Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to machines which form
shells of can dimensions.
This application is a divisional application
of copending Canadian application l~o. 245,507 filed Feb. 11,
1976 to which reference may be made for a full discussion of the
background to the present invention~ Attention is also directed
to our copending application Nos. filed
on even date therewith.
In said copending application No. 245,507 there is dis-,
closed and claimed a machine for producing elongated shells of
can dimension and comprising (a) die means at a forming station
to produce an elongated shell from a cup or shorter length; (b)
a punch assembly afforded by a punch member'and a complemented
stripper member, coupled one to another both for unitary forward
movement into and through the die forming station to form the
sheel from the cup and for retracting movement of the punch mem-
ber independently of the stripper member which is to be held
stationary after the shell is formed, said stripper member in ¦
its stationary state stripping the shell while said punch mem-
ber is being rétracted, and said ~tripper member having a piston
thereon disposed in a cylinder presented by the punch assembly;
(c) a fixed bushing for supporting and guiding said punch
assembly and having a chamber for receiving and transmitting
fluid under pressure to said cylinder to hold said stripper
member stationary at the end of forward travel, said punch
assembly being provided with a transfer port communicating with
said cylinder and so located as to traverse said bushing chamber
over a path of predetermined extent during reverse movement f
said punch member whereby fluid under pressure in said bushing
chamber may be communicated to said cylinder at the end of for- r
ward punch travel and allowed to prevail until said port clears
the bushing chamber to depressurize the latter during reverse
movement of said punch member; and (d) means to retrac-t the
stripper member after said point has so cleared the bushing
chamber.
According to the present invention there is provided
in a machine which forms shells of can dimension from a blank
by moving the blank through a deforming means including a die and
in which the blank is of a material having given electromagnetic
properties, the combination of (a) a punch of material having
different electromagnetic properties, on which the shell is
1~ supported on emerging from the deforming means, (b) an electro-
magnetic field sensor positioned to create an electromagnetic
field in an area corresponding to the free end of a shell of
correct length, and (c) means to discriminate between the field
altered by the punch element alone, not supporting a shell of
correct length, and the field altered by a shell of correct
length on the punch.
The present invention will be described with reference
to the accompanying drawings in which:
Figures 1, 2 and 3, are detail views of a short shell
detector means embodying the present invention.
The short shell detector embodying the invention is
incorporated in a machine as described in siad copending applica-
tions, to which reference should be made for a full description
of the details of machine construction, operation and control.
In the instance of thin-walled shells, a proximity
detector, physically contacting the shell to determine correct
length, is not always reliable and in addition requires consid-
erable maintenance. What is more, physical contact can result
in a damaged shell, necessitating shutdown.
The short shell detector 68 embodied in the present
machine operates by detec-ting metallurgical differences in
materials based on changes in an electromagnetic field and
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emitting a corresponding signal. In one specific form, it
includes an oscillator having an operating frequency which
depends upon the inductance of a particular sensing coil If
one substance interdicts the field of the coil, the inductance
will be of a particular value, but if another substance of
markedly different magnetic properties interdicts the field the
inductance will be of different value.
~ eferring to Fig. 1 the short shell detector com-
prises an inductor 250, represented by a core 251 and a coil
252, that is connected into the frequency-determining circuit
of an oscillator 254. A phase-locked-loop frequency comparator
255 is coupled to the oscillator 254.
The inductance of coil 252 will vary, depending upon
whether there is merely an air gap beneath the coil, whether
there is only the punch element 35L (tungsten carbide) adjacent
the coil, or whether the steel shell 26 projects into the field
space at the end of the coil.
In the null state, with neither a punch nor a shell in
the inductance field, Fig. 1 the fre~uency of the oscillator 254
may be taken as a value fO, significant of an air gap.
An event-se~-point switch in the machine's logic con-
trol is electrically triggered at the time when shell formation
of correct length should have taken place. The sensing probe,
the inductance 250, is located to sense whether a shell of
correct length has been obtained; if this is indeed the case,
Fig. 2, the variance in the inductance results in an oscillator
frequency fl, Fig. 2. The output from the phase-locked-loop,
delivered to -the logic control, signifies "no error".
On the other l~and, the inductance of the sensing coil
252 will be different if, at the time of event-set-point switch-
ing, only the material of the punch element interdicts the
inductance field, resulting in a frequency f2 emitted by the
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oscillator, Fig. 3. This frequency is detected by the internal
comparator of the phase-locked-loop; the output characterizes an
error. The logic control, informed of the error, signals the
appropriate servo valves, which thereupon set the main cylinder
valve and the cup feeder valves of the machine to null position.
In the illustrated short shell detector, f2~ fo> f
and the differences between the sensor frequencies can readily
be made great enough for consistent and accurate detection in
the comparator 255. It will be recognized that other frequency
relations may obtain if the magnetic properties of the punch and
shell are changed (e.g., an aluminium shell on a steel punch) or
if the sensor is modified to constitute a capacitive sensing
device instead of an inductive device.
Similar event-set-point switching in the logic control
is employed for the various "no error" machine functions: cup
feed rod fully extended; cup in position; punch body fully
extended; shell being stripped and so on.
