EP0685298B1 - 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 6. Such a method and such a device are known from EP-A-0 453 094.

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. The surface is polished by contact between a polishing tool and the workpiece. 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 radii. 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 macro shape changes on 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.

EP-A2-0 453 094 describes a method and an apparatus for grinding toric lenses. A rotating diamond pot tool is guided with a preselected head angle in a pivoting movement over a linearly supplied lens blank, the pivoting radius of the tool being able to be changed during a pivoting movement. The pot tool is arranged on a pivotable cross slide which is mounted on a base slide which can be pivoted about a horizontal axis and can be extended and retracted in the radial direction. In order to keep the head angle and thus the vertical curvature of the lens constant along the changing circular path, this becomes Readjust the tool according to its position using the cross slide. To calculate the required alignment or readjustment of both carriages along their path using a microprocessor, the pivoting angle of the basic carriage is detected by a sensor during its movement. After a swivel movement has ended, the lens blank is adjusted by means of a feed drive. This process is repeated until a desired lens thickness is reached.

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 object of the invention is to significantly improve and accelerate the cost-effective manufacture of aspherical lenses while overcoming the disadvantages of the prior art. Furthermore, a short duration of processing with high precision should be achieved without the need for post-processing. An important goal is to minimize the effects of tool wear on the lens shape.

The invention is set out in independent claims 1 and 6. Refinements are the subject of dependent claims 2 to 5 and 7 to 10.

In a method for producing aspherical surfaces on lens blanks, namely made of glass, using a CNC machine tool with a control unit and with an adjustable pot tool rotating about an axis for grinding and / or polishing the lens blank in a holder, which runs along a first feed axis is movable into a machining position, the pot tool being pivotable about a pivot axis perpendicular to the first feed axis and linearly adjustable along a second feed axis perpendicular to the pivot axis and the first feed axis, and wherein the pot tool is guided on the lens blank along a machining contour specified via the control unit, that between the longitudinal axis of the pot tool and the tangent in its point of contact on the workpiece a selectable lead angle (head angle) is kept constant, the invention provides according to the characterizing part of claim 1 that the pot tool eug with a contact point that can be selected via the lead angle, is moved towards the workpiece rotating about the first feed axis and is guided from the edge of the workpiece to its center and beyond along the machining contour by means of the control unit by interpolation control of the feed axes and also the swivel axis, during or after the Machining of the workpiece Samples of its surface are obtained and taken into account in subsequent processing by changing the path of the pot tool in the control unit.

Due to its preselected inclination, the pot tool brought up to the workpiece by means of the feed axes only comes into contact with the corresponding preselected contact point on the workpiece, ie the peripheral edge of the pot tool only touches the workpiece at a relatively narrow point of contact.

According to claim 2, the lead angle is 0 °, from the starting position at the beginning of the machining. The lead angle can be modified beforehand in order to optimize the grinding process 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.

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 not changeable during the cutting process, so that precise individual adjustments are not possible.

It is extremely advantageous if the machining parameters for determining the machining contour 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 according to claim 4 on the basis of the lens geometry for the workpiece in micro steps and the associated parameters for the current 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.

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 specified aspherical machining contour. This greatly facilitates the subsequent aspherical processing.

According to independent claim 6, a device for producing aspherical surfaces on lens blanks, in particular for carrying out the aforementioned method, is characterized in that the pot tool can be brought up to the workpiece rotating by means of a rotary drive about the first feed axis with a contact point which can be selected via the lead angle and by means of the Control unit can be guided by interpolation control of the feed axes and the swivel axis from the edge of the workpiece to its center and beyond along the machining contour, with each axis being assigned a measuring system whose measured values and samples of the machined surface can be fed into the control unit and in a microprocessor computer Comparison of the actual surface profile with the machining contour for correcting the tool path can be evaluated.

This simple and compact structure of a grinding machine is clear, very precisely controllable and economical, especially since conventional linear drives are used can find. Thanks to the integrated measuring and correction system, it ensures a precise and cost-effective implementation of the method according to the invention and also enables the rational production of both extremely precise aspherical lens surfaces and so-called free-form surfaces, even in small series or one-off production.

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.

To produce free-form surfaces, the invention provides, according to claim 7, that for the rotation of the workpiece there is an additional drive for the central axis with respect to the workpiece holder, in particular in close proximity to the second feed drive, which is favorable for the structural arrangement is. 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. This enables rotational asymmetrical surface processing with very little effort.

The device according to claim 9 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 linear feed drive. According to claim 10, this can be arranged parallel to the feed direction of the first drive, which offers structural advantages, for example a simplified frame and slide design.

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 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 starts at 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 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 contact point P is always behind the puncture point of the outer axis line A ', and the contact line 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 second feed drive I. A first 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. It can be seen, however, 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.

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 on 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 (10)

1. Procedure of fabricating aspheric surfaces on lens blanks (L), particularly glass blanks, using a CNC machine tool with a control unit (E) and a movable cup-shaped tool (T) rotating around an axis (A) for grinding and/or polishing the lens blank (L) in a holder (H) that can be moved into a processing position along a first feeding axis (Z), the cup-shaped tool (T) being rotatable around an axis of rotation (B) at right angles to the first feeding axis (Z) and being linearly adjustable along a second feeding axis (X) at right angles to the axis of rotation (B) and to the first feeding axis (Z), and the cup-shaped tool (T) being guided on the lens blank (L) along a processing contour (K) predetermined by the control unit (E) so that a selectable angle of lead (heading angle (β) is maintained between the longitudinal axis (A) of the cup-shaped tool (T) and the tangent line (F) in its point of contact (P) on the workpiece (W), characterized by the following features:

   a) the cup-shaped tool (T) rotated by a rotary drive is moved with a point of contact (P) adjustable via the angle of lead (β) towards the workpiece (W) rotating around the first feeding axis (Z),

   b) the cup-shaped tool (T) is guided from the edge of the workpiece (W) to its center and beyond along the processing contour (K) by means of the control unit (E) through interpolation control of the feeding axes (X, Z) as well as of the axis of rotation (B),

   c) during or after processing of the workpiece (W), scanning values of its surface are obtained and considered in the control unit (E) for subsequent processing through a changed trajectory guidance of the cup-shaped tool (T).
2. Procedure according to claim 1, wherein the angle of lead (β) is 0 degrees.
3. Procedure according to claim 1 or claim 2, wherein the processing parameters for determination of the processing contour (K) are entered into a microprocessor computer (R), e.g. via a keyboard under monitor control or via an interface, whereupon the tool trajectory data are calculated in the control unit (E).
4. Procedure according to any one of claims 1 to 3, wherein the processing contour (K) is defined for the workpiece (W) in microsteps based upon the lens geometry and wherein the corresponding parameters for the instantaneous movement control of the cup-shaped tool (T) and the holder (H) are programmed.
5. Procedure according to any one of claims 1 to 4, wherein the surface of the lens blank (L) is rough-worked to a spherical shape that is utterly approximated to the predetermined aspheric processing contour (K).
6. Device for fabricating aspheric surfaces on lens blanks (L), particularly glass blanks, comprising a CNC machine tool with a control unit (E), an adjustable cup-shaped tool (T) rotated around an axis (A) for grinding and/or polishing the lens blank (L), a holder (H) for receiving the lens blank (L), which holder (H) can be moved into a processing position along a first feeding axis (Z) by means of a first feeding drive (II), the cup-shaped tool (T) being rotatable around an axis of rotation (B) at right angles to the first feeding axis (Z) as well as being linearly adjustable along a second feeding axis (X) at right angles to the axis of rotation (B) and to the first feeding axis (Z) by means of a second feeding drive (I), in particular for performing the procedure according to any one of claims 1 to 5, wherein the cup-shaped tool (T) is advanceable to the workpiece (W) which rotates around the first feeding axis (Z) by means of a rotary drive (30), with a point of contact (P) being adjustable via the angle of lead (β), and which is guidable from the edge of the workpiece (W) to its center and beyond along the processing contour (K) by means of the control unit (E) through interpolation control of the feeding axes (X, Z) as well as of the axis of rotation (B), with a measuring system (M1, M2, M3, M4) assigned to each axis (X, Z, B) the values of which can, together with the scanning values of the processed surface, be fed into the control unit (E) and be evaluated in a microprocessor computer (R) for correcting the tool trajectory by comparison of the actual surface with the processing contour (K).
7. Device according to claim 6, wherein for rotation of the workpiece (W), especially in the vicinity of the first feeding drive (II), an additional drive (IV) is available for an additional axis (C) that is centric with regard to the workpiece holder (H).
8. Device according to claim 7, wherein there is a switch-over device (60) for changing the holder operation from pure rotation to controlled circular motion operation and vice-versa.
9. Device according to any one of claims 6 to 8, wherein the cup-shaped tool (T) is arranged at a fixed distance to the transverse axis (B) and is adjustable by means of a linear third feeding drive (III).
10. Device according to claim 9, wherein the third feeding drive (III) is arranged parallel to the feeding direction of the second drive (I).

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