The present invention concerns a method of machining rotationally symmetrical parts to put them into the condition in which they are capable of use, wherein the parts in the course of the manufacturing procedure are at least partially subjected to a hardening procedure. In particulars this concerns crankshafts, in particular the bearing surfaces (both of the big-end bearings and also the fitting bearings, the sealing bearings and central bearings) of crankshafts, in regard to which the condition in which they are capable of use is that condition in which the crankshaft can be fitted in the engine without further removal of material at the bearing surfaces. In that respect bearing surfaces is used to denote both the peripheral surfaces, that is to say the bearing diameter, and also the surfaces which are referred to as the mirror or thrust surfaces, that is to say the end faces which adjoin the peripheral surfaces generally in perpendicular relationship and which serve for example for axial thrust support. In this respects particularly in the case of steel crankshafts, the bearing surfaces are hardened in the regions thereof which are in the proximity of their surface. That serves both for increased resistance to abrasive wear of the bearing locations, and also serves to protect those bearing surfaces from damage in the course of handling throughout the entire manufacturing procedure as well as serving to influence the strength properties involved.
Crankshafts, in particular the crankshafts for private motor vehicles which have a large number of cylinders, are known to be workpieces which are unstable during machining and thus difficult to machine. Assessment of the dimensional accuracy of a finished crankshaft is effected primarily, besides the axial bearing width, by assessment of the following parameters:
Diameter deviation, which is deviation from the predetermined reference or target diameter of the bearing journal,
roundness, which is macroscopic deviation from the round reference or target contour of the bearing journal,
concentricity, which is a diameter deviation in the case of a rotating workpiece, that is to say, for example, the deviation from the reference positional contour which a main bearing location effects during the rotary movement of the crankshaft by virtues on the one hands of the non-ideal roundness of that main bearing journal and, on the other hand, of flexing of the crankshaft which in that case is clamped only at its ends,
roughness Ra, which is a value which is ascertained by calculation and which represents the microscopic roughness of the surface of the bearing location, and
percentage contact area, which is the load-bearing surface proportion of the surface structure, considered microscopically, which comes into contact with a co-operating or counterpart surface pressed thereon.
In addition, in regard to the big-end bearing locations, there is an assessment of:
stroke deviation, which is the dimensional, percentage deviation of the actual stroke (spacing of the actual center of the big-end bearing journal from the actual center of the main bearings), from the reference or target stroke, and
angle deviation, which is the deviation of the actual angular position of the big-end bearing journal from its reference or target angular position relative to the main bearing axis and with respect to the angular position in relation to the other big-end bearing journals, the angle deviation being specified in degrees or as a length dimension in the peripheral direction, related to the stroke.
Observing the desired tolerances in regard to those parameters is made difficult less due to the available machining methods than the instability of the workpiece and the machining forces involved. The efficiency and economy of the method also play a large part in a practical context.
Hitherto the removal of material from the bearing locations on the crankshaft in its original form, that is to say as cast or forged, was effected in succession in four machining steps:
First Step:
Cutting machining with a given cutting edge; this involved using the processes of turning, rotary broaching, turning-rotary broaching, internal round milling and external milling, rotary milling, in particular in the form of high-speed milling or combinations of such procedures. The magnitude of the material to be removed was in the millimeter range.
Second Step:
Hardening of the desired regions, in particular the bearing surfaces, wherein heating and subsequent cooling of the crankshaft and the change in the material and configurational structure, in particular due to the irregular way in which those procedures take place, causes distortion and dimensional variation of the crankshaft or one of the regions thereof, and that has to be compensated by the subsequent operations of removing material, in consideration of the reference or target dimensions of the final condition.
Third Step:
Grinding by means of a hard, massive grinding tool, for example a grinding wheel, which generally rotates with its axis of rotation in parallel relationship with the axis of rotation of the crankshaft to be machined; the amount of material to be removed was in the tenths-of-millimeter range.
In the case of crankshafts which are difficult to machine, in particular crankshafts which are long and thus highly unstable, the grinding machining operation was also effected in a multi-stage procedure, for example in a two-stage procedure by preliminary and finishing grinding.
Fourth Step:
Finishing by generally a stationary grinding means (grinding belt or grinding stone) which is pressed against the external periphery of the rotating bearing location; the amount of material to be removed is at the present time in the range of hundredths of a millimeter or even mm.
In that respect, a distinction is also to be drawn in regard to the machining operation, in respect of the material of the crankshaft (steel or cast iron), in which connection in particular steel crankshafts which are preferably used for situations of use involving a high loading are hardened after the cutting machining operation.
In order to reduce the costs involved in crankshaft machining, the endeavour is to reduce the machining of the bearing locations from four to three different machining stages.
This means however when the cutting machining operation is omitted that in particular the removal of material which is to be implemented by the grinding operation must be greater than in the case of the four-stage method.
Removing material by means of grinding however involves the following disadvantages:
because of the cooling/lubricating agent which is to be added, the grinding slurry which is produced gives rise to problems and is extremely costly to dispose of,
because of the oil contained in the cooling/lubricating agent, for example in the case of CBN-grinding, there is always a latent risk of explosion,
in the grinding operation the amount of cooling/lubricating agent used is substantially greater than in the case of cutting machining procedures as the cooling/lubricating agent is additionally employed in order to remove the grinding dust and swarf from the surface of the grinding wheel again, by jetting the cooling/lubricating agent on to same under high pressure,
nonetheless the danger of the workpiece suffering from overheating is very high,
the machining pressures acting on the workpiece are higher than in the case of cutting machining, and
a microscopic surface structure is produced, in which the grain boundaries which are torn open by the grinding grain are smeared closed again by the subsequent grinding grains, with removed workpiece material, that is to say this is a surface structure with relatively few steep peaks, but with more or less flat, bent-over peaks which partly overlap in scale-like relationship.
A primary object of the invention is to simplify the removal of material when machining bearing locations on a crankshaft to be hardened.
This object is attained by virtue of the grinding machining operation being omitted so that the machining sequence is reduced from four to only three machining procedures which are, in principles different. This eliminates not only all disposal problems in regard to the grinding slurry or swarf, but also the quite considerable capital investment costs for grinding machines, the costs involved in tool consumption and not least the required stock of workpieces, which is increased due to the grinding operation, by virtue of prolonged turn-around times for the workpieces. Disposal of the cuttings or swarf from the cutting machining operation does not give rise to any problems as either cutting is effected dry (high-speed milling) or separation of oil and cuttings or swarf is entirely possible by virtue of the much lower specific surface area of the cuttings or swarf in relation to grinding dust.
So that, in the procedure for removing material, the finishing operation can directly follow the cutting machining operation with a given cutting edge, hereinafter referred to for the sake of brevity as cutting machining, the degree of the admissible deviation of the actual values from the reference or target values, as occur after the cutting machining operation, must be so established that, in the totality of the machining procedures (cutting machining+finishing), the complication and expenditure involved must be technically as low as possible, with at the same time an overall machining time that is as short as possible.
Under some circumstances it is also possible to effect cutting machining with such a good final result that it is possible completely to omit the finishing operation. Overall the present invention therefore concerns the following combinations of processing sequences until the condition in which, for example, the crankshaft is capable of use is achieved:
cutting-hardening-cutting-finishing
cutting-hardening-cutting
cutting-hardening-finishing
cutting-hardening.
In that respect it is not sufficient in the cutting machining operation to strive for reference or target dimensions which come as close as possible to the final dimensions after the finishing operation, so that the oversizes which are to be dealt with by finishing and thus by relatively slow removal of material can remain as small as possible.
It must be taken into account that the stroke deviation and the angle deviation of the big-end bearing journals can no longer or can be only very slightly compensated by the finishing operation.
It must also be borne in mind that in the finishing operation in any case firstly the amounts of material that can be removed (reduction in diameter) are very small, that is to say up to about 200 mm can be achieved at economically viable expense and in addition secondly the finishing operation primarily provides for an increase in the percentage contact area, more specifically by a reduction in roughness, with the aim of achieving a percentage contact area of about 95%. A percentage contact area of 100% is unwanted as then there would no longer be any depressions which are necessary in order to maintain a film of lubricant at the bearing.
In regard to the microscopic surface structure, the cutting machining operation gives a surface in which the grain boundaries are partially torn open by virtue of the cutting edge pulling the grains apart as it cuts through the material. As a result the surface has a relatively large number of pointed raised portions, interrupted by valleys in the form of opened grain boundaries. A surface structure of that kind is conducive to the finishing operation by virtue of the fact that the many pointed raised portions not only facilitate the removal of material by the finishing procedure, but at the same time they also delay clogging of the finishing belt and the like member which is used in the finishing operation, insofar as the pointed raised portions of the workpiece surface provide that the material which is already deposited in the finishing belt is partially torn out of same again.
The limitations of this procedure are already encountered in the operation of reducing roundness deviations by finishing, insofar as the amount of time involved or the roundness deviations which can be dealt with depend not only on the absolute value of the roundness deviation to be equalised or levelled, but also the configuration thereof:
If the non-roundness is such that there are only a few (for example 2-3) troughs and raised portions, distributed over the periphery (thus constituting long-wave non-roundness), then, with the same absolute value in terms of non-roundness, for equalisation by means of finishing, a substantially greater amount of time is required or, under some circumstances, it is not possible to provide for complete equalisation, in comparison with short-wave non-roundness involving about 5 and preferably even about 30 or more troughs per periphery of the bearing location, with the same absolute value in respect of non-roundness.
It is also to be borne in mind that in the finishing operation at the same time on the one hand the degree of roughness is reduced and thus the percentage contact area is improved, while on the other hand the existing non-roundness is equalised or levelled. Those two effects can scarcely be decoupled from each other, or can be decoupled only to a very limited extent. If therefore, starting from an initial roughness, the desired roughness is achieved in the finishing operation after a given period of time, the finishing procedure is stopped as a given percentage contact area should not be exceeded. The equalisation or levelling effect in respect of the roundness deviation, which is achieved in that condition, is then accepted as a final result, and cannot be advanced separately any further.
Accordingly, if the procedure is commenced from a given initial condition in respect of those two parameters from the finishing operation, roughness and percentage contact area cannot be machined independently of each other to afford desired final values.
For the direct succession of the finishing operation after the cutting machining operation therefore what is recommended is in particular specific coupling of the input parameters in regard to the finishing operation and therewith the output parameters in regard to the cutting machining operation, in respect on the one hand of the absolute value and degree of roundness deviation and on the other hand in respect of microscopic roughness and the percentage contact area which applies in that case.
In the cutting machining procedure using turning methods and also rotary broaching methods and possibly also when using slow milling methods, long-wave roundness deviations rather occur by virtue of long-wave oscillations in the machine structure, the tools and the workpieces involved in cutting machining. In contrast more especially high-speed external milling gives rise to short-wave roundness deviations. Therefore, the use of high-speed external milling in which an external round milling tool of a diameter of about 700 cm, which is very large in diameter in comparison with the crankshaft, rotates at a cutting speed of between 150 and 1000 m/min beside the relatively slowly rotating workpiece about an axis which is parallel with respect to the crankshaft means that it is possible to achieve roundness deviations with many raised portions along a circumference of a bearing.
Rotary milling, in particular if it is implemented with high cutting speeds, also rather tends to involve short-wave oscillations and thus short-wave roundness deviations. For, rotary milling involves milling with a kind of end-milling cutter which is arranged in parallel displaced relationship with respect to the radial direction of the peripheral surface, which is to be machined, of the bearing location, insofar as the peripheral surface is machined by means of the one or some cutting edges arranged on the end of the end-milling cutter. In that case, in particular machining with only one single cutting edge has been found to be advantageous if in that case operation is implemented with very high speeds of milling cutter rotation and the workpiece rotates comparatively slowly. If in that case the above-mentioned mirror surfaces or thrust surfaces of a bearing location are also to be subjected to machining, the end-milling cutter is also provided with one or more cutting edges on its peripheral surface.
It must also be taken into account that, in the finishing operation, the grinding means which bears against the workpiece, for example a finishing belt, is generally not changed during the procedure. The grinding means therefore becomes increasingly clogged at its surface during the finishing operation and the amount of material removed per unit of time progressively decreases.
How fast the grinding means begins to suffer from clogging in particular at the beginning of the finishing operation depends not only on the initial roughness of the surface but also on the percentage contact area thereof:
The lower the percentage contact area with a given level of roughness at the beginning of the finishing operation, that is to say the more pointed the microscopic surface structure is with correspondingly steeper flanks, then all the more readily can the material particles which have been removed from the surface and deposited in the finishing belt be removed from the finishing belt or the finishing stone or comparable finishing means at the beginning, when dealing with such a surface structure. With increasing equalisation or levelling of the microscopic surface the surface of the grinding means also becomes progressively more quickly clogged in the finishing operation.
This means that, with the same degree of roughness, a percentage contact area which is low at the beginning of the finishing operation is advantageous for high initial removal of material and thus a finishing operation which is short in time.
This also means that the degrees of roughness which can be handled by the finishing operation increase, in inverse proportion to the percentage contact area involved with those greater levels of roughness.
With the previous grinding operation, which was always effected following hardening, the roughness of the surface was admittedly reduced in comparison with preliminary cutting machining, but in-that respect in the same way the percentage contact area was either kept constant or even increased as the preliminary cutting machining operation left behind a microscopic surface structure which involves a low percentage contact area as machining with the given cutting edge, in the regions near the surface, means that in part the grain boundaries in the metal structure are torn open, extending radially from the outside inwardly.
In that way it is possible to finish in an economic fashion directly after the cutting machining operation insofar as efficient removal of material in the finishing operation is promoted and assisted by the preliminary machining procedure, insofar as on the one hand the choice of the correct cutting machining procedure means that the roughness achieved in that case has a low percentage contact area, and the roundness deviation achieved in that case is a roundness deviation which is as short-wave as possible.
In that respect it must further be taken into consideration that, in the previous grinding of bearing locations, the roundness deviations resulting from the preliminary cutting machining were generally only reduced by the grinding operation in terms of their absolute value but not in terms of their characteristic. Therefore, the grinding operation did not result in long-wave roundness deviations becoming short-wave roundness deviations, but the number of troughs was either retained or even reduced, with the consequence that a further improvement in roundness deviations by the finishing operation, considered as an improvement in result per unit of time, was made rather more difficult in the finishing procedure.
That means that a finishing operation directly after the cutting machining procedure is particularly economical when, after the cutting machining operation, the roundness deviations are less than 60 mm and in particular less than 40 mm, the diameter deviation is less than 200 mm, in particular less than 150 mm and the roughness Ra is less than 10 mm, in particular less than 6 mm. In that respect, the aim to be sought is roundness deviations with a short-wave nature of at least 5 waves per circumference, which applies in regard to bearing diameters of about 50 mm, but which, with rising or falling bearing diameters, should only change in a sub-proportional fashion, that is to say for example a 100% change in circumference produces only a maximum 30% change in the number of waves.
Furthermore, in that respect, the aim to be sought is a rather lower percentage contact area in respect of the roughness achieved after the cutting procedure, than is obtained after the grinding procedure.
In the case of big-end bearings in addition the angle deviation after the cutting machining operation should be less than 0.4xc2x0, in particular less than 0.2xc2x0, and the stroke deviation should be less than 0.40%, in particular less than 0.20%, which corresponds to the tolerances to be observed in regard to the crankshaft when ready for use, as those parameters can no longer be changed in an economically viable manner by the finishing operation.
A suitable form of the cutting machining procedure is therefore external milling or rotary milling, in particular in the form of high-speed milling, in consideration of the above-described interrelationships.
Particularly when dealing with heavy workpieces, it can be found that a combination of the deviations between reference or target values and actual values of the relevant parameters, such deviations occurring after the cutting machining operation and prior to the finishing operation, wherein such combination is desirable in terms of direct coupling of cutting machining and finishing, can only be achieved if the cutting machining procedure is effected in a plurality of stages, in particular in two stages (preliminary cutting and finishing cutting). In that respect, high-speed external milling or high-speed rotary milling is to be preferred both when dealing with big-end bearings and also when dealing with main bearings, for the second stage, the finishing cutting operation.
The first stage, the preliminary cutting operation, when dealing with big-end bearings will also be effected using external milling, in particular using high-speed external milling, while when dealing with main bearings this can also be effected by turning or rotary broaching or turning-rotary broaching.
If the cutting operation is implemented in two or even more stages, the oversizes which are to be dealt with in the finishing cutting operation range in the optimum fashion between 0.1 and 0.4 mm in order further to improve in particular roundness and diameter deviation by virtue of the removal of very thin cuttings or swarf and in order to achieve a roughness which remains as equal as possible in the course of each individual step in the high-speed milling procedure, with the percentage contact area remaining uniformly low, from a microscopic point of view.
Whether hardening is effected only after conclusion of the single-stage or two-stage cutting machining procedure, and material can be removed after the hardening operation only by means of finishing, depends primarily on the extent of the distortion which occurs in the hardening operation. The distortion depends on a series of individual factors such as configuration and dimensioning of the crankshaft, the nature of the hardening process, whether relatively small or relatively large surface regions or indeed the entire surface are or is hardened, the depth of penetration of the hardening procedure and the configuration in respect of time of the hardening procedure, and so forth. In that respect the probable maximum distortion which occurs in the hardening operation, together with that after the last cutting machining step effected prior to the hardening operation, is not to exceed the threshold which can be managed at commercially acceptable cost by finishing.
On the other hand after the hardening operation a material-removal step is again implemented by cutting machining, in particular the second cutting machining step (finishing cutting) of a two-stage cutting procedure.
As the removal of material in the cutting machining operation is more difficult after hardening than when working on the unhardened workpiece, in this case the oversize which is to be dealt with by the finishing cutting operation in the hardened condition is kept at a small level.
The use of belt-type finishers is recommended for the finishing operation, in which case the grinding belts are pressed by means of contact pressure shell members against the rotating bearing location and at the same time a relative oscillation is produced between the grinding belt and the workpiece, in the longitudinal direction. In that case, the contact pressure shell members should embrace the workpiece by at least 120xc2x0 in each case, preferably by up to 180xc2x0, of cooling/lubricating agents to the machining location, although this cannot always be achieved.