Method and installation for making a workpiece comprising at least a tubular section

#The invention relates to a method and to an installation for making a workpiece comprising at least one tubular part obtained by bending a metal sheet. This method and this installation are particularly adapted to the manufacture of a shadow mask support frame for a cathode-ray display tube.

#Such a frame is used for example in the domain of the manufacture of colour televisions. The colour display cathode-ray tubes comprise a metal foil pierced with a plurality of holes or slots, called “shadow mask” and disposed between the electron gun and the display screen. Such a shadow mask is supported by a frame, generally rectangular in shape, which holds it in position in the vicinity of the display screen and, if necessary, ensures that the mask is taut in order to limit the deformations resulting from the local overheatings created by the electron beams.

#It is known from EP-A-0 809 272 to produce a shadow mask frame from two thin metal strips joined to each other and comprising ribs for rigidification. Such a frame is light and rigid, which allows it to be used for a taut shadow mask and for a crimped shadow mask. Application FR-A-2 790 140 discloses a shadow mask frame comprising tubular parts formed by bending a metal sheet. The sheet may be bent by different methods, in particilar by hand, but a manual bending is not compatible with high-rate industrial production. Furthermore, the known methods of bending with the aid of machines comprising an apron, sometimes called “support strap”, on which a metal sheet to be bent is held with the aid of a holding clamp, and an articulated bending flap, do not allow tubular parts to be easily manufactured.

#The invention will be more readily understood and other advantages thereof will appear more clearly in the light of the following description of two forms of embodiment of an installation for manufacturing a shadow mask support frame and of its method of implementation, given solely by way of example and made with reference to the accompanying drawings, in which:

#FIG. 1 schematically shows a shadow mask frame on which is disposed a partially shown shadow mask.

#FIG. 2 schematically shows a perspective view of a part of an installation for manufacturing the frame of FIG. 1, the frame not having been shown.

#FIG. 1 schematically shows a shadow mask 1 mounted on a shadow mask supporting frame 2. The frame is generally rectangular in shape and comprises two lateral uprights 3, 3′ and two end uprights 4, 4′. The shadow mask is constituted by a thin metal foil, for example of iron-nickel alloy with a low coefficient of expansion, pierced with holes 5 and fixed by welding on upper edges 6, 6′ of the end uprights 4, 4′. Different forms of shadow mask are possible as a function of the cathode-ray tube with which it is to be associated. In the example shown, the shadow mask is substantially planar, but it may equally well be curved into a portion of cylinder. The mask I may in addition be taut, i.e. subjected to a tension parallel to the lateral uprights 3, 3′ of the frame 2.

#The uprights 3, 3′, 4 and 4′ are formed by bending a sheet metal so as each to constitute a tubular element. These four uprights may be obtained by bending a single metal plate. It is also possible to provide using one bent plate for each upright or one plate for two contiguous uprights.

#In the example illustrated in the Figures, a single metal plate 10 is used for making the four tubular uprights 3, 3′, 4, 4′. This plate is partially visible in FIG. 3 after steps of cutting out assembly tongues 11 and of pre-marking longitudinal edges 12 to 17.