Gear cutting – milling – or planing – Milling – Process
Reexamination Certificate
1998-12-22
2003-10-07
Briggs, William (Department: 3722)
Gear cutting, milling, or planing
Milling
Process
C409S199000, C409S200000
Reexamination Certificate
active
06629806
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
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 peripher
Kiefer Herbert
Kohlhase Matthias
Santorius Rolf
Scharpf Paul Dieter
Schreiber Leo
Boehringer Werkzeugmaschinen GmbH
Briggs William
Norris & McLaughlin & Marcus
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