Device for the finish machining of circumferential surfaces of substantially rotationally symmetrical workpiece portions on shaft-like workpieces

The invention relates to a device for the finish machining of circumferential surfaces of substantially rotationally symmetrical workpiece portions on workpieces, which have a workpiece axis and several workpiece portions arranged in offset manner along the workpiece axis, particularly for the precision machining of bearing points on camshafts and crankshafts, according to the preamble of claim 1.

Finishing is a precision machining method, in which the circumferential surfaces of substantially rotationally symmetrical workpiece portions on workpieces such as crankshafts, camshafts, gear shafts and other components for machines and motors are worked for producing a desired precision surface structure. Finishing involves a machining tool having a granular cutting compound being pressed by a pressing device onto the circumferential surface to be worked. To produce the cutting speed necessary for material removal, in many method variants the workpiece is rotated about its workpiece axis and simultaneously an oscillating relative movement parallel to the workpiece axis is produced between the workpiece and the machining tool engaging on the circumferential surface. As a result of the combination of the rotary movement of the workpiece and the superimposed oscillatory movement a so-called crosshatch pattern can be produced, so that the worked workpiece circumferential surfaces are particularly suitable e.g. as contact surfaces for roller or plain bearings, etc.

The workpieces can e.g. be camshafts or crankshafts. Such crankshaft-like workpieces are provided in axially spaced manner with mutually rotationally symmetrical workpiece portions serving as bearing points. The so-called main bearings or centre bearings are positioned coaxially to the workpiece axis and serve to mount the shaft for its rotary movement. Immediately alongside a main bearing are provided one or two so-called stroke or lift bearings, whose axes are parallel and displaced eccentrically to the workpiece axis. Other parts of a machine, e.g. the connecting rods of pistons of an internal combustion engine or a pump or a compressor engage on the lift bearings.

During the finish machining of such workpieces, use is generally made of devices making it possible to simultaneously work or machine several of the workpiece portions to be worked or machined. Such finishing devices have a number of finishing units, each of said finishing units having at least one finishing arm with a pressing device for pressing cutting compounds onto the workpiece surface and with each of the workpiece portions to be machined in a machining phase is associated a finishing unit. Examples of devices with which it is possible to carry out a parallel machining of juxtaposed bearing surfaces on crankshafts and the like are described in EP 318 966 B1, U.S. Pat. No. 5,951,377 or EP 997 229 B1.

EP 997 229 B1 discloses a belt finishing machine, which for the simultaneous machining of main bearings and rod bearings has several axially juxtaposed finishing units placed on a common support. Each of the finishing units has two which are pincer-like and pivotable towards and away from one another and on each of which is fixed a pressing device in the form of a workpiece-dependently dimensioning finishing shell, with the aid of which finishing belt is pressed onto the workpiece surface to be machined. In each case the finishing units are very narrow, so as to be able to simultaneously machine juxtaposed bearing points. The width of the finishing units (measured parallel to the workpiece axis) cannot exceed the axially measured width of the workpiece portion to be machined, plus the proportionate, half axial dimensions of adjacent crank webs.

The axial spacing of the centres of immediately adjacent bearings is here also referred to as the bearing spacing and in the case of crankshafts for internal combustion engines corresponds to half the cylinder spacing, i.e. the bore spacing, also referred to as the inside micrometer, of directly adjacent cylinder bores. Typical cylinder spacings for in-line four cylinder motors with an approximately two litre capacity can e.g. be approximately 90 mm, so that a value of approximately 45 mm, as a function of the precise distribution of the shaft, is obtained for the maximum width of the juxtaposed finishing units. On the basis of smaller motors with capacities of e.g. 1.2 to 1.4 litres, the admissible widths of adjacent finishing units become smaller and can e.g. be approximately 40 mm. Therefore the possible arm spacings, i.e. the possible spacings of corresponding elements (e.g. the centre axes) of adjacent finishing units in conventional crankshaft finishing machines is of the same order of magnitude.

The finishing units of belt finishing machines generally comprise not only the fixing arms provided for supporting the pressing devices, together with optionally swivel bearings and linear guides, but also devices for finishing belt conveying and sensors for monitoring the machine functions and supply equipment. Also in the case of finishing devices operating with finishing stones, the finishing units are complex assemblies, which can only undergo a size reduction with considerable effort and cost. If the bearing spacings present on the workpiece drop below the minimum possible arm spacings of the finishing units, the finishing units necessary for machining have hitherto been distributed over a number of stations, which leads to corresponding increased costs and space requirements, as well as longer machining times for the overall machining process.

The problem of the invention is to provide a device for the finish machining of circumferential surfaces of substantially rotationally symmetrical workpiece portions on shaft-like workpieces enabling the economic machining of workpieces with different dimensions. The device must in particular make it possible to economically machine workpieces with bearing spacings of less than 40 mm. These aims are to be achieved whilst maintaining accessibility to the workpieces to be machined for workpiece conveying and preferably whilst bringing about a reduction in manufacturing costs, whilst at the same time the machining quality of conventional finishing devices must be equalled and preferably improved.

For solving this problem the invention proposes a finish machining device having the features of claim 1. Advantageous further developments are given in the dependent claims. By express reference the wording of all the claims is made into part of the content of the description.

In the case of the finish machining device according to the invention, the finishing units are subdivided into a first group of finishing units and a second group of finishing units and axially directly adjacent finishing units are alternately associated with the first and second groups and the first group of finishing units is angularly displaced from the second group of finishing units with respect to the workpiece axis.

In all two or more finishing units can be provided. As a rule there are far more than two finishing units, e.g. 4 or more, 6 or more, 8 or more or 10 or more. A group of finishing units can consist of a single finishing unit, but normally such a group has two or more finishing units.

As a result of the inventive arrangement, directly adjacent finishing units, which are intended to machine immediately adjacent workpiece portions, are displaced relative to the workpiece axis in the workpiece circumferential direction, so that the corresponding pressing devices of immediately adjacent finishing units engage from directions oriented in angularly displaced manner to the workpiece axis on the workpiece. Thus, a finish machining device with separate finishing packages for the lift bearings and the main bearings can be provided. As a result of the angularly displaced arrangement of groups of finishing units, the conventional restriction with respect to the minimum arm spacing can be obviated, so that a maximum width of a finishing unit, measured parallel to the workpiece axis, can be greater than the minimum settable distance between corresponding structural elements of directly adjacent pressing devices. Whereas hitherto the maximum width of a finishing unit could be no greater than the bearing spacing of immediately adjacent bearings of a shaft to be machined, this restriction is eliminated as a result of the angularly displaced arrangement. If e.g. immediately adjacent finishing units of a group are in each case arranged in parallel juxtaposed manner, i.e. in a row, now the maximum width of adjacent finishing units of a group is only limited by the cylinder spacing which is twice the bearing spacing. This constructionally simplifies the making available of all the functional elements necessary for the functioning of the finishing units, such as bearings, guides, optionally belt conveyors, sensor systems, etc., which cuts down costs and may be able to improve the functionality of the finishing units.

Thus, in the case of many constructional embodiments, it is possible for the fixing units in the vicinity of the pressing devices to have a width of less than 40 mm and said width can also be 35 mm or less or 30 mm or less or 25 mm or less.

Although the finishing units of a group can also be angularly displaced against one another, generally the finishing units of a group are arranged in a row. This simplifies the mounting of the finishing machine bed and it is possible to easily adjust the reciprocal spacings of the finishing units.

In an embodiment the first finishing arms of the first finishing units of the first group are linearly displaceably mounted for machining first workpiece portions, arranged coaxially to the workpiece axis, in the radial direction of said axis. The pressing device of a finishing arm can consequently be moved by a translatory movement towards or away from the workpiece, which simplifies the construction of the first finishing units.

A first finishing unit preferably only has a single finishing arm. As a result of the one-sided pressing, the pincer-like shape of conventional finishing units can be obviated and as a result a constructionally relatively easily implementable, operationally reliable and stable one-sided construction of the first finishing units is obtained.

The first finishing units can e.g. be placed below a horizontal plane defined by the workpiece axis and in particular in such a way that the finishing arms of the first finishing units perform a substantially vertical displacement movement between a retracted rest position (e.g. for tool change) and a machining position with engagement on the workpiece.

Preferably second finishing units of second workpiece portions (lift bearings) arranged eccentrically to the workpiece axis are equipped with at least one swivel arm swivellable about a swivel axis for supporting at least one pressing device. The second finishing units can be constructed in pincer-like, conventional manner. Preferably the second finishing units have only one finishing arm in the form of a swivel arm with a pressing device, so that there is a one-sided construction for the second finishing units.

A one-sided or one-armed construction of the second finishing units is inter alia advantageous because the lift bearings to be machined therewith are fitted eccentrically to the workpiece and on rotating the workpiece about its axis perform a circular motion about said axis which must be followed by the finishing unit. For this purpose an individual drive is not normally used and instead the finishing arms are carried along by the workpiece. As a result of the one-armed construction, the carried along masses of the second finishing units provided for the lift bearings can be significantly reduced compared with pincer-like constructions, so that inertia-caused fluctuations of the pressing force and the consequent quality losses when machining the lift bearings can be reduced.

A support device for the second finishing units can be placed alongside the workpiece, e.g. in such a way that the swivel axis of the swivel arm is in a horizontal plane with the workpiece axis. The swivel arm of the second finishing unit can be so positioned that in its machining position it is substantially horizontally oriented and engages from above on the workpiece and can be swivelled away upwards from the machining position with workpiece engagement into a rest position.

The overall arrangement of the finishing units in certain embodiments is chosen in such a way that first finishing units and second finishing units are so positioned that the associated pressing devices engage on the workpiece essentially from opposite sides. Thus, e.g. linearly displaceable finishing arms of first finishing units for the main bearings can engage from below, whereas swivellable finishing arms of second finishing units for the lift bearings in the machining position engage on the workpiece essentially from the opposite top side. Thus, the finishing units which are in machining engagement provide closely juxtaposed bearing points on alternately opposing sides, so that the pressing forces of the first and second finishing arms compensate one another and consequently a sagging of the workpiece during machining possible in the case of only one-sided pressing forces is avoided.

Inventive finish machining devices can be set up for different finishing machining processes.

In some embodiments the workpiece is rotated about its axis for producing the cutting speed necessary for material removal. At the same time an oscillatory relative movement between the workpiece and the machining tool engaging on the circumferential surface parallel to the workpiece axis is produced. For this purpose the workpiece can be given an axial oscillatory movement. It is alternatively or additionally possible to produce the oscillatory movement through the machining tool.