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Not all the faces of a workpiece are suitable for plane locating.In the case of flat workpieces, only the large faces, and in the case of long workpieces, only the long faces are suitable.
The dimensions of candidate features for plane locating must be compared with the three overall dimensions of the workpiece.
3.4.2 Surfaces suitable for side locating
Suitable features are established by each side locating type.
a) Suitable surfaces for side locating type p1. Side locating can be pided into guide locating and endwise locating. Hence, suitability tests are to perform separately for guiding and for endwise locating.
Suitability for guiding must be tested from three aspects, i.e., shape of surface, dimension, and position of surface.
• According to the shape of the guide locating element the useable faces are: planar face, two planar faces, two cylindrical faces, combination of cylindrical and planar surfaces, and single cylindrical surface.
• The typical dimension of a surface must not be less than 35% of the longest dimension of the plane locating face.
• According to the position of the guide locating element, they must belong to adjoining faces of the plane locating face.
Suitability for endwise locating must be tested from two aspects, i.e., according to shape of surface and position of surface.
• According to the shape of the surface, planar or cylindrical surface is useable for endwise locating.
• According to the position of the endwise locating elements,they must be on the face, which is the adjoining face of both the plane locating face and guiding face.
b) Suitable surfaces for side locating type p2. According to the shape of surface there should be two holes on the plane locating face. The distance between the holes must not be less than 35% of the greatest length of the plane locating face. Suitable surfaces for other side locating types are defined in a similar way.
3.4.3 Suitable surfaces for clamping
Suitable clamping surfaces have been established for every clamping type.
4 General solution concept of operation sequence and clamping fixtures
Rectangular castings can almost always be machined on a horizontal machining centre in two operations. The operation in which the functional surfaces, or most of them, are machined, is called the main clamping, while the operation in which the rest of the surfaces are machined is called additional clamping. Fixtures for both the main and additional clamping make a clamping fixture set. The additional clamping takes precedence in most cases.Regardless of the clamping sequence, it is the main clamping that is solved prior to the additional clamping.
4.1 General solution concept of main clamping fixture
On a horizontal machining centre in one clamping, a maximum of four faces of the workpiece are workable. When solving the main clamping, one must try to find a position for the workpiece in the workspace a of machining centre in which the machining of all the connected faces is possible. In this way the greatest accuracy of the workpiece can be obtained and the accuracy requirement and the complexity of the fixture are the lowest. This position of the workpiece is called the “technologically ideal
workpiece position”. The fixture determined in this way is the best possible fixture solution.
However, in several cases the disposition of connected faces means that the workpiece can not be held in a technologically ideal workpiece position. In this case, it is necesary to find a position of the workpieces in which at least the machining of strictly connected faces is possible in one clamping. In other words, the loosely connected faces in this stage are left out of consideration.In this way, the fixture solution is still “fair”, but the accuracy requirements refer only to the parallelism, or rather to the perpendicularity
of any fixture surfaces.