The group of strictly tolerance-related connections consists of accuracy-related requirements which can be achieved only with difficulty in cases when the connected surfaces are machined in two separate clampings. They need a high accuracy of the fixture and of the locating elements of the workpiece. For these connecting types the connected surfaces need to be machined in one clamping or, such surfaces should be machined in two separate clampings, only when there is no other solution.
   The faces of a workpiece, depending on the surfaces they contain, have appropriate attributes. The face is: “machined” when it contains a machined surface, it is “connected” when it contains a connected surface, it is “strictly connected” when it contains a strictly connected surface, and it is “loosely connected” when it contains a loosely connected surface.
3 Identification of the locating and clamping entities
3.1 Plane locating types of the workpiece
  Determining of the plane locating types was with the help of the analysis of technological facilities of horizontal machining centers and existing clamping fixtures. There are three types of plane locations (Fig. 4): horizontal (denoted with “pos1”), vertical (“pos2”), and vertical with partial machining of the locating face (“pos3”).
  The selection of a plane locating face with a given plane locating type determines the set of workable faces.
3.2 Side locating types
From an analysis of existing clamping fixtures and the corresponding literature, there are four basic types of side location(Fig. 5) [8, 10]:
  (1) Side location by using surfaces, which are on the adjoining faces of the plane locating face
  (2) Side location using two inside diameter on the plane locating face
  (3) Side location by using one inside diameter on the plane locating face and a face on one of the adjoining faces
  (4) Side location by using two threaded joints on the plane locating face
 
Fig. 4. Plane locating types
 
Fig. 5. Types of side locating

3.3 Clamping types
   According to the direction of clamping forces, the clamping can be perpendicular to the plane locating surface (type s1) or parallel with the plane locating surface (type s2). The basic type s1,depending on the location of clamping faces, can be further pided into subtypes s11, s12, and s13. In the case of s11, the clamping surfaces are on the adjoining sides of the plane locating face. In the case of s12, the clamping surface (or surfaces) is on the opposite side of the plane locating face, and in type s13, the clamping is on the through hole (Fig. 6).
 
  Fig. 6. Types of clamping
   One of the specific ways of clamping is clamping by screws and threaded joints on the plane locating face which is called type s3.
   The number of clamping points is also a very important characteristic of clamping. According to the number of clamping points, clamping is differentiated in one, two, three, and four points. Adding these to the previous basic types, the possible clamping types are: s11_2, s11_3, s11_4; s12_2, s12_3, s12_4;s13_1, s13_2; s2_1, s2_2; s3_2, s3_3, and s3_4.
  In this notation, the last number means the number of clamping points.
3.4 Suitable surfaces of the workpiece for locating and clamping
3.4.1 Suitable surfaces for plane locating
  The suitability of surfaces for plane locating depends on the shape and dimension of the surface.
  According to the shape, the following surfaces are suitable for plane locating: planar surfaces, intermittent planar surfaces, group of planar surfaces in same plane, group of planar surfaces in two different planes, cylindrical surfaces (with parallel axes), and combination of cylindrical and planar surfaces.
  The surfaces (features) so far identified are not equally suitable.The suitability decreases in the order of enumeration. In the expert system prototype, only the first three features are builtin. Next to the fair shape (feature), the plane locating surface must be sufficiently large in order to be usable for plane locating. Prismatic workpieces, according to their global structure, can be pided into: cubical, flat and long workpieces. 齿轮箱外壳综合设置和夹具规划系统英文文献和中文翻译(2):http://www.youerw.com/fanyi/lunwen_33217.html