System of polishing external pieces for a timepiece

A system of polishing a concave surface of an external piece for a timepiece, including a securing device including a support that carries the piece, and a grinding device including an abrasive mechanism rotatably mounted along a first axis and configured to polish the piece along a first curvature. The securing device further includes a moving mechanism of the support so that the support imparts a back-and-forth motion along a second axis and a contact surface of the abrasive mechanism is curved to polish the piece along a second curvature in addition to the first curvature. The system can be applied to the field of crystals for a timepiece.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a National Phase Application in the United States of International patent application PCT/EP2012/052743 filed on Feb. 17, 2012 which claims priority on European patent application No. 11158458.7 filed Mar. 16, 2011. The entire disclosures of the above patent applications are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a system of polishing an external piece for a timepiece and in particular the concave surface of a piece of this type.

BACKGROUND OF THE INVENTION

It is known to form sapphire watch crystals which are highly scratch resistant. These crystals are generally manufactured by placing a rotating grinding wheel in contact against the surface of a drum carrying several crystals. The resulting grinding operation enables a cylindrical or spherical crystal to be formed. However, it becomes necessary to form asymmetrical crystals to be fitted, for example, to timepiece displays which are not centred with respect to the timepiece case, which is not possible using current series manufacturing techniques.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome all of part of the aforecited drawbacks by proposing a piece-by-piece polishing system which does not damage the blanks and allows improved polishing of complex concave surfaces with a very low reject rate.

The invention therefore relates to a system of polishing a concave surface of an external piece for a timepiece comprising a securing device including a support which carries said piece, a grinding device including abrasive means rotatably mounted on a first axis and intended to polish said piece along a first curvature, characterized in that the securing device further includes moving means of the support so that the support imparts a back-and-forth motion along a second axis and in that the contact surface of the abrasive means is curved to polish said piece along a second curvature in addition to the first curvature.

It is thus clear that the polishing is carried out piece-by-piece via the contact of the piece against the abrasive means. The back-and-forth motion of the support thus forces the piece to follow the curved contact surface of the abrasive means. Polishing is thus performed by the movement of the pieces one-by-one against the abrasive means, rotatably mounted along a fixed axis, which provides a very low reject rate by avoiding damage to the rough concave surface before polishing.

In accordance with other advantageous features of the invention:the moving means is formed by a rotating actuator driving a crank-shaft which is connected off-centre to a connecting rod integral with said second axis to form said back-and-forth motion;the moving means is mounted on a set of selectively movable carriages so as to compel the piece to be polished to exert a force against said abrasive means;said first axis and said second axis are substantially perpendicular;the abrasive means is formed by a grinding wheel;the piece is formed from crystallised alumina.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention relates to an external piece for a timepiece such as a crystal, case or dial made of crystallised alumina-based material, such as sapphire, corundum or ruby. The invention relates to new manufacturing systems for producing blanks and then polishing pieces of complex shape. Naturally, although the invention was developed for the field of horology, it is not limited thereto. Other applications may also be envisaged such as optics, tableware or electronics.

As illustrated inFIG. 1, a manufacturing system1has been developed in order to produce external pieces3comprising surfaces whose curvatures C1, C2are concave. Manufacturing system1includes a securing device5and a machining device7.

Securing device5includes a drum11, rotatably mounted along a first axis A1, and carrying at least one blank3′ of the future piece3. Preferably, as seen inFIG. 1, drum11is a ring comprising a facetted inner wall, i.e. provided with successive planes P. As illustrated inFIG. 1, each successive plane Pxreceives a blank3′ which may be secured, for example, by bonding.

Machining device7includes an abrasive means13which is rotatably mounted along a second axis A2and which is intended to machine each blank3′. Preferably, abrasive means13is moved in the hollow of ring-shaped drum11. The abrasive means13shown inFIG. 1is formed by a conventional grinding wheel, i.e. whose contact area does not have any particular shape. Of course, abrasive means13may be different and, for example, take the form of a curved or conical sabot.

Advantageously according to the invention, the machining device7includes moving means15of the second axis A2so that said device is movably mounted along a curved directrix to selectively form a second curvature C2in each blank3′. It is thus clear that manufacturing system1can form first and second concave curvatures C1, C2.

According to the invention, moving means15may, in a non-limiting manner, be formed by an actuator moved back-and-forth against the profile of a fixed cam corresponding to the second curvature C2or, for example, an automated device programmed to move along said second curvature.

Thus, the first curvature C1is generated perpendicular to axis Alby the radius extending between axis Aland the contact area between abrasive means13and each blank3′. Since drum11is moved in rotation along axis A1, each blank3′ is thus hollowed out transversely along a single radius forming the first concave curvature C1.

Moreover, the second curvature C2is directly obtained by selectively moving the second axis A2. Thus while the first curvature C1is being generated, the contact area between abrasive means13and each blank3′ is gradually moved relative to the thickness of ring-shaped drum11. Consequently, each blank3′ is hollowed out longitudinally along a curved directrix forming the second concave curvature C2.

It is therefore immediately clear that the curved directrix of moving means15may or may not be symmetrical in order to form the second curvature C2on one or several radii. By way of example, it is possible to start with a blank3″ comprising a top face12and a bottom face14shown inFIG. 3. After modification by manufacturing system1, the resulting blank3′ may then include one of these faces12,14with a transverse concave surface comprising a curvature C1and a longitudinal concave surface comprising a curvature C2.

Finally, preferably according to the invention, the first axis Aland the second axis A2are perpendicular so that the machining lines intersect. This feature advantageously facilitates the subsequent polishing of external pieces3.

Polishing a concave surface like that formed from curvatures C1and C2was attempted with tools similar to the system of manufacturing system1, i.e. mainly by replacing the type of abrasive means. However, this attempt did not provide satisfaction, since this type of polishing caused a deformation of curvatures C1, C2, particularly on the edges of the blanks3′ to be polished and consequently resulted in too high a reject rate.

Consequently, a manufacturing system21was developed for pieces of the type3′ illustrated inFIG. 4, namely comprising surfaces whose curvatures C1, C2are concave. As illustrated inFIG. 2, the manufacturing system21includes a securing device25and a grinding device27.

Grinding device27includes an abrasive means33rotatably mounted along an axis A3and designed to polish each piece3′ along a first curvature C1. Preferably according to the invention, the contact surface of abrasive means33includes a curved surface for polishing piece3′ along a second curvature C2in addition to said first curvature C1. The abrasive means33shown inFIG. 2is preferably formed by a disc whose grinding surface includes a convex surface, for example made of metal, which is regularly coated with a polishing liquid.

Securing device25includes a support31which carries the piece3′ to be polished. Preferably according to the invention, the securing device25further includes moving means35of the support31to impart a back-and-forth motion along an axis A4. It should be noted inFIG. 2that axis A4is substantially perpendicular relative to the axis A3of rotation of abrasive means33.

Thus, moving means35enables the blank3′ of piece3to be both pressed and to move in order to force the friction of abrasive means33against each blank3′ in order to polish said pieces selectively along second curvature C2. It is thus clear that polishing system21can polish first and second concave curvatures C1, C2.

According to the invention, moving means35enables blank3′ of piece3to be pressed and moved relative to abrasive means33. The moving means35will be better understood with reference toFIGS. 5 to 8.

Preferably according to the invention, the moving means35is formed by a rotating actuator32driving a substantially discoid crank-shaft34which is connected off-centre to a connecting rod36integral with axis A4in order to form the desired back-and-forth motion B.

The connecting rod36in the example illustrated inFIGS. 5 to 8includes two arbours37and38. Arbour37thus connects the stud39of crank-shaft34, which is off-centre with respect to the axis of actuator32, to arbour38. Arbour38, pivotably mounted along axis A4, connects arbour37and support31. An illustration of motion B is shown inFIGS. 6 to 8.

FIG. 6shows the moving means35in one of the extreme positions of motion B. In this first extreme position, the two arbours37,38of connecting rod36form an acute angle with respect to each other. It is thus clear that when crank-shaft34imparts a backward rotation, as illustrated inFIG. 6, this causes a trigonometric rotation of arm38and, incidentally, of support31with respect to axis A4.

FIG. 7shows moving means35in a substantially central position of motion B. In this position, the two arbours37,38of connecting rod36form a substantially right angle with respect to each other. It is thus clear that if crank-shaft34continues its backward rotation illustrated inFIG. 7, this again causes a trigonometric rotation of arm38and, incidentally, of support31with respect to axis A4.

FIG. 8shows moving means35in the second extreme position of motion B. In this second extreme position, the two arbours37,38of connecting rod36form an obtuse angle with respect to each other. It is thus clear that when crank-shaft34imparts a backward rotation, as illustrated inFIG. 8, this causes a backward rotation of arm38and, incidentally, of support31with respect to axis A4until the return to the first extreme position passing through the same substantially central position.

Consequently, the rotating motion of actuator32is converted by the crank-shaft34-connecting rod36assembly into a back-and-forth motion of support31.

Preferably according to the invention, the moving means35is also mounted on a set41of carriages that are moveable selectively longitudinally C, transversely D, and vertically E. This not only enables support31to be precisely positioned with respect to abrasive means33, but also provides the desired pressing action. Indeed, in order to force the piece to be polished via support31to exert a force against abrasive means33, the longitudinal carriage is controlled in a motion C greater than the space between blank3′ of piece3to be polished and abrasive means33.

It is thus clear that polishing is performed piece-by-piece by pressing blank3′ against abrasive means33, the back-and-forth motion of support31forcing blank3′ to follow the curved contact surface of abrasive means33. Polishing is thus performed by moving a blank3′ piece-by-piece against abrasive means33rotatably mounted along a fixed axis A3which provides a very low reject rate.

The piece3obtained after modification of blank3′ by polishing system21thus includes one of these faces12,14with a transverse concave surface comprising a curvature C1and a longitudinal concave surface comprising a curvature C2which are perfectly polished. Pieces3may also be subjected to a final chemical super-polishing step to further improve their appearance.

Of course, this invention is not limited to the illustrated example but is capable of various variants and alterations that will appear to those skilled in the art. In particular, abrasive means33may be different and, for example, take the form of a curved or conical sabot.

It is also understood that the moving means35may be of a different nature to obtain the same type of pressing action and back-and-forth motions B.