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Procedia Materials Science (2014) 000–132

www.elsevier.com/locate/procedia

6M.E.Mohana Sundaram / Procedia Materials Science 00 (2014) 000–132

M.E.Mohana Sundaram / Procedia Materials Science 00 (2014) 000–132

5

International Conference on Advances in Manufacturing and Materials Engineering, AMME 2014

Machining of Free Form Surface with Enclosed Wall of Three Bladed Mini Propeller Mould

M.E.Mohana Sundarama, R. Prathapanayakab*, N. Vinod kumarc, S.J.Krishnamurthyd, M.Radha Krishnae

aTechnical officer - Propulsion division, CSIR –National Aerospace Laboratoires, Bangalore, India

bPrincipal scientist - Propulsion division, CSIR –National Aerospace Laboratoires, Bangalore, India

cScientist - Propulsion division CSIR –National Aerospace Laboratoires, Bangalore, India

dTechnical consultant - Propulsion division CSIR –National Aerospace Laboratoires, Bangalore, India

ePrincipal Technical Officer - Propulsion division, CSIR –National Aerospace Laboratoires, Bangalore, India

Abstract

The conventional machining of miniature components is an interesting and very challenging task. Machining of miniature components will take long lead time to get the required size and tolerance due to the limitations of size of the machining tool and limited material removal rate. This paper presents the process of machining metal mould using aluminum-2024 material in 3-Axis CNC milling machine with enclosed wall of free form surfaces. The mini propeller mould is divided in to two halves namely top and bottom. The top mould has the twisted cavity contours of suction surface of the propeller blade airfoil. The bottom mould has the twisted cavity contours of pressure surface of the propeller blade airfoil. Smooth matching of leading and trailing edges over the span of the blade are also accomplished. To avoid the cutting tool interference during machining of the space between two blades, smooth contours in the mould is given and machined accordingly. The machining operations are divided in to ‘rough machining’ and ‘finishing operation’ to minimize the machining time. To avoid local gouging on the profile, sufficient allowance is kept in comparison to actual material to be removed by leaving extra material for finishing operation. During the finishing operation on the propeller blade profile which consists of free form surface, two boundaries were introduced instead of single boundary method to generate the tool path in span wise direction. The matching of two boundaries profile after finishing operation is within the accepted tolerance of + 0.04mm. The machined moulds were inspected and compared with the IGES model of the mould. New machining method adopted here has saved 40% machining time without compromising in accuracy of machining top and bottom moulds of the propellers. The geometrical accuracies and surface finish of propellers blade obtained through this process are good in comparison to RPT process.

© 2014 The Authors. Published by Elsevier Ltd.

Selection and peer-review under responsibility of Organizing Committee of AMME 2014.

Keywords: Mini propeller; free form surface; MRR; Optimization; CAM; CNC milling

1. Introduction

Development of micro and nano air vehicles has gained significant momentum in recent past. CSIR-NAL under the sponsorship of DRDO-ADE and NP-MICAV program has developed BLACK KITE (300 mm wing span), GOLDEN HAWK (450 mm wing span) and PUSHPAK (450 mm wing span) micro air vehicles. All these MAVs are of fixed wing type and driven by electric propulsion system. The propulsion system of NAL MAVs is battery driven electric brushless motor and propeller. Propulsion division of CSIR-NAL is involved in the design and development of miniature propellers for CSIR-NAL MAVs published by Prathapanayaka et al. (2013) shown in Fig. 1.

Fig. 1. Black Kite MAV

Design and development of 3 bladed, 6 inch diameter, fixed pitch mini propeller is under taken in CSIR-NAL under the NP-MICAV program for 300mm wing span micro air vehicle (MAV). Given the thin section of the propeller blade profile and geometry, machining of the blade using metal or any other material is impractical. Rapid Prototype (RPT) fabrication using poly carbonate, poly urethane or ABS material has failed to meet the desired structural integrity, surface finish and accuracy. In view of this, effort is being made to fabricate the mini propeller using the metal mould with CFRP material through injection moulding process under controlled environment to contain the blowholes. Generating the CAD model of the propeller mould itself is an hurricane task in view of complex surface matching and creating an accurate cavity for the thin section of the propeller blade profile as well as propeller hub section is accomplished using commercial CAD software SOLIDWORKS. Aluminium – 2024 material is preferred due to its higher percentage of copper which provides good machinability, dimensional accuracy, surface finish, without distortion, warp age, pitting. The propeller mould consists of two halves of cavity (top & bottom) made out of Aluminum-2024 alloy. The shape of the propeller blade cavities in moulds are free form surfaces, thickness & twist of the propeller blades are varying from hub to tip. Machining of this mould is taken up in 3-axis CNC machine available in Propulsion Division of CSIR-NAL.

In the aviation sector complex contours and aerofoil sections having large opened surfaces and enclosed free form surfaces are machined from 5-axis CNC machine using ball nose end mill cutter within the acceptable accuracies. In recent study, it is stated that the use of ball-end mill for 3-axis machining is preferred for any free form surface machining, but is an inefficient material removal process for machining of surfaces defined within a cavity explained by Jensen et al. (2008). They also used another method described as Curvature Matched Machining (CM2). It is a 5-axis machining technique that uses Flat and Filleted-end mills to machine free-form surfaces efficiently and accurately for large free-form surfaces, but has primarily been applied to open surfaces by Jensen et al. (2008). It is also reported, that the Flat end mill on a 5-axis milling machine, primarily been applied to open surfaces by Jensen et al. (2002) and Grind free tool path generation algorithms which avoids gouging, surface finishes are within waviness tolerances by Pi et al. (1998). In the above studies, that they have used Flat end mill on a 5-axis CNC milling machine for machining the free form open surfaces and for curvature matched machining.

In the present case, complex contours of enclosed free form surface of mini propeller mould is machined using DMU 50, DECKEL MAHO, 3-axis CNC universal milling machine adopting two stages of machining. Thus machining lead time can be reduced by deploying flat end mill cutter instead of ball nose end mill. This may lead to local gouging on the profile, but at the same time, it increases the material removal rate compared to ball nose end mill. The local gouging can be avoided by offsetting the required profile by leaving extra material for finishing operation using ball end nose mill.

Two types of gouging may occur while machining the complex surfaces with the help of Flat or Ball nose end mill. The first one is Global gouging and the other one is local gouging as depicted in Fig. 2.

Local gouging: When the bottom trocoidal portion of the cutter removes more material than the allowable tolerance of the profile surface, specified in the design, is called as ‘Local gouging’.

Global gouging: When any other part of the Tool, Tool holder or Spindle interferes with any surface region of the component it results in ‘Global gouging’.

Fig. 2 . Local and global gouging

Adopting the above methods in 3-axis CNC machine for miniature components with the use of different size of ball nose end mill by boundary cut method during finishing stage, which results in faster material removal rate without any compromise in specified geometrical accuracies.

Abbreviations

NP-MICAV National Program on Micro Air Vehicles

CNC Computer Numerical Control

RPT Rapid Proto Type

CSIR-NAL CSIR-National Aerospace Laboratories

CFRP Carbon Fiber Reinforced Plastic

ABS Acrylonitrile Butadiene Styrene

MRR Material Removal Rate

Ra Roughness average

2. Design of 6 inch diameter, fixed pitch, 3 bladed propellers

The 6 inch diameter, fixed pitch, 3 bladed mini propeller suitable for 300mm wingspan MAV has been designed using ‘minimum induced loss’ method and Eppler-193 and NACA 66-021 aerofoils are used in the design by Prathapanayaka et al. (2013). The propeller in general has a complex geometry consisting of two profiles and varying twist from hub to tip has open surfaces. Fig.3.shows chord and twist distribution of the propeller along the span. It was decided to fabricate the propellers using CFRP material because of its properties, like light weight and structural strength. It is required to use two halves of moulds to cast and extract the finished propellers using CFRP material. Fig.4. shows the propeller blade geometry and Reynolds number at various sections. Fig. 5. shows CAD model of the propeller.

Chord distribution

Twist distribution

Fig. 3. Chord and twist distribution

Fig. 4. Profile distributions along the Blade height

Fig. 5. CAD model of the propeller

3. Mould Fabrication

3.1 Mould drawing

The mould drawing has been carried out using the CAD model of the propeller. The pressure surface and suction surface is extracted from the propeller CAD model shown in Fig.5. The mould consists of two halves of cavity plate’s namely as Top mould and Bottom mould. The surfaces extracted from CAD model are used to develop the top and bottom mould CAD models. Top mould consists of suction surface, runner, gate, core pin, and dowel and ejection holes. Bottom mould consists of pressure surface & dowel holes for aligning the top and bottom mould. 3D arc at exactly same radial stations were used for both the halves of mould to ensure identical blade to blade connectivity. Projections with appropriate tolerance were made near the propeller blade surfaces on both halves of the mould for proper alignment of moulds. Although the profile of the propellers is a free open surface, but the profile to be machined in each mould is enclosed with wall. Bottom and top moulds are shown in Fig.6.

Bottom mould

Top mould

Fig. 6. Propeller moulds

3.2. Mould machining

Trail machining of mould is carried out using Ural material blank to check the tool path interference, curved surface topography and geometrical dimensions using the 3axis CNC machine and its program settings. Fig.7. (a) and 7. (b) show Ural material mould machining and the machined mould respectively. In Fig.7. (b) Portion 1 represents the space between propeller blades on the mould and portion 2 represents the propeller blade cavity surfaces on the mould respectively. Aluminum-2024 alloy blank is machined using conventional milling machine with close geometrical allowance with a side step for handling and clamping purposes. The blank are clamped in a fixture on the table of CNC machine. Proper alignment is ensured by a dial gauge, the datum setting of work piece (X0.0, Y0.0, Z0.0) are set with the help of Heidenhain infrared measuring touch probe. The tool height and diameter are measured in UNO 115 tool preset and the values are entered into tool table of CNC machine. The dowel holes of 8mm diameter, 4 numbers were drilled and reamed on the CNC machine. Since the CNC machine tool path and other dimensions are confirmed as stated earlier the same program is carried out using the aluminum mould.

Fig. 7. (a) Mould machining of Ural material Fig. 7. (b) Ural material top and bottom mould

The machining program of moulds can be classified as

1. Rough and Semi finish machining program,

2. Finish machining program.

3.2.1. Rough and semi- finish machining program (Space between propeller blades, outer portion1)

Machining of free form surface normally the ball nose end mills are employed to remove the extra material by small thin chips to get the shape and dimension. Outer portion 1 as shown in Fig.7. (b) Consists of free form surface with enclosed wall. The profile within outer portion 1 is machined by cavity mill operation with ‘follow periphery cut method’. In this method, flat end mill of 10 mm diameter is used with the step over of 7mm and constant depth of cut 1.5mm, stock 1mm, speed 2547rpm and feed 611mm/min. Thus the material removal rate is higher compared to ball nose end mill as shown in Table-1. The machining time is also very less as shown in Table-2. Roughing operation is followed by semi finishing with fixed contour method by using a ball nose end mill of diameter 8mm, stock 0.4 mm, step over 0.5mm. Finally semi finish machining is followed by a finishing operation with a 6mm ball nose end mill.

3.2.2. Finish machining Program (Blade profile, portion 2)

For achieving the required airfoil shape, tolerance and surface finish of the propeller blade profile in the mould halves, two boundaries are created to machine the full blade profile in NX – CAD/CAM 7.5 software. Portion 2 is as shown in Fig. 7. (b) consists of free form surface with enclosed wall of suction and pressure surfaces of the propeller blade starting from hub to tip. These cavity surfaces are subdivided in to two boundaries along the span for easy machining and to accommodate different diameter cutting tools. The first boundary starts from the hub section and extends till 70% of the blade span. The remaining portion of the blade span is called second boundary (boundary-2) till the tip section of the blade. As the tip of the blade is only 3.7mm in width and a thickness varying from 0.5mm to 0.2mm, boundaries are created to generate the tool path of blade contour. Finishing cut started at first boundary with a 5mm ball nose end mill, step over 0.2mm has resulted in good surface finish, but at the same time tool is unable to reach the tip portion of the blade because the remaining portion at tip of the blade is small and is less than 5mm. To machine this portion it is required to use a cutting tool which is less than the tip portion of the blade. As the tip of the blade is only 3.7mm, use of less than 3.7mm diameter tool for machining the entire blade will lead to higher machining time, the percentage of tool breakage chance is also high. To optimize the cutting time 5mm ball nose end mill cutter was used up to 70% of blade height (first boundary) and 2.5mm ball nose end mill cutter is used in the remaining portion of the blade height (second boundary). Not to distort the tip section of the propeller blade from flow of resin and to provide recess for riser, the tip portion is extended by another 15mm using 2mm cylindrical end mill in the mould. While programming the machine tool path of the 3-axis CNC machine under contour machining, out of many options, two options are adopted for the present case, firstly fixed contour machining and secondly boundary contour machining. Ball end nose cutter of 2.5mm diameter is used in both the options. The comparison of the time taken for this finish machining is shown in Table-3. Fig. 8. (a) and Fig.8. (b) Show the tool path contour obtained after the machining with the above options. It can be observed from these Fig.8. (a), that the fixed contour machining did not complete the propeller blade tip section properly. On the other hand, Fig. 8. (b) shows the complete machining of the propeller tip section. Fig. 9. Shows finished top and bottom moulds. Fig. 10. shows the assembled mould. Subsequently provision for the core pin, ejection holes and runner, gate is machined.

a b

Fixed contour option at tip section Boundary contour option at tip

Fig. 8. Effect of tip section machining using different options`

Fig. 9. Aluminium material Top and Bottom mould

Fig. 10. Assembled view of the mould

Table 1. Material Removal Rate (MRR), Space between propeller blades, outer portion 1

Cutter size, mm

Step over,

mm

Depth of cut,

mm

Cutting Speed

80m/sec

Feed(0.06P/F)

MRR

Cm3/Min

Flat end cutter, 10

7

1.5

2547, rpm

611

6.415

Ball end nose mill,

10

0.7

1.0

2547, rpm

611

0.427

Table 2. Machining time, Space between propeller blades, outer portion 1

S.No

Operation type

Ball end nose mill (10mm),

time taken, minutes

Flat end mill (10mm),

Time taken, minutes

1.

Cavity mill roughing

153

93

Table 3. Machining time for fixed contour and Boundary contour options using 2.5mm ball end nose cutter

S.No

Operation type

Hub to tip

Time, minutes

With split boundaries,

Boundary -1 &Boundary-2,

Time minutes

Remarks

1

Fixed contour option

( blade profile)

25

26

Incomplete machining

2

Boundary contour option ( blade profile)

22

16

Complete

3.3. Mould Inspection

The machined moulds are subjected to inspection. Aberlink coordinate measuring machine is used to measure the blade profile and is compared with the IGES model of the top and bottom moulds. A sufficient number of profile coordinate points of propeller blade surface contours in the machined mould are randomly selected and is used for the comparison. It is found that machined moulds profile coordinates average deviations are within a tolerance of +0.04 mm as shown in the Fig.11. Fig.12. shows the measured X, Y and Z deviations on the mould from CAD model. The surface roughness is measured using Surface Roughness Tester. Fig.13. shows the comparative average values of surface roughness of mould, as well as CFRP and RPT propeller blade.

Fig. 13. Comparative values of average surface roughnes of Mould, as well as CFRP and RPT blades

4. Conclusions

Micro air vehicles mini propeller having 6 inch diameter, fixed pitch, and 3 blades mould is machined using aluminum-2024 alloy in 3-axis CNC machine. The mould machining is carried out by dividing operations into ‘rough machining’ and ‘finishing operation’. Rough machining is carried out using Flat end mill cutter. Finishing operation is carried out using ball nose end mill cutter. The finished mould dimensions are within the tolerance limit. The method adopted to machine this mould saved 40% machining process time. During finish machining, 3-axis CNC machine program setting to boundary contour option and also splitting of blade profile in to two boundaries produced complete machining of the blade cavity contours within the satisfactory tolerance limit. CFRP propellers fabricated using this mould gave satisfactory geometric accuracy, surface finish, structural integrity and aerodynamic performance.

Acknowledgements

The authors are thankful to Director, CSIR- National Aerospace Laboratories, Bangalore and Mr. M.Jayaraman, Head, Propulsion Division for giving this opportunity. Authors also thank NPMICAV for sponsoring this work. The authors would like to thank Mr. M. N.Varadarajan, propulsion workshop In-charge and their colleagues of Propulsion Workshop for giving timely help and support during the process of machining.

References

1. Prathapanayaka, R., Vinod kumar, N., Krishna murthy, S.J., 2013. “Design and development of three bladed propeller

for Micro air vehicles” ICIUS-2013-69, 9th International Conference on Intelligent Unmanned Systems, Jaipur, India.

2. Jensen, C. G., Red W.E., Ernst, C., 2008 “Machining Free-Form Surface Cavities using a Combination of

Traditional and Non-Traditional Multi-Axis Machining Methods” Computer – Aided Design and Applications, 5(1-4), 241 – 253.

3. Jensen, C. G., Red. E., Pi. J., 2002 “Tool Selection for Five-Axis Curvature Matched Machining”,

Computer-Aided Design, 34(3), 251-266,

4. Pi, J., Red, W. E., Jensen, C. G., 1998 “Grind-free Tool Path Generation for Five-Axis Surface

Machining, Computer Integrated Manufacturing Systems”, 11(4), 337-350.

� EMBED PBrush ���

� EMBED PBrush ���

Fig. � SEQ Figure \* ARABIC �1�1. Average profile deviation at profile surface

Co-ordinate points in the mould.

Fig.12. Measured X,Y and Z deviation of mould from

CAD model

* Corresponding author. Tel.: +91-80-25051605; fax: +91-80-25222494.

E-mail address: prathap@nal.res.in

2211-8128 © 2014 The Authors. Published by Elsevier Ltd.Selection and peer-review under responsibility of Organizing Committee of AMME 2014.