edge loop belonging to each face in the STEP format with the basic format entities. Then, they regularly represent them in the basic format. Each face representation in basic format begins a face number followed by face attributes such as surface type, radius, maximum radius, minimum radius, angle, local origin, direction in accordance with face type and, terminates with attributes of the edge loop bounded each face such as curve type, start and end coordinates of the curve. With this format, each face on the component is generated a face section. All the attributes and coordinates belonging to all surface types are represented in their face section. Surface types handled in this study are planar,

Fig. 4. (a) STEP representation of intake valve; (b) stitched faces of the intake valve.

Fig. 6. Intake valve and FORM of the intake valve.

cylindrical, spherical, conical, toroidal, bounded, b_spline and quasi_uniform surfaces. Edge types are line, arc, ellipse, bounded curve, b_spline curve and quasi_uniform curve. All these surfaces and edge curves are clearly represented in the basic format. Conversion of STEP file into basic format for a conical face is shown in Fig. 2.

The basic format has two sections. The first section begins with face number and represents face attributes. The second section starts ‘‘outer edge loop’’ entities and represents the attributes, coordinates and curve types of the edge curves of the outer and/or inner edge loops bounding a face (Fig. 2).

3.2. Stitching the equivalent faces

Cylindrical, conical, toroidal, spherical surfaces in STEP graphic standard are represented with two faces. Because an expert system is utilized in part recognition algorithm of the program, to define a rule for such parts in the knowledge base, it is rather time-consuming and difficult. Therefore, use of stitched faces on the component for rule writing is available than STEP representation of a part.

For example, faces 1–2, 4–5 and 7–8 of plain rivet in Fig. 3a is inpidual faces in the STEP file. To define rule, equivalent

faces should be stitched into a unique face as shown in Fig. 3b. Thus, rule definition processes in the knowledge base are fairly simplified. For this reason, the program developed automati-cally stitches pairs of the equivalent faces. To stitch a pair of equivalent faces, some conditions must be satisfied. For example, attributes of two faces should be the same and two faces must share at least a common edge in their edge loops.

The pairs of faces 2–3, 4–5, 7–8, 10–11, 12–13, 15–16, 17– 18 on intake valve shown in Fig. 4a are symmetric and equivalent. Therefore, they must be stitched. After stitching process of faces, pairs of faces 2–3, 4–5, 7–8, 10–11, 12–13, 15–16, 17–18 in Fig. 3a, new faces are re-numbered as faces 2, 3, 5, 7, 8, 10, 11, respectively (Fig. 4b). The part model with stitched faces has been used as input for both building of FORM and optional and automatic rule writing module in the system.

4. Construction of FORM

Part representation schemes can be important communica-tion tool and input for many CAD systems. Parts are represented in CAD systems using B-Rep and/or constructive solid geometry (CSG). A part in the B-Rep is represented by vertices, edges, loops and surfaces. A part in the CSG is

Fig. 7. Plain rivet (Ø5) and the plain rivet represented in FORM. Fig. 8. Plain rivet (Ø6) and the plain rivet represented in FORM.

738 A. C¸ ic¸ek, M. Gu¨lesin / Computers in Industry 58 (2007) 733–746