EP0346425B1 - Process for dressing a grinding wheel - Google Patents

Abstract

In a process for dressing a grinding wheel (12), a dressing roller (40) is moved along a rough surface (30, 31) of the grinding wheel (12) by a numerically-controlled multicoordinate moving unit (46, 47), thereby producing a predetermined circumferential profile on the grinding wheel (12). In order to simplify the grinding process and in particular to optimize with respect to time a high-speed rough grinding process which produces large quantities of chips in a given time, a circumferential profile is first determined in function of parameters of a grinding process to be carried out on a workpiece (20) by means of the grinding wheel (12) after dressing, and is then produced on the grinding wheel (12) by the dressing roller (40).

Description

The invention relates to a method for grinding a workpiece by generating the circumferential profile of a grinding wheel, which has a main and a secondary cutting surface and optionally a free surface, in which a dressing roller uses a numerically controlled multi-coordinate movement unit along a raw surface of the grinding wheel to produce a predetermined circumferential profile Grinding wheel is moved, as well as by subsequent high-speed peeling grinding of the workpiece with a high chip removal rate, in which peeling grinding the main cutting surface of the grinding wheel with a high axial feed speed creates a helicoidal transition between the raw dimension and the finished dimension, and the secondary cutting surface rests on the finished dimension of the workpiece.

It is known to provide grinding wheels on their machining peripheral surfaces with a predetermined peripheral profile in order to be able to grind predetermined peripheral profiles of workpieces in one operation. For example, when plunge grinding, an annular groove of a given width and with a given fillet in the transition from the cylindrical base surface to the radial side surfaces can be ground in a single operation by providing the outer surface of the grinding wheel with a peripheral profile that precisely matches the dimensions of the contour of the Ring groove corresponds. The same applies to various external cylindrical grinding processes, in which, for example, bushes, flanges and the like are ground with almost any rotationally symmetrical contours in one setting by providing the grinding wheel with a corresponding negative peripheral profile beforehand.

General methods of the type mentioned above are known from the "Handbook of Manufacturing Technology" by G. Spur and Th. Stöferle, Volume 3/2, Carl Hanser Verlag, Munich, 1980, pages 151 to 157.

In order to generate the desired circumferential profile on the grinding wheel, the numerically controlled multi-coordinate movement unit of the dressing roller is given a data record which corresponds to the spatial coordinates of the workpiece profile to be ground. Depending on the data set, the dressing roller is then guided over the circumferential surface of the grinding wheel in such a way that the desired circumferential profile is created. In this way it is possible to prepare grinding wheels for a specific machining task or to dress them between successive machining operations of the same type.

On the other hand, it is known to include process parameters in the control in the numerical control of a grinding machine, ie when setting the movement sequence between the workpiece and the grinding wheel, in order to optimize the grinding process in this way. It is known, for example, to enter data sets into a numerical control of the grinding machine which reflect the geometry of the workpiece, its material properties, the desired surface quality of the workpiece and the material properties of the grinding wheel itself. The numerical control of the grinding machine uses this to calculate the rigidity and thus the flexibility of the workpiece . Known controls also have the property of taking into account the rigidity of the grinding machine itself and in this way determining overall what pressure may be exerted on the workpiece in order to achieve the specified parameters, in particular dimensional accuracy and surface quality.

From GB-A-986 427 a method for grinding a workpiece is known, in which a grinding wheel with a conical grinding area is used. The length of the conical grinding area is about half the width of the grinding wheel. The length mentioned is determined by the cone angle and the total depth of cut as well as by the depth of the chips and the feed per revolution of the grinding wheel. Since the total width of the grinding wheel is constant during a process, the length of the secondary cutting surface is dimensioned opposite to the cone length depending on the parameters mentioned.

The invention is based on the object of developing a method of the type mentioned in such a way that specifications of a grinding process can already be taken into account when dressing the grinding wheel and not only during grinding, so that as a result the overall control of the grinding machine, in particular the sequence of movements during the grinding process, remains unaffected.

This object is achieved according to the invention in that for the purpose of specifying the circumferential profile as a function of the execution conditions of the subsequent high-speed peeling process to be carried out on the workpiece by means of the grinding wheel, namely a roughness depth value specified as a measure of the surface quality to be achieved and on the process parameters determining the grinding conditions. in particular the degree of coverage of the secondary cutting surface when the workpiece rotates, the ratio of the peripheral speeds of the grinding wheel and the workpiece, the diameter of the workpiece, the axial feed speed of the workpiece relative to the grinding wheel and the peripheral speed of the grinding wheel, the axial length of the secondary cutting surface is dimensioned.

The object on which the invention is based is completely achieved in this way, because for the first time a dressing strategy is used in which not only the workpiece geometry but also process parameters, i.e. Path speeds, feed speeds, material properties, surface qualities and the like. In this way, a completely new dimension of the control of grinding processes is made available because, as has already been mentioned, it has so far been limited to seeing the dressing process on the grinding wheel exclusively in the light of the workpiece geometry and a process parameter-dependent control only for the movement sequence the grinding machine itself, but not for the movement of the dressing roller.

Practical tests have shown that with the novel dressing strategy according to the invention, even before the actual grinding process begins, i.e. Before the workpiece machining begins, the necessary influencing variables can be taken into account, so that the subsequent numerical control of the machine tool can be made correspondingly simpler. This has an effect above all because the movement units for a dressing roller are much easier to manufacture and handle than the movement units of the grinding wheel spindle and the workpiece spindle due to the smaller masses to be moved.

In the invention, a method for dressing a grinding wheel for a subsequent high-speed peeling grinding process (external cylindrical grinding with a high rate of chip removal) is used, in which a main cutting surface of the grinding wheel with a high axial feed speed produces a helicoidal transition between the raw size and the finished size and an auxiliary cutting surface rests on the finished dimension of the workpiece. According to the invention, the axial position of the secondary cutting surface is set in this case as a function of the process parameters.

This has the advantage that the shape of the grinding wheel can be optimized for this application in such a way that a grinding process is set for a predetermined surface quality, which produces the desired surface quality with a minimum machining time.

dimensioniert.In a preferred embodiment of the method according to the invention, the axial length of the secondary cutting surface is based on the relationship:

dimensioned.

The method according to the invention can be used with different types of grinding wheels, in which either the grinding wheel axis is inclined to the axis of the workpiece or parallel to it.

In a preferred embodiment of the invention, the peripheral profile of the grinding wheel is generated in such a way that the main cutting surface encloses an angle of 90 ° with the axis of the workpiece.

This measure has the advantage that essentially axial forces are exerted on the workpiece, so that both the heat development and the deflection of the workpiece due to radial loading are minimized.

The method according to the invention can, however, also advantageously be used in circumstances in which the main cutting surface is produced in such a way that it lies against the helicoidal transition along a direction inclined to a radial direction.

Although greater radial forces arise in this variant of the method according to the invention, the inclination angle of the main cutting surface provides another freely selectable parameter in order to optimize the grinding process.

Fig. 1

eine äußerst schematisierte Gesamtansicht einer Schleifmaschine zur Erläuterung des erfindungsgemäßen Verfahrens;

Fig. 2

in stark vergrößertem Maßstab den Eingriffsbereich einer Schleifscheibe beim HochgeschwindigkeitsSchälschleifen zur Erläuterung einer Variante des erfindungsgemäßen Verfahrens;

Fig. 3

eine Darstellung, ähnlich Fig. 2, jedoch für eine Abwandlung des erfindungsgemäßen Verfahrens;

Fig. 4

eine weitere Darstellung ähnlich Fig. 2, jedoch für eine noch andere Variante des erfindungsgemäßen Verfahrens.

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

Fig. 1

an extremely schematic overall view of a grinding machine to explain the method according to the invention;

Fig. 2

on a greatly enlarged scale the engagement area of a grinding wheel in high-speed peeling grinding to explain a variant of the method according to the invention;

Fig. 3

a representation, similar to Figure 2, but for a modification of the inventive method.

Fig. 4

another representation similar to FIG. 2, but for yet another variant of the method according to the invention.

In FIG. 1, 10 designates a grinding machine of the usual type, in which a grinding spindle 11 rotates a grinding wheel 12 about a grinding wheel axis 13 at a speed n _s . Taking into account a grinding wheel diameter d _s, this results in a path speed v _s on the outer circumference of the grinding wheel 12.

In the exemplary embodiment shown in FIG. 1, the grinding wheel axis 13 is inclined at an angle 14 to a workpiece axis 15, as is known per se.

A workpiece 20 is clamped between a conventional chuck 21 of a workpiece spindle and a center point 22.

The workpiece 20 can be displaced along its axis by means of a drive (not shown in FIG. 1) at an axial feed speed v _z . It goes without saying that, alternatively, the workpiece 20 can also stand still and the grinding wheel 12 can be displaced in the direction of the workpiece axis 15 if this is more favorable in individual cases.

The workpiece 20 is rotated by the chuck 21 at a speed n _w , which leads to a path speed v _w at the periphery of the workpiece 20 with a workpiece diameter d _w .

M in FIG. 1 denotes the material properties of the workpiece 20, in particular its modulus of elasticity, Toughness etc., while O is intended to symbolize a desired surface quality on the machined workpiece 20.

The grinding wheel 12 is initially provided with raw, conical surfaces 30 and 31, of which surface lines each run parallel or perpendicular to the surface of the workpiece 20 to be machined.

A dressing roller 40 is provided in order to provide the raw surfaces 30, 31 with a desired circumferential profile. The dressing roller 40 is preferably of a double-conical shape with two conical surfaces 41, 42 which intersect at an acute angle along a circumferential line 43. In the area of the circumferential line 43, the dressing roller 40 is set with diamonds 44. A shaft 45 of the dressing roller 40 is connected to a drive connection 46, which leads to a three-coordinate drive 47.

The drive 47 is designed so that it can not only set the speed n of the dressing roller 40 but also its position in space according to three spatial coordinates.

For example, the drive 47 can move the dressing roller 40 vertically via the connection 46 in FIG. 1, as indicated by an arrow 48. In this case, the dressing roller 40 can dress the raw surface 30 of the grinding wheel 12.

The drive 47 can also rotate the dressing roller 40 via the drive connection 46, for example, by 90 ° and move in a direction parallel to the workpiece axis 15, as indicated by an arrow 49 in FIG. 1. With such a The second raw surface 31 of the grinding wheel 12 can then be machined.

It goes without saying, however, that in addition to these simple linear movement processes of the dressing roller 40, any inclined or curved movement paths can also be carried out, as is known per se from multi-coordinate drives.

To control the drive 47, a control device 55 is used which has a large number of inputs 56 and an output 57 which is connected to the drive 47.

Signals are present at the inputs 56, which were detected as state parameters, for example by manual digital preselection or also by sensors.

From the input signals, the control device 55 determines a circumferential profile for the grinding wheel 12, which is then subsequently generated on the grinding wheel 12 by actuating the drive 47 by means of the dressing roller 40.

Fig. 2 shows a first application example of such a dressing process.

The workpiece 20 has already been partially machined in the illustration in FIG. 2, and in the left half of FIG. 2 a raw dimension of the still unprocessed workpiece 20 can be seen at 60, while a finished dimension is symbolized in the right half of FIG. 2 at 61 .

In this variant, the grinding wheel 12 is provided with three conical surfaces 62, 63, 64. In this case, the first conical surface 62 serves as the main cutting edge, the second conical surface 63 as the secondary cutting edge and the third conical surface 64 as the non-engaging rear surface of the grinding wheel 12. The action of the main cutting surface 62 creates a helicoidal transition 66 between the raw dimensions 60 and Finished dimension 61. Depending on how the speed n _w and the feed speed v _{z are} set, there is an overlap u, that is to say the slope of the helical helical transition 66 generated by the main cutting surface 62.

The secondary cutting surface 63 lies on the finished dimension 61 of the workpiece 20 over an axial length l _N.

In the embodiment of FIG. 2, the main cutting edge angle is set at 90 °, so that the main cutting surface 62 bears against the helicoidal transition 66 along a radial line. The third conical surface 64, on the other hand, includes a clearance angle κ 'with the finished dimension 61 of the workpiece 20.

The dressing strategy explained above now proceeds such that it preferably determines the length l _{N of} the secondary cutting surface 63 from the process parameters mentioned and, if appropriate, additionally the angles κ and κ ', so that the dressing roller 40 then the resulting circumferential profile on the grinding wheel 12 can generate.

The machining process shown schematically in FIG. 2 can preferably run with the following parameters:
A diamond form roller with the designation SG71P-200-1.7 is used as the dressing tool. The roll has a diameter D _R of 200 mm. The grain size d _{K of} the diamonds is 200 µm. The grain distribution is statistically scattered. The packing density is 50 Δ / cm³ in the positive process. The bond is galvanic and the covering thickness is 0.85 x2, the covering height being 10 mm.

A CBN grinding wheel with the type designation B 151MSS387V420 is used as the grinding tool. The grinding wheel diameter D _s is 600 mm. The grain size d _{K of} the CBN grains is 151 µm. There is a metallic bond MSS with a concentration of 42% vol. The base of the grinding wheel is made of steel and the edge radius r is 2 mm.

For the grinding process, a grinding wheel peripheral speed v _s of 140 m / s, a workpiece diameter d _w of 30 mm, an oversize a of 1 mm, a machining length L of 120 mm, a surface quality R _a of 0.8 µm and the material of the The workpiece is normally machinable with a characteristic value q of 70.

The characteristic value q denotes the speed ratio of the grinding wheel and workpiece. This characteristic value is determined according to the machinability of the workpiece material. The machinability of metallic materials is significantly influenced by their chemical composition and structure. The microstructure in turn depends on the production of the material (casting, hot or cold forming) and on the post-treatments used (hardening, annealing). The materials with a high alloy content, the additional carbides contain, due to different deformability different chip types, which determine the behavior of machinability. Short-chipping or long-chipping materials can be meaningfully assigned according to the speed ratio if you consider machinability based on hardness and toughness. If you compare the machinability of the different materials according to this criterion, the greater the speed ratio, the greater the carbide content (cementite) and the more or less homogeneous fine structure (martensite).

In practice, the following ranges can be defined for the speed ratio q:
56 <q ≦ 70 for materials that are difficult to machine
70 <q ≦ 140 for normally machinable materials
140 <q ≦ 280 for easily machinable materials.

Bei diesem Ausdruck ist q, wie erwähnt, das Verhältnis der Umfangsgeschwindigkeiten von Schleifscheibe und Werkstück. Für die nachfolgende Beispielsrechnung sei q = 70 angenommen.In the present example, the geometry of the grinding wheel is defined by the dressing process according to the invention in that a length l _{N of} the secondary cutting edge is set. In the present example, this is done according to the relationship:

In this expression, q, as mentioned, is the ratio of the peripheral speeds of the grinding wheel and the workpiece. For the example calculation below, assume q = 70.

The size d _w is the workpiece diameter and for the following example a value d _w = 30 mm is assumed.

The size v _s is the grinding wheel peripheral speed. For the following example, a value of v _s = 140 m / s is assumed.

In dieser Beziehung ist r der Kantenradius des Schleifwerkzeuges, und für das vorliegende Beispiel sei von einem Wert r = 2 mm ausgegangen.U in the above-mentioned relationship denotes the so-called degree of overlap which arises because the minor cutting edge of length l _N covers areas during rotation of the workpiece and axial infeed which, in the previous rotation of the workpiece, also partially covers the grinding wheel had been. The relationship applies to the degree of coverage U:

In this regard, r is the edge radius of the grinding tool, and for the present example a value of r = 2 mm is assumed.

R _t denotes the desired roughness depth of the ground workpiece and for the following example a value of R _t = 6.4 µm is assumed.

The value k is a constant with dimensions, which is assumed for the present calculation example with k = 1 mm / revolution.

A degree of coverage U = 3.125 is then calculated from the variables mentioned above.

gilt.In the relationship given above for the length l _{N of} the minor cutting edge, v _{fa is} the axial infeed speed for which the relationship:

applies.

In this regard, n _{w is} the workpiece speed. If the grinding wheel peripheral speed v _{s is} assumed to be 140 m / s and the peripheral speed ratio q has a value of 70, this results in a workpiece peripheral speed v _w of 2 m / s. With a workpiece diameter d _w of 30 mm, the workpiece speed n _w = 1273.88 revolutions / minute is then calculated according to the known relationships.

With the value given above and the constant k = 1 mm / revolution already explained, the axial infeed speed is then calculated to be v _fa = 636.94 mm / min.

If the calculated values of the degree of coverage U and the axial infeed speed v _{fa are used} in the relationship given above for the length l _{N of} the secondary cutting edge, a value ultimately results:
l _N = 1.562 mm.

In practice, the clearance angle κ 'is set as small as possible, for example with an angle κ' = 1 °. The reason for setting the clearance angle κ 'so small is as follows:
The grinding wheel is not only delivered once in the process before it is even used, it must also be used later during the Use, as is generally customary, to be dressed so that the grinding properties of the grinding wheel are retained or restored even after prolonged use.

If, however, the grinding wheel in the area of the minor cutting edge is dressed in a direction perpendicular to it, the length l _{N of} the minor cutting edge increases because the further section adjoins the lifting cutting edge at the rear under the clearance angle κ '. In order to ensure that the length l _{N of} the minor cutting edge remains constant, the main cutting edge must also be dressed in order to shorten the axial length l _{N of} the minor cutting edge again. It can now be shown that with the same large infeed of the dressing tool on the main cutting edge and on the secondary cutting edge, as is desired anyway for technological reasons, the length l _{N of} the secondary cutting edge remains unchanged when the clearance angle κ 'is set as small as possible , for example in the range 1 ° to 3 °.

The sizes l _N = 1.562 mm and κ '= 1 ° are therefore specified for the control of the grinding tool in the example mentioned.

When dressing, the grinding wheel _{peripheral speed} v _{sd is} expediently about 30 m / s. The ratio of the peripheral speeds of the grinding wheel and the dressing roller q _D is, for example, set to the value 0.8. These values result in a surface structure of the grinding wheel which corresponds precisely to the desired effective roughness depth R _t of 6.4 µm.

Taking into account a grinding wheel diameter D _s of 600 mm, the grinding wheel peripheral speed v _sd and the peripheral speed ratio q _{D are used to} calculate a grinding wheel speed n _s of 955.42 min -1. The peripheral speed of the dressing roller v _{R is} calculated in a corresponding manner at 24 m / s. The speed of the dressing roller is then calculated with a roller diameter D _R of 200 mm to 2293 min⁻¹.

In practice, the values for the axial and radial infeed result from the grain size d _K , which in the present case can be 0.2 mm, multiplied by a constant factor. The axial infeed s _D then results with a factor of 0.4 to 0.08 and the radial infeed a _D in a corresponding manner to 0.005.

The infeed speed of the dressing roller v _fd is then calculated from the axial infeed s _d per grinding wheel revolution and the grinding wheel speed n _s to 76.43 mm / min.

With the values calculated above for an example, the grinding machine with the control device 55 controls the dressing roller 40, for example, from a position in FIG. 1 to the right of the grinding wheel 12 to a point which is an extension of the third conical surface 64 of the grinding wheel 12. In this position, the grinding wheel 12 and the dressing roller 40 are rotated and at the same time a coolant is supplied. From the first point in the extension of the third conical surface 64, the dressing roller 40 is now guided in an XZ control along the third conical surface 64 until a point at the transition from the third conical surface 64 to the second conical surface 63, ie the minor cutting edge, is reached. The grinding wheel is now guided along the minor cutting edge over the length l _N , specifically in the Z direction, as indicated by the arrow 49 in FIG. 1. After passing through the length l _N , the control is switched over again, so that the dressing roller 40 is moved along the main cutting edge 30 in the X direction, ie in the direction of the arrow 48 in FIG. 1.

The control unit 55 now moves the dressing roller 40 back to the starting point in rapid traverse.

The grinding wheel 12 conditioned in this way is now used for high-speed peripheral grinding, a value of 981.05 mm 3 / mm s resulting from the known relationships for the related chip removal volume Q ' _w , the peripheral speed v _{w of} the workpiece being 2 m / s and therefore the speed n _{w of} the workpiece at 1273.88 U / min. lies. This results in the infeed per pass a _{e of} 0.5 mm, from which it follows that a total of two passes are required for the predetermined measurement of 1 mm.

Due to the length l _{N of} the secondary cutting edge of 1.562 mm, the desired surface quality is then obtained with the process parameters mentioned.

In the variant of the embodiment shown in FIG. 2 shown in FIG. 3, there is a difference in that the main cutting edge angle κ is no longer 90 ° but, for example, 45 °, so that the helical transition 66 'is formed as a helically conical conical surface. In this case by freely choosing the main cutting edge angle α, a third parameter besides l _N and α 'in order to generate optimal process conditions.

Finally, FIG. 4 shows yet another variant, which differs from those described so far in that the grinding wheel axis runs parallel to the workpiece axis. As a result, the secondary cutting surface 63 ″ is a circular cylindrical peripheral surface. The main cutting surface 62 ″ can, as shown in FIG. 4, be a conical surface, so that a main cutting angle α arises which is less than 90 °. Of course, the main cutting surface in this embodiment can be radial, i.e. be flat, surface, so that a main cutting edge angle α of 90 ° would also arise here.

It goes without saying that the above-mentioned exemplary embodiments represent only a small selection from the possible alternative solutions which can be implemented within the scope of the invention specified in the claims. It also goes without saying that the exemplary embodiments described relate to the case of machining metallic workpieces, in particular shafts, journals and the like. However, suitable adjustment of the process variables can also be used to adapt them to other types of workpieces.

Finally, the method according to the invention is in principle independent of the type of grinding wheel used. However, in the desired high-speed peeling grinding process, in which the difference between raw size 60 and finished size 61 can be on the order of a few millimeters, a grinding wheel 12 is preferred which has CBN crystals (CBN = cubic boron nitride) at least on its circumference is.

Claims (3)

1. A method of grinding a workpiece (20) by generating a circumferential profile of a grinding wheel (12), the circumferential profile comprising a primary cutting surface (62), a secondary cutting surface (63), and, as the case may be, a free surface (64), wherein a dressing roll (40) is guided along a rough surface (30, 31) of the grinding wheel (12) by means of a numerically controlled multicoordinate displacement unit (46, 47) for generating a predetermined circumferential profile of the grinding wheel (12), and by subsequently high-speed circumferentially grinding the workpiece (20) under high time-chipping-volume conditions, the primary cutting surface (62) of the grinding wheel (12) generating a helicoidal transition (66) between a rough dimension surface (60) and a finished dimension surface (61) under high axial infeed speed (v_z) conditions, the secondary cutting surface (63) contacting the finished dimension surface (61) of the workpiece (20), characterized in that for predetermining the circumferential profile as a function of the execution conditions of the subsequent high-speed circumferential grinding process to be executed on the workpiece (20) by means of the grinding wheel (12), the execution conditions being the roughness depth value (R_z) corresponding to the achieved surface quality, as well as of process parameters determining the grinding conditions, in particular the overlapping ratio (U) of the secondary cutting surface (63) during rotation of the workpiece (20), the ratio (q) of the grinding wheel (12) circumferential speed and the workpiece (20) circumferential speed, the workpiece (20) diameter (d_w), the axial infeed speed (v_fa) of the workpiece (20) relative to the grinding wheel (12), and the circumferential speed (v_s) of the grinding wheel (12), the axial length (l_N) of the secondary cutting surface (63) is determined.
2. The method of claim 1, characterized in that the axial length (l_N) of the secondary cutting surface (63) is dimensioned according to the formula:

   
3. The method of claim 1 or 2, characterized in that the circumferential profile (62 - 64) of the grinding wheel (12) is generated such that the primary cutting surface (62) together with the axis (15) of the workpiece (20) encloses an angle (κ) of 90°.

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