tailieunhanh - Dimensioning and Tolerancing Handbook Episode 1 Part 10

Tham khảo tài liệu 'dimensioning and tolerancing handbook episode 1 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Geometric Dimensioning and Tolerancing 5-131 Note All boundaries are oriented perpendicular to primary datum A. Figure 5-121 FRTZF virtual condition boundaries for Fig. 5-119 With a Secondary Datum in the Lower Segment With composite control there s no explicit congruence requirement between the PLTZF and the FRTZF. But if features are to conform to both tolerances the FRTZF will have to drift to where its virtual condition boundaries or central tolerance zones have enough overlap with those of the PLTZF. Fig. 5-122 shows for our example one possible valid relationship between the PLTZF and FRTZF. Again the virtual condition boundaries are based on a substitute boss. Notice that the PLTZF virtual conditions are so large they allow considerable rotation of the pattern of tapped holes. The FRTZF offers no restraint at all of the pattern relative to datums B or C. This could allow a handle to be visibly crooked on the box. In Fig. 5-123 we ve corrected this limitation by simply referencing datum B as a secondary datum in the lower segment. Now the orientation rotation of the FRTZF is restrained normal to the datum B plane. Although datum B could also restrain the basic location of the FRTZF in a composite control such as this it s not allowed to. Thus while the pattern of tapped holes is now squared up it can still shift around nearly as much as before. Rules for Composite Control Datum References Since the lower segment provides specialized refinementonly within the constraints of the upper segment the lower segment may never reference any datum s that contradicts the DRF of the upper segment. Neither shall there be any mismatch of material condition modifier symbols. This leaves four options for referencing datums in the lower segment. 1. Reference no datums. 2. Copy only the primary datum and its modifier if any . 3. Copy the primary and secondary datums and their modifiers in order. 4. Copy the primary secondary and tertiary datums and their .