Patent Application
20130319063
The invention relates to a machining method for machining a curved workpiece surface of a workpiece, in particular for machining the internal surface of a bore in a workpiece, according to the preamble of claim 1, and to a machining tool, which can be used in the implementing of the method, for machining a curved workpiece surface of a workpiece according to the preamble of claim 9. Furthermore, the invention relates to a workpiece obtainable with the aid of the machining method and/or the machining tool.
Embodiments of the invention can be used, inter alia, in the production of sliding surfaces on workpieces, the sliding surfaces being tribologically stressed, rotationally symmetrical or curved in another manner, preferably for the construction of engines. A particularly preferred field of use is in the production of cylinder faces in internal combustion engines, i.e. in the machining of internal surfaces of cylinder bores in an engine block or in a cylinder sleeve to be installed in an engine block.
For the quality-determining finish-machining of cylinder faces, use is generally made of honing processes in order to satisfy the extremely high requirements with regard to dimensional and geometrical tolerances and with regard to the desired surface structure. Honing is a cutting process using geometrically undefined cutting edges, in which multi-cutter honing tools carry out a cutting movement which consists of two components and results in a characteristic surface structure of the machined workpiece surface with intersecting finishing marks.
In some workpieces, the sliding surface is produced directly on the basic material of the workpiece. For example, in the case of engine blocks (cylinder crankcases) made of gray cast iron, the cylinder faces are frequently produced directly on the gray cast iron material. If, by contrast, the material used for the workpiece is not suitable because of its material properties for serving the tribologically stressable surface following fine machining of the workpiece surface, the basic material can be provided with a coating, the surface of which is then optionally mechanically reworked in order to serve as the sliding surface. For example, it is known to provide cylinder bores in engine blocks consisting of aluminum material by means of thermal spraying with a coating consisting of an iron-containing coating material. The surface of the spray layer is normally also mechanically reworked after the coating operation. During thermal spraying, additional materials, what are referred to as spray additives, are melted down, fused or melted open within or outside a spray burner, are accelerated in the form of spray particles in a gas stream and held onto the surface of the workpiece to be coated. In the process, the workpiece surface is generally not fused and is subject to thermal loading only to a small degree. The heated spray particles strike against the workpiece surface and bond therewith. Lamination takes place, since the spray particles are flattened to a greater or lesser extent depending on the process and material as they strike against the workpiece surface, remain stuck thereto primarily by means of mechanical clamping and build up the coating, which is referred to as a spray layer, in layers.
The adhesive strength of the coating of the basis material of the workpiece is decisively determined by the preliminary machining of the workpiece surface directly before the coating process. Sufficient adhesive strength can be achieved by roughening the workpiece surface before the coating. Different processes have already been proposed for this purpose.
The article “Prozesskette zur Herstellung thermisch gespritzter Zylinderbohrungen [Process chain for the production of thermally sprayed cylinder bores]” by G. Flores, A. Schwenk and C. Schnell in: VDI Reports No. 2109, 2010 describes processes for producing thermally sprayed cylinder bores. The workpiece surface is roughened before coating by a variant of fine boring, in which profile undercuts are introduced into the workpiece surface. As a result, the subsequently applied thermal spray layer is intended to obtain a high degree of adhesive strength on the cylinder wall.
DE 196 14 328 A1 describes processes, in which workpiece surfaces are textured in a regular pattern by means of laser beams for the preliminary preparation of the coating.
It is known from EP 1 141 438 E1 to roughen the cylinder bores in a cylinder crankcase by means of corundum or sandblasting before the coating.
It is pointed out in EP 0 607 779 E1 (DE 694 23 373 T2) that sandblasting is an additional working step which increases the coating costs. In some cases, such as, for example, in cylinder bores of internal combustion engines, there is also the risk of sand particles being able to remain behind embedded in the blasted workpiece surface and subsequently resulting in the engine being damaged or even destroyed.
DE 10 2004 038 177 A1 describes processes for preparing a cast cylinder bore for thermal coating, in which the surface of the cylinder bore is roughened by means of a jet, preferably by means of a liquid jet containing solid particles, and/or by means of a water jet, wherein, during the roughening, material from the cylinder bore is removed to a certain thickness at the same time.
One problem addressed by the invention is to provide a machining method for mechanically machining a curved workpiece surface of a workpiece, said machining method permitting efficient and cost-effective conditioning of curved workpiece surfaces, for example for preparing for a subsequent coating by means of thermal spraying. A further problem addressed by the invention is to provide a machining tool suitable for carrying out the method. Furthermore, workpieces having improved use properties when used as directed are intended to be provided.
To solve this and other problems, the invention provides a machining method having the features of claim 1. Furthermore, a machining tool having the features of claim 9 is provided. Furthermore, a workpiece having the features of claim 20 is provided.
Advantageous developments are specified in the dependent claims. The claims are all worded by reference to the content of the description.
The machining method is characterized by a roll-embossing operation, in which at least one rolling element of a machining tool is rolled under a predeterminable contact pressure against a section to be machined of the curved workpiece surface. In this case, an external surface of the rolling element has a rough surface structure in at least one working section intended for rolling contact with the workpiece surface. The contact pressure is set in such a manner that, during the rolling of the rolling element on the curved workpiece surface, a rough roll-embossed structure is produced by local deformation of workpiece material in the region of the workpiece surface. The design or the formation of the roll-embossed structure is dependent, inter alia, on the contact pressure and on the roughness or the structure of the working section. For this purpose, the workpiece consists, at least in the region of the workpiece surface to be machined, of a plastically deformable workpiece material, for example of a metallic material. During the roll-embossing operation, a border layer adjacent to the workpiece surface is plastically deformed. The “rough surface structure” of the working section of the rolling element is a surface structure having elevations and depressions which are lower than the elevations. The spatial distribution of the elevations and depressions may be irregular, but it is also possible for the structure-forming elevations and depressions to be distributed regularly over the working section. Accordingly, the rough roll-embossed structure which is produced by deformation can likewise consist of elevations and of depressions set back from the elevations, wherein, during the roll-embossing operation, normally an elevation on the working section of the rolling element produces a depression in the roll-embossed structure by displacement of material.
A “rough surface structure” refers here to a surface structure in which elevations and/or depressions project in relation to, or are set back from, a mathematically smooth reference surface by a number of micrometers. An intended and consciously produced roughness is involved here in contrast to unavoidable residual roughnesses which may be produced, for example, on nominally smooth technical surfaces by polishing.
As a rule, the average peak to valley height Rz of the rough surface structure in the working section lies within the range of a number of micrometers, in particular around 10 micrometers or more. In preferred embodiments, the roll-embossed structure having an average peak to valley height Rz of more than 10 micrometers is produced on the workpiece surface by the roll-embossing operation, wherein the average peak to valley height can lie, for example, within the range of between Rz=30 μm and Rz=100 μm. Average peak to valley heights in this range can be advantageous in particular if a subsequent coating is provided. The average peak to valley height after the roll-embossing operation may also be greater than 100 μm.
The roll-embossing operation is basically a method which is very substantially non-cutting, since the rough roll-embossed structure is essentially not produced by removal of material but rather by deformation of material or displacement of material and redistribution of material without substantial removal of material. In comparison to machining methods with removal of material, substantially longer tool downtimes are generally produced, thus enabling the manufacturing costs to be kept low.
In some method variants, the operation can be carried out without the assistance of liquid manufacturing aids, for example cooling lubricant, i.e. by dry machining. This permits a particularly cost-effective texturing of the surface. If required, however, the operation can be also be carried out under wet conditions, for example in order to reduce the adhesion between the rough tool surface and workpiece surface.
The contact pressure can optionally be set at such a high level that work-hardening of the workpiece material occurs in the border layer close to the surface. In this case, the term work-hardening refers to an increase in the strength of the workpiece material, said increase in strength being able to be achieved upon plastic deformation of the material in a substantially cold state. Said increase in strength in the vicinity of the surface can be favorable in particular in the case of relatively soft metallic materials, such as light metal materials on the basis of aluminum or magnesium.
In some method variants, a coating is applied to the workpiece surface after the roll-embossing step. The coating can be applied directly onto the rough roll-embossed structure produced by the roll-embossing step, in particular if said roll-embossed structure has been produced by the dry method. It is also possible to rework said roll-embossed structure after the roll-embossing step with the aid of one or more reworking methods, in order thereby to produce a modified rough roll-embossed structure which is then coated.
The coating is preferably applied by a thermal spraying process, such as, for example, plasma spraying or the high velocity flame spraying process (HVOF process). Depending on the application, the coating can consist, for example, of a ceramic coating material, a metallic coating material or a metal matrix composite (MMC), in particular a metal-ceramic laminated material.
It has proven advantageous if the roll-embossed structure or the modified roll-embossed structure has an average peak to valley height of between Rz=30 μm and Rz=100 μm before the coating. If the lower limit is significantly fallen short of, in the case of some material combinations the adhesive strength of the coating in the critical range is not reliably adequate. If, by contrast, the upper limit is significantly exceeded, more coating material than actually necessary for the coating is generally required in order to fill the heavily roughened structure. The coating process is thereby unnecessarily extended. Average peak to valley heights within the preferred range therefore constitute a good compromise between reliable adhesive strength and efficiency during the production of the layers.
The roll-embossing operation may be the final machining step on the workpiece surface. In some cases, one or more further treatment steps or machining steps may follow.
In some variants of the method, cleaning of the workpiece surface provided with a roll-embossed structure is carried out during and/or after the roll-embossing operation. The roll-embossing operation is basically a non-cutting deformation process in which removal of material is neither intended nor occurs to a relatively large extent. Nevertheless, in the case of some materials, the roll-embossing operation on the workpiece surface may produce material particles which are only weakly connected to the material surface, if at all, for example in the form of flat lamellae or tinsel, which could impair the function of the roll-embossed structure. In some variants of the method, the workpiece surface which is provided with the roll-embossed structure is therefore cleaned after the roll-embossing operation in order to eliminate material particles which are produced by the roll-embossing operation and are only weakly bound, if at all. The cleaning can also be partially or entirely carried out during the roll-embossing operation, i.e. at the same time as the roll-embossing operation. The cleaning is preferably shape-maintaining, i.e. the shape of the roll-embossed structure is virtually unchanged as a result.
Dry or wet cleaning processes are possible. In some variants of the method, the roll-embossed structure is cleaned with the aid of relatively soft, optionally rotating brushes. The bristles of the brushes are non-abrasive and have such softness that the form of the roll-embossed structure remains unchanged. As an alternative or in addition, the workpiece surface which is provided with a roll-embossed structure can be pneumatically cleaned with the aid of compressed air or another compressed gas. The compressed gas can be conducted onto the roll-embossed structure externally and/or through the machining tool by means of suitable compressed gas passages and out of outlet openings. As an alternative or in addition, during and/or after the roll-embossing operation, chips and other machining residues from the roll-embossing operation can be extracted by suction, wherein, preferably, in addition to the extraction by suction, the workpiece surface is blown clear.
Wet cleaning with the aid of a cleaning liquid is also possible. During the preparation for a subsequent coating, use may be made, for example, of a grease-detaching cleaning liquid (alcohol, acetone or the like). Cleaning with water is also possible.
In some variants of the method, the workpiece surface which is provided with a roll-embossed structure is also slightly changed in its shape in order, by reworking, to provide a modified roll-embossed structure. For this purpose, some variants of the method provide for eliminating peaks in the region of elevations of the roll-embossed structure in order to produce a plateau structure with flattened elevations and depressions located in between on the workpiece surface.
In some variants of the method, the peaks in the region of elevations are eliminated by material-removing fine machining. For this purpose, a honing operation with relatively fine-grained cutting means is normally carried out in order, in the manner of plateau honing, to provide a modified roll-embossed structure which has flattened elevations with plateau-like, flat surfaces which have a fine groove structure. The depressions produced by the roll-embossing operation lie in between. In such variants of the method, the roll-embossing operation can replace the conventional basic honing (honing with comparatively coarse cutting means). In contrast to conventional workpiece surfaces, which first of all are machined with basic honing and then with plateau honing in a multi-stage machining process, a method with a roll-embossing step and subsequent plateau honing has the advantage, inter alia, that the region close to the surface during the roll-embossing step can additionally still obtain increased strength by means of work-hardening. Although work-hardening can also occur during basic honing, the work-hardening zone reaches deeper during the roll-embossing operation than during basic honing, since, during basic honing, a possible work-hardening layer is largely removed again immediately.
The material-removing elimination of peaks may be the final machining step in a multi-stage machining of the workpiece surface such that the workpiece surface is subsequently finished for use as directed. However, other machining steps may also follow.
In another variant of the method, the peaks in the region of elevations of the roll-embossed structure are eliminated by deformation by, for example, rolling elements having a smooth external surface being rolled under suitable contact pressure along the roll-embossed structure. The peaks can thereby be pressed flat without removal of material. Said material-deforming reworking may cause laterally protruding material lugs to be produced at the sides of the resulting plateaus. Such structures with an undercut are particularly suitable for a subsequent coating, since the coating material entering the depressions can interlock in a form-fitting manner with the workpiece surface in the region of the lateral lugs, thus enabling particularly good layer adhesion on the workpiece surface to be achieved.
There are various possibilities of realizing the machining method.
In one embodiment of the machining method, the at least one rolling element is rolled on that section of the curved workpiece surface which is to be machined in such a manner that the machining tool is guided along a predefined path at a predefined distance from the workpiece surface. The course of the path of the machining tool is selected here in such a manner that the rolling element rolls with the required or desired contact pressure on the workpiece surface to be machined. For this purpose, the machining tool can be coupled, for example, to a working spindle of a machining center, in particular a 3-, 4- or 5-axis CNC machining center. By means of a machining center controller, the course of the path can be controlled and regulated in dependence on the geometry of the workpiece surface, in particular depending on the curvature thereof.
In one embodiment of the machining method, which can be used in particular for machining the internal surfaces of cylindrical bores, a rotation tool which is rotationally driven about the tool axis thereof is used as the machining tool with at least one rolling element arranged eccentrically with respect to the tool axis. By rotation of the machining tool, the external surface of the rolling element rolls on the workpiece surface to be machined, and the tool axis can be fixed in position during the machining. Such a machining method can be carried out, given an appropriate configuration of a coupling device of the machining tool, for example on a honing machine by the machining tool being arranged on a working spindle of the honing machine, in a manner similar to a honing tool for internal cylindrical honing. The machining tool can optionally substantially fill the bore to be machined, and therefore the effective outside diameter of the machining tool virtually corresponds to the inside diameter of the bore.
The invention also relates to a machining tool, which is suitable for carrying out the method, for machining a curved workpiece surface of a workpiece. The machining tool has a tool body which defines a tool axis. The machining tool has at least one rolling element which is carried by the tool body, is rotatable about a rolling element axis and has an external surface. The rolling element can rotate about the rolling element axis thereof during the machining. The external surface of the rolling element has a rough surface structure in at least one working section intended for rolling contact with the workpiece surface. The working section normally has the abovementioned elevations and depressions in order to produce the rough roll-embossed structure on the machined workpiece surface by a roll-embossing operation.
The machining tool can be configured as a rotation tool which is driven in a rotational manner during use of the machining tool as directed, and therefore the tool body or the entire machining tool rotates about the tool axis. If, in this case, a rolling element has a rolling element axis lying outside the tool axis, the external surface of said rolling element, or a section of said external surface, can roll on the machined internal surface and the sought structure can be produced.
A rolling element normally has a rotatable mounted rolling element body which can be composed, for example, of a metallic material of suitable hardness, of a ceramic material or of a metal-ceramic composite. The rough surface structure can be introduced directly into a rotationally symmetrical external surface of the rolling element body. The rough surface structure can be produced, for example, by laser machining or by an etching process. For example, the external surface of a metallic roller can be etched chemically or electrochemically or mechanically (for example by sandblasting) in such a manner that depressions are produced and elevations protrude in between. In order to improve the wear resistance, a thin layer of hard material can be applied to the rough surface produced by this means, for example by means of a PVD (physical vapor deposition) process or a CVD (chemical vapor deposition) process.
The rolling element body can also be composed completely of a hard material with bound grains of hard material, in the manner of a sharpening stone, in order to form a rough external surface.
In preferred embodiments, the rolling element has a rolling element body which, in the working section, bears a coating with a rough surface structure. This construction is distinguished, inter alia, by simple producibility and great flexibility in the configuration of the geometry and other embossing properties of the rough surface structure. The rolling element body can be a simple turned part consisting of a metallic material, while the hardness, which is required for the roll-embossing step, of the rough surface structure and the geometry thereof can be determined by the coating.
In some embodiments, the coating has a multiplicity of grains of hard material which are bound in a binding and protrude from the binding on the external side and thereby form the elevations of the rough surface structure. The average grain size of the grains of hard material is preferably around at least 15 μm. In particular, the average grain size can lie within the range of approx. 30 μm to approximately 250 μm. As a result, for example when machining workpieces made from aluminum or an aluminum alloy, averaged peak to valley heights of between approx. 10 μm and approx. 100 μm can be produced on the workpiece surface. In many cases, embossed structures having said characteristic roughness values are particularly suitable for a subsequent coating.
The grains of hard material can consist, for example, of diamond, corundum, cubic boron nitride or other hard materials, the hardness of which is preferably significantly greater than the hardness of steel. The binding is preferably electroplated in order to achieve good adhesion on a metallic rolling element body and at the same time high wear resistance to grains of hard material breaking out of the binding.
Some embodiments of a machining tool, which can be used in particular for machining the internal surface of a cylindrical bore, have an outside diameter which substantially corresponds to the inside diameter of the bore to be machined, and therefore the machining tool virtually fills the bore. The machining tool is preferably designed as a rotation tool and can be driven in a rotating manner about the tool axis thereof for use as directed. The machining tool can be designed in a similar manner to a honing tool for internal cylindrical honing.
The machining tool has at least one rolling element which is carried by the tool body and has a rotationally symmetrical external surface having a rough surface structure which serves to produce the roll-embossed structure. The rolling element is rotatable about a rolling element axis which can be arranged eccentrically with respect to the tool axis, i.e. at a distance outside the tool axis. During the roll-embossing operation, rotation of the machining tool about the tool axis thereof then causes the rough surface structure of the external surface of the rolling element to roll on the internal surface of the bore to be machined and to produce the roll-embossed structure.
The rolling element axis of a rolling element can run parallel to the tool axis. As an alternative, the rolling element axis may also run in the radial direction obliquely with respect to the tool axis. Rolling elements with a rolling element axis running parallel to the tool axis are provided for machining a workpiece surface extending parallel to the tool axis. By means of rolling elements which have an obliquely running rolling element axis, workpiece surfaces running obliquely with respect to the tool axis can be machined, in particular bevels at the entry and/or exit of bores or the like.
Some embodiments have at least one rolling element having a rolling element axis running parallel to the tool axis and, in addition, at least one rolling element having a rolling element axis running obliquely with respect to the tool axis. By means of the use of such a machining tool, it is therefore possible, in a workpiece fixture, to machine both the internal surface of a bore and also at least one bevel of the bore and to produce the roll-embossed structure not only in the internal surface but also in the bevel surface. The coating properties of the bevel or of the transition between the cylindrical bore section and bevel can thereby be improved with such a machining tool, for example without an additional manufacturing step.
Some machining tools have one or more rolling elements which, adjacent to a cylinder-jacket-shaped external surface having a rough surface structure, have at least one further rotationally symmetrical conical external surface having a rough surface structure, wherein said external surface generally tapers, but optionally also widens, at increasing distance from the cylinder-jacket-shaped external surface. A conical external surface can be adjacent to the cylinder-jacket-shaped external surface on the side facing away from the spindle and/or on the side facing the spindle. With such a machining tool, the bevels and the cylindrical internal surface of a bore can undergo a roll-embossing operation in a fixture in a manner offset in time.
Some embodiments having a rolling element or rolling elements arranged eccentrically with respect to the tool axis are characterized by a rolling element infeed system for the variably controllable infeed of the rolling element in an infeed direction running radially with respect to the tool axis. By these means, it is possible infinitely to change the effective diameter of the tool over a predeterminable infeed region such that, for example, a machining tool can first of all be introduced with the rolling elements retracted into a bore before the rolling elements are then fed outward in order to obtain contact with the internal surface of the bore and the machining begins. By means of a variably controllable infeed, the contact pressure is also exactly settable and can optionally be changed during the machining.
A machining tool can have a single rolling element. A rolling element can be in single-part form or segmented into a plurality of rotatable, coaxial rolling element segments which are independent of one another. As a rule, a plurality of rolling elements distributed uniformly or non-uniformly around the circumference are arranged on the tool body. An embodiment has three rolling elements distributed uniformly around the circumference. It has turned out that, by this means, the machining tool can optimally follow the bore shape resulting from the preceding machining steps, and therefore the same contact pressures without pressure peaks are essentially set at all of the rolling elements and a uniformly textured surface is produced. If a plurality of rolling elements are provided, said rolling elements can preferably be infed together via a common rolling element infeed system. A separate infeed independently of one another is likewise possible.
As an alternative or in addition, a machining tool can have a plurality of rolling elements in a longitudinal direction of the tool. Said rolling elements can be arranged on the tool body distributed uniformly or non-uniformly in the longitudinal direction.
As a rule, the rolling elements which are distributed on the circumference are arranged annularly with the rolling element axes thereof or centerpoints thereof at the same distance from the tool axis. However, the rolling elements may also be arranged distributed eccentrically on the circumference at different distances from the tool axis.
Embodiments are also possible, in which one or more rolling elements are arranged on a limb of an L-shaped tool body, and the limb having the rolling elements is rotatable about the other limb, which defines the tool axis. If such a machining tool is arranged on the working spindle of a machining center, an eccentric rolling movement of the rolling elements can be produced by a corresponding continuous-path control.
A rolling element can be mounted directly on the tool body or in a bearing mounted on the tool body. However, at least one carrier element which is movable relative to the tool body radially with respect to the tool axis is preferably arranged on the tool body, and the rolling element is mounted rotatable on the carrier element. Machining tools for machining the internal surface of bores can be constructed to this extent in a similar manner as honing tools for internal cylindrical honing, but, inter alia, instead of the abrasive honing stones required for the honing, one or more non-abrasive rolling elements are provided.
In order to permit the radial infeed of rolling elements, in some embodiments an infeed element which is movable parallel to the tool axis and has at least one wedge section, which can be in the form of, for example, a cone or a truncated cone, is arranged in the tool body. The oblique surfaces of said infeed element can interact with oblique surfaces on the inner side of carrier elements for the rolling elements in the manner of a wedge drive such that an axial movement of the infeed element results in a radial movement of carrier elements and of the rolling elements mounted thereon.
In some embodiments, the external surface of the rolling element, in the working section which forms the rough surface structure, bears a thin anti-adhesion coating, by means of which the adhesion between the rough external surface of the working section and the machined workpiece surface is reduced in relation to an external surface without an anti-adhesion coating. The anti-adhesion coating can be, for example, a TiN coating or a DLC (diamond like carbon) coating. If the adhesion is reduced, then, during the rolling operation, the rough surface structure of the exterior region, after penetrating the workpiece material, is more easily detached from the workpiece material textured by this means without small adhering material particles being torn out of the textured surface and then being able to impair machining steps which may follow. This makes it possible to dispense with the use of liquid auxiliary manufacturing materials, such as, for example, cooling lubricant or the like, during the roll-embossing machining, and therefore the roll-embossing operation can be carried out as a dry machining step. By this means, manufacturing costs can be considerably saved. In addition, roll-embossed structures produced under dry conditions are generally immediately suitable for coating without further reworking.
Some embodiments of machining tools are suitable to be operated in a path-guided manner. As a rule, said machining tools are designed for use in machining centers, in particular in 3-, 4- or 5-axis CNC machining centers, and can have corresponding coupling structures on the tool body for connection to the working spindles of a machining center. With such machining tools, rotationally symmetrical internal surfaces and also internal surfaces of non-round or non-cylindrical bores, in particular oval recesses or the like, can be machined. Such a machining tool can be used in particular to produce roll-embossed structures in workpiece surfaces, such as, for example, piston bearing surfaces of Wankel engines. However, it is also possible to use such machining tools to machine non-curved or planar workpiece surfaces or planar sections of partially curved workpiece surfaces.
Machining tools which are provided for use in machining centers generally have a significantly smaller outside diameter than the recess being machined. Said machining tools are usable in a flexible manner, and therefore a corresponding and matching machining tool which virtually fills the bore does not have to be kept ready for each bore diameter. Furthermore, the rolling element infeed system can generally be omitted in path-guided machining tools, since the required contact pressure of the rolling elements against the workpiece surface to be machined can be controlled via the path guide of the machining tool.##
In a path-guided machining tool, the rolling element axis of at least one rolling element can lie concentrically or coaxially with respect to the tool axis. It is not required to drive the tool as a whole in a rotating manner, since, with the path guide, the at least one rolling element is rotated about the rolling element axis thereof by the rolling contact with the machined internal surface.
The invention also relates to a workpiece which is obtainable or is produced or can be produced using the machining method and/or using the machining tool. The workpiece may be in particular a cylinder crankcase made of light metal material or a cylinder sleeve to be installed in a cylinder bore. It may also be a connecting rod, in which at least one connecting rod bore has been machined with the aid of a roll-embossing operation, optionally followed by coating. The workpiece may also be an extrusion pipe having an internal coating.
A path-guided machining tool can also be used to machine non-cylindrical internal surfaces of a workpiece, in particular the piston bearing surface of a Wankel engine housing.
These and further features are apparent from the description and the drawings as well as from the claims, wherein each individual feature can be realized on its own or a plurality thereof can be realized in the form of sub-combinations in an embodiment of the invention and in other fields and can constitute advantageous embodiments patentable on their own. Exemplary embodiments are illustrated in the drawings and are explained in more detail below.
From that end of the tool body which faces away from the spindle, a central, cylindrical receiving bore 118 having a planar end surface 120 leads into the tool body, and therefore said section is configured in a tubular manner. The tube wall formed as a result contains three substantially rectangular passage slots 122, which are distributed uniformly about the circumference and pass radially from the inside to the outside, for receiving carrier elements (explained in more detail further on) for rolling elements. From that end of the coupling section 114 which faces the spindle, a further cylindrical receiving bore 124 having a planar basic surface leads into the coupling section. A cylindrical passage bore 126 having again a smaller diameter is located between the receiving bores 124 and 118.
The coaxial bores 118, 126 and 124 serve to receive tool-internal elements of a rolling element infeed system. This includes a double-conical expansion cone 142 which sits in an axially displaceable and play-free manner in the receiving opening 118 facing away from the spindle. That end surface of the expansion cone which faces the spindle is provided with a central blind hole bore with an internal thread, into which, during assembly of the machining tool, a threaded section, which is provided with an external thread, of a push rod 144 is screwed, said push rod being inserted from the side facing the spindle into the receiving opening 124 of the coupling section. The push rod has a widened cylindrical head section 146 which fits in a manner substantially free from lateral play, but in an axially movable manner, in the receiving bore 124. During the assembly, the push rod reaches through a spiral compression spring 148 which is inserted into the receiving bore 124 and, after the push rod is screwed to the expansion cone, is supported at one end on the widened head section 146 and at the other end on the planar base of the receiving bore 124. The effect achieved by this is that, in the absence of external forces, the push rod and the expansion cone screwed thereto take up their upper stop position, which is shown in
The hollow driving rod 150 contains an infeed rod 152 which is displaceable axially with the aid of an infeed drive of the honing machine and the free end side of which, with machining tools coupled, acts on that end side of the push rod 144 which is on the spindle side.
Each of the radially continuous passage slots 122, which are circumferentially offset by 120°, contains a strip-shaped carrier element 160 which fits into the passage slot in a radially movable manner and so as to be substantially free from play axially and in the circumferential direction. The inner side of each carrier element has two oblique surfaces which lie axially one above the other and, when the tool is assembled, interact with the cone surfaces, which are located axially one above another, of the expansion cone 142 in the manner of a wedge drive in such a manner that an axial displacement of the expansion cone brings about a radial displacement of the carrier element.
Each carrier element has a base section 162 which is located on the inside and on which the oblique surfaces located on the inside are mounted, and also bearing sections 164A, 164B which in each case protrude outward on the narrow end sides. Each carrier element bears a rolling element 180A, 180B, 180C which is mounted rotatably on the bearing sections of the carrier element in such a manner that the axis of rotation of the rolling element, which axis of rotation is referred to as the rolling element axis 182, runs parallel to the tool axis 112 at a distance outside same.
The carrier elements are prevented from dropping out when the machining tool is fully assembled by means of encircling restoring springs 170A, 170B which are held in grooves located on the outside of the bearing sections and exert a resetting force on the carrier elements in the direction of the tool axis. In the absence of pressure against the upper end surface of the push rod 144, the carrier elements are in their maximally retracted position which corresponds to the minimum effective diameter of the machining tool. The effective diameter can be enlarged by the infeed rod 152 being displaced in the direction of the machining tool until the push rod is pressed in the direction of the end remote from the spindle. This also moves the expansion cone toward the end remote from the spindle, and therefore the oblique surfaces slide along each other in the region of the cone sections, and the supporting strips for increasing the distance between tool axis and rolling element axis are pressed outward counter to the force of the compression spring 148 and the restoring springs 170A, 170B. For pulling-back purposes, the infeed rod 152 merely needs to be pulled back, and the resetting force then comes from the spring elements.
Each rolling element has a rotationally symmetrical, substantially cylindrical rolling element body 184 which is made from tool steel and the opposite end sides of which are provided with blind hole bores. An external ring of an encapsulated needle bearing 186 sits in each of the bores. The internal element of a needle bearing is formed by a cylindrical bolt which is fastened in the bearing section of the carrier element during assembly. The needle bearing which moves with little rolling resistance and is highly stable to radial forces is encapsulated to the outside against the penetration of contaminants, for example tinsel-like material particles, in order to permit interference-free rotation of the rolling element in relation to the carrier element during the machining.
It can be seen particularly readily in the enlarged detail illustration of
The width of the working section 190A, as measured parallel to the tool axis and to the rolling element axis, is only approx. one third of the axial length of the rolling element body. As can readily be seen in
For the machining of the internal surface of a bore, the machining tool is first of all driven into the bore to an extent such that the combined machining region of the individual rolling elements covers the beginning of a section to be machined. This insertion takes place with the rolling elements pulled back. The rolling elements are then fed outward with the aid of the rolling element infeed system until the rolling elements bear under suitable contact pressure against the internal surface. Before said radial feed motion, after said radial feed motion or during said radial feed motion, the tool is set into rotation about the tool axis via the rotary drive of the honing spindle. In the process, the working regions of the rolling elements are pressed onto the internal surface in such a manner that the rough surface structure thereof presses into the workpiece surface and substantially produces a complementary roll-embossed structure with elevations and depressions located next to one another, i.e. a roll-embossed structure which substantially constitutes a negative image corresponding to the structure of the roll-embossing tool. After one or more rotations, the roll-embossing operation for the corresponding axial section of the bore is ended and the carrier elements are set back inward, thereby removing the touching contact between rolling elements and internal surface of the bore.
If the internal bore section to be machined is longer than the working width of the combined working regions, the machining tool is then advanced in the axial direction until the tool lies level with the next bore section to be machined. The sequence of working steps with a radial feed motion outward, beginning of the rotational movement, roll embossing and subsequent pulling back is then repeated.
During the roll embossing, i.e. during the machining engagement of the rolling elements on the internal surface of the bore, the working spindle or the machining tool is not advanced axially.
The contact pressure crucial for the deformation of material during the roll-embossing operation can be set in a temporally variable manner with the aid of the rolling element infeed system. For example, it is thus possible to produce two or more differently machined sections by roll embossing on one and the same internal surface of the bore, during the machining of which sections the rolling elements were pressed on with a different contact pressure. It is also possible to leave unembossed sections next to roll-embossed sections. For example, the roll-embossing operation can be restricted to the upper and/or lower interstitial region of a cylinder bore.
The machining tool 100 shown in
In other variants of combination tools, in addition to one or more rolling elements having a rough surface (for the roll-embossing operation), smooth rolling elements (for example smooth rollers) which can be infed separately, or non-rolling smoothing sticks having a hard, smooth working surface for work-hardening without substantial removal of material are provided.
The substantially tubular tool body 310 has six radially continuous passage openings, which are distributed uniformly around the circumference, are elongated axially and are relatively narrow in the circumferential direction, for receiving carrier elements for the machining elements. Carrier elements 360 are seated in three passage openings, which are offset in each case by 120° on the circumference, for the rolling elements 380 mounted rotatably therein. The construction and arrangement of said carrier elements can be substantially as described in conjunction with
The inner sides of the supporting elements 360 for the rolling elements, which are rotatable about the rolling element axes 312, have oblique surfaces which interact with conical oblique surfaces on the outer side of a tubular infeed rod 350 which surrounds the inner infeed rod 340. The rolling element infeed system operates independently of the infeed system for the honing sticks, and therefore it is possible, in particular alternately, only for the rolling elements or only for the honing sticks to be brought into machining engagement with the internal surface to be machined of the bore. If the machining tool is intended to be used, for example, exclusively for roll embossing, the inner infeed rod 340 can remain pulled back such that the honing sticks are permanently disengaged from the workpiece surface.
The machining tool can be used in particular in the case of machining methods in which first of all the internal surface of the bore is intended to be honed and then the honed surface subjected to a roll-embossing operation. It is also possible to use the honing tool in methods in which the internal surface of the bore is first of all prepared by other methods and in which a combination tool is then used first of all to carry out a roll-embossing operation and then a honing operation. It would also be possible to use the machining tool first of all to hone an internal surface with the aid of the honing sticks and then to subject the honed workpiece surface to a roll-embossing operation and then to rework the roll-embossed structures produced as a result with the same honing sticks again, but with less removal of material. In the second honing operation, the operation is preferably carried out under a contact pressure which is reduced in comparison to the first honing operation, in order to produce a finer surface structure on plateau surfaces.
The limited movability of the coupling between machining tool and the honing spindle driving the latter enables the machining tool to track the shape of the bore to a certain extent during the roll-embossing operations, and therefore unacceptable constraining forces do not occur even if the shape of the bore deviates slightly from an ideally circular-cylindrical shape.
In some embodiments, the machining quality during the roll-embossing operation is further improved and evened out by the mounting of the rolling elements having a certain elastic flexibility in order, as a result, to be able to recede, for example when rolling over surface elevations, and/or to compensate for irregularities in the shape of the bore by a slight inclination of the rolling element axis.
The supporting element 460 in
With reference to
In the version of the method of
With reference to
Directly after the roll-embossing operation, the roll-embossed structure is ready without further treatment for subsequent coating by means of thermal spraying. In the case of the example, an iron-containing coating 860 is applied directly onto the roll-embossed structure 858 with the aid of a flame spraying process. By means of the roughness of the roll-embossed structure, the coating material readily meshes with the metallic base material of the workpiece, thus producing firm adhesion between workpiece and coating. The coating surface, which is relatively rough after the thermal spraying, can then be subject to mechanical reworking, for example by means of honing, in order to produce the desired final roughness of the surface and the desired final size.
In the machining method explained with reference to
The crucial operations for the final surface structure (roll-embossing operations and subsequent plateau honing operation) can optionally be produced with the same machining tool, which can be constructed, for example, in the manner of the machining tool of
Further exemplary embodiments of machining tools for roll embossing, which can be used within the scope of the machining method, are described below.
The rolling elements 1181 also each have a rolling element axis 1182C which runs parallel to the tool axis 1112 and likewise lies outside the tool axis. However, these rolling elements 1181 have two rolling element segments, an upper one and a lower one, which are each mounted so as to be rotatable separately.
The machining tool 1100 furthermore has rolling elements 1183 which are arranged in the upper third of the machining tool and are likewise mounted so as to be rotatable about a rolling element axis 1182A. However, in contrast to the rolling element axes 1182B and 1182C, the rolling element axis 1182A does not run parallel to the tool axis 1112, but rather obliquely with respect thereto and in the radial direction. The clearance angle, measured between the rolling element axis 1182A and the tool axis 1112, is approx. 45°, it may also be smaller or larger and lie, for example, between 30° and 60°.
The machining tool illustrated in
The machining tool 1307 is coupled to the spindle 1302 of the machining center 1300 by means of a suitable coupling structure. The spindle does not rotate during the machining. The outside diameter of the machining tool 1307 is significantly smaller than the inside diameter of the bore to be machined, for example is at least 10% or at least 20° smaller. In order to machine the internal surface 1308, the machining tool 1307, by activation of the axis drives of the machining center, is guided on a predefined path in such a manner that the external surfaces 1304, 1305, 1306 of the rolling element 1380 roll with the rough surface structure thereof on the workpiece surfaces to be machined (cylindrical internal surface and bevel) and the desired roll-embossed structure is produced. The contact pressure of the rolling element 1380 can be controlled or regulated via the course of the path. The cylindrical external surface 1304 is provided for machining the vertically running internal surface 1308, whereas the lower conical external surface 1305, which is arranged on that side of the external surface 1304 which faces away from the spindle, is provided for machining the upper bevel 1303. By means of the upper conical external surface 1306, a bevel present on a lower side (not illustrated here) of the workpiece 1301 can be machined. All of the surfaces can be machined successively in a fixture without changing tools.
A number of exemplary embodiments of machining methods with a roll-embossing operation have been explained using the example of machining curved workpiece surfaces in the region of cylinder bores in order to obtain tribologically highly stressable cylinder faces. However, the methods can also be used in other workpieces. For example, the internal machining of bores in a small or large connecting rod eye is also possible using roll-embossing operations. For example, the small connecting rod eye can first of all be machined by roll embossing and, subsequently, the internal surface which is provided with the roll-embossed structure can be covered with a coating made from a bearing material. As an alternative, the rough roll-embossed structure can serve as a means for securing against rotation for an inserted bearing shell, the external surface of which can optionally likewise be roughened by means of roll embossing.
By means of the path-guided machining method explained by way of example, non-cylindrical recesses, for example the piston bearing surfaces of a Wankel engine housing, can also be machined by roll embossing. Furthermore, it is optionally possible for not only curved, but also planar or plane regions of workpiece surfaces to thereby be subject to a roll-embossing operation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 063 704.1 | Dec 2010 | DE | national |
| 10 2011 079 757.2 | Jul 2011 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP11/72647 | 12/13/2011 | WO | 00 | 8/23/2013 |
