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If this attempt is not successful, then one must give up the idea of machining all of strictly connected faces in one clamping. There are also cases when not the entire face, but only the strictly connected features (holes) on the face are workable with other strictly connected faces in one clamping (see Sect. 4.1.3).
Lastly, when none of these attempts is successful the strictly tolerance-related features must be machined in separate clampings. In this case, the accuracy requirements of the fixture are very high.
Taking into consideration the abovementioned aspects, four strategies are devised for solving the main clamping.
4.1.1 Technologically ideal clamping
This strategy is aimed at finding the position of the workpiece in the machine workspace, which would allow machining of all the connected faces in one clamping. The potential position of a workpiece in the machine workspace is determined from the requirement that all connected faces should be machined in one clamping. Depending on the plane locating type, this can be formulated in the following manner:
• In the case of horizontal plane locating type pos1, the plane locating face and its opposite face must not be tolerancerelated faces, because it is not possible to machine them together with other faces in the same clamping.
• For a vertical plane locating type pos2, the plane locating face, the face which is facing the machine table, and its opposite face, must not be tolerance-related faces.
In order to select the workpiece position, it should be satisfactory for both plane and side locating and clamping (Fig. 7). Since the horizontal plane locating (pos1) allows machining of four faces (while pos2 allows only three faces to be machined) and the fixture construction is simpler, the system first attempts to generate a fixture solution for horizontal plane locating type. If that attempt fails, a second attempt is made at solving the fixture for vertical plane locating.
Suitability for plane locating. The selected workpiece position is suitable for plane locating if the plane locating face contains surfaces which are suitable for plane locating. If the workpiece position is suitable for plane locating, then in the next step, a check for the suitability for side locating is performed.
Suitability for side locating. The clamping sequence mainly determines the applicability of certain types of side locating: if the second clamping is the main clamping, then the side locating type can be either p2.2 or p4, and if the first clamping is the main clamping, then the plane locating type can be p1 or, alternatively,p3.
If, for some of the possible types of side locating, there are suitable surfaces on the appropriate workpiece face, then the
Fig. 7. A part of AND-OR graph for fixture solution in a technologically ideal workpiece position
proposed workpiece position in the machine workspace is considered suitable for side locating and the suitability for clamping is checked in the next step.
In the opposite case, the proposed workpiece position is eliminated and a new attempt is made at finding an alternative workpiece position.
Suitability for clamping. Possible types of clamping are selected depending on the clamping sequence, i.e., the finish quality of locating surfaces. If, for some of the possible types of clamping, there exist suitable surfaces on the appropriate workpiece face(or faces), then the proposed workpiece position is considered to be suitable for clamping. The assessment of the suitability of some surface for clamping is performed through the application of several criteria, such as: shape, size and location suitability, and superposition of clamping forces, as well as some specific criteria related to certain types of clamping. Since the suitability for plane locating and side locating has already been checked at the previous stages, the fixture solution has been reached for a technologically idealworkpiece position. If the proposed workpiece position is not suitable for clamping, the position is eliminated and a new position is selected which is again checked for plane locating, side locating, and clamping suitability.