1. Field of the Invention
The invention relates to a method for finishing surfaces, a smoothing tool suited for the method and a workpiece.
2. Description of the Related Arts
Methods and smoothing tools of this type are used, for instance, in finishing surfaces of a connecting rod. In internal combustion engines a piston is supported via a piston pin at a small connecting rod eye of the connecting rod, whose large connecting rod eye is connected to a crankshaft. So far a bearing bush has been inserted in the small connecting rod eye. In the course of efforts made for light-weight manufacture and for cost minimization, this bearing bush is to be dispensed with so that the small connecting rod eye directly encompasses the piston pin. It is necessary in this context to manufacture the bearing surface of the small connecting rod eye with high precision. So far the small connecting rod eye has been formed to be round, elliptical and/or trumpet-like in the longitudinal axis of the piston pin by precision lathing. It has turned out that the surface quality to be achieved by precision lathing is not good enough to ensure the durability of the bearing connection.
Another problem consists in the fact that increased wear has been observed especially with bush-less forged connecting rods. Applicant stated that this increased wear has to be traced back to segregations in the cast blank. Such segregations (black cores) are decompositions of the melt during the casting operation. These areas have a higher hardness than the regular structure of the workpiece. If from such a casting blank a connecting rod is manufactured by forging, the segregations will occur especially in the central circumferential areas of the large and the small connecting rod eyes so that the bearing combination is subjected to wear in this area.
Compared to that, the object underlying the invention is to provide a method and a tool by which the surface quality of such surfaces is improved with simple efforts in terms of devices. Moreover, a workpiece having an improved surface quality and wear resistance is to be provided.
The method by the combination of features of a method for finishing surfaces, a smoothing tool as well as a workpiece.
According to the invention, the finishing of the surface is performed by partial reshaping, wherein a smoothing tool is used which is pressed with a convex surface against the surface to be machined and is moved along the same. Thus the invention does not make use of conventional finishing methods in which the surface is smoothed by chip removal. The peculiarity of the method according to the invention consists in the fact that a convex smoothing member is merely pressed against the surface and itself performs no rotation or the like—as for instance during a spinning operation.
The method according to the invention and the smoothing tool according to the invention can be used with particularly great success in finishing a connecting rod eye.
The shaft of the smoothing tool is preferably clamped in a radially feedable infeed head. For instance, the infeed head is a diaphragm tilting head. By such infeed head bearing recesses which are oval in cross-section or trumpet-like in axial direction, surfaces provided with recesses or structured in any other way can be provided or machined.
In a preferred embodiment the convex smoothing member is supported at a substantially rigid tool shaft and thus is not pre-stressed in engaging direction by spring bias or the like. However, in particular cases of application it may be necessary to elastically pre-stress the convex smoothing member in the engaging position. In this case the area receiving the smoothing member can be elastic. In accordance with an especially preferred embodiment of the invention, the tool shaft is in the form of a parallel link which is cut clear from the tool shaft by eroding, for instance. In an alternative variant of the invention, the tool shaft is in the form of a spring link by means of at least one spring, especially a leaf spring. By process control with a micro measuring stylus which detects the deflection of the parallel link it is confirmed that the diameter generation has taken place rotation-symmetrically for instance with a share of smoothing of 5 μm.
As smoothing member preferably a natural diamond or a correspondingly hard material is formed. For instance, use is made of a diamond ball or a diamond ball segment. The radius of a smoothing surface of the smoothing member in a preferred embodiment of the invention is within the range of from approx. 2 to 6 mm.
In an especially preferred variant of the method the smoothing tool is guided approximately along the same track of motion as that of the previously used machining tool.
The smoothing tool is preferably pressed against the surface at a predetermined surface pressure or a defined bias. This is achieved by the fact that the surface pressure of the smoothing tool or the biasing force of the smoothing tool to the surface is kept within a predetermined tolerance range. By such a method substantially lower roughness depths than by conventional methods can be achieved. The roughness depth obtained in preliminary tests was less than 1/1000 mm. It is confirmed by an appropriate process control of the pressing force that the diameter generation has taken place rotation-symmetrically for instance at a share of smoothing of 5 μm. The desired constant pressing force can be obtained, for instance, due to the centrifugal force acting upon the smoothing member and is adjustable by the speed of the smoothing tool.
In the case of particular applications it is advantageous when recesses are formed in the surface to be treated; they can be in the form of a circumferential groove, circumferentially spiral-shaped or a cross-shaped or else can be slots formed in sections. By appropriately controlling the smoothing tool it is advantageous in some cases when the smoothing tool is disengaged in the area of said recesses in order to avoid stress concentrations and thus damage of the workpiece and/or the smoothing tool. Moreover, it is possible to introduce recesses, for instance lubricating troughs, into the surface of the component by the smoothing tool. Such tool can be a multi-purpose tool having a small ball for introducing the trough and a large ball for smoothing the diameter.
The dimensional stability of the surface to be machined can be further improved, when the pre-machining tool is first adjusted to the theoretical adjusting dimension and subsequently the pre-machining, for instance fine boring, is performed by means of controlling or adjusting the tool cutting edge, until the center of any other value within the tolerance range is reached.
In a subsequent step this value is picked up as zero dimension in the machine and is transferred to a measuring station—a so-called master—and is fixed.
In the case of tool change zero measure can be picked up from the master and transferred to the tool in the machine. This transfer can be performed by the machine control or directly by adjustment in the tool cutting edge (infeed tool).
In a preferred variant of the invention it is checked after smoothing whether the smoothing tool is in position. In this way it can be determined, for instance, whether the smoothing member is damaged or is still existing at all.
The bias mentioned in the beginning is adjusted so that also the deformation (flattening) of the smoothing tool during smoothing can be compensated.
It has turned out to be especially advantageous in terms of manufacture when the smoothing tool is a multi-purpose tool having at least one pre-machining cutting edge.
The pre-machining cutting edge is preferably arranged approximately diametrically with respect to the smoothing member so that optionally the smoothing tool or the pre-machining cutting edge can be engaged by a swivel movement.
The present application suggests for improving the wear behavior to form a circumferential groove especially in forged parts in the central portion of the circumferential walls of bearing bores/recesses—for instance the large and small connecting rod eyes. Said circumferential groove is designed such that the segregations are partly abraded and thus are no longer located in the main supporting area of the bearing bore so that surprisingly the wear resistance can be substantially improved vis-à-vis conventional solutions.
Under certain circumstances it can also be sufficient, however, to machine the circumferential wall of the bearing recess and the circumferential groove only by precision-turning, wherein it is essential that the segregations are no longer located in the main supporting area of the bearing. The applicant reserves the right to direct a separate independent claim to the formation of a groove—independently of the machining method.
Other advantageous further developments of the invention are the subject matter of further subclaims.
These, and other aspects and objects of the present invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the present invention, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.
Preferred embodiments of the invention shall be explained in detail hereinafter by way of schematic representations, in which:
This tool arrangement can then be inserted, according to
In accordance with
In the case of special requirements it might be necessary to design the tool shaft 6 to exhibit certain elasticity. In this case the portion receiving the smoothing member 8 can be elastic. In the embodiment shown in
As one can take especially from
The pre-machining and the smoothing are carried out for instance at two stations. When machining a connecting rod 1 (cf.
The radially adjustable infeed head permits to machine also surfaces having recesses—for instance for forming lubricating slots etc. It is preferred to disengage the smoothing member in the area of said lubricating slots by adjusting the infeed head so that after passing said recess the smoothing member is not exposed to impact stress, because it is engaged again only after having passed said recess.
After performing the smoothing operation it is checked by a measuring device whether the smoothing member 8 is still in position and whether there is possibly wear which can then be appropriately compensated. A respective measurement of wear is also carried out during pre-machining (fine-boring), wherein the tool is measured after each machining of a workpiece and possibly existing wear is stored and compensated by a station-oriented wear compensation (offset correction). This correction can be made using a sliding mean value, wherein for instance during a tool change a measuring unit rejection free of primary processing times is performed with up-to-date reading and possible offset correction.
According to
In
According to
Of course, the method and the smoothing tool 4 are not restricted to the use in connecting rod eyes 2 but can also be used for other surfaces having a high surface quality. The method according to the invention and the smoothing tool 4 can also be employed in machining cylinder bores, wherein the prescribed lubrication slots (spiral-shaped, diamond pattern . . . ) can be formed in the circumferential walls. The method according to the invention and the smoothing tool 4 can be used as an alternative of honing which requires considerable efforts in terms of machine tools and control.
As explained in the beginning, the invention is especially suited also for use with workpieces in which segregations occur in the area of the circumferential walls of the bearing recesses. As said segregations are formed substantially in the central circumferential area in workpieces forged of round bars, in accordance with the invention a circumferential groove is introduced into said center portion of the circumferential walls. Said groove can have a width of one or more millimeters, for instance about 3 mm, in the case of a connecting rod, the depth merely amounting to a fraction of the width. So the wear behavior of the workpiece can be improved already with grooves having a depth of between 1 μm and 5 μm. I.e. the width of the groove and the depth of the groove are designed, according to the invention, at a ratio of more than 100:1, preferably more than 1000:1. The groove can be formed by precision-turning or else according to the method as set forth by the invention by smoothing. In the latter case, the groove can be initially pre-machined by precision-turning and then be finished by smoothing. On principle, it is also possible to form the groove by smoothing only.
Of course, the method according to the invention can also be used for oval bearing recesses (connecting rod eyes) or for a cross-section deviating from the circular shape, because practically any shape of recess can be tracked by the infeed head. The principal advantage of a circumferential groove machined by smoothing resides in the fact that a clean oil passage is formed in which a continuous oil film is formed without cut-off so that the wear behavior of the workpiece or the bush-less connecting rod is further improved. Due to the small depth of the groove, it is also referred to as micro-groove.
The invention relates to a method and a smoothing tool for finishing surfaces as well as a workpiece that is machined by smoothing. In accordance with the invention the surface that is to be machined is smoothed by a shaping process with the aid of a convex smoothing tool.
Although the best mode contemplated by the inventors of carrying out the present invention is disclosed above, practice of the above invention is not limited thereto. It will be manifest that various additions, modifications and rearrangements of the features of the present invention may be made without deviating from the spirit and the scope of the underlying inventive concept.
Number | Date | Country | Kind |
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10 2006 017 468.2 | Apr 2006 | DE | national |
10 2006 038 117.3 | Aug 2006 | DE | national |
10 2007 012 764.4 | Mar 2007 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/DE2007/000666 | 4/16/2007 | WO | 00 | 2/2/2009 |