Airfoil Surface Forming and Conformity Test Using Laser Tracker

In this study, NACA0018 airfoil surface conformity test was conducted using API tracker3 in combination with SpatialAnalyzer (SA) and modeling software SolidWorks. Plaster of Paris is used as a plug making material and a woven-type fiberglass is used as mold and airfoil surface makingmaterial. For airfoil surface analysis, three-dimensional model of the airfoil surface was developed in SolidWorks software and imported in IGES file format to SpatialAnalyzer (SA) software. ,en, measurements were taken from manufactured airfoil surface using laser tracker through surface scanning method. Surface conformity test was conducted through fitting of measured points to surface model imported from SolidWorks to SpatialAnalyzer (SA) software. ,e optimized fit summary result shows that the average fit difference is 0.0 having standard deviation from 0.22224 from the average and zero with RMS of 0.2210. ,e maximum magnitude of the difference including x and y together is 0.5336 and the minimum −0.5077. ,us, with a given range of surface quality specification, laser tracker is an easy and reliable measurement and inspection tool to be considered.


Introduction
Fiber reinforced composites (FRC) are now becoming the natural material selection for aerospace industry due to their large strength and stiffness to weight ratio compared with conventional structural materials [1]. With the features of high stiffness, strength, and low weight, the high-performance composites were extensively utilized not only in the aerospace industry, but also in marine, armor, automotive, and civil engineering applications [2]. An obvious advantage of fiber-reinforced composites over metals is the shape potential, particularly the ability to produce large double curvature geometry [3]. is shape potential ability of the FRC should be supported with appropriate measuring technologies for its shape and geometry conformity test, alignment, and assembly. Among these technologies, commercial laser trackers are the pioneer technology available for various applications.
A laser tracker is a portable device that makes threedimensional measurements which is extremely appealing to metrology instruments for numerous reasons including size, portability, repeatability, accuracy, and ability to capture large volumes of 3-dimensional coordinate data quickly and in real-time [4]. Aerospace industry was an early adopter of laser tracker technology where large scale of metrology, involving in-place inspection of large parts and assemblies, is required. e automotive industry uses many similar tooling applications to those found in aerospace and can apply laser trackers to similar ends. Laser trackers are not just used for part inspection only; they are often used to monitor the condition of a fixture or tool over time and provide real-time feedback on the wear or movement of the fixture or tool [5]. Laser tracking system enables measuring three-dimensional coordinates based on the principle of trilateration with high accuracy. Laser-based instrument emits a laser from a gimbaled head ( Figure 1); in the case of a laser tracker, a spherically mounted retroreflector (SMR) is then used to reflect the laser back to the unit allowing the distance to be measured [6].
Literature reveals the use of laser tracker as a reference instrument for articulated arm coordinate measuring machines (AACMMs) verification processes by [7], as a calibration system for a coordinate measuring machine (CMM) by [8], as a metrology tool for aligning optical systems, including the use of mirrors and windows by [9], and as an instrumentation tool used to make large scale measurements within aerospace assembly by [6]. Laser tracker technology applications in the manufacturing sector include machining equipment and tool alignments, inspection of parts, complex assembly of components, equipment, machine and tool calibrations, and data collection of dimensional information for reverse engineering of components with three-dimensional information to complete solid parametric models that can be used to manufacture [4,10,11]. e objective of this paper is to produce airfoil surface from fiber glass and use API laser tracker3 in combination with spatial software as surface conformity testing tool.

Methods and Materials
is paper covers surface forming of NACA0018 airfoil from fiberglass and conduct conformity test using laser tracker. erefore, the procedure of experimental work starts with plug making, mold making, casting/surface forming, and conducting the conformity test. Table 1 shows a list of materials and their purpose to manufacture the airfoil.

Plug
Making. e first step in fiberglass mold making is to make a plug which is typically a representation of the finished part and can be an actual part or a mockup of a part [12]. Plug is made from a variety of different materials which include, but are not limited to, wood, plaster, polyester resin, fiberglass, polyurethane foam, etc. [13]. For the purpose of this study, a white fine-grained plaster commonly known as Plaster of Paris is chosen as plug material due to it being readily available at constructional material stores, inexpensive, easy to use, able to take shape without shrinkage and crack, rapid cure, and being fairly strong for the required purpose. Figure 2 shows the schematic representation of plug forming process.
Since mold making requires use of a full-scale exact geometry of the product to fabricate, plug is assumed to be split pattern having exact shape of half side of the airfoil. Manual extrusion forming process with a movable extrusion die approach was adopted to shape plug as shown in Figure 2(a). A template that has the exact curvature of airfoil was made from sheet metal to represent movable extrusion die. e curvature was prepared using special sniper cutting tool and polished with fine grit sand paper. To guide the template along an edge of molding board in plug shaping process, a special fixture similar to try square was carefully glued at one end of the template as shown in Figure 2(b). To prevent flexing of the template, it is backed with rigid material made of metal bars. e fixture is prepared to have a smooth surface finish with two of its surfaces perpendicular to each other. Shaping process of the plug was done by gently sliding the template over properly mixed plaster with water over molding board using two hands. During the shaping process, the guiding plate enables the template to slide over the surface of molding board at equidistance from the edge of the molding board. e left-hand controls movement of the template across the boarding edge and the right-hand controls movement of the template over surface of the molding board.
To complete overall shaping of the plug, several rough shaping operations across the length were conducted. Finally, the plug shaping process was completed and the plug was allowed to dehydrate before use. After proper dehydration of the plug was confirmed, the surface was sealed to cover porosity of the plug with automotive body filler and sand finished at 220 grit size.
Mold Making. For the purpose of this study, 400gsm woven cloth type fiberglass was chosen as mold and airfoil material. Since the airfoil profile is symmetrical, both upper half and lower half mold have been made from the formed plug after one another. Before proceeding with mold making, the plug surface was finished up to the desired level and PVA was applied on the surface as the mold release and surface protection of finished plug for next use. en, epoxy was prepared by mixing with catalyst and hardener at the recommended ratio. e prepared epoxy was then brushed on the surface of the plug and molding board surface at an equidistance of 40 mm from both sides of the plug. When the epoxy densification begins, settlement of fiber glass layers laid up took place as shown in Figure 3(a). is extra layup is used to make mold with flanged for bolting purpose as shown in Figure 3(b). After the layer was cured, 2 layers of the fiberglass were laid up at a time until the desired thickness achieved. Much layer was added on the flanged part to make the mold rigid. After the mold fully cured, the mold was removed from the plug, followed by trimming of excess laminates, corrections of defected surfaces, cleaning the mold, and finishing. For correcting defected surfaces, special polyester putty grey was used. e other half mold was similarly made from the same plug and prepared for assembly as shown in Figure 3 Some of the precautions required in the mold making process are (1) to make the working area free of dust and dirt as the mold release may trap it and become part of the surface as it dries; (2) plug surface finish should be to the required quality and surface imperfection needs to be corrected as the surface seen on the plug is the surface finish obtained on the mold; (3) proper ratio of hardener and epoxy needs to be used as faster drying rate or lower drying rate affects mold quality; (4) proper wet-out of glass is required to avoid air bubbles which results in surface defect; and (5) proper pressure application is required to distribute epoxy uniformly over the plug so that surface defect due to wet problem can be minimized.
Airfoil Surface Forming. Before proceeding with airfoil surface forming, PVA was applied on the surface of the mold to facilitate release of the airfoil. e airfoil surface was made in two halves of the mold as shown in Figure 4(a). After the  Advances in Materials Science and Engineering layup was finished and cured, excess portions were cut and the edges were prepared for assembly. After assembly, the two halves of the airfoil were then glued together from the inside at the leading and trailing edge using epoxy and fiberglass. e assembly was dismantled and a complete airfoil with excess materials at the parting line ( Figure 4(b)) was removed from the mold. Excess materials were removed, defected surfaces were corrected using special polyester putty grey, and the surface was finished up to the required level.

Surface Conformity
Testing. e API model used in the experimental testing was an API tracker3 in combination with SpatialAnalyzer (SA) software. As shown in Table 2, API tracker3 has angular accuracy of 3.5 µm/m, ADM accuracy of ±15 µm, and IFM accuracy of >±0.5 ppm. In this experiment, the laser tracker is located within the range of 4 m from the target point so that the correction point or deviation distance of the laser tracker from the target point would be plus or minus 0.5 µm. In the process of surface conformity test, flatness and linearity of the molding board were first checked. For this purpose, random measurements were taken from top surface of the molding board with tracker through scanning method of measurement taking as shown in Figure 5(a). e points were then fitted to make a plane to check the flatness of the molding board. For checking the linearity of the molding board edge, points were measured randomly along the edge as shown in Figure 5(b). A line was then fitted to the points for checking linearity. To check linearity of template movement along the edge of molding board in the process of mold making, Spherically Mounted Retro Reflector (SMR) was attached to the template to take reading while the template made shaping process of the plug. Surface flatness and linearity were then analyzed for the measured points using Spatia-lAnalyzer (SA) software.
To test surface conformity of the airfoil, one side of the whole length of the airfoil was divided into 15 equal sections.
en across each division line, measurements were taken along the chord length starting from leading edge toward tail through surface scanning method. To analyze the airfoil surface with measured points taken, airfoil design model was developed in SolidWorks software and imported in IGES file format to SpatialAnalyzer software. Figure 6 shows measured points using laser tracker and Figure 7 shows measured points imported to SpatialAnalyzer (SA) software. Table 3 shows the measurements randomly taken at different points to check the flatness and linearity of the molding board. e points were chosen to fit a plane using Spatial Analyzer (SA) software. Accordingly, the flatness of the plane created using the points became 0.0386 with root mean square (RMS) of 0.0079. So, it can be generalized that the flatness of the molding board is equal to the flatness of this plane. Table 4 shows the tracker reading taken along the edge of the molding board to check linearity. Checking the linearity is important as the edge is used to guide the template in the plug making process. e points were connected to fit line SpatialAnalyzer (SA) software. e linearity analyzed using the SpatialAnalyzer software indicates 0.2067 with RMS of 0.0908. Table 4 shows the reading taken by Spherically Mounted Retro Reflector (SMR) attached to the template to take reading while the template makes shaping process of the plug. e measured points were fit to make a line to check linearity test of the movement in SpatialAnalyzer software and the result shows 0.2179 linearity with RMS of 0.0918. When we compare the linearity of the molding edge to the template movement along the molding edge, the result shows linearity difference of 0.0112 with RMS difference 0.001. is proves that the template movement is almost parallel to the edge of the molding board without deflection from the molding board surface. Table 5 shows the quick alignment points first selected and the optimization parameters of points to object fit. Table 6 shows the optimized fits of the points measured to the airfoil surface model imported from SolidWorks   e standard deviation of the fit from the average and zero is 0.22224 with root mean square of 0.2210. e maximum magnitude of the difference including x and y together is 0.5336 and the minimum −0.5077. It can be generalized from this fitting that, considering error duplications from surface of molding board, template movement, and personal errors, and so on, the result indicates a good fit of the produced airfoil surface with the model created.

Results and Discussion
In the study conducted, the inspection result shows that the linearity of molding board edge and template movement is in agreement. at means the error that can be created due to template movement is very low. Concerning the quality of produced airfoil surface, based on the average value of the fitness of the measured points on the airfoil model created in SolidWorks, it revealed a good result from the RMS value.     From the above result, it is possible to conclude that fitting of measured points taken by laser tracker to surface of a model created in SolidWorks imported to SpatialAnalyzer software can be used as an inspection tool against a given specification. Various applications of a laser tracker have been discussed in [7][8][9]. e use of a laser tracker as instrumentation tool and inspection tool is in agreement with [4][5][6]11]. A comparison of commercial metrology instruments conducted by [6] on Metris K610, Metris MV224, Faro tracker, and V-stars shows the ability of laser tracker to work in higher working range within the same level of uncertainty with compared instruments.

Conclusions
A laser tracker was chosen as an inspection tool to study the surface conformity test of airfoil surface. e following conclusions were drawn from the current work: (i) Measurements are taken from the airfoil surface using laser tracker through surface scanning method and these measured points are fitted to three-dimensional surface model imported to SpatialAnalyzer software (ii) From the test results, effect of error coming from the molding board flatness and linearity is negligible. Effect of manual shaping of the plug also is considered negligible (iii) From the surface conformity test, since the measured points are perfectly fitted with three-dimensional surface model created, the manufactured product quality is good (iv) It is also confirmed that laser tracker is one of the important measurements and inspections that can be easily utilized within the acceptable range of accuracy

Data Availability
For airfoil surface analysis, three-dimensional model of the airfoil surface was developed in SolidWorks software and imported in IGES file format to SpatialAnalyzer (SA) software. en, measurements were taken from the manufactured airfoil surface using laser tracker through surface scanning method. Surface conformity test was conducted through fitting of measured points to surface model imported from SolidWorks to SpatialAnalyzer (SA) software.

Conflicts of Interest
e authors declare no conflicts of interest. Advances in Materials Science and Engineering 9