Ruminal bolus for electronic identification of a ruminant

The bolus is intended to be received in the reticulum of a ruminant. It comprises a body (10) having a housing (12) for containing a data interchange device (14). According to the invention, the body (10) is made of a material based on alumina, Al.sub.2 O.sub.3, and/or on silica, SiO.sub.2.

The present invention relates to a ruminal bolus for electronically
 identifying ruminants. The invention is particularly, but not exclusively,
 intended for use with ruminants living under agricultural conditions
 (cattle, buffalo, sheep, and goats), but is also applicable to industrial,
 hunting, or wild uses (reindeer, deer, and ruminants in general). The
 bolus can be used with animals of large weight (above about 25 kg), or of
 small weight (less than 25 kg), depending on the variant of the invention.
 A conventional ruminal bolus is constituted by a body having an electronic
 device for storing and interchanging data, which device resides in one of
 the stomachs or pre-stomachs of a ruminant for tracking purposes. Its main
 use is identifying animals, monitoring production (weight gain, milk
 production, monitoring reproduction, state of health, . . .) or for
 automating common operations concerning the management of such animals,
 such as feeding, or controlling access to restricted areas or controlling
 gates of classification runs, etc. In conventional manner, such
 information is delivered and picked up by means of electromagnetic waves.
 Document U.S.-A 4 262 632 discloses an electronic identification system for
 herds of ruminants based on boluses, each comprising a transmitter which
 is administered orally as an alimentary bolus, and which is optimized to
 penetrate into the second pre-stomach referred to below as the "reticulum"
 (honeycomb) of a ruminant. That device is in the form of a cylinder that
 is about 75 mm long, having a diameter of about 18 mm. To prevent any
 regurgitation phenomenon, it is recommended for the density of the device
 to exceed 2 g/cm.sup.3. To this end, provision is made to incorporate a
 weight inside the bolus, in the vicinity of the data interchange device.
 Other types of known bolus can be received equally well in the reticulum or
 in the rumen (first pre-stomach).
 By way of example, document WO 93--A-05 648 describes a bolus in the form
 of a cylinder or a flat capsule made of resin or of high density glass,
 and including an electronic device. That bolus is also provided with a
 continuous visible display system enabling the animal to be identified
 when the bolus is taken from a dead animal.
 Document AU-A 64 92 12 describes an apparatus for identifying an animal
 based on a passive transducer included in a porcelain capsule and having a
 density of not less than 1.75 g/cm.sup.3. That system includes a magnetic
 block making it possible to take hold of the system after the animal has
 been slaughtered. The porcelain capsule is filled with a dense liquid in
 order to obtain the desired specific gravity for maintaining the
 transponder in an operational position.
 In other types of bolus, an elongate body is used having one end ballasted
 with a mass of metal so as to confer the required density thereto and so
 as to ensure that the body remains implanted in a vertical position in the
 rumen or in the reticulum. One such bolus is described, for example, in
 document WO-A-95 17 809.
 Although the presence of a mass of metal is advantageous in that it makes
 it possible to increase the density of the bolus, it nevertheless suffers
 from the following drawbacks. Firstly, it interferes with radio
 transmission between the electronic device within the bolus and the
 outside, in particular by shifting the frequencies of tuned circuits.
 Secondly, it frequently happens that the stomachs of ruminants contain,
 whether deliberately or involuntarily, foreign bodies or magnetized pieces
 which tend to collect around the metal piece, thereby attenuating
 transmission and reception signals, or causing the bolus to be rejected or
 expelled.
 In most cases, it is observed that known boluses tend, either deliberately
 or accidentally, to take up final residence in the rumen. They therefore
 suffer from the drawback of being incapable of being used successfully
 until the suckler animal has developed a rumen. The range at which they
 can be read, and the effectiveness of such reading are also limited by the
 large size of the rumen and by the frequently random orientation of the
 electronic device in use.
 An object of the invention is thus to provide a high density bolus enabling
 small dimensions to be used and as a result making it possible to fix the
 bolus reliably in the reticulum (second pre-stomach) of the ruminant,
 accurately located against the left costal wall, behind the heart.
 Another object of the invention is to make it possible to mass-produce
 boluses at low cost.
 A more general further object of the present invention is to make it
 possible to obtain a bolus that avoids the drawbacks of prior art boluses.
 According to the invention, these objects are achieved by a bolus for
 electronically identifying a ruminant, the bolus being designed to be
 received in the reticulum of the ruminant and comprising a body, itself
 having a housing designed to contain a data interchange device (such as an
 electronic transponder), the bolus being characterized in that the body is
 made of a material based on alumina, Al.sub.2 O.sub.3, and/or on silica,
 SiO.sub.2.
 It has been discovered that alumina and silica provide two advantages,
 firstly they impart very high density to the body while presenting high
 resistance to the digestive juices and processes that take place in the
 pre-stomachs of ruminants. Secondly these materials are cheap and easy to
 use in mass-production methods.
 In addition, these materials are non-magnetic and present excellent
 transparency at the radiofrequencies used for remotely interchanging data
 with the device housed in the body.
 When the material is based on alumina, the content of the alumina present
 in the material is preferably not less than 60% by weight. Its content may
 lie in the range 75% to 99.5% by weight, and preferably lies in the range
 80% to 99.5% by weight.
 In a variant of the invention, silica represents the major portion by
 weight of the material. Under such circumstances, the silica can be
 present in the material at a content of not less than 40% by weight,
 preferably at a content of not less than 50% by weight, and more
 preferably, of not less than 55% by weight.
 When silica is the majority material, alumina is preferably present in
 small quantities in the material, with the content of alumina preferably
 being less than 10% by weight, and more preferably less than 5% by weight.
 In some variants of the invention, the alumina and the silica together
 constitute not less than 80% by weight of the composition of the material,
 and preferably not less than 90% by weight of the composition of the
 material.
 To optimize the chemical composition of the material, it is possible to add
 thereto one or more of the following compounds: MgO, CaO, BaO, Na.sub.2 O,
 K.sub.2 O, Fe.sub.2 O.sub.3, TiO.sub.2. Each of these compounds may be
 present at a content lying in the range 0.1% to 2% by weight.
 When the bolus is made with a silica-based material, it is possible to
 provide for the compound MgO and for the silica together to constitute not
 less than 70% by weight, preferably 80% by weight, and more preferably
 still 85% by weight of the composition of the material. Under such
 circumstances, the MgO content may exceed 2%, and may even exceed 25% by
 weight.
 Advantageously, the bolus does not include any added metal pieces (apart
 from the electronic circuit of the data interchange device) so as to
 benefit from the non-magnetic nature of the material used.
 The composition of the material in the above-specified ranges makes it easy
 to achieve density of not less than 2.5 g/cm.sup.3. The density may be not
 less than 3 g/cm.sup.3, or indeed not less than 3.5 g/cm.sup.3.
 Advantageously, the bolus is constituted by a symmetrical body with
 uniformly-distributed mass, such that its center of gravity and its
 geometrical center coincide.
 In order to ensure that the bolus is properly fixed in the reticulum, the
 bolus is cylindrical in shape and circular in cross-section, with the
 edges of each of its ends being chamfered or rounded. This shape, in
 combination with an appropriate choice of dimensions, makes it possible in
 particular to fix the bolus in the direction of the major axis of the
 reticulum, in a position that is oblique and parallel to the diaphragm.
 This confers great stability and makes it possible to obtain optimum and
 uniform conditions for the electromagnetic link.
 Such fixing can be obtained reliably when the ratio between the length and
 the radius of the bolus body lies in the preferred range of 2:1 to 5:1.
 Advantageously, this ratio lies in the range 2.5:1 to 4:1.
 In a preferred embodiment for a bolus that is for use in ruminants of
 weight exceeding about 25 kg, and possibly reaching as much as 1,000 kg to
 1,300 kg for bulls, the length of the body lies in the range 50 mm to 90
 mm.
 When the bolus is more particularly intended for use with ruminants of
 weight not exceeding about 25 kg, the length of the body preferably lies
 in the range 30 mm to 70 mm.
 According to another characteristic of the bolus of the present invention,
 the housing for the electronic data interchange device comprises a cavity
 that is accessible from one end or from both opposite ends of the body.
 The cavity can be in the form of a blind hole on the main axis of the body
 or in the form of a through hole on the main axis of the body. Once the
 data interchange device has been put into place, the hole can be closed by
 means of an epoxy resin or a plastic cement that withstands the
 environment that obtains in the ruminant reticulum.
 In a variant, the above-mentioned hole is closed at the or each end by
 means of a male part such as a pressure screw or a self-locking rivet
 suitable for fixing against the wall of the hole.
 When the cavity is in the form of a through hole, it can be closed at each
 end by a self-locking rivet constituted by two separate elements, each
 having a portion of rod with a head at one end thereof, the two respective
 rods being disposed in such a manner as to engage via their free ends and
 to lock one within the other, clamping between them the data interchange
 device.
 This disposition has the advantage of enabling the data interchange device
 to be installed prior to final assembly, and to ensure that it is
 accurately positioned automatically within the cavity. It also makes it
 possible to reduce the number of successive operations that need to be
 performed on the body of the bolus, since the device is put into place
 simultaneously with the rivet.
 In a preferred embodiment of the invention, the bolus also includes a
 sleeve of resilient material designed to receive the data interchange
 device and having an outside diameter which enables it to fit without
 clearance inside the housing.
 The present invention also provides a method of manufacturing a bolus of
 the kind described above, characterized in that it comprises the following
 steps: preparing a blend based on alumina, Al.sub.2 O.sub.3, and/or based
 on silica, SiO.sub.2 ; forming a preform for the body of the bolus from
 the blend; and subjecting the preform to a firing step.
 If necessary, the method further comprises a step of giving the bolus body
 preform its final dimensions and of finishing it after the firing step.
 The firing step may be performed at a temperature lying in the range
 1000.degree. C. to 2500.degree. C. Nevertheless, it has been found that
 excellent results for the intended application are obtained when the
 firing temperature is substantially equal to 1400.degree. C.
 In a preferred embodiment, the preform is made by extrusion and the
 extruded piece is cut to the general shape of the bolus body. Under such
 circumstances, the housing in the bolus body can be made by drilling prior
 to the firing step.
 Nevertheless, it is also possible to make the bolus body by a molding
 method in which case the housing is formed simultaneously with the body.

As shown in FIGS. 1A and 1B, the bolus 10 constituting the first embodiment
 of the invention is in the form of a cylindrical capsule of circular
 cross-section, having chamfered edges 10a at both ends.
 Inside the capsule there is a cavity 12 of cylindrical shape and of
 circular section for receiving a data interchange device in the form of an
 electronic transponder 14 (FIG. 2). In this first embodiment, the cavity
 12 is in the form of a blind hold on the main axis A--A' of the capsule.
 It will be observed that the geometrical center and the center of gravity
 of the capsule coincide substantially (that is to say the capsule is not
 significantly unbalanced by the presence of the cavity 12 in the form of
 blind hole).
 The transponder 14 used is of conventional type, comprising a passive
 transmitter/receiver activated at radiofrequency, with incorporated
 sensors enabling identification or data collection to be performed from
 the body of the ruminant, together with a storage circuit containing
 programmed or programmable code. This type of transponder is generally
 encapsulated in a cylinder of glass or clear plastic.
 The data interchange device 14 is received in a sleeve of resilient
 material 16 such as an elastomer (FIG. 3). The outside diameter of the
 sleeve matches the cavity 12 of the bolus so that the transponder/sleeve
 assembly is received therein i.e. snugly, substantially without play. It
 has been found that the sleeve 16 provides excellent protection to the
 transponder 14 against mechanical and thermal shock while being
 transparent to radiofrequencies.
 When the transponder 14 is in place, the blind hole is closed by means of
 epoxy resin or plastic cement that withstands the environment of the
 reticulum.
 In a variant, the hole can be closed by a rivet as shown in FIG. 4.
 For this purpose, it is also possible to use a pressure screw of plastics
 material, or any other known means enabling closure to be performed that
 is proof against and that withstands the digestive juices in the animal's
 reticulum.
 FIG. 5 shows a bolus constituting a second embodiment of the invention. It
 differs from the bolus described above mainly by the fact that the cavity
 12 is implemented in the form of a through hole on the main axis A--A' of
 the cylinder.
 In this embodiment, the hole may also be closed by a self-locking rivet
 device specially adapted to receive the transponder 14, as explained below
 with reference to FIG. 6.
 The self-locking rivet device is constituted by two separate elements 21
 and 22 each having a shank portion 23, 24 with a head 25, 26 at one end
 thereof. It is preferably made of a plastics material of the ABS type. The
 two shanks 23 and 24 are made so as to engage mutually via their free ends
 remote from their heads, and to lock one within the other while enclosing
 between them the transponder 14.
 To this end, the shank 23 of a first one of the elements 21 is in the form
 of a cylinder of dimensions suitable for being engaged snugly in the hole
 forming the cavity 12 in the body 10 of the bolus. This shank 23 itself
 includes a recess 27 in the form of a blind hole that is accessible via
 its free end and that serves to receive the transponder 14. The end wall
 27a of the blind hole is concave so as to fit closely over a first end of
 the transponder 14. A portion 23a of the shank 23 close to its free end is
 of smaller section having one or more longitudinal slots (not shown)
 enabling it to be engaged with the shank 24 of the other element by
 resilient co-operation. This smaller-section portion 23a includes a
 peripheral rib 28 on its inside surface constituting a portion of
 snap-fastening means.
 The shank 24 of the second element 22 is circular in section and
 dimensioned so as to fit closely against the inside surface of the smaller
 section portion 23a of the shank of the first element 21. It includes a
 peripheral groove 29 for receiving the rib 28 on the first element 21,
 thereby forming the other half of the snap-fastening means. The free end
 of the shank 24 of the second element 22 has a hollow 24a on the
 longitudinal axis and suitable for fitting closely over the second end of
 the transponder 14.
 Together the two elements 21 and 22 are shaped so that when the rib 28 of
 the first element is engaged in the groove 29 of the second element, the
 inside faces 25a and 26b of the respective heads are spaced apart by a
 distance corresponding to the length of the body 10 of the bolus. This
 disposition is achieved by the fact that the shank 23 of the first element
 has the same length as the body of the bolus; its end thus comes into
 abutment against the inside face 26a of the head of the second element
 when the self-locking rivet is in the snap-fastening position. Also, the
 respective positions of the end wall 27a of the blind hole in the first
 element 21 and the hollowed end 24a in the second element 22, once the
 elements are engaged one with the other, are such that the transponder 14
 is received snugly in the axial direction of the bolus, and is located at
 the geometrical center thereof.
 While the transponder 14 is being mounted in the body 10 of the bolus, the
 transponder is initially inserted into the blind hole 27 formed in the
 first element 21 of the self-locking rivet. This operation can be
 performed outside the body of the bolus, thereby making it possible to
 reduce the number of successive operations performed thereon.
 Once the transponder 14 is retained in the first element 21, it is inserted
 via one end into the through hole 12 in the body of the bolus. The second
 element 22 of the self-locking rivet is then inserted via the other end of
 the body of the bolus and force is applied between the respective heads 25
 and 26 until the rib 28 snaps into the groove 29. The rivet is then locked
 with the inside faces 25a and 26a of the heads bearing against the
 respective end faces of the body of the bolus.
 It will be observed that in this second embodiment, the heads 25 and 26 can
 cover the entire surface of the end faces of the body 10 and can have
 outside surfaces that are dome-shaped. Under such circumstances, the edges
 of the body 10 need not be chamfered.
 The particular choice of composition for the material of the bolus body,
 whether it is based on alumina or on silica, depends on several factors:
 manufacturing method, desired density, dimensions of bolus, etc. . . . ,
 and these factors must take account specifically of the weight and the
 type of ruminant for which the bolus is intended.
 In numerous cases, it is possible to envisage using a material comprising
 about 80% by weight alumina, Al.sub.2 O.sub.3, and about 15% by weight
 silica, SiO.sub.2.
 The balance, in percentage by weight, may be shared in the range 0.1% to 2%
 by weight between the following substances: MgO; CaO; BaO; Na.sub.2 O;
 K.sub.2 O; Fe.sub.2 O.sub.3 ; and TiO.sub.2. Naturally, it is possible to
 use only one or only some of those substances, with appropriate matching
 of contents.
 With such a composition, the density of the material may exceed 3.2
 g/cm.sup.3.
 The porosity of such a material based on high density silica, SiO.sub.2,
 and/or alumina, Al.sub.2 O.sub.3, is negligible.
 Three specific examples of composition for the material having a high
 alumina content and having the characteristics specified above are given
 below.
 EXAMPLE 1
 Composition of an alumina-based material for forming the body of the bolus
 and having a density of 3.2 g/cm.sup.3. Substance Al.sub.2 O.sub.3
 SiO.sub.2 MgO CaO BaO Na.sub.2 O K.sub.2 O Fe.sub.2 O.sub.3 TiO.sub.2
 Other % by weight 80.615.1 1.1 0.9 0.8 0.6 0.5 0.2 0.2 &lt;0.1
 First and second embodiment boluses of the invention have been made using
 the above composition. Their characteristics are summarized in Table I.
 Table I
 Characteristics of a first or a second embodiment of the bolus
 Shape and dimensions
 Cylindrical with flattened ends
 Weight lying in the range 65 g to 70 g
 Length=69 mm, diameter=20 mm
 Cylindrical orifice (blind hole): length=45 mm, diameter=6.5 mm
 Cylindrical orifice (through hole): diameter=8 mm
 Physical characteristics of the material:
 Rich in alumina and white in color
 Porosity (%)=0
 Density (g/cm.sup.3)&gt;3.2
 Dielectric strength (kV/mm)&lt;10
 Thermal shock&gt;1400.degree.C.
 Thermal conductivity from 20.degree. C. to 100.degree. C. (w/mkg)=10-16
 Coefficient of linear expansion at 600.degree. C. (microns)=6-8
 Bending strength (MPa)&gt;200
 EXAMPLE 2
 Composition of the alumina-based material forming the body of the bolus and
 having a density of 3.5 g/cm.sup.3. Substance Al.sub.2 O.sub.3 SiO.sub.2
 MgO CaO BaO Na.sub.2 O K.sub.2 O Fe.sub.2 O.sub.3 TiO.sub.2 Other % by
 weight 95.03.0 0.6 0.5 0.2 0.2 0.2 0.1 0.2&lt;0.1
 EXAMPLE 3
 Composition of the alumina-based material forming the body of the bolus and
 having a density &gt;3.8 g/cm.sup.3. Substance Al.sub.2 O.sub.3 SiO.sub.2 MgO
 CaO Other % by weight 99.00.5 0.2 0.2 &lt;0.1
 The material of Examples 2 and 3 is particularly well adapted to ruminants
 of small size, of weight less than about 25 kg, such as young sheep or
 goats. It is thus possible to obtain a bolus having the general shape of
 the first and second embodiments, having a mass greater than 45 g or even
 65 g, but with dimensions that are relatively small (e.g. 55 mm.times.15
 mm or 60 mm.times.17 mm).
 In a variant of the invention, the material forming the body of the bolus
 is based on silica SiO.sub.2. In this case, the alumina content can be
 much smaller, e.g. being less than 10% or even less than 5%, by weight.
 A silica-based composition is particularly suitable for use with a bolus of
 relatively large dimensions, e.g. greater than 65mm.times.20 mm, for use
 with ruminants of large weight (cattle, buffalo).
 An example of such a material used for making a 75 mm.times.20 mm bolus is
 given below.
 EXAMPLE 4
 Composition of the silica-based material forming the body of the bolus and
 having a density of 2.8 g/cm.sup.3. Sub-Al.sub.2 O.sub.3 SiO.sub.2 MgO CaO
 BaO Na.sub.2 O K.sub.2 O Fe.sub.2 O.sub.3 TiO.sub.2 P.sub.2 O.sub.3 Other
 stance % by 3.1 60 28.0 0.7 6.6 0.2 0.4 0.5 0.2 0.1 &lt;0.1weight
 It will be observed that it is preferable to make up the silica content
 with another compound, such as MgO or the like, such that the silica and
 the other compound together constitute not less than 70%, or 80%, or
 indeed 85% of the weight of the material.
 The characteristics of density, general shape, and dimensions, given in the
 above-mentioned examples enable the bolus to remain permanently in the
 ruminant's reticulum, and this is done in the direction of the major axis
 of the reticulum (an oblique position), as shown in FIG. 7.
 The use of a material based on high density silica or alumina for making
 the body of the bolus makes it possible to achieve dimensions that are
 optimized for administration by the mouth to numerous species of ruminant
 animals, and of any age.
 When the boluses of the first and second embodiments described above are
 specifically configured for use with ruminants of weight exceeding 25 kg,
 the esophagus is large enough to enable the capsule to move down into
 position after being inserted into the back of the animal's mouth (close
 to the gullet), until it becomes permanently located in the reticulum of
 the ruminant (FIG. 7).
 Because of its relatively small dimensions, the bolus can be administered
 merely by means of a medicine or "balling" gun as is commonly used by
 stockmen.
 The design and characteristics of the capsule prevent it from coming to
 final rest in the rumen of the animal, the first and largest pre-stomach
 of ruminants (while food is moving during digestion and rumination). Nor
 can it be regurgitated back towards the mouth, nor can it pass into the
 subsequent portions of the digestive system of a ruminant (omasum, or
 third pre-stomach).
 The design of a bolus and the characteristics of alumina or of silica make
 it possible to use a single type of capsule for all species of ruminant,
 regardless of age, providing they have appropriate weight. There is
 therefore no need to wait for the rumen to develop, as normally happens
 with age approaching the end of the suckling period, given that the
 capsule is placed in a fixed position solely in the reticulum.
 The permanent localization of the capsule in the reticulum makes it
 possible to avoid erratic movement within the rumen. The large volume of
 the rumen reduces opportunities for defining the position of the capsule
 relative to the reticulum. The large volume of the rumen also reduces the
 effectiveness and the distance at which the electronic system placed in
 the capsule can be read.
 The small size of the reticulum, its contractile characteristics, and the
 longitudinal disposition of its muscular fibers in the antero-posterior
 direction all contribute to ensuring that the capsule and the data
 interchange device is oriented fixedly in the same direction. This
 orientation increases the distance from which it is possible to read the
 device in the crano-caudal (longitudinal) axis of the animal and decreases
 the possible reading distance on the costal axis (transverse axis). This
 increases reading effectiveness when the animal comes up to a read point
 and reduces the possibility of confusion with other animals nearby.
 There follows a description with reference to FIGS. 8A and 8B of the main
 steps in manufacturing the bolus in a preferred implementation of the
 present invention.
 Firstly (FIG. 8A), a blend is prepared in a vessel 30 of the constituents
 of the silica- or alumina-based material that is to form the body of the
 bolus. These basic ingredients are fed to the vessel via respective ducts
 32 to enable the method to be performed continuously.
 The vessel 30 includes stirring means 34 and possibly also heating means
 (not shown) to ensure that the consistency and the blending of the
 material are optimal for extrusion.
 The material is removed via the bottom of the vessel and is conveyed along
 a duct 36 leading to an extruder 38. The extruder is fitted with an
 extrusion head 40 in the form of a single orifice of outlet section
 corresponding to the section of the body of the bolus. At the outlet from
 the extrusion head, the material is in the form of a solid and continuous
 cylinder 42 that is suspended vertically down therefrom. This cylinder is
 laid on a conveyor belt 44 moving in a horizontal direction to various
 manufacturing stations.
 Once on the conveyor belt 44, the cylinder of material 42 passes through a
 station 46 where it is cut up into cylindrical bars 42a of length
 corresponding substantially to the length of the finished product. In this
 example, the station 46 has a blade 46a oscillating at a frequency that
 matches the speed of the conveyor belt 44.
 The bars 42a are then stood upright and pass through a drilling station 48
 (FIG. 8B) to form a through hole in the axial direction of the body, said
 hole corresponding to the housing 12 in the first embodiment. When
 manufacturing a bolus body in accordance with the second embodiment,
 drilling is stopped before passing through both ends, in order to form a
 blind hole.
 The bars then go through a first station 50a for forming a chamfered edge
 10a on one of the ends of the bar 42a. After this operation, the bars are
 turned upside-down and pass through a second station 50b for forming a
 chambered edge, identical to the first, from which they are delivered
 having chamfers 10a at both ends, suitable for both the first and second
 embodiments of the invention. After these operations, a preform is
 obtained for the body having dimensions that correspond substantially to
 those of the final object.
 The preform then passes to a firing station 52 where it is fired for a
 predetermined length of time. The firing temperature varies as a function
 of the specific composition of the material and as a function of the
 desired mechanical properties. In general, this temperature will lie in
 the range 1000.degree. C. to 2500.degree. C. For the composition of
 material specified in the example (Table I) and also for variants thereof,
 it has been found that a firing temperature of 1400.degree. C. is
 preferable.
 After firing, the preform is hardened and stabilized in dimensions. It then
 passes to a last station 54 (or set of stations) in which it is rectified
 to obtain a bolus having the final desired dimensions. In this example,
 the preform is subjected after firing to a reboring operation (station 52)
 to ensure that the housing is accurately dimensioned, so that it is sure
 to be closed properly with a self-locking rivet or with a pressure screw.
 Naturally, the present invention makes it possible to implement numerous
 variants, as to shape, dimension, and specific composition of the bolus,
 and as to choice of data interchange device to be incorporated therein.