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If the workpiece is six-sided and that, in principle, any one face can be chosen for the plane locating face, the workpiece has six possible positions in the machine workspace for each of the plane locating types, pos1 and pos2. If none of the possible workpiece positions meet the requirements, the fixture solution strategy based on the ideal workpiece position cannot yield a result and should be abandoned.
4.1.2 Fixture solution based on the overlook of the loosely tolerance-related faces
This strategy is used in cases when the first one fails to produce a fixture solution. The eventual workpiece position is selected under the condition that all strictly tolerance-related surfaces must be machined in one clamping. This strategy is similar to the previous one, with the exception that the loosely tolerancerelated faces are omitted during the process of selection of the workpiece position in the machine workspace.
In order for the assumed workpiece position to be adopted according to the previous requirement, it has to be suitable for locating and clamping. With this strategy, the main clamping is always the second one, which implies that only finished surfaces can be used for locating. The suitability of a workpiece position is checked as for the first strategy. This strategy still generates a fairly acceptable fixture solution, considering that fixture
accuracy is critical only with respect to the parallelism or perpendicularity of certain surfaces of the main clamping fixture. In cases when this strategy fails to produce a solution, another strategy should be applied.
4.1.3 Fixture solution based on the reduction of a strictly tolerance-related side into loosely tolerance-related surfaces and strictly tolerance-related surfaces
This employs the analysis of strictly tolerance-related faces in order to find a face, which, besides including tolerance-related surfaces, also includes tolerance-unrelated or loosely tolerancerelated faces, which are suitable for locating and clamping. If such a face exists, then all the included surfaces, except the strictly tolerance-related ones, are machined in an additional clamping. That face will be the plane locating face in the main clamping, where all the strictly tolerance-related workpiece surfaces will be machined.
Fig. 8. Surface quality of the workpiece plane locating surface
Fig. 9. Typical cases of relationships on the plane locating face and possible types of side
locating
Since in the main clamping, some surfaces are machined on the plane locating face, the plane locating type will always be pos3. A more precise definition of workpiece position follows from the condition that the face facing the machine table and its opposite face must not be strictly tolerance-related. Plane locating, side locating, and clamping suitability are checked for in a way similar to the one described in the previous strategy.
This fixture solution permits machining accuracy without special requirements in respect to fixture accuracy, but nevertheless increases fixture complexity.
4.1.4 Fixture solution by disintegration of strict functional relationships
Should none of the discussed strategies succeed in generating the desired solution, one is forced to resort to machining the strictly tolerance-related surfaces in separate clampings. For this reason, the fixture for the second clamping must be highly accurate,
which often presents a key issue in workpiece accuracy.
The first step involves an attempt to find, by horizontal plane locating, a workpiece position in the machine workspace, which ensures that the face opposite to the plane face is not strictly tolerance-related. If this attempt fails, then both the plane face and its opposite face are allowed to be strictly tolerance-related, on the condition that the plane face is not one of the wide faces on a thin workpiece. In the case of thin workpieces, the plane locating type used in the main clamping is pos2. For each assumed workpiece position, the suitability for plane locating, side locating, and clamping are checked according to the previous solution methods.