Patent Application
20150113778
I. FIELD OF THE INVENTION
The invention relates to a method and a device for machining rotation symmetrical and also non rotation symmetrical components, in particular crank shafts and mass production, in particular bearing surfaces (of crank pin bearings and also journal bearings) of crank shafts to a useable condition, thus the condition when the crank shaft can be installed in an engine without additional material removal at the bearing surfaces.
Thus bearing surfaces are enveloping surfaces, thus a width of the bearing, and also the so called transom surfaces, thus the faces adjacent to the bearing width which are used for example for axial support.
Crank shafts in particular crank shafts for car engines with a high number of cylinders are known to be work pieces that are instable during machining and thus difficult to work on. Determining dimensional compliance of a finished crank shaft is primarily provided besides axial bearing width by assessing the following parameters:
Typically material removal from the bearing of the formed thus cast or forged crank shaft was performed in three material removing machining steps:
Chip removing machining through a defined edge, thus, the methods turning, turn-broaching, turn-turn-broaching, internal circular milling and external circular milling, face milling, in particular performed as high speed milling or combinations of these methods are used. The excess material to be removed is in a range of one-tenth millimeters.
Step 2. Fine dimension Machining:
Wet grinding through a hard, massive grinding tool, for example a grinding disc which typically rotates with its rotation axis parallel to the rotation axis of the crank shaft to be machined; the excess material to be removed is in a range of one-tenth millimeters.
For crank shafts that are difficult to machine, thus in particular long and thus very unstable crankshafts the grinding is also performed in several stages, for example in two stages by pre grinding and finish grinding.
Step 3: Primary Surface Structuring: Finishing through a typically stationary grinding device (grinding band or grinding stone) which is pressed against a circumference of the rotating bearing; the removed access material is presently in a 1/100 mm range or even pm range.
Thus, the processing has to be differentiated based on the material of the crank shaft (steel or cast iron) wherein in particular steel crank shafts which are preferably used for highly loaded components are hardened at the surfaces of the bearings after the first chip removing machining step. This causes renewed warping of the crank shaft which had to be compensated by grinding and finishing. Hardening cast iron crank shafts is currently typically omitted and can be completely avoided by using a cast material with greater hardness like e.g. GGG 60 or 70 and improved strength values.
In order to reduce the cost of crank shaft machining it is desired to reduce the machining of the bearings from three different machining steps to two different machining steps.
Omitting the first processing step in that forming, typically forging is performed precisely enough, so that only fine machining is required thereafter, has not been successful in practical applications at least for mass production so far. At least this, however, would have the consequence that in particular the material removal to be provided by grinding has to be greater than for a 3 step method. For material removal through wet grinding it is disadvantageous however that
Thus it was attempted in the past to minimize the complexity, thus the amount of investment and also machining times and similar for partially hardened work pieces, thus in particular machining after hardening.
Thus it was attempted in particular to eliminate wet grinding and to transition from chip removing machining for example directly to finishing as suggested by DE 197 146 677 A1 while predetermining defined transfer conditions with respect to the individual dimensional parameters.
Also EP 2 338 625 A1 proposes particular fine machining with a defined edge which shall replace the step of wet grinding, however, a finishing is optionally provided thereafter which shall not only improve shape and surface but also dimensional precision to a lesser extent.
Prior optimization attempts, however, do not sufficiently consider the options and in particular the possible combinations of the new machining methods with a defined edge and also with a non defined edge and without edge which meanwhile are also provided in variants for hard machining, thus for machining hardened work piece surfaces and can thus be used at the work piece after hardening.
A bearing can be machined with a single turning tool that is feedable in X-direction, moveable in Y-direction and additionally rotatable about a B-axis (single point turning) so that the bearing can be turned without producing a shoulder.
The second step and the third step of finishing with finer grit produces material removal in a range of 5 μm and is performed time based and eventually used for surface structuring.
Furthermore there is electrochemical etching of surfaces today which is used for structuring surfaces, thus in particular removing the peaks of the microscopic surface structure.
It is well known that it is not only relevant for structuring to remove the peaks but it is also important to keep the valleys open to maintain them as oil reservoirs. In case this is not sufficiently achievable with the known methods like finishing, the known methods must be actively included, for example by including laser beam treatment.
Certainly the precision requirements on the customer side have also increased which are typically at 5 μm regarding circularity, quality level 6 with respect to diameter precision, thus for a car crank shaft approximately 16 μm and with respect to concentricity between 0.05 and 0.1 mm.
Thus it is an object of the invention to reduce fine machining of the work pieces recited supra to provide usability in particular after hardening, in particular to reduce the number of process steps.
The object is achieved by the characterizing features of claims 1, 2 and 24. Advantageous embodiments can be derived from the dependent claims.
Thus, it is an object of the present invention to machine the work pieces recited supra and in particular their bearings after chip removing rough machining which achieves a precision of one-tenth mm and possible subsequent hardening which causes additional warping.
The subsequently recited processing steps typically relate to the same machining location.
According to the invention it is presumed that a first finishing step is required after that, wherein the first finishing step is used for achieving dimensional precision and a second finishing step is used for achieving the respective surface quality.
The first fine machining step is a chipping with a defined edge. This can either be turn milling with an external milling bit which rotates parallel to the work piece during machining or an orthogonal milling bit whose rotation axis is oriented perpendicular or at a slant angle to the rotation axis of the work piece, or the turning, in particular in the form of single point turning which are all capable to machine down to tolerances of approximately 10 μm which, however, shall not always be fully utilized in the process chain according to the invention.
For the second fine machining step in particular material removal with an undefined edge like e.g. fine—dry grinding or finishing, thus in particular the fine steps of dimensional form finishing are available or also electrochemical hardening with or without pulsating loading of the electrodes.
Ideally the process chain only includes the first and second fine machining step.
If necessary a fine intermediary step is performed there between. The following is available dry grinding which only provides removing much smaller amounts of material compared to wet grinding for example at the most 150 μm, or tangential turning thus a method with a defined edge, or the coarse step of dimensional form finishing, or single point turning is another option in case this was not already selected for the first fine machining step.
It largely depends on customer requirements if a final fine finishing step is required after the second fine finishing step for structuring the surface.
A fine completion step of this type, in particular through ECM or band finishing can preferably only be performed in a particular circumferential segment of the work piece when there is a segment in which the load is higher than in other portions as it is the case for example for bearings of crank shafts.
This can be used in particular for introducing cavities as oil reservoirs into the work piece surface for improving lubrication and thus sliding properties for a longer time period.
For this purpose in particular a targeted laser impact can be used for achieving such cavities or in turn electrochemical etching, in case this was not already selected as a machining method for the second fine machining step.
Namely in this case the respective protrusions for burning the cavities into the work piece are already machined into the electrode for electro chemical etching and the cavities are introduced and the peaks of the microscopic surface structure are clipped off in one process step.
This way a shortening of the process chain is provided over the conventional process chain and in spite of increased customer requirements. This has the advantage that in particular wet grinding is prevented and additionally depending on the particular combination several process steps can be performed in the same machine and with the same clamping step.
Thus, the machining methods of the first and second fine processing step, besides electrochemical etching, can jointly by implemented in one machine and thus the work piece can be machined in one clamping step.
Even an additional fine intermediary step can be included therein regardless of the actual choice of the machining method for this fine intermediary step.
Even a laser unit for impacting the work piece surface can be additionally used in a machine that is a turning machine as a matter of principle, thus for a work piece that is drivable during processing and defined with respect to its rotational position (C-axis).
According to the present invention after the first fine machining step which is performed with a defined edge, a dry grinding or fine dry grinding shall be performed immediately thereafter, wherein this can be performed preferably in the same machine and clamping step as the first fine machining step with defined edge.
Preferably turn milling is used as a first fine machine step which is performed as external milling, especially grinding is in particular performed with a grinding disc that also rotates about a rotation axis parallel to the turning axis during machining. Thus, drive units for grinding disc and disc cutter can be used which are configured very similar or even identical. When fine dry grinding is performed directly after the first fine processing step, machining is performed to a precision of 15 μm for circularity and 15 μm or better for diameter. These are excess dimensions which are still removable with the very fine grit, and thus depending on the required final precision it is also possible to leave the fine dry grinding as a finishing step thus in particular not to let any band finishing follow, but at the most one fine finishing step for surface structuring, for example laser impact or introducing cavities in the work piece or leveling the surface peaks with electrochemical etching.
Another option is to perform dry grinding with a coarser grit and thus achieve greater material removal, but then perform finishing or electro chemical etching as a second fine machining step. In this case the first fine machining step with a defined edge is only performed to a precision of 25 μm for circularity and 30 μm for diameter, since these greater excess dimensions can still be removed through dry grinding due to the coarser grit used.
During fine dry grinding and also during coarser dry grinding preferably a dry cooling and cleaning of the grinding device for example with compressed air is provided at or proximal to the machining location.
Since in the instant case irrespective of the particular selected processing combination only the electrochemical etching is a wet machining, all other machining steps can be performed successively in the same machine and thus at the work piece in the same clamping step at the individual machining locations. Additionally this facilitates simultaneous machining of different machining locations in different processing steps.
Thus, not only the investment for the process is reduced, but in particular an occurrence of additional dimensional imprecisions is avoided which are inevitably provided when clamping and re clamping the work piece when machining is performed with the precisions cited.
Another advantage of the combination of peripheral milling with dry grinding is that both tools due to their basic disc shape and rotation about the Z-direction are not only configured similar but can be used on similar or even identical tool supports.
Thus, even the typically much higher cutting velocity of a grinding disc over a disc cutter can be omitted to use this advantage. Thus, for example the speed and/or cutting velocity of the grinding disc can be set at the most to three times, better only two times the value for the disc cutter which facilitates using partially or completely identically configured supports.
In particular for this purpose it is also useful to define the diameter of the grinding disc approximately in the order of magnitude of the diameter of the disc cutter, at the most, however 20% larger.
In order to further improve precision in the first fine machining step tools are used in which the cutting edges can be subjected to a fine alignment relative to the base element of the tool through wedge systems in order to achieve precisions in a range of 10 μm or below.
Additionally when using an orthogonal cutter, a cutter with two-eight cutting edges, in particular 4-6 cutting edges at the face is used which preferably may be distributed unevenly over the circumference in order not to cause any resonance frequency.
Additionally the orthogonal cutter is moved in engagement at the enveloping surface to be processed, typically starting at an outer circumference of the face of the orthogonal cutter in Y-direction relative to the rotation axis of the work piece during the engagement, thus by at least 40% better at least 50% in particular at the most 60% of the diameter of the orthogonal cutter, so that the problem of the cutting performance and cutting direction that is reduced in the center of the orthogonal cutter or which is not present at all due to the lack of cutting edges is solved in that the continuously performed axle offset causes all length portions of the bearing to be machined with sufficient precision.
For this purpose the work piece rotates at least five times while performing the axis offset of the orthogonal cutter, the work piece better rotates at least 10 times or even better at least 20 times.
The speed of the orthogonal cutter should thus be at least 80 times, better 100 times or even better 130 times the speed of the work piece.
For an external cutter, however, the diameter of the cutter shall be at least 40 times, better 45 times of a stroke of the crank shaft to be machined.
When processing hardened surfaces the cutting edges of the chipping tools with defined edge are typically made from CBM or hard metal, however, are preferably made with a grit of below . . . and thus rather elastic in spite of having sufficient hardness.
In case electrochemical etching is selected in the second fine processing step, thus a material removal of 30 μm at the most, better only 20 μm is performed, but a removal of at least 5 μm since only this achieves sufficient smoothing of the microscopic surface structure to a arm portion of at least 50% which is the general goal for the second fine machining step.
A further acceleration of the production process can be achieved in that the second fine machining step, in particular electrochemical etching only machines the circumferential portion of the lift bearing, thus the rod bearing at the crank shaft which is loaded with the pressure of the connecting rod upon ignition which is always the same circumferential portion.
In particular only the respective half circumference of the rod bearing is processed in the second fine machining step.
This way the first fine machining step can be used for machining the lift bearings thus the rod bearings in the same clamping step and in particular the same clamping step as the proceeding coarse machining which is of interest in particular when hardening is not performed in between or an inductive hardening is also performed in the same machine and in the same clamping step.
In particular in the second fine machining step, this can certainly also be performed in the first fine machining step, the crankshaft is supported with a vertical support and thus at a bearing directly adjacent to the bearing to be processed.
This generates imprints of the stationary support on the supported bearing circumferences, wherein the imprints are not relevant with respect to dimensions and surface quality but shall be finished for optical reasons in that the imprints are removed in a last fine machining step which is facilitated in that the support through the adjacent stationary support is always on the side of the advance direction of the last fine machining step.
In the first fine machining step the flange and the pinion are advantageously machined while the crank shaft is supported at least in radial direction at the main bearing that is respectively adjacent to the respective machining location, in particular with a vertical support or also at adjacent bearings directly with a clamping chuck.
Embodiments of the invention are subsequently described in more detail with reference to drawing figures, wherein:
a, b: illustrates a typical crank shaft in a side view and an enlarged individual bearing;
a, b: illustrates a turning machine with supports arranged above and also below the turning axis;
a, b: illustrates a turning machine with supports only arranged above the turning axis;
a, b: illustrates different processing situations at a symbolized work piece;
a illustrates a side view of a typical crank shaft 1 of a four cylinder combustion engine, thus with four eccentrical lift- or rod bearings PL1-PL4 and a total of 5 main bearings HL1-HL5 arranged adjacent thereto, wherein the main bearings are arranged on the subsequent rotation axis on which the crank shaft 1 is clamped in a turning machine that is not illustrated in more detail, wherein the rotation axis is also designated as rotation axis 2 in the illustration of
The invention relates in particular to machining the enveloping surfaces of the bearings, thus the main bearings and the rod bearings including the adjacent side surfaces, the so called mirror surfaces.
Above and below the crank shaft 1 machining tools are illustrated in an exemplary manner from the top left to the right:
Due to the large number of typically 80 cutting edges or cutting plates 23 which have to be adjusted at a disc cutter 8 with for example 700 mm diameter the exact adjustment in radial and in axial direction in sync with all cutting plates is very time consuming.
Below the crank shaft a turning tool 10 configured as a single point turning tool is illustrated, wherein the turning tool does not extend exactly in X-direction but at a slight slant angle thereto in a direction towards the bearing and can contact the bearing in order to be able to also turn one of the corners of the bearing.
In order to turn both corners including the enveloping surfaces without stopping and thus without a shoulder with the same turning tool 10, this turning tool 10 as illustrated in
It is appreciated that machining one of the rod bearings PL1-PL4 at the crank shaft rotating about the main bearing axis, the engaging tools additionally have to perform a feed movement in X-direction and for the end mill 7 and for the cutting tool 10 an additional feed movement in Y-direction is required in order to be able to follow the orbiting rod bearing.
a and b illustrate an embodiment of a turning machine in a frontal view in Z-direction which can be used for machining work pieces like crank shafts with the methods according to the invention.
As illustrated in
On the front side of the bed 11 below the rotation axis and on the flat top side of the bed 11 longitudinal guides 15 are arranged respectively extending in pairs in Z-direction, wherein tool units are moveable on the longitudinal guides, in this case one tool unit on the lower longitudinal guides and two tool units on the upper longitudinal guides 15.
Each tool unit is made from a Z-slide 16 that is moveable along the longitudinal guides 15 and an X-slide 17 extending on the Z-slide and moveable in X-direction, wherein the tool or the tool unit are mounted on the X-slide.
In the unit below the rotation axis 2 this is a typical tool revolver 18 with a turning tool 10 inserted therein configured as a star revolver and with a pivot axis that extends in Z-direction.
The left upper unit is an individual turning tool 10 in single point configuration, thus pivotable about the B-axis which extends approximately in X-direction and thus moveable in X-direction also in accordance with the pivot movement.
The right upper unit is a finishing tool 19 which can make a circumferential surface at the work piece smoother.
In
Also a single point turning tool is illustrated again in this view adjacent there to in
a on the other hand side illustrates a turning-milling machine in which in turn a crank shaft 1 is supported again as a work piece by spindle stock and opposite spindle stock 14 between two clamping chucks oriented against one another drive able in rotation about the rotation axis 2 which is configured as a C-axis, like in the turning machine of
In this case longitudinal guides 15 are only arranged at the machine bed 11 above the turning axis 2, wherein two tool units with Z-slides 16 and X-slides 17 running thereon are provided.
In this case the right X-slide 17 supports a disc cutter 8 which rotates parallel to the rotation axis as indicated in
Additionally a measuring unit 22 is provided at the right X-slide 17, wherein the measuring unit can be activated and deactivate by pivoting in order to perform measurements at a circumferential surface with respect to diameter, circularity, etc. without unclamping or re clamping the work piece in that a measuring probe to be approached in X-direction contacts the circumferential surface.
In
a illustrates processing a portion of a circumferential surface not with reference to a crank shaft but with reference to a circumferential work piece which could be the circumferential surface of the lift bearing or rod bearing, through tangential turning.
Thus, a straight or concave cutting edge that is arranged skewed to the rotation axis of the rotating work piece is moved in a tangential moving direction 24 contacting at the circumferential surface of the work piece, for a straight edge in a tangential in a straight direction and for a convex edge in a tangential, arcuate direction about a pivot axis which extends parallel to the rotation axis 2.
Thus, only very small excess dimensions can be removed; however the machining result is very precise and has an excellent surface.
In
Thus, an EMC electrode 25 whose contact surface is advantageously adapted to the contour of the circumference of the work piece produced and which includes a respective cavity is moved towards the work piece, wherein an electric current or an electric voltage is applied between the work piece on the on hand side and the electrode 25 on the other hand side and additionally a salt solution or acid is introduced between both of them.
When these parameters are selected accordingly, portions proximal to the surface, in particular the peaks of the microscopic surface structure of the work piece are etched off and carried away in the salt solution. For improvement purposes the electrode 25 can be moved in a pulsating manner radially and axially in order to optimize extraction through salt solution or acid.
As a matter of principle the work piece can be rotated about the rotation axis 2.
However, when a plurality of small microscopic protrusions 26 is provided on the contact surface of the electrode surface 25 like in the illustrated case which are used for producing respective microscopic cavities in the surface of the work piece which are subsequently used as oil reservoirs, the work piece certainly has to be machined while standing still.
Otherwise such microscopically fine cavities, typically only with a depth of a few μm, can also be produced through laser impact.
Thus
Longitudinal turning yields a typically uniform saw tooth profile whose roughness Rz is in the range of 10-20 μm.
The surface structure after tangential turning is similar as a matter of principle; however it has a much lesser distance between peaks and valleys with an Rz of approximately 5 μm.
For external circular milling, however, it is typical that the surface structure includes portions thereafter which are microscopically on different levels according to the impact of the individual milling blades after one another on the work piece and the very fine facets on the work piece thus formed.
The lower portion
Thus, it also becomes clear that with increasing support portion, thus no matter whether through finishing or fine grinding, the surface to be treated with the tool becomes larger and larger and thus the removal in radial direction becomes slower and slower.
Thus in order to determine circularity an inner enveloping circle and an outer enveloping circle is applied to the actual contour and the distance of the two enveloping circles defines circularity.
Additionally also an actual center of the respective bearing may not exactly coincide with the nominal center which is the case in particular for rod bearing pinions and has a negative influence on concentricity precision.
Furthermore, the nominal contour after finishing is defined, thus the final contour which is accordingly radially within the nominal contour after chipping with the defined edge is completed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102011113757.6 | Sep 2011 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2012/068311 | 9/18/2012 | WO | 00 |
