Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1
METHOD FOR ASSEMBLING SHEETS BY RIVETING
TECHNICAL FIELD
The present invention relates generally to the
field of assemblinq sheets or thin metallic structures
by riveting, this technique being widespread in
aviation construction operations.
The invention can in fact be applied most suitably
but non-limiting in the field of robot assembly of
aircraft sheets by riveting, having a sharply curved
piercing/riveting surface, such as for example the
leading edge of a wing, or else a weaker curve, such as
an aircraft fuselage panel.
PRIOR ART
In the prior art, the methods for assemblina
sheets by riveting are usually employed by suc:~essively
performing a step for piercing a hole through the
sheets to be assembled, then a step for placing a rivet
in the pierced hole, this combination of steps able to
be repeated as many time as necessary at different
points on the sheets.
This type of method, which can optionally be
carried out by means unique tooling at the same time
incorporating a piercing system and a riveting system
has never proven entirely satisfactory to date in terms
of the quality of resulting holes and/or the millings,
especially in the field of assembling sheets making up
a leading edge of an aircraft wing. In this respect, it
should be noted that these millings are aenerally
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provided for accommodating the head of 7-he rivet
located in its corresponding hole.
In fact, irrespective of the type of tooling
employed, it has been ascertained that using these
assembling methods clearly did not guarantee the
formation of a perfectly circuiar hole and/or even
milling on the sheets to be assembled.
OBJECT OF THE INVENTION
The aim of the invention is thus to propose a
method for assembling sheets by riveting rectifying the
problems mentioned hereinabove, and relative to the
executions of the prior art.
To this end, the object of the invention is a
method for assembling sheets by riveting, comprising a
step for piercing a hole through the sheets fcllowed by
a step for placing a rivet in the pierced hole, the
step for piercing a hole being performed by providing
an advance speed instruction of a piercing tool as well
as a rotation speed instruction of this tool. According
to the invention, a previous step for determining
information on the local stiffness of the sheets
Info stiffness is also employed at the level of the
hole to be pierced, the advance speed instruction and
rotation speed instruction of the tool being a function
of this information on the local stiffness of the
sheets.
Therefore, considering information on the local
stiffness of the sheets for controlling the piercing
operation of a hole, which conventionally but non-
limiting comprises making this hole as well as
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preferably that of milling intended for taking up the
rivet head, it is advantageously possible to guarantee
forming a perfectly circular hole and even milling at
one end of the latter. Effectively, correction of the
advance speed and rotation speed instructions of the
tool as a function of the stiffness of the sheets at
the particular point where piercing is subsequently
carried out considerably eases, or even completely
eradicates, the problems encountered in the prior art,
such as ovalisation of the hole, delaminating of the
composite, the fins in the form of a crater at. the exit
of the hole, or even producing an undesired rough
surface. In fact, the abovementioned instructions are
corrected with the local information on the stiffness
so that the thrust generated by the tool on the sheets
during oiercina does not cause contact rupture between
the sheet press system and these same sheets.
The step for determining information on the local
stiffness of the sheets Info stiffness at the level of
the hole to be pierced is preferably performed by
carrying out a clamping operation aimed at sinking a
sheet press system in the sheets at the level of the
hole to be pierced over a clamping distance D clamping
reaching a final value D clamping final on completion
of the clamping operation, an operation during which is
determined periodically the value of a resistance force
of the sheets to clamping F2 resulting from sinking of
the sheet press system in the sheets, to then determine
a resistance force value of the sheets on completion of
clamping F2 final, at the end of the clamping
operation. It is noted that since updating of the value
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of this force F2 during the clamping operation can
occur for example every 5 ms, especially allowina
tracking of the evolution of --he latter.
In addition, the fact of also precisely tracking
the shift distance of the sheet press system ciuring the
clamping operation, called clamping distance,
D clamping, helps discover the real position of the
restricted sheets due to the fina_'_ value
D clamping final on completion of the clamping
oberation, and thus of performing milling having
exactly the desired deptri.
Determining the value of the resistance fcrce of
the sheets to clamping F2 i-s preferably conducted by
determinina the value of the motor power P2 absorbed
absorbed by the sheet press system sinking into the
sheets, this value of the absorbed motor power
P2 absorbed then being converted by a converter to
obtain the value of the resistance force of the sheets
to the clamping F2.
The clamping operation is preferably completed
when the resistance force of the sheets to the
determined clamping F2 has reached a target value
F2 target or when the clamping distance D_clamping has
reached a target value D clamping target.
More preferably, the step for determining
information on the local stiffness of the sheets
Info stiffness at the level of the hole to be pierced
is also carried out, prior to initiating the clamping
operation, by providing a docking operation of the
sheet press system on the sheets at the level of the
hole to be pierced, an operation during which the value
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of a resistance force of the sheets to docking Fi
resulting from sinki ng the sheet press svstenl into the
sheets is determined periodically so as to determine a
resistance force value of the sheets on completion of
docking FI final, at the end of the docking operation.
In such a case, it is provided that the clamping
operation is started with the sheet press system
located in a position such as occupi-ed on com-clet;-on of
the docking operation, by marking a stop time between
the two successive operations.
The information on the local stiffness of the
sheets Info stiffness is preferably obtained oy making
the ratio between on the one hand the difference
between the value of the resistance force of the sheets
on completion of clamping F2 final and -:~he valae of the
resistance force of the sheets on completion of docking
Fl final, and on the other hand the final value of the
clamping distance D clamping final. Nevertheless, it is
noted that this information could be alternatively
obtained by making the ratio between the value of the
resistance force of the sheets on completion of
clamping F2 final and the final value of the clamping
distance D clamping final, without departing from the
scope of the invention.
In addition, it is also preferably provided that
the advance speed instruction and rotation speed
instruction of the tool are also a function of the
nature of the material of the sheets to be assembled,
and of the type of piercing tool employed.
More preferably, during the piercing step of the
hole, the value of a resistance force of the sheets F3
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resulting from the support of the sheet press system on
the sheets is determined periodically, and the latter
is compared to a minimal value F3 min so as to order a
decrease in the advance speed instruction of the
piercing tool when it is detected that the value of
this resistance force of the sheets F3 is less than
said minimal value F3 min, for example fixed at 5 N.
Therefore, this additional security, added to that
resulting from the predisposition of the rotation and
advance speeds of the tool as a function of the
information on the local stiffness, arrests the advance
motor of the tool to avoid the thrust of this tool
causing rupture of the contact between the sheet press
head and the sheets to be assembled.
Also, the object of the invention is also a
control system for a device intended for assembling
sheets by riveting, capable of ensuring execution of
the method described hereinabove. This system thus
comprises means for delivering an advance speed
instruction of a piercing tool of the device, as well
as a rotation speed instruction of this tool, these
instructions being a function of informatiori on the
local stiffness of the sheets at the level of a hole to
be pierced intended to receive a rivet.
Other advantages and characteristics of the
invention will emerge from the detailed non-limiting
description hereinbelow.
BRIEF DESCRIPTION OF THE DRAWINGS
This description will be given with reference to
the attached drawings, in which ;
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- figure 1 illustrates a partial perspective view
of a device for assembling sheets by riveting, intended
for carrying out an assembling method by rivetina
according to a preferred embodiment of the present
invention ;
- figure 2 illustrates an exploded perspective
view of the device shown in figure 1 ;
- figures 3 to 5 illustrate schematic views of
different parts of a control system according to a
'creferred embodiment of the present invention, this
system equipping the device shown in figures 1 and 2
and
- figures 6a to 6c show the front part of the
device of figures 1 and 2 at different stages during
execution of the assembling method by riveting
according to said preferred embodiment of the present
invention, and more particularly during the step for
determining information on the local stiffness of the
sheets at the level of said hole to be pierced.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Initially in reference to both figures 1 and 2,
these show the front part of a device 1 for assembling
sheets by riveting, of the metallic type or made of any
other material such as composite material, this device
1 being intended for executing an assembling method by
riveting according to a preferred embodiment of the
present invention. Naturally, this device 1 is
described only by way of indication, and it must of
course be understood that the assembling method by
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riveting can be carried out by any other type of
device.
The method according to the invention, which is
aoplied most suitably in the field of aircraft
construction, can be adapted for aliow automatic
placing of any type of rivets, such as draw rivets,
and/or struck rivets, and/or crushed rivets, without
departing from the scope of the invention. Hcwever, it
is noted that the device 1 is preferabiy designed to
work blind, with draw rivets.
The front part of the device 1 illustrated in
figures 1 and 2 concerns only an end portiorl of this
device, and preferably constitutes a
mountable/demountable tool intended to be assembled at
the end of a robotic arm (not shown) preferabiy forming
an integral front part of this device. For the sake of
clarity, the description of the device 1 will be given
in reference to a system of axes of this device, which
is specifically attached to a chassis 2 of the latter,
also called a tool chassis. Therefore, X is the
longitudinal direction of the device, Y the direction
oriented transversally relative to this device, and Z
the vertical direction or the height, these three
directions being orthogonal to one another. Naturally,
it must be understood that the abovementioned axes
system moves according to the same movement as that of
the chassis 2, controlled by the robot arm.
The device 1 thus comprises overall three systems
attached to the chassis 2 and intended to ensure
different functions, specifically a piercing system 4,
a riveting system 6, and a sheet press system B. By way
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of information, it is indicated that these svstems are
also called actuators.
With respect to the piercing system 4, the latter
has a first carriage 10 supporting the piercing spindle
12 assembly, having at the level of its front part a
piercing head 14 equipped with a piercing tool 17 and
defining a piercing head axis 16, also called a
piercing tool axis, accordinq to which --his same tool
is arranged. More precisely, the spindle 12 is mounted
fixed on the carriage 10, such that the relative
position between the piercing head axis 16 oriented
according to the direction X, and this same carriage
10, is intended to remain identical --hroughout an
assembling cycle by riveting. By way of indication, the
iDiercing head 14 conventionallv comprises the piercing
tool 17, as well as the support of this too-_, of the
mandrel type or similar.
The first carriage 10 is mounted on the chassis 2
so as to be able to slide in a rectilinear direction
relative to the latter, according to a direction of
slide 18 parallel to the direction X. To this end, the
carriage 10 is mounted sliding on two guide rails 20
oriented according to the direction X, these --wo rails
20 being spaced from one another in the direction Y.
To allow securing to the rails 20, the carriage 10
is equipped with a plurality of ball skids 22 in the
form of a stirrup, for example provided with four in
number, with two of them linked to the rails 20, and
the two others linked to the other of these rails.
To allow shift in the direction of slide 18 of the
first carriage 10 relative to the chassis 2, the
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piercing system 4 integrates movement means 24 which
preferably take the form of a lirear motor integrating
a primary mobile element 26 seated on the first
carriage 10, and a secondary fixed element -2:8 mounted
5 on the chassis 2.
As is clearly visible in figures i and 2, the
chassis 2 has in section according to a plane YZ a
general U-shape, at both ends of which are fixed the
two rails 20. Provided between the two branches of this
10 U is a magnetic track made of permanent rear-earth
magnets, alternating all along this same --rack the
north and south polarisations. This track, placed under
the first carriage 10, thus constitutes the secondary
fixed element 28 of the linear motor 24.
Therefore, activation of the solenoid equipping
the primary mobile element 26 of the linear motor 24
creates electromagnetic forces on the one hand ensuring
shift according to the direction X of the first
carriage 10 on the rails 20, and on the other hand
attraction according to the direction Z of this same
carriage 10 to the secondary fixed element 28.
To obtain micrometric precision in the shift of
the carriage 10, it is provided that the Latter is
equipped with a reading head 30 cooperating with an
optical rule 32 placed on the chassis 2, according to
the direction X. This rule 32 is preferably constituted
by a glass rod bearing very high-precision graduations.
Therefore, the reading head 30 converts the detection
of engraving read on the rule 32 into electronic
signals during passage of the carriage 10, to give its
exact position on the guide rails 20.
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Still in reference to figures 1 and 2, the
riveting system 6 itself comprises a second carriacte 34
supporting the assembly of the riveting tool 36 or
riveter, which comprises in its front part a rivet
heading 38, defining a rivet heading axis 4C parallel
to the directions X and 18. More precisely, the rivet
heading 38, and more generaily the riveting tool
assembly 36, is mounted solid at the front of a
deportation arm 42 extending broadly according to the
direction X, and whereof the rear part is attached
mechanically to the carriage 34.
The abovementioned mechanical attachment is made
by way of movement means (hidden in the fiaures)
designed to the bale to place in rotation the arm 42
and the head 38 integral therewith relative to the
carriage 34 around an axis of rotation 44, with the aim
of shifting this same rivet heading 38 between a rest
position in which the piercing head axis 16 and the
rivet heading axis 40 are distinct and parallel, as
shown in figure 1, and a work position in which these
axes 16, 40 are joined. The movement means preferably
take the form of a classic rotary motor, whereof the
axis of rotation 44 is preferably parallel to the
directions X and 18, and naturally distinct from the
piercing head and rivet heading axes 16, 40. Due to
this, starting up the rotary motor causes a movement of
the head 38 relative to the carriage 34, this movement
describing a trajectory corresponding to a portion of a
circle situated in a plane YZ.
The second carriage 34 is mounted on the chassis 2
so as to be able to slide in a rectilinear direction
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relative to the latter according to the direction of
sliding 18. To this end, the second carriage 34 is
mounted sliding on a guide rail 46 preferably distinct
from the two guide rails 20 of the carriage 10, but
also oriented according to the directions X and 18.
o allow securing on the rail 46, the carriage 34
is equipped with one or a plurality of ball skids 48 ir_
the form of a stirrup, two of which for example are
provided, spaced according to the direction X.
The carriage 34 of the riveting system 6
preferably comprises no inherent translation means, but
is provided to be able to couple with the carriage of
the piercing system 4, and is consequently likely to be
set in motion according to the direction 18 under the
startup effect of the first linear motor 24 described
earlier.
In fact, coupling means 50 are provided for
coupling in translation the carriages 10, 34 to one
another, when in an active state, according to the
direction 18, and, when in an inactive state, for
enabling relative sliding between these same carriages.
As for the sheet press system 8, the latter has a
third carriage 60 supporting a sheet press head 62,
also called a pressurisation cylinder, and which
defines a sheet press head axis 64 oriented according
to the directions X and 18. As known to those skilled
in the art, the head 62, intended to contact the sheets
to be assembled during the piercing and riveting
operations is provided with a continuous orifice 66
arranged according to the sheet press head axis 64 and
intended to alternatively have the piercing tool 17 and
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the rivet heading 38 pass through it. More iDrecisely,
this head 62 or c.vlinder is mounted fixed on the
carriage 60, such that the relative position between
the sheet press head axis oriented according to the
direction X and this same carriage 60 is intended to
remain identical throughout an assembling by rivetina
cycle. Also, the axes 64 and 16 are permanently joinea
during an assembling by riveting cycle.
The third carriage 60 is mounted on the chassis 2
to be able to slide in a rectilinear direction relative
to the latter according to the direction of sliding 18.
To this end, the carriage 60 is mounted sliding on the
two guide rails 20, at the front relative to the first
carriage 10 of the piercing system, given naturally
that the front and rear are determined here as a
function of the orientation of the piercing tool 17
employed by the system 4.
To allow securing on the rails 20, the carriage 60
is equipped with a plurality of ball skids 68 in the
form of a stirrup, two of which for example are
provided, each associated with the two rails. To allow
shift in the direction of sliding 18 of the third
carriage 60 relative to the chassis 2 the sheet press
system 8 integrates movement means 70 which preferably
take the form of a linear motor integrating a primary
mobile element 72 on board the third carriage 60, as
well as a secondary fixed element 28 mounted on the
chassis 2, and which is preferably the same as that
used for the first linear motor, with the aim of
limiting to the maximum the number of kinematic
components necessary for running the device 1.
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Therefore, here also, activation of the solenoid
equipping the primary mobile element 72 of the linear
motor 70 creates electromagnetic forces ensuring on the
one hand the shift according to the direction X of the
third carriage 60 on the rails 20, and on the other
hand attraction according to the direction Z of this
same carriage 60 to the secondary fixed element 28 of
the permanent magnet track type.
To also produce micrometric precisior. in the shift
of the carriage 60 it is provided that the latter is
equipped with a reading head 74 cooperating with the
abovementioned optical rule 32 placed on the chassis 2.
Because of this, it is thus possible to fully control
the relative spread of the two carriages 10 and 60, the
advantage of which is to fully control the depth of the
holes and the millings made by means of the piercing
tool.
To be able to control this device 1 as wanted, it
is also equipped with a control system 83 shown
schematically in figures 3 to S. Overall, this system
83 comprises first control means 84 which are linked to
the sheet press system 8, and second control means 86
which are linked to the piercing system 4, these means
84, 86 naturally being able to be combined inside the
same equipment.
As for the first means 84 shown in figure 3, they
comprise a first digital control unit 88 linked to a
closed loop control card 90 of the linear motor 70 of
the sheet press system 8. The unit 88 is thus capable
of delivering instructions on position, advance speed
and power to the card 90, which thus gives feedback
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control on position, advance speed and power, by
supplying appropriate current to the motor 70 to which
this card 90 is linked.
In return, the closed loop control card 90
5 receives from the reading head 74 information on the
real position of the carriage 60, this information
being returned to the unit 88. In addition, this closed
loop control card 90 is also capable of returning tc
the unit 88 measurements on the advance speed of the
10 carriaae 60 and the effective power, this effective
power allowing the unit 88 to determine the motor power
absorbed by the system 8 during the docking and
clamping operations.
With respect to the second means 86 shown in
15 figure 5, they comprise a second digital control unit
92 attached to a closed loop control card 94 of the
linear motor 24 of the piercing system 4. The unit 92
is thus capable of delivering instructions on position,
advance speed and power to the card 94, which then
gives feedback control on position, advance speed and
power, by supplying appropriate current to the motor 24
to which this card 94 is attached. In return, the
closed loop control card 94 receives from the reading
head 30 information on the real position of the
carriage 10, this information being returned to the
unit 92. In addition, this closed loop control card 94
is also capable of returning to the unit 92
measurements on the advance speed of the carriage 10
and optionally the effective power.
Also, the digital control unit 92 is attached to a
closed ioop control card 96 of the rotary motor of the
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spindle 12. The unit 92 is thus capable of supplying
instructions on rotation speed and power to the card
96, which then gives feedback control on rotation speed
and power, bv supplying appropriate current to the
rotary motor to which this card 96 is attached. In
return, it can be provided that this closed loop
control card 96 returns to the unit 92 measurements on
the rotation speed of the tool 17 and the effective
power.
In this respect, it is indicated that one of the
barticular features of the invention is that the unit
92 comprises means 82 for delivering, respectively to
--he cards 94 and 96, advance speed instruction on the
~~ool and rotation speed instruction on this tool which
are a function of information on the local stiffness of
the sheets at the level of the hole to be pierced
intended to receive a rivet, this information being
called Info stiffness.
More specifically in reference to figure 4 it is
evident that these means 82 for example take the form
of a correction matrix of both abovementioned
instructions, this matrix thus not only considering the
information Info stiffness determined earlier, but also
optionally the nature of the material and the type of
the piercing tool whereof the data are pre-registered
in a specific program. Of course, this correction
matrix is designed for the advance speed and rotation
instructions it supplies to the cards 94, 96 to carry
out piercing with as high as possible quality and
precision.
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The assembling by riveting method carried out bv
means of the device 1 presented hereinabove will now be
described, this method comprising a step for
determining informat_Jon on the local stiffness of the
sheets at the level of the hole to be pierced, followed
by a piercing step aimed at making the hole as well as
the associated milling, then finally a step for placing
a rivet in said pierced hole, these three steps being
repeated as many times as there are rivets to be placed
on the sheets to be assembled.
In reference to figures 6a to 6c, these show a
front part of the device 1 at different stages during
execution of the step for determining the information
Info stiffness, this determining step essentially being
carried out during docking and clamping operations made
with the sheet press system 8, as will be described
hereinbelow.
In reference to figure 6a, this shows that the
chassis 2 is first guided by the robot arm to near the
sheets 80 to be assembled so that the front end of the
sheet press head 62 is located at a standard distance
D stand from the sheets 80 according to the direction
of sliding 18 and that of the axis 64, this distance
able to be of the order of 15 mm. At this stage, the
carriage 60 is in a position such that its central
point C is located at the level of a reference point R
of the optical rule 32.
Then, the docking operation is initiated by
commanding linear shift of the carriage 60 with the
unit 88, so as to make contact between the head 62 and
the sheets 80. It is noted that from establishment of
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the abovementioned contact the control unit 88
periodically determines the value of the absorbed motor
power Pl absorbed by the svstem 8, th.is value
Pl absorbed then being converted by a converter
integrated into the unit 88 to produce a value of the
resistance force of the sheets to the docking Fl. By
way of indication, it is noted that this force Fl,
updated every 5 ms, also corresoonds in value to a
sinking effort of the sheet press system 8 aaainst the
sheets 80.
Command of this docking operation is provided so
that the shift of the system 8, and more specificallv
that of its carriaae 60, is completed when the
determined force Fl has reached a target value
Fl target, which can for example be fixed to a weak
value of the order of 1 N. As shown in figure 6b, on
completion of the docking operation the carriage 60 has
thus travelled a distance Dl final between point R and
point Cl of the rule 32 at the level of which the point
C of the carriage 60 is situated, the value of this
distance Dl final measured by means of the rule 32
being returned to the unit 88. In addition, at this
instant the value of the resistance force of the sheets
on completion of docking, called Fl_final, which is
naturally substantially identical to the force
F1 target, becomes known and is registered by means of
the unit 88.
In addition, error detection by means of the value
of the distance Dl final registered can be carried out.
In fact, if this value is not in a predetermined range,
it can then be concluded that the device is incorrectly
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positioned relative to the sheets, or that these sheets
comprise a form beyond tolerance.
Then, the clamping operation is initiated, which
is started as soon as the docking operation ends, with
optionally a stop time between these two operations.
Identically to that encountered within the scope of the
previous operation clamping is performed bv controllina
linear shift of the carriage 60 with the unit 88 to
produce reinforced adherence between the head 62 and
the sheets 80 in contact. It is noted that during this
operation the control unit 88 periodically determines
on the one hand the value of the absorbed motor power
P2_absorbed by the system, 8, this value P2 absorbed
then being converted by the converter to produce a
value of the resistance force of the sheets to the
clamping F2, and on the other hand the clamping
distance D_clamping corresponding to the real distance
travelled by the point C of the carriage between the
point of the optical rule 32 at the level of which it
is located at the instant t in question, and the point
Cl of this rule. Here too, it is specified that the
force F2, updated every 5 ms as is the value
D_clamping, also corresponds in value to a sinking
effort of the sheet press system 8 against the sheets
80.
Command of this clamping operation is provided so
that shifting of the carriage 60 is completed when the
determined force F2 has reached a target value
F2_target, or when the clamping distance D clamping has
reached a target value D_clamping_target, the clamping
SP 2-83 47 AP CA 02647156 2008-09-25
operation thus being completed as soon as any one of
these two target values has been reached.
By way of indication, the target value F2 target
car_~ for examrle be fixed at a value of ~-he order of 150
5 N, and the target value D clamping target can for
example be fixed at a value of the order of 500 pm. As
shown in figure 6c on completion of the clamping
operation the carriage 60 has travelled a distance
D2 final between point R and point C2 of the rule 32 at
10 the level of which the point C of the carriage 60 is
located, the value of this distance D2 final measured
by means of the rule 32 being returned to the unit 88.
This then produces the final clamoing distance
D clamping final actually covered by the system 8, by
15 subtracting Dl final from D2 final. Also, Knowledge on
the one hand of the dimensions of the system 8 and on
the other hand of the real position of the latter on
the chassis 2 on completion of the clamping operation
helps determine the exact position of the restricted
20 sheets 80 relative to the chassis 2. In this respect,
the unit 88 can then determine then store the distance
D sheets final corresponding to the distance according
to the direction 18 between the point R of the rule 32
and the front end of the sheet press head 62 on
completion of the clamping operation.
This specificity is advantageous since it best
optimises the linear shift of the piercing system 4
during the subsequent piercing step, to the extent
where this system 4 can be controlled at high speed
over a precise distance fixed as a function of the
distance D sheets final, before being slowed to the
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advance speed of the tool previously determined. In
addition, knowing this distance D sheets final, of the
order of 200 mm, precisely fixes the distance of change
in rotation speed of the piercing tool for tr:e milling
attack, when a boring-milling tool stage is used.
Finaliy, another advantage is that the depth of the
milling can be fully respected. In this way, it is
indicated that the course of subsequent milling can
also be corrected as a function of the information
Info stiffness determined as described hereinbelow, and
also optionally as a function of diverse
characteristics of the rivets employed. Tn this
respect, it is noted that the weaker the local
stiffness of the sheets, the more these are deformed by
the thrust of the sheet press head, and thus the more
the centre of this sheet press head is moved away from
these same deformed sheets. Therefore, the weaker the
local stiffness of the sheets, the greater importance
of the course of milling relative to the sheet press
system to obtain a determined depth of milling.
In addition, error detection can also be conducted
by means of the value of the registered distance
D clamping final. In fact, if this value is not in a
predetermined range it can be concluded that the device
is incorrectly positioned relative to the sheets, or
these sheets comprise a form beyond tolerance. In
addition, at the end of the clamping operation stopped
when the target value D_clamping_target has been
reached, the value of the resistance force of the
sheets on completion of clamping, called E2_final,
becomes known and is registered by means of the unit
SP 2834 7 AP CA 02647156 2008-09-25
22
88. If this value is too low, it can be considered that
the structure constituted by the sheets is non-
existent.
With the value of the resistance force o` the
sheets on completion of clamping F2 final it is
possible to determine, still by means of the unit 88,
the information Info stiffness by establishing the
following ratio :
Info stiffness=(F2 final-FI final)/D clamping final
This information on the local stiffness of the
sheets, whereof the value is for example of the order
of 30 kg/mm, is then supplied to the second control
means 86 linked to the piercing system 4, and more
particularly to the correction matrix 82 equipping the
unit 92. As indicated previously, this information
Info stiffness is provided to predispose the advance
speed and rotation speed instructions of the tool 17
used during control of the piercing step.
Then, the piercing step is effectively undertaken,
consisting of setting in motion the carriage 10 of the
piercing system 4 such that the latter passes through
the sheet press system 8, and also passes through the
two sheets 80 to be assembled to produce the desired
hole and milling.
Piercing is carried out by controlling the linear
shift of the carriage 10 with the advance speed
instruction of the tool such as previously determined
and originating from the matrix 82, and by
simultaneously controlling rotation of the spindle 12
sP 28347 AP CA 02647156 2008-09-25
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with the rotation speed instruction of the tool also
originating from this matrix 82, these instructions
being supplied to the feedback control cards 94 and 96
r~.'spectlvely.
During this piercing step, the value of a
resistance force of the sheets F3 resulting from
support of the sheet press system 8 on the sheets 80 is
determined periodicallv. This determination of F3 is
preferably executed in the same way as that adopted for
determining Fl and F2. In this way, it is indicated
that the motor linked to the carriage 60 of the sheet
press system continues to be fed during piercing, and
that it is synchronised in position such that the
carriage 60 retains its position in C2 on the chassis
2.
By way of indication, F3 is updated every 5 ms and
corresponds in value to a sinking effort of the sheet
press head 62 in the sheets 80, during piercing.
This allows periodical comparison during piercing,
by means of the unit 92, of the value of this force F3
to a minimal value F3 min, the minimal value F3 min
able for example to be fixed at 5 N.
When it is detected that F3 is less than F3 min, a
decrease in the advance speed instruction of the
piercing tool is then ordered via the matrix 82, so
that the value of the force F3 rises above the minimal
value F3 min. Therefore, this way of operating
advantageously ensures that the sheet press head 62
does not lose contact with the sheets 80 during the
piercing operation, following excessive thrust of the
piercing tool 17 on these sheets.
CA 02647156 2008-09-25
SP 28347 AP
24
Finally, once the piercing step is complete the
step of placing the rivet can be started by placing in
motion the riveting system 6 in the appropriate way.
Of course, various modifications can be made by
the expert to the invention described hereinabove,
purely by way of non-limiting example.
