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Paper n. 3
"PROPELLER BLADE MANUFACTURING - FINCANTIERI ADVANCED
TECHNOLOGY OF PRODUCTION BY CAMPROCEDURE"
P. NaVarra, G. Rivara, G. RöI!ando (Mech
Dept.), L. Grossi (Design
Dept) - FINCANTIERI
Naval Shipbuilding bivision
L. Accardo - MARIÑA MILITARE ITALIANA
NAV '88 - WEMT '88 SYMPOSIUM
ARCH IEF
ABSTRACT
A few years ago the Mechanical and Design Dept. of FINANTIERI Naval Shipbuilding Division started and applied a research program,
partial-ly supported by a CETENA financing in order to achieve a quality hi-gher degree in the propeller blade manufacturing, by developing a CAN procedure suitably adapted to an already existing numerically
control-led working machine and fully connected with the propeller CAD-procedure.
This paper, bes.des a brief discussiott on the good grounds for a
hi-gher degree of accuracy in the propeller blade manufacturing describes the CAD-CAM software developed by FINCANTIERI-CNI as well as the
exi-sting hardware.
The high achieved level is substantiated by illustrating the results obtained by this manúfacturing procedure, applied to the propeller blades of an Oceanographic Research Vessel för the NATO SACLANT CEN-TRE, recently delivered by FINCANTIERI-CNI, for which the required to-lerances wére very demanding.
i. FOREWORD
In the làst few years, the world shipbuilding industry. has trièd the
utmost to optimize ships design production and maintenance
Many efforts were devoted to achieve shipbuilding and operational
costs reduction with a view to increasing performances and ship requi-rements.
Propeller plays an important role in both ship propulsion. systeth and performances such as maximum obtained speed, fuel consumption, hull vibrations, accommodation comfort and radiated noise These problems
make sometimes the propeller the most critical component for the
ship's successful outcome. .
In fact, the propeller must meet an.increasing number of rigbrous re-quirements, such as, high efficiency, cavitation reduction, low levels of hull induced excitation and onboard as well as underwater radiated
noise.
These requirêments led to a high propeller engineering level both in the design and manufacturing process. In the last decade, a conside-rable. progress in the propeller design technichs has been achieved, by. utilizing airfoil vortex theories and computer codes, based upon stea-dy and unsteady lifting surface methods, which suitably reflect the three dimensional hydrodynamic propeller behaviour and its cavitating pattern.
These numerIcal methods, together with the model and full-scale trial acquired experience, allows the propeller designer to acquire a detai-led knowdetai-ledge of the hydrodynaxn,ic behaviour even in the design of the up-to-date complex propeller blade shapes, such as high skew, radially changing pitch and unconventional sections thickness. and camber.
Extensive use of reliable computer codes and model tests has well fo-cused the importance of some particular blade details, surface finish,
leading and trailing edge shapes, camber and thickness radial and
chordwise distributions on both propeller efficiency and cavitation performances.
Many authors [1] and [2] have demonstrated how moderate leading edge geometrical differences can drastically change the propeller's cavi-tation behaviour Sometimes these differences are withir' the current
International Standards allowed tolerances. For example, the ISO
484/1 HIghest Class (S) applied to a 3 m dia. propeller allows manu-facturing inaccuracies in thé bÏade shape of 10-15 i while design re quirements as often occurs for naval and special propellers may re-quire tolerances lower than 0.5 mm.
Although purpose of this paper is not to review and/or criticize the
currént ISO International Standard, the above example clearly proposes the concept that manufacturing tolerances and related Rules should re-flect the required level of design efforts and should be functions of ship's and propeller's actual performance requirements.
Current manufacturing procedures of propellers, based upon moulding, casting and grinding, cannot allow so narrow tôlerances as required by high performance propellers unless a considerable and unacceptable cost and time increase, due to an extreme. accurate hand-finishing, is involved.
Fortunately, in connection with the design procedure improvements, néw powerful production tools, numerical controlled machines and
robo-tics have now become available.
Of course, these new tools allow to achieve higher degrees of quality
in propeller bläde dimensional tolerances, but they have especially led to a conceptual change regarding the propeller production, by
con-sidering the propeller no more a traditional casting product but à mo= dem machining product.
According to this point of view, FINCANTIERI Naval and Mechanical Di-vision, partially supported by CETENA, has developed a CAD-CAN
proce-dure, integrated with the existing computer hardware and numerica].ly
controlled machine, to tackle and solve the problem of the propeller production (more exactly very demanding c.p. propellers) from the
design to the manufacturing and quality control phases.
A brief description of this procedure as well as dimensional
toleran-ces achieved in a propeller manufacturing protoleran-cess are reported in the following.
.2. F.INCANT.IERI CAD-CAN SYSTEM
FINCANTIERI propeller CAD-CAN system is based upon and summarizes the many experiences carried Out fl the past years [.3], [4], [5] by both CNI Merchant and Naval Divisions and CETENA in propeller design,
manu-facturing and verification activities both i:n model and full-scale.
As it is summarized in the Flow Chart of Fig.1, the System can be
di-vided n three main phases : Design, Surface Definition,
Manufactu-ring.
These three phases, that will be describec. in the following para-graphs, are carriêd out. by computer codes and numerically controlled machines fully integrated and connected.
Computer codes have been set up on computer UNISYS .1100/80 and manu-facturing is performed in the Riva Trigoso Shipyard by the exj..st:i.ng
milling achines, three axes numerically controlled.
Of course, the best solution for milling a propeller is the use of a
five axes numerIcally controlled machine. The lack of twO numerically controlled axes has been overcomed by optinu...ing CAD and mi1ling pro-cedures and by mounting the blade to be machined on a rotating (step
by step) platform, which practically adds a non-numerically control-led fourth degree of freedom. Therefore, milling tools, blade posi-tioning and computer codes for surface definition have been adapted for athree/four axes machine. This procedure hà been optimized for blades of controllable pitch propellers, that represent àlmost 90% of the Naval DivisiOn.
2.1. PROPELLER DESIGN
Propeller design procedure in use at FINCAÑTIERI CNI takes into
ac-count the acquired experience [3], [4] in this field The necessity of high performance propellers with uncommon blade shapes requires a
hy-drodynamic behaviour in steady and unsteady working conditions.
As Shown in Fig. 1, the design. phase of the CAD-CAN procedure utilizes computer codes based upon
upto-date
théories on three_dimensional propeller flow.The propeller design starts with the choIce of main parameters by a quick and easy lifting line computer code ELIPACK Detailed blade geo-metry is then calculated by a steady state lifting surface computer program PESP [6], meeting the performance and cavitation propeller re-quirements.
Undésired phenomena as cavitation, noise and hull-induced vibrations are mainly generated by unsteady behaviour of the propeller operating in the ship's wake, so that an unsteady lifting surface computer code PRESS [1] has been set up and widely utilizéd for the calcu].ation of bearing forces and unsteady pressures associated field.
Propellers with high cavitation and noise performançe requirements are usually tested in cavitation tunnel, on model-scale, in order to check and investigate on its cavitating behaiour, before the definition of the final design.
The highly skewed propellers strength is verified by finite elements
method computer programs NASTRAN and/or SESAM.
2.2. SURFACE DEFINITION
For traditional propeller manufacturing a drawing of blade shape and dimensions at no more than ten blade sections is generally necessary.
Smoothing of the blade geometric characteristiéS is devoted to the blade draftsman, while fairing and smoothing of blade shapes and de-tails, such as leading edges, tip and hub root fillets are devoted to grinding-men.
In the numerically controlled milling procedure, blade surfaces have to be matheffiaticaily well defined in details, smoothed and faired in any particular without man intervention.
The surface definit-ition has been realizéd by two main computer
pro-grams ELIGRAPH for the propeller geometry detailed definition and
smoothing; APT for surface back and face fairing and milling cutter trajectory generation.
The ELIGRAPH computer code, fully developed by FINGANTIERI CNI and
CE-TENA, utilizes the propeller geometry data generated by the design
computer program PESP and allows the propeller designer to develope quite a deal of functions.
cha-racteristics of the propeller (pitch, chord length, skew, thickness and camber distributions) fairéd by splinesunder.-tensiofl mathematical algorithms. No automatic smoothing is allowed, but the designer can interactively operate, changing data and smoothing curves until the desired result is obtained.
This is a very important characteristic of this program as it allows
the operator to obtain by video-terminal smoothing curves without
loosing the direct control of the propeller parameters defined by the design procedure.
An accurate smoothing operation, that does not take more than fifteen minutes, is of paramount importance in the correct development of the
subsequent phases of the manufacturing process.
The ELIGRAPH computer code, af.ter smoothing process generátes the de-tailed drawing of the propeller and sufficient. data for the APT compu-ter code.
The propeller drawing in the conmton traditional propeller manufactu-ring represents the essential and often the only liaison between desi-gner and manufacturer. In the CAD-CAM procedure described hereby, the propeller drawings are generated for control and documentation but
do not intervene in the machining process, as the
designer-manufacturer liaison function is assumed by the APT data.
The APT (Automatically Programmed Tool) computer code, being known the surface to be realized and the cutting-tool type tobe used, gives to the numerically controlled machine the trajectory of the milling cut-ter.
As said above, while a five-axis controlled machine permits the mil-ling of the whole surface in one. step only, (with the milli:ng tool
al-ways perpendicular to the shape of the piece to be machined), the
a-vailability of a three-axes controlled machine does not allow a con-tinuous perpendicularity between the blade surface and the milling tool which should work with relative angles within 0° and 180°.
This problem led to the necessity of a blade surface milling process by steps, bearing in mind its actual physical dimensions and optimi-zing, for each surface to be worked, the best positioning of the blade in the machine respect to the tool, by means of the rotating platform. The utlization of a three-axes controlled machine further invOlves the necessity to utilize a spheric-head milling cutter and to compensate
its trajectory through suitable software, to take into account its
physical dimensions and its progressive consumption.
In practice, the blade surface is divided into three main parts: fa-ce, back and contour, and before proceeding to the detailed definition of each sculptured surface,(face, baòk and contour profile), the most
suitable blade. positioning/S respect to the operating machine are
de-fined basing especially on preliminary drawings of the design section profiles, and on the availability of the rotating platform.
As a further problem inherént a three axes machine, the definition of face and back surface, only by solid points on section profile, does not allow a complete leading and trailing edge cutting
This problem has been solved again by software generation of extra points for both leading and trailing edges, in açcordance with the ac-tuai curvature of each section, as shown in Fig. 2. Details of the leading edge machining are shown in Fig 3, where it can be seen how
the remaining part of the material is cut off by N planes contour, which allow the leading edge itself to be almost completely finished,
according to its exact geomet-ry.
As already said, APT package generates all the information for the machine-tool, considering the tool itself as a part physically inte-grated in the geometry of the blade to machine.
In short, the employ of the APT program is realizéd through two ma.n subsequent steps:
* generation of the sculptured surface to be realized during the actual contact between the bla.e and the milling cutter;
* prograuing of the milling tool technologic parameters (rotating speed, advance speed. etc.) for machining.
Being known the milling tool dimensions as well as the cut intervals, the tool path is generated in such a way that an. actual point of con-tact with the blade describes a helical trajectory.
This type of path has been seléc.ted according to cylindrical
coordina-tes (not rectangular cartesian coordinates) as it follows exactly the
propeller design sections and better suits the path of the water flow on the operating propeller blade.
Ïn general, sOme drawings of the tool path are visualized and plotted according to different points of view, as a control of the exact fai-ring of the surface to be machined.
The abóvementiofled géoetrical and technological data, processed by the APT, are passed, through an intermediate file (CLFILE), to a post-processor for the particular machine on whjch the piece is to be wor-ked by the machine-tool, transformed into machine language, processed and transferred directly to the nuierical control of the operating
2.3. MANUFACTURING
The working machine consists Of a milling boring-machine with movable pillar, numerically controlled with ECS control on the three axes. The piece is positioned on a piece-holder fitted on a swiveling board at high precision at the blade design pitch and then rotated by the
quantity establihed on the program
As for the working phases, the performance takes place by sequences contour rough machining, face and back rough-shaping, face
finish-machini.ng (including blade root-fillet finishing), contour finishing
(even with more planes).
The blade surface smoothing and finishing is made by hand, taking as
reference the hollOw of each milling Cut.
After the hand-finishing, the blade is repOsitioned on the numerically controlled machine for the necessary dimentional verifications.
The verificat-ion procedüre is made by the same operating machine by
replacing the milling tool with an electronic comparator provided with a tracer point.
Of course the contrOl procedure utilizes the same design points of the various sections, suitably rotated and transformed (by software) in corresponding three-dimensional coordinates in the space, respect to
the operating machine The operating machine through these data is positioned in the theoretical exact points and the differences read on the comparator quantify the section shape tolerances.
The details of the leading and trailing edges at required radii
(ran-ging from 6 to 8) for both face and back of blades are controlled
through templates, worked out by numerically controlled machine.
2.4. BALANCING
The usual procedure of static balancing of controllable pitch propel-ler is based on
* a preliminary static balañcing of thé hub and separately of a few
component of the same;
* an inspection on the static unbalance in the radial and tangential directions of each single bla4e;
an optimum choice of the blade assenbly in order to minimize the final resultant propeller unbalance;
* an inspection with cooipletely assembled screw propeller of the
Stan-dard Rules.
*
For Naval propellers, manufactuted in a conventional way, some Na vies (Italian Navy included) require a dynamic balancing, evenif
the MIL-STD-l67 requires dynamic balance only for very high revolu-tions (> 1000 RPM) propellers and ISO does not require dynamic ba-lance at all.
*
For Nàval propellérs manufactured by a numerically controlled. ma-chine (with high level accuracy requirements), dynamic balancing is considered quite useless. and unnecessary.3. APPLICATIVE RESULTS
An example of. application of this procedure consists in the manu-facturing by Fincantieri of the blades for the propellers of an
O-ceanographic Research Vessel recently delivered to NATO SACLANT
CENTRE. Due to the very demanding requests of. high cavitation per-formance and very low noise levels, the propeller design led to the definition of unconventional geometrical characteristics, high skew, extreme tip and robt pitch unloéding and, finally, to the
re-quest of a very high degree of dimensional tolerances 0.25 0.5
mm for blade section.shape, 1 mm for axial rake and 0.12 degrées for local section's pitch.
The design process for this very demanding propeller has been
car-ried out also through the manufacturing of some propeller models, widely tested and compared iñ the Italian Navy cavitation tunnel
(CEIMM) for the blade cavitation radiated noise aspects. Blade
strength has been verified in detail for all various propeller ope-rating conditions by means of FEM stress calculation procedure in use at Fincantieri's [Fig. 4, 5, 6).
Without dealing with the design and verification procedure, applied to this propeller, which is not the target of this paper, and going. back to the manufacturing, the whole CAD proceure illustrated abo-ve, has been applied.
The trajectory of the milling cutter has been extensively verified by drawings, in a first time following the blade design sections (Fig. 7), in a second time with the actual milliñg tool trajectory seen from many perspective views (Fig. 8, 9, 10).
After each blade milling, only a few hours of machining by skilled workmen were required for finishing the blade tip and root and for smoothing the milling grooves on the blade surface, (already of an average value lower than 100 p), to surface finishing degree of
3.1.. MANUFACTURING ACCURACY RESULTS
The blade shape controlled through the above procedure has given di-mensional tolerances generally lower than the values requested by the
Contract Specification.
During the static balance, the weight difference between the various blades resulted lover than 0.09% of their average weight. This result
led to consider as superfluous any adjustment with removal of
mate-rial, thus getting a residual static unbalance of the assembled
pro-peller equal to 0.4 kg.m, which represents 13% of the allowed
unbalan-ce according to the most restrictive class of the International
Stan-dard Rules (Class S).
During an additional test of the whole propeller assembled aÍid fitted
on the dynamic balancing machine (however not requested by the Con-tract Specification but performed by the Shipyard on its own account), values of 0.4 kg..m were obtained, equal to those ones resulted during
the static balancing.
As a consequence of the blade manufacturing at such, a high precisionj
degree, the unbalánce obtained on the propeller in question is practi-cally only static so as to consider superfluous any dynamic balancing on propellers manufactured with such a kind of procedure.
3.2. SEA TRIALS RESULTS
During the Ship's delivery trials, many propellèr cavitation obsrva-tions by strobo-lamp, were performed Such observations, according to the Yard's practice, were compared with the scaled cavitation
pat-terns derived from the extensive tests carried out in model-scale at the cavitation tunnel.
In Fig. 11 it is possible to observe the cavitation inception bucket
as surveyed on model-scale and the same inception surveys predicted in full scale-condition, by ut!lizing a semi-empirical methodology based on quite a flumber of model and fUll-scale condition
correla-tions, on propellers manufactured by traditional methods.
According to such methodology a blade pressure side sheet cavitation inception was expected at a ship's speed of about 13 knots and a tip vortex cavitation inception at about 16 knots.
As a matter of fact, the full-scale stroböscopic observations have shown a propeller blade free from any cavitation phenomenon up to at
least 16.0 knots.
The same investigation as been made for a different propeller pitch.
setting, according to the special ship's operative requirements.
vor-tex inception was predicted at a ship's speed of about 10.5
knots and the face and back cavitation inception at 11.6 knots. The full-scale stroboscopic observations have shown no face and back cavitation and only a tip vortex inception at 12 knots, which is the max speed
for this ship operating condition.
Bearing in mind that the cavitation inception speed estimate
methodo-logy, with empiric coefficients derived from model_full-scale
corre-lations on propellers manufactured according to the traditional mode, in the past has always given good results wIth evaluation errors
not higher than 0 5 knots, we can deduce and confirm that the applied pro-peller CAN manufacturing methodology allows considerable advantages from the propeller cavitative point o view.
In fact, in the case under examination, an improvement of at least 1+2 knots was obtained as far as the tip vortex is concerned and of at least 3 knots as far ás the sheet cavitation phenomena are concerned.
4. CONCLUSIONS
The propel-lers designer has today enough powerful tools and methods
(theoreticS and experiments in model scale) to arrive at an optimum solution, complying with even very demanding performance requirements. Conventional propeller manufacturing processes are often a
considera-ble constraint in achieving the final high performance objective. Numerically controlled machining processes, such as those desçribed in this paper, are avalab1e today and represent a significant
improve-ment in the propellét manufacturing capabilities from a technical
point of view and at an aceptab1e total cost.
The degree of accuracy and associated tolerance requirements in the propeller manufacturing should derive of course frqm an objective as-sessment of the propeller criticality of the ship operational criteria. and performances.
When ship powering, propeller cavitation, associated noise and propel-1er strength re4urements are so high as to have a considerbie and di-rect impact on ship operability, the traditional propeller manufactu-ring process, even if producing a propeller complying with the highest ISO Class tolerance requirements, cannot be a sufficient guarantee of
its performances.
In such cases, numerically controlled machining becomes compulsory. The described (CAD-CAN) manufacturi-Ug procedure, developed and set up by Fincantieri allow, as a consequence, to reduce and solve the pro-blem of propeller balancing by making useless any dynamic test, as de-monstrated in the illustrated application case.
As illustrated in the paper, even a three-axis machine (with an asso-ciated rotating platform) can be suitably adopted to machining almost completely even à complicated sculptured three-dimensional body, such as a propeller blade. Software developments have been extremely use-ful to overcome machine degrees of freedom limitations.
Surface finishing with associated roughness levels, is still left to the ability and experience of hand-workers.
The normal surface finishing achieved in this way is generally in
ac-cordance with ISO tolerances and seems tó be at a suf.ficient level of efficiency/cost ratio, even if research on the assessment of the effi-cient value is going on.
On the latter argument it seems that, more than for newly manufactured propellers, the problem arises during the propeller life an4 simply consists of a proper maintenance problem.
REFERENCES
[lj
- D. T. Valentine."The Effect of Nose Radius on the Cavitation Inception Characte-ristics of two Dimensional Hydrofoils't DTMB Report No. 38133
-July 1974.
-[2] - T.. Brdcket.t
Minimum Pressure Envelopes for Modified Naca 66 Sections with Naca a = 0 8 Camber and Buships Type I and type II sections
DTMB Report No. 1780 - February. 1966.
(3] - F. Ba - G. Bellone - B. Chilò - A. Colombo
"Propeller Design Optimization an Integrated Theoretical and
Experïmefltal Procedure". Paper presented at "propeller '81"
-SNAME Symposium-MaY 1981.
[4] - L. Accardo - F. Baü A. Colombo - L.. Grossi
"An Integrated Theoretical and Experimental Procedure for Pro-peller Design". Paper presented at "ISSHES '83" EL PARDO (MA-DRID) - September 1983.
(5] - A. Tufano - G. Borgogna - L. Grossi
"Naval Propeller Design and Cavitation Verificatior by Lifting Surface Theory Theoretic Experimental Comparisons" Paper pre-sented at SYMPOSIUM NAy '82 - Naples - December 1982.
[6] - L. Grossi
"PESP - Design Program of Naval Propellers with the theory of the lifting surface - User's Handbook" CETENA Report no. 1737 - April 19ã3. .
(7] - B. Chilò - A. COlombO
"PROGRAM PRESS - USER'S HAÑDBOOK" - CETENA Report no. 1.736 - A-pril 1983.
pROCEDURE STEPS
DESIGN
SURFACE
DEFINITION
MA N UF ACT URI NG--MODEL TESTS
WORKING
DRAWÏ NGCHECK
DRAWING
DESIGN LIFTING LINE
DESIGN LIFTING SURF.
s-L
e
UNSTEADY
LIFTING SURF.
VERIFICATION
VIDEO GRAPHIC
SMOOTHING AND
CONTROL
FAIRING AND MILLING
CUTTER TRAJECTORY
GENERATION
POST-PROCESSOR
DIMENSIONAL
TÓLERANCES
VERIFICATION
¿BALANCING
Fig. I
FINCANTIERI CAD-CAM SY:STEM FLOW-CHART
COMPUTER CODE
ELIPACK
PESP
PRESS
NASTRAN
S E SAMELIGRAPH
AP T
1mFEM STRENGTH
VERIFICATION
eNUMERICALLY
CONTROLLED tIILLING
MACHINE
cP
2 0.5 OT.E.0
1 .2BACK
.4 5 .6 .7 FACEFig.2 - BACK AND FACE
SURFACE GENERATION WITH
XTRA-POINTS
Fig. 4 - BLADE LOADING DISTRIBUTION
"B
1,5
--F
.8
.9(Cp
:çcvi-)
Fig.
5FEM BLADE SCHEMATIZATION
f
lo - TOP VIEW OF BACK MILLING CUTTER TRAJECTORY
2
I
MODEL OBtIQVAT)OPJ
_--- PULL SCALI PVs$IOMS
0/
\j
4
\
/
SI.w
/ i4
i
1114 1213
1.Fig.11 - CAVITATION BUCKET
(MODEL) AND FULL SCALE PREVISIONS
e7
6
s
