Automatic machine for grinding and bevelling ophthalmic glasses

The invention provides a machine for grinding and bevelling ophthalmic glasses of the type comprising a carriage (1) mounted to be movable in translation and in oscillation in a direction parallel with and perpendicular to a shaft (5) carrying at least one grinding wheel (4, 4A), and means for measuring the distance (L1) between the plane (PO) tangent to the pole (0) of the convex face of the glass (3) and the edge of the convex face, characterized in that the measuring means comprise means defining a reference surface (S) which is connected to move axially in translation with the grinding wheel, extends in a radial plane perpendicular to the axis of rotation of the grinding wheel and with which at least one point of the edge (A) of the convex face of the glass (3) is brought into contact by a movement in translation of the carriage (1), a sensor (18) for measuring the movements in translation of the carriage (1) relative to the grinding wheel (4) and means (12) for analyzing and processing signals from the sensor (18) for detecting the contact of the edge of the glass (3) with the reference surface (S) and deducing therefrom the distance (L1).

BACKGROUND OF THE INVENTION 
The present invention relates to an automatic machine for grinding and 
bevelling glasses of spectacles. 
Conventional machines for grinding and bevelling ophthalmic glasses 
comprise a U-shaped carriage which is slidably and pivotally mounted on a 
fixed horizontal shaft, a second horizontal shaft rotatively mounted in a 
perpendicular manner between the branches of the U-shaped carriage and 
consisting of two parts between which the glass to be machined is gripped, 
and a third rotatable horizontal shaft carrying at least one grinding 
wheel, and more particularly edge-trimming and bevelling grinding wheels. 
When an optical glass blank having a thick edge surface has been cut to the 
shape of the frame, the edge surface of the glass has a cylindrical shape 
and a bevelling operation must be carried out by guiding the glass so as 
to form a V-sectioned bevel on the edge surface of the glass which must be 
located between the edges of its periphery, said projecting bevel being 
adapted to engage in the groove of the rim of the frame. 
Machining such a bevel with manual control means is extremely difficult. 
It has already been proposed to employ a "free bevel" system in which the 
edge surface of the blank is brought into a V-sectioned groove of a 
grinding wheel and the shaft carrying the glass is left free to move in 
translation during the grinding operation. 
However, this system does not give a fully satisfactory result. 
It is indeed desirable to construct a device which 
an permits obtaining a guided bevel on the edge surface of ophthalmic glass 
automatically and with high precision. 
Document EP-A-0281,480 proposed for this purpose a machine for grinding and 
bevelling ophthalmic glasses of the aforementioned type which comprises a 
follower adapted to remain in contact with the edge surface of a shaped 
blank, in a position perpendicular to said edge surface, and adapted to 
measure the distance L1 from the edge of the front face of the glass 
relative to that of a flat glass in which L1=0. 
This solution is not fully satisfactory, since the construction of the 
sensor, which is a mechanical sensor of complex and costly structure, is 
difficult to achieve. Further, the follower is of fragile construction and 
the contact of the latter with the edge surface of the glass results in a 
wear of the follower; moreover, the precision of the data obtained is too 
closely dependent on the construction of the follower. 
It has already been proposed in document DE-A-3,842,601 to use a detecting 
fork each of the two teeth of which extends on one of the two sides of the 
blank grinding wheel and against which the front convex face and rear 
concave face of the glass to be ground abut. 
The arrangement described in this document is particularly complex and 
costly and the two branches of the detecting fork, which must be 
resiliently deformable, are particularly fragile. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a grinding machine in 
which the means for measuring the distance L1 between the plane tangent to 
the pole of the convex face of the glass and the edge of the face are 
particularly simple to use, cheap and strong. 
The invention therefore provides a grinding machine of the aforementioned 
type, characterized in that the measuring means comprise means defining a 
reference surface which is connected to move in axial translation with the 
grinding wheel, extends in a radial plane perpendicular to the axis of 
rotation of the grinding wheel and with which at least one point of the 
convex face of the glass is brought into contact by translation of the 
carriage, a sensor for measuring the movements in translation of the 
carriage relative to the grinding wheel, and means for analyzing and 
processing signals delivered by the sensor for detecting the contact of 
the edge of the glass with said reference surface and deduce therefrom 
said distance L1. 
The arrangement according to the invention is therefore particularly simple 
and the movement sensor is in fact a sensor of the movements in 
translation of the carriage of conventional design and is disposed in a 
region remote from the grinding region. 
According to other features of the invention: 
the reference surface is an annular radial reference face of an element 
connected to move in translation with the grinding wheel; 
the grinding wheel comprises two adjacent grinding wheel sections, namely a 
roughing section and a finishing section, and the annular reference face 
is provided in the connection zone between the two sections of the 
grinding wheel; 
the annular reference surface is defined by the radial face of a shoulder 
on the grinding wheel; 
the reference surface is defined by the annular peripheral portion of one 
of the two opposite faces of a washer which bears against a radial face of 
the grinding wheel; 
the washer is provided between the adjacent radial faces of the two 
grinding wheel sections; 
the washer is connected to rotate with the grinding wheel; 
the washer is freely rotatively mounted on the driving shaft of the 
grinding wheel; 
the machine comprises a sensor for determining the angular position of the 
glass about an axis of a shaft of the carriage which carries the glass so 
as to measure said distance L1 for at least one given point of the edge of 
the convex face of the glass; 
said distance L1 is measured in succession for four measuring points 
angularly spaced substantially 90.degree. apart.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference to FIGS. 1 and 2, a conventional grinding and bevelling 
machine comprises a U-shaped carriage 1 which is pivotally mounted on a 
horizontal shaft (not shown) and between the branches of which is 
rotatively mounted, and driven in rotation by a motor M, a second 
horizontal shaft 2 in two parts between which is gripped a blank 3 of an 
ophthalmic glass the contour of which is ground by a grinding wheel 4 
which is rotatively mounted and driven in rotation by a third horizontal 
shaft 5 driven in rotation by a motor M'. 
In the known manner, the machine also includes means 6, such as a sensor 
having a code wheel, which gives at each instant the angular position of 
the shaft 2, and therefore the angular position of the glass 3, about the 
axis of rotation of the shaft 2. 
According to the invention, the machine comprises means for measuring the 
distance L1 between a reference plane PO, which is tangent to the pole O 
of the convex spherical face of the glass, and the edge A of the convex 
face of the glass. 
These measuring means comprise a sensor 18, diagrammatically represented in 
FIG. 1 in the form of an electric rheostatic potentiometer, which permits 
measuring the movements in translation of the carriage 1, and consequently 
of the glass 3, relative to the frame of the machine and therefore 
relative to the grinding wheel. 
The measuring means further comprise a reference surface S which, in the 
embodiment illustrated in the Figures, is defined by the annular face of a 
radial shoulder on the grinding wheel which separates the roughing section 
4 from the finishing section 4A of the grinding wheel which extend axially 
on opposite sides of the surface S, respectively to the left and to the 
right as viewed in FIG. 1. 
In a known design, the grinding wheel is most commonly constructed in the 
form of two adjacent and distinct grinding wheel sections which are 
mounted on a common driving shaft 5. 
The radial dimension of the annular reference surface, which results from 
the difference between the diameters of the two adjacent faces of the two 
sections of the grinding wheel, is small and for example on the order of 1 
to 2 mm. 
The machine further comprises electronic means for analyzing and processing 
signals, diagrammatically illustrated in the form of a block 12 which 
comprises in particular analog-to-digital converters 13 and 14 and 
calculating means 15 which form from data delivered by the sensors 6 and 
18, as will be explained hereinafter, signals controlling the movements of 
the carriage 1 which are employed when bevelling the edge surface on the 
finishing section 4A of the grinding wheel. 
In the case of numerically or digitally controlled grinding machines which 
do not employ a shape template and in which this template is replaced by a 
circular disc concentric with the shaft 2, these machines operate from a 
register of memorized shapes which permits the automatic control of the 
movement key 17. 
With reference to FIGS. 3 and 4, it can be seen that the edge surface of 
the glass may be represented in the form of two substantially parallel 
lateral edges A and B constituted by series of points a1-b1, a2-b2, ai-bi, 
etc. corresponding to different angular positions of the glass. An 
intermediate line C represents the trace of the free edge of the bevel 9 
in relief formed on the edge surface 8 of the glass. 
When it is desired to measure the distance L1, the control means of the 
machine cause the shaft 2 to stop rotating, then the carriage I to move in 
translation, from the left toward the right as viewed in FIG. 1, so as to 
cause the peripheral edge of tile convex face of the glass to bear against 
the annular reference surface S. 
In the course of this movement in translation, the movement sensor 18 
transmits signals to the unit 12 which, on one hand, deduces therefrom, 
when the movement in translation ceases, that the peripheral edge A of the 
convex face is bearing against the radial shoulder or surface S and, on 
the other hand, calculates the distance L1. 
The measuring operation may be repeated for example at four points a1, a2, 
a3 and a4 of the edge of the front face of the glass which are for example 
angularly evenly spaced 90.degree. apart, as illustrated in FIG. 2, and 
substantially correspond to the four corners of the glass. 
The machine according to the invention therefore employs a process for 
measuring the distance L1 which merely requires putting the convex face of 
the glass in contact with the reference surface S. 
Inasmuch as the bevel 9 has a small width "1", usually less than 3 mm, it 
is certain that it will always be located within the thickness ai, bi. 
As can be seen from FIG. 2, and owing to the design of the invention, the 
contact point of the peripheral edge of the convex front face is located 
on the same radius as the grinding point PM and it is therefore 
unnecessary to resort to calculated corrections as is required in some 
devices of the prior art. 
When the radial shoulder constituting the reference surface S is 
constituted by a shoulder on the grinding wheel itself, and bearing in 
mind that the grinding wheel rotates at high speed, it is desirable to 
cover this shoulder with a non-abrasive material. 
However, in the case where the surface S itself is abrasive, a very slight 
marking is produced on the edge of the convex face of the glass, but this 
marking occurs in an extra thick region which will disappear in the 
finishing operation forming the bevel. 
Subsequent to the measuring operation, the calculating means 12 compare the 
position of the measured points ai with the theoretical position of the 
programmed and memorized bevel. If there is no concordance, the 
calculating means 12 modify the parameters of the control of the grinding 
machine so as to correct this lack of concordance. 
This correction is carried out by causing the theoretical sphere to rotate 
in space. If after this correction the concordance with the programmed 
bevel is not attained, a different theoretical sphere is employed. 
It will be observed that the operation for measuring the distance L1 
requires neither stopping the rotation of the grinding wheel nor 
temporarily stopping the spraying. 
In an alternative embodiment, the reference surface S may be defined by one 
of the two opposite radial faces of a reference washer interposed between 
the two sections of the grinding wheel and mounted on the driving shaft 5 
of the latter. 
The washer may be driven in rotation with the grinding wheel or comprise 
means, such as for example a rolling bearing, so that it may be freely 
rotatively mounted on the shaft 5 so as to avoid the phenomena marking the 
edge of the convex outer face of the glass 3. 
If the washer is driven in rotation with the grinding wheel, it is also 
possible to cover the radial reference surface S with a material having a 
very low coefficient of friction. 
The measurement may be carried out in any number of points by stopping or 
not stopping the rotation of the glass.