Process of unknown variable fillet definition realized in order to substitute hand production by NC manufacturing.

Monkova, Katarina ; Monka, Peter

1. INTRODUCTION

At the production of parts on NC/CNC machines, which dimensional and shape topography characteristics are unknown, it is useful to obtain needed data in digital form. The digitizing of real three dimensional objects is time consuming process that is known as Reverse engineering. In conventional computer aided design, the representation of object is created by means of commands typically using an interactive design program with 3D graphics. This representation is used for further design, analysis and optionally for numerically controlled manufacturing. While conventional engineering transforms engineering concepts and models into real parts, in reverse engineering real parts are transformed into such computer models which are suitable to exploit all advantages of modern CAD/CAM technologies and transfer real objects into virtual reality environments. Typical applications include reproducing and redesigning parts, when no original drawings or documentation are available. In areas where aesthetic design is particularly important--such as in the automobile industry real-scale wood or clay models are often needed, because stylists prefer to evaluate real 3D objects rather than 2D screen images at reduced scale. (Weyrich & Drews, 1999)

Basis for the work inside this method are special equipments so called 3D scanners. In the most general terms, 3D scanning, also referred to as 3D digitizing, is the utilization of a three dimensional data acquisition device to acquire a multitude of X, Y, Z coordinates on the surface of a physical object. Each discrete X, Y, Z coordinate is referred to as a point. The conglomeration of all these points is referred to as a "point cloud". Polygonal mesh representation of the point cloud is known as an STL file format. (Varady, 2001) This manner of data obtaining is used for specific and complex part shapes which topography and dimensions are unknown. A template for stator winding of electromotor (Fig. 1) belongs to the group of parts and it is a component that has to exactly fit into assembly. (Fig.2)

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2. PROCESS OF UNKNOWN VARIABLE RADIUS DEFINING

In mechanical engineering, a fillet is defined as a concave easing of an interior corner of a part design. A rounding of an exterior corner is called a "round". Fillets can be quickly designed onto parts using 3D solid modelling engineering CAD software by invoking the function and picking edges of interest. Once these features are included in the CAD design of a part, they are often manufactured automatically using computer-numerical control. (Lukovics & Bilek, 2008)

The variable fillet is achieved by adding variable attributes at any location chosed along a selected edge. When the fillet is complete, the variable attributes will govern the shape of the fillet by constraining the radius to the value of the attribute at given points. These options are used to add a variable attribute at a point or points on an edge. The attribute controls the fillet radius at the point. Multiple attributes can be added along an edge to create a variable fillet along the edge.

Variable radius flow is defined by following entities (Fig.3)

* edge for the round

* points on the edge and its position

* radius in the points

The same radius can be created by various values of the entities listed above. Combination of point position and value of radius has many solutions and their determination, mainly if the fillet is unknown, is very difficult. Therefore the special technique is needed for the manufacturing conditions improvement.

Variable radius fillet of the template for stator winding of electromotor was up to now abroad produced in a way that its finite shape underwent a hand grinding into an anti-template but the drawing documentation of the resultant topography was not available.

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The average delivery time was longer than 3 month. For the productivity increasing and delivery time shortening it was needed to use manufacturing technology with NC/CNC machines application. To take advantage of suggested technology the Reverse Engineering technique of data obtaining was used.

The unknown geometrical characteristics obtaining at the complex shaped parts is time consuming process. It was realized at FMT TU Kosice with seat in Presov for this part by means of laser scanner LPX 250 and CAD/CAM software Pro/Engineer. The whole process consists of several steps:

1. Real steel part preparation

2. Scanning

3. Data processing and transferring

4. 3D model creation

5. CL data generation and Production

1. Real steel part preparation. Considering the surface of the original component was too reflexive for the laser beam (it was polished), it was necessary to decrease its gloss values, e.g. by spray-painting with a gray undercoat colour. At the same time, it was essential to evenly apply the sprayed layer as this factor may also affects the approximation rate of a created model toward its original and a finite accuracy of the component created on the basis of a virtual 3D model. (Jankura et al., 2008)

2. Scanning. Plannar scanning from 6 various positions was chosed in this case. (Fig. 4)

3. Data processing and transferring. For next processing it was necessary to export the acquired data from the scanner software, transferred it and subsequently imports into CAD/CAM software. Conrete procedure used on FMT TU Kosice with seat in Presov is shown on Fig.5. (Monkova et al., 2008)

4. Modelling. The modelling of the part in this phase was done with the goal to achieve the real geometry as precisely as it is possible. It was used various techniques, tools and features that the selected CAD/CAM system Pro/Engineer proposes to user. The curves that created the boundary of round were smoothed by means of Matlab software. The final version of created 3D model is shown on the Fig.6; created assembly is presented on Fig.7.

5. CL data generation and NC/CNC manufacturing. Within the software Pro/Engineer it is possible to compare the created 3D model with the scanned data by geometry. The modelled volume equals to the origin steel part inside needed tolerances and so it can be used for the producing of new part by means of NC machine. Machining process was simulated in CAM module of software where specific parameters were defined in regard to the tool steel machining. CL data, that are direct output of module Pro/Manufacturing in CAD/CAM system Pro/Engineer, were transformed by postprocessor to NC program for specific control system of NC/CNC machine.

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3. CONCLUSION

Present situation in the industry is characterized as a period of intense progress of technologies at the significant computer aid in all branches of industry. The product must be competitive, it must be up to qualitative and functional standard, it must have reasonable price, efficacious design, it must have regard for safety, ergonomic and another aspects, which decided about its marketability. In connection with the technical advance it is increasing the pressure on the manufacturers to develop and make the products as soon as possible at the minimal cost in required quality. On the other hand, the evolution of all-new product, or its innovation, is the process very difficult and time-consuming in regard to requirements listed above. Various technologies are used at the part manufacturing; NC machines are applied in many cases. The machining by means of CL data as output of CAM system is very quickly, simple and precise. (Valicek et al., 2009)

Modern production methods with computer aid were applied in described process, too. Time to delivery of real template for rotor winding after digitizing was abbreviated from three month to several days and the production costs decrease ten times at average.

4. REFERENCES

Jankura, D., Brezinova, J. & Draganovska, D. (2008). Technical materials, TU Kosice, ISBN 978-80-8073-959-1, Slovakia

Lukovics, I. & Bilek, O. (2008). High Speed Grinding Process, Manufacturing Technology, vol.8, p. 12-18, ISSN 1213248-9.

Monkova, K., Hatala, M. & Cep, R. (2008). Some problems that originate at the creating of 3D model with difficult shapes without the parameters and dimensions of real part. Proceedings of scientific works of VSB TU, Mechanical Engineering Part, Vol. 54, no. 1, Ostrava, p. 159-163, ISBN 978-80-248-1891-7, ISSN 1210-0471.

Valicek, J., Hloch, S. & Kozak, D. (2009). Surface geometric parameters proposal for the advanced control of abrasive waterjet technology. International Journal of Advanced Manufacturing Technology. vol. 41, no. 3-4, p. 323-328. ISSN 0268-3768.

Varady, T. (2001). Reverse Engineering Shapes, ERCIMNews, No.44, p. 19-20, ISSN 0926-4981

Weyrich, M. & Drews, P. (1999). An interactive environment for virtual manufacturing: the virtual workbench, Computers in Industry, no. 38, p. 5-15, ISSN 0166-3615