ISSN 1068�798X, Russian Engineering Research, 2014, Vol. 34, No. 3, pp. 195–197. © Allerton Press, Inc., 2014.Original Russian Text © E.S. Kiselev, E.N. Leksin, 2013, published in STIN, 2013, No. 8, pp. 35–38.

195

The life of gas�turbine engines is determined bytheir most stressed components: the turbine’s rotorblades (Fig. 1a). The front and rear of the blade expe�rience variable stress due to thermal and vibrationalloads, whose frequency and amplitude vary widely.They are also subject to flexure and torsion under theaction of the gas flux. The temperature at the blades inthe first stage of the turbine may reach 1000°C ormore. In turbine operation, the front and rear edges ofthe blade are heated and cooled considerably morerapidly than is the central region. At the edges, com�pressive stress arises on heating, and tensile stress oncooling.

The cycle of alternating stresses leads to blade fail�ure as a result of thermal fatigue. The thermal microc�racks initially formed turn into fatigue cracks.

Note that such components operate in high�tem�perature aggressive gas, which may result in gas corro�sion. In a wet and dusty air flux, the blades constantlyexperience the action of abrasive particles, with conse�quent wear of their front and rear edges.

To ensure the required blade reliability and motorperformance in airplanes, special requirements mustbe imposed on the manufacturing technology. Often,this process is designed on the basis of the blade geom�etry, the thermophysical and mechanical properties ofthe blank, and the specified operating conditions.

Today, gas�turbine blades are generally produced onmulticoordinate machining centers capable of operat�

ing under the direction of CAD–CAM–CAE controlprograms. To ensure minimal manufacturing costs,with the required blade quality, continuous five�coordi�nate machining is actively employed. In that case, theblade is machined on a minimum of equipment.

The simplest machining method, which does notrequire a highly skilled operator, is to align the axis ofthe face mill along the normal to the machined surface(Fig. 1b). Although the simplest in mathematicalterms, this method is the least effective in practice onaccount of the extremely low cutting speed close to thetool’s axis of rotation, which is generally associatedwith poor surface quality. In addition, in many cases,this method cannot produce the required part withoutthe introduction of cuts.

The best method involves the use of lead angles(Fig. 1c) and inclination (Fig. 1d) of the face mill, sothat the point of tool–blank contact may be shiftedfrom the critical cutting zone (close to the tool’s axis ofrotation) toward the periphery. When the blade geome�try does not permit machining by means of these angles,the mill position is reversed, with a lag angle and a neg�ative inclination.

These parameters have a particularly importantinfluence on the productivity in the final machiningoperations, when it is inexpedient to reduce themachining parameters (cutting speed, supply, cuttingdepth, etc.) for economic reasons. With incorrectselection of the inclination of the tool axis and the lead

High�Speed Turbine�Blade Production with Five�Coordinate Machining by Face Mills

E. S. Kiselev and E. N. LeksinUlyanovsk State Technical University, Ulyanovsk, Russia

e�mail: stinedit@yandex.ru

DOI: 10.3103/S1068798X14030071

δ

γ(a) (b) (c) (d)

Fig. 1. Gas�turbine blades made of KhN35VTYu high�temperature alloy (a) and methods of machining the blade’s front profile bymeans of a monolithic spherical mill: (b) perpendicular to the machined surface; (c) with a lead angle δ; (d) with an inclination γ.

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RUSSIAN ENGINEERING RESEARCH Vol. 34 No. 3 2014

KISELEV, LEKSIN

angle, the frictional force at tool–blank contact maybe impermissibly large. That increases the cuttingforces and affects the final deformation. In addition, atthe end of its path, the cutting tool will have wear at itsfront surface that considerably exceeds the tolerances.That will impair the linear precision in machining.

In an experiment, we want to minimize the strain atthe front of a KhN35VTYu high�temperature alloy bladein final milling. We assume constant cutting parameters,which remain unchanged throughout the whole series ofexperiments: cutting speed v = 75 m/min (rotary speed7960 rpm); supply s = 0.03 mm/tooth (240 mm/min);cutting depth t = 0.15 mm. The tool employed is anSGS 57 MB spherical end mill (diameter D = 3 mm;number of teeth z = 2) manufactured in the UnitedStates. An Integrex 200�IV ST multipurpose machin�ing center (Japan) is used. The strain at the front of theblade is measured on a Carl Zeiss coordinate�measur�ing machine (Germany). The wear at the front surfaceof the mill is monitored on a Speroni Esperia instru�ment (Italy).

The experiment is based on a 22 multifactorialdesign. We now describe the design matrix for the22 multifactorial experiment. The variables are theinclination γ of the tool axis and the mill’s lead angleδ. Measurements are made in two cross sections of thefront profile, on two sides. In Fig. 2a, as an example,we show the measurement of the blade’s front profilein the fourth cross section. In Fig. 2b, we show thestrain�measurement points for all four cross sections:

These values of γ and δ are selected on account oftheir practical applicability. Russian experience in manu�facturing gas�turbine blades shows that the variation ofthese angles is generally within the ranges indicated.

In Fig. 3, we show the experimental results. Hori�zontally, we plot 20 points on the front edge of theblade in each of the four cross sections. (The first pointis most remote from the rear of the blade.) Vertically,we plot the corresponding strain Δ at the front profile.

According to measurements on the Speroni Esperiasystem, the wear of the mill at the front surface in exper�iments 1–4 is 0.036, 0.033, 0.024, and 0.019 mm,respectively.

Analysis indicates that the efficiency of machining isgreatest in experiment 4: with those values of γ and δ, thewear at the front surface is no more than 0.02 mm. Thatis about a third of the corresponding tolerance, withallowance for the tolerance in final grinding.

The results may be explained as follows. Greaterinclination γ of the mill increases the effective toolradius, which reduces the frictional force in the con�tact zone and hence the stress associated with the pro�cess. Small γ does not permit adequate reduction inthe friction or stress.

Experiment 1 2 3 4Angle γ, deg 5 5 10 10Angle δ, deg 1 5 1 5Fig. 2. Measurement of the blade’s front profile (a) and

strain�measurement points for four cross sections (b).

0–0.1

5 10 15 20

0

0.1

0.2

N

Δ, mm

0

–0.1

5 10 15 20

0

0.1

0.2

N

Δ, mm

0–0.2

5 10 15 20

00.1

0.3

N

Δ, mm

0–0.2

5 10 15 20

00.10.2

N

Δ, mm

–0.1

0.3

–0.1

0.4

0.2

12

3

4

11

12

2

3

2

3

44

4

3

(a) (b)

(c) (d)

Fig. 3. Results for the strain Δ at the blade’s front profile for cross sections 1–4 (numbers given on the curves) in experiments 1(a), 2 (b), 3 (c), and 4 (d): N = 1–20 denotes the numbers of the points at the blade’s front profile in each cross section.

3.1

3.2

3.N

3.19

3.20

1.1 2.1

4.12.24.2

2.N

4.N

2.19

4.192.20

4.20

1.19

1.20

1.N

1.2

(a) (b)

RUSSIAN ENGINEERING RESEARCH Vol. 34 No. 3 2014

HIGH�SPEED TURBINE�BLADE PRODUCTION 197

At the same time, small δ (up to 5°) improves chipremoval. Further increase in δ has no significant influ�ence on the strain at the front surface but increasestool wear and reduces operational safety, with greaterrisk of collision of the machine�tool components(according to 3D simulation). The use of the supplymechanisms is also ineffective.

Thus, a significant benefit of continuous five�coor�dinate milling is the possibility of machining relativelyinaccessible zones and undercutting, by appropriateinclination of the tool. However, high moments ofinertia (and hence large inertial forces) may preventsufficient angular accelerations and velocities of thetool components. Therefore, in continuous five�coor�dinate milling, high�speed machining is very uncom�mon. At high cutting speeds, large supply rates areemployed, so that rotation of the blank (or spindlehead) may prove impossible. That impairs the linearand angular supply mechanisms.

A regression equation describing the formation ofthe residual strain as a function of the machiningparameters may be derived by means of a computa�tional table created using the Microsoft Excel 2007editor. By means of that software, the experimentalresults may be ordered, and the dispersion of the opti�mization parameter with respect to the mean may becalculated. The uniformity of the dispersion and theadequacy of the regression equation may be verified.

In Fig. 4, we plot the maximum strain of the blade’sfront profile measured for each experiment.

The corresponding regression equation for the strain(mm) takes the form Δ = 0.197 – 0.02γ – 0.05δ+0.0043γδ.

Two monofactorial experiments are conducted formore precise assessment of how each parameteraffects the residual strain. In Fig. 5, we show the

dependence of the strain Δ at the front profile on theinclination γ and lead angle δ of the tool’s axis.

Analysis of Fig. 5 yields a conclusion consistentwith the total factorial experiment. The inclination γof the mill axis has the primary influence on the strain.However, the scope for increase in γ may be quicklyexhausted in the equipment employed. Even smallchanges in the lead angle δ may reduce the strain at thefront profile. However, increase in δ beyond around 4°is inexpedient.

CONCLUSIONS

(1) The machining of the blade’s front surface incontinuous five�coordinate milling may be effectivelycontrolled by means of just two parameters: the incli�nation γ of the tool axis and its lead angle δ withrespect to the machined surface. The final surfacequality may be significantly enhanced here, as a resultof decrease in the frictional force between the tool’srear surface and the surface of the blank, withoutincrease in the machining time.

(2) We have determined the optimal inclination(11°) and lead angle (4°) for continuous five�coordi�nate machining of the blade’s front profile.

(3) When using optimal parameters, the wear at thetool’s front surface is no more than 0.02 mm.

ACKNOWLEDGMENTS

Financial support was provided by OOO IPKKhALTEK, Ulyanovsk.

Translated by Bernard Gilbert

10

0.1

0.2

0.3

2 3 4

Δ, mm

Fig. 4. Maximum deviation of the blade’s front profile (strain Δ) according to a total factorial experiment (Fig. 3).

γ1

0

0.1

0.2

12 3

4

γ2

1 2 3 4

γ3

1 2 34

γ4

1 2 3 4

Δ, mm

δ1

0

0.06

0.161 2 3 4

δ2

1 2 3 4

δ3

1 2 3 4

δ4

1 2 3 4

Δ, mm

0.08

0.12

(a) (b)

Fig. 5. Dependence of the strain Δ at the blade’s front profile on the inclination γ of the tool’s axis (a) and its lead angle δ (b): C1–C4,numbers of the cross sections in Fig. 2b; δ1 = 2°; δ2 = 3°; δ3 = 4°; δ4 = 5°; γ1 = 5°; γ2 = 7°; γ3 = 9°; γ4 = 11°.