EP0685298A1 - Procedure of and device for fabricating aspheric lens surfaces - Google Patents

Abstract

The workpiece in its holder (H) is ground and/or polished by a tool (T) guided along a machining contour predetermined by the control unit. One rectilinear drive (I) is pivoted w.r.t. another (II) about a transverse axis (B), pref. by interpolative control of a third drive (III) to guide the tool from the edge to the centre of the workpiece and along a meridional line. An additional drive permits rotation of the holder about a central axis. The path of the tool is corrected by comparison of measurements at sample points with the predetermined contour. <IMAGE>

Description

The invention relates to a method and a device for producing aspherical lens surfaces according to the preambles of claim 1 and claim 9, respectively.

First, reference is made to the production of optical lenses with spherical surfaces, which is conventionally carried out on glass blanks in the surface grinding process followed by a polishing process. Surface contact between a polishing tool and the workpiece can also be used to eliminate form errors. However, it is disadvantageous that different tools must be available for different ball radii, the condition of which also depends on the accuracy of the finished lens.

Another manufacturing process for spherical lenses processes the mostly preformed glass compacts with a diamond cup wheel by means of ball grinding. The feed takes place either with the tool spindle or with the workpiece spindle to which the diamond cup wheel is at a defined angle. The spherical radius on the lens is determined by this setting angle, so that different spherical shapes can be produced within one limit with one and the same tool. However, its abrasive coating changes its shape due to wear during the grinding process by adapting to the changing ball radius. If changing ball radii are to be machined with the same pot tool, the grinding diameter to be generated cannot be determined in advance. Due to different radii and tool wear, unpredictable engagement zones are created on the tool, which lead to unwanted changes in the macro shape of the workpiece surface. Such processing errors must be eliminated by subsequent fine grinding processes. For smaller series with frequently changing ball radii, the share of tool costs in the total costs and the manufacturing effort for grinding is very large. In addition, grinding is often carried out in two stages with decreasing diamond grit, so that the overall machining time during grinding is considerably extended.

Optical lenses with aspherical surfaces offer a number of advantages. The imaging performance compared to spherical lenses is significantly increased, image errors are better corrected, and the number of lenses can be reduced in optical systems by using aspherical lenses. So far, these advantages could only be exploited to a very limited extent. The manufacturing outlay and the associated high unit costs conventionally restricted the use of aspherical, translucent components to special and special applications. Up to now, one-off and small series productions were not possible, or only to a limited extent, due to cost reasons.

For example, DE-A-2 441 976 describes a suction holder for lens blanks which are to be given an aspherical surface. The negative contour recessed in the middle is formed on the top of a rigid glass block. This is drilled centrally and clamped on a plate chuck, which is rotatably driven via a hollow shaft with a pump connection. The sucked-in blank can be turned after processing its top and processed on a similarly shaped second glass block on its underside. The method requires thin, bendable blanks and has the disadvantages mentioned above.

The grinding of aspherical glass lenses with high precision requires a subsequent polishing process, which, however, cannot easily be done in surface contact because of the non-spherical shape of the lens surface. Rather, flexible and very small polishing tools are necessary in order to achieve the most accurate representation of the asphere. Conventionally, there is the disadvantage that, due to the mechanical structure and the flexibility of the tool, movements which correspond exactly to the aspherical geometry cannot be carried out easily. The shape deviation increases with increasing polishing time and wear.

The invention has for its object to significantly improve and accelerate the inexpensive manufacture of aspherical lenses while overcoming the disadvantages of the prior art. Furthermore, a short processing time should be long Precision can be achieved without the need for post-processing. An important goal is to minimize the effects of tool wear on the lens shape.

Main features of the invention are set out in independent claims 1 and 9. Refinements are the subject of dependent claims 2 to 8 and 10 to 16.

In a method for producing aspherical surfaces on lens blanks, in particular made of glass, using a CNC machine tool with a control unit and a rotatingly adjustable pot tool for grinding and / or polishing a workpiece in a holder which can be moved along a feed axis into a processing position , wherein the axes of the pot tool and holder form an angle to one another, the invention provides according to the characterizing part of claim 1 that the pot tool is guided on the lens blank along a machining contour predetermined via the control unit such that between the longitudinal axis of the pot tool and the tangent a selectable lead angle is constantly maintained at its point of contact on the workpiece. As a result, the pot tool remains aligned from the grinding point insert throughout the machining with respect to the machining point on the workpiece, so that changing aspheres or spherical radii are machined true to the contour with changing series. This is done with the same grinding diameter throughout, so that wear on the pot tool is minimized perpendicular to the contact surface. In addition, because of the lead angle, the pot tool only comes into contact with the workpiece at an incline, so that point contact increases - after a long service life - to a very small segment or arc area.

According to claim 2, the lead angle is 0 °, from the starting position at the beginning of the machining. In order to optimize the grinding process, it can be modified beforehand in such a way that a specifically desired grinding area diameter is set on the pot tool and / or the material is removed in the most favorable manner. This is particularly important for targeted aspherical machining with the pot tool.

In an embodiment of the method with linear movement of the pot tool and the holder by feed drives along axes, claim 3 provides that at least one feed drive is pivoted with respect to another around a transverse axis, preferably by means of a further one, with respect to another, in order to produce aspheres and spheres of various shapes Drive. So there are two linear axes and a rotary axis together in order to guide the pot tool over the workpiece, which preferably occurs along a meridian line from the edge of the workpiece to its center and above. This can be controlled with relatively little design effort and extremely precisely.

Especially for soft contact lenses with a spherical back, a method described in EP-A2-0 304 106 uses a flat control surface which, with a selectable axis offset, lies tangentially on the rotating workpiece and, together with a tool, can be pivoted on a circular path about a common axis of rotation. The extent of the axis offset determines the asphericity during machining, which takes place in the dewatered state of the lens blank on a lathe. However, every setting must be specified and cannot be changed during the cutting process, so that precise individual adjustments are not possible.

A further development according to claim 5 is that the surface of the lens blank is brought to a spherical shape by pre-processing, which is as close as possible to the predetermined machining contour. This greatly facilitates the subsequent aspherical processing.

It is extremely advantageous if the machining parameters are entered into a microprocessor computer, e.g. Via a keyboard under screen control or via an interface, whereupon the tool path data are calculated in the control unit. This control can therefore be used as an open system to meet high requirements with regard to computing power, storage capacity, processing speed and process representation on the basis of derivative software. For this purpose, the machining contour can be defined in micro steps using the lens geometry for the workpiece and the associated parameters for the momentary movement control of the pot tool and holder can be programmed. As a result, the machining is largely automated and at the same time refined, so that larger deviations are excluded and the fine machining is achieved with the greatest possible accuracy and speed.

In addition, corrections are possible in that, according to claim 8, sample values of its surface are obtained during or after the machining of the workpiece and are taken into account in the control unit during subsequent machining by changing the path guidance of the pot tool. This can be done in segments and enables processing that compensates for any distortions that may occur during subsequent polishing.

According to independent claim 9, a device in particular for performing the aforementioned method is characterized in that a relative pivoting about a transverse axis can be carried out between the pot tool and the holder, the pot tool being movable into a machining position by a first feed drive and the holder by a second feed drive is. Such a structure is clear, very precisely controllable and economical, especially since conventional linear drives can be used. The device according to claim 10 is preferably designed such that the pot tool is arranged at a fixed distance from the transverse axis and can be pivoted by means of a third feed drive. According to claim 11, this can be arranged parallel to the feed direction of the first drive, which offers structural advantages, e.g. a simplified frame and slide design.

What is important is the measure of claim 12, according to which the pot tool can be moved along a meridian line of the workpiece from its edge to beyond the center, which can be achieved simply and precisely according to the invention with a corresponding arrangement of the drives.

Significant advantages of the invention are based on the fact that contour-accurate, rotationally symmetrical lens surfaces can be produced independently of central planning and calculation with pot tools, which can have different diamond grits, but do not require any dressing processes which would influence the lens geometry. In order to produce so-called free-form surfaces, the invention provides for an additional feed drive for an axis which is central with respect to the workpiece holder, in particular in close proximity to the second feed drive, which is advantageous for the structural arrangement. The workpiece spindle can optionally rotate continuously, in a conventional manner counter to the direction of rotation of the pot tool, or it can be converted into a controlled rotary axis by switching axes according to claim 14. This enables off-center surface processing with very little effort.

For precision machining, a measuring system is assigned to each axis, so that actual and target values can be compared and corresponding control data can be supplied to the control unit. According to claim 16, the invention further provides that the samples of the machined surface can be fed into the control unit and evaluated in a microprocessor computer by comparing the actual surface profile with the machining contour to correct the tool path. The easiest way to do this is to place a correction data curve over the default contour and derive the corrected tool path from it.

Fig. 1

eine schematische Seitenansicht einer Bearbeitungsgeometrie mit unterschiedlich geneigtem Topfwerkzeug,

Fig. 1a

eine vergrößerte Seitenansicht der Werkzeugstellung bei Arbeitsbeginn,

Fig. 1b

eine vergrößerte Seitenansicht der Werkzeugstellung bei Arbeitsende,

Fig. 2

eine schematische Seitenansicht eines Topfwerkzeuges bei sphärischer Linsenbearbeitung,

Fig. 3

eine schematisierte Schrägansicht einer CNC-Werkzeugmaschine mit drei Achsen,

Fig. 4

eine schmematisierte Schrägansicht einer CNC-Werkzeugmaschine mit vier Achsen,

Fig. 5

ein Flußdiagramm eines Arbeitsablaufs und

Fig. 6

ein Grundschema einer CNC-Werkzeugmaschnine.

Further features, details and advantages of the invention emerge from the wording of the claims and from the following description of exemplary embodiments with reference to the drawings. In it show:

Fig. 1

1 shows a schematic side view of a machining geometry with a differently inclined pot tool,

Fig. 1a

an enlarged side view of the tool position at the start of work,

Fig. 1b

an enlarged side view of the tool position at the end of work,

Fig. 2

1 shows a schematic side view of a pot tool with spherical lens processing,

Fig. 3

a schematic oblique view of a CNC machine tool with three axes,

Fig. 4

a schematized oblique view of a CNC machine tool with four axes,

Fig. 5

a flow chart of a workflow and

Fig. 6

a basic scheme of a CNC machine tool.

1, 1a, 1b, 2, a workpiece W is shown, which is machined in the form of a lens blank L by a pot tool T with an inclined axis A along a machining contour K. A peripheral part U attaches to the edge of the workpiece W and touches it at point P, the outer axis line A 'enclosing a constant lead angle β to the tangent F. The workpiece W is preferably driven in rotation and rotates in the opposite direction to the pot tool T, which is guided from the edge of the workpiece W beyond its center. The lead angle β, which can also be seen between the outer axis line A 'and the normal N to the tangent F (FIG. 1), remains the same throughout. It can be 0 ° at the start of work, but can also be set differently if necessary. It can be seen that in the course of processing the tangent angle τ to the surface O becomes smaller and smaller. The point of contact P is always behind the piercing point of the outer axis line A ', and the line of contact or ring surface of the peripheral part U ensures a uniform and gentle material removal. Depending on the effective diameter D of the peripheral part U, the inclination of the pot tool T and the lead angle β can be optimally set for the respective grinding or polishing task. If a preprocessing with a spherical surface O is to be carried out, a constant inclination of the pot tool T to the axis Z of the valuable spindle S is predefined with a setting angle (FIG. 2).

The structural design is illustrated in Fig. 3. The CNC machine tool, designated as a whole by 10, has a frame 12 with a table 14 on which a horizontal frame 16 is arranged. A slide 18 with a housing 20 is slidably arranged thereon. A head 22 is connected to the housing 20, which contains a deflecting gear 24 and holds a rotatably driven tool spindle V. A carriage 28 with a rotary drive 30 for a rotary spindle S is arranged on a vertical frame 26 and carries a holder H for the workpiece W.

The carriage 18 is movable in the direction of an axis X by means of a first feed drive I. A second feed drive II is provided for the slide 28, which enables movement in the direction of an axis Z. The head 22 is pivotable about a transverse axis B, for which a third feed drive III is used, which is arranged parallel to the axis X. It can be seen that only by the interaction of two linear drives in the direction of the axes X and Z and by a pivoting movement about the transverse axis B, the pot tool T with its peripheral part U can be controlled on the holder H with respect to the workpiece (omitted in FIG. 3).

FIG. 4 shows a modified embodiment which corresponds in general to the construction of the CNC machine tool 10 from FIG. 3. In addition, however, there is a further feed drive IV which is arranged centrally with respect to the axis Z and which, after switching over from the rotary drive of the workpiece spindle S, enables control thereof by means of an additional rotary axis C.

A general flow chart of the work flow is shown in FIG. 5. The machining type is first selected depending on whether an aspherical or spherical machining contour K (FIG. 1) is specified. This is followed by the selection of the type of geometry, which can be convex, concave or flat. The associated geometry parameters such as radius of curvature, outside diameter, center thickness of the lens etc. and the tool or machining parameters such as the effective diameter of the peripheral part U, its lip radius, lead angle b, feed speed and speed of the pot tool are then entered. From this, the tool path is calculated in the control unit, whereupon the lens is machined along the machining contour K. Following this grinding and / or polishing operation, the surface O is scanned, which is used to obtain correction data which can be used to correct the tool path for subsequent processing.

The basic structure of a suitable device is shown schematically in FIG. 6. The CNC machine tool 10 has an operating panel 40, preferably with a screen, and an input / output part 50, which can be designed as a keyboard. Both units are connected to a microprocessor computer R, to which measuring systems M1 to M4 are assigned. The latter are connected to a control unit E, which directly influences the feed drives I to III. A changeover device or switch 60 serves to selectively control only the rotary drive 30 for the workpiece spindle S or the fourth feed drive IV for the axis C.

The machine 10 is expediently of modular construction and equipped with (not shown) highly dynamic servo motors. Interpolators, not shown, ensure that the tool guide in the finest steps - i.e. the machining contour K quasi-continuously - can be controlled and thus guarantees the production of usable aspherical surfaces. Compensatory movements can be taken into account as well as any polishing allowances that can be provided with an extremely aspherical contour to compensate for non-linear material removal.

It can be seen that, according to the method according to the invention and with the CNC machine tool according to the invention, the universal lens processing with aspherical but also spherical surfaces can advantageously be carried out. The arrangement is particularly suitable for small series or one-off production.

The additional C-axis for the workpiece spindle S also allows free-form surface machining using the same principle. Here too, a servo drive and a rotation measuring system are provided for the controlled rotary axis, so that after axis switching, off-center surface machining can be carried out if necessary. If these are not required, the switch 60 switches to a pure rotary drive 30 for the workpiece spindle S.

The invention is not restricted to the exemplary embodiments shown; rather, numerous modifications are possible. However, it can be seen that in a method for producing aspherical surfaces on lens blanks L, in particular made of glass, a CNC machine tool with a rotatingly adjustable pot tool T is preferably used according to the invention for grinding and / or polishing a workpiece W in a holder H. The pot tool T is guided along a machining contour K specified via a control unit E in such a way that an optional lead angle β of, for example, 0 ° is maintained between the longitudinal axis A of the pot tool T and the tangent F in its contact point P on the workpiece W. At least one feed drive (for example I) is pivoted with respect to another (for example II) about a transverse axis B, preferably under interpolation control of a further drive III to guide the pot tool T from the edge of the workpiece W to its center and above it along a meridian line. An additional feed drive IV enables control of the workpiece holder H about a central axis C. Each axis X, Z, B, C is assigned a measuring system M1, M2, M3, M4, the measured values and scanning values of the machined surface can be fed into the control unit E and can be evaluated in a microprocessor computer R by comparing the actual surface profile with the machining contour K for recalculating the tool path. The surface of the lens blank L can be pre-machined to a spherical shape that is as close as possible to the specified machining contour K.

All of the features and advantages arising from the claims, the description and the drawing, including constructive details, spatial arrangements and method steps, can be essential to the invention both individually and in the most varied of combinations.

Reference symbol list

α

Entering angle

β

Lead angle

τ

Tangent angle

I, II, III, IV

Feed drives

A

Axis (from T)

A '

Outer axis line

B

Transverse axis

C.

central axis

D

Diameter (from U)

E

Control unit

F

tangent

H

bracket

K

Machining contour

L

Lens blank

M1, M2, M3, M4

Measuring systems

N

Normal (to F)

O

Surface (from W)

P

Point of contact (T to W)

R

Microprocessor calculator

S

(Rotation) spindle

T

Potting tool

U

Circumferential part

V

Tool spindle

W

workpiece

X

axis

Z.

axis

10th

CNC machine tool

12th

frame

14

table

16

Horizontal frame

18th

carriage

20th

casing

22

head

24th

Deflection gear

26

Vertical frame

28

carriage

30th

Rotary drive (for A / W)

40

Control panel

50

Input / output unit

60

Switchover device / switch

Claims (16)

Method for producing aspherical surfaces on lens blanks (L), especially made of glass, using a CNC machine tool with a control unit (E) and a rotatingly adjustable pot tool (T) for grinding and / or polishing a workpiece (W) in a holder (H), which can be moved along a feed axis (Z) into a machining position, the axes (A or Z) of the pot tool (T) and holder (H) enclosing an angle to one another, characterized in that the pot tool (T) is guided on the lens blank (L) along a machining contour (K) specified via the control unit (E) in such a way that between the longitudinal axis (A) of the pot tool (T) and the tangent (F) at its point of contact (P) on the workpiece (W ) a selectable lead angle (β) is kept constant. Method according to claim 1, characterized in that the lead angle (β) is 0 °. Method according to claim 1 or 2, wherein the pot tool (T) and the holder (H) can be moved linearly along axes (X, Z) by feed drives (I, II), characterized in that for producing aspheres and spheres of various shapes Interpolation control pivots at least one feed drive (eg I) with respect to another (eg II) about a transverse axis (B), preferably by means of a further drive (III). Method according to one of claims 1 to 3, characterized in that the pot tool (T) is guided from the edge of the workpiece (W) to its center and above, in particular along a meridian line. Method according to one of claims 1 to 4, characterized in that the surface of the lens blank (L) is brought to a spherical shape by pre-machining, which is largely approximated to the predetermined machining contour (K). Method according to one of claims 1 to 5, characterized in that the processing parameters (K) are entered into a microprocessor computer (R), for example via a keyboard under screen control or via an interface, whereupon the control unit (E) Tool path data can be calculated. Method according to Claim 6, characterized in that the machining contour (K) is defined in micro-steps on the basis of the lens geometry for the workpiece (W) and in that the associated parameters for the momentary movement control of the pot tool (T) and holder (H) are programmed become. Method according to one of claims 1 to 7, characterized in that during or after machining of the workpiece (W) samples of its surface are obtained and are taken into account in the control unit (E) during subsequent machining by changing the path of the pot tool (T). Device for the production of aspherical surfaces on lens blanks (L) by means of a CNC machine tool with a control unit (E), with a rotatably driven pot tool (T) which can be changed in position by a first feed drive (I) and with a workpiece holder (H), which can also be used a second feed drive (II) can be moved into a machining position, the axes (A or Z) of the tool (T) and holder (H) being at an angle to one another, in particular for carrying out the method according to one of claims 1 to 8, characterized in that a relative pivoting about a transverse axis (B) can be carried out between the pot tool (T) and the holder (H). Apparatus according to claim 9, characterized in that the pot tool (T) is arranged at a fixed distance from the transverse axis (B) and can be pivoted by means of a third feed drive (III). Apparatus according to claims 9 and 10, characterized in that the third feed drive (III) is arranged parallel to the feed direction of the first drive (I). Apparatus according to claim 10 or 11, characterized in that the pot tool (T) can be moved along a meridian line of the workpiece (W) from its edge to beyond the center. Device according to one of claims 9 to 12, characterized in that an additional feed drive (IV) is provided for an axis (C) central with respect to the workpiece holder (H), in particular in close proximity to the second feed drive (II). Apparatus according to claim 13, characterized in that a switchover device (60) is provided for the transition from the holder operation from pure rotary movement to controlled rotary axis operation and vice versa. Device according to one of claims 9 to 14, characterized in that a measuring system (M1, M2, M3, M4) is assigned to each axis (X, Z, B, C). Apparatus according to claim 15, characterized in that the measured values of the measuring systems (M1, M2, M3, M4) and scanning values of the processed surface can be fed into the control unit (E) and in a microprocessor computer (R) by comparing the actual surface profile with the machining contour (K) can be evaluated to correct the tool path.

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