TAILIEUCHUNG - Dimensioning and Tolerancing Handbook Episode 1 Part 8

Tham khảo tài liệu 'dimensioning and tolerancing handbook episode 1 part 8', 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 5-81 feature surface s . Rather than achieving a unique and repeatable fit the fixed-size TGC can achieve a variety of orientations and or locations relative to its datum feature as shown in Fig. 5-73. This effect called datum reference frame DRF displacement is considered a virtue not a bug since it emulates the variety of assembly relationships achievable between potential mating parts. 3 65 .O2 Drawing Two possible orientations of part in TGC A BlS CtS l Figure 5-73 Two possible locations and orientations resulting from datum reference frame DRF displacement Usually a looser fit between two mating parts eases assembly. You may have experienced situations where screws can t seem to find their holes until you jiggle the parts around a little then the screws drop right through. Where a designer can maximize the assembly clearances between piloting features those clearances can be exploited to allow greater tolerances for such secondary features as screw holes. This may reduce manufacturing costs without harming assemblability. Relative to a Boundary of Perfect Form TGC In Fig. 5-74 we have three parts shaft collar and pin. Let s assume our only design concern is that the pin can fit through both the collar and the shaft. We ve identified as datum features the shaft s diameter and the collar s inside diameter. Notice that the smaller the shaft is made the farther its cross-hole can stray from center and the pin will still assemble. Likewise the larger the collar s inside diameter the farther off-center its cross-hole can be and the pin will still assemble. On the shaft or the collar we can make the hole s 5-82 Chapter Five Figure 5-74 DRF displacement relative to a boundary of perfect form TGC positional tolerance interact with the actual size of the respective datum feature always permitting the maximum positional tolerance. We ll explain the tolerance calculations in Chapter 22 but right now we re concerned with how .

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