Techniques of obtaining geometric models.

Cosma, Cristian ; Cioana, Cristian ; Stan, Daniel 等

1. INTRODUCTION

The process of obtaining digital models from an existing physical model by acquiring surface information using scanning or measurement devices is usually called reverse engineering.

From the 3D physical models, reverse engineering techniques and systems such as RapidFormTM, can be used to construct surface from unorganized 3D measured points; while for 3D sketching, there are some challenges in constructing surface from unorganized curves (Moustakas et al., 2009).

2. METHODOLOGY PROPOSE

In the following will present the two methods of making geometric models from a physical model. The part that was done study (digitized mouse) was initially measured on a machine coordinate measured (figure 1). Measurements where defined by a probe attached to the third moving axis of this machine. There are two types of collecting the data (1) contact type (mechanical type) and (2) non-contact type (optical type).

We used a mechanical probe, which is used to collect the data, by touching the surface at various points on part profile that is coordinate point's xyz.

2.1 Getting geometrical model using reverse engineering techniques

For obtaining the cloud points, and so the geometric characteristics of the model we used the scanning machine Modela MDX 15. The machine is automated and uses a mechanical probe which is drawn along the surface of the object. The automated scanning is more accurate than manual scanning. However, the scanning method is slightly less accurate than individual point method. The software allows the machine setting area which is intended to be scanned and the pitch will be used.

After the scan, the file obtained (cloud points) is saved in STL format. Then, using special software from reverse engineering, through various Boolean operations (intersection, merge) is obtained the geometric model of the scanned part--figure 1 (Cosma et al., 2008).

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2.2 Getting part using 3D sketch method

The paper presents a style for 3D modeling, which includes the integration of scanned images, drawings, in the modeling process. These images serve as a guide for the user when they are modeling the virtual object. We can say that this method is part of Top-Down methodology. One can say this, because the designer is using the drawings to make a better image of the new product. This method is based on creating 3D models sketching the important curves of the product, thus making the frame. The curves which are composing the skeleton are drawn up according to the 2D drawings that represent the views of the product: Top view, Front view and Side view. This method combines 3D sketch technique with traditional design (Kara et al., 2007).

a). Placing the *.* jpg files in the appropriate planes

We insert the images into CAD software where we resize them according to the gauge of the physical model, obtained in previous stage. To achieve skeleton part we can use several methods. One method consists in building plane and 3D sketches, based on the contour of the image, loft them together to form surfaces. After trimming or extending the surfaces later we can knit them together, shelled and split into various components. The method takes a lot of time and patience, but the result is better than using other methods.

b). Drawing the spline skeleton of the model

The skeleton of the model is composed of both plane sketches but also 3D sketches (Sapidis et al., 2005). Using the points of the spline curves, was followed the contour of the pictures, trying to adjust the spline in such way that it will take the form of the model. In figure 2 is shown the tracing of a sketch using spline curves, obtaining the desired contour by altering the position of points. It creates several sketches which accurately copy the lines which define the contours of the product (figure 3). Using the method to create surfaces by Sweep function is not needed to use Mirror function, the model is achieving in its entirety.

c). Creating surfaces that will form the model

We mentioned the use of spline curves (B-spline) without mentioning the definition and their usage; thereof, curve splines such are interpolation curves controlled by points which are respect the condition and continuity of the curve. Shape control is made through the change of the points and angle of contact; this makes them more difficult to handle for the creation of so called Free forms.

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Otherwise, are noted generating surfaces that will shape the product? We can generate two types of surfaces; surfaces obtained by using lines or curves and also NURBS (Non Uniform Rational Basis Spline) which are called Freeform surfaces consist of B-Splines, obtained using 3D outlines, which allow the formation of links with spline points in different areas (Seiculescu et al., 2009).

Areas with freeform have no fixed points, they can change like the designer wants, resulting in new models. Designer can modify the surface by changing the position of points in space. Forms can be in a wide range of surfaces whose shape cannot be measured, only approximated.

The main functions used were those of Sweep, Planar Surface and Surface Extrude. After, being used all drawings which formed the skeleton of the part.

Finally, using different modifications and complete details of the model we obtain the geometrical model (figure 4).

As it can be observed, the differences between physical model values and virtual model values (Tulcan et al., 2004) are presented in table 1.

In the figure below (figure 5) is presented values of the virtual geometric models obtained by the two methods outlined above.

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Once the geometric model is obtained we can manufacture the replica or the insertions of mold (Figure 6).

3. CONCLUSION

This paper presents two methods of obtaining certain products for which there is no technical documentation. The first one appeals to modern techniques of reverse engineering (hardware and software) is a relatively simple method, but not cheap.

The second method involves more modest resources, but requires knowledge of the applicant further in terms of geometrical modeling.

Constraints that have emerged in the research were held by the complexity of parts. If in the case of reverse engineering are less obvious, in the 3D sketch method with increasing complexity of the parts the graders difficulty in obtaining their geometric models is increasing.

This tool should improve the method considerably because the user will be focused in the creative process instead of computer interface problems.

4. REFERENCES

Cosma, C.; Dume, A.; Tulcan, A. & Iclanzan T. (2008). Reverse Engineering for Injection Parts, Materiale plastice, Vol. 2, No. 45 (June 2008) page number (208-213), ISSN 0025/5289

Kara, L.B.; Shimada, K. & Marmalefsky, S.D. (2007). An evaluation of user experience with a sketch-based 3D modeling system. Computers & Graphics, Vol. 31, No. 4, (August 2007), pages numbers (580-597), ISSN: 0097-8493

Moustakas, K.; Nikolakis, G.; Tzovaras, D.; Carbini, S.; Bernier, O. & Viallet, J.E. (2009). 3D content-based search using sketches. Personal and Ubiquitous Computing, Vol. 13, No. 1 (January, 2009) page numbers (1-8), ISSN 1617-4909 (Print) 1617-4917 (Online)

Sapidis, N.; Kyratzi, S. & Azariadis, P. (2005). Improved Computational Tools for Concept Development based on Sketches and Advanced CAD Technologies. Computer-Aided Design & Applications, Vol. 2, No. 6, (September 2005), page numbers (707-716), ISSN 1686-4360

Seiculescu, V.; Tulcan, A. & Stan, D. (2009). Virtual 3D CAD model of complex bodies realised under solid works environment, Academic Journal of Manufacturing Engineering, Vol. 7, No.1, (january 2009) page number (89-94), ISSN 1583-7904

Tulcan, A.; Grozav, I.; Turc, C. & Tulcan, L. (2004). Tesa Micro-Ms 343 coordinate measuring machine accuracy evaluation, Academic Journal of Manufacturing Engineering, Vol. 2, No.3, (september 2004) page number (38-42), ISSN 1583-7904

Tab. 1. Comparisons between real and virtual values

Axis Physical Virtual Virtual model value
 model model through Sketch based
 value [mm] value [mm] 3D modeling [mm]

X 107.9875 108.2766 107,24
Y 65.6573 65.7346 64,36
Z 28.4314 28.674 26,91