EQUIPMENT COMPRISING A ROTATABLE CRADLE AND INTENDED
FOR ENCASING THE AIRFOIL OF A TURBINE BLADE IN ORDER TO
MACHINE THE ROOT
5 The present invention relates to the field of aeronautics and, more
specifically, to the field of manufacturing turbine blades for turbomachines.
Turbomachine blades generally consist of three parts: an upper part (or
shroud), a middle part (or airfoil) and a lower part (or root), the upper and lower
parts being separated from the airfoil by a platform. The airfoil is designed to be
10 positioned in the gas flow in order to extract work through expansion of the gases
in the flow, enabling it to drive the rotor of the turbomachine. An essential element
in the production of a blade is ensuring the correct orientation of the airfoil relative
to the root because, as the root is attached to a disk and is therefore immobile
relative to the rotor of the turbomachine, this orientation determines the position of
15 the airfoil in the flow of gases and, consequently, its aerodynamic efficiency.
The first stage in the manufacture of a blade is generally the creation of a
casting which has the finished dimensions as regards the airfoil but is only a blank
as regards the root and the shroud. The root and the shroud must then be machined
to give them their definitive shape. Machining the root, in particular, is very
2 0 important because it is this process which dictates the correct orientation of the
airfoil relative to the flow of air. It is important to carry out this step of machining
the root without increasing the uncertainty regarding the positioning of the airfoil
relative to the root, in particular by avoiding excessive machining tolerances being
- added to the tolerances linked with the production of the airfoil.
25 Machining the root is traditionally carried out by positioning the blade in a
known reference frame, connected with the machine tool, and embodied by six
contact points which act as stops, against which the blade must press. A reference
plane, oriented parallel to the direction of the leading edge of the blade and
defining the orientation of the airfoil of the blade relative to a face of the root
30 thereof the purpose of which is to come into contact with the slot of the disk on
which the blade is to be mounted, is generally defined from these six points. The
optimum aerodynamic efficiency of the airfoil is obtained if machining its root
results in this reference plane of the blade corresponding to an ideal plane of
orientation of the blade, also defined relative to the same face of the root.
Some of the points of the blade which are in contact with the stops are
embodied by a reference point on the outer surface of the blade, which reference
5 point ii generated during forging or casting. The next step, which is essential for
achieving correct positioning of the root relative to the airfoil, involves clamping
the blade so that it does not move during machining of the root and so that the
sides of the bulb or fir-tree shape of the root are correctly oriented. One of the
difficulties associated with this operation is due to the three-dimensional shape of
10 the airfoil which has no planar surface against which a perfectly braced pressure
can be applied.
One known technique involves mechanically clamping the airfoil in a
reference frame embodied by six points linked with the machine tool by pressing a
gripping part against the airfoil. Once the airfoil has the correct orientation, i.e.
15 once it has been rotated through what is referred to as a preselection angle which
will impart to the airfoil the correct angle of attack in the flow of gases in the
turbomachine, a face of the root which will act as a spatial reference for all
subsequent steps is machined first of all. The following steps of machining the
shape of the root and then the shape of the shroud are then carried out by
2 0 positioning the machined face of the root against an appropriate reference frame
which is defined on the machine tool.
First of all, this technique does not guarantee perfect stability during
machining of the face of the root acting as the spatial reference because the
pressure, which is generally provided at the center of the suction face of the airfoil,
25 is limited by the fact that it acts in only one direction. The pressure which can be
applied is also limited by the strength of the airfoil and by the strength of the skin
which represents the thickness of this suction face. Without sufficient pressure, the
airfoil can move during machining; excessive pressure, however, would result in
the suction-face surface being marked, which could be accompanied by
30 deformation of the profile of this suction face, which is relatively thin. This
technique then accumulates uncertainties regarding the position of the elements to
be machined, since first of all a reference frame linked to the airfoil is used when
placing the latter on the machine tool, then the root is machined in a reference
frame linked to the machine tool, and finally the shroud is machined in a reference
frame linked to a face of the root.
Another frequently used technique involves embedding the airfoil, or at least
a substantial portion thereof, in an encasing block made of a low-melting point
5 material, such as an alloy of tin and bismuth. A face of the blade root is machined
first of all. This face then serves as a spatial reference for positioning the blade in a
six-point reference frame. The airfoil is then embedded in the encasing block of
low-melting point material. Thus, the issue of precise positioning of the airfoil is
becomes that of the positioning of the encasing block which is designed to have
10 planar surfaces which will act as reference planes for the subsequent machining
steps. After machining of the root and the shroud, the block is removed by melting
and the blade regains its normal outer shape. However, this method still has the
drawback of increasing the uncertainties in the positioning of the root by adding
the tolerances resulting from placing the block around the airfoil to the tolerances
15 of manufacturing the airfoil.
The object of the present invention is to provide a solution to these
drawbacks by proposing a method for defining a reference frame for the spatial
positioning of a blade for machining the root thereof, which method does not
20 involve the drawbacks of the prior art and, in particular, does away with the
uncertainties caused by the changes in planes of reference used for machining the
ends of said blade. The object of the invention is also to propose equipment
allowing the root of a turbine blade to be machined using this method.
-
25 To that end, the invention relates to equipment for encasing the airfoil of a
turbomachine blade in an encasing block, comprising a base on which a molding
block is positioned for casting the encasing metal around said airfoil, said molding
block comprising an imprint which defines a reference plane on the block acting as
a support for positioning the block in space, said base further carrying a cradle for
30 holding the blade by means of six contact points forming a reference frame for
positioning said blade in space and defining what is referred to as an orientation
plane which represents the angular position of the airfoil relative to the root of the
blade, said reference planes and the orientation plane being oriented substantially
parallel to the direction of the leading edge of the blade when the latter is
positioned against said six contact points, characterized in that the cradle is able to
move in rotation relative to said base about an axis oriented substantially parallel to
the direction of the leading edge of the blade, so as to allow the angle formed
5 between said reference and orientation planes to be changed.
The result is improved precision in positioning the airfoil of the blade in the
flow of gases of the turbomachine, as the equipment makes it possible to use just
one change of reference frame for positioning the airfoil in space, this being done
directly from the six points of the blade toward a plane of the encasing block.
10 Subsequently, machining the root and the shroud is carried out, for all the blades,
from a same reference plane, which is that located on the encasing block.
In one particular embodiment, the cradle is formed of two elements
positioned on either side of the molding block and connected rigidly to one another
by a connection means.
15 More preferably, the contact points are distributed over the two elements.
The means for holding the blade are thus distributed further from each other,
which represents an advantage for more stable holding of the blade.
Advantageously, the imprints of the molding block define two planes
oriented substantially parallel to the direction taken by one of the lateral faces of
2 0 the platform of the root of the blade in position on said equipment, each being able
to serve as first reference plane.
Such a configuration makes the subsequent machining steps easier.
Moreover, the volume of the encasing block is smaller; this reduced size provides
for more flexibility for installing the equipment for machining the root and the
25 shroud and therefore offers the possibility of carrying out both steps
simultaneously.
The equipment preferably comprises a means for adjusting the angle of
rotation of the cradle, the effective range of which is plus or minus 2".
The invention also claims a method for encasing a turbomachine blade using
30 a low-melting point metal for holding said blade during machining of its root
and/or of its shroud, the airfoil of said blade having its finished dimensions, said
root being in the state of a blank and having a face oriented substantially parallel to
the direction of the leading edge of the blade, forming a spatial reference for the
rotational position of the airfoil in space, said method comprising a step of
measuring the preselection angle made by a plane referred to as an orientation
plane which represents the rotational position of the airfoil in space, with the ideal
orientation plane for said airfoil defined as giving the optimum orientation for the
5 airfoil in the flow of gases of the tdrbomachine, the two planes being oriented
substantially parallel to the direction of the leading edge of the blade and their
orientations being defined in terms of angles relative to the face of said root which
acts as spatial reference, said method further comprising a step of casting said lowmelting
point metal around said airfoil in a molding block to form an encasing
10 block. It is characterized in that it comprises, before casting the low-melting point
metal, a step of rotating said airfoil relative to the molding block through an angle
equal to the measured preselection angle.
By means of the relative position given to the walls of the block relative to
the airfoil, this rotation allows the airfoil to be oriented directly in the correct
15 position for machining the facets of the root thereof, and also the shroud thereof.
In conclusion, the invention claims a method for producing a turbine blade
comprising a step of encasing the airfoil thereof using the method as described
above.
Advantageously, the above method provides that the root and the shroud are
2 0 machined simultaneously on the same machine tool, the blade being held on said
machine by means of the encasing block.
There is thus an appreciable time saving in the production of the turbine
blades, with no loss of quality as regards the orientation of the airfoils relative to
their ideal position in the flow of gases of the turbomachine.
25
The invention will be better understood, and other objects, details, features
and advantages thereof will appear more clearly during the following purely
illustrative and non-limiting detailed explanatory description of one embodiment of
the invention, with reference to the attached schematic drawings.
30 In these drawings:
- figure 1 is a perspective view of a turbine blade of a turbomachine in its
raw as-cast state, before the machining of its root and shroud;
- figure 2 is a perspective view of a turbine blade embedded in an encasing
block;
- figure 3 is a view from below of the encased turbine blade of figure 2;
- figure 4 is a perspective view of part of a piece of equipment for encasing a
turbine blade, according to one embodiment of the invention;
- figure 5 is a detail view of figure 4, showing one particular element of the
encasing equipment, and
- figure 6 is a second perspective representation of the part of the encasing
equipment of figure 4.
Figure 1 shows a turbine blade 1 having, from bottom to top (i.e. from left to
right in the figure), a root 2, an airfoil 3 and a shroud 4. The blade 1 is cast with its
airfoil 3 having its finished dimensions, i.e. one which needs no further shaping,
whereas the root 2 and the shroud 4 are blanks which need to be machined in order
to have their final shape. In particular, the precise orientation of the root relative to
the airfoil must be defined during this machining, by means of rotation, relative to
a current flank of the root, by a preselection angle which can vary by +/- 2", in
order to obtain the best possible aerodynamic performance for the blade. The
shroud will then be machined to give, in a known manner, sealing lips.
Figure 2 shows a blade 1 imprisoned in an encasing block 5, with the
majority of its airfoil 3 embedded in the low-melting point metal, while the root 2
and the shroud 4 are free and can be fashioned by a machine tool. The encasing
block 5 has parallel planes 5a, 5b which extend substantially parallel to the leading
edge 8 and which have precise angular orientations, relative to the orientation of
the airfoil, so as to serve as reference plane for positioning the airfoil in space,
during installation thereof on a machine tool.
Figure 3 also shows the blade 1, seen from below, embedded in the block 5,
with its root 2 and the platform 7 delimiting the root. The block 5 is shown here in
the form of a right-angled parallelepiped with two parallel faces 5a, 5b, at least one
of which will act as a plane of reference during machining of the ends of the blade.
These two faces are positioned substantially parallel to two of the lateral faces of
the platform 7 of the blade root.
WO 2012/052666 PCT/PR2011/052411
7
In parallel, the airfoil 3 has what is referred to as an orientation plane 6,
oriented parallel to the direction of the leading edge 8 of the blade, which in this
case corresponds substantially to the central surface of the airfoil, and which is
defined precisely in relation to'the six reference points of the blade which define
the position of the airfoil thereof in space. The orientation of this plane 6 relative to
a flank 2a of the root 2, or of a lateral face 7a of the platform 7 thereof, which is
thus taken as that face of the root which acts as a spatial reference from which the
rotational position of the blade in space is defined, controls the angle of attack
which the airfoil of the blade will have relative to the flow of gases when the blade
is installed in the turbomachine. As mentioned before, the optimum aerodynamic
efficiency of the airfoil is obtained if machining its root results in the plane of
orientation of the airfoil corresponding to an ideal plane of orientation of the
airfoil. This ideal plane, also defined relative to a face of the platform 7 or a flank
of the root 2 of the blade, is supplied by the company's design department.
Controlling the angle that the plane of face 5a or 5b of the encasing block 5 makes
with the plane of orientation 6 of the airfoil is precisely the object of the invention.
It is shown as equal to zero in figure 3.
Referring now to figure 4, this figure shows the main pieces of equipment 10
for the purpose of casting the encasing block 5 onto the airfoil 3 of the blade 1.
This equipment 10 essentially consists of three parts: a base 11 fixed to the floor, a
molding block 12 which is also fixed and borne rigidly by the base 11, and a cradle
13 borne by the base 11 and able to rotate about a horizontal spindle 30 relative to
this base 11 and to the molding block 12. The equipment 10 here carries a blade 1 . .-
which can be seen in the cradle 13. This blade is arranged with its leading edge 8
in a substantially horizontal position, parallel to the spindle axis 30.
The molding block 12 is positioned between two elements 13a and 13b of the
cradle, which grip it and which are connected to one another by a rigid connection
means, located behind the equipment 10. This is a conventional mold for casting a
low-melting point metal around an airfoil having, in the first instance, an imprint
designed to produce the parallel faces 5a and 5b of the block 5 with high precision
as regards the orientation thereof and, in the second instance, a casting aperture
located in the upper portion (not shown) and two plates 22 located on either side of
the molding block, between the molding block 12 and the two elements of the
moveable cradle 13, so as to contain the liquid metal during pouring and to limit
the extent of the block 5 along the airfoil 3. The molding block is shown here only
in part, a complementary part being placed in front of the airfoil 2 of the blade in
order to encase it completely during pouring.
5 The cradle 13 can rotate about the horizontal spindle 30, which is borne by
two bearings held by the base 11. An adjusting means 32 adjusts ;he rotation of the
cradle 13 and, as a result, the angular position of the cradle relative to the base 11,
and hence relative to the molding block 12, can be known and adjusted. It is this
means that allows the preselection angle, i.e. specifically the desired angle of
10 separation there is to be between the plane of orientation 6 and the planes of the
faces 5a and 5b of the encasing block, to be set.
Figure 5 shows the moveable cradle 13 on its own, without the blade 1. It has
six contact points for holding a blade at the level of the six reference points thereof
for positioning it in space. Five contact points X1, X2, X3, Y1 and Y2 define
15 precisely the spatial position that the airfoil 2 must have when the low-melting
point metal is poured and, in particular, the orientation of its plane of orientation 6
in the rotation about the spindle 30. A sixth point Z defines the longitudinal
position of the airfoil on the spindle 30 by cooperation with the shroud 4 of the
blade; an uncertainty regarding the position of the airfoil on this spindle has no
2 0 negative consequences, as it results in the end in a simple longitudinal offset of the
block 5 on the airfoil 3.
Figure 6 shows the equipment of figure 4 without the encasing block so that
the position of the blade 1 is apparent, with four contact points in position against
four of the six reference points for positioning the blade in spaceithe last two not
2 5 being visible.
There follows a description of the process for mounting and then encasing a
blade 1 in a block 5 on a pouring equipment 10 in accordance with one
embodiment of the invention.
30 The as-cast blade I is analyzed during its quality control after the casting
step, during which quality control its orientation with respect to a lateral face of the
as-cast platform 7 is checked. The angular measurement carried out makes it
possible to define by what angle the root 2 must be offset by adjusting the
orientation of the flanks thereof or of the lateral faces of the platform 7 during
machining thereof, so that the airfoil 3 is correctly oriented in the flow of gases of
the turbomachine. The preselection angle is equal to the angle between the plane of
orientation 6 and the ideal plane of orientation of the airfoil in space, i.e. that
5 which would give the latter an ideal orientation in the flow of gases. This
, preselection angle is then measured, as in the prior art, by comparing the
orientation of one of the lateral faces 7a of the platform 7 or of a flank 2a of the
root 2 of the blade, with the ideal orientation which is defined in connection with
the six reference points of the blade for positioning the blade in space.
10 Once this angle is known, the blade is positioned in the cradle by aligning its
six positioning reference points against the six contact points X1, X2, X3, Y1, Y2
and Z of the cradle 13. By using the adjustment means 32, the operative causes the
cradle 13 to rotate on its spindle 30, relative to the base 11, by an angle equal to the
previously measured preselection angle, and in a direction which corrects the
15 observed angular offset.
The operative then pours low-melting point metal, in the liquid phase, into
the molding block 12 through the casting aperture. The metal spreads around the
airfoil 3 of the blade, within the limit allowed by the internal imprints of the mold
and by the two lateral plates 22. Once the encasing block 5 has solidified, the
2 0 encased blade is removed from the equipment 10 and is ready to be mounted in a
machine tool for machining its root 2 and shroud 4. Once the preselection angle
has been corrected by the rotation performed on the airfoil before encasing, the
faces 5a and 5b of the block are correctly oriented relative to the plane of
-orientation 6 of the airfoil; they will then be able to serve as positioning reference
25 for the airfoil and therefore as reference for positioning the block in the machine
tool.
There is no longer any need to transfer the reference of the airfoil 3 onto one
of the lateral faces of the platform 7 in order to position the blade before machining
the root 2 thereof, or to wait for machining of the root to be complete to use this as
3 0 reference for machining the shroud 4. It is thus possible to machine these two ends
simultaneously by placing the encased blade in a machine tool comprising, for
example, twin grinding wheels.
The position of the airfoil of the blade being thus correctly defined right from
the casting of the block (9, there isho longer any need to change the settings of
the machine tools when changing from one blade to the next, as long as the roots of
one and the same type of blade are being machined.
5 An appreciable time saving in the producfion of a turbine blade is thus
achieved, as well as better positioning thanks, to the reduction in the number of
successive transfers of reference frame.

CLAIMS
1. Equipment for encasing the airfoil (3) of a turbomachine blade (1) in an
encasing block (5), comprising a base (11) on which a molding block (12) is
positioned for casting an encasing material around said airfoil (3), said molding
block comprising an imprint which defines a reference plane (5a, 5b) on the block
(5) acting as a support for positioning the block (5) in space, said base (1) further
carrying a means (1 3) for holding the blade (1) by means of six contact points (XI,
X2, X3, Y1, Y2, 2) forming a reference frame for positioning said blade in space
and defining what is referred to as an orientation plane (6) which represents the
angular position of the airfoil (3) relative to the root (2) of the blade (I), said
reference planes (5a, 5b) and the orientation plane (6) being oriented substantially
parallel to the direction of the leading edge (8) of the blade (1) when the latter is
positioned against said six contact points,
characterized in that the holding means (13) is able to move in rotation relative to
said base (1 1) about a spindle (30) oriented substantially parallel to the direction of
the leading edge of the blade (I), so as to allow the angle formed between said
reference and orientation planes to be changed.
2. The equipment as claimed in claim 1, in which the holding means (13) is
formed of two elements (13a, 13b) positioned on either side of the molding block
(12) and connected rigidly to one another by a connection means.
3. The equipment as claimed in claim 2, in which the contact points are
distributed over the two elements (1 3a, 13b).
4. The equipment as claimed in one of claims 1 to 3, in which the imprints of
the molding block (12) define two planes (5% 5b) oriented substantially parallel to
the direction taken by one of the lateral faces (7a) of the platform (7) of the root of
the blade (1) in position on said equipment, each being able to serve as first
reference plane.
5. The equipment as claimed in one of claims 1 to 4, comprising a means
(32) for adjusting the angle of rotation of the holding means (13), the effective
range of which is plus or minus 2".
6. A method for encasing a turbomachine blade (1) using a meltable material
for holding said blade (1) during machining of its root (2) andlor of its shroud (4),
the airfoil (3) of said blade having its finished dimensions, said root (2) having a
face (2a) oriented substantially parallel to the direction of the leading edge (8) of
I the blade, forming a spatial reference* for the rotational position of the airfoil in I
I I space, said method comprising a step of measuring the .preselection angle made by i
I 5 a plane referred to as an orieMation plane (6) which represents the rotational i
position of the airfoil (3) in,space, with the ideal orientation plane for said airfoil
defined as giving the optimum orientation for the airfoil (3) in the flow of gases of
the turbomachine, the two orientation planes being oriented substantially parallel to I
the direction of the leading edge (8) of the blade and their orientations being - -
10 defined relative to the face of said root which acts as spatial reference, said method 1
further comprising a step of casting said meltable material around said airfoil in a
molding block (12) to form an encasing block (3,
characterized in that it comprises, before casting the material, a step of rotating
said airfoil (3) relative to the molding block (12) through an angle equal to the
15 measured preselection angle.
. . 7. A method for producing a turbine blade comprising a step of encasing the
' , .' - .
.A> 6:-
*?, :< <
airfoil (3) thereof using the method as claimed in claim 6.
- -
,' - , . 8. The method as claimed in claim 7, in which the root (2) and the shroud (4)
' - -
s : ~ . - " - , , are machined simultaneously on'the same machine tool, the blade (1) being held on
.L ;p>**.,?. # . I ,. r
OF REMFRY &. SAGAR
ATTORNEY FOR THE APPLICANT[S]