EP1338382B1 - Method for attaching a blocking tool to an ophtalmic lens blank - Google Patents

Abstract

The unfinished lens (1) is positioned on a fixed base (19) on the sloping face of a consol (7) which has motorized adjustment (10) and a display (9). The positioning is such that contact is made at points (S1,S2,S3). A gripping block is orientated to the lens and then fixed to the one finished surface. Orientation takes into account the form of the finished face and also the prescribed prism Independent claims are also included for the following: (1) Locking equipment for orientating and securing the unfinished lens and for fixing the gripping block to the finished lens face; (2) Support ring for positioning unfinished lens to the locking equipment in order to facilitate placing finished side on gripping block.

Description

The invention relates to a method for laying on a semi-finished blank. of ophthalmic lens of a gripping block.

In the production of ophthalmic lenses, a finished lens is formed. from a blank with a cylindrical edge, the raw faces of which are obtained by molding or machining, are surfaced, that is to say they are successively soft and polished.

The faces, one of which is generally concave while the other is convex, are surfaced one after the other. Generally, for reasons practical, the convex side is surfaced before the concave side. A draft of lens of which only one of the faces is finished, that is to say that it has been surfaced, is called semi-finished blank.

Surfacing the second side is a more delicate operation which requires better precision, because it is not only a question of giving this second face the surface condition and the required curvature, but also orient it in space extremely precisely so that the finished lens have the desired optical properties.

This orientation may require one or two predetermined adjustments one of which is called "prism" and the other "axis".

The prism, which is generally a prescription prism measured in diopters and determined by the ophthalmologist, consists of a tilting of the second face compared to the first, while the axis consists of a rotation of the second face with respect to the first, around the optical axis of The lens.

• positionner l'ébauche sur un socle fixe, de manière centrée et angulairement définie et de telle sorte que la face finie porte conjointement sur une pluralité de points d'appui dudit socle,
• définir une orientation du bloc de préhension par rapport à l'ébauche,
• orienter le bloc de préhension de la manière définie,
• fixer le bloc de préhension sur la face finie en conservant son orientation.

Generally, the installation of a gripping block on the semi-finished blank of an ophthalmic lens intended to present a predetermined prism consists in:

• positioning the blank on a fixed base, in a centered and angularly defined manner and in such a way that the finished face bears jointly on a plurality of support points of said base,
• define an orientation of the gripping block relative to the blank,
• orient the gripping block in the defined manner,
• fix the gripping block on the finished face, keeping its orientation.

American patent US-4,714,232 in the name of the applicant describes a process of this type.

Ordinarily, semi-finished blanks of ophthalmic lenses are supplied with marks on the finished face. In general, a point mark materializes the reference point of the prism, hereinafter referred to as PRP in accordance with current practice and through which the optical axis passes, and a line or a succession of aligned lines materializing a locating axis for the mounting the lens in a spectacle frame.

This tracking axis corresponds in practice to the horizontal nose-ear axis, with respect to which the ophthalmologist generally indicates the axis adjustment.

When positioning it on the base, centering the blank consists in placing the PRP on a fixed centering axis defined in relation to the base, while the angular orientation of the blank consists in placing the axis of marking in a fixed plane defined with respect to the base and containing the axis of centering.

Given the curvature of the finished face, it occurs, during its contact with all the support points with conservation of the centering and angular orientation, tilting of the blank, i.e. pivoting from the optical axis of the lens relative to the centering axis.

This results in the appearance, when positioning the blank on the base, of an uncontrolled prism which it is necessary to compensate during the orientation of the gripping block. The finished faces with progressively variable curvatures are by nature the most likely to generate the appearance of such a prism and randomize the positioning of the blank on the base.

One solution to control this positioning is to provide a base different for each type of finished face. Obviously such a solution is extremely expensive and requires a lot of handling, not only to select each of the bases from a very wide range given the variety of faces with gradually variable curvature, but also to position the base on its support.

In addition, it is necessary to ensure that, whatever the curvature of the finished face, the location of the PRP is substantially constant on the axis of centering, so that the distance from the PRP to the grip block varies little - or not at all - from one lens to another.

Indeed, if the lens must be sufficiently far from the block of gripping so as not to hit it, it must be close enough to it so that the block + lens assembly is, ultimately, sufficiently rigid.

As the curvature of the front face varies from one lens to another, we have for the habit of providing rings of different heights to compensate displacement of the PRP along the centering axis, which requires a large number of different rings.

Another solution, described by patent US Pat. No. 4,714,232 introduced above, proposes to make a base in the form of a support ring comprising three support zones for contact with a semi-finished blank, distributed circularly around an axis at the vertices of an isosceles triangle, each of these support zones comprising a plurality of facets which, taken together, form between them a globally convex set.

At the time of filing of this patent, such a provision was particularly advantageous compared to known techniques, the same ring suitable for processing a range of semi-finished blanks.

In fact, the support zones are distributed angularly so that two of them are in contact with the far vision portion of the face finished, while the third is in contact with the near vision portion.

Therefore, we understand that it is necessary to classify by types the different finished faces with progressively variable curvature, depending on the analogy of their topographies so that the same ring suits them. We must therefore provide for a number of rings equal to the number of different types of finished faces with progressively variable curvature. The same ring cannot therefore suitable for the whole range of lenses produced.

In addition, if this solution makes it possible to minimize the risks linked to the appearance of a prism during the positioning of the semi-finished blank, it does not allow however not to get rid of it.

Anyway, whatever the technique used for laying on a semi-finished ophthalmic lens blank, the final optical properties of the lens never very precisely correspond to the prescription of the ophthalmologist, but one generally accepts this inaccuracy.

The invention aims to solve in particular the aforementioned drawbacks of known techniques by proposing a solution which, by making it possible to control the risks linked to the appearance of a positioning prism, allows a faster and cheaper production of ophthalmic lenses with increased optical qualities.

• positionner l'ébauche sur un socle fixe, de manière centrée et angulairement définie et de telle sorte que la face finie de l'ébauche porte conjointement sur une pluralité de points d'appui dudit socle,
• définir une orientation du bloc de préhension par rapport à l'ébauche,
• orienter le bloc de préhension de la manière définie,
• fixer le bloc de préhension sur la face finie en conservant son orientation,

caractérisé en ce que l'étape de définition de l'orientation du bloc de préhension comporte les étapes consistant à :

• prendre en compte la forme tridimensionnelle de la face finie et la position desdits points d'appui,
• en déduire l'orientation de la face finie lorsque l'ébauche est positionnée sur le socle;
• prendre en compte le prisme prédéterminé,
• déduire de l'orientation de la face finie et du prisme prédéterminé l'orientation du bloc de préhension par rapport à la face finie.

To this end, the invention proposes, according to a first aspect, a method for installing a gripping block on a semi-finished blank of an ophthalmic lens intended to present a predetermined prism, which comprises the steps consisting in:

• positioning the blank on a fixed base, in a centered and angularly defined manner and in such a way that the finished face of the blank bears jointly on a plurality of support points of said base,
• define an orientation of the gripping block relative to the blank,
• orient the gripping block in the defined manner,
• fix the gripping block on the finished face while maintaining its orientation,

characterized in that the step of defining the orientation of the gripping block comprises the steps consisting in:

• take into account the three-dimensional shape of the finished face and the position of said support points,
• deduce the orientation of the finished face when the blank is positioned on the base;
• take into account the predetermined prism,
• deduce from the orientation of the finished face and from the predetermined prism the orientation of the gripping block with respect to the finished face.

In this way, it is possible to compensate very precisely for the possible tilting of the blank when it is placed on the base so that the actual prism introduced into the blank during positioning of the block grip effectively corresponds to the predetermined prism.

For example, to get the orientation of the finished face when the roughing is positioned on the base, we calculate a positioning prism resulting from the tilting of the blank during its installation on the base.

où:

• AngH et AngV sont définis comme suit :
  
  
  où f_N est une fonction du type z = f_N (x,y) définissant la forme de la face finie dans un repère XYZ fixe par rapport au socle et où x,y,z sont les coordonnées cartésiennes liées respectivement aux axes X, Y et Z dudit repère fixe, L étant défini par la formule suivante : L = 1 + ∂f N ∂x 2 x=0,y=0 + ∂f N ∂y 2 x=0,y=0
• AngV ₀ est définit comme suit :
  
  Pr V ₀ étant défini comme suit : PrV 0 = K × add où add est l'addition de puissance de la lentille ophtalmique que l'on souhaite obtenir, et K un indice de proportionnalité, de préférence égal à 2 / 3.

More precisely, to define the orientation of the gripping block, one can calculate two angles γ and  defined by the following formulas:

or:

• AngH and AngV are defined as follows:
  
  
  where f _N is a function of the type z = f _N ( x , y ) defining the shape of the finished face in a frame XYZ fixed with respect to the base and where x, y, z are the Cartesian coordinates linked respectively to the axes X, Y and Z of said fixed coordinate system, L being defined by the following formula: L = 1 + ∂ f NOT ∂ x 2 x = 0, there = 0 + ∂ f NOT ∂ there 2 x = 0, there = 0
• AngV ₀ is defined as follows:
  
  Pr V ₀ being defined as follows: Pr V 0 = K × add where add is the addition of power of the ophthalmic lens that one wishes to obtain, and K an index of proportionality, preferably equal to 2/3.

• calculer une fonction f_p définissant la forme tridimensionnelle de la face finie dans le repère fixe XYZ,
• calculer les profondeurs z _i , liées à l'axe Z du repère fixe XYZ, des projections des points d'appui sur la face finie suivant l'axe Z, au moyen de la formule suivante : z _i = f_p(x_i,y_i) où, pour chaque point d'appui, x_i et y_i sont ses coordonnées liées respectivement à l'axe X et l'axe Y du repère fixe XYZ,
• calculer la valeur de l'écart maximal ε entre les profondeurs z_i ,
• comparer la valeur de l'écart ε avec une valeur prédéterminée ε₀,
• calculer les angles α_p et β_p définis par les formules suivantes : αp = Arc tan(a) β p = Arc tan(b) où a et b sont les coefficients directeur du plan A_p passant par les projections sur la face finie des points d'appui,
• basculer la face finie suivant deux rotations, à savoir une première rotation d'ange α_p dans le plan X, Z et une deuxième rotation d'angle β _p dans le plan Y, Z,
• incrémenter p d'une unité,
  tant que la valeur de l'écart ε est supérieure à la valeur prédéterminée ε₀, où :
• i est un entier compris entre 1 et 3,
• p est un entier initialement égal à 1, avec f 1 = f où f est une fonction prédéterminée du type z'= f(x',y') définissant la forme tridimensionnelle de la face finie dans un repère orthogonal X'Y'Z' lié à la face finie, x',y',z' étant les coordonnées cartésiennes liées respectivement aux axes X', Y', Z' du repère lié X' Y' Z',
• N est la valeur de p lorsque la valeur de l'écart ε devient inférieure à la valeur prédéterminée ε₀.

Three support points being provided on the base, the function f _N can be obtained by repeating the succession of the following steps:

• compute a function f _p defining the three-dimensional shape of the finite face in the fixed coordinate system XYZ,
• calculate the depths z _i , linked to the Z axis of the fixed coordinate system XYZ, of the projections of the support points on the finished face along the Z axis, by means of the following formula: z _i = f _p (x _i , y _i ) where, for each fulcrum, x _i and y _i are its coordinates respectively linked to the X axis and the Y axis of the fixed coordinate system XYZ,
• calculate the value of the maximum difference ε between the depths z _i ,
• compare the value of the difference ε with a predetermined value ε ₀ ,
• calculate the angles α _p and β _p defined by the following formulas: α p = Bow tan ( at ) β p = Bow tan ( b ) where a and b are the directing coefficients of the plane A _p passing through the projections on the finished face of the support points,
• tilt the finished face according to two rotations, namely a first angel rotation α _p in the plane X, Z and a second angle rotation β _p in the plane Y, Z,
• increment p by one,
  as long as the value of the deviation ε is greater than the predetermined value ε ₀ , where:
• i is an integer between 1 and 3,
• p is an integer initially equal to 1, with f 1 = f where f is a predetermined function of the type z ' = f (x', y ') defining the three-dimensional shape of the finite face in an orthogonal coordinate system X'Y'Z' linked to the finite face, x ', y', z ' being the Cartesian coordinates linked respectively to the axes X', Y ', Z' of the linked coordinate system X 'Y' Z ',
• N is the value of p when the value of the difference ε becomes less than the predetermined value ε ₀ .

The difference ε is for example defined as follows: ε = max ( z 1 - z 2 , z 1 - z 3 , z 2 - z 3 ).

In addition, the plane A _p being defined in the fixed frame of reference XYZ by the equation z = ax + by + vs, the coefficients a and b are defined as follows:

• l'angle que son axe Z" forme avec l'axe Z du repère fixe XYZ soit égal à l'angle γ , et
• l'angle que forme la projection de son axe Z" dans le plan formé par les axes X, Y du repère fixe XYZ avec l'axe X de ce repère fixe soit égal à l'angle .

The gripping block, which has a Z "axis, is oriented so that:

• the angle which its axis Z "forms with the axis Z of the fixed frame of reference XYZ is equal to the angle γ, and
• the angle formed by the projection of its axis Z "in the plane formed by the axes X, Y of the fixed coordinate system XYZ with the axis X of this fixed coordinate system is equal to the angle .

The fixing of the gripping block on the finished face can consist of pouring a metal with a low melting point in a cavity formed between the finished face and the gripping block and cool or allow this metal to cool.

• un socle fixe pour le positionnement de l'ébauche semi-finie,
• des moyens pour centrer et orienter de manière définie l'ébauche par rapport au socle,
• des moyens de maintien de l'ébauche sur le socle,
• des moyens pour fixer le bloc de préhension sur la face finie,
• des moyens pour définir l'orientation du bloc de préhension en fonction de la forme tridimensionnelle de la face finie, et
• des moyens pour faire varier l'orientation du bloc de préhension par rapport au socle en fonction de l'orientation définie.

According to a second aspect, the invention provides a blocking device for fitting a gripping block to a semi-finished ophthalmic lens blank, which comprises:

• a fixed base for positioning the semi-finished blank,
• means for centering and orienting the blank in a defined manner with respect to the base,
• means for holding the blank on the base,
• means for fixing the gripping block to the finished face,
• means for defining the orientation of the gripping block as a function of the three-dimensional shape of the finished face, and
• means for varying the orientation of the gripping block relative to the base as a function of the defined orientation.

For example, the means to define the orientation of the gripping block include a calculator.

According to a third aspect, the invention proposes a support ring for positioning a semi-finished ophthalmic lens blank on a blocking device for laying on the finished face of the blank a block of gripping, this ring comprising a plurality of support points against which is suitable for applying the finished face of the blank, the points each located on a spherical surface with a small diameter in front of the radius of curvature of the finished face of the blank.

The diameter of said spherical surface is for example between 1.5 mm and 3 mm, preferably equal to 2 mm.

According to one embodiment, each spherical surface belongs to a protruding pawn, which can be brought back.

According to one embodiment, the ring has three pins.

The ring generally having a symmetry of revolution around a Z axis, the vertices of the pawns are preferably located in the same plane perpendicular to the Z axis, being for example distributed at the vertices of a triangle whose circumscribed circle is centered on the Z axis.

This circumscribed circle can have a diameter between 50 and 60 mm, preferably equal to 55 mm.

According to one embodiment, the angles at the vertices of said triangle are respectively between 60 ° and 80 °, between 50 ° and 70 °, and between 40 ° and 60 °.

In addition, the ring may have, for the casting of a metal at low point of fusion, a hollow channel extending along a radial axis.

According to one embodiment, one of the pawns is located near the channel.

For example, the pawn located near the channel is angularly offset with respect to it by an angle between 5 ° and 15 °, preferably equal to 10 °.

Alternatively, one of the pawns is in the axis of the channel being diametrically opposite with respect to it.

• la figure 1 est une vue en élévation de côté et en arraché partiel d'un appareil conforme à l'invention pour la pose sur une ébauche semi-finie de lentille ophtalmique d'un bloc de préhension ;
• la figure 2 est une vue de face, en plan, d'une face finie à courbure progressivement variable d'une ébauche semi-finie de lentille ophtalmique, sur laquelle sont tracés les lignes isohypses ;
• la figure 3a est une vue en perspective d'une bague d'appui selon un premier mode de réalisation, destinée à recevoir une ébauche semi-finie de lentille ophtalmique pour l'oeil gauche d'un porteur ;
• la figure 3b est une vue analogue à la figure 3a, suivant un autre angle de vue, d'une bague d'appui suivant un premier mode de réalisation, destinée par contre à recevoir une ébauche semi-finie de lentille ophtalmique pour l'oeil droit d'un porteur ;
• la figure 4 est une vue en plan de dessus d'une bague d'appui selon un deuxième mode de réalisation, destinée à recevoir, indifféremment, une ébauche semi-finie pour l'oeil gauche ou l'oeil droit d'un porteur ;
• la figure 5 est une vue en plan de dessus de la bague d'appui de la figure 3 a;
• la figure 6 est une vue d'élévation en coupe de la bague de la figure 5, suivant le plan de coupe matérialisé sur cette figure par la ligne VI-VI ;
• la figure 7 est une vue à plus grande échelle de la bague d'appui de la figure 6, suivant un détail matérialisé sur cette figure par l'encart VII; une ébauche semi-finie de lentille ophtalmique, partiellement représentée en traits mixtes, est posée sur la bague ;
• la figure 8 est une vue d'élévation en coupe illustrant une bague d'appui conforme à l'invention, sur laquelle est positionnée une ébauche semi-finie de lentille ophtalmique représentée en traits mixtes, ainsi qu'un arbre mobile assurant le positionnement du bloc de préhension par rapport à la lentille, dans une position où la bague et l'arbre sont coaxiaux ;
• la figure 9 est une vue analogue à la figure 8, l'arbre étant désaxé par rapport à la bague d'appui ;
• la figure 10 est un schéma géométrique simplifié illustrant la face finie de l'ébauche semi-finie en appui sur les points d'appui d'une bague d'appui conforme à l'invention ;
• la figure 11 est un schéma géométrique simplifié représentant la lentille en coupe et deux points d'appui supposés diamétralement opposés, illustrant une étape du calcul de l'orientation de l'ébauche ;
• la figure 12 est un schéma analogue à la figure 11, illustrant une étape consécutive du calcul de l'orientation de l'ébauche ;
• les figures 13 et 14 sont des diagrammes illustrant les différentes étapes d'un procédé conforme à l'invention,
• la figure 15 est une vue en perspective illustrant un ensemble formé d'une ébauche semi-finie de lentille ophtalmique sur laquelle a été posé un bloc de préhension suivant un procédé conforme à l'invention.

Other characteristics and advantages of the invention will become apparent in the light of the following description of an embodiment given by way of nonlimiting example, description made with reference to the appended drawings in which:

• Figure 1 is a side elevational view in partial cutaway of an apparatus according to the invention for fitting on a semi-finished ophthalmic lens blank of a gripping block;
• FIG. 2 is a front view, in plan, of a finished face with progressively variable curvature of a semi-finished blank of ophthalmic lens, on which the isohypse lines are drawn;
• FIG. 3a is a perspective view of a support ring according to a first embodiment, intended to receive a semi-finished blank of ophthalmic lens for the left eye of a wearer;
• FIG. 3b is a view similar to FIG. 3a, from another angle of view, of a support ring according to a first embodiment, intended on the other hand to receive a semi-finished blank of ophthalmic lens for the eye right of a holder;
• Figure 4 is a top plan view of a support ring according to a second embodiment, intended to receive, indifferently, a semi-finished blank for the left eye or the right eye of a wearer;
• Figure 5 is a top plan view of the support ring of Figure 3a;
• Figure 6 is an elevational sectional view of the ring of Figure 5, along the section plane shown in this figure by the line VI-VI;
• Figure 7 is an enlarged view of the support ring of Figure 6, according to a detail shown in this figure by the insert VII; a semi-finished ophthalmic lens blank, partially shown in phantom, is placed on the ring;
• Figure 8 is a sectional elevation view illustrating a support ring according to the invention, on which is positioned a semi-finished ophthalmic lens blank shown in phantom, as well as a movable shaft ensuring the positioning of the gripping block relative to the lens, in a position where the ring and the shaft are coaxial;
• Figure 9 is a view similar to Figure 8, the shaft being offset from the support ring;
• FIG. 10 is a simplified geometrical diagram illustrating the finished face of the semi-finished blank bearing on the support points of a support ring according to the invention;
• FIG. 11 is a simplified geometrical diagram representing the lens in section and two support points assumed to be diametrically opposite, illustrating a step in the calculation of the orientation of the blank;
• FIG. 12 is a diagram similar to FIG. 11, illustrating a consecutive step in the calculation of the orientation of the blank;
• FIGS. 13 and 14 are diagrams illustrating the different stages of a method according to the invention,
• Figure 15 is a perspective view illustrating an assembly formed of a semi-finished ophthalmic lens blank on which has been placed a gripping block according to a method according to the invention.

A semi-finished ophthalmic lens blank 1 has a front face 2, assumed to be convex, and an opposite rear face 3, assumed to be concave, connected by a cylindrical section 4.

In what follows, it is further assumed - which corresponds generally in practice - that the front face 2 is finished, that is to say that it has already been surfaced, while the rear face 3 is roughly molded or machining.

In Figure 1 is shown a locking device 5 for fixing on the blank 1 of a gripping block 6 intended to be itself attached to the spindle of a finishing machine (not shown) for surface facing gross 3.

The three-dimensional shape of the front face 2 is arbitrary (spherical, aspherical, toric, atoric, etc.) but we chose it here, for its complexity, gradually variable curvature for the production of a lens progressive.

This front face 2 has a far vision area VL, as well as a near vision zone VP diametrically opposite. As shown in FIG. 2, the near vision zone VP is not in the wearing position horizontal, vertical to the far vision area VL, but it is slightly offset from this vertical, the blank 1 shown here being intended to a right eye.

In order to give an overview of the three-dimensional shape of the front panel 2 from outline 1, we have drawn in Figure 2 its isohypse lines in the areas of the front face 2 located on either side of a far vision zone axis VL - near vision zone VP.

This front face 2 carries two registration marks, namely a point corresponding to the PRP of the blank, through which the optical axis of the latter passes, and, on both sides of the PRP, a succession of aligned lines forming an axis A marking corresponding to the horizontal nose-ear axis in the wearing position normal by the user.

As will be seen later, these marks are intended to allow respectively the centering and the angular orientation of the blank 1 during its positioning on the locking device 5.

As shown in Figure 1, the locking device 5 includes a frame 7, defining an inclined console 8, and surmounted by a display screen 9.

The device 5 also includes a positioning device 10 extending inside the frame 7 and comprising two circular plates spaced apart from each other and substantially parallel, namely an upper plate 11 fixed to the console 8, and a floating lower plate 12 carrying a sheath 13 in which a support shaft 14 is introduced, an upper end of which forms a housing 15 intended to receive the gripping block 6.

The sleeve 13 is rigidly fixed by a lower end to the plate lower 12, while it is connected to the upper plate 11 by a ball joint (not shown).

Furthermore, the lower plate 12 is connected to the upper plate 11 by through three parallel rods 16a, 16b, 16c, each being rigidly fixed to the lower plate 12 while it is connected to the upper plate 11 by a ball joint 17.

One 16a of these rods is of fixed length, while the other two 16b, 16c are of variable length thanks to an adjustment system 18 of the type motorized screw / nut.

For more details on the construction of this device positioning 10, reference may in particular be made to patent n ° US-4,372,368 in the name of the plaintiff.

As will be understood, it is possible, thanks to these rods 16a, 16b, 16c, orient the position of the support shaft 14 along three perpendicular axes - and consequently that of the gripping block 6 - relative to the upper plate 11.

On the upper plate 11 is fixed, in the axis of the sheath 13, a base 19 for positioning the semi-finished blank 1 on the locking device 5.

As can be seen in Figures 3 to 5 in particular, this base 19 is an annular support ring having generally a symmetry of revolution around a Z axis.

The ring 19 has an outer ring 20 by which it is adapted to be fixed on the upper plate 11. Two through holes 21a, 21b diametrically opposite axes Z1, Z2 parallel to the axis Z are formed in the crown 20, for their fitting on two pins 21 'provided on the plate 11 for the precise positioning and orientation of the ring 19.

The ring 19 has a flat lower support face 22 through which it rests on the upper plate 11.

Inside the crown 20, the ring 19 has, opposite the face support 22, a seat 23 with a frustoconical surface, extended towards the center of the ring 19 by a bore 24, the seat 23 and the bore 24 being centered on the axis Z of the ring 19.

As can be seen in FIG. 5, the ring 19 is truncated and has a flat support face 25 extending parallel to a plane containing the axis Z of the ring and the axes Z1 and Z2 of the through holes 21a, 21b.

A channel 26 opening out is further formed in the ring 19. This channel 26, which has a section substantially in an arc, extends along a radial direction perpendicular to the bearing face 25 and hollow the ring 19 on part of its thickness by cutting successively, from the outside towards the interior, the crown 20 and the seat 23, possibly the bore 24.

Furthermore, around and near seat 23, it is practiced in the crown 20 a groove 27, concentric with the seat 23, which is interrupted on both sides and else and near canal 26.

In this groove 27 is added by overmolding, gluing or the like, a seal 28 with a frustoconical lip 29 projecting from the crown 20.

Three holes 30 of circular cross section with an axis parallel to the axis Z are formed in the seat 23. In each of these holes 30 is reported a pin 31a, 31b, 31c having a cylindrical body 32 fitted in the hole 30, extended by a head 33 with a spherical surface projecting from the seat 23 and having at its upper end a point S ₁ , S ₂ , S ₃ called the apex.

The diameter of the pins 31a, 31b, 31c is much smaller than the other dimensions of the ring 19, so that one can assimilate, according to an acceptable approximation, each head 33 at its apex S ₁ , S ₂ , S ₃ .

The pins 31a, 31b, 31c, or more precisely their respective vertices S ₁ , S ₂ , S ₃ , jointly form the vertices of a triangle whose circumscribed circle is centered on the Z axis of the ring 19.

Through these three vertices S ₁ , S ₂ , S ₃ passes a single reference plane parallel to the lower support face 22 of the ring 19 and perpendicular to its axis Z.

In this reference plane, we define two intersecting perpendicular axes on the Z axis, namely an X axis passing through the Z1, Z2 axes of the through holes 21a, 21b, and a Y axis coincident with the axis of the channel 26.

We therefore have an orthogonal coordinate system XYZ defined with respect to the ring 19 and which, when the latter is fixed to the upper plate 11, is fixed by relative to the locking device 5. We denote by O the center of this fixed reference by report to which will be defined, in what follows, the positions of the draft 1 and the gripping block 6.

The blank 1 must be positioned very precisely on the blocking 5.

Indeed, it is desired that the optical properties of the finished lens correspond exactly to the prescription of the ophthalmologist.

In particular, the prism and the axis produced between the front face 2 and the face rear 3 must correspond very precisely to the prism and axis adjustment defined during prescription, respectively.

• de manière centrée, c'est-à-dire que le PRP se trouve sur l'axe Z de la bague 19,
• de manière angulairement définie, l'axe A de repérage étant situé dans le plan XOZ formé par les axes X et Z, et
• de telle sorte que la face finie 2 porte simultanément sur les trois pions 31a, 31b, 31c, les points de contact par lesquelles la face finie 2 est en appui sur les pions 31a, 31b, 31c étant pratiquement confondus avec leurs sommets respectifs S₁, S₂, S₃.

To this end, the blank 1 is positioned on the support ring 19:

• in a centered way, that is to say that the PRP is on the Z axis of the ring 19,
• angularly defined, the axis A of location being located in the plane XOZ formed by the axes X and Z, and
• so that the finished face 2 bears simultaneously on the three pins 31a, 31b, 31c, the contact points by which the finished face 2 is supported on the pins 31a, 31b, 31c being practically coincident with their respective vertices S ₁ , S ₂ , S ₃ .

In order to facilitate the positioning of the blank 1 by an operator, the apparatus 5 comprises a camera 34 carried by a bracket 35 fixed on the console 8 so that the camera 34 is plumb and in the Z axis of the support ring 19. The image of the ring 19 formed by the camera 34 is worn on the display screen 9.

As can be seen in Figure 1, it is also shown on the screen of visualization 9 an orthogonal coordinate system formed by two perpendicular axes appearing in broken lines, namely a horizontal axis X1 materializing at screen 9 the X axis and a vertical axis Y1 materializing on screen 9 to the Y axis of the coordinate system fixed XYZ.

Thus, to ensure correct positioning, as defined above, of the blank 1 on the support ring 19, it suffices for the operator to ensure that, on the image provided by the display screen 9, the PRP is confused with the cross of axes X1 and Y1 while axis A of location is confused with the X1 axis.

The locking device 5 further comprises a holding arm 36 having a free end 37 curved, this arm 36 being mounted articulated by relative to the frame 7, between an open position in which its free end 37 is at a distance from the support ring 19 (position shown in lines interrupted in Figure 1) and a so-called closed position in which its free end 37 abuts against the rough face 4 of the blank 1 in now it pressed against the support ring 19 (position shown in solid lines in Figure 1).

Once the blank 1 positioned on the support ring 19, the operator controls the tilting of the holding arm 36 towards its closed position so maintain the positioning of the blank 1 during operations which will follow, for fixing the gripping block 6 to the finished face 2.

As will be seen later, these operations include the orientation of the gripping block 6 and the casting of a metal with a low melting point between the block grip 6 and the finished face 2 of the blank 1.

These operations are coordinated by a control unit 38 comprising a computer 39 into which the values of the adjustments of prism and / or prescription axis which must take into account the orientation of the block of grip 6.

Given the gradually variable curvature of the finished face 2, when the blank 1 is positioned on the support ring 19, the vertices S ₁ , S ₂ , S ₃ forming the support points of the blank 1 are not on the same isohypse line, which generates a tilting of the blank 1 and the subsequent appearance of a so-called positioning prism which will be defined below and whose value, expressed in diopters, depends on the three-dimensional shape of the finished face 2 and of the position of the support points S ₁ , S ₂ , S ₃ .

As will be seen below, the definition of the orientation of the block of grip 6 will take into account very precisely the value of the prism of positioning to compensate for it when the block is actually positioned grip 6, so that the value of the final prism produced on the finished lens is actually equal to the value of the prescription prism (even, and in particular, if this value is zero).

To this end, we define a local orthogonal coordinate system X'Y'Z 'linked to the blank 1, whose axis Z 'coincides with the optical axis of the blank 1, and whose axes X' and Y 'correspond respectively to the projection, in the plane tangent to the face ended at the PRP, from the tracking axis A, and from the vertical meridian passing through the PRP in normal wearing position.

• le PRP, qui est par définition le centre du repère lié X'Y'Z', se trouve sur l'axe Z de la bague 19, ce qui correspond au centrage de l'ébauche 1 sur la bague 19,
• l'axe X' se trouve dans le plan XOZ formé par les axes X et Z, en étant incliné dans ce plan par rapport à l'axe X, et
• l'axe Y' se trouve dans le plan YOZ formé par les axes Y et Z, en étant incliné dans ce plan par rapport à l'axe Y, ce qui résulte du choix de l'orientation angulaire de l'ébauche 1 sur la bague 19.

Once the blank is positioned:

• the PRP, which is by definition the center of the linked coordinate system X'Y'Z ', is located on the Z axis of the ring 19, which corresponds to the centering of the blank 1 on the ring 19,
• the axis X ′ lies in the plane XOZ formed by the axes X and Z, being inclined in this plane relative to the axis X, and
• the Y 'axis is in the YOZ plane formed by the Y and Z axes, being inclined in this plane relative to the Y axis, which results from the choice of the angular orientation of the blank 1 on the ring 19.

We note α the angle formed in the plane XOZ by the axes X and X ', and β the angle formed in the plane YOZ by the axes Y and Y '. The values of the angles α and β define the orientation of the finished face 2 with respect to the fixed coordinate system XYZ and are characteristics of the positioning prism introduced above.

To obtain the values of the angles α and β from the three-dimensional shape of the finished face 2 and the position of the support points S ₁ , S ₂ , S ₃ , it is carried out by iterative calculation, as described below. below.

By convention, we note for any point in space x, y and z its Cartesian coordinates (abscissa, ordinate, depth) in the coordinate system fixes XYZ and x ', y' and z 'its Cartesian coordinates in the linked coordinate system X'Y'Z'.

As we saw previously, the support points S ₁ , S ₂ , S ₃ are located on a circle centered on the axis Z. Let R be the radius of this circle. The position of any point P in the fixed coordinate system XYZ can be expressed in cylindrical coordinates ρ, , z where ρ is the distance from the point to the center O and  the angle made by the vector OP with the X axis.

où i = 1 à 3.Thus, the cylindrical coordinates of the support points S ₁ , S ₂ , S ₃ , can be expressed as follows:

where i = 1 to 3.

   où i=1 à 3.We deduce the Cartesian coordinates of the support points S ₁ , S ₂ , S ₃ :

where i = 1 to 3.

Furthermore, in the linked coordinate system X'Y'Z ', the three-dimensional shape of the finished face 2 is known; it is defined by a determined function f , such that, for a point (x ', y', z ') of the finite face, we have: z '= f ( x ' , y ' ).

In the fixed coordinate system XYZ, the three-dimensional shape of the finite face is, on the other hand, defined by another function f _p such that, for a point (x, y, z) of the finite face, we have: z = f p ( x, y ) where p is the (integer) index of the iteration.

A first step E1 of the calculation consists in superimposing the linked reference frame X'Y'Z 'on the fixed reference frame XYZ. At the same time, the value 1 is assigned to the index p , which means that this is the first iteration of the calculation.

This situation is illustrated in FIG. 11 where, for reasons of convenience, only two diametrically opposed support points S ₂ , S _{3 have been shown,} both situated on the axis X.

A second step E2 of the calculation consists in defining the function f _p . For the first iteration ( i = 1), the fixed coordinate system XYZ and the linked coordinate system being combined, the function f ₁ is identical to the function f: f ₁ = f .

We denote S p / 1, S p / 2, S p / 3 the points of the finished face 2 resulting from the projection, parallel to the axis Z, of the support points S ₁ , S ₂ , S ₃ on the finished face 2. This projection preserving the abscissas and the ordinates, the coordinates of the points S p / 1, S p / 2, S p / 3 are therefore the following:

A third step E3 consists in calculating the depths z _i , i = 1 to 3, of the points S p / 1, S p / 2, S p / 3.

A fourth step E4 consists in calculating the maximum difference ε between the depths z _i of the projected points by means of the following formula: ε = max ( z 1 - z 2 , z 1 - z 3 , z 2 - z 3 ).

A fifth step E5 then consists in comparing the value of this difference ε with a predetermined value ε ₀ , for example equal to 1 micron.

As long as ε> ε _{0 '} we continue the calculation as described below.

Through the projected points S p / 1, S p / 2, S p / 3 passes a single plane A _p whose equation in the fixed frame of reference XYZ can be expressed as follows: z = ax + by + vs.

The coefficients a, b, c can be obtained by solving the system of three linear equations with three unknowns: f p ( x i , there i ) = ax i + by i + vs , i = 1 to 3.

In matrix form, this system is written:

The coefficients a, b, c are obtained by inversion of the previous system:

The intersection lines of the plane A _p respectively form, in the XOZ and YOZ planes, angles α _p and β _p with the axes X and Y. As, by definition: at = tan (α p ) b = tan (β p ) ' we can deduce: α p = Arc tan ( at ) β p = Bow tan (B) .

A sixth step E6 consists in calculating the angles α _p and β _p as provided above.

   et

A seventh step E7 consists in tilting the linked reference frame X'Y'Z '(and, consequently, the finished face 2) relative to the linked reference frame XYZ, so that the axis X' pivots according to the angle α _p relative to the X axis in the XOZ plane and the Y 'axis pivots at the angle β _p relative to the Y axis in the YOZ plane. It is therefore a combination of two rotations whose respective matrices in the fixed frame are the following, by definition:

and

The characteristic matrix R of the combined rotation is defined by the relation R = R 1 × R 2.

As a result of this combined rotation, the plane A _p is parallel to the plane XOY in which the support points S ₁ , S ₂ , S _{3 are located} (FIG. 12).

However, in view of this tilting, the projections S p / 1, S p / 2, S p / 3 of the support points S ₁ , S ₂ , S _{3 are} no longer directly over the latter.

An eighth step E8 therefore consists in incrementing the index p by one unit to launch a new iteration: p becomes p +1.

In this new iteration, the new function f _{p +1} is redefined by calculation, defining in the fixed coordinate system XYZ the three-dimensional shape of the finished tilted face. For this, it suffices to carry out a simple change of reference mark of matrix R.

We then redo all the calculations described above on the basis of this new function f _{p +1} .

As many iterations are carried out as necessary, that is to say steps E2 to E8 are repeated until the value of the difference ε obtained in step E4 is less than the predetermined value ε ₀ in step E5. We denote by N the index of the corresponding iteration.

As soon as ε <ε ₀ , we indeed consider that the orientation of the finished face 2 corresponds to its orientation when it is positioned on the support ring 19. According to this approximation, the single plane A _N passing through the projections S p / 1, S p / 2, S p / 3 is decreed parallel to the XOZ plane passing through the support points S ₁ , S ₂ , S ₃ .

Ultimately, the linked reference frame X'Y'Z 'was tilted according to the angles α and β, equal respectively to the sum of the successive tilting angles α _p and β _p , that is:

We thus have a geometric definition of the prism of positioning. However, the prescribed prism being expressed in diopters, the value of angles α and β cannot be used directly.

To this end, the positioning prism can be defined by two prismatic deviations Pr H and Pr V in the XOZ and YOZ planes, respectively.

These prismatic deviations Pr H and Pr V are defined as follows: Pr H = 100 × tan (( not -1) × ANGH ) Pr V = 100 × tan (( not -1) × AngV ) where n is the refractive index of the material in which the blank is produced, and where AngH and AngV are the angles, with respect to the X and Y axes, of the projections of the normal to the finished face at PRP in the XOZ planes and YOZ, respectively.

   où : L = 1 + ∂f N ∂x 2 x=0,y=0 + ∂f N ∂y 2 x=0,y=0 Mathematically, the angles AngH and AngV are defined as follows:

or : The = 1 + ∂ f NOT ∂ x 2 x = 0, there = 0 + ∂ f NOT ∂ there 2 x = 0, there = 0

As we will have understood, ∂ f NOT ∂ x x = 0, there = 0 and ∂ f NOT ∂ there x = 0, are the partial derivatives, at PRP, of the function f _N defining the finite face 2 in the last iteration.

A ninth step E9 consists in calculating the angles AngH and AngV of the positioning prism.

The prescription prism is defined by the prismatic deviations Pr H ₀ and Pr V ₀ defined as follows: Pr H 0 = 0 Pr V 0 = K × add where add is the addition of power of the ophthalmic lens that one wishes to obtain, and K an index of proportionality, generally equal to 2/3.

Using formulas (1) and (2) above, we can characterize the prescription prism by the angles AngH ₀ and AngV ₀ defined as follows: ANGH 0 = 0

We can deduce from the above the geometric angular difference between the prescription prism and positioning prism.

We define this angular deviation by two angles, noted respectively γ and  and defined as follows:

These angles γ and  in fact make it possible to define, in the fixed coordinate system XYZ, the orientation of the support shaft 14 - or, which amounts to the same thing, the orientation of the gripping block 6 - to compensate for the positioning prism, γ being defined as the angle formed by the axis Z "of the support shaft 14 with the axis Z, while  is defined as the angle that forms with respect to the X axis the projection of the axis Z "of the support shaft 14 in the plane XOY.

A tenth step E10 consists in calculating the angles γ and .

Steps E1 to E10 described above for defining the orientation of the gripping block 6, which are gathered in the diagram of FIG. 14, can be programmed, in the form of a calculation algorithm, in the control unit 39 computer 38.

Before describing as a whole the process followed to pose on the blank 1 the gripping block 6, let's give some additional details concerning the production of the support ring 19.

Note that, on the console 8, the ring 19 is positioned so that the axis X is horizontal, with the support face 25 facing upwards.

According to a first embodiment, illustrated by FIGS. 3a and 3b, two support rings 19.1, 19.2 are provided, depending on whether one wishes to install a gripping block 6 on a blank intended for a left eye or on a blank for a right eye. These rings 19.1, 19.2 differ from one of the other by the location of their pawns 31a, 31b, 31c.

Each of these rings 19.1, 19.2 is, with the exception of the seal 28, made entirely in steel, the pins 31a, 31b, 31c preferably being made of hardened steel.

Each head 33 has a diameter between 1.5 and 3 mm. In practical, this diameter will preferably be chosen equal to 2 mm.

The average radius of curvature of the finished face 2 being generally between 100 and 150 mm, the diameter of the heads 33 is much less than this radius, so that the approximation, made above, according to which the points of support of the finished face 2 against the pins 31a, 31b, 31c, are more or less confused with the vertices S ₁ , S ₂ , S ₃ , is justified.

Conventionally, the diameter of the edge 4 of a semi-finished blank of ophthalmic lens is 65 mm.

The diameter of the circle circumscribed by the triangle defined by the vertices S ₁ , S ₂ , S ₃ of the pins 31a, 31b, 31c will therefore be chosen less than 65 mm, for example between 50 and 60 mm.

Preferably, the diameter of the circumscribed circle is equal to 55 mm, which is large enough, relative to the diameter of the blank 1, to guarantee perfect stability of the latter, but also small enough to overcome variations in depth of the PRP when moving from one draft to another.

As a result, the depth of the PRP, i.e., in practice, its distance from the support shaft 14 remains more or less constant from one blank to another; in all case, it remains included in a range of values for which one is assured that the blank will not strike the support 14, and for which the fixing of the shaft support 14 at the preform will be rigid enough to absorb the engine torque and the machining torque when finishing the rough face 3.

One of the support rings 19.1, 19.2 is shown in Figure 3a. he in this case it is a ring 19.1 intended for the positioning of a blank 1 intended for a left eye.

As we saw above in the description of the calculation of the orientation of the gripping block 6, the location of the vertices S ₁ , S ₂ , S ₃ on the ring 19 can be defined, relative to the fixed reference, by their cylindrical coordinates. Their depth being zero since by definition of the fixed coordinate system the vertices are located in the plane XOY, their coordinates are reduced to ρ _i and  _i where i = 1 to 3.

Whatever i is, ρ _i is equal to the radius of the circle circumscribed to the triangle formed by the vertices, of which a range of values has been given above. Thus, whatever i, ρ _i is between 25 and 30 mm, preferably equal to 22.5 mm.

The first pin 31a is located angularly close to the channel while that the second 31b and the third 31c are angularly relatively distant, without however being diametrically opposed to it.

Furthermore, their location is such that the angular opening between the pawns 31a, 31b, 31c, taken two by two, is always greater than 90 °.

Thus, the angular coordinate  ₁ of the first vertex S ₁ is between 95 ° and 105 °, preferably equal to 100 °. In other words, the angle formed by the vector OS ₁ with the axis Y is between 5 ° and 15 °, preferably equal to 10 ° (Figure 5).

The angular coordinate  ₂ of the second vertex S ₂ is, for its part, between 195 ° and 205 °, preferably equal to 200 °. In other words, the angle formed by the vector OS ₂ with the axis X is between 15 ° and 25 °, preferably equal to 20 ° (Figure 5).

Finally, the angular coordinate  ₃ of the third vertex S ₃ , equal in absolute value to the angle formed by the vector OS ₃ with the axis X, is between -15 ° and -25 °, preferably equal to -20 ° (figure 5).

This means that the values of the angles at the vertices of the triangle S ₁ S ₂ S ₃ , i.e. the angles (S ₁ S ₂ , S ₁ S ₃ ), (S ₂ S ₁ , S ₂ S ₃ ) , (S ₃ S ₂ , S ₃ S ₁ ), are respectively between 60 ° and 80 °, between 50 ° and 70 °, and between 40 ° and 60 °.

From the operator's point of view when ring 19.1 is located positioned on the console 8, which corresponds to the orientation presented on the Figure 5, the first pin 31a is located on the left of channel 26.

The other support ring 19.2, provided for positioning a blank intended for a right eye, is shown in FIG. 4.

This ring 19.2 can be deduced from the ring 19.1 which has just been described by a plane symmetry with respect to the YOZ plane.

Thus, compared to the previous ring 19.1, only the angular coordinate  ₁ of the first vertex S ₁ changes, here between 75 ° and 85 °, preferably equal to 80 °. The angle formed by the vector OS ₁ with the Y axis is always between 5 ° and 15 °, preferably equal to 10 °.

From the operator's point of view when ring 19.2 is located positioned on the console 8, the first pin 19a is therefore located on the right of the channel 26.

According to a second embodiment, a single ring 19.3, illustrated by FIG. 4, is provided, this ring being adapted to receive, indifferently, a blank intended for a left eye or a blank intended for a right eye.

This ring 19.3 has all the characteristics of the rings 19.1, 19.2 described above, except the positioning of the vertices S ₁ , S ₂ , S ₃ , that is to say of the pins 31a, 31b, 31c. Their common elements, of course, have the same references.

The first pin 31a is located here in the axis of the channel 26 while being diametrically opposite to the latter. As a result, the vertex S ₁ is on the axis Y, as shown in FIG. 4.

Thus, the angular coordinate  ₁ of the first vertex S ₁ is here equal, or substantially equal, to 270 °. As for the vertices S ₂ and S ₃ , that is to say the pins 31b and 31c, they are located on the other side of the axis X, relative to the first pin 31a.

In other words, the angle formed by the vector OS ₁ and the axis Y is zero, or almost zero (that is to say less than 5 °).

Their angular coordinates  ₂ ,  ₃ are preferably respectively equal to 160 ° and 20 °, but they can be understood, respectively, between 155 ° and 165 °, and between 15 ° and 25 °.

In other words, the angle formed by the vector OS ₂ with the X axis is between -15 ° and -25 °, preferably equal to -20 °, while the angle formed by the vector OS ₃ with the axis X is, for its part, between 15 ° and 25 °, preferably equal to 20 °.

Whichever ring 19.1, 19.2, 19.3 is used, when a blank 1 positioned correctly on the ring, the apex S ₁ of the first pin 31a comes into contact with a point on the finished face 2 located in the vision zone closely VP, while the vertices S ₂ and S ₃ of the second and third pins 31b, 31c come into contact with points of the finished face 2 each located in a transition zone between the far vision zone VL and the zone near vision VP, while being closer to the far vision area VL.

To place the gripping block 6 on a semi-finished blank 1, we proceed as described below. It is assumed that a gripping block 6 is is correctly placed in the housing 15 of the support shaft 14, and that the support ring 19, chosen according to the type of blank (left or right eye) on which one wishes to place the gripping block 6, is correctly positioned and fixed on the upper plate 11.

A first operation F1 consists in introducing into the computer 39 the predetermined function f defining the three-dimensional shape of the finished face 2 of the blank 1.

A second operation F2 consists in introducing into the control unit 38, that is to say into its computer 39, the cylindrical or Cartesian coordinates of the vertices S ₁ , S ₂ , S ₃ . This operation is optional at this stage insofar as these coordinates can just as well have been memorized beforehand in order to allow their re-use. The diagram in Figure 13 takes this possibility into account.

A third operation F3 consists in defining the orientation of the tree support 14. This operation is carried out by the computer 39 according to the method in 10 steps E1 to E10 described above.

A fourth operation F4 consists in positioning the support shaft 14 following the orientation defined above during the third operation F3. This positioning is controlled by the control unit 39.

A fifth operation F5 consists in positioning and fixing the blank 1 on the support ring 19 respecting the centering and the angular orientation defined above.

The blank 1 is held on the support ring 19 by the arm of support 36. In this position, the finished face 2 is in contact with the lip 29 of the seal 28, as shown in FIG. 7, so that a sealing between the seal 28 and the finished face 2, except of course to the right of the channel 26.

It is thus defined, between the finished face 2 and the gripping block 6 opposite, a mold cavity delimited by the finished face 2, the lip 29 of the seal 28, the seat 23, the bore 24 and the gripping block 6.

A sixth operation F6 consists in fixing the gripping block 6 on the finished face 2 of the blank 1.

This operation consists in pouring through channel 26 a metal with a low point of fusion in the cavity 40. The channel 26 overhanging the cavity 40 (which results from the orientation of the support ring 19 and the tilt of the console 8), the gravity facilitates this operation.

To this end, the apparatus comprises a reservoir 41 connected to the cavity 40 by a flexible 42. The control unit 39 controls the supply of metal to the cavity 40 from tank 41.

The metal is then cooled. You can also let it cool, which is however longer.

The order of operations F1 to F6 described above is indicative. Some operations can be moved. In particular, operation F5 of positioning of the blank 1 can equally well be carried out first.

After placing the holding arm 36 in its open position, it suffices then remove from the apparatus 5 the now rigid assembly 43 formed by the blank 1, the gripping block 6 and the interface 44 made of low point metal fusion. In order to facilitate this removal, the bore 24 of the ring 19 can be produced slightly undercut, as shown in Figure 7.

Given the presence of channel 26 on the ring 19, it persists on the set 43 a metal core.

The fact that the ring 19 is truncated, as we saw above, allows to minimize the length of channel 26, and therefore the length of this carrot, and save the low-melting metal, which turns out to be a expensive commodity.

The definition of the orientation of the gripping block 6 taking into account the exact three-dimensional shape of the finished face 2, it is understood that the same ring 19 is suitable for receiving the whole range of semi-finished blanks whatever the type to which the finished face belongs.

Also, although the above description was made for a finished face 2 with gradually variable curvature, is the same ring 19 suitable for all other types of finished faces, in particular spherical finished faces, aspherical, toric, or atoric.

Claims (25)

1. A method of fitting a holding block (6) to a semifinished ophthalmic lens blank (1) intended to have a predetermined prism, which method includes the following steps:

   positioning the blank (1) on a fixed base (19), in a centered and angularly defined manner, so that the finished face (2) of the blank (1) bears conjointly on a plurality of bearing points (S₁, S₂, S₃) of said base (19),

   defining an orientation of the holding block (6) relative to the blank (1),

   orienting the holding block (6) in the defined manner, and

   fixing the holding block (6) to the finished face (2) while maintaining orientation,

      characterized in that the step of defining the orientation of the holding block (6) includes the following steps:

   taking account of the three-dimensional shape of the finished face (2) and the position of said bearing points (S₁, S₂, S₃),

   deducing therefrom the orientation of the finished face (2) when the blank (1) is positioned on the base (19),

   taking account of a predetermined prism, and

   deducing from the orientation of the finished face (2) and the predetermined prism the orientation of the holding block (6) relative to the finished face.
2. A method according to claim 1, characterized in that, to orient the finished face (2) when the blank (1) is positioned on the base (19), a positioning prism resulting from tilting of the blank (1) when it is placed on the base is calculated.
3. A method according to claim 2, characterized in that, to define the orientation of the holding block (6), two angles γ and  are calculated defined by the following equations:

   

   

   in which:

   AngH and AngV are defined as follows:

   

   

   where f_N is a function of the type z= f_N(x,y) defining the shape of the finished face (2) in a system of axes XYZ fixed relative to the base (19) and x,y,z are the cartesian coordinates linked respectively to the axes X, Y and Z of said fixed system of axes, L being defined by the following formula: L = 1 + ∂f N ∂x 2 x=0,y=0 + ∂f N ∂y 2 x=0,y=0

   AngV ₀ is defined as follows:

   

   Pr V ₀ being defined as follows: Pr V 0 =K × add where add is the power addition of the ophthalmic lens to be obtained and K is an index of proportionality preferably equal to 2 / 3.
4. A method according to claim 3, characterized in that three bearing points (S₁, S₂, S₃) are provided on the base (19) and in that the function f_N is obtained by repeating the following succession of steps:

   calculating a function f_p defining the three-dimensional shape of the finished face (2) in the fixed system of axes XYZ,

   calculating the depths z _i tied to the axis Z of the fixed system of axes XYZ of the projections of the bearing points (S₁, S₂, S₃) onto the finished face (2) in the direction of the axis Z by means of the following formula: z_i = f_p(x_i,y_i) where, for each bearing point (S_i), x_i and y_i are its coordinates respectively tied to the axis X and the axis Y of the fixed system of axes XYZ,

   calculating the maximum difference ε between the depths z_i ,

   comparing the difference ε with a predetermined value ε₀,

   calculating the angles α_p and β _p defined by the following equations: α p = Arc tan(a) βp = Arc tan(b)    where a and b are the director coefficients of the plane A_p passing through the projections of the bearing points (S₁, S₂, S₃) onto the finished face (2),

   tilting the finished face (2) through two rotations with a first rotation through an angle α _p in the plane X, Z and a second rotation through an angle β_p in the plane Y, Z,

   incrementing p by one unit,
      for as long as the difference ε is greater than the predetermined value ε₀,
      where:

   i is an integer from 1 to 3,

   p is an integer initially equal to 1, with f 1 = f    where f is a predetermined function of the type z'= f(x',y') defining the three-dimensional shape of the finished face (2) in an orthogonal system of axes X'Y'Z' tied to the finished face (2), x',y',z' being the cartesian coordinates respectively tied to the axes X', Y', Z' of the tied system of axes X' Y' Z',

   N is the value of p when the difference ε becomes less than the predetermined value ε₀.
5. A method according to claim 4, characterized in that the difference ε is defined as follows: ε = max( z 1 - z 2 , z 1 - z 3 , z 2 - z 3 ).
6. A method according to claim 4 or claim 5, characterized in that, the plane A_p being defined in the fixed system of axes XYZ by the equation: z = ax + by + c, the coefficients a and b are defined as follows:

   
7. A method according to any of claims 3 to 6, characterized in that the holding block (6), which has an axis Z", is oriented so that:

   the angle between its axis Z" and the axis Z of the fixed system of axes XYZ is equal to the angle γ, and

   the angle between the projection of its axis Z" in the plane formed by the axes X, Y of the fixed system of axes XYZ and the axis X of that fixed system of axes is equal to the angle .
8. A method according to any preceding claim, characterized in that the holding block (6) is fixed to the finished face (2) by pouring a low melting point metal into a cavity (40) formed between the finished face (2) and the holding block (6) and cooling the metal or allowing it to cool.
9. Blocking apparatus (5) for fitting a holding block (6) to a semifinished ophthalmic lens blank (1), which apparatus includes:

   a fixed base (19) for positioning the semifinished blank (1),

   means (34, 9) for centering and orienting in a defined manner the blank (1) relative to the support (19),

   means (36) for retaining the blank (1) on the base (19),and

   means (41, 42, 44) for fixing the holding block (6) to the finished face (2),

      characterized in that it includes:

   means (39) for defining the orientation of the holding block (6) as a function of the three-dimensional shape of the finished face (2), and

   means (10) for varying the orientation of the holding block (6) relative to the base (19) as a function of the defined orientation.
10. Blocking apparatus (5) according to claim 9, characterized in that the means (39) for defining the orientation of the holding block include a calculator.
11. A bearing ring for positioning a semifinished ophthalmic lens blank (1) on blocking apparatus (5) for the purpose of fitting to the finished face (2) of the blank (1) a holding block (6), the ring (19) including a plurality of bearing points (S₁, S₂, S₃) against which the finished face (2) of the blank (1) is pressed, characterized in that the bearing points (S₁, S₂, S₃) are each on a spherical surface (33) whose diameter is small compared to the radius of curvature of the finished face (2) of the blank (1).
12. A ring according to claim 11, characterized in that the diameter of said spherical surface (33) is from 1.5 mm to 3 mm.
13. A ring according to claim 12, characterized in that the diameter of said spherical surface (33) is equal to 2 mm.
14. A ring according to any of claims 11 to 13, characterized in that each spherical surface (33) is part of a projecting peg (31a, 31b, 31c).
15. A ring according to claim 14, characterized in that the peg (31a, 31b, 31c) is an add-on.
16. A ring according to claim 14 or claim 15, characterized in that it includes three pegs (31a, 31b, 31c).
17. A ring according to claim 16, characterized in that the ring is globally circularly symmetrical about an axis Z and the summits of the pegs are in a common plane perpendicular to the axis Z.
18. A ring according to claim 17, characterized in that the pegs are at the vertices of a triangle whose circumscribed circle is centered on the axis Z.
19. A ring according to claim 18, characterized in that said circumscribed circle has a diameter from 50 to 60 mm.
20. A ring according to claim 19, characterized in that said circumscribed circle has a diameter equal to 55 mm.
21. A ring according to any of claims 18 to 20, characterized in that the angles at the vertices of said triangle are respectively from 60° to 80°, from 50° to 70°, and from 40° to 60°.
22. A ring according to any of claims 16 to 21, characterized in that it has a recessed channel (26) extending along a radial axis.
23. A ring according to claim 22, characterized in that one of the pegs (31a) is near the channel (26).
24. A ring according to claim 22, characterized in that the peg (31a) near the channel (26) is offset angularly relative thereto by an angle from 5° to 15° and preferably equal to 10°.
25. A ring according to claim 22, characterized in that one of the pegs (31a) is diametrically opposite and on the axis of the passage (26).

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