EP0212338B1 - Method for machining the surface of a cam - Google Patents

Description

The invention relates to a method for machining the surface of a cam, the outer contour of which has a base circle section, a secondary circle section, and two flanks which continuously connect the circle sections, in which the cam extends around a first, spatially fixed, through the center of the base circle going axis is rotatable, wherein a grinding wheel rotatable about a second axis parallel to the first axis is provided and the spacing of the axes is adjustable, so that, starting from a raw contour, a nominal contour can be generated by removing the surface by changing the grinding wheel by varying the distance the axes are moved while the cam is rotating, so that on the one hand a section of the grinding wheel which is in engagement on the surface can be guided along the contour in web operation and on the other hand can be adjusted in the feed operation by the surface distance between the raw contour and the desired contour.

A method of the type mentioned above is known from EP-A-0 093 352.

In the known method, the camshaft is rotatably arranged about a fixed axis, and a grinding wheel is rotatably mounted about an axis parallel to the camshaft axis. In order to grind a predetermined target contour of the cams, the distance between the grinding wheel axis and the camshaft axis is changed with the camshaft rotating slowly in such a way that the surface section of the grinding wheel engaged in each case removes the surface of the cam in such a way that the desired target contour is finally created.

The variation of the distance between the grinding wheel axis and the camshaft axis obeys two boundary conditions:

On the one hand, the distance variation is necessary in order to compensate for the respective contour of the cam to be machined. This part of the distance variation is referred to as "railway operation".

On the other hand, however, the distance between the axes must be varied in such a way that a certain infeed of the grinding wheel takes place, i.e. an approximation to the cam to be machined by the amount of the distance from the raw dimension to the target dimension. This part of the distance variation is referred to as "delivery operation".

The known methods, which usually use numerically controlled processing machines, provide for simultaneous execution of rail operations and delivery operations.

In one of these known methods, the next point to be approached is continuously interpolated depending on the rotation of the camshaft, the path movement and the infeed movement being numerically superposed as part of the interpolation.

In another known method, the grinding wheel is mounted on a two-part carriage, one part of which runs on the other part, one part of the carriage being controlled as a function of the rail operation and the other as a function of the infeed operation. In this known method, the required superposition is achieved by mechanical superimposition.

The two known methods explained above thus have in common that the point of a surface section of the grinding wheel which is in engagement in each case moves from the raw contour to the desired contour in a spiral path which is wound several times, this movement always having a radial component due to constant infeed and - as already mentioned - must be set by continuous superposition of rail operations and delivery operations.

The two known methods therefore have the following major disadvantages:

On the one hand, the constant superposition of rail operations and delivery operations requires considerable effort. In the case of a superposition within the framework of the interpolation of the next processing point to be approached, either a very fast and therefore expensive computing unit is required, or a simpler computing unit is used, which, however, increases the processing time drastically because the computing time required to determine the next one to be approached Point becomes unacceptably high.

The mechanical superposition in accordance with the second of the known methods described gives rise to considerable mechanical problems, in particular because circular areas, for example, occur in sections when machining cam contours, during which the web operation has a constant control variable. The part of the feed intended for railway operation must therefore stand still during the machining of this circular section, but this is hardly possible within the scope of the precision of the machining required in the present context. In practice, tremendous movements, which result from small control cycles around a constant position, occur rather with still justifiable control expenditure for the positioning of the railroad operating feed.

Finally, the known methods have the common disadvantage that, as a result of the continuous infeed, there is always a radial component of the machining direction, which can either lead to problems with the surface quality or make a separate aftertreatment necessary.

From US-A-3,344,559 a numerically controlled camshaft grinding machine is known, in which the control is carried out via two different punched strips. One of the punched tapes contains the information about the cam shape, while the other punched tape contains general processing parameters, for example for infeed, dressing, firing and the like. In this known Grinding machine, the two punched strips are processed alternately by first adjusting the grinding machine to the cam by a certain delivery amount using the second punched tape and then generating the desired cam profile using the first mentioned punched tape. However, the cam is stopped during the infeed of the grinding wheel and in a position in which the grinding wheel lies against the cam tip. The cam tip is also the zero point for the polar coordinate system, in which the cam contour is stored in the former punched tape and which is called up again as soon as a further infeed step has been carried out by means of the second punched tape.

A similar numerically controlled camshaft grinding machine in which the infeed is carried out at the cam tip when the cam is stationary is also described in EP-A-0 085 225.

Furthermore, a camshaft grinding machine is described in US Pat. No. 3,919,614, in which, however, the cam profile is not stored in a numerical memory, but rather is tapped mechanically from a master cam.

From DE-Z "wt magazine for industrial production", 1979, pages 613 to 620, it is known to allow the grinding wheel to be radially immersed into the workpiece when grinding external milling and drilling tools for grinding a closed annular groove, before the actual deep grinding begins by rotating the workpiece. Alternatively, it is known from DE-Z to superimpose this immersion and the subsequent cutting movement (rotation of the workpiece), which results in an oblique immersion in the closed annular groove. Finally, elsewhere in DE-Z it is also stated that the deep grinding principle can also be used for grinding camshafts, although it is expressly stated as a disadvantage that the infeed specified with the workpiece allowance is not large enough to achieve high cutting performance and work simultaneously in a workpiece speed range that is sufficiently low in relation to the workpiece edge zone temperature.

From DE-C-26 21 430 a compensation system for a numerically controlled machine tool is known which is used for a cam grinding machine. The known system uses a certain mathematical algorithm for adjusting the distance between the axis of the workpiece and the axis of rotation of the grinding wheel and for adjusting the angular position of the workpiece. The known system therefore uses the interpolation already described above to calculate the path points in "path operation".

Finally, a camshaft grinding machine is also known from DE-A-28 21 753, in which a number of cams are ground as a function of profiles of pattern cams, which are scanned by means of a button. This known machine is therefore a conventional copy grinding machine, geared to the needs of camshaft grinding.

The invention is based on the object of developing a method of the type mentioned in such a way that the desired target contours can be achieved without loss of accuracy with significantly reduced effort and high processing speed.

This object is achieved according to the invention in that the section is first guided from a first point on the surface of the base circle of the raw contour only in the delivery mode to a second point of the base circle of the target contour and then is guided along the target contour by switching the operation only in rail operation, whereby the first point and the second point define an angle of rotation of the first axis which is between 20 ° and 180 °.

The object on which the invention is based is thus completely achieved because subsequent process steps are achieved by the strict time separation of delivery operation and rail operation that the position control only has to work from one point of view, so that the disadvantages described in detail above, which result from the simultaneous Operation result, be avoided.

When using a numerical control, only one of the movements can be interpolated during each of the two successive phases, which shortens the computing time and / or reduces the demands on the required computing unit. When using a mechanical superposition, on the other hand, while the one feed part is taking effect, the other feed part can be mechanically locked in a controlled manner, so that no errors can occur.

The measure to start the feed operation in the first phase from a point on the surface of the base circle of the raw contour has the advantage that there is a cylindrical outer contour, so that the operation in a particularly simple manner by suitable coordination of the angular speed of the cam and the Feed speed of the grinding axis can be adjusted. The measure of doing this in an angular range between 20 and 180 ° has the advantage that suitable time / machining volumes can be set, depending on how this is expedient for the respective material or workpiece.

In a preferred embodiment of the invention, after the section has been guided on the target contour, it is first guided from a third point on the target contour only in the delivery mode to a fourth point of a second target contour and then guided along the second target contour by switching the operation only in rail mode.

This measure, thus taking the steps several times in succession, the desired effect can be achieved even with large volumes of material to be removed. This is particularly advantageous if the raw contour is very irregular, so that the required target contour cannot be achieved with a single machining process for machining reasons. In contrast to the prior art, however, even with this procedure, the point of the machining tool that is in engagement is not guided along a spiral that is wound several times, but the infeed area is always limited to a spatially narrow surface area, while the rest of the time, in turn, is driven exclusively in rail operation. Apart from the small infeed area, the location curve of the processing point thus has the shape of several location curves concentrically running at a parallel distance.

Finally, a further development of the invention is preferred in which the machining tool is guided in the infeed mode with a uniform infeed.

With this procedure, the locus of the respective processing point has the form of an Archimedean spiral, which can be handled particularly easily for control tasks in numerical control.

Further advantages result from the description and the attached drawing.

• Fig. 1 und 2 eine Vorrichtung zum Durchführen des erfindungsgemäßen Verfahrens in zwei senkrecht zueinander liegenden Ansichten;
• Fig. 3 und 4 einen stark vergrößerten Ausschnitt aus der Vorrichtung gemäß den Fig. 1 und 2 zur Erläuterung des erfindungsgemäßerr Verfahrens in zwei Bearbeitungsphasen;
• Fig. 5 eine schematische Darstellung einer Ortskurve eines Bearbeitungspunktes bei in mehreren Schichten erfolgender Bearbeitung des Profils. In den Fig. 1 und 2 ist mit 10 gesamthaft eine Schleifmaschine bezeichnet, wie sie zur Durchführung des erfindungsgemäßen Verfahrens verwendet werden kann.

Embodiments of the invention are shown in the drawing and are explained in more detail in the following description. Show it:

• Figures 1 and 2 an apparatus for performing the method according to the invention in two perpendicular views;
• 3 and 4 show a greatly enlarged detail from the device according to FIGS. 1 and 2 to explain the method according to the invention in two processing phases;
• Fig. 5 is a schematic representation of a locus of a machining point when machining the profile in several layers. 1 and 2, a grinding machine is designated by 10 as it can be used to carry out the method according to the invention.

A camshaft 12 is rotatably arranged about a fixed axis 11, which is also referred to in technical terms as the C axis. For this purpose, the camshaft 12 is clamped between two tips 13 and 14 of a headstock 15 or a tailstock 16, and a rotationally fixed connection 17 between the camshaft 12 and a spindle of the headstock 15 ensures that the camshaft 12 is driven.

In the operating position shown in FIGS. 2 and 2, a cam 18 of the camshaft 12 is being machined by means of a grinding wheel 19. The grinding wheel 19 is actuated by a drive 20 which can be moved by means of a feed 21 relative to a fixed base 22 along an axis 23, which is also referred to in technical terminology as the x-axis. The grinding wheel 19 itself is rotatable about an axis 24, so that the feed 21 is able to adjust the distance between the axes 11 and 24 in the direction of the axis 23 perpendicular thereto.

For the sake of clarity, FIGS. 1 and 2 do not show the control and regulating units which derive control signals for the feed 21 from the respective rotational position of the camshaft 12 in a manner known per se.

3 and 4 show, in a greatly enlarged representation and rotated 90 ° clockwise in relation to the representation of FIG. 1, the conditions when machining the cam 18 by means of the grinding wheel 19.

3 shows the starting position of the cam 18. The cam 18 is provided with an outer contour in which a base circle 30 and a secondary circle 31 occur, which are connected to one another by straight or curved flanks 32. The sections 30, 31, 32 drawn thick in FIGS. 3 and 4 denote a raw contour, i.e. a not yet finished cam, while the reference numerals 30a, 31a, 32a denote the corresponding elements of a target contour that is to be produced. For this purpose, an infeed 33 is required, which corresponds to the distance of the contour 30/31/32 from the contour 30a / 31a / 32a.

An arrow 34 indicates the direction of rotation of the cam 18, and an arrow 35 indicates the direction of rotation of the grinding wheel 19.

Fig. 3 shows the cam 18 in the starting position. The grinding wheel 19 has been moved up to the point where the cam 18 comes into contact with it, namely that the cam 18 in this basic position is aligned in its rotational position in such a way that the cam 18 and grinding wheel 19 meet one another at a first point 40 in the transition from the flank 32 to the base circle Touch 30.

In a first process step, the cam 18 is now rotated in the direction of the arrow 34, and at the same time the grinding wheel 19 is moved linearly along the axis 23 to the left. With a suitable coordination of the angular speed of the cam 18 and the feed speed of the grinding wheel 19, a locus 41 drawn in FIG. 4 thus arises from the first point 40 on the raw contour to a second point 42 on the target contour, which is within a rotation angle ψ of, for example, 120 ° is achieved. Due to the linear feed of the grinding wheel 19, the locus 41 has the shape of an Archimedean spiral.

In the illustration of FIG. 4, the grinding wheel 19 has thus been moved in a linearly controlled manner from the position 19 ′ shown in broken lines to the position 19 shown in solid lines.

After reaching the second point 42, a transition is now made to a second process step.

In this second step of the method, the deflection of the grinding wheel 19 in the direction of the x-axis 23 is set such that the point on the surface of the grinding wheel 19 that is engaged in each case follows the desired contour 30a / 31a / 32a exactly.

This has among other things the effect that in this process section the material is always removed tangentially with respect to the target contour 30a / 31a / 32a. This is illustrated in FIG. 4 by means of a position 45 of the grinding wheel 19 relative to the cam 18, and one can clearly see that the grinding wheel 19 touches the desired contour in this position 45, in that case the flank 32a at a single point 46.

Finally, FIG. 5 shows a variant in which the sequence of the two process sections described above is repeated cyclically.

The thick curve in Fig. 5 of the respective processing point again starts at the first point 40 and runs in the manner described in the pure delivery mode along the locus 51 to the second point 42, where the method changes to the rail mode, so that the path curve now runs along the desired contour 30a / 31a / 32a, as already described: this railway operation now continues to a third point 40a, which lies radially next to the first point 40. When the third point 40a is reached, the method returns to the pure infeed mode, and the path curve of the respective processing point in turn runs along a locus 41a which runs within the locus 41 already described. The pure infeed operation now continues up to a fourth point 42a, which is located radially next to the second point 42, at which point the method again switches to pure rail operation, so that a further target contour 30b / 31b / 32b is created. This further target contour is continued via a fifth point 40b located radially next to the first point 40 and the third point 40a, so that the further target contour 30b / 31b / 32b is traversed to the fourth point 42a, so that the desired further target contour 30b / 31b / 32b is now completely processed.

In a practical embodiment of the method according to the invention, a four-cylinder camshaft was machined at a peripheral speed of the grinding wheel 19 of 45 m / s, the cams of which had a base circle of 38 mm in diameter and a cam stroke of approximately 10 mm. The radial grinding allowance was between 2 and 2.5 mm.

The cam was rotated a total of four times for pre-grinding, a feed operation and a rail operation being set successively in the manner described. It was followed by a turn in pure rail operations. In the subsequent finish grinding, three revolutions with infeed operation and subsequent rail operation were set, and five revolutions without infeed operation followed.

The ratio of the infeed speed and the angular speed of the cam was chosen so that an angle ψ of 30 ° was set during pre-grinding and an angle ψ of 60 ° during finish-grinding.

In this way, it was possible to reduce the total grinding time per cam to less than half compared to the conventional method.

Claims (3)

1. A method for machining a surface of a cam (18) in a chip removing operation, the cam outer profile comprising a section (30) of a base circle, a section (31) of a secondary circle and two flanges (32) steadily interconnecting said circle sections (30,31), said cam (18) being rotatable about a first stationary axis (11) intersecting the base circle (30) center, a grinding wheel (19) being rotatable about a second axis (24) parallel to said first axis (11) with the distance of said axes (11, 24) being adjustable such that, starting from a raw profile (30/31/32), a finished profile (30a/31a/32a) is generated by removing said surface through displacement of said grinding wheel (19) with a variation of said distance of said axes (11, 24) while said cam (18) is rotated, such that, on the one hand side, during a profiling mode a section of said grinding wheel (19) engaging said surface may be guided along said profile while, on the other hand side, during a feeding mode said grinding wheel section may be fed in by the surface distance between said raw profile (30/31/ 32) and said finished profile (30a/31a/32a), characterized in that said grinding wheel section is first guided solely in said feeding mode from a first point (40) on the base circle (39) surface of said raw profile (30/31/32) to a second point (42) on said base circle (30a) of said finished profile (30a/ 31a/32a) and then, by switching-over between said modes, solely in said profiling mode along said finished profile (30a/31a/32a), with said first point (40) and said second point (41) defining an angle (ψ) of rotation about said first axis (11), said angle being between 20° and 180°.

2. The method of claim 1, characterized in that after guiding of said grinding wheel section along said finished profile (30a/31a/32a), said grinding wheel section is first guided solely in said feeding mode from a third point (40a) on said finished contour (30a/31a/32a) to a fourth point (42a) of a second finished contour (30b/31b/32b) and, then, by switching-over between said modes, is guided solely in said feeding mode along said second finished contour (30b/31b/32b).

3. The method of claim 1 or 2, characterized in that the machining tool during said feeding mode is guided with continuous feeding-in.

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