Method for grinding workpieces, in particular for centering grinding of workpieces such as optical lenses

The invention relates to a method for centering grinding of workpieces, for example optical lenses by a grinding tool using an actuator for generating an advancing movement between the grinding tool and the workpiece, wherein the actuator and a current regulator for an actuator current which determines an advancing force of the actuator are integrated in a position control loop using a predetermined control cycle. For each control cycle: (i) a desired direction of movement (Rsoll(n)) of the advancing movement and an actual direction of movement (Rist(n)) of the advancing movement are ascertained; then (ii) the ascertained actual and desired directions of movement are compared to one another; and (iii) when the comparison results in a deviation between the actual and desired directions of movement, a predetermined current limit (Isollmax) for the actuator current emitted via the current regulator is decreased in a defined manner.

TECHNICAL FIELD

The present invention relates generally to a method of grinding workpieces by a grinding tool with use of an actuator for producing a relative advancing movement between grinding tool and workpiece, wherein the actuator together with a current controller for an actuator current, which determines an advance force of the actuator, is integrated in a position control circuit which is run through with a predetermined control cycle.

In particular, the invention relates to a method for centered grinding of workpieces from the fields of use of high-precision optics (optical glasses), horological industry (timepiece glasses) and semiconductor industry (wafers), where workpieces are initially to be subject to centered clamping by centering machines and subsequently ground at the edge.

PRIOR ART

Lenses for objectives or the like are, after processing of the optical surfaces, “centered” so that the optical axis, the position of which is characterized by a straight line running through the two points of curvature of the optical surfaces, also passes through the geometric centre center of the lens. The lens is for this purpose initially so aligned and clamped between two aligned centering spindles that the two center points of curvature of the lens coincide with the common axis of rotation of the centering spindle. The edge of the lens is subsequently processed in such a defined relationship to the optical axis of the lens as is later necessary for fitting the lens in a frame. In that case the edge is provided with a defined geometry both in plan view of the lens, i.e. circumferential profile of the lens, and as seen in radial section, i.e. profile of the edge, for example rectilinear formation or formation with a step or steps/facet or facets, by machining. This is carried out, in particular in the case of glass lenses, by a grinding process. If in connection with the present invention reference is generally made to “grinding”, this, however, also embraces “finish-grinding” and “polishing”, where processing is similarly by geometrically indeterminate cutting.

Sofar as the mechanisms, which are used in centering, for producing the relative advancing movement between grinding tool and workpiece are concerned, in the case of the older cam-controlled centering machines “LZ 80” of LOH Optikmaschinen AG, Wetzlar, Germany (predecessor in law of Satisloh GmbH), the two grinding spindles for the rotary drive of the grinding tools (grinding wheels) were adjusted with the use of settable weights by way of a cable pull. The maximum adjusting movement of the grinding spindles themselves was in that regard controlled by way of slowly rotating cam discs on which a scanning roller, coupled with the respective grinding spindle, ran as a fixed stop. Although this very simple mechanical solution had advantages with respect to the processing speed possible, because the advance largely set itself in dependence on the capability of the grinding wheels and the ground substrate material itself, there was the serious disadvantage that an individual cam disc had to be provided for every workpiece geometry.

Solutions are also known (see, for example, specification EP-A-1 693 151, which does not, however, relate to a centering machine) in which the grinding force is set by way of the bias of springs acting on the grinding spindle. However, the use of springs for setting the grinding force has disadvantages when grinding of non-circular, in particular polygonal, geometries of rotating workpieces is involved. In particular, at the corners the workpiece “strives” to urge the grinding disc away against the direction of advance, in which case the bias of the springs acting on the grinding spindle increases. This in turn produces an undesired increase in the grinding force, which can have the consequence of a trough, thus a shape fault, arising in the region of the corners of the workpiece pressing on the grinding wheel.

In modern CNC-controlled centering machines, which by way of appropriate track guidance of tool and/or workpiece enable grinding of any workpiece shapes, a constrained advance control is usually provided. However, if in that case the speed of advance is selected to be too rapid, overloading of the grinding tool and, in certain circumstances, also “burning” of the workpiece at the point of contact between tool and workpiece can occur, which can lead to resonances and significant consequential damage to, among other parts, the centering machine particularly when mineral oil is used as cooling lubricant. Programmed safety spacings can indeed provide a remedy here, for example in such a manner that the speed of advance is set to be high up to a predetermined spacing between tool and workpiece and, when this spacing is reached, is switched over to a lower speed of advance. However, such safety mechanisms necessarily occasion longer processing times.

Finally, so-called “adaptive control” solutions are also known (see, for example, specification U.S. Pat. No. 7,413,499) in which the power consumption of the grinding spindle and/or the rotary drive for the workpiece or, however, signals from specifically provided force-pick-ups are used as input variables for limitation of advance. A disadvantage of control of advance in dependence on power consumption of the grinding spindle is that, due to the high cutting speeds required for the grinding, the latter is sluggish as a consequence of the mass inertia of grinding spindle and grinding tool and therefore reacts only with a delay, possibly too late. Conversely, the use of a force sensor has, in particular, the disadvantage that this always has to be arranged between tool and machine or workpiece and machine, which as a consequence of function leads to a degree of pliancy of the machine, which can be detrimental to high workpiece quality and accuracy.

What is desired is to provide a method of grinding workpieces, particularly for centered grinding of workpieces such as optical lenses, which addresses the problems discussed above with respect to the prior art. In particular, in that regard the advancing movement between grinding tool and workpiece shall be such that on the one hand during grinding neither overloading of the grinding tool nor “burning” or faulty shaping of the workpiece occurs or arises and on the other hand the speed of advance and material machining are nevertheless carried out rapidly and efficiently as possible.

SUMMARY OF THE INVENTION

According to invention in a method for grinding workpieces, particularly for centered grinding of workpieces such as optical lenses, by a grinding tool with use of an actuator for producing a relative advancing movement between grinding tool and workpiece, which actuator is integrated together with a current controller for an actuator current, which determines an advance force of the actuator, in a position control circuit which is run through with a predetermined control cycle, for each control circuit initially: (i) a target movement direction of the advancing movement and an actual movement direction of the advancing movement are determined; then (ii) the determined actual movement direction of the advancing movement is compared with the determined target movement direction of the advancing movement; and finally (iii) if the comparison shows a difference between the actual movement direction of the advancing movement and the target movement direction of the advancing movement a predetermined current limit for the actuator current delivered by way of the current controller is reduced in defined manner in order to reduce the advance force of the actuator.

Through this method—in which a variable advance force is preset for the advancing motor (actuator) by way of the motor current, a conclusion about the instantaneous force relationships is made on the basis of the target and actual directions of the advancing movement and as a result thereof the advance force is influenced by way of the motor current in dependence on the process—there is optimization of, in particular, the machining capability during grinding, especially in the centering of non-circular workpieces. By comparison with the prior art the result is significant reductions in processing times, elimination of safety spacings, simple recognition of cutting start and reliable prevention of overload states of tool and workpiece due to excessive speeds of advance or due to collisions. The actual speed of advance is here ultimately determined by way of the machining capability of the tool, which can change during the course of processing due to, for example, blunting or clogging of the abrasive coating or a change in the coolant and lubricant capabilities. Ultimately, external force pick-ups or the like are rendered superfluous through the evaluation of the target and actual directions of the advancing movement and the utilization of the force/current dependence of the advancing motor; pliancies which may be detrimental to workpiece quality and accuracy are thus avoided.

For preference, for ascertaining or determining the movement directions of the advancing movement in the above step (i) the target and actual positions of the actuator are evaluated from the present control cycle and from the preceding control cycle, which can be derived without problems from the position control circuit.

With respect to a good possibility of influencing the behavior of the change in current it is additionally preferred if in the comparison of the determined actual movement direction of the advancing movement and the determined target movement direction of the advancing movement in the above step (ii) a comparison signal is generated which produces a current reduction signal by way of a PI or PID transfer element, wherein in the step (iii) a signal for the predetermined current limit reduced by the respective current reduction signal is then applied to the current controller as current limitation signal.

In order to optimize the grinding method for the processing of non-circular geometries, which can be “polygonal” to a greater or lesser extent, use is preferably made of different parameter sets for the proportional component (amplification KP) and the integral component (reset time TN) of the PI or PID transfer element in dependence on the shape of the workpiece to be ground.

Although any actuators can be used as advancing drive for the grinding method according to the invention, provided these have a defined force/current dependence, it is ultimately preferred, particularly with respect to a high level of sensitivity of the regulation, a rapid reaction behavior, an easy motion and freedom from self-locking, etc., if a linear motor is used as the actuator for producing the relative advancing movement between grinding tool and workpiece.

DETAILED DESCRIPTION OF THE EMBODIMENT

A CNC-controlled centering machine10for grinding workpieces, particularly optical lenses L, is illustrated inFIG. 1merely schematically and only to the extent that appears necessary for an understanding of the present invention. Further details with respect to the construction and functioning of the centering machine10can be inferred from U.S. Publication 2013/0316624 A1, to which an incorporation by reference is hereby expressly made.

InFIG. 1there can be seen on the left two centering spindles12,14, which are arranged in alignment with respect to a centering axis C and the centering spindle shafts16,18of which are rotationally drivable independently of one another and with positional regulation with respect to rotational angle (workpiece axes of rotation C1, C2). Synchronism of the centering spindle shafts16,18is in that case by CNC technology in a manner known per se. The centering spindle shafts16,18are respectively constructed at mutually facing ends for mounting a clamping bell20,22such as known from German Standard DIN 58736-3. The optical lens L is firmly clamped in place between the clamping bells20,22for grinding of its edge. The stroke and clamping devices, which are required for that purpose and which enable a defined movement of or force application to one of the centering spindles12,14along the centering axis C, are not shown inFIG. 1. The centering spindles12,14are fixed, i.e. immovable, in a direction perpendicular to the centering axis C.

Provided at the tool side is a (at least one) tool spindle24with a rotary drive for a tool spindle shaft26at which a grinding wheel G as grinding tool is mounted. The grinding wheel G is thus rotationally drivable with controlled rotational speed in correspondence with the arrow inFIG. 1(tool axis of rotation A) in order to effect, by its circumferential surface U, material removal from the workpiece L.

The tool spindle24is additionally mounted on an X slide28which is linearly movable to the right or left inFIG. 1under CNC positional regulation (linear axis X; advancing movement). For that purpose the X slide28is guided by way of guide carriages (not shown here) at two parallel extending guide rails30,32mounted on a machine bed (not illustrated). Serving for the drive of the X slide28is a linear motor34as actuator, of which inFIG. 1the stator36, which is fixed to the machine bed, with its magnets can be seen. The rotor (coils) of the linear motor34is mounted under the X slide28and cannot be seen inFIG. 1. Arranged above the X slide28inFIG. 1is a linear travel measuring system38by which the axial position (xist) of the X slide28can be detected in a manner known per se.

Finally, additionally indicated inFIG. 1above the linear travel measuring system38or the centering spindle14are, on the right, the advance force FV, which acts in the direction of the centering axis C and which can be exerted by the linear motor34on the X slide28, the magnitude of the force being proportional to the current I applied to the rotor of the linear motor34, and, on the left, the processing force component FP, which opposes the advance force FValong the x direction and which is dependent on the rotational speed and rotational direction of the workpiece L, the rotational speed and rotational direction of the grinding wheel G (same sense/opposite sense), the material and geometry of the workpiece L, the material, geometry and state of wear of the grinding wheel G, the cooling and lubrication (friction) at the point of action between workpiece and grinding wheel G, etc.

FIG. 2illustrates a centered grinding process in general form; an advancing movement V of the grinding wheel G rotating about the tool axis of rotation A is produced in correspondence with the arrow by way of the linear motor3. In that case, the X axis is to be so positionally controlled that the optical lens L, which is rotationally driven about the centering axis C (workpiece axis of rotation C1) and which can at the outset have any external profile AK (octagonal in the illustrated example), is centered with respect to a final profile EK defined by an NC program. In the case of a non-circular final profile EK, such as the slightly elliptical final profile EK shown here, the axis X of advance is additionally co-ordinated in a manner known per se with the workpiece axis C1of rotation, for which purpose the latter is provided with a high-resolution angle measuring system WM (seeFIG. 1). It is evident that the grinding wheel G in the case of non-circular processing of workpieces L cannot be continuously moved in an advancing direction, i.e. only to the left inFIG. 2, but rather—at least at the end of the processing—has to be moved back and forth along the axis X of advance in dependence on the rotational angle of the workpiece L about the centering axis C so as to be able to generate the non-circular final profile EK.

With help of a simplified block circuit diagramFIG. 3shows the position control circuit40for the linear motor34(advancing drive) of the centering machine10according toFIG. 1, with which is associated a special current controlling or limiting circuit, current limitation42for short, for the actuator current I for performance of the grinding method according to the invention. The position control circuit40comprises in a manner known per se—cf., for example, the reference work “Werkzeugmaschinen Band 3, Automatisierung and Steuerungstechnik” by Prof Dr.-Ing. Manfred Weck, 3rd Edition 1989, VDI-Verlag, Dusseldorf, p. 195,FIG. 8-3which is hereby incorporated by reference—a position controller44, a speed controller46, a current controller48and the actuator controlled thereby (the linear motor34in the present case) as well as in the context of positional feedback a summation point50for the target position xsolland the actual position xist. The linear travel measuring system38, which supplies the actual position xistis shown to no greater extent inFIG. 3than the NC control presetting the target position xsoll. In addition, subordinate speed and current feedbacks, which can be provided within the scope of cascade regulation, are not illustrated. The position control circuit40is, as usual, run through with a predetermined control cycle, for example with a cycle time or scanning rate of 2 ms.

Finally, it is also to be mentioned at this point that Isollin the position control circuit40according toFIG. 3denotes the target current preset for the current controller48, which—optionally in accordance with current feedback—is preset in the position control circuit40with the objective of so controlling the linear motor that the position actual value (actual position xist) as control circuit output follows the position target value (target position xsoll) as control circuit input as free of error as possible. However, the actuator current I delivered by way of the current controller48is limited in defined manner and, in particular, with consideration even of larger lag errors, for which purpose the current limitation42to be described in the following is provided.

Serving as input variables for the current limitation42are, as apparent, the target position xsollpredetermined by the NC control for the axis X of advance, the actual position xist, which is detected by the linear travel measuring system38, of the axis X of advance and a maximum target advance force FVsollmax, which is similarly predetermined by the NC control and from which a pre-defined current limit Isollmaxresults, this being explained in more detail later.

The target positions xsoll(n), xsoll(n−1)of the linear motor34are evaluated in the function element52at the top left inFIG. 3from the present control cycle (n) and from the preceding control cycle (n−1) by a signum function (“Sgn”). The abbreviation “d/dt” (derivation over time) in this connection stands for the following relationship:
d/dt=(xsoll(n)−xsoll(n−1)/(t(n)−t(n−1)

Since the scanning rate is constant, this can be simplified by (t(n)−(t(n−1))=const. to:
d/dt=(xsoll(n)−xsoll(n−1))

The result of the formed signum function is the target movement direction Rsoll(n)of the advancing movement V in the present control cycle (n). In that regard, the following three cases are possible:

In analogous manner the detected actual positions xist(n), xist(n−1)of the linear motor34are evaluated in the function element54at the top right inFIG. 3from the present control cycle (n) and from the preceding control cycle (n−1) by a signum function. In that case:
d/dt=(Xist(n)−xist(n−1))/(t(n)−t(n−1)

This expression is turn simplified by (t(n)−tn−1)=const. to:
d/dt=(xist(n)−xist(n−1))

Accordingly, the following three cases are possible for the actual movement direction Rist(n)of the advancing movement in the present control cycle (n):

In other words, in the first case (1) there is tendency to forward movement of the grinding disc G with respect to the centering axis C, in the second case (2) the spacing of the grinding disc G from the centering axis C does not change, i.e. the grinding disc G is stationary (no movement), and in the third case (3) there is tendency to rearward movement of the grinding disc G with respect to the centering axis C.

The thus-determined directional values (1, 0 or −1) for the target movement direction Rsolland the actual movement direction Ristof the advancing movement V are then respectively applied to a proportionally acting transfer element (P element)56or58, which issues the respective signal with a settable amplification. This amplification can be varied in order to weight the influence of the respective signal.

The signals amplified in that manner for the target movement direction Rsolland the actual movement direction Ristof the advancing movement V are thereafter applied to a summation point60, which carries out comparison of the determined actual movement direction Ristof the advancing movement V with the determined target movement direction Rsollof the advancing movement V by a difference formation (target value minus actual value). If in that case the determined target and actual movement directions Rsolland Rist, respectively, of the advancing movement V correspond—
Rsoll(n)=+1=Rist(n)(a)
or
Rsoll(n)=−1=Rist(n)(b)
i.e. (a) the grinding wheel G shall have a tendency to forward movement with respect to the centering axis C and actually also moves forwardly or (b) the grinding wheel G shall have a tendency to rearward movement with respect to the centering axis C and in fact also moves rearwardly, then the output of the summation point60is equal to zero. The same also applies to the boundary case of the intentionally stationary axis X of advance—
Rsoll(n)=0=Rist(n)(c)
i.e. if (c) no advancing movement V of the grinding wheel G is to take place and in addition is not present. The grinding process in these cases runs as desired; the grinding wheel G is sharp.

The possible difference cases in the afore-described comparison at the summation point60comprise, in particular, the states:
Rsoll(n)=+1≠Rist(n)=0  (d)
and
Rsoll(n)=+1≠Rist(n)=−1  (e)

In the first-mentioned difference case (d) the grinding wheel G is to move in the direction of the centering axis C (advancing movement V inFIG. 2), but does not do this (blocking of the axis X of advance). Accordingly, at this time instant the processing force component FPopposing the advance force FVis at least equal to the advance force FV(cf.FIG. 1), in which case the grinding wheel G is prevented from further advancing movement V thereof. The cause of that can be, for example, a blunted or worn grinding wheel G or an insufficient cooling lubricant feed.

The second-mentioned difference case (e) can arise when grinding of a non-circular geometry of the workpiece L is carried out if the processing force component FPexceeds the advance force FV, since—due to change of the point of action in dependence on angle—variations in amount and effective direction of the grinding force arise, in which case the workpiece L urges away the grinding wheel G against the advancing direction as a consequence of the non-circular external profile AK of the workpiece L. This is illustrated inFIG. 4: the rotating workpiece L pushes, by its radius—which changes over the circumference—with respect to the centering axis C or by its profile sections “jutting out” in radial direction, the grinding wheel G to the right against the advance direction inFIG. 4by an amount Δx.

In the described difference cases there is a risk of over-stressing or overloading of workpiece L and/or tool G, which can lead to “burning” at the point of action and in the case of non-circular processing additionally the risk of “digging in” of the grinding wheel G into the workpiece L and thus of errors in shape at the workpiece L. In order, in these cases, to facilitate yielding of the axis X of advance and also to eliminate the associated initial breakaway torque of the linear guides30,32the force limit of the axis X of advance is dynamically reduced by way of the actuator current I.

More precisely, in the comparison of the determined actual movement direction Rist(n)of the advancing movement V with the determined target movement direction Rsoll(n)of the advancing movement V there is generated at the summation point60a comparison signal which produces a current reduction signal Ired(n)by way of a transfer element62with proportional-integral action (PI element). Alternatively, use can also be made here of a fast PID element with, for example, a differential or derivative action time TVof zero or almost zero, which acts similarly to a PI controller.

The current reduction signal Ired(n)is applied as a subtrahend to a further summation point64. The predetermined current limit forms the minuend at the summation point64, i.e. a signal for a maximum target current Isollmax, which arises via a further proportionally acting transfer element66(P element) from the maximum target advance force FVsollmaxwhich has already been mentioned above and which is preset by the NC control. In this preset for the maximum target advance force FVsollmax(for example 100 N) on the one hand there is consideration of the advance force which is desired for the actual grinding process and which can be input by the user; on the one hand, the force fluctuations of the adjusting axis X due to the influence of cogging torques of the linear motor34as well as force losses due to friction in the linear guides30,32and at the covers (not shown) of the work area are taken into consideration, which are determined on a single occasion in exemplifying form and included as an additive value in the target advance force FVsollmax.

The summation point64ultimately issues a current limitation signal Imax(n)(maximum target current Isollmaxminus the respective current reduction Ired(n)), which is applied to the current controller48. As a result, the actuator current I, which determines the advance force FVof the linear motor34, delivered by the current controller48to the linear motor34is dynamically limited to the current Imax(n), i.e. notwithstanding a possibly present higher current preset Isoll(n)) in the position control circuit40the current controller48delivers merely the limited current Imax(n)to the linear motor34. In the above movement direction difference cases (d) and (e) this leads to a reduction in the advance force FV(n)of the linear motor34(illustrated by force arrows of different length for the advance force FVinFIG. 4at the top right and bottom right). Thereagainst, in the above cases (a) to (c), in which still no difference of the actual and target movement directions of the advancing movement V is present, the predetermined current limit, i.e. the maximum target current Isollmax, is not reduced, since the summation point60outputs zero and consequently also the current reduction signal Ired(n)is zero.

If a movement direction difference according to the cases (d) and (e) is present over several control cycles n then the current reduction signal Ired(n)is correspondingly increased by way of the PI element62; after the summation point64the permitted current Imax(n)is consequently ever smaller from control cycle to control cycle. The control behavior of the PI element62—such as fast, “hard” or “soft”—can, as known, in that case be influenced by way of the parameters for the proportional component (amplification KP) and the integral component (reset time TN) and also optimized with respect to the processed material. Advantageously, different parameter sets for the amplification KPand the reset time TNare used from grinding process to grinding process in dependence on the circularity or the polygonality of the workpiece geometry to be ground, but then continuously for the respective grinding process. Thus, for a polygonal, for example square, external profile AK the amplification KPis preselected to be quite high, but the reset time TNrather small, and for a round or cornerless, for example elliptical, external profile AK the amplification KPis preselected to be rather lower and consequently the reset time PNto have a tendency to higher. The actual values for the controller parameterization are to be individually optimized for the respective centering machine10and respective grinding process, so that a quantification shall not take place here. If, ultimately, in the comparison of the actual and target movement directions there is no longer a difference at the summation point60the actuator current I is increased by way of the current controller48back to at most the preset current limit Isollmax, whereby the advance force FVof the linear motor34correspondingly increases again.

FIG. 5shows—recorded over time t in a diagram by way of example for a centered grinding process with the afore-described selectably switchable-on or switchable-off actuator current limitation or force limitation at the linear motor34—at the top the advance travel x (solid or dotted line) of the X slide28, consequently of the workpiece spindle24together with the grinding wheel G, and below that the lag error (dot-dashed line) building up due to the limitation of the actuator current I. The X slide28starts off at the point a at a preselected speed of advance, which does not have to be coupled to the machining capability of the tool and with respect to the most rapid and efficient material machining possible is preferably selected to be higher than possible by material removal by grinding. At the point b the grinding wheel G impinges on the workpiece L. Whereas the actual position xistfollows the target position xsollsubstantially free of error up to the point b, the actual position xist(solid line) and target position xsoll(dotted line) thereafter “come apart”; a lag error (dot-dashed line at the bottom) arises. In that case, a brief blockage of the advancing movement V is to be expected at the point b (not visible in the graph), which as described above induces a reduction of the advance force FVby way of the current limitation42, so that overloading of workpiece L or tool G does not take place. As a consequence, the position control circuit40“strives” to compensate for the lag error, but notwithstanding an appropriate current preset Isollat the current controller48the supply of current to the linear motor34is limited by the current limitation42(Imax). Only from the point c, when the end value of the target position xsollis reached, does the lag error diminish until the actual position xisthas also reached its end value at the point d. In other words, between the points b and d the actual positions xistof the grinding wheel G and the speed of the advancing movement V (gradient of the graph) come about merely as a consequence of the advance force FVpermitted by way of the current limitation42. The advance force is of such magnitude between the points b and d as a consequence of the current limitation42that a more lengthy deviation between the actual movement direction Ristand the target movement direction Rsollof the advancing movement V does not arise, thus will always be a maximum amount within the scope permitted. The described power grinding process can be concluded when at d a settable limit value for the lag error (for example 0.01 mm) is fallen below during a complete revolution of the workpiece L.

Whereas (inter alia) at the point b inFIG. 5the difference case (d) described further above is expected to be present (blocking of the axis X of advance), the detail DV, which is of substantially increased scale in x direction and t direction, inFIG. 5illustrates the situation in the difference case (e), which was explained above with reference toFIG. 4, when the rotating workpiece L pushes away the grinding wheel G against the direction of advance by an amount Δx. In that case, the point e in the detail DVcorresponds with the state inFIG. 4at the top, whereas the point f in the detail DVrepresents the state inFIG. 4at the bottom. Accordingly, increases in the lag error which are repetitive in sawtooth-like manner (not repeatedly illustrated) arise.

When the current limitation42is activated the amount of the preselected speed of advance is basically equal, because the target actuator current Isolldelivered by the speed controller46may in any case be limited (Imax) in the current controller48during the processing. Thus, processing is also possible with different preselected speeds of advance, for example with a rapid movement towards fast approach of tool G and workpiece L and a working cycle, which is slower by comparison therewith, during the machining. The switchover point between fast motion and working cycle can in that case be found simply and reliably by continuous evaluation of the lag error of the axis X of advance (recognition of initial cut), because at the instant of contact between tool G and workpiece L the lag error of the axis X of advance increases rapidly and strongly due to the absence of force reserve or limited advance force FVof the linear motor34(cf. the lag error rapidly building up after the point b inFIG. 5). A safety spacing from the workpiece L, which is usual in the prior art and which is accompanied by a substantial loss of time due to “grinding in mid-air in the working cycle”, is not necessary, since as a consequence of the force reduction of the linear motor34a critical overloading or destruction of the tool G and/or workpiece L cannot occur.

A method particularly for centered grinding of workpieces such as optical lenses by a grinding tool with use of an actuator for producing a relative advancing movement between grinding tool and workpiece is disclosed, wherein the actuator together with a current controller for an actuator current, which determines an advance force of the actuator, is integrated in a position control circuit, which is run through at a predetermined control cycle. In the case of the method, for each control cycle: (i) a target movement direction of the advancing movement as well as an actual movement direction of the advancing movement are determined; then (ii) the determined actual and target movement directions are compared with one another; and finally (iii) if the comparison shows a difference between the actual and target movement directions a predetermined current limit for the actuator current delivered by way of the current controller is reduced in defined manner in order to reduce the advance force of the actuator. As a result, the advancing movement and material machining are carried out quickly and efficiently without overloading of tool or workpiece being able to occur.

Variations and modifications are possible without departing from the scope and spirit of the present invention as defined by the appended claims.