March 1975
MITSUBISHI HEAVY IHDUSTRIES, LTD.
lab. y. Scheepsbouwkond
Technische HogschooI
D.Wt
s
IiITsuHIsiII TECHNiCAL BULLETIN No.99
A Numerically Controllèd Model Propeller
Hidetake Tan ibayashi* Takao Sasajima**
To achieve the higher accuracy and timely preparation of model tests in both towing tank and cavitation tunnel, a numerically controlled model propeller milling machine has been developed by Mitsubishi Heavy Industries. This machine is controlled by
FANUC 23OSb in open-loop system. A software system was a/so developed by ourselves, which enables any type of propellers
to be machined.
More than 40 model propellers have been machined already in these two years, since the machine was put into operation. Test results, using model propellers machined by this NC machine, show clearly improvements in accuracy of geometrical shape of the blade section and in uniformity among blades of a propeller.
The technique of developing this machine and the software system has been applied to the development of the NC ship propeller milling machine, which was put into operation last year at Nagasaki Shipyard and Engine Works of our company.
t Introduction operation in 1972 after several cutting tests together with
To cope with rapidly changing situation of the world modification of the software system. Since then about
at present, the Experimental Tank has been making every 40 model propellers have been machined with successful
effort to supply shipbuilders and shipowners the most results.
reliable information as soon as possible based on model Experience of the development of the machine and the experiments. Improvements and extension of the facilities software system was applied to the automatic milling have been introduced into various fields of the activities machine of ship propellers in our Nagasaki Shipyard.
such as measurement in the tank, data processing of the
measured results, and the preparation of the models for 2 Design of Machine experiments. This paper gives a description of an automatic 2.1 PrincipIe of Design
model propeller milling machine developed by Mitsubishi The basic requirements for the design of this machine Heavy Industries, as a part of the recent modernization of are summarized as follows:
their Experimental Tank. (1) The maximum size of model propeller to be machined
Formerly model propellers were finished by hand using shall be 500mm in diameter.
templates of blade sections, as is still done in the majority (2) The material of model propellers shall be corrosion of the model basins in the world. The accuracy of model resisting aluminium, high tensile brass, nickel aluminium
propellers and accordingly the reliability of the test results bronze as well as tin-alloy conventionally used.
were dependent on the skill of the workers and it took (3) The whole blade area shall be machined except the
several weeks to reach the acceptable level of accuracy. fillet at the hub. Formerly the overlapped part could
Sometimes the model propellers were modified or renewed not be point-drilled by the conventional method. in case the test results suggested inadequacies arising from (4) The accuracy of the machined surface shall be within
the accuracy of the models. Such a hand work was reduced 0.02mm from the designed offset of blade surface. by the use of a point-drilling machine.(1) The blade surface To achieve these requirements it was decided to choose to be finished is specified by points drilled by the machine, a type of milling each blade section by a single cutter Accuracy of the model propellers was largely improved path, viz, milling the back and the face sides of blades
and the finishing time was considerably reduced. This without changing the propeller setting. If each side of a
method was thought the best way to manufacture model blade is milled separately, as in the case when milled by a propellers as early as 1960, when the machine was universal milling machine, propeller models should be
introduced in the Experimental Tank, turned upside down to mill another side and therefore
With the recent development in numerical control great care should be taken to repeat the same propeller
technique, NC milling machines are now available with setting with high accuracy. For this mode of cutting, it three axes controlled simultaneously. It was decided in is most convenient to rotate the propeller axis and simul-1969 to develop an NC machine specifically designed for taneously to move the cutter in axial direction to follow
milling model propellers under the cooperation with a blade section shape, shift of blade section being given by
Hiroshima Machine Tool Works of our company. The a pick feed in radial direction. Such a line of consideration
machine was completed in 1971 and put into routine led to a structure as shown in Fig. 1, specifically designed Manager, Ship Experimental Tank, Nagasaki Technical Institute, Technical Headquarters
MTB 99 March 1975
for milling model propellers. Details of the structure are described ¡n the following chapter.
Fig. i General View of Numerically Controlled Model
Propeller Milling Machine
2.2 Structure of Machine
The machine was designed and constructed by Hiroshima
Machine Tool Works, Mitsubishi Heavy Industries. The structure of the machine is shown in Fig. 2. The machine consists of a column with a saddle for cutter and a flat
2
1'Circu1atJ
JIr
Operation FANUC,23OSbPa n el Pu se Drive Motor' Unit 15. Stroke 12 16 400mm
Fig. 2 Structure of Machine
bed with a slide table on which a circular table is mounted.
A propeller model is set on the circular table and can be
rotated around its vertical axis. The cutter moves vertically
along the column, while the radial position of the cutter relative to the propeller can be changed by shifting the
slide table.
Linear motion of the saddle and the slide table are
transmitted from the electric-hydraulic pulse motors
through ball screws with tolerance o,f O.Ol5mm/300mm.
The rotational motion of the circular table ¡s given through a worm gear and wheel with double leads.
Accuracy of the machine is largely dependent on the setup of the sliding parts. Attempts were made to design
a simple and reliable structure and as a result, a mechanism
as shown in Fig. 3 was adopted. The saddle and the slide
table are supported by roller slide bearings with flat guides
made of Teflon-type synthetic resin. Owing to low static and dynamic friction and appropriate damping of such combination of materials, the saddle and the slide table can follow the pulse motors without causing significant fluctuation of loads and therefore vibration or stickslip effects can be avoided. About 2/3 of the weight of the
saddle ¡s balanced by a counter weight to reduce the load
on the ball screw. An electric brake is fitted to clamp the
ball screw whenever the vertical motion of the saddle stops.
The cutter can be swiveled up to ±30 degrees in a vertical plane so as to be adjusted to the rake angle of propeller blades. The adjustment is done by hand since setting has to be made only once for one propeller. The cutter spindle ¡s driven by a 2-pole, 3-phase induction motor, the number of revolutions of which ¡s 4040 rpm.
Hardened by cementation
Flat surface guide made of synthetic
resin
Roller elide bearing
Pivot to awing the cutter
Cutter hea
Column
Fig. 3 Structure of Sliding Part
2.3 Control
The machine is controlled by a FANUC 230 Sb
con-troller, made by Fujitsu Ltd. Electro-hydraulic pulse motors
are driven by incremental pulse signals transmitted from
FANUC 230 Sb.
Paper tapes with EIA code are used to supply informa-tion to FANUC 230 Sb. Thus an open-loop system was adopted to control the pulse motors, since the machine is
2400 mro
1. Electro-hydraulic pulse motor 9. Cutter head
2. Electric brake 10. Model propeller holder
3. Sliding surface of saddle IChip holder)
4. Ball screw 11. Block gauge to find zero point
5. Chain to support the weight of machine coordinate of saddle 12. Optical reader
6. Saddle 13. Circular table
7. Swing mechanism of cutter 14. Slide table
head 15. Sliding surface of table
so designed as to have an accurate and reliable structure.
A simple control system implies high reliability because of
less vulnerable elements besides lower production cost. Incremental movement of the saddle and the slide table per pulse is chosen to be 0.005mm considering the accuracy
of the machined propeller. Incremental rotating angle per
pulse is also chosen to be 0.005 degree, which corresponds
to 0.01mm at the radius of 125mm. Feed rate can be
changed from 10 to 2000 pulse/sec stepwisely in 56 cases, except for the rapid feed where 8000 pulse/sec. is set.
2.4 Cutter
Cutters of ball-end end mill type, as shown in Fig. 4,
were designed for this machine. The cutter has four blades,
two of which located in the opposite side are connected
with one another at the end of the ball. With such
arrange-ment of blades any part of the ball serves as cutter and therefore the whole propeller blades can be cut without
changing swivel angle of cutter irrespective of relative position and angle between the propeller and the cutter. The tolerance of spherical blade outline of the cutter is checked by an optical instrument and kept within 0.01mm
from the prescribed values.
Fig. 4 Ball End Cutters
3. Software System
To cut
a complicate three dimensional surface, an automatic programming system is indispensable, of whichAPT(2) is widely known. But, for this specially designed propeller milling machine, such a universal system is too expensive and time consuming to be applied. Thus a
special software system was developed on the basis of the
"DIMNUC" system' originally developed in our company
to controll the NC three dimensional milling machine for
the press-mould of the body shape of automobiles.
The new software system is called "PRPCUT" system
and is composed of two parts, i.e. "PSURF" and "PRPCUT". The flow chart of the computation is shown
in Fig. 5.
3.1 PSURF
The function of this software is to generate the propeller
surface as systematically arranged points from the input data and to store them together with their normal vectors in a disk. Surface generation is made in accordance with
Coons surface modeling technique. The complex blade surface is considered as sets of piecewise continuous surface
NO NO Principal Dimension of Propeller Blade Surface (Contour) Generation "PSURF" Data for Controlling Cutter Path (PART PROGRAM) Calculation of Cutter Location (Control Point) PRPCUT (1)' Post Processor "PRPCUT (2)"
ìH
(Paper Tape)Fig. 5 Flow Chart of "PRPCUT" System
patches, the corner points consisting of input data, and any point in the patch is defined by a bicubic equation of two independent parameters. Special care was taken in treating the part near the leading edge, which usually
consists of a circular section, because Coons surface
satisfies the condition of continuity up to only first-order
siope at the patch boundaries.
The smoothness of the generated surface and the normal
vectors are checked by an automatic plotter by which the trajectory of the center of the ball-end end mill is plotted
as shown in Fig. 6. 3.2 PRPCUT
The function of this software is to calculate the cutter
path and to supply a paper tape for FANUC 230 Sb. Gouges and other inadequate movements of the cutter are checked by the automatic plotter.
Typical modes of cutting blades are illustrated in Fig. 7.
For cutting a blade section, the cutter starts from the
trailing edge of back side, turning around the leading edge
7'lad
Surface (Contour) Data Cutter Locahon Data (Disk) (Disk)MTB 99 March 1975 Approach to work and pick __ Extended surface &
I
Face side I Back side Back side -Cutting point Ball cutter -Control point 4 Back side't
\
(a> Blade Section Cutting
Offset
contour of
trailing edge i Face side
Back side
(b) Blade Contour Cutting Fig. 7 Cutting Mode
and returns to the trailing edge of face side. The cutter does not turn around the trailing edge, since the blade near the trailing edge is too thin. The cutter proceeds
undercutting the blades, thus the vibration of the blade being reduced considerably in comparison with upcutting the blades. The blade contour is cut by a zigzag mode since the edges of blades as cast are thick in the early
stage of machining. In Fig. 8 is shown a propeller, a blade of which is being cut by the machine.
Fig. 6 An Example of Plotter Output (PSURF)
Face side
Leading edge
z
/
r
Trajectory of cutter center
Trajectory of cutter center
Offset contour of leading edge Leading edge Trajectory of Cutter center -Ball cutter Control point
Trajectory of the center of a sphere which is tangent to the blade section at
constant radius
rO
Fig. 8 Blade Section Cutting
4. Accuracy
4.1 Integrated Accuracy of the Machine
Statical accuracy of the machine was checked according
to JIS tolerance rule for hobbers(6) and the results were
found to be satisfactory. Then the integrated accuracy of the machine under the control of FANUC 230 Sb was
checked by a laser interference meter. The results are well within the prescribed values as shown in Table 1.
4.2 Measurement of Blades
On completion of model propellers, blade sections are measured by the point-drilling machine. Fig. 9 shows an example of the measurement, in which a blade section numerically milled and the one point-drilled and finished
by hand are compared. Considerable improvement in
accuracy by the use of the NC milling machine can be seen,
but there appears room for further improvement near the
leading and trailing edges.
Table 2 is a summary of measurement for 4 model
propellers milled by the NC machine in which the
dif-ference of the measured and designed thickness are
tabulated at the representative radial and chordwise loca-tions. The difference tends to increase towards tip and edge of blades, which are ascribed to deflection of blades
PARTNO PSORF-BS VIEW '(Z-PLANE VIEW
-Max. thickness t=3.5Omm
Chord length C=77.7mm Designed section Point Drilling Machine
- NC Machine
Fig. 9 Blade Section Measurement at 0.7R of a Model Propeller for a Tanker
(D = 250.00, corrosion resisting aluminium)
Table 2 Thickness Difference of Blades
due to the cutter pressure. Effect of the deflection will be eliminated by offsetting the coordinate of blade sections, the amount of which will be obtained from accumulation
of such measured data and analysis of them.
5. Examples of Test Results on Propellers Numerically
Machined
Two examples are quoted in the following to demon-strate the improvement in accuracy of the blade shape by
trn i measured thickness
At = I tm - td I thickness difference
Table i Integrated Accuracy of the Machine
the use of the NC machine.
5.1 Open-Water Tests on a Series of Propellers
Openwater tests on a methodical series of propellers are conducted as a basis of the propeller design charts. A series consists usually of propellers with the same
geometrical particulars, except for the pitch ratio...
The thrust and torque characteristics KT, AQ
J
arethen made fair against pitch ratio to draw a design chart. Table 3 shows a comparison of the standard deviation of
Axis Stroke Positioning accuracy Repeatability Dead zone Tolerance ±0.02 ±0.01 0.01 Horizontal (Slide Tsblel 350.0mm Measurement ® ±0.008mm 0.004mm 0.003mm ® ±0.008mm Tolerance ±30" 20" 60" Circumferential Measurement 4" 1" (Circular Table) 360° 0±5.5" Tolerance ±0.02 ±0.01 0.01 Vertical ISaddlel 350.0mm Measurement 0±0.012mm ±0.004mm 0.003mm 0±0.012mm 0.9 r/R = 0.5 At At M.T. L.E. T. E. M.T. LE. 0.020 0.031 0.045 0.003 0.064 0.064 0.022 0.1 22 6.25 5.76 6.21 7.50 0.041 0.054 0.066 0.074 0.016 0.017 0.04 1 0.034 0.064 0.047 0.024 0.059 0.032 0.068 I 0.058 0.027 0.049 unit mm td designed thickness Material Model prop.
r/R = max D T. E. Corrosion resisting al um i n um A B C D 250.00 0.03 7 0.094 0.040 0.109 Mean 0.070
Enlarged trailing edge part Enlarged leading edge part
MTB 99 March 1975
Kr
and KQ between the series machined by the NC milling machine and those manufactured by the point-drilling method. lt is clear from the table that the variation of propeller characteristics among different propellers arereduced considerably by the use of the NC milling machine. 5.2 Cavitation Inception Tests
Inception of face cavitation is very sensitive to the shape of leading edges. In Fig. 10 are plotted desinent cavitation
6
8.0
o
Description D24, Kempf & Remmers
Hamilton, DR., Propeller Manufacture by a Numerically Con-trolled Milling Process, NAVWEP Report 8595 (NOTS
Tech-nical Publication 36071, (1965)
Takehara, A., Kamiya, K. and Fujino, K., Machining of Master Model and Press-mould for Body Shape of Automobil by Numerically Controlled Machine (in Japanese), Journal of
Table 3 Standard Deviation of Thrust and Torque Coefficients
for Mitsubishi-Type Series Propellers
References
numbers of each blade of a propeller machined by the NC milling machine and of that manufactured by point-drilling method. The scatter of the plots has been largely reduced by the use of the NC milling machine, which implies considerable improvement in accuracy and
uni-formity among different blades. 6. Concluding Remarks
A numerically controlled model propeller milling machine was developed and put into routine operation in
Mitsubishi Experimental Tank. The software system, which
dominates the effectiveness of NC machine, was also developed by ourselves. About 80% of the blade surface of a model propeller can be machined within three days,
contributing to reduction of finishing work as well as
improvement in accuracy.
Comparing with the blade section data of the propellers made by point-drilling method, accuracy of the geometrical
shape and uniformity among different blades are certainly improved. This is supported by model test results such as
cavitation inception tests and open-water tests
on a
methodical series of propellers.
There is room for further improvement, however, i.e. reduction of deflection and vibration of the blade during
cutting. Effect of deflection will be eliminated by off-setting blade section data, the amount of which will be obtained by accumulation and analysis of blade measure-ment data. Variation of feed rate and rotational speed of cutter will serve the reduction of vibration of blades. Then the simple open-loop system can still be applied and the
work is under way at present, the result of which is
expected to be obtained in the near future.
JSME, Vol. 73, No. 615 (1970), p.955
Coons, SA., Surface for Computer-Aided Design cf Space Forms, MAC-TA-41, Project MAC, MIT 119671
Almond, D.B., Numerical Control for Machining Complex
Surfaces, IBM SYST J, No. 2 (1972), p. 150
Japan Industrial Standard, 86216 (19711 and B 6427 (1971)
O Model prop
--- Model prop
A NC Machine
B: Point Drilling M achine
s
-.
o
Slip ratio
S 1JIP
s.,'
.
0.1
0.05 o 0.05Fig. 10 Cavitation Inception Test
Standard Deviation
Method of (Number of blades - Thrust coeff. Torque coeff.
100 xexpanded Finishing
area ratio) GK T 0KQ
5-55 0.0018 0.00020
5-65 0.00 16 0.00021 Drilled by
Point-5-85 0.0009 0.00019 Drilling Machine and then finished
6-55 0.0009 0.00012 by hand-work 6-65 0.0011 0.00024 6-85 0,0007 0.00009 Machined by NC Machine 6.0 -o b E 4.0 't 't o a 2.0
