# Geometrical Tolerances--Separate, Combined or Simultaneous?
## 2 New Rules That Reduce Specification Ambiguity
In Figure 4a, the two cylindrical tolerance zones for the position tolerance establish the combined tolerance zone (the CZ modifier is indicated)--their axes are located in one plane that is perpendicular to datum A and passes through datum B as well as the tolerance zone axes that are at a radius of 20 mm from datum B. The two tolerance zones for the symmetry indicator form the next combined tolerance zone (the CZ modifier is indicated). One common symmetry plane for the two pairs of two parallel plains establishing each of the two symmetry tolerance zones is perpendicular to datum A and passes through datum B (Figure 4b). This means that the actual part shown in Figure 3b shall be rejected according to the requirements given in Figure 4a. In Figure 5a, the axes of the tolerance zones and the symmetry planes of the tolerance zones are located on two mutually perpendicular planes that are perpendicular to datum A and pass through datum B, as defined by the SIM modifier--the simultaneous requirement. Moreover, the axes of the cylindrical tolerance zones are at a radius of 20 mm from datum B. The use of a SIM modifier transforms a set of more than one geometrical specification into a combined specification (pattern specification). This means that the tolerance zones for all the given specifications are locked together with location and orientation constraints (Figure 5b). According to the requirements shown in Figure 5a, the actual parts shown in Figures 3b and 4b shall be rejected. Figure 3: (**a**) Specification—there are no constraints for the angle between the toleranced holes. There are no constraints for the angle between the toleranced grooves. There are no constraints for the angle between the holes and the grooves. (**b**) The actual part that fulfils the specification from (**a**). Derived median lines of the top (1) and bottom (2) holes and derived median surfaces of the left (3) and right (4) grooves are in the tolerance zones. Each tolerance zone is separately established with respect to datum system A\(\,\)B. The above discussion may be used to correct the specifications given in Figures 1 and 2. In Figure 1, the CZ modifier shall be added after the tolerance value T = 0.1 mm to set the pattern of two holes with a diameter of 10 mm. The CZ modifier determines that the tolerance feature is defined as the collection of two extracted median lines. The tolerance is the tolerance zone pattern composed of two cylindrical tolerance zones with a diameter of 0.1 mm, where their axes are perpendicular to datum A (implicit theoretically exact dimension, TED = 90\({}^{\circ}\)[8]), that are internally constrained in a location at a distance of TED = 40 mm apart and are diametrically opposed, i.e., the axes of the tolerance zones are located on the plane perpendicular to datum A that passes through datum B. Moreover, due to the CZ modifier, the tolerance zones are constrained in the location from datum B at a distance of 20 mm. Alternatively, as shown in Figure 1, when the SZ modifier (separate zones) is specified, each individual cylindrical tolerance zone shall be independently considered. This means that the angular location between the holes is not fixed. The axis of each tolerance zone is located in the half-plane that starts from datum B, but the angular location between the two half-planes is optional. In Figure 2, the SIM modifiers shall be indicated on the right side of the two position tolerance indicators specified for two holes with a diameter of 10 mm. The specification of the SIM modifiers transforms the specifications that are individually given for each hole with a diameter of 10 mm into the combined specification. It denotes that the tolerance zones for two holes are locked together with location and orientation constraints into the pattern specification. So, in the considered case, the specification of two SIM modifiers is functionally equivalent to the discussed specification with the CZ modifier.