1. Field of the Invention
The present invention is generally concerned with spectacle frames of the xe2x80x9cmetal supraxe2x80x9d type, i.e. frames like those sold under the trade name xe2x80x9cNylorxe2x80x9d in which each rim or surround has a rigid part and a filament extending from one end of the rigid part to the other.
2. Description of the Prior Art
A groove must be machined into the edge of the lens to be mounted in a rim or surround of the above kind to trap the filament.
In practise, the groove extends continuously all around the lens and the rigid part of the rim or surround of the spectacle frame has a projecting bead, commonly referred to as a cushion, adapted to engage in the groove where it is not filled by the filament.
Only the shape of the lens, i.e. the shape of the contour of the lens, which is in practise determined by a contour reader relating to a flat template of the appropriate shape, is currently taken into account in determining the trajectory of the groove on the edge of the lens for controlling the grooving machine when machining the groove.
Then the groove is merely made to remain within the boundaries of the edge of the lens.
To this end, the groove is usually machined at a fixed distance from one face of the lens or halfway between its two faces.
The groove machined in this way therefore at best follows the inherent curvature of the lens.
Although the filament can adapt to this curvature of its own accord, given its inherent flexibility, this is clearly not the case for the rigid part of the rims or surrounds of the spectacle frame.
This rigid part is itself manufactured with some degree of curvature, which is not necessarily the same as that of the lens to be mounted in it.
Accordingly, when it is practicable, i.e. when the spectacle frame lends itself to this, as is the case when the spectacle frame is made of metal, for example, practitioners fitting the lens must apply an estimated twist to the rigid part of the rims or surrounds of the spectacle frame to match the curvature of the rigid part as closely as they can to that of the groove on the lens to be mounted in the frame.
This twisting is a delicate operation and difficult to control.
If practitioners do not apply this twist, or if this is impossible because the spectacle frame does not lend itself to twisting, the lens is not securely held in the rim or surround in which it is mounted and the aesthetic result is generally not entirely satisfactory.
A general object of the present invention is an arrangement intended to overcome this problem.
The present invention consists in a method of determining the trajectory of a groove to be machined in an edge of a lens intended to be fitted to a spectacle frame in which each rim or surround has a rigid part and a filament which extends from one end of the rigid part to the other end thereof, wherein the curvature of an active portion of the rigid part is systematically taken into account over a portion of the trajectory of the groove corresponding to the rigid part concerned of the spectacle frame.
Accordingly, over the portion of its trajectory corresponding to the rigid part of the spectacle frame, the groove follows the inherent curvature of the rigid part as well as it can, even perfectly, instead of following that of the lens, it being understood that the two curvatures must obviously be compatible for the results to be satisfactory.
The cushion provided for this purpose on the rigid part can then nest exactly and throughout its length in the groove in the lens, to the benefit of the retention thereof.
Over the portion of its trajectory corresponding to the filament, the groove is formed in the usual manner.
However, within limits set by the curvature and thickness of the lens, the groove preferably of itself corresponds to a minimal curvature so that, following the shortest possible trajectory, the filament is itself tensioned as well as it can be, which again is to the benefit of the retention of the lens.
Be this as it may, because of the arrangement in accordance with the invention, the groove formed on the edge of the lens has two separate portions of different appearance, namely a portion corresponding to the rigid part of the spectacle frame which is more or less sinuous and a portion corresponding to the filament which is substantially rectilinear.
Of course, the two portions of the groove are continuous with each other, joining tangentially to each other, to prevent any distortion at the corresponding junction points.
The features and advantages of the invention emerge from the following description given by way of example and with reference to the accompanying diagrammatic drawing.