The invention relates to the metal-removing machining of workpieces, in particular crankshafts, in particular of crankshafts for engines for passenger vehicles.
For various reasons, crankshafts are difficult to machine, since firstly they have eccentrically positioned rotationally-symmetrical surfaces, namely the peripheral surfaces of the big-end journals, and moreover when chucked only at their ends form a workpiece which is relatively unstable in the transverse direction and the longitudinal direction.
Since in particular crankshafts for passenger vehicles are produced in very large numbers and are subject to considerable pressure on prices, a focal point with regard to keeping production costs as low as possible is a machining time which is as short as possible and has a small number of operating steps while maintaining sufficient machining quality and dimensional accuracy.
To date, passenger vehicle crankshafts, which currently still predominantly consist of grey cast iron, e.g. GGG60 or 70, have been machined in the unhardened state by turning, internal milling, rotary turn broaching or similar machining processes on the necessary surfaces, that is to say the peripheral surfaces of the bearing locations and on the end faces of the webs, cutting speeds of 100 to 160 m/min being customary for example for milling. In this case, the cutting edges of the milling tools, which are generally formed as internal circular-milling cutters, have a negative tool geometry. The milling has been followed by hardening and then rough-grinding and precision-grinding
The specific machining processes which are currently customary, i.e. turning, rotary turn broaching and internal milling and internal milling (that is to say milling with an internally toothed, annular milling cutter, in the interior of which the workpiece is arranged) have specific advantages and disadvantages:
Internal milling has preferably been used to machine the big-end journals of the crankshaft and the adjoining web surfaces. The advantage consists, on the one hand, in that in the process the workpiece does not move or is rotated at only a low rotational speed and the internal milling cutter is moved around the journal to be machined. The cutting speed is thus produced solely or primarily by the tool, so that a plurality of tools can operate simultaneously and independently on the same workpiece.
This process is suitable above all for high metal-removal rates per unit time, the disadvantages of such rates being the corollary effects of high cutting forces and high tool and chip temperatures.
Internal milling is less suitable in particular for unstable workpieces, such as for example split-pin crankshafts (in which two crankpin journals, which partially overlap one another in the radial direction, are situated only a very small distance apart by comparison with the width of the journal; as required for V-engines).
Internal milling is to be preferred to rotary turn broaching for cost reasons, since it requires a shorter machining time per journal; however, the roundness deviations in internal milling are greater by a multiple than in rotary turn broaching.
The advantages of rotary turn broaching are thus primarily the good dimensional accuracy, in particular the low roundness deviations.
However, in rotary turn broaching the workpiece, e.g. the crankshaft, rotates, in contrast to internal milling, specifically at a considerably higher speed than the tool itself, which may even, under certain circumstances, not execute a complete revolution but rather only a pivoting motion, in order to bring into action on the workpiece the cutting edges which are arranged one behind the other on the external circumference of the tool.
The cutting speed is thus primarily produced by the rotation of the workpiece, resulting in the disadvantage that where specific cutting speeds are to be observed it is not possible for a plurality of tool units to work independently of one another on the workpiece, but rather only on mutually corresponding parts of the workpiece, that is to say, for example, on a plurality of centre bearings or on the two big-end journals, arranged in an identical angular position, of a crankshaft for a 4-cylinder in-line engine.
For this reason, rotary turn broaching machining has been adopted primarily for machining the centre bearings.
The object in accordance with the present invention is to provide a metal-removing machining process in particular for crankshafts, which allow [sic] a short machining time but nevertheless a high machining quality and thus low production costs for the crankshaft.
The two competing parameters here are chip thickness and cutting speed:
For reasons of keeping the introduction of cutting forces into the workpiece as low as possible, in order to minimize the deflection thereof, low chip thicknesses are sought. However, this increases the machining time and can only be compensated for by increasing the cutting speed. In addition, the cutting speed frequently affects the service life, i.e. the overall machining capacity of the cutting means, so that additional boundary conditions apply in this respect too.
This object is achieved by means of the features of Claim 1. Advantageous embodiments emerge from the subclaims.
The shorter machining time is achieved in that the cutting speeds are drastically increased in particular for milling, specifically to at least 180 m/min, in particular to 250 to 600 in/min, in particular to 450 to 600 m/min, for roughing and to at least 200 m/min, in particular to 300 to 800 m/min, in particular to 650 to 850 in/min, for finishing, and with certain cutting materials, such as for example cemented oxides, CBN, PCD (=polycrystalline diamond), cermets (hard metal-ceramic mixture), including coated cermets, e.g. TiAlN-coated cermets, to over 1000 m/min. These cutting speeds are achieved, for example in the case of a disc-like milling cutter with a diameter of 800 mm, in that the tool rotates at 50-2000, in particular 200-400, in particular 200-250 revolutions per minute and at the same time the workpiece rotates at 0-60, in particular 15-20 revolutions/minute in the case of a crankpin journal having a diameter of, for example, 50 mm. Particularly when milling the crankshaft, in particular by means of a large disc-like external milling cutter, this has the effect of reducing the introduction of force into the workpiece per cutting-edge action, owing to the considerably higher frequency of interruption to the cut during milling.
The above is assisted by the fact that a positive tool geometry is employed instead of the previous negative tool geometry and sometimes new materials are employed for the cutting means. In this process, the cutting edges on the disc-like milling cutter are positioned either on the outer circumference of the milling cutter and/or on the end face in the corner region between end face and circumferential surface of the milling cutter, thus permitting the machining not only of the peripheral surfaces of the crankshaft but also of the various, in particular end-side, web surfaces. Since, however, the volume of metal to be removed during the end-face machining of a web is usually considerably greater than when machining the peripheral surfaces of a journal of a crankshaft, it is preferable for a separate milling cutter to be used for the machining of the webs and also a separate milling cutter for the machining of the journals.
In addition, it is generally necessary to make radial recesses, the so-called undercuts, at the transition between the journal surface and the web surface.
A number of different cutting distributions are conceivable in order nevertheless to be able to machine more than just a specific axial length of a bearing using a tool: either a separate milling cutter is provided in each case for the left-hand end region of the journal peripheral surface and for the right-hand end region, in the case of which cutter, following the cutting-edge surfaces for the peripheral surface of the journal, protruding cutting-edge parts is [sic] present for producing the undercut.
The cutting-edge area for machining the peripheral surface in this case has to be sufficiently large in the axial direction for these cutting areas of the two milling cutters for the left-hand and right-hand halves of a journal to overlap in the normal situation, in order thus to permit axial compensation for bearings with different axial lengths. At the overlapping end, the cutting edges for the peripheral surface shallow out gently by means of a slight drop-off or a considerable rounding, in order not to produce an encircling edge in the overlap region.
The other possibility consists in making the undercuts with a special milling cutter, while another milling cutter only has cutting edges for machining the journals, i.e. does not have any protruding cutting-edge areas for an undercut. A milling cutter of this kind which is purely for cutting journals may be of relatively narrow design, that is to say narrower than the shortest expected axial extent of a bearing journal. In order to machine the entire axial length of the bearing journal, this milling unit is additionally moved slowly in the longitudinal direction, that is to say the Z-direction, so that a helical, strip-like path is machined along the peripheral surface of the journal.
This results, by comparison with the known machining of axially adjacent, slightly overlapping annular regions, in there being no annular step (which can never be completely avoided) between the individual annular regions.
Owing to the high cutting speeds, it is also possible to keep the feed and chip thickness at low levels, and thus to achieve high machining quality while nevertheless retaining a low machining time.
High-speed metal-removing machining, in particular high-speed milling, therefore combines a number of advantages:
The workpiece is only chucked centrically, that is to say on the longitudinal axis of the big-end journal positions, and is only driven slowly. Imbalances of the workpiece itself therefore have scarcely any effect on the machining process and the cutting speed is primarily achieved by the rotation of the, for example disc-like, external milling cutter. It is thus possible in principle to have a plurality of such tool units working independently of one another on the workpiece. The big-end journals too are machined with the workpiece chucked centrically, the external milling cutter only having to be displaceable along the X-axis. Owing to the relatively slow rotation of the workpiece, the milling tool can constantly follow the crankpin journal, during the rotation of the latter, by means of this X-displacement.
It is also possible to machine tangential, planar surfaces on the workpiece and/or to machine non-circular external surfaces, and even to mill cavities in the workpiece surface, as long as the radius of curvature thereof is greater than the radius of the external milling cutter.
Tangential milling off on the workpiece also makes it possible, for example, to carry out a balancing operation directly during clamping of the work, in that in the determined angular position mass is removed from the workpiece until the imbalance is eliminated.
Despite the bevelling of the surface, which is typical of the milling of peripheral surfaces, it is possible by means of high-speed external milling cutting to achieve roundness deviations which are as low as those which can be achieved by means of rotary turn broaching; these are in no way attainable by means of internal milling. Nevertheless, the machining time for high-speed milling is lower by a factor of about 3-5 than the machining time for rotary turn broaching and is lower by a factor of approximately 2 than the machining time for internal milling.
Owing to the extremely low chip thickness (what is meant here is the average or the maximum chip thickness, since in the case of, for example, the machining of the peripheral surfaces by means of external circular milling the thickness of the chip changes as the operation progresses) and a relatively short chip length, on the one hand the cutting forces introduced into the workpiece are low, and on the other hand the major part of the process heat is introduced into the chip but not into the workpiece and tool, so that, for dynamic and thermal reasons, very good dimensional accuracy is achieved on the workpiece.
Owing to the cutting parameters and cutting materials, the workpiece is generally machined dry, that is to say without the use of a cooling lubricant, which considerably simplifies disposal of the chips.
If, in addition, the diameter of the milling cutter is selected to be larger than the necessary penetration depth for the crankshaft to be produced at the big-end journal remote from the tool, that is to say for a crankshaft throw of, for example, 120 mm and a penetration depth of about 200-250 mm, the diameter of the external milling cutter is selected to be at least 700, preferably 800-1000 mm, only relatively low rotational speeds of the tool of 50-300 revolutions/minute are necessary to achieve the desired cutting speed. Owing to the large diameter of the milling cutter, the time between two successive cutting interventions by one and the same cutting edge increases, consequently also increasing the time available for cooling the cutting edge and the adjoining tool body, thus reducing the thermal loading on the tool and consequently increasing its service life.
Aligning the individual cutting edges on a precise circular path with respect to the tool body is also facilitated as the diameter of the milling cutter increases.
The additional use of a positive tool geometry instead of the negative tool geometry which was previously used in milling and which nevertheless, primarily in connection with the low average or maximum chip thicknesses, leads to a sufficient tool life of the cutting means, in turn results in a reduction in the cutting forces and consequently also in a reduction in the driving powers required for the tool, which powers, for the size ratios indicated, is [sic] only about half to one third of the power required for internal milling or rotary turn broaching. In addition to the lower energy costs, this also minimizes the waste heat problems of the drives, which always have a negative effect on the overall machine and the machining result.
The high-speed milling according to the invention may in this case be carried out, in particular, not only on the unhardened workpiece but also on the hardened (e.g. Rockwell hardness HRC of 60 to 62, in particular fully hardened) workpiece. In this case, the cutting material preferably used is cermet or polycrystalline boron nitride (PCB), and in the case of the latter in particular cubic boron nitride (CBN). In this case, it is preferable firstly to sinter a carbide cutting tool tip which, however, has cavities in the cutting-edge area, e.g. in the tool face open towards the cutting edge. CBN powder is placed in these cavities in the base body and is then sintered.
It is not only the noses of throw-away cutting tool tips which can be reinforced in this manner, but also an entire cutting edge can be reinforced by arranging a plurality of CBN pellets next to one another along a cutting edge, or else by providing a bar-shaped CBN insert. It is consequently also possible to machine unhardened steel or cast iron, even by milling.
These cutting materials can also be used without cooling lubricant, that is to say dry, thus saving on disposal costs and environmental problems.
It is thus possible even as early as during the metal-removing machining to eliminate the distortion of the workpiece which due to the hardening process occurs in conventional production (metal-removing machining prior to hardening). Since, when using high-speed milling and in particular when using high-speed milling on the hardened workpiece, it is possible to achieve surface qualities which are acceptable as the final state of the workpiece, it is consequently possible to dispense with at least the rough-grinding operation altogether.
When machining the journal and web surfaces on crankshafts which consist of cast iron or steel and are machining [sic] in the unhardened state by means of an external circular-milling cutter, in particular by means of a disc-like milling cutter with cutting edges on the circumferential region, it has proven particularly advantageous to observe the following parameters:
Cutting speed during the roughing machining: at least 180, preferably 250-600 m/min,
Cutting speed during the finishing machining: at least 200, preferably 300-800 m/min,
Chip thickness: 0.05-0.5 mm in particular 0.1-0.3 mm.
The tool used here is generally a disc-like tool body driven in rotation and having inserted throw-away cutting tool tips. In this case, the configuration of the cutting tool tips differs depending on their intended purpose (machining of the end faces on the webs, machining of the peripheral surfaces on the journals of the main bearing point and big-end journal points, production of the undercuts at the transition between peripheral surfaces and end faces) and they are also positioned differently with respect to the tool carrier or to the workpiece:
Data on the basic material relates to the known ISO application groups, in which: K10: consists of 94.2% tungsten carbide (TC), 5.5% cobalt (Co) and 0.3% . . . (Ta/C) K20: consists of 93.2% TC, 6%, Co and 0.6%; Ta/C and 0.2% titanium carbide (TiC)
The flexural strength is 1900 N/m2 for K10 and 2000 N/m2 for K20.
In the coatings specified, the individual compounds are applied in layers one after the other in the sequence specified from-the inside outwards.