential velocity of the shaft and the radius of the bearing seat to be machined. In this connection, for example, the following machining speeds are achieved: Relative feed between approximately 200 mm/min and 9000 mm/min, preferably between approximately 600 mm/min and 1500 mm/min; cutting speed approximately between 60 m/min and 600 m/min, preferably between approximately 80 m/min and 120 m/min. The offset of the axis of the milling cutter relative to the axis of the bearing seat to be machined depends on the bearing geometry and corresponds preferably to 0.1 to 0.25 times the value, particularly preferably 0.15 to 0.2 times the value of the diameter of the milling cutter. Because of the eccentricity of the milling cutter, the end cutters can embrace the complete bearing seat. The eccentricity can be optimized to reduce the vibration behavior. The shaft can be rotated in the direction of the eccentricity or in the opposite direction during rotary milling machining.
According to a special aspect of the present invention, the workpiece is gauged after rough rotary milling and the infeed of the milling head for finish rotary milling is determined as a function of the result of this measurement. This gauging of the bearing seats after rough rotary milling is performed on the clamped shaft, so that rough rotary milling, gauging and finish rotary milling take place in a direct sequence. Depending on the individual conditions, an equally large or a different machining allowance of the machining face can be removed during the individual steps of rotary milling machining of the bearing seats. The second of these options is regularly particularly favorable, specifically when organized such that preferably approximately 60 to 80% and particularly preferably approximately 65 to 75% of the machining allowance remaining after hardening, is removed during rough rotary milling and the rest is removed during finish rotary milling.
Depending on the requirements of surface quality as well as on other boundary conditions, rotary milling may or may not be followed by finishing of the bearing seats. From cost viewpoints, a procedure without finishing is particularly preferred. This is also feasible in principle, since the surface irregularities still present after finish rotary machining, because of the nature of the method, run transverse and not parallel relative to the direction of movement in the bearing. Thus they are evened out more rapidly during operation, and the danger that the lubricant film will break away from them locally is smaller than in the case of marks running along the direction of movement in the bearing.
According to another preferred improvement of the invention, the infeed of the milling cutter during immersion at the beginning of rough rotary milling and at the beginning of finish rotary milling respectively has only an axial component relative to the axis of the milling cutter. In other words, in each case the milling cutter is infed along its longitudinal axis and bears radially on the workpiece, while the shaft is being rotated by a certain amount (see hereinafter). This is advantageous in preventing the production of a “dent” in the material to be removed during immersion of the milling cutter. Because such denting can be avoided in this way, the machining allowance existing before rotary milling can be correspondingly small (for example, only 0.35 mm), which is favorable for the economy of multi-stage rotary milling machining. It has proved particularly favorable when immersion of the milling cutter in the material to be removed, at its position that is critical for rough rotary milling, extends over an angle of rotation of approximately 3 to 15°, particularly preferably approximately 5° in the crankshaft or other shaft. An analogous condition applies for immersion of the milling cutter during finish rotary milling.
Another preferred improvement of the invention is characterized in that the diameter of the milling cutter used for rotary milling is larger than the width of the bearing seat to be machined. Particularly preferably, the diameter of the milling cutter used for rotary milling is approximately 1.15 to 1.35 times the amount of the width of the bearing seat to be machined. In this case, a relief groove in which the milling cutter can run out during rotary milling machining is expediently made during rough-shaping on both sides of the bearing seat to be produced. Taking these parameters into consideration, the profile of the irregularities remaining on the surface of the bearing seats after rotary milling machining is favorable for maintaining a lubricant film.
If the present invention is used for machining the bearing seats of a crankshaft, it is advantageous for the crankshaft to be clamped for multi-stage rotary milling machining at its flange end in a first rotatable chuck and at its journal end in a second rotatable chuck. Particularly preferably, these two chucks can be driven synchronously and rotated with a speed of revolution of between 1 and 100 rpm.
The bearing seats of the individual main bearings of a crankshaft are expediently machined successively with a single rotary milling tool, while the crankshaft preferably is braced simultaneously in radial direction by one or by two steadies on at least one adjacent main bearing. By bracing with steadies, it is ensured that the crankshaft will not bow under the machining forces and therefore that the machining result will not be impaired.
In analogous manner, the bearing seats of the individual thrust bearings are expediently machined successively with a single rotary milling cutter, while the crankshaft is braced simultaneously in radial direction by a steady on at least one main bearing adjacent to the thrust bearing being machined.
Alternatively, a plurality of main or thrust bearings may each be machined simultaneously with a plurality of rotary milling tools. An appropriate plurality of rotary milling tools is provided for this purpose.
It is especially advantageous for multi-stage rotary milling machining if each bearing seat has its own individual NC control program running. A prerequisite for such individual control is the existence of measuring devices and measuring programs, with which the individual machining result of rough milling machining is recorded directly and used in the NC program of the machine for machining the bearing seat in question during finish rotary milling machining. Preferably, as explained, the measurements are made after the rough cutting step of multi-stage rotary milling machining, and then infeed is undertaken for the finish-cutting step. By virtue of recent developments, such complex NC control of the machining machine is now possible without difficulty.
The invention also relates to a machine tool for performing the method. The machine tool possesses:
During machining of the bearings of a crankshaft in a machine with two chucks that can be driven in rotation, the first chuck holds the crankshaft at its flange end and the second chuck holds the crankshaft at its journal. In each case, the crankshaft is clamped with its axis of rotation along the C axis (in Z direction) of the machine tool.
The axis of rotation (A axis) of the tool spindle containing the rotary milling tool runs parallel to the X direction of the machine, which in turn runs orthogonally relative to the Z direction. The tool spindle for the rotary milling tool is also provided with a device in order to make the tool spindle adjustable for adjusting the eccentricity (the offset) between the axis of the bearing to be machined and the tool axis in the Y direction of the machine. Finally, the tool spindle can also be displaced and immobilized in the Z direction of the machine tool, in order that the individual bearing seats can be machined in succession.
Particularly preferably, the machine controller makes it possible, instead of the above-explained infeed movement of the milling cutter alone in its longitudinal direction, also to execute an infeed movement of the rotary milling tool comprising both an axial component (in X direction) and a radial component (in Y direction) during immersion in the bearing location to be machined at the beginning of rough rotary milling and of finish rotary milling. This makes it possible to react flexibly to special circumstances of the individual case, by an infeed movement that is particularly suitable for the specific application.
For machining of seat faces of thrust bearings, the tool spindle is provided with a device allowing it to oscillate in coordinated manner in the direction of the X axis and of the Y axis, so that it can follow the circular movements of the thrust bearing during rotation of the crankshaft around its main axis.
The tool spindle is preferably equipped to hold a milling finger, whose shank has a length-to-diameter ratio ranging between 10:1.5 and 10:3. Such slenderness of the milling finger is the prerequisite for all bearing faces (generated faces) to be machinable with the tool while the crankshaft is rotating. Another prerequisite for slenderness, however, is that the shank of the milling finger has high flexural strength. It is advantageous for the shank of the milling finger to be made of hard metal—or other materials with high flexural strength. The milling finger is preferably clamped in a shrink-fit chuck.
To achieve steady and uniform cutting, preferably at least three end cutters comprising CBN (cubic boron nitride) cutting tips fixed by solder or other suitable means or comprising one cutting tip made of a suitable other cutting material are provided on the finger cutter.
The CBN or other cutting tips each preferably have a chamfer, so that the height of the cutting edges above a normal plane decreases by a small amount from the outside circumference of the milling finger to its axis. By such a chamfer it is possible to achieve a convex (crowned) form of the bearing faces, which is favorable in particular for crankshafts. This chamfer of the end tool cutters, produced by surface grinding, can be in particular approximately 0.04 to 0.1 mm. Preferably the ratio of the speeds of the chuck or chucks to the speed of the tool spindle can range from 1:400 to 1:2000. The cutting speeds are preferably on the order of magnitude of between 80 and 600 m/min. If necessary, the tool can be cooled internally.
Bracing of the respective bearing seats not currently being machined is preferably to be provided by a steady (see above). It is expediently achieved on a main bearing, at three points, one of which lies in the X direction (relative to the rotary milling tool). In order to preclude disturbances of bracing by the necessary oil bores in the main and thrust bearings, the bracing faces of the steady are preferably designed as sliding blocks, which in the region of the oil bores of the main-bearing seats each have a recess in the form of a groove. In addition, the sliding blocks can be adapted to the surface contour of the generating line of the bearing, especially via hollow grinding matched to the bearing diameter. This prevents ingress of chips in the region of the bracing of the shaft on the sliding block or steady in question, and in this way it prevents damage to the bearing used for bracing the shaft.
As mentioned hereinabove, the special advantages of the present invention are evident in particular for crankshafts and other shafts whose bearing seats are hardened. However, if hardening of the bearing seats is not necessary either in regard to protecting them during further handling or in regard to the desired wear resistance, it is conceivable that the inventive procedure can be used without hardening of the bearing seats. In this case, less stringent requirements would be imposed on the cutters of the rotary milling tool. In addition, the machining allowance for rotary milling machining could be smaller than for hardened bearing seats, since the shrinkage that occurs regularly during hardening would not be a factor, and this is favorable from viewpoints of manufacturing economy. To this extent, the Applicant reserves the right to claim separate protection for such a procedure via a divisional application.
The invention will be described in more detail hereinafter with reference to a practical example.
In the drawing, which is not to scale and in some cases is greatly simplified,
a shows a vector diagram of the relative feed during various phases of rotary milling machining,
Crankshaft 1—rough-shaped by chip removal, hardened and passed through dressing rollers—is clamped to rotate around its main axis of rotation 2 in the machine tool (not illustrated), in which its main and thrust bearing seats will be machined. Beginning at journal 3, the main bearings are designated successively as HL 1 to HL 5. The numbering of the thrust bearings is similar. Beginning at journal 3, they are designated successively as PL 1 to PL 4 in the drawing. The end of crankshaft 1 opposite journal 3 is flange 4. In the present example, crankshaft 1 is clamped at flange 4 by a chuck, two jaws 5 of which are illustrated. As shown by the direction of arrows 6, the clamping forces act in radial direction on flange 4.
Axis of rotation 2 of crankshaft 1 is also the C axis of the machine tool running in Z direction. Beginning at flange 4, main bearings HL 5 to HL 1 of crankshaft 1 are machined successively according to arrow 9. During machining in the direction of arrow 9, which is the X direction of the machine tool, crankshaft 1 is braced in the direction of arrow 10, which is opposite to the machining direction. Bracing in the direction of arrow 10 is accomplished by one or two steadies (not illustrated) of the machine tool. This machining of HL 5 takes place with bracing at HL 4, machining of HL 4 takes place with bracing at HL 3, machining of HL 3 takes place with bracing at HL 4 and/or HL 2, machining of HL 2 takes place with bracing at HL 3 and machining of HL 1 takes place with bracing at HL 2. Bracing during machining of thrust bearings PL 1 to PL 4 is provided in similar manner. During machining of PL 1, for example in machining direction 11, bracing is provided at HL 1 and/or HL 2. Machining of PL 2 in turn takes place with bracing at HL 2 and/or HL 3, machining of PL 3 takes place with bracing at HL 3 and/or HL 4, and finally machining of PL 4 takes place with bracing at HL 4 or HL 5. For simplicity, it is assumed that machining direction 11 corresponds to machining direction 9 and therefore to the X direction of the machine tool.
A milling finger 12 as illustrated in
Viewed from the direction of arrow A (
Machining allowance 22 (for example, 0.35 mm) is provided for multi-stage rotary milling machining of main bearing HL. During rough cutting, in which crankshaft 1 performs approximately one complete revolution in direction 20, an outer layer 23 with a predetermined thickness (rough machining allowance, for example 0.25 mm) is removed from main bearing HL. Immediately thereafter, or in other words without reclamping of the shaft, once the bearing has been gauged after rough cutting, inner layer 24 (finish machining allowance, for example 0.1 mm) is removed by precision cutting, during which crankshaft 1 is rotated once again in direction of rotation 20 and milling finger 12 is rotated in direction of rotation 21. Reversal of directions of rotation 20 and 21 for precision cutting is not provided, but is entirely possible. During precision cutting, crankshaft 1 performs somewhat more than one complete revolution. Together with the range of angle of rotation for immersion of the tool at the beginning of the precision cutting stage of rotary milling machining, the tool is engaged here over an angular interval of approximately 420°.
By means of a vector diagram,
As already mentioned, crankshaft 1 is braced by one or two steadies (not illustrated) of the machine tool during multi-stage rotary milling machining of a main bearing HL. Bracing is provided primarily in the X direction, which is illustrated in vertical direction in
| Number | Date | Country | Kind |
|---|---|---|---|
| 102004057111.2 | Nov 2004 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP05/12634 | 11/25/2005 | WO | 7/6/2007 |
