The domain of the invention relates to
calibration heads for the drilling of shafts,
particularly turbine shafts. In particular, the
domain of the invention deals with the structure
of a calibration head adapted to provide better
straightness, greater stability and better reaming
precision.
STATE OF PRIOR ART
The turbine shaft calibration operation is a
special method that requires taking account of very
15 restrictive dimensional constraints such as the surface
condition, the straightness of the reaming with low
tolerance of the order of a few hundredths of a
millimetre to guarantee a constant shaft diameter and
particularly the thickness of machined shafts and the
20 variation of the Lhickness: the delta thickness.
One dlificulty is in making a reaming when it is
done blind, in other words over a considerable length
inside a part.
In general, the calibration operation is done
25 using a drilling machine with a bar fitted with a
calibration head at one end. This operation governs
the finishing operation that is done using a "bottle"
type form with a so-called "recessing" drilling head,
this term usually being used to refer to the extension
30 and retraction of tools.
A calibration head forms a particularly sensitive
part of a drilling tool and it has to be designed to
satisfy severe shaft drilling constraints. In general,
a calibration head according to the state of the art is
5 fitted with machining inserts and guide pads arranged
around the outside surface of said head. Machining
inserts may relate to rough machining inserts and/or
finishing inserts.
The distribution of guide pads and machining
10 inserts on the head surface and the positioning
precision of their adjustment must be such that
straight guidance is possible during drilling so that
machining can be guaranteed while minimising surface
defects to respect dimensional constraints.
15 Figure 1 shows an example tool for drilling a
shaft 4. The tool comprises a steering shaft 2 and a
calibration head 1 fixed to one of the ends of the
steering shaft 2 with a rotation axis 6. The tool is
previously inserted into a cavity in the shaft to be
20 drilled for pre-calibration.
The shaft to be drilled is usually prc-drilled
during a pre-drilling operation so as to remove a large
proportion of the material from the shaft to be drilled
so that a first cavity 5 and a second cavity 3 can be
25 drilled in said shaft.
The pre-drilling and pre-calibration operations
are used to optimise preparation of the finishing phase
using a tool comprising a calibration head.
It is found that guidance using two pads located
30 on the front of the calibration head does not result in
head stability thus creating vibrations with the
consequence of causing a degradation in the surface
condition. Other defects may also appear, due to a
non-optimised arrangement of the guide pads. In
particular, potential degradation includes scratches,
5 concentricity defects and work-hardened surface
defects.
Figure 2 shows a front view of a calibration head
according to the state of the art showing two rough
machining inserts 12, 13 and a finishing insert 14
10 slightly set back from the end of the calibration head
1. The front guide pads 11 and 15 are also shown, and
are located in front of the calibration head, in other
words in the part of the head that will be inserted
first into the internal cavity of the shaft to be
15 drilled.
Two back pads are also shown nearby but they are
not referenced.
Nevertheless, at the present time, calibration
heads have many disadvantages when they are used to
20 drill shafts over a long distance.
A first problem with existing calibration heads is
that they induce a bad distribution of cutting and
guidance forces. The number of pads appears to be
insufficient, but increasing the number of pads could
25 increase friction.
Consequently, degradation on the calibration head
itself can quickly occur, particularly on the front
guide pads that can break.
A second problem that is difficult with existing
30 solutions is that it is very difficult to adjust rough
machining and finishing inserts and guide pads.
For the rough machining and finishing inserts,
scaling can occur around the periphery of the
attachment bolt hole. This phenomenon is due to random
packing because there is no sufficiently precise
5 adjustment system for this type of drilling.
There is also a third problem that results from
the position of the cutting angle of finishing inserts
that is generally very small. Vibrations during
drilling may 'be amplified if the adjustment is not
10 adjusted as a function of the adjustment of the rough
machining inserts and if the latter adjustment is not
sufficiently precise. The repercussion of vibrations
on the stability of the tool and the calibration head
during drilling can lead to errors and degradation to
15 the inside surface of the shaft.
Consequently, calibration heads according to the
state of the art have a number of disadvantages such
that shafts cannot be drilled guaranteeing sufficient
drilling precision, stability during drilling and
20 straightness of the drilling direction. These
disadvantage,^ also apply to the heads themselves that
can be degraded.
It is then necessary to limit the repercussions of
these constraints by different actions, for example
25 such as replacement of parts or tools that may be very
expensive.
Furthermore, the ratio of drilled and rejected
shafts is high. One problem lies in the domain of
turbines, particularly in the aeronautical sector for
30 which parts are very expensive, large and for which
excessive errors cannot be tolerated particularly when
machining the parts.
SUMMARY OF THE INVENTION
The invention can solve the above-mentioned
disadvantages.
The purpose of the invention is a cylindrical
calibration head for drilling a shaft. The calibration
head according to the invention comprises three flutes
uniformly distributed around the circumference so that
they are separated from each other by an angle of 120"
in rotation about the axis of the calibration head, the
flutes being used for evacuation of chips and passage
of the lubrication during drilling, each of the flutes
containing a machining insert, the position of which is
adjusted in the flute by means of an adjustment
cartridge.
Advantageously, the calibration head comprises at
least one front guide pad arranged on the outside
surface of the head in a zone corresponding to the
front end of the head, and at least one back guide pad
placed on the outside surface of said head in a second
zone defining the back zone of said head, one or each
back guide pad being placed behind one or each front
guide pad, along the same axis as the front guide pad.
Advantageously, the calibration head comprises
three front guide pads located in a zone defining a
front end of the calibration head, the three pads being
arranged uniformly on the outside surface of the head
so that they are separated from each other by an angle
of 120" relative to rotation about the axis of the
calibration head.
Advantageously, the positions of the front guide
pads and the flutes on the outside surface of the
5 calibration head are alternated such that the angle
between each front guide pad following a flute along
the circumference of the calibration head forms an
angle of 60" relative to rotation about the axis of the
calibration head.
10 Advantageously, the calibration head comprises a
plurality of back guide pads arranged on the outside
surface of said head in a second zone defining the back
zone of the head. The plurality of pads may correspond
to four back pads. Two back pads are then arranged
15 symmetrically relative to each other about the axis of
the head. According to one embodiment, at least one
back pad is axially offset from a front pad.
According to another compatible embodiment of this
embodiment, each of two other back guide pads is
20 located behind a front pad.
Advantaqeously, the calibration head comprises a
set of guide pads arranged on the outside surface of
the head, with a number and thickness so that a minimum
lubricant flow can be maintained while minimising
25 friction.
Advantageously, one flute contains a finishing
insert and each of two flutes contains a rough
machining insert.
According to one embodiment, one of the two back
30 pads that are positioned symmetrically relative to the
axis of the head, is placed at 30" from the rough
machining insert or a finishing insert.
Advantageously, each flute is tapered at an angle
equal to approximately 25" within 10%.
5 Advantageously, the flutes are sized to allow the
injection of a flow of 380L/min of lubricant within 15%
for a pressure of 15 bars within 15% in the calibration
head in the shaft to be drilled.
Advantageously, the length of the flutes along the
10 axis of the calibration head is more than 5 0 h f the
length of the calibration head.
Adva~tageously, each machining insert has three
edges that can be configured so as to select a working
edge by means of the adjustment cartridge.
15
BRIEF DESCRIPTION OF THE FIGURES
Other characteristics and advantages of the
invention will become clear after reading the following
detailed description with reference to the appended
20 figures that show:
- figurc 1: a drilling tool comprising a
calibration head according to prior art;
- figure 2: a front view of a calibration head
including machining inserts and guide pads;
- figure 3: a drilling tool comprising a
calibration head according to the invention;
- figure 4: a front view of a calibration head
according to the invention;
- figure 5: a perspective of a flute in a
calibration head according to the invention;
-. figure 6: a rough machining insert of a
calibration head according to the invention;
- figure 7: sectional views of the front and
back zone of a calibration head according to
the invention.
DESCRIPTION
In the remainder of the description, "drilling
straightness" refers to the straight and fixed
10 direction along which a shaft is drilled. The
straightness requirement is respected when it is
possible to drill straight along a fixed direction,
particularly in a shaft.
In the remainder of the description, we will
15 consider that general shape of the calibration head is
approximately cylindrical. The average cylinder
comprises a central axis that corresponds to the axis 6
of the calibration head.
In the remainder of the description, a "thickness"
20 refers to the difference between the inside diameter
and the outside diameter. The "delta thickness" then
corresponds to the variation in this thickness.
A calibration head according to the invention
discloses an arrangement of machining inserts on the
25 outside surface of the calibration head to prevent any
unbalance of the head during drilling.
"Regular arrangement" refers to inserts or flutes,
an arrangement of these inserts or flutes so that the
geometric centre of gravity of the calibration head is
30 made to coincide, at least approximately, with the
centre of gravity of the calibration head. A regular
arrangement of the inserts or flutes on the calibration
head may be considered either by taking account of the
arrangement of the pads, or without taking them into
account.
5 In particular, when the number of pads distributed
on the head surface, at a given position of the head
axis, is not a multiple of the number of flutes or
machining inserts, then the regular arrangement of the
flutes or inserts on the head does not take account of
10 the distribution, number and arrangement of the guide
pads.
An appropriate distribution of machining inserts
can enable straight drilling, the required lubricant
flow as far as the end of the head and sufficient
15 evacuation of cut chips of material. Finally, the
balancing between appropriate drilling pressure and the
necessary lubricant flow is improved.
Figure 3 shows a calibration head 1 according to
the invention. This head comprises three flutes 20,
20 21, 22 that are also shown in the front view of the
head in figure 4. The flutes are preferably
geometrically approximately identical.
The calibration head according to the invention
comprises three flutes machined at the end of the
25 calibration head and extending along the direction of
the axis of rotation 6 of the head towards the back of
the head. The flutes are machined such that they are
distributed at equal distances from each other around
the circumference of said head. The three flutes form
30 an equilateral triangle in the plane normal to the axis
6, considering three points each taken at the same
position on each flute.
The axial distribution is also symmetric, the
three flutes have approximately identical lengths and
5 depths.
Therefore, the weight of the calibration head is
distributed symmetrically to define a centre of gravity
with circular symmetry properties about the axis 6.
In a configuration of axial symmetry, the flutes
10 are located at 120" from each other, considering the
axis 6 of the head as the reference axis of the
symmetry.
In this configuration the machining inserts,
including the rough machining and finishing inserts,
15 are arranged so that they are held in place in each of
the flutes. They are preferably held in place on the
same side of each of the flutes with which they are
associated. With this configuration, a uniform
symmetry of the head can be respected with a good
20 weight distribution. The geometric centre of gravity
of the calibration head is approximately coincident
with its centre of gravity.
Means of attachment and adjustment are provided so
that the inserts can be firmly fixed while enabling a
25 simple disassembly solution to either replace an insert
or to modify its orientation so as to replace the
cutting edge.
In one embodiment, the attachment and adjustment
means are adjustment cartridges that enable fine
30 adjustment of the cutting angle of the edge and the
height projecting from the flute. Furthermore, the
adjustment cartridges can act as a support to hold the
machining inserts 30, 32, 35 in position. Figure 4
shows the adjustment cartridges 31, 33, 34 that hold
the inserts in position in the flutes.
5 According to one configuration of the calibration
head to increase the efficiency of cutting the shaft to
be drilled, the angle of attack of a rough machining or
finishing insert may be high, for example greater than
20" from the axis of a radius of the axis 6 of the
10 head.
The use of adjustment cartridges contributes to
maintaining a constant cuttlng force through the
drllling period.
The calibration head according to the invention
15 also includes guide pads. The guide pads on the
calibration head include three front pads present on
the outside surface of the head in a zone corresponding
to the front end of the head.
The pads are present on the outside surface of the
20 head and they are of the order of 1 to 2 mm thick. In
one preferred embodiment, the thickness of the pads is
1 mm. This thickness of 1 mm within 30% is
particularly advantageous for limiting friction, while
enabling a lubricant flow circulating around the
25 outside surface of the calibration head.
According to the invention, the three front guide
pads 36, 37, 38 are distributed around the
circumference of the outside surface of the head in a
balanced distribution. The front guide pads are then
30 arranged at a spacing of an angle of 120" from each
other relative to the central axis 6 of the calibration
head. They also form an isosceles triangle in a plane
normal to the axis 6. Each front guide pad is
equidistant from the other front guide pads.
Thus, the three pads balance the weight of the
calibration head and the forces applied to it during
its rotation.
In one improved embodiment of the invention, the
three front pads and the three flutes are distributed
alternately and successively around the circumference
of the head along a direction of rotation about the
axis 6. A flute following a guide pad around the
circumference of the calibration head forms an angle
equal to approximately 60' with said pad, in rotation
about the axis 6 of the head.
Thus, the arrangement of the flutes and the front
guide pads at the front end and on the outside surface
of the calibration head enables a balanced distribution
of the weight of the head preventing the generation of
vibrations during drilling.
The position of the two rough machining inserts
must be such that cutting forces can be balanced and
therefore they are arranged so as to distribute the
weight on the head in a balanced manner with the
finishing insert and the guide pads. The machining
inserts are located at an angular position of 120" from
each other because they are held in position in the
flutes in the head. They are arranged alternately with
the circumferential position of the front guide pads
that are themselves at a spacing of 120".
Considering a reference angle equal to 0" in a
cutting plane perpendicular to the axis 6 of the head
when the finishing insert is in the low position, the
calibration head according to the invention enables
alternate distribution of a front guide pad and a rough
machining insert every 60". This arrangement enables
5 perfect distribution of the three pads and the three
machining inserts. i I In one embodiment, each machining insert located
! jl
in the inner plane in a flute is held in its plane by
an adjustment cartridge f it.ted with attachment. means.
10 The working edges of each of the two rough machining
inserts are located at equal distance from the end of
the head along the axis 6 of the head. On the other
hand, the working edge of the finishing insert is
/ slightly set back by a distance equal to a few
i
I 15 millimetres from the working edges of the rough
I
I machining inserts. In one preferred embodiment of the
I invention, the distance between the working edge of the
finishing insert and a working edge of a rough
machining insert is approximately equal to 8 mm.
20 The "working edge" is the edge positioned such
that it is in contact with material to be removed in
the shaft to be drilled.
The position and angle of the machining inserts
can be adjusted so as to guarantee:
25 - straightness better than 0.01 mm variation from
a reference axis, and;
- a machined diameter with a variation of not
more than 0.02 mm.
These latter two parameters guarantee an
30 approximately constant delta thickness within an
acceptable tolerance so that the machined part does not
have to be scrapped.
The calibration head comprises flutes with a
geometry that increases the stability of the head and
5 improves the ratio of the drilling pressure to the
required lubricant flow to be injected at the end of
the head during drilling.
In particular, the increased cross-section of the
flute can carry a lubricant flow to its front end more
10 easily. The flow is better controlled and can transfer
lubricant continuously and regularly throughout the
drilling period. A special optimised "funnel" shape
gives better cooling, particularly for machining
inserts, and prevents jamming by chips during drilling.
15 The width of the flute at the circumference of the
calibration head is greater than its depth, the depth
of the flute being parallel to the axis 6 of the head
and located as close as possible to said head axis 6.
Figure 5 shows a perspective view in which the depth 46
20 is shown together with the taper corresponding to an
angle 45 that facilitates evacuation of chips and the
distribution of lubricant.
In one variant embodiment, the depth can vary
throughout the length of the flute, for example
25 reducing uniformly. Thus, the depth 46 can be reduced
along the length of the flute towards the back of the
calibration head 1 particularly to improve the
efficiency of lubricant transfer towards the front of
the head. The average taper angle 45 is between 10"
30 and 35" depending on example embodiments. According to
one example embodiment that can be combined with the
latter embodiment, the variation in the depth 46 along
the flute can vary from 5 to 30% of the maximum depth.
Thus, chips are transferred towards the back of
the calibration head after they have been cut in the
5 shaft. The calibration head thus prevents jamming
configurations. The lubricant also flows naturally
forwards and it is carried towards the outside edges of
1 the calibration head 1 under the effect of the
centrifugal force.
10 The shapes of the three flutes are identical.
Figure 5 shows a simplified perspective view of a flute
of the calibration head according to the invention.
Each of the flutes has a length 47 that projects by
half 23 of the length of the calibration head 1. On
15 the other hand, they only extend approximately as far
as half of the second half 24 of the length of the
I calibration head. This characteristic contributes to
improving the ratio between the pressure applied by the
head in to the shaft to be drilled and the lubricant
20 flow that can be carried to the front of the head. The
flutes facilitate lubrication so that ~ h eli fe of the
tool can be achieved and chips can be evacuated.
The lubricant transfer flow may be more than 350 L
per minute and can be continuously of the order of
25 420 L per minute while maintaining a drilling pressure
of the order of 15 bars.
I A calibration head according to the invention
I guarantees a lubricant flow of about 380 L per minute
within 20% for a pressure of 15 bars withln 20%.
30 The taper of the flutes around the periphery of
the head facilitates adjustment and placement of the
three cartridges to support and maintain each machining
insert.
The circumferential and axial distribution of the
three machining inserts and the three guide pads can
5 increase machining stability over the length of the
head. This eliminates and/or reduces vibrations.
In one embodiment, the finishing insert comprises
an appendage that is used to position it in the low
position in the pre-calibrated cavity of the shaft, see
10 portion 3 in figure 1. Advantageously, the finishing
insert is preferably arranged to enable symmetric
insertion of the calibration head, making use of a fool
proofing system positioned in the shaft pre-calibration
cavity 3. This initial configuration improves
15 placement of the tool to improve straightness and
reduce vibrations during drilling.
The calibration head according to the invention
also comprises back guide pads references 25, 26, 25'
for the pads visible in figure 3 and in figure 4,
20 placed at the back of said head. The function of the
back guide pads is to maintain guidance during
drilling. They can improve the straightness of
drilling. They can also absorb some of the vibrations
and keep the front end of the calibration head in line.
25 Finally, the front guide pads and the back guide pads
reduce friction and improve the drilling pressure.
According to one embodiment, the calibration head
according to the invention comprises four guide pads
arranged at the back of the head as shown in figure 3.
30 One advantageous distribution is as follows:
- first and second back guide pads are arranged
along the same axis as a front guide pad, and
an angle of 120" separates the first and second
back guide pad axes; preferably, the two front
gulde pads and the two back guide pads are on
each side of the flute inside which the
finishing insert is held;
- the two other back guide pads are arranged
symmetrically around the circumference of the
10 calibration head and at the same length along
the axis of said head.
The calibration head according to the invention is
capable of maintaining a good surface condition.
Furthermore the cutting speed is better, and speeds of
15 the order of 0.25 mrn per turn are possible. A nonnegligible
time saving is posslble when machining a
shaft to be drilled.
A calibration head is particularly efficient for
drilling large dimension shafts, for example with a
20 large diameter of the order of 60 to 120 mm.
The invention also applies to rough machiriing and
finishing inserts that can be fixed to the calibration
head. Figure 6 shows an example profile 30 of the
rough machining insert. This rough machining insert
25 may be the same material grade as the finishing insert,
in other words it may have the same metallurgical
composition, all that is modified is the profile of the
inserts that is not the same for the finishing inserts
and the rough machining inserts. One particularly
30 advantageous profile is a profile with three cutting
edges 50, 51, 52 that can be used to machine several
parts without replacing the insert. All that is
necessary when an edge is worn or damaged is to turn
the machining insert over and to put it back into the
adjustment cartridge.
5 Figure 7 shows one embodiment representing back
guide pads positioned behind the front guide pads with
the same alignment along the axial direction 6 of the
head 1. This figure clarifies the embodiment described
above with reference to figure 3.
10 A simp]-ified view of the calibration head 1 is
shown with no flutes and without the machining inserts.
Two sections are shown, with a first section AA' in the
zone 23 defining a front end of the calibration head 1
and a second section BB' in the zone 24 defining a back
15 end of the calibration head 1. The zones are shown as
in figure 3 such that they cover approximately half of
the length of the calibration head 1.
In section AA' located in zone 23, the front pads
37 and 38 are circumferentially spaced by an angle
20 approximately equal to 120". In section BB' located in
zone 24, the back pads 26 and 27 are also
circumferentially spaced by an angle equal to
approximately 120". The front pad 37 and the back pad
26 are aligned with each other, with one behind the
25 other. Note that the alignment of the pads is parallel
to the axis 6 of the head 1. Similarly, the front pad
38 and the back pad 27 are aligned with each other,
with one behind the other and are arranged on the
surface of the head 1.
This arrangement is identical to the arrangement
in figure 3, however in figure 3, this alignment cannot
be shown for pads 38 and 27.
One technical effect of this configuration is that
5 the guidance of the calibration head can be improved
throughout its length when drilling, particularly
during insertion of the head into the zone to be
drilled.
One advantage compared with the configuration in
10 which a single pad extends over the entire length of
the head is that there is a saving of mass and
precision is better in the head fabrication method.
In the embodiment shown in figure 3 and figure 7,
the two front pads are circumferentially spaced by an
15 angle of 120" which enables precise positioning of the
head in the zone to be drilled.
In one variant embodiment, there is only one front
pad, for example pad 37, placed on the surface of the
head in the zone 23. This pad is then in line with a
20 back pad 26. In this embodiment, a single front pad
and a single back pad in line with the front pad can be
defined.
This configuration using only two pads in line
with each other at the front and at the back may be
25 combined with other front pads (front pads 25 and 25')
not necessarily in line with other back pads as shown
in figure 4.
According to one variant embodiment, the back pad
25' in figure 4 is combined with a second back pad 25
30 arranged symmetrically, in other words diametrically
opposite on the back of the calibration head 1. This
arrangement enables optimum balancing of the front pads
added to this configuration and of the arrangement of
the flutes.
The pad 25' is ideally arranged to be angularly
5 close to the rough machining insert 30 or 35 or the
finishing insert 32, at approximately 30" in the
clockwise direction as shown in figure 4. This
arrangement holds the head in position, although under
its own weight the head would sag when it is stopped
10 and consequently prevents an axial scratch when the
head is removed, particularly when the head is stopped.
According to one example, the pad 25' is arranged
at an angle of approximately 30" from the finishing
insert 32 when rotating in the clockwise direction
15 according to figure 4, to prevent lubricant from
escaping from flute 20.

WE CLAIMS:-
1. Cylindrical calibration head (1) for drilling
a shaft (4) comprising three flutes (20, 21, 22)
extending along the axis of said head and uniformly
distributed around the circumference so that they are
separated from each other by an angle of 120" in
rotation about the axis (6) of the calibration head
(I), the flutes being used for evacuation of chips and
passage of the lubrication during drilling, each of the
flutes comprising a machining insert, the position of
which is adjusted in the flute by means of an
adjustment cartridge, characterised in that it
comprises at leasst one front guide pad (36, 37, 38)
arranged on the outside surface of the head in a zone
(23) corresponding to the front end of the head (I),
and at least one back guide pad (25, 26, 25') placed on
the outside surface of said head (1) in a second zone
(24) defining the back zone of said head, one (26) or
each back guide pad being placed behind one (37) or
each front guide pad, along the same axis as the front
guide pad.
i 2. Cylindrical calibration head (1) for drilling I
a shaft (4) according to claim 1, characterised in that
it comprises three front guide pads located in a zone
(23) defining a front end of the calibration head, the
I
#I 30 three pads (36, 37, 38) being arranged uniformly on the
:I
I I outside surface of the head (1) so that they are
separated from each other by an angle of 120" in
rotation about the axis (6) of the calibration head
(1)
3. Cylindrical calibration head for drilling a
shaft (4) according to claim 2, characterised in that
the positions of the front guide pads (36, 37, 38) and
the flutes (20, 21, 22) on the outside surface of the
calibration head (3) are alternated such that the angle
10 between each front guide pad (36, 37, 38) following a
flute along the circumference of the calibration head
forms an angle of 60" in rotation about the axis (6) of
the calibration head (1).
15 4. Cylindrical calibration head (1) for drilling
a shaft (4) according to any one of claims 1 to 3,
characterised in that it comprises a plurality of back
guide pads (25, 26, 25') arranged on the outside
surface of said head (1) in a second zone (24) defining
20 the back zone of the head, two back guide pads (25,
25') being arranged symmetrically relative to each
other about the axis (6) of the head (1) and of which
at least one back pad (25') is axially offsel from a
front pad ( 3 8 ) .
25
5. Cylindrical calibration head (1) for drilling
a shaft (4) according to any one of claims 1 to 4,
characterised in that it comprises four back guide pads
(25, 26, 25') arranged on the outside surface of said
30 head (1) in a second zone (24) defining the back zone
of the head, two back guide pads (25, 25') being
arranged symmetrically relative to each other about the
axis of the head and each of two other back guide pads
(26) being placed behind a front pad.
6. Cylindrical calibration head (1) for drilling
a shaft (4) according to any one of claims 1 to 5,
characterised in that it comprises a set of guide pads
(36, 37, 38, 25, 26, 25') arranged on the outside
5 surface of the head, with a number and thickness so
that a minimum lubricant flow can be maintained while
minimising friction.
7. Cylindrical calibration head for drilling a
10 shaft (4) according to any one of claims 1 to 6,
characterised in that one flute contains a finishing
insert and each of two flutes contains a rough
machining insert.
15 8. Cylindrical calibration head for dr'illing a
shaft (4) according to claim 8, characterised in that
one of the two back pads (25, 25') that are positioned
symmetrically relative to the axis (6) of the head (I),
is placed at 30° from the rough machining insert (30,
20 35) or a finishing insert (32).
9. Cylindrical calibration head for drilling a
shaft (4) according to any one of claims 1 to 8,
characterised in that each flute is tapered at an angle
25 equal to approximately 25' within 10%.
10. Cylindrical calibration head for drilling a
shaft (4) according to any one of claims 1 to 9,
characterised in that the flutes are sized to allow the
30 injection of a flow of 380L/min of lubricant within 15%
for a pressure of 15 bars within 15% in the calibration
head in the shaft to be drilled.
11. Cylindrical calibration head for drilling a
35 shaft (4) according to any one of claims 1 to 10,
characterised in that the length of the flutes along
the axis of the calibration head is more than 50% of
the length of the calibration head.
1'2. Cylindrical calibration head for drilling a
shaft (4) according to any one of claims 1 to 11,
characterised in that each machining' insert has three
edges that can be configured so as to select a working.
edge by means of the adjustment cartridge.