Blade machining by mikesanye

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									     Blade machining




     At the heart of any gas turbine, whatever       machining challenges due to their tougher
     its particular design or function, the action   materials and more complex designs. The
     of the turbine blades is crucial for the tur-   stationary blades, also called vanes, have
     bine to carry out its intended function.        simpler designs and are primarily used to
     Their operating environment can be              direct the airflow. Hence they are usually
     extremely demanding, with large variations      regarded as being easier to machine than
     in temperature and pressure as well as the      rotating blades, although the quality of
     physical stresses of high speed rotation,       their manufacture is still critical for turbine
     and any flaws in their structural integrity     efficiency.
     may lead to rapid failure. Consequently         For either class of blade, the raw material can
     the choices of material and manufacturing       be bar stock, forging, or precision castings.
     process in blade production, and the
     security and efficiency of the machining
     operations employed, are vital.
     Blades of many different sizes and geome-
     tries are utilised in gas turbines, and can
     perform different functions within the
     turbine. Some are stationary blades, while
     others are rotating, and it is usually the
     rotating blades which present the greater


86
Cutting tools for turbine blades
The commonest workpiece materials for              als, account for the majority of workpieces
turbine blades are stainless steel, heat resist-   used.
ant super alloys (HRSA) and titanium, but
of these stainless steel, ISO class M materi-



 For cutting data see page 134, stainless steel insert recommendations
 see page 82, feed recommendations see page 85.




Blade machining strategies
A variety of machine tools are suitable for        Deciding which strategy is best in a partic-
blade machining, including 3-, 4-, or 5-axis       ular situation depends on numerous factors,
machines, but the 3-axis machines are not          including:
recommended for the majority of modern             the philosophy in the blade shop
blade machining operations. They can only
                                                   the different types and sizes of blade
effectively produce the simplest shapes and
geometries, and although such machines are         the design of the blade
still widely available and profitable for          the number of blades of each size
maintenance operations, they are not rec-          the machining operations involved
ommended for new investments or most
                                                   the calculated gross profit and the amortisa-
modern machining processes.
                                                   tion time
4-axis machining is more common, particu-
                                                   the process flow
larly in older machines which have been
upgraded with NC-programming capabil-              the CAD/CAM systems
ity. But the modern trend is towards 5-axis        the operating performance
machines, which allow maximum flexibility          the staff.
and versatility while still using standard
cutting tools.                                     In general, for the efficient manufacture of
The choice of overall machining strategy is        large quantities of similar components, per-
important, and will greatly influence the          haps thousands of comparable blades per
subsequent machining parameters. These             year, a machining cell is the best solution.
strategies fall into two classes:                  Alternatively, for production of many dif-
Machining with one (or more) individual            ferent blade designs, a single machining
machining centre(s)                                centre which is able to accommodate a
                                                   variety of fixtures and/or special tools,
or:
                                                   will provide more flexibility and higher
Machining with a dedicated machining cell          productivity.

                                                                                                   87
     Machining the fixturing elements
     The fixturing elements at the head and               Consequently the design of these elements
     root of the blade structure are ultimately           and the tools used to machine them must
     removed to leave the final shaped item, but          be selected carefully, and will be discussed
     during the machining process itself their            here in some detail.
     accuracy and form have a crucial impact on
     the success of the overall operation.


     Element                          Tool                                   Remarks

     Root                             Standard endmill – only roughing       Wide tolerance possible, fixturing in
     Rectangle                        necessary                              jaws (screw). Secured in two axis.

     Root                             Special – roughing and finishing       Close tolerance. Secured in all three
     Trapezoid or dovetail            necessary                              axis.

     Head                             Standard – counterbore                 Axial pressure (tailstock) for fixturing
     Countersinking                                                          necessary.

     Head rectangle                   Standard endmill – only roughing       Special equipment. Large size.
     Standard                         necessary                              Transmission of torque possible.

     Head                             Standard endmill – roughing and        Fixturing in a pull-in collet. Secured
     Cylindrical                      finishing                              in all three axis.




     Whichever processing methods are                     390 are also good alternatives.
     employed, the first step is to machine the           It may also be possible in this operation to
     reference surfaces by which the workpiece            also machine the clearances necessary for
     will be clamped during the subsequent                subsequent processes, if the machining
     machining. Several Coromant tools are                strategy would benefit from this.
     suitable for this operation, and the
     CoroMill 390 long edge cutter is particu-
     larly recommended. CoroMill 200, 300 and




                                                                         n

                                                                              n




                                                               vf Down milling
                                 Clearance
             Fixturing element

88
                                     Bending = 0,005 x L (mm)




                                              L




It is possible that the blade workpiece may       The recommended procedure for such
deform or bend during subsequent stages of        reworking on a 5-axis machine is:
the machining process, the result of              G   opening the fixturing system on the blade
machining away 80% of the original rolled             head and moving it back, so that the
or annealed raw material and the residual             blade is now secured only by the root.
stresses thus created. This is particularly
                                                  G   creating a new centre line for the work-
possible for large blades, 400–600 mm long,
                                                      piece, by counter-boring or turnmilling.
which may bend by as much as 2 mm.
Reworking the fixturing elements during           G   fixing the blade by the new element.
the machining process, so that the position       An alternative is to modify the adaptor
of the workpiece in the machining centres         itself, so that the position of the workpiece
is modified to account for the deformation,       is suitably adjusted when the modified
can counteract this phenomenon.                   adaptor is held in the machine, without any
                                                  changes to the fixturing elements.




                                                                                                  89
     Machining the root of the blade
     The machining process to shape the root of
     the blade will depend on several factors,
     notably the dimensions of the finished
                                                                                             160 mm
     item. Small blades are often machined
     directly from round bar stock, which is
     then is milled to a square shape.



     Larger blades are often made from rectan-
     gular bar stock or forging. Normally these
     blades are first machined with cutting tools,
     and then broached or ground.                                                            500 mm




     Turbine blades can be divided into two          Rotor blades may have different mounting
     classes, stator and rotor blades, and in nor-   systems, such as a “Christmas tree” profile,
     mal practice these two designs have differ-     or deep slots machined in a trapezoidal
     ent mounting systems and different styles       cross-section.
     of root, to accommodate the different load-     These variations in the profile and geome-
     ings they receive in use.                       try of the blade’s root will require different
     Stator blades normally have one small slot      machining strategies:
     in one side of the root, which is relatively
     easy to machine with solid carbide or
     indexable insert endmills.


     Machining a ‘Christmas tree’ profile:
     For machining the Christmas tree profile        edge milling cutters, applying down milling
     on a blade, it can be helpful to change the     for each side of the profile, will allow max-
     fixturing arrangement, and make the tool        imised metal removal rates and tool life.
     axis parallel to the blade length.
     It may also then be possible to use a special
     adaptor on the Christmas tree profile to
     hold the blade during subsequent roughing
     operations, and so avoid the need for
     machining (and later removing) separate
     fixturing elements onto the workpiece.
     A milling strategy using CoroMill 390 long

90
1. Roughing with the long edge cutter in
   different ap-steps, using down milling
Calculate a suitable ae/Dc ratio so as to
bring more than one effective tooth into cut
during the cutting cycle.



2. Roughing completed.




3. Machining the christmas tree profile,
   with special HSS tooling.




Roughing the christmas tree profile may
also be performed by CoroMill 331 side
and face milling cutters in different diame-
ters, to achieve the stair-like shape on the
component.




However, using a set of different diameter     An alternative is to employ solid tools,
cutters mounted in this manner results in      particularly if there are difficulties with
large differences in effective cutting speed   accessibility or the complexity of the
between the largest and the smallest cutter.   shapes being produced.



                                                                                             91
     Machining a deep slot in the blade root by endmilling
     The type of workpiece material will have a      sated for when the item is remounted prior
     large influence over the machining parame-      to the finishing operations, an approach
     ters when machining slots into the blade        which should also help to maximise the
     roots. In many cases it will be stainless       quality of the final blade.
     steel, and thus problems of chip adhesion       In general, machining deep slots in the
     to the cutting tool will occur. However,        blade root can be divided into:
     carefully selected tooling and the correct
                                                     slot milling (L-style with endmill)
     machining methods will counteract these
     difficulties.                                   plunge milling (with endmill)

     The blade’s size and material, and the slot’s   trochoidal milling (with endmill)
     position and form, will determine the
     machining strategy. In most cases it will be
     better to leave the machining of the slots,
     along with their roughing and finishing,
     until after the other machining operations
     are complete. That way the machining of
     the blade profile itself can be carried out
     without any slots in the blade root which
     might conceivably affect the clamping and
     stability of the workpiece. In addition any
     bending or deformation in the workpiece
     that occurs during profiling, due to the
     release of internal stresses, can be compen-




     Slot milling
                                                     L-style milling is a technique for cutting
                                                     deep slots which can be beneficial to both
                                                     power consumption and tool life. After an
                                                     initial channel has been cut by the first pass,
                                                     subsequent steps down towards the final
                                                     slot depth are made with the tool cutting a
                                                     sequence of L-shape shoulders around the
                                                     cavity perimeter, rather than further full-
                                                     scale cutting engagements.
                                                     Machining with CoroMill 390 endmills,


92
with -11 or -17 size inserts mounted in dif-    Recommended cutting speed/feeds can be
ferential pitch, will enable tool life and      found in Coromant publications.
power consumption to be optimised, and
allow the machining operation to be com-
pleted in a minimum number of steps. The
larger depths of cut should be carried out in
down milling.
The widths of the slot should be 1,2–1,4
times larger than the diameter of the end-
mill, giving an overlap of 20–40% in the
down milling action.
For the finishing operation a special tool is
required to produce a trapezoidal profile in
the slot, often HSS or brazed carbide.




Plunge milling




Plunge milling is a very effective method to    R390 endmill, inserts size -11 or size -17
achieve a maximum chip volume per               The width of the slot should be 1,2 to 1,4
minute, low power consumption, and              times larger than the diameter of the end-
increased tool life at higher cutting speeds.   mill.
This method is the first choice when            For the finishing operation you need a spe-
machining stainless steels with an austenitic   cial tool according to the trapezoidal profile
structure (e.g. CMC 5.21 and others). In        in the slot, often HSS or brazed carbide.
the cutting action, the chip has no opportu-
                                                Recommended cutting speed/feeds can be
nity to glue onto the cutting edge, and both
                                                found in Coromant publications.
sides of the cutting edge can be employed.
It is also the first choice when machining in
weak fixturing conditions.
Recommended tooling is in line with the
recommendations for slot milling, i.e.:


                                                                                                 93
     Troichoidal milling:

                            Tool path

                                                             Tool path




     Conventional method.               Trochoidal method.

     In trochoidal milling, an approach particu-       grooves, and can also be employed for high
     larly suited to CoroMill Plura solid carbide      speed machining (HSM) operations, as the
     endmills, the cutter removes repeated             curved path enables the maximum feed rate
     ‘slices’ of material in a sequence of spiral      to be maintained during the entire machin-
     paths, combining large axial cuts with small      ing process.
     radial cutting depths. This is ideal for
     machining slots, as well as pockets and



     Machining the rotor – power generation turbines
     Although machining the detailed design            milling cutter or a 3-dimensional bell
     features of the rotor sections of turbines is     milling cutter is required. Both tools are
     beyond the scope of this book, some gen-          highly specialised.
     eral information and basic principles can be      The basic machining strategy will include:
     given.
                                                       Machining the first slot with a slot milling
     Very specialised equipment is required to         cutter such as CoroMill 331.
     machine the grooves in a rotor, the grooves
                                                       Opening the slot as wide as possible.
     into which the root of each individual blade
     will ultimately be fitted. These grooves may      Using the bell milling cutter or slot milling
     be straight or curved in geometry.                cutter to machine the profile, or using a
                                                       special “Christmas tree milling cutter”, an
     For machining, the rotor is normally fixed
                                                       endmill with indexable inserts.
     on a heavy duty turning machine with an
     integrated milling unit.
     The form of the groove, straight or curved,
     will determine whether a 3-dimensional slot




94
Machining the blade body
Machining the blade rhombus is a critical
step in blade manufacture, and a wide vari-
ety of potential machining solutions are
available depending on the design of the
blade and the types of cutting machinery
available. A comprehensive description of
all these different methods is beyond the
scope of this book, but the basic principles
can be outlined, emphasising the machining
principles which underlie them: optimising
the cutting tool engagement, reducing
vibrations, using the tooling as effectively as
possible, and maximising productivity.




Roughing the rhombus – parallel to the blade axis, using one tool
                                                  This is a very common machining approach,
                                                  using two separate cutting steps to reach
                                                  the full depth of cut. In most cases this
                                                  method allows the cutting force to be
                                                  reduced more effectively than by reducing
                                                  the feed per tooth, as it allows the chip
                                                  thickness to be modified towards the
                                                  recommended target values.




                                                   Material – CMC 5.2
                                                   Tool – R200-L, Dc 63 mm, zn 6
                                                   Insert – RCKT 1204M0-MM 2040
                                                   vc 220 m/min, fz 0,21 mm, ap 2–4 mm,
                                                   ae 30–63 mm




                                                                                              95
     To achieve the full benefits of this
     approach, the milling strategy must use
     down milling, and a 45° angle of cutting
     entry into the workpiece.
     The tool path must not change through 90°
     angles. Instead, change the feed direction
     incrementally through small changes of
     radii.
     Ensure a tool engagement of 60–80%, if
     necessary by changing the tool diameter or
     cutting path.
     Employ a different depth of cut in each of
     the two passes, to minimise notch wear on
     the cutting insert.
     Maximise the larger depth of cut as much as
     possible.




     Vibrations and heavy axial pressure on the
     inserts will occur if the feed forces cause
     any movement or deflection of the work-
     piece. If this occurs the feed direction
     should be modified so the forces act in
     directions where the blade fixturing
     arrangement supports the workpiece most
     effectively.




96
                                                Vibrations can also be reduced by adopting
                                                cutting paths which machine the metal in
                                                small triangular steps, in both the longitudi-
                                                nal and lateral directions. This approach
                                                requires modifications to the cutting speed
                                                and feed, along with no more than 60% of
                                                the usual maximum depth of cut, and the
                                                modified cutting forces will also produce
                                                changes in the wear patterns seen on the
                                                cutting inserts.




Roughing the rhombus – parallel to the blade axis,
using two tools of different diameter
The use of two different tools to machine
the rhombus is an effective strategy in
many situations.
A first cut, producing a slot perpendicular
to the blade axis, can be made with an end-
mill such as CoroMill 390 (using L-milling
or plunge milling) or a slot milling cutter
such as CoroMill 331. This slot then pro-
vides clearance for a subsequent cutting
tool of different diameter, which should
experience a less severe cutting environment
and generate lower vibrations while it
machines along the blade’s longitudinal axis.

For this milling strategy to be effective, a
round insert cutter is recommended for this
second cutting stage, such as CoroMill 200
with RCKT 1204 inserts in a 40–80 mm
diameter cutting head .




                                                                                                 97
     Roughing the rhombus – machining the roof slopes
     This penultimate operation in roughing the
     blade’s contour uses a roughing tool whose
     size will depend on the design of the blade,
     and on the radius between the roof slope
     and the blade’s root.
     A round insert milling cutter is normally
     recommended, such as CoroMill 200,
     or alternatively an endmill such as
     CoroMill 390.




     Roughing the pressure side – peripheral milling
     Roughing the pressure side of the blade –       workpiece as the machining operation pro-
     the concave side – is usually the last stage    ceeds.
     of the roughing process, and also one of the    Peripheral milling is an effective way to
     most complex.                                   carry out this operation, with a depth of
     Modern designs of turbine blades maximise       cut between 1–5 mm. Round inserts are
     their efficiency through complicated surface    recommended, such as CoroMill 200, as the
     geometries, and machining these surfaces        round geometry ensures that the minimum
     requires a careful machining strategy to        residual metal remains after each cutting
     account for both the profile of the blade,      pass – as long as the correct cutting para-
     and changes in the effective stiffness of the   meters are used. The feed direction should


98
be away from the fixturing in the root, and
a left hand tool such as CoroMill 200 may
be required.




                                               Square shoulder
                                               cutter, 90°              Stock to be
                                                                        removed




  Smooth transitions –
  little stock
                                                Much material
                                                remaining after
                                                roughing




Roughing the pressure side – waterline milling, parallel to the blade axis
An alternative strategy to machine the         circular interpolation, with the initial feed
pressure side is “waterline milling”, an       direction always away from the solid fix-
approach originally derived from 3-axis        turing at the root of the blade.
milling in the die and mould industry, now     Recommended tool: round insert cutter,
adapted to 5-axis milling machines.            such as CoroMill 200.
In this technique, the cutting operation
consists of a sequence of 2-dimensional lay-
ers, each completed before the tool moves
down to the next. Transitions between the
layers are carried out by helical ramping or




                                                Material – CMC 5.2 (1.4418)
                                                Tool – R200-L, Dc 63 mm, zn 6
                                                Insert – RCKT 1204M0-MM 2040
                                                vc 216 m/min, fz 0,175 mm, ap 4,0 mm,
                                                ae 40 mm



                                                                                               99
      Roughing the pressure side – plunge milling
      Plunge milling is a very effective milling    On turbine blades, plunge milling can be
      strategy, especially when producing long      used to machine deep roof surfaces, as well
      blades or when fixturing conditions are       as the concave deep pressure sides.
      weak, and is also useful when machining
      austenitic stainless steel.




                                                                       ap




                                                              ae = step size

                                                    Typical cutting recommendations include:
                                                    CoroMill 390 endmill with –11 or –17 size
                                                    inserts, an insert radius rε of 2 mm or less
                                                    to reduce radial pressure during the cut, and
                                                    a step size 60–70% of Dc.
                                                    The recommended radial depth of cut will
                                                    depend on the insert:
                                                    For -11 insert: 5,5 mm
                                                    For -17 insert: 8,5 mm

                                                    Other speed and feed recommendations can
                                                    be found in Coromant publications.

                                                    Machining example for plunge milling
                                                    pressure side:
                                                    G   Plunge milling the roof surface
                                                    G   Plunge milling the pressure side


                                                        Material – CMC 5.21, Tool – CoroMill 390,
                                                        32 mm diameter cutting head, holding two
                                                        inserts grade 2040
                                                        Cutting data: vc – 210 m/min, fz – 0,18
                                                        mm/tooth, ae – 8,5 mm, step size – 20 mm



100
Roughing blades using turnmilling techniques



                               n tool
                                                      Torque
                                                               9   8   7   6   5   4   3   2   1




                     n blade = vf




Turnmilling centres are versatile machines        G   the highest values for chip volumes per
in which the cutting head can either hold a           minute
single cutting edge which remains station-        G   regular engagement of the tool, leading to
ary (in which case the machining operation            a smooth cut
is fundamentally a turning process) or a
                                                  G   the minimum number of necessary tools
multi-tool cutting head rotating in place (as
in a milling process). In such machines,          G   a balanced spindle load
when in ‘milling’ mode, the workpiece can         G   machining the whole surface profile along
still turn along its longest axis – as it would       a component’s length with one cut
in a turning operation, although now at           G   an alternating depth of cut, therefore
slower revolutions – enabling the rotating            reducing the notch wear
tool’s attitude to the workpiece to be
                                                  G   a constant movement in all 5 axis, reduc-
changed as desired by moving the work-
                                                      ing any reciprocating movement in the
piece as well as the tool. Complex profiles
                                                      fixturing system
can be machined in this way.
                                                  G   short tool overhangs
Such techniques are among the most
effective methods for roughing small and
medium size blades up to 600 mm in
length, but although the basic principles are
simple, in practice this approach requires a
very flexible CAD-CAM system, with
specialised software and optimised NC
systems.
If correctly applied however, turnmilling
has several attractive advantages over other
strategies, in both machining centres and
machining cells:




                                                                                                   101
      Correctly applied, this technique can be        The normangle of the tool is not constant,
      used for all roughing operations on a blade,    and will alter as the workpiece rotation
      including the head and root as well as the      presents different aspects to the tool. The
      blade body.                                     first engagement should be in a direction
      When turnmilling a blade structure, the         away from the blade root, to correspond
      workpiece slowly rotates along its long axis    with the blades inherent flexural strength,
      in front of the tool, while the cutting head    and downmilling should always be used.
      moves in two dimensions, perpendicular to       Recommended tools for turnmilling when
      the cross-section of the blade profile, and     machining blades with stiff cross sections
      along its length. In practice, the profile is   are round insert milling cutters such as the
      thus machined in a single cutting engage-       CoroMill 200 cutter, with RCKT 1204
      ment, and the tool moves slowly down the        inserts.
      length of the spinning workpiece, in a          The step size should be app. 80% of the Dc.
      single helical cut. Therefore the concave
      pressure side can also be machined in the
      same operation, without having to alter the
      rotation of the workpiece, and only one
      revolution per blade is required.




                                         n tool
                                                      Recommended tools for turnmilling when
                                             A-Axis
                                                      machining thin blades (e.g. compressor
                                                      blades) are face milling cutters such as
                                                      CoroMill 390, using inserts with a radius rε
                                                      of 0,2–4,0 mm. The axial pressure on such a
                                                      tool is much lower than on a round insert
                                                      milling cutter, which causes less vibration
                             vf                       during machining.
              Roughing            Prefinishing
            ae = 0,8 x Dc         ae = 0,3 x Dc



      Semi-finishing the blade
      The semi finishing operation requires a         milling. The recommended tool is an end-
      5-axis milling operation, and will directly     mill with indexable inserts, such as the
      influence the surface quality of the final      CoroMill 390, or a round insert milling
      finished blade. Therefore the aim should        cutter such as CoroMill 300. The choice of
      always be to achieve a very regular, uniform    tool will depend on the profile of the blade
      level of residual material – if necessary,      and its size.
      through two separate semi finishing opera-      A variety of tool paths can be employed.
      tions.                                          One common technique, especially when
      Normally this operation is done by turn-        machining large cast blades, is to use a feed

102
direction along the blade length, but other
possibilities are shown in the diagram. For
example, the blade can be shaped by milling
across the blade, either using several passes
in one direction with a rapid return move-
ment between passes, or in a single continu-
ous helical cut around the blade.




Machining the transitional radius
                                                Before finishing a blade, the transitional
                                                radius between the root of the blade and the
                                                head must be machined. This is specialised
                                                stand-alone operation between the semi-
                                                finishing and finishing stages.
                                                This job is also usually a 5-axis turnmilling
                                                operation. The recommended tool is a
                                                conical solid carbide endmill, employing a
                                                very small width of step between passes
                                                (0,2–0,5 mm) to reduce the stress concen-
                                                tration in the radii.

Finishing the blade
Finishing the blade is probably the most
difficult 5-axis machining operation, but its
success will greatly depend on the quality
of the other machining steps carried out
previously.
The most suitable tool depends on the type
and size of the blade, and also on the spin-
dle speed and the feed available in the


                                                                                                103
      machining centre. The capabilities of the
      machines employed can often be the limit-
      ing factors.
      In general, it is possible to use solid carbide
      endmills like CoroMill Plura 216.24, or
      endmills with indexable inserts, such as the
      CoroMill 390 with inserts R390 11T3 31E-
      PM 1025).
                                                                   Material – CMC 5.2
      The tool diameters vary, e.g. between                        Tool – R390-025A25 11-H
                                                                   Insert – R390-11T324E-ML 1025
      10–20 mm.                                                    vc 320 m/min, fz 0,22 mm, ap 0,5 mm, ae 1,75 mm
                                                                   Tc 45 min/tool, Dry machining




      The principal problems when finishing are                   face, and always in a downmilling manner.
      vibrations, and the quality of the pre-fin-                 In this way, and combined with an oil mist
      ished surfaces. Using tools with a smaller                  coolant, the resulting surface can be highly
      radius, rε, or using a different number of                  polished.
      inserts in the cutting head can help combat                 With suitable optimised equipment it is
      vibrations, in line with the recommenda-                    possible to achieve a surface roughness of
      tions given in Coromant publications                        Ra < 0,4 µm, although the final surface
      During the cutting process the tool follows                 quality will strongly depend on the combi-
      a helical path around the blade, a path con-                nation of normangle, feed and cutting
      trolled by a specialised CAD-CAM system.                    engagement.
      To achieve the best surface quality and
      structure, the tool has to maintain a con-
      stant normangle at each point on the sur-




      Spindle speed n = 12 000 rpm, cutter diameter = 6 mm
       ƒz/ae                0,05             0,075         0,1            0,15          0,2            0,25
       Feed vf              F1200            F1800         F2400          F3600         F4800          F6000
       Cusp./h              h0,0001          h0,0002       h0,0004        h0,0009       h0,002         h0,003
       Time (min)           10               4,44          2,50           1,11          0,62           0,40
                                                                             R =radius of cutter   h = cusp. height
                      ae
                                                                             h = ae2l(8 x R)       Feed = rpm x ƒz x z

      fz


                 fz <ae            fz = ae
                                                ae
                                                       R                          h=
                                                                                    √R – 2R2–ae2      or      h~
                                                                                                                   ae2
                                        h                                                4                         8R



104
Adjustable guide blades – turning or turnmilling
Some turbines contain adjustable “guide         When using these turnmilling variants,
blades”, usually made from titanium (in         chipbreaking problems can be avoided, and
aerospace) or stainless steel (in power         with suitable tools and programming a
generation), used to direct or control the      complete machining process is possible. For
air flow within a turbine. They may be          these specialised operations various differ-
fixed and stationary, or fitted onto rotating   ent tools are required, such as CoroMill
shanks by suitable grooves and fixturing        390 endmills, CoroMill 331, CoroMill Ball
elements. Normally these blades are             Nose and CoroMill Plura. The most
between 300 mm and 1200 mm long,                important tool is the face milling cutter
including the shank.                            which creates the finished surface on the
When machining such a blade there are two       shank to the correct tolerances of rough-
main problems: the forged skin of the           ness and roundness.
material, and the unbalanced nature of the
workpiece.                                      3. Combined machining with a turnmill
                                                   centre (roughing) and a turning machine
Three different machining methods are
                                                   (finishing)
recommended:
                                                This combined method to machine these
1. Machining on a turning machine
                                                blades is probably the most profitable tech-
   (roughing – finishing)
                                                nique, using a turnmill centre for the
A very stiff and powerful machine is neces-     roughing applications, as described above,
sary, preferably with a self balancing chuck.   followed by a separate turning stage.
After pre-machining and creation of the
                                                To achieve the highest surface quality and
fixturing elements, a complete machining
                                                accuracy, a stable turning machine with a
process is then possible, but due to the
                                                self balancing chuck is recommended.
imbalance of the workpiece the cutting
speed must be low. To improve chip-break-       When using a Coromant Capto system
ing, Coromant Capto with an integrated          with integrated Jetbreak cooling and a
high pressure cooling system (Jetbreak) is      Wiper insert, both the feed and speed can
recommended. With this equipment, cut-          be maximised, and the machining time
ting speed and tool life can be optimised,      reduced dramatically.
mainly due to the optimised chip breaking.

2. Complete machining on a turnmill
   centre (orthogonal-, longitudinal- and
                                                                     Dimensions in mm
   plunge turnmilling)
                                                   Ø 50




                                                       140

                                                                    700

                                                                                               105
      Machining examples for adjustable guide blades
      G   Roughing the shank: CoroMill 390, insert
          R390-170450E-MM 2030
      To achieve the highest productivity, the cut-
      ting tool diameter should roughly equal the
      shank diameter.




      G   Roughing the cone: CoroMill 390, insert
          R390-170450E-MM 2030.




      G   Machining the sealing lip: solid carbide
          endmill CoroMill Plura 216.22, with
          zn = 3.
      For the highest productivity, the sealing lip
      can be machined in one cut.




      G   Finishing the shank: CoroMill 390, inserts
          R390-11T308E-PL 1025.




106
Other blade machining operations
Short blades
Certain blade designs consist solely of a          Recommended tools for roughing:
short blade body without head or root ele-         CoroMill 200 or CoroMill 300
ments, which can be machined using a               Recommended tools for finishing:
“helimilling” method.                              CoroMill 300
In this approach the tool axis and the blade
axis are parallel, and the tool rotates around
the blade following a helix.
This method is very effective for small
blades with lengths <150 mm, and as long
as the set up is sufficiently rigid, it is poss-
ible to machine the blade with just one
roughing cut and one finishing cut.




Machining with an automatic bar fed machine
An automatic bar machine is normally only          Due to the low RPM on such machining
capable of 4 axis machining, but this can          equipment, solid carbide endmills are not
still be an effective and profitable way to        recommended.
machine small blades. The short overhangs
between the blade and the tool will pro-
mote a rigid set up, so large depths of cut
and feed per tooth can be employed,
although subsequent grinding of the blade
may be necessary.
Cutting paths which follow small triangular
steps in the longitudinal and lateral direc-
tions, similar to those described above for
roughing of the blade rhombus, can be
utilised.
Recommended tools:
CoroMill 200; CoroMill 390; CoroMill 245
for roughing
CoroMill 390 in different diameters for
semi-finishing and finishing




                                                                                               107
      Peel milling (Multi-Spindle)
      Peel milling is one of the most traditional      ing and renovating old blades, and the
      methods to machine turbine blades.               multi-spindle machines can allow up to six
      It can be performed on multi spindle copy        blades to be machined at one time.
      milling machines, with side and face milling
      cutters such as CoroMill 331, or with an
      endmill such as CoroMill 200.
      This is a very effective technique for repair-




      After finishing
      After the finishing operation is completed
      there will inevitably be residual stresses
      remaining in the blade surface, induced by
      the machining operations themselves.
      Typically these stresses could reach 200 to
      400 MPa, down to a depth of 0,4 mm, and
      must be counteracted in order for the blade
      to maintain its structural integrity and
      rigidity.
      The common techniques for addressing
      residual stresses include grinding, press
      polishing and vibratory grinding, of which
      grinding, vibratory grinding and blasting
      are the methods most often employed.
      Press polishing requires specialised equip-
      ment and knowledge, as does the use of
      ultra sonic methods to monitor and observe
      these residual stresses.




108
Trouble shooting and general advice
If excessive vibrations, poor tool life, or      axial depth of cut and increase the cutting
bad surface finish should occur, the first       speed.
step in trouble shooting should always be        Reduce the overhang of the tool.
to check that the correct recommended
                                                 Use rigid set-ups, preferably Coromant
tools and cutting parameters are being
                                                 Capto throughout.
employed. Check also the rigidity of the
tool set-up, together with the power con-        Check the feed direction. In some cases it
sumption and torque of the spindle at the        can be helpful to reverse the feed direction,
particular RPM being used.                       using the same tool. Be sure to employ
                                                 down milling techniques.
If problems still remain, check the engage-
ment of the tools. Some CAD/CAM sys-             Control the cutting force acting on the
tems have their own interpretation of the        tool. Large radii on the insert will increase
basic programming instructions, which may        the security of the overall process, but at
need to be adjusted. Check that the ae and       the same time they will increase the axial
Dc values are in the recommended ranges.         pressures created. Change the radii on the
Using a smaller tool diameter at the begin-      insert for a smoother cutting action.
ning of the cut may be of benefit.
Check the axial depth of cut. Some fixtur-
ing systems are not rigid enough to handle
a feed direction perpendicular to the long
axis of the blade. In that case reduce the



Special machining parameters for HRSA and Ti
Usually these blades are cast or precision       Due to the tendency for work-hardening
forged, and the machining is concentrated        when machining HRSA, it is not desirable
on the root.                                     to machine again a surface which has
For finishing the root, use coated solid car-    already been through a previous cutting
bide endmills, or endmills with indexable        operation. Therefore, all the metal removal
inserts. If a coated insert is impractical for   required by the design of the blade should
health and safety reasons, use uncoated cut-     preferably be carried out with one cut. Or,
ting tools in line with the published            if this is not possible, use a minimum depth
Coromant recommendations.                        of cut of 1,0 mm.
Recommended tool geometries: sharp
geometries, such as -ML.
Recommended grade of indexable insert:
1025.
Recommended grade for solid carbide:
1010.


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