C_2920_18_ENG_086_109

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Blade machining

At the heart of any gas turbine, whateverits particular design or function, the actionof the turbine blades is crucial for the tur-bine to carry out its intended function.Their operating environment can beextremely demanding, with large variationsin temperature and pressure as well as thephysical stresses of high speed rotation, and any flaws in their structural integrity may lead to rapid failure. Consequently the choices of material and manufacturingprocess in blade production, and thesecurity and efficiency of the machiningoperations employed, are vital.

Blades of many different sizes and geome-tries are utilised in gas turbines, and canperform different functions within theturbine. Some are stationary blades, whileothers are rotating, and it is usually therotating blades which present the greater

machining challenges due to their toughermaterials and more complex designs. Thestationary blades, also called vanes, havesimpler designs and are primarily used todirect the airflow. Hence they are usuallyregarded as being easier to machine thanrotating blades, although the quality oftheir manufacture is still critical for turbineefficiency.

For either class of blade, the raw material canbe bar stock, forging, or precision castings.

87

Cutting tools for turbine bladesThe commonest workpiece materials forturbine blades are stainless steel, heat resist-ant super alloys (HRSA) and titanium, butof these stainless steel, ISO class M materi-

als, account for the majority of workpiecesused.

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

Blade machining strategiesA variety of machine tools are suitable forblade machining, including 3-, 4-, or 5-axismachines, but the 3-axis machines are notrecommended for the majority of modernblade machining operations. They can onlyeffectively produce the simplest shapes andgeometries, and although such machines arestill widely available and profitable formaintenance operations, they are not rec-ommended for new investments or mostmodern machining processes.

4-axis machining is more common, particu-larly in older machines which have beenupgraded with NC-programming capabil-ity. But the modern trend is towards 5-axismachines, which allow maximum flexibilityand versatility while still using standardcutting tools.

The choice of overall machining strategy isimportant, and will greatly influence thesubsequent machining parameters. Thesestrategies fall into two classes:

Machining with one (or more) individualmachining centre(s)

or:

Machining with a dedicated machining cell

Deciding which strategy is best in a partic-ular situation depends on numerous factors,including:

the philosophy in the blade shop

the different types and sizes of blade

the design of the blade

the number of blades of each size

the machining operations involved

the calculated gross profit and the amortisa-tion time

the process flow

the CAD/CAM systems

the operating performance

the staff.

In general, for the efficient manufacture oflarge quantities of similar components, per-haps thousands of comparable blades peryear, a machining cell is the best solution.

Alternatively, for production of many dif-ferent blade designs, a single machiningcentre which is able to accommodate avariety of fixtures and/or special tools, will provide more flexibility and higherproductivity.

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vf

n

n

Machining the fixturing elements

The fixturing elements at the head and root of the blade structure are ultimatelyremoved to leave the final shaped item, butduring the machining process itself theiraccuracy and form have a crucial impact onthe success of the overall operation.

Consequently the design of these elementsand the tools used to machine them mustbe selected carefully, and will be discussedhere in some detail.

Element Tool

RootRectangle

RootTrapezoid or dovetail

HeadCountersinking

Head rectangleStandard

HeadCylindrical

Standard endmill – only roughingnecessary

Special – roughing and finishingnecessary

Standard – counterbore

Standard endmill – only roughingnecessary

Standard endmill – roughing andfinishing

Remarks

Wide tolerance possible, fixturing injaws (screw). Secured in two axis.

Close tolerance. Secured in all threeaxis.

Axial pressure (tailstock) for fixturingnecessary.

Special equipment. Large size.Transmission of torque possible.

Fixturing in a pull-in collet. Securedin all three axis.

Whichever processing methods areemployed, the first step is to machine thereference surfaces by which the workpiecewill be clamped during the subsequentmachining. Several Coromant tools aresuitable for this operation, and theCoroMill 390 long edge cutter is particu-larly recommended. CoroMill 200, 300 and

390 are also good alternatives.

It may also be possible in this operation toalso machine the clearances necessary forsubsequent processes, if the machiningstrategy would benefit from this.

Down millingClearance

Fixturing element

89

L

It is possible that the blade workpiece maydeform or bend during subsequent stages ofthe machining process, the result ofmachining away 80% of the original rolledor annealed raw material and the residualstresses thus created. This is particularlypossible for large blades, 400–600 mm long,which may bend by as much as 2 mm.Reworking the fixturing elements duringthe machining process, so that the positionof the workpiece in the machining centresis modified to account for the deformation,can counteract this phenomenon.

The recommended procedure for suchreworking on a 5-axis machine is:

� opening the fixturing system on the bladehead and moving it back, so that theblade is now secured only by the root.

� creating a new centre line for the work-piece, by counter-boring or turnmilling.

� fixing the blade by the new element.

An alternative is to modify the adaptoritself, so that the position of the workpieceis suitably adjusted when the modifiedadaptor is held in the machine, without anychanges to the fixturing elements.

Bending = 0,005 x L (mm)

90

Machining the root of the blade

The machining process to shape the root ofthe blade will depend on several factors,notably the dimensions of the finisheditem. Small blades are often machineddirectly from round bar stock, which isthen is milled to a square shape.

160 mm

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

Turbine blades can be divided into twoclasses, stator and rotor blades, and in nor-mal practice these two designs have differ-ent mounting systems and different stylesof root, to accommodate the different load-ings they receive in use.

Stator blades normally have one small slotin one side of the root, which is relativelyeasy to machine with solid carbide orindexable insert endmills.

Rotor blades may have different mountingsystems, such as a “Christmas tree” profile,or deep slots machined in a trapezoidalcross-section.

These variations in the profile and geome-try of the blade’s root will require differentmachining strategies:

Machining a ‘Christmas tree’ profile:

For machining the Christmas tree profileon a blade, it can be helpful to change thefixturing arrangement, and make the toolaxis parallel to the blade length.

It may also then be possible to use a specialadaptor on the Christmas tree profile tohold the blade during subsequent roughingoperations, and so avoid the need formachining (and later removing) separatefixturing elements onto the workpiece.

A milling strategy using CoroMill 390 long

edge milling cutters, applying down millingfor each side of the profile, will allow max-imised metal removal rates and tool life.

500 mm

91

1. Roughing with the long edge cutter indifferent ap-steps, using down milling

Calculate a suitable ae/Dc ratio so as tobring more than one effective tooth into cutduring the cutting cycle.

2. Roughing completed.

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

Roughing the christmas tree profile mayalso be performed by CoroMill 331 sideand face milling cutters in different diame-ters, to achieve the stair-like shape on thecomponent.

However, using a set of different diametercutters mounted in this manner results inlarge differences in effective cutting speedbetween the largest and the smallest cutter.

An alternative is to employ solid tools,particularly if there are difficulties withaccessibility or the complexity of the shapes being produced.

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Machining a deep slot in the blade root by endmilling

The type of workpiece material will have alarge influence over the machining parame-ters when machining slots into the bladeroots. In many cases it will be stainlesssteel, and thus problems of chip adhesionto the cutting tool will occur. However,carefully selected tooling and the correctmachining methods will counteract thesedifficulties.

The blade’s size and material, and the slot’sposition and form, will determine themachining strategy. In most cases it will bebetter to leave the machining of the slots,along with their roughing and finishing,until after the other machining operationsare complete. That way the machining ofthe blade profile itself can be carried outwithout any slots in the blade root whichmight conceivably affect the clamping andstability of the workpiece. In addition anybending or deformation in the workpiecethat occurs during profiling, due to therelease of internal stresses, can be compen-

sated for when the item is remounted priorto the finishing operations, an approachwhich should also help to maximise thequality of the final blade.

In general, machining deep slots in theblade root can be divided into:

slot milling (L-style with endmill)

plunge milling (with endmill)

trochoidal milling (with endmill)

Slot milling

L-style milling is a technique for cuttingdeep slots which can be beneficial to bothpower consumption and tool life. After aninitial channel has been cut by the first pass,subsequent steps down towards the finalslot depth are made with the tool cutting asequence of L-shape shoulders around thecavity perimeter, rather than further full-scale cutting engagements.

Machining with CoroMill 390 endmills,

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with -11 or -17 size inserts mounted in dif-ferential pitch, will enable tool life andpower consumption to be optimised, andallow the machining operation to be com-pleted in a minimum number of steps. Thelarger depths of cut should be carried out indown milling.

The widths of the slot should be 1,2–1,4times larger than the diameter of the end-mill, giving an overlap of 20–40% in thedown milling action.

For the finishing operation a special tool isrequired to produce a trapezoidal profile inthe slot, often HSS or brazed carbide.

Recommended cutting speed/feeds can befound in Coromant publications.

Plunge milling is a very effective method toachieve a maximum chip volume perminute, low power consumption, andincreased tool life at higher cutting speeds.

This method is the first choice whenmachining stainless steels with an austeniticstructure (e.g. CMC 5.21 and others). Inthe cutting action, the chip has no opportu-nity to glue onto the cutting edge, and bothsides of the cutting edge can be employed.

It is also the first choice when machining inweak fixturing conditions.

Recommended tooling is in line with therecommendations for slot milling, i.e.:

R390 endmill, inserts size -11 or size -17

The width of the slot should be 1,2 to 1,4times larger than the diameter of the end-mill.

For the finishing operation you need a spe-cial tool according to the trapezoidal profilein the slot, often HSS or brazed carbide.

Recommended cutting speed/feeds can befound in Coromant publications.

Plunge milling

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Tool path

Conventional method. Trochoidal method.

Tool path

Troichoidal milling:

In trochoidal milling, an approach particu-larly suited to CoroMill Plura solid carbideendmills, the cutter removes repeated‘slices’ of material in a sequence of spiralpaths, combining large axial cuts with smallradial cutting depths. This is ideal formachining slots, as well as pockets and

grooves, and can also be employed for highspeed machining (HSM) operations, as thecurved path enables the maximum feed rateto be maintained during the entire machin-ing process.

Machining the rotor – power generation turbines

Although machining the detailed designfeatures of the rotor sections of turbines isbeyond the scope of this book, some gen-eral information and basic principles can begiven.

Very specialised equipment is required tomachine the grooves in a rotor, the groovesinto which the root of each individual bladewill ultimately be fitted. These grooves maybe straight or curved in geometry.

For machining, the rotor is normally fixedon a heavy duty turning machine with anintegrated milling unit.

The form of the groove, straight or curved,will determine whether a 3-dimensional slot

milling cutter or a 3-dimensional bellmilling cutter is required. Both tools arehighly specialised.

The basic machining strategy will include:

Machining the first slot with a slot millingcutter such as CoroMill 331.

Opening the slot as wide as possible.

Using the bell milling cutter or slot millingcutter to machine the profile, or using aspecial “Christmas tree milling cutter”, anendmill with indexable inserts.

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Machining the blade body

Machining the blade rhombus is a criticalstep in blade manufacture, and a wide vari-ety of potential machining solutions areavailable depending on the design of theblade and the types of cutting machineryavailable. A comprehensive description ofall these different methods is beyond thescope of this book, but the basic principlescan be outlined, emphasising the machiningprinciples which underlie them: optimisingthe cutting tool engagement, reducingvibrations, using the tooling as effectively aspossible, 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 reachthe full depth of cut. In most cases thismethod allows the cutting force to bereduced more effectively than by reducingthe feed per tooth, as it allows the chipthickness to be modified towards therecommended target values.

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

96

To achieve the full benefits of thisapproach, the milling strategy must usedown milling, and a 45° angle of cuttingentry into the workpiece.

The tool path must not change through 90°angles. Instead, change the feed directionincrementally through small changes ofradii.

Ensure a tool engagement of 60–80%, ifnecessary by changing the tool diameter orcutting path.

Employ a different depth of cut in each ofthe two passes, to minimise notch wear onthe cutting insert.

Maximise the larger depth of cut as much aspossible.

Vibrations and heavy axial pressure on theinserts will occur if the feed forces causeany movement or deflection of the work-piece. If this occurs the feed directionshould be modified so the forces act indirections where the blade fixturingarrangement supports the workpiece mosteffectively.

97

Vibrations can also be reduced by adoptingcutting paths which machine the metal insmall triangular steps, in both the longitudi-nal and lateral directions. This approachrequires modifications to the cutting speedand feed, along with no more than 60% ofthe usual maximum depth of cut, and themodified cutting forces will also producechanges in the wear patterns seen on thecutting inserts.

Roughing the rhombus – parallel to the blade axis,using two tools of different diameter

The use of two different tools to machinethe rhombus is an effective strategy inmany situations.

A first cut, producing a slot perpendicularto the blade axis, can be made with an end-mill such as CoroMill 390 (using L-millingor plunge milling) or a slot milling cuttersuch as CoroMill 331. This slot then pro-vides clearance for a subsequent cuttingtool of different diameter, which shouldexperience a less severe cutting environmentand generate lower vibrations while itmachines along the blade’s longitudinal axis.

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

98

Roughing the rhombus – machining the roof slopes

This penultimate operation in roughing theblade’s contour uses a roughing tool whosesize will depend on the design of the blade,and on the radius between the roof slopeand the blade’s root.

A round insert milling cutter is normallyrecommended, such as CoroMill 200, or alternatively an endmill such asCoroMill 390.

Roughing the pressure side – peripheral milling

Roughing the pressure side of the blade –the concave side – is usually the last stageof the roughing process, and also one of themost complex.

Modern designs of turbine blades maximisetheir efficiency through complicated surfacegeometries, and machining these surfacesrequires a careful machining strategy toaccount for both the profile of the blade,and changes in the effective stiffness of the

workpiece as the machining operation pro-ceeds.

Peripheral milling is an effective way tocarry out this operation, with a depth ofcut between 1–5 mm. Round inserts arerecommended, such as CoroMill 200, as theround geometry ensures that the minimumresidual metal remains after each cuttingpass – as long as the correct cutting para-meters are used. The feed direction should

99

be away from the fixturing in the root, anda left hand tool such as CoroMill 200 maybe required.

Roughing the pressure side – waterline milling, parallel to the blade axis

An alternative strategy to machine thepressure side is “waterline milling”, anapproach originally derived from 3-axismilling in the die and mould industry, nowadapted to 5-axis milling machines.

In this technique, the cutting operationconsists of a sequence of 2-dimensional lay-ers, each completed before the tool movesdown to the next. Transitions between thelayers are carried out by helical ramping or

circular interpolation, with the initial feeddirection always away from the solid fix-turing at the root of the blade.

Recommended tool: round insert cutter,such as CoroMill 200.

Smooth transitions –little stock

Square shouldercutter, 90° Stock to be

removed

Much materialremaining afterroughing

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

100

ap

Roughing the pressure side – plunge milling

Plunge milling is a very effective millingstrategy, especially when producing longblades or when fixturing conditions areweak, and is also useful when machiningaustenitic stainless steel.

On turbine blades, plunge milling can beused to machine deep roof surfaces, as wellas the concave deep pressure sides.

Typical cutting recommendations include:CoroMill 390 endmill with –11 or –17 sizeinserts, an insert radius rε of 2 mm or lessto reduce radial pressure during the cut, anda step size 60–70% of Dc.

The recommended radial depth of cut willdepend on the insert:

For -11 insert: 5,5 mm

For -17 insert: 8,5 mm

Other speed and feed recommendations canbe found in Coromant publications.

Machining example for plunge millingpressure side:

� Plunge milling the roof surface

� Plunge milling the pressure side

Material – CMC 5.21, Tool – CoroMill 390, 32 mm diameter cutting head, holding twoinserts grade 2040

Cutting data: vc – 210 m/min, fz – 0,18mm/tooth, ae – 8,5 mm, step size – 20 mm

ae = step size

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9 8 7 6 5 4 3 2 1

Roughing blades using turnmilling techniques

Turnmilling centres are versatile machinesin which the cutting head can either hold asingle cutting edge which remains station-ary (in which case the machining operationis fundamentally a turning process) or amulti-tool cutting head rotating in place (asin a milling process). In such machines,when in ‘milling’ mode, the workpiece canstill turn along its longest axis – as it wouldin a turning operation, although now atslower revolutions – enabling the rotatingtool’s attitude to the workpiece to bechanged as desired by moving the work-piece as well as the tool. Complex profilescan be machined in this way.

Such techniques are among the mosteffective methods for roughing small andmedium size blades up to 600 mm inlength, but although the basic principles aresimple, in practice this approach requires avery flexible CAD-CAM system, withspecialised software and optimised NC systems.

If correctly applied however, turnmillinghas several attractive advantages over otherstrategies, in both machining centres andmachining cells:

� the highest values for chip volumes perminute

� regular engagement of the tool, leading toa smooth cut

� the minimum number of necessary tools

� a balanced spindle load

� machining the whole surface profile alonga component’s length with one cut

� an alternating depth of cut, thereforereducing the notch wear

� a constant movement in all 5 axis, reduc-ing any reciprocating movement in thefixturing system

� short tool overhangs

n tool

n blade = vf

Torque

102

vf

Correctly applied, this technique can beused for all roughing operations on a blade,including the head and root as well as theblade body.

When turnmilling a blade structure, theworkpiece slowly rotates along its long axisin front of the tool, while the cutting headmoves in two dimensions, perpendicular tothe cross-section of the blade profile, andalong its length. In practice, the profile isthus machined in a single cutting engage-ment, and the tool moves slowly down thelength of the spinning workpiece, in asingle helical cut. Therefore the concavepressure side can also be machined in thesame operation, without having to alter therotation of the workpiece, and only onerevolution per blade is required.

The normangle of the tool is not constant,and will alter as the workpiece rotationpresents different aspects to the tool. Thefirst engagement should be in a directionaway from the blade root, to correspondwith the blades inherent flexural strength,and downmilling should always be used.

Recommended tools for turnmilling whenmachining blades with stiff cross sectionsare round insert milling cutters such as theCoroMill 200 cutter, with RCKT 1204inserts.

The step size should be app. 80% of the Dc.

Recommended tools for turnmilling whenmachining thin blades (e.g. compressorblades) are face milling cutters such asCoroMill 390, using inserts with a radius rεof 0,2–4,0 mm. The axial pressure on such atool is much lower than on a round insertmilling cutter, which causes less vibrationduring machining.

Semi-finishing the blade

The semi finishing operation requires a 5-axis milling operation, and will directlyinfluence the surface quality of the finalfinished blade. Therefore the aim shouldalways be to achieve a very regular, uniformlevel of residual material – if necessary,through two separate semi finishing opera-tions.

Normally this operation is done by turn-

milling. The recommended tool is an end-mill with indexable inserts, such as theCoroMill 390, or a round insert millingcutter such as CoroMill 300. The choice oftool will depend on the profile of the bladeand its size.

A variety of tool paths can be employed.One common technique, especially whenmachining large cast blades, is to use a feed

n tool

Roughing ae = 0,8 x Dc

Prefinishing ae = 0,3 x Dc

A-Axis

103

direction along the blade length, but otherpossibilities are shown in the diagram. Forexample, the blade can be shaped by millingacross the blade, either using several passesin 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 transitionalradius between the root of the blade and thehead must be machined. This is specialisedstand-alone operation between the semi-finishing and finishing stages.

This job is also usually a 5-axis turnmillingoperation. The recommended tool is aconical solid carbide endmill, employing avery 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 mostdifficult 5-axis machining operation, but itssuccess will greatly depend on the qualityof the other machining steps carried outpreviously.

The most suitable tool depends on the typeand size of the blade, and also on the spin-dle speed and the feed available in the

104

ae

hR

ae

fz

fz <ae fz = ae

The principal problems when finishing arevibrations, and the quality of the pre-fin-ished surfaces. Using tools with a smallerradius, rε, or using a different number ofinserts in the cutting head can help combatvibrations, in line with the recommenda-tions given in Coromant publications

During the cutting process the tool followsa helical path around the blade, a path con-trolled by a specialised CAD-CAM system.To achieve the best surface quality andstructure, the tool has to maintain a con-stant normangle at each point on the sur-

face, and always in a downmilling manner.In this way, and combined with an oil mistcoolant, the resulting surface can be highlypolished.

With suitable optimised equipment it ispossible to achieve a surface roughness ofRa < 0,4 µm, although the final surfacequality will strongly depend on the combi-nation of normangle, feed and cuttingengagement.

R =radius of cutter h = cusp. height

h = ae2l(8 x R) Feed = rpm x ƒz x z

machining centre. The capabilities of themachines employed can often be the limit-ing factors.

In general, it is possible to use solid carbideendmills like CoroMill Plura 216.24, orendmills with indexable inserts, such as theCoroMill 390 with inserts R390 11T3 31E-PM 1025).

The tool diameters vary, e.g. between 10–20 mm.

Material – CMC 5.2Tool – R390-025A25 11-HInsert – R390-11T324E-ML 1025vc 320 m/min, fz 0,22 mm, ap 0,5 mm, ae 1,75 mmTc 45 min/tool, Dry machining

Spindle speed n = 12 000 rpm, cutter diameter = 6 mm

ƒz/ae

Feed vf

Cusp./h

Time (min)

0,05

F1200

h0,0001

10

0,075

F1800

h0,0002

4,44

0,1

F2400

h0,0004

2,50

0,15

F3600

h0,0009

1,11

0,2

F4800

h0,002

0,62

0,25

F6000

h0,003

0,40

orh =R – 2R2–ae

2

4h ~

ae2

8R√

105

Adjustable guide blades – turning or turnmilling

Some turbines contain adjustable “guideblades”, usually made from titanium (inaerospace) or stainless steel (in powergeneration), used to direct or control the air flow within a turbine. They may befixed and stationary, or fitted onto rotatingshanks by suitable grooves and fixturingelements. Normally these blades arebetween 300 mm and 1200 mm long,including the shank.

When machining such a blade there are twomain problems: the forged skin of thematerial, and the unbalanced nature of theworkpiece.

Three different machining methods arerecommended:

1. Machining on a turning machine (roughing – finishing)

A very stiff and powerful machine is neces-sary, preferably with a self balancing chuck.After pre-machining and creation of thefixturing elements, a complete machiningprocess is then possible, but due to theimbalance of the workpiece the cuttingspeed must be low. To improve chip-break-ing, Coromant Capto with an integratedhigh pressure cooling system (Jetbreak) isrecommended. With this equipment, cut-ting speed and tool life can be optimised,mainly due to the optimised chip breaking.

2. Complete machining on a turnmillcentre (orthogonal-, longitudinal- andplunge turnmilling)

When using these turnmilling variants,chipbreaking problems can be avoided, andwith suitable tools and programming acomplete machining process is possible. Forthese specialised operations various differ-ent tools are required, such as CoroMill390 endmills, CoroMill 331, CoroMill BallNose and CoroMill Plura. The mostimportant tool is the face milling cutterwhich creates the finished surface on theshank to the correct tolerances of rough-ness and roundness.

3. Combined machining with a turnmillcentre (roughing) and a turning machine(finishing)

This combined method to machine theseblades is probably the most profitable tech-nique, using a turnmill centre for theroughing applications, as described above,followed by a separate turning stage.

To achieve the highest surface quality andaccuracy, a stable turning machine with aself balancing chuck is recommended.

When using a Coromant Capto systemwith integrated Jetbreak cooling and aWiper insert, both the feed and speed canbe maximised, and the machining timereduced dramatically.

700

140

Ø 50

Dimensions in mm

106

Machining examples for adjustable guide blades

� Roughing the shank: CoroMill 390, insertR390-170450E-MM 2030

To achieve the highest productivity, the cut-ting tool diameter should roughly equal theshank diameter.

� Roughing the cone: CoroMill 390, insertR390-170450E-MM 2030.

� Machining the sealing lip: solid carbideendmill CoroMill Plura 216.22, with zn = 3.

For the highest productivity, the sealing lipcan be machined in one cut.

� Finishing the shank: CoroMill 390, insertsR390-11T308E-PL 1025.

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Other blade machining operations

Certain blade designs consist solely of ashort blade body without head or root ele-ments, which can be machined using a“helimilling” method.

In this approach the tool axis and the bladeaxis are parallel, and the tool rotates aroundthe blade following a helix.

This method is very effective for smallblades with lengths <150 mm, and as longas the set up is sufficiently rigid, it is poss-ible to machine the blade with just oneroughing cut and one finishing cut.

Recommended tools for roughing:CoroMill 200 or CoroMill 300

Recommended tools for finishing: CoroMill 300

Short blades

An automatic bar machine is normally onlycapable of 4 axis machining, but this canstill be an effective and profitable way tomachine small blades. The short overhangsbetween the blade and the tool will pro-mote a rigid set up, so large depths of cutand feed per tooth can be employed,although subsequent grinding of the blademay be necessary.

Cutting paths which follow small triangularsteps in the longitudinal and lateral direc-tions, similar to those described above forroughing of the blade rhombus, can beutilised.

Recommended tools:

CoroMill 200; CoroMill 390; CoroMill 245for roughing

CoroMill 390 in different diameters forsemi-finishing and finishing

Due to the low RPM on such machiningequipment, solid carbide endmills are notrecommended.

Machining with an automatic bar fed machine

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Peel milling is one of the most traditionalmethods to machine turbine blades.

It can be performed on multi spindle copymilling machines, with side and face millingcutters such as CoroMill 331, or with anendmill such as CoroMill 200.

This is a very effective technique for repair-

ing and renovating old blades, and themulti-spindle machines can allow up to sixblades to be machined at one time.

Peel milling (Multi-Spindle)

After the finishing operation is completedthere will inevitably be residual stressesremaining in the blade surface, induced bythe machining operations themselves.Typically these stresses could reach 200 to400 MPa, down to a depth of 0,4 mm, andmust be counteracted in order for the bladeto maintain its structural integrity andrigidity.

The common techniques for addressingresidual stresses include grinding, presspolishing and vibratory grinding, of whichgrinding, vibratory grinding and blastingare the methods most often employed.Press polishing requires specialised equip-ment and knowledge, as does the use ofultra sonic methods to monitor and observethese residual stresses.

After finishing

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Trouble shooting and general adviceIf excessive vibrations, poor tool life, orbad surface finish should occur, the firststep in trouble shooting should always beto check that the correct recommendedtools and cutting parameters are beingemployed. Check also the rigidity of thetool set-up, together with the power con-sumption and torque of the spindle at theparticular RPM being used.

If problems still remain, check the engage-ment of the tools. Some CAD/CAM sys-tems have their own interpretation of thebasic programming instructions, which mayneed to be adjusted. Check that the ae andDc values are in the recommended ranges.Using a smaller tool diameter at the begin-ning of the cut may be of benefit.

Check the axial depth of cut. Some fixtur-ing systems are not rigid enough to handlea feed direction perpendicular to the longaxis of the blade. In that case reduce the

axial depth of cut and increase the cuttingspeed.

Reduce the overhang of the tool.

Use rigid set-ups, preferably CoromantCapto throughout.

Check the feed direction. In some cases itcan be helpful to reverse the feed direction,using the same tool. Be sure to employdown milling techniques.

Control the cutting force acting on thetool. Large radii on the insert will increasethe security of the overall process, but atthe same time they will increase the axialpressures created. Change the radii on theinsert for a smoother cutting action.

Special machining parameters for HRSA and TiUsually these blades are cast or precisionforged, and the machining is concentratedon the root.

For finishing the root, use coated solid car-bide endmills, or endmills with indexableinserts. If a coated insert is impractical forhealth and safety reasons, use uncoated cut-ting tools in line with the publishedCoromant recommendations.

Recommended tool geometries: sharpgeometries, such as -ML.

Recommended grade of indexable insert:1025.

Recommended grade for solid carbide:1010.

Due to the tendency for work-hardeningwhen machining HRSA, it is not desirableto machine again a surface which hasalready been through a previous cuttingoperation. Therefore, all the metal removalrequired by the design of the blade shouldpreferably be carried out with one cut. Or,if this is not possible, use a minimum depthof cut of 1,0 mm.